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Title: The Ancient Volcanoes of Great Britain, Volume 1 (of 2)

Author: Archibald Geikie

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*** START OF THE PROJECT GUTENBERG EBOOK THE ANCIENT
VOLCANOES OF GREAT BRITAIN, VOLUME 1 (OF 2) ***

THE ANCIENT VOLCANOES OF GREAT
BRITAIN

THE

ANCIENT VOLCANOES
OF

GREAT BRITAIN
BY

SIR ARCHIBALD GEIKIE, F.R.S.
D.C.L. Oxf., D. Sc. Camb., Dubl.; LL.D. St. And., Edinb.

DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF GREAT
BRITAIN AND IRELAND; CORRESPONDENT OF THE INSTITUTE
OF FRANCE; OF THE ACADEMIES OF BERLIN, VIENNA, MUNICH,
TURIN, BELGIUM, STOCKHOLM, GÖTTINGEN, NEW YORK; OF
THE IMPERIAL MINERALOGICAL SOCIETY AND SOCIETY OF
NATURALISTS, ST. PETERSBURG; NATURAL HISTORY SOCIETY,
MOSCOW; SCIENTIFIC SOCIETY, CHRISTIANIA; AMERICAN
PHILOSOPHICAL SOCIETY; OF THE GEOLOGICAL SOCIETIES OF
LONDON, FRANCE, BELGIUM, STOCKHOLM, ETC.

WITH SEVEN MAPS AND NUMEROUS ILLUSTRATIONS

IN TWO VOLUMES

VOL. I

London
MACMILLAN AND CO., Limited.
NEW YORK: THE MACMILLAN COMPANY.
1897
All rights reserved

TO

M. Ferdinand Fouqué
MEMBER OF THE INSTITUTE

PROFESSOR OF THE NATURAL HISTORY OF INORGANIC BODIES
IN THE COLLÈGE DE FRANCE

AND

M. Auguste Michel-Lévy
MEMBER OF THE INSTITUTE

DIRECTOR OF THE GEOLOGICAL SURVEY OF FRANCE

DISTINGUISHED REPRESENTATIVES
OF THAT FRENCH SCHOOL OF GEOLOGY
WHICH BY THE HANDS OF DESMAREST FOUNDED THE
STUDY OF ANCIENT VOLCANOES
AND HAS SINCE DONE SO MUCH TO
PROMOTE ITS PROGRESS
THESE VOLUMES ARE INSCRIBED
WITH THE HIGHEST ADMIRATION AND
ESTEEM

PREFACE
In no department of science is the slow and chequered progress of
investigation more conspicuous than in that branch of Geology which treats
of volcanoes. Although from the earliest dawn of history, men had been
familiar with the stupendous events of volcanic eruptions, they were
singularly slow in recognizing these phenomena as definite and important
parts of the natural history of the earth. Even within the present century, the
dominant geological school in Europe taught that volcanoes were mere
accidents, due to the combustion of subterranean beds of coal casually set
on fire by lightning, or by the decomposition of pyrites. Burning mountains,
as they were called, were believed to be only local and fortuitous
appearances, depending on the position of the coal-fields, and having no
essential connection with the internal structure and past condition of our
planet. So long as such fantastic conceptions prevailed, it was impossible
that any solid progress could be made in this branch of science. A juster
appreciation of the nature of the earth's interior was needed before men
could recognize that volcanic action had once been vigorous and prolonged
in many countries, where no remains of volcanoes can now be seen.
To France, which has led the way in so many departments of human
inquiry, belongs the merit of having laid the foundations of the systematic
study of ancient volcanoes. Her groups of Puys furnished the earliest
inspiration in this subject, and have ever since been classic ground to which
the geological pilgrim has made his way from all parts of the world. As far
back as the year 1752, Guettard recognised that these marvellous hills were
volcanic cones that had poured forth streams of lava. But it was reserved for
Desmarest twelve years later to examine the question in detail, and to
establish the investigation of former volcanic action upon a broad and firm
basis of careful observation and sagacious inference. His method of
research was as well conceived as the region of Auvergne was admirably
fitted to be the field of exploration. He soon discovered that the volcanoes
of Central France were not all of one age, but had made their appearance in
a long series, whereof the individual members became less perfect and
distinct in proportion to their antiquity. Beginning with the cones, craters,

and lava-streams which stand out so fresh that they might almost be
supposed to have been erupted only a few generations ago, Desmarest
traced the volcanic series backward in time, through successive stages of
the decay and degradation wrought upon them by the influence of the
atmosphere, rain and running water. He was thus able, as it were, to watch
the gradual obliteration of the cones, the removal of the ashes and scoriæ,
and the erosion of the lava-streams, until he could point to mere isolated
remnants of lava, perched upon the hills, and overlooking the valleys which
had been excavated through them. He showed how every step in this
process of denudation could be illustrated by examples of its occurrence in
Auvergne, and how, in this way, the various eruptions could be grouped
according to their place in the chronological sequence. To this illustrious
Frenchman geology is thus indebted, not only for the foundation of the
scientific study of former volcanic action, but for the first carefully worked
out example of the potency of subærial erosion in the excavation of valleys
and the transformation of the scenery of the land.
While these fruitful researches were in progress in France, others of hardly
less moment were advancing in Scotland. There likewise Nature had
provided ample material to arrest the attention of all who cared to make
themselves acquainted with the past history of our globe. Hutton, as a part
of his immortal Theory of the Earth, had conceived the idea that much
molten material had been injected from below into the terrestrial crust, and
he had found many proofs of such intrusion among the rocks alike of the
Lowlands and Highlands of his native country. His observations, confirmed
and extended by Playfair and Hall, and subsequently by Macculloch,
opened up the investigation of the subterranean phases of ancient volcanic
action.
Under the influence of these great pioneers, volcanic geology would have
made steady and perhaps rapid progress in the later decades of last century,
and the earlier years of the present, but for the theoretical views
unfortunately adopted by Werner. That illustrious teacher, to whom
volcanoes seemed to be a blot on the system of nature which he had
devised, did all in his power to depreciate their importance. Adopting the
old and absurd notion that they were caused by the combustion of coal
under ground, he laboured to show that they were mere modern accidents,
and had no connection with his universal formations. He proclaimed, as an

obvious axiom in science, that the basalts, so widely spread over Central
and Western Europe, and which the observations of Desmarest had shown
to mark the sites of old volcanoes, were really chemical precipitates from a
primeval universal ocean. Yet he had actually before him in Saxony
examples of basalt hills which entirely disprove his assertions.
Fortunately for the progress of natural knowledge, Werner disliked the
manual labour of penmanship. Consequently he wrote little. But his wide
range of acquirement, not in mineralogy only, his precision of statement, his
absolute certainty about the truth of his own opinions, and his hardly
disguised contempt for opinions that differed from them, combined with his
enthusiasm, eloquence and personal charm, fired his pupils with emulation
of his zeal and turned them into veritable propagandists. Misled as to the
structure of the country in which their master taught, and undisciplined to
investigate nature with an impartial mind, they travelled into other lands for
the purpose of applying there the artificial system which they had learnt at
Freiberg. The methodical but cumbrous terminology in which Werner had
trained them was translated by them into their own languages, where it
looked still more uncouth than in its native German. Besides imbibing their
teacher's system, they acquired and even improved upon his somewhat
disdainful manner towards all conclusions different from those of the Saxon
Mining School.
Such was the spirit in which the pupils of Werner proceeded to set the
"geognosy" of Europe to rights. The views, announced by Desmarest, that
various rocks, far removed from any active volcano, were yet of volcanic
origin, had been slowly gaining ground when the militant students from
Saxony spread themselves over the Continent. These views, however, being
irreconcilable with the tenets enunciated from the Freiberg Chair, were now
either ignored or contemptuously rejected. Werner's disciples loved to call
themselves by their teacher's term "geognosts," and claimed that they
confined themselves to the strict investigation of fact with regard to the
structure of the earth, in apparent unconsciousness that their terminology
and methods were founded on baseless assumptions and almost puerile
hypotheses.
With such elements ready for controversy, it was no wonder that before
long a battle arose over the origin of basalt and the part played by volcanoes
in the past history of the globe. The disciples of Werner, champions of a

universal ocean and the deposition of everything from water, were dubbed
Neptunists, while their opponents, equally stubborn in defence of the
potency of volcanic fire, were known as Vulcanists or Plutonists. For more
than a generation this futile warfare was waged with extraordinary
bitterness—dogmatism and authority doing their best to stop the progress of
impartial observation and honest opinion.
One of the most notable incidents in the campaign is to be found in the way
in which the tide of battle was at last turned against the Wernerians. Cuvier
tells us that when some of the ardent upholders of the Freiberg faith came to
consult Desmarest, the old man, who took no part in the fray, would only
answer, "Go and see." He felt that in his memoir and maps he had
demonstrated the truth of his conclusions, and that an unprejudiced observer
had only to visit Auvergne to be convinced.
By a curious irony of fate it was from that very Auvergne that the light
broke which finally chased away the Wernerian darkness, and it was by two
of Werner's most distinguished disciples that the reaction was begun.
Daubuisson, a favourite pupil of the Freiberg professor, had written and
published at Paris in 1803 a volume on the Basalts of Saxony, conceived in
the true Wernerian spirit, and treating these rocks, as he had been taught to
regard then, as chemical precipitates from a former universal ocean. In the
following year the young and accomplished Frenchman went to Auvergne
and the Vivarais that he might see with his own eyes the alleged proofs of
the volcanic origin of basalt. Greatly no doubt to his own surprise, he found
these proofs to be irrefragable. With praiseworthy frankness he lost no time
in publicly announcing his recantation of the Wernerian doctrine on the
subject, and ever afterwards he did good service in making the cause of
truth and progress prevail.
Still more sensational was the conversion of a yet more illustrious prophet
of the Freiberg school—the great Leopold von Buch. He too had been
educated in the strictest Wernerian faith. But eventually, after a journey to
Italy, he made his way to Auvergne in 1802, and there, in presence of the
astonishing volcanic records of that region, the scales seem to have fallen
from his eyes also. With evident reluctance he began to doubt his master's
teaching in regard to basalt and volcanoes. He went into raptures over the
clear presentation of volcanic phenomena to be found in Central France,

traced each detail among the puys, as in the examination of a series of vast
models, and remarked that while we may infer what takes place at Vesuvius,
we can actually see what has transpired at the Puy de Pariou. With the
enthusiasm of a convert he rushed into the discussion of the phenomena, but
somehow omitted to make any mention of Desmarest, who had taught the
truth so many years before.
Impressed by the example of such men as Daubuisson and Von Buch, the
Wernerian disciples gradually slackened in zeal for their master's tenets.
They clung to their errors longer perhaps in Scotland than anywhere else
out of Germany—a singular paradox only explicable by another personal
influence. Jameson, trained at Freiberg, carried thence to the University of
Edinburgh the most implicit acceptance of the tenets of the Saxon school,
and continued to maintain the aqueous origin of basalt for many years after
the notion had been abandoned by some of his most distinguished
contemporaries. But the error, though it died hard, was confessed at last
even by Jameson.
After the close of this protracted and animated controversy the study of
former volcanic action resumed its place among the accepted subjects of
geological research. From the peculiarly favourable structure of the country,
Britain has been enabled to make many important contributions to the
investigation of the subject. De la Beche, Murchison and Sedgwick led the
way in recognizing, even among the most ancient stratified formations of
England and Wales, the records of contemporaneous volcanoes and of their
subterranean intrusions. Scrope threw himself with ardour into the study of
the volcanoes of Italy and of Central France. Maclaren made known the
structure of some of the volcanic groups of the lowlands of Scotland.
Ramsay, Selwyn, and Jukes, following these pioneers, were the first to map
out a Palæozoic volcanic region in ample detail. Sorby, applying to the
study of rocks the method of microscopic examination by thin slices,
devised by William Nicol of Edinburgh for the study of fossil plants,
opened up a new and vast field in the domain of observational geology, and
furnished the geologist with a key to solve many of the problems of
volcanism. Thus, alike from the stratigraphical and petrographical sides, the
igneous rocks of this country have received constantly increasing attention.
The present work is intended to offer a summary of what has now been
ascertained regarding the former volcanoes of the British Isles. The subject

has occupied much of my time and thought all through life. Born among the
crags that mark the sites of some of these volcanoes, I was led in my
boyhood to interest myself in their structure and history. The fascination
which they then exercised has lasted till now, impelling me to make myself
acquainted with the volcanic records all over our islands, and to travel into
the volcanic regions of Europe and Western America for the purpose of
gaining clearer conceptions of the phenomena.
From time to time during a period of almost forty years I have
communicated chiefly to the Geological Society of London and the Royal
Society of Edinburgh the results of my researches. As materials
accumulated, the desire arose to combine them into a general narrative of
the whole progress of volcanic action from the remotest geological periods
down to the time when the latest eruptions ceased. An opportunity of
partially putting this design into execution occurred when, as President of
the Geological Society, the duty devolved upon me of giving the Annual
Addresses in 1891 and 1892. Within the limits permissible to such essays, it
was not possible to present more than a full summary of the subject. Since
that time I have continued my researches in the field, especially among the
Tertiary volcanic areas, and have now expanded the two Addresses by the
incorporation of a large amount of new matter and of portions of my
published papers.
In the onward march of science a book which is abreast of our knowledge
to-day begins to be left behind to-morrow. Nevertheless it may serve a
useful purpose if it does no more than make a definite presentation of the
condition of that knowledge at a particular time. Such a statement becomes
a kind of landmark by which subsequent progress may be measured. It may
also be of service in indicating the gaps that have to be filled up, and the
fields where fresh research may most hopefully be undertaken.
I have to thank the Councils of the Royal Society of Edinburgh and the
Geological Society for their permission to use a number of the illustrations
which have accompanied my papers published in their Transactions and
Journal. To Colonel Evans and Miss Thom of Canna I am indebted for the
photographs which they have kindly taken for me. To those of my
colleagues in the Geological Survey who have furnished me with
information my best thanks are due. Their contributions are acknowledged
where they have been made use of in the text.

The illustrations of these volumes are chiefly from my own note-books and
sketch-books. But besides the photographs just referred to, I have availed
myself of a series taken by Mr. Robert Lunn for the Geological Survey
among the volcanic districts of Central Scotland.
Geological Survey Office,
28 Jermyn Street, London,
1st January 1897.

CONTENTS

BOOK I
GENERAL PRINCIPLES AND METHODS OF
INVESTIGATION

CHAPTER I
PAGE
Earliest Knowledge of Volcanoes—Their Influence on Mythology
and Superstition—Part taken by Volcanic Rocks in Scenery—
Progress of the Denudation of Volcanoes—Value of the Records of
former Volcanoes as illustrating Modern Volcanic Action—
Favourable Position of Britain for the Study of this Subject 1

CHAPTER II
The Nature and Causes of Volcanic Action—Modern Volcanoes
10

CHAPTER III
Ancient Volcanoes: Proofs of their existence derived from the Nature
of the Rocks erupted from the Earth's Interior. A. Materials erupted at
the Surface—Extrusive Series. i. Lavas, their General Characters.
Volcanic Cycles. ii. Agglomerates, Breccias and Tuffs 14

CHAPTER IV

Materials erupted at the Surface—Extrusive Series—continued. iii.
Types of old Volcanoes—1. The Vesuvian Type; 2. The Plateau or
Fissure Type; 3. The Puy Type. iv. Determination of the relative
Geological Dates of Ancient Volcanoes. v. How the Physical
Geography associated with Ancient Volcanoes is ascertained 39

CHAPTER V
Underground Phases of Volcanic Action. B. Materials injected or
consolidated beneath the Surface—Intrusive Series: I. Vents of
Eruption—i. Necks of Fragmentary Materials; ii. Necks of Lava-
form Materials; iii. Distribution of Vents in relation to Geological
Structure-Lines; iv. Metamorphism in and around Volcanic Cones,
Solfataric Action; v. Inward Dip of Rocks towards Necks; vi.
Influence of contemporaneous Denudation upon Volcanic Cones; vii.
Stages in the History of old Volcanic Vents 52

CHAPTER VI
Underground Phases of Volcanic Action—continued. II.
Subterranean Movements of the Magma: i. Dykes and Veins; ii. Sills
and Laccolites; iii. Bosses (Stocks, Culots), Conditions that govern
the Intrusion of Molten Rock within the Terrestrial Crust 77

CHAPTER VII
Influence of Volcanic Rocks on the Scenery of the Land—Effects of
Denudation 100

BOOK II
VOLCANIC ACTION IN PRE-CAMBRIAN
TIME

CHAPTER VIII
Pre-Cambrian Volcanoes
The Beginnings of Geological History—Difficulties in fixing on a
generally applicable Terminology—i. The Lewisian (Archæan)
Gneiss; ii. The Dalradian or Younger Schists of Scotland; iii. The
Gneisses and Schists of Anglesey; iv. The Uriconian Volcanoes; v.
The Malvern Volcano; vi. The Charnwood Forest Volcano 109

BOOK III
THE CAMBRIAN VOLCANOES

CHAPTER IX
Characteristics of the Cambrian System in Britain
The Physical Geography of the Cambrian Period—The Pioneers of
Palæozoic Geology in Britain—Work of the Geological Survey in
Wales—Subdivisions of the Cambrian System in Britain 139

CHAPTER X
The Cambrian Volcanoes of South Wales 145

CHAPTER XI
The Cambrian Volcanoes of North Wales, the Malvern Hills and
Warwickshire 159

BOOK IV
THE SILURIAN VOLCANOES

CHAPTER XII
Characters of the Silurian System in Britain. The Arenig Volcanoes
The Land and Sea of Silurian time—Classification of the Silurian
System—General Petrography of the Silurian Volcanic Rocks—I.
The Eruptions of Arenig Age 173

CHAPTER XIII
The Eruptions of Llandeilo and Bala Age
i. The Builth Volcano—ii. The Volcanoes of Pembrokeshire—iii. The
Caernarvonshire Volcanoes of the Bala Period—iv. The Volcanic
District of the Berwyn Hills—v. The Volcanoes of Anglesey—vi. The
Volcanoes of the Lake District; Arenig to close of Bala Period—vii.
Upper Silurian (?) Volcanoes of Gloucestershire 202

CHAPTER XIV
The Silurian Volcanoes of Ireland 239

BOOK V
THE VOLCANOES OF DEVONIAN AND OLD
RED SANDSTONE TIME

CHAPTER XV
The Devonian Volcanoes 257

CHAPTER XVI
The Volcanoes of the Old Red Sandstone

Geological Revolutions at the close of the Silurian Period Physical
Geography of the Old Red Sandstone—Old Lake-basins, their Flora
and Fauna—Abundance of Volcanoes—History of Investigation in
the Subject 263

CHAPTER XVII
Distribution of the Volcanic Centres in the Lower Old Red
Sandstone—Characters of the Materials Erupted by the
Volcanoes 271

CHAPTER XVIII
Structure and Arrangement of the Lower Old Red Sandstone
Volcanic Rocks in the Field 281

CHAPTER XIX
Volcanoes of the Lower Old Red Sandstone of "Lake Caledonia"
Description of the several Volcanic Districts: "Lake Caledonia," its
Chains of Volcanoes—The Northern Chain: Montrose Group—Ochil
and Sidlaw Hills—the Arran and Cantyre Centre—the Ulster Centre 294

CHAPTER XX
Volcanoes of the Lower Old Red Sandstone of "Lake Caledonia"—
continued
The Southern Chain—The Pentland Volcano—The Biggar Centre—
The Duneaton Centre—The Ayrshire Volcanoes 317

CHAPTER XXI
Volcanoes of the Lower Old Red Sandstone of the Cheviot Hills,
Lorne, "Lake Orcadie" and Killarney 336

CHAPTER XXII
Volcanoes of the Upper Old Red Sandstone—The South-West of
Ireland, the North of Scotland 348

BOOK VI
THE CARBONIFEROUS VOLCANOES

CHAPTER XXIII
The Carboniferous System of Britain and its Volcanic Records
Geography and Scenery of the Carboniferous Period—Range of
Volcanic Eruptions during that time—I. The Carboniferous
Volcanoes of Scotland—Distribution, Arrangement and Local
Characters of the Carboniferous System in Scotland—Sketch of the
Work of previous Observers in this Subject 355

CHAPTER XXIV
Carboniferous Volcanic Plateaux of Scotland
I. The Plateau-type restricted to Scotland—i. Distribution in the
Different Areas of Eruption—ii. Nature of the Materials Erupted 367

CHAPTER XXV
Geological Structure of the Carboniferous Volcanic Plateaux of
Scotland
1. Bedded Lavas and Tuffs; Upper Limits and Original Areas and
Slopes of the Plateaux; 2. Vents; Necks of Agglomerate and Tuff;
Necks of Massive Rock; Composite Necks; 3. Dykes and Sills; 4.
Close of the Plateau-eruptions 383

CHAPTER XXVI
The Carboniferous Puys of Scotland
i. General Character and Distribution of the Puys; ii. Nature of the
Materials Erupted—Lavas Ejected at the Surface—Intrusive Sheets
—Necks and Dykes—Tuffs 414

CHAPTER XXVII
Geological Structure of the Carboniferous Puys of Scotland
1. Vents: Relation of the Necks to the Rocks through which they rise
—Evidence of the probable Subærial Character of some of the Cones
or Puys of Tuff—Entombment of the Volcanic Cones and their
Relation to the Superficial Ejections. 2. Bedded Tuffs and Lavas—
Effects of Subsequent Dislocations. 3. Sills, Bosses, and Dykes 424

CHAPTER XXVIII
Illustrative Examples of the Carboniferous Puys of Scotland
The Basin of the Firth of Forth—North Ayrshire—Liddesdale 462

LIST OF ILLUSTRATIONS
FIG. PAGE
1. Vesicular structure, Lava from Ascension Island, slightly less than
natural size
15
2. Elongation and branching of steam-vesicles in a lava, Kilninian, Isle
of Mull, a little less than natural size 17
3. Microlites of the Pitchstone of Arran (magnified 70 diameters)
19
4. Perlitic structure in Felsitic Glass, Isle of Mull (magnified) 19
5. Spherulitic structure (magnified)
19
6. Micropegmatitic or Granophyric structure in Granophyre, Mull
(magnified)
20
7. Ophitic structure in Dolerite, Gortacloghan, Co. Derry (magnified) 20
8. Variolitic or orbicular structure, Napoleonite, Corsica (nat. size) 22
9. Flow-structure in Rhyolite, Antrim, slightly reduced 23
10. Lumpy, irregular trachytic lava-streams (Carboniferous), East
Linton, Haddingtonshire
24
11. View at the entrance of the Svinofjord, Faroe Islands, illustrating
the terraced forms assumed by basic lavas
25
12. Sack-like or pillow-form structure of basic lavas (Lower Silurian),
Bennan Head, Ballantrae, Ayrshire 26
13. Alternations of coarser and finer Tuff
34
14. Alternations of Tuff with non-volcanic sediment 35
15. Ejected block of basalt which has fallen among Carboniferous
shales and limestones, shore, Pettycur, Fife
37
16. Effects of denudation on a Vesuvian cone
40
17. Section to illustrate the structure of the Plateau type 43
18. Diagram illustrating the structure and denudation of Puys
45
19. Section illustrating submarine eruptions; alternations of lavas and
tuffs with limestones and shales full of marine organisms 48
20. Diagram illustrating volcanic eruptions on a river-plain
49
21. Diagram illustrating volcanic eruptions on a land-surface
50
22. Ground-plans of some volcanic vents from the Carboniferous
districts of Scotland
55
23. View of an old volcanic "Neck" (The Knock, Largs, Ayrshire, a vent
of Lower Carboniferous age) 56

24. Section of neck of agglomerate, rising through sandstones and
shales
58
25. Neck filled with stratified tuff
64
26. Section of neck of agglomerate with plug of lava 65
27. Section of agglomerate neck with dykes and veins
66
28. Section of neck filled with massive rock
68
29. Successive shiftings of vents giving rise to double or triple cones
70
30. Section to show the connection of a neck with a cone and
surrounding bedded tuffs
71
31. Diagram illustrating the gradual emergence of buried volcanic
cones through the influence of prolonged denudation 75
32. Dyke, Vein, and Sill
80
33. Section of Sill or Intrusive Sheet
83
34. Ideal section of three Laccolites. (After Mr. Gilbert)
86
35. Diagram illustrating the stratigraphical relations of the pre-
Cambrian and Cambrian rocks of the North-west Highlands of
Scotland
112
36. Map of a portion of the Lewisian gneiss of Ross-shire 118
37. Section showing the position of sills in the mica-schist series
between Loch Tay and Amulree
124
38. Sketch of crushed basic igneous rock among the schists, E. side of
Porth-tywyn-mawr, E. side of Holyhead Straits 128
39. Section across the Uriconian series of Caer Caradoc
132
40. Map of the volcanic district of St. David's
146
41. Section showing the interstratification of tuff and conglomerate
above Lower Mill, St. David's 154
42. Basic dyke traversing quartz-porphyry and converted into a kind of
slate by cleavage. West side of Llyn Padarn
162
43. Section of well-cleaved tuff, grit and breccia passing up into rudely-
cleaved conglomerate and well-bedded cleaved fine
conglomerate and grit. East side of Llyn Padarn
163
44. Section of Clegyr on the north-east side of Llyn Padarn, near the
lower end
164
45. Section across the Cambrian formations of the Malvern Hills,
showing the position of the intercalated igneous rocks. After
Phillips
170
46. Section across Rhobell Fawr 178

47. Section at the Slate Quarry, Penrhyn Gwyn, north slopes of Cader
Idris
180
48. Sketch-section across Cader Idris
182
49. Section across the Moelwyn Range 185
50. Section across the anticline of Corndon
190
51. Structure in finely-amygdaloidal diabase lava, south of mouth of
Stinchar River, Ayrshire
193
52. View of Knockdolian Hill from the east
194
53. Section across the Lower Silurian volcanic series in the south of
Ayrshire (B. N. Peach)
197
54. Section of part of the Arenig volcanic group, stream south of
Bennane Head, Ayrshire 198
55. Flow-structure in the lowest felsite on the track from Llanberis to
the top of Snowdon
211
56. Section of Snowdon
212
57. Section across the Berwyn Hills. (Reduced from Horizontal Section,
Geol. Surv. Sheet 35)
219
58. Section of the strata on the shore at Porth Wen, west of Amlwch
223
59. Section of intercalated black shale in the volcanic series at Porth yr
hwch, south of Carmel Point, Anglesey 224
60. Green slates overlain with volcanic breccia, Carmel Point
224
61. Blue shale or slate passing into volcanic breccia east of Porth
Padrig, near Carmel Point
225
62. Section of felsites in the Coniston Limestone group, west of
Stockdale
232
63. Fine tuff with coarser bands near Quayfoot Quarries, Borrowdale
234
64. Diagram of the general relations of the different groups of rock in
the Lower Silurian volcanic district along the western shore of
Lough Mask 253
65. Veins and nests of sandstone due to the washing of sand into
fissures and cavities of an Old Red Sandstone lava. Turnberry
Point, Ayrshire
283
66. Ground-plan of reticulated cracks in the upper surface of an Old
Red Sandstone lava filled in with sandstone. Red Head,
Forfarshire 284
67. Section across the volcanic series of Forfarshire
286
68. Section across two necks above Tillicoultry, Ochil Hills
288
69. Section of the granite core between Merrick and Corscrine 290

70. Section across the three Dirrington Laws, Berwickshire
291
71. Section of Papa Stour, Shetlands, showing sill of spherulitic felsite
traversing Old Red Sandstone and bedded porphyrites (Messrs.
Peach and Horne) 292
72. Section across Northmavine, from Okrea Head to Skea Ness,
Shetland, showing dykes and connected sill of granite and
felsite (Messrs. Peach and Horne) 292
73. Section at the edge of one of the bays of Lower Old Red Sandstone
along the northern margin of Lake Caledonia, near Ochtertyre 295
74. Craig Beinn-nan-Eun (2067 feet), east of Uam Var, Braes of Doune.
Old Red Conglomerate, with the truncated ends of the strata
looking across into the Highlands; moraines of Corry Beach in
the foreground 296
75. Section showing the top of the volcanic series at the foot of the
precipice of the Red Head, Forfarshire
300
76. Andesite with sandstone veinings and overlying conglomerate.
Todhead, south of Caterline, coast of Kincardineshire 303
77. Section across the Boundary-fault of the Highlands at Glen Turrit,
Perthshire 305
78. Section across the chain of the Sidlaw Hills near Kilspindie 306
79. Section across the Eastern Ochil Hills from near Newburgh to near
Auchtermuchty
307
80. Generalized section across the heart of the Ochil Hills from
Dunning on the north to the Fife coal-field near Saline on the
south
308
81. Diagram of the volcanic series of the Western Ochil Hills
309
82. View of Cnoc Garbh, Southend, Campbeltown. A volcanic neck of
Lower Old Red Sandstone age, about 400 yards wide in its
longer diameter 312
83. Section of volcanic series on beach, Southend, Campbeltown 313
84. Section of the base of the volcanic series, Reclain, five miles south
of Pomeroy
316
85. Section of shales and breccias at Crossna Chapel, north-east of
Boyle 316
86. Section across the north end of the Pentland Hills, from Warklaw
Hill to Pentland Mains. Length about five miles
318
87. View of the lava-escarpments of Warklaw Hill, Pentland chain,
from the north-west 319
88. Section across the Pentland Hills through North Black Hill and
Scald Law. Length about three miles 322

89. Section from the valley of the Gutterford Burn through Green Law
and Braid Law to Eight-Mile Burn 322
90. Section across the north end of the Pentland Hills, and the southern
edge of the Braid Hill vent. Length about two miles
324
91. Section across the northern end of the Biggar volcanic group, from
Fadden Hill to beyond Mendick Hill 326
92. Section across the southern part of the Biggar volcanic group from
Covington to Culter 328
93. Section from Thankerton Moor across Tinto to Lamington 328
94. Section across the Duneaton volcanic district from the head of the
Duneaton Water to Kirklea Hill
330
95. Cavernous spaces in andesite, filled in with sandstone, John o'
Groats Port, Turnberry, Ayrshire 333
96. Section of andesites, Turnberry Castle, Ayrshire 334
97. Lenticular form of a brecciated andesite (shown in Fig. 96),
Turnberry, Ayrshire
334
98. Section across the volcanic area of St. Abb's Head (after Prof. J.
Geikie) 339
99. View of terraced andesite hills resting on massive conglomerate,
south of Oban
341
100. Section of lava-escarpment at Beinn Lora, north side of mouth of
Loch Etive, Argyllshire 342
101. Section across Strathbogie, below Rhyme, showing the position of
the volcanic band 344
102. View of Knockfeerina, Limerick, from the north-east—a volcanic
neck of Upper Old Red Sandstone age
349
103. Section of the volcanic zone in the Upper Old Red Sandstone, Cam
of Hoy, Orkney 351
104. Section of the volcanic zone in the Upper Old Red Sandstone at
Black Ness, Rackwick, Hoy
351
105. Section across the volcanic band and its associated necks, Hoy,
Orkney 352
106. Ground-plan of volcanic neck piercing the Caithness Flagstone
series on the beach near John o' Groat's House 353
107. View of the escarpment of the Clyde Plateau in the Little Cumbrae,
from the south-west
368
108. View of the edge of the Volcanic Plateau south of Campbeltown,
Argyllshire 370
109. View of North Berwick Law from the east, a phonolite neck 371
marking one of the chief vents of the Garleton Plateau. (From a

photograph)
110. The Bass Rock, a trachytic neck belonging to the Garleton plateau,
from the shore at Canty Bay
372
111. Corston Hill—a fragment of the Midlothian Plateau, seen from the
north 373
112. View of Arthur Seat from Calton Hill to the north 374
113. View of Arkleton Fell, part of the Solway Plateau, from the south-
west
376
114. Vertical sections of the escarpment of the Clyde plateau from north-
east to south-west 384
115. Section of Craiglockhart Hill, Edinburgh
387
116. Section of the bottom of the Midlothian Plateau, Linnhouse Water
above Mid-Calder Oilworks 387
117. Section of the top of the Midlothian Plateau in the Murieston Water 388
118. Section of Calton Hill, Edinburgh 389
119. Cliff of tuff and agglomerate, east side of Oxroad Bay, a little east
from Tantallon Castle, East Lothian
391
120. Section across part of the Clyde Plateau to the west of Bowling
(reduced from Sheet 6 of the Horizontal Sections of the
Geological Survey of Scotland) 392
121. Diagram illustrating the thinning away southwards of the lavas of
the Clyde Plateau between Largs and Ardrossan. Length about
10 miles 393
122. Diagram illustrating the thinning away eastwards of the lavas of the
Clyde Plateau in the Fintry Hills. Length about 12 miles 394
123. View of the two necks Dumgoyn and Dumfoyn, Stirlingshire, taken
from the south
395
124. Ground-plan of Plateau-vents near Strathblane, Stirlingshire, on the
scale of 6 inches to a mile 395
125. Ground-plans of double and triple necks in the Plateau series, on the
scale of 6 inches to a mile
396
126. Ground-plan of tuff-neck, shore east of Dunbar 398
127. Section across the vents Dumgoyn and Dumfoyn, and the edge of
the Clyde plateau above Strathblane, Stirlingshire 400
128. Section through the large vent of the Campsie Hills 400
129. Diagrammatic section across the central vent of the Clyde plateau in
Renfrewshire
400
130. Section across Southern Berwickshire, to show the relation of the
volcanic plateau to the vents lying south from it 401

131. Section of south end of Dumbuck Hill. East of Dumbarton
403
132. Section across the East Lothian plateau, to show the relative
position of one of the necks 403
133. View of Traprain Law from the south, a phonolite neck of the
Garleton Plateau 405
134. Veins and dykes traversing the agglomerate and tuff of the great
Renfrewshire vent
408
135. "The Yellow Man," a dyke in volcanic tuff and conglomerate on the
shore a little east of North Berwick 409
136. Trachytic sills, Knockvadie, Kilpatrick Hills 410
137. Section across the edge of the Clyde plateau, south-east of Beith
411
138. Section across the upper part of the Clyde plateau at Kilbirnie,
Ayrshire 411
139. Section across the upper surface of the Clyde volcanic plateau,
Burnhead, north-west of Kilsyth 412
140. Section across the upper surface of the Clyde volcanic plateau at
Campsie 412
141. Section across western edge of the Garlton plateau
412
142. Section across the Solway plateau 413
143. Section of volcanic vent at East Grange, Perthshire coal-field,
constructed by Mr. B. N. Peach from the rocks exposed in a
railway-cutting, and from plans of ironstone- and coal-pits 426
144. View of the Binn of Burntisland—a volcanic neck of agglomerate 428
145. View of part of the cliffs of vertical agglomerate, Binn of
Burntisland 431
146. Diagram of buried volcanic cone near Dalry, Ayrshire. Constructed
from information obtained in mining operations 434
147. Diagram to illustrate how Volcanic Necks may be concealed and
exposed
434
148. Section across the Saline Hills, Fife 435
149. Section across the Binn of Burntisland, in an east and west direction 436
150. Section in old quarry, west of Wester Ochiltree, Linlithgowshire.
Calciferous Sandstone series 437
151. Ejected volcanic block in Carboniferous strata, Burntisland 438
152. View of volcanic agglomerate becoming finer above east end of
Kingswood Craig, two miles east from Burntisland
439
153. Alternations of basalt and tuff, with shale, etc., of Kingswood Craig,
Burntisland 441

154. Section of the upper surface of a diabase ("leckstone") sheet, Skolie
Burn, south-east of Bathgate 443
155. Section across the volcanic ridge of the Linlithgow and Bathgate
Hills, showing the intercalation of limestones that mark
important stratigraphical horizons 444
156. Section in Wardlaw Quarry, Linlithgowshire 445
157. Section from Linlithgow Loch to the Firth of Forth 446
158. Section across the Campsie Fells illustrating the contrast between
the sills below and above the plateau-lavas 447
159. Section showing the position of the basic sills in relation to the
volcanic series at Burntisland, Fife
448
160. Sills between shales and sandstones, Hound Point, Linlithgowshire 449
161. Section of Sill, Cramond Railway, Barnton, near Edinburgh 450
162. Intrusive dolerite sheet enclosing and sending threads into portions
of shale, Salisbury Crags, Edinburgh 452
163. Intrusive sheet invading limestone and shale, Dodhead Quarry, near
Burntisland 452
164. Spheroidal weathering of dolerite sill, quarry east of North
Queensferry, Fife.
455
165. Two thin sills of "white trap" injected into black carbonaceous shale
overlying the Hurlet Limestone, Hillhouse Quarry, Linlithgow 456
166. Dyke cutting the agglomerate of a neck. Binn of Burntisland 457
167. Boss of diabase cutting the Burdiehouse Limestone and sending
sills and veins into the overlying shales. Railway cutting, West
Quarry, East Calder, Midlothian 458
168. Side of columnar basalt-dyke in the same agglomerate as in Fig.
166 459
169. Dyke rising through the Hurlet Limestone and its overlying shales.
Silvermine Quarry, Linlithgowshire
460
170. Junction of amygdaloidal basalt with shales and limestone, shore,
half a mile east from Kinghorn, Fife 464
171. Columnar basalt, Pettycur, Kinghorn, Fife 469
172. Section across the Fife band of Sills 473
173. Section across the upper volcanic band of north Ayrshire. Length
about four miles 474
174. Section showing the connection of the two volcanic bands in
Liddesdale 476

175. Diagram to show the position of a mass of Upper Old Red
Sandstone which has fallen into the great vent near Tudhope
Hill, east of Mosspaul
476

MAPS
I. General map of the Volcanic districts of the British Isles—At the end of the volume
II. Map of the Cambrian and Silurian volcanic region of North Wales To face p. 256
III. Map of the Old Red Sandstone volcanic region of "Lake Caledonia"
in Central Scotland and North Ireland To face p. 334
IV. Map of the Carboniferous volcanic districts of Scotland To face p. 476

BOOK I
GENERAL PRINCIPLES AND METHODS OF
INVESTIGATION

CHAPTER I
Earliest Knowledge of Volcanoes—Their Influence on Mythology
and Superstition—Part taken by Volcanic Rocks in Scenery
—Progress of the Denudation of Volcanoes—Value of the
Records of former Volcanoes as illustrating Modern Volcanic
Action—Favourable Position of Britain for the Study of this
Subject.
Among the influences which affected the infancy of mankind, the most
potent were those of environment. Whatever in outer nature stimulated or
repressed courage, inventiveness, endurance, whatever tended to harden or
to weaken the bodily faculties, whatever appealed to the imagination or
excited the fancy, became a powerful factor in human development.
Thus, in the dawn of civilization, the frequent recurrence of earthquakes
and volcanic eruptions throughout the basin of the Mediterranean could not
but have a marked effect on the peoples that dwelt by the borders of that
sea. While every part of the region was from time to time shaken by
underground commotion, there were certain places that became specially
noteworthy for the wonder and terror of their catastrophes. When, after
successive convulsions, vast clouds of black smoke rose from a mountain
and overspread the sky, when the brightness of noon was rapidly replaced
by the darkness of midnight, when the air grew thick with stifling dust and a
rain of stones and ashes fell from it on all the surrounding country, when
streams of what looked like liquid fire poured forth and desolated gardens,
vineyards, fields and villages—then did men feel sure that the gods were
angry. The contrast between the peacefulness and beauty of the ordinary
landscape and the hideous warfare of the elements at these times of
volcanic fury could not but powerfully impress the imagination and give a
colour to early human conceptions of nature and religion.
It was not only in one limited district that these manifestations of
underground convulsion showed themselves. The islands of the Ægean had
their volcanoes, and the Greeks who dwelt among them watched their
glowing fires by night and their clouds of steam by day, culminating now

and then in a stupendous explosion, like that which, in prehistoric time,
destroyed the island of Santorin. As the islanders voyaged eastward they
would see, on the coast of Asia Minor, the black bristling lavas of the
"Burnt Country," perhaps even then flowing from their rugged heaps of
cinders. Or when, more adventurously still, they sailed westward into the
Tyrrhenian waters, they beheld the snowy cone of Etna, with its dark
canopy of smoke and the lurid nocturnal gleam of its fires; while from time
to time they witnessed there on a still more stupendous scale the horrors of
a great volcanic eruption.
From all sides, therefore, the early Greek voyagers would carry back to the
mother-country marvellous tales of convulsion and disaster. They would tell
how the sky rapidly darkened even in the blaze of mid-day, how the land
was smothered with dust and stones, how over the sea there spread such a
covering of ashes that the oarsmen could hardly drive their vessels onward,
how red-hot stones, whirling high overhead, rained down on sails and deck,
and crushed or burnt whatever they fell upon, and how, as the earth shook
and the sea rose in sudden waves and the mountain gave forth an appalling
din of constant explosion, it verily seemed that the end of the world had
come.
To the actual horrors of such scenes there could hardly fail to be added the
usual embellishments of travellers' tales. Thus, in the end, the volcanoes of
the Mediterranean basin came to play a not unimportant part in Hellenic
mythology. They seemed to stand up as everlasting memorials of the victory
of Zeus over the giants and monsters of an earlier time. And as the lively
Greek beheld Mount Etna in eruption, his imagination readily pictured the
imprisoned Titan buried under the burning roots of the mountain, breathing
forth fire and smoke, and convulsing the country far and near, as he turned
himself on his uneasy pallet.
When in later centuries the scientific spirit began to displace the popular
and mythological interpretation of natural phenomena, the existence of
volcanoes and their extraordinary phenomena offered a fruitful field for
speculation and conjecture. As men journeyed outward from the
Mediterranean cradle of civilization, they met with volcanic manifestations
in many other parts of the world. When they eventually penetrated into the
Far East, they encountered volcanoes on a colossal scale and in astonishing
abundance. When they had discovered the New World they learnt that, in

that hemisphere also, "burning mountains" were numerous and of gigantic
dimensions. Gradually it was ascertained that vast lines of volcanic activity
encircle the globe. By slow degrees the volcano was recognized to be as
normal a part of the mechanism of our planet as the rivers that flow on the
terrestrial surface. And now at last men devote themselves to the task of
critically watching the operations of volcanoes with as much enthusiasm as
they display in the investigation of any other department of nature. They
feel that their knowledge of the earth extends to little beyond its mere outer
skin, and that the mystery which still hangs over the vast interior of the
planet can only, if ever, be dispelled by the patient study of these vents of
communication between the interior and the surface.
If, however, we desire to form some adequate idea of the part which
volcanic action has played in the past history of the earth, we should be
misled were we to confine our attention to the phenomena of the eruptions
of the present day. An attentive examination of any modern volcano will
convince us that of some of the most startling features of an eruption no
enduring memorial remains. The convulsive earthquakes that accompany a
great volcanic paroxysm, unless where they actually fissure the ground,
leave little or no trace behind them. Lamentably destructive as they are to
human life and property, the havoc which they work is mostly superficial.
In a year or two the ruins have been cleared away, the earth-falls have been
healed over, the prostrated trees have been removed, and, save in the
memories and chronicles of the inhabitants, no record of the catastrophe
may survive. The clouds of dust and showers of ashes which destroyed the
crops and crushed in the roofs of houses soon disappear from the air, and
the covering which they leave over the surface of a district gradually
mingles with the soil. Vegetation eventually regains its place, and the
landscape becomes again as smiling as before.
Even where the materials thrown out from the crater accumulate in much
greater mass, where thick deposits of ashes or solid sheets of lava bury the
old land-surface, the look of barren desolation, though in some cases it may
endure for long centuries, may in others vanish in a few years. The surface-
features of the district are altered indeed, but the new topography soon
ceases to look new. Another generation of inhabitants loses recollection of
the old landmarks, and can hardly realize that what has become so familiar
to itself differs so much from what was familiar to its fathers.

But even when the volcanic covering, thus thrown athwart a wide tract of
country, has been concealed under a new growth of soil and vegetation, it
still remains a prey to the ceaseless processes of decay and degradation
which everywhere affect the surface of the land. No feature of a modern
volcano is more impressive than the lesson which it conveys of the reality
and potency of this continual waste. The northern slopes of Vesuvius, for
example, are trenched with deep ravines, which in the course of centuries
have been dug out of the lavas and tuffs of Monte Somma by rain and
melted snow. Year by year these chasms are growing deeper and wider,
while the ridges between them are becoming narrower. In some cases,
indeed, the intervening ridges have been reduced to sharp crests which are
split up and lowered by the unceasing influence of the weather. The slopes
of such a volcanic cone have been aptly compared to a half-opened
umbrella. It requires little effort of imagination to picture a time, by no
means remote in a geological sense, when, unless renovated by the effects
of fresh eruptions, the cone will have been so levelled with the surrounding
country that the peasants of the future will trail their vines and build their
cots over the site of the old volcano, in happy ignorance of what has been
the history of the ground beneath their feet.
What is here predicted as probable or certain in the future has undoubtedly
happened again and again in the past. Over many districts of Europe and
Western America extinct volcanoes may be seen in every stage of decay.
The youngest may still show, perfect and bare of vegetation, their cones and
their craters, with the streams of lava that escaped from them. Those of
older date have been worn down into mere low rounded hills, or the whole
cone has been cleared away, and there is only left the hard core of material
that solidified in the funnel below the surface. The lava-sheets have been
cut through by streams, and now remain in mere scattered patches capping
detached hills, which only a trained eye can recognize as relics of a once
continuous level sheet of solid rock.
By this resistless degradation, a volcanic district is step by step stripped of
every trace of its original surface. All that the eruptions did to change the
face of the landscape may be entirely obliterated. Cones and craters, ashes
and lavas, may be gradually effaced. And yet enough may be left to enable a
geologist to make sure that volcanic action was once rife there. As the
volcano marks a channel of direct communication between the interior of

the earth and the atmosphere outside, there are subterranean as well as
superficial manifestations of its activity, and while the latter are removed by
denudation, the former are one by one brought into light. The progress of
denudation is a process of dissection, whereby every detail in the structure
of a volcano is successively cut down and laid bare. But for this process,
our knowledge of the mechanism and history of volcanic action would be
much less full and definite than happily it is. In active volcanoes the
internal and subterranean structure can only be conjectured; in those of
ancient date, which have been deeply eroded, this underground structure is
open to the closest examination.
By gathering together evidence of this nature over the surface of the globe,
we learn that abundantly as still active volcanoes are distributed on that
surface, they form but a small fraction of the total number of vents which
have at various times been in eruption. In Italy, for example, while Vesuvius
is active on the mainland, and Etna, Stromboli and Volcano display their
vigour among the islands, there are scores of old volcanoes that have been
silent and cold ever since the beginning of history, yet show by their cones
of cinders and streams of bristling lava that they were energetic enough in
their day. But the Italian volcanic region is only one of many to be found on
the European Continent. If we travel eastward into Hungary, or northward
into the Eifel, or into the heart of France, we encounter abundant cones and
craters, many of them so fresh that, though there is no historical record of
their activity, they look as if they had been in eruption only a few
generations ago.
But when the geologist begins to search among rocks of still older date than
these comparatively recent volcanic memorials, he meets with abundant
relics of far earlier eruptions. And as he arranges the chronicles of the
earth's history, he discovers that each section of the long cycle of geological
ages has preserved its records of former volcanoes. In a research of this
kind he can best realize how much he owes to the process of denudation.
The volcanic remains of former geological periods have in most cases been
buried under younger deposits, and have sunk sometimes thousands of feet
below the level of the sea. They have been dislocated and upheaved again
during successive commotions of the terrestrial crust, and have at last been
revealed by the gradual removal of the pile of material under which they
had lain.

Hence we learn that the active volcanoes of the present time, which really
embrace but a small part of the volcanic history of our planet, are the
descendants of a long line of ancestors. Their distribution and activity
should be considered not merely from the evidence they themselves supply,
but in the light derived from a study of that ancestry. It is only when we
take this broad view of the subject that we can be in a position to form some
adequate conception of the nature and history of volcanoes in the geological
evolution of the globe.
In this research it is obvious that the presently active volcano must be the
basis and starting-point of inquiry. At that channel of communication
between the unknown inside and the familiar outside of our globe, we can
watch what takes place in times of quiescence or of activity. We can there
study each successive phase of an eruption, measure temperatures,
photograph passing phenomena, collect gases and vapours, register the fall
of ashes or the flow of lavas, and gather a vast body of facts regarding the
materials that are ejected from the interior, and the manner of their
emission.
Indispensable as this information is for the comprehension of volcanic
action, it obviously affords after all but a superficial glimpse of that action.
We cannot see beyond the bottom of the crater. We cannot tell anything
about the subterranean ducts, or how the molten and fragmental materials
behave in them. All the underground mechanism of volcanoes is necessarily
hidden from our eyes. But much of this concealed structure has been
revealed in the case of ancient volcanic masses, which have been buried and
afterwards upraised and laid bare by denudation.
In yet another important aspect modern volcanoes do not permit us to
obtain full knowledge of the subject. The terrestrial vents, from which we
derive our information, by no means represent all the existing points of
direct connection between the interior and the exterior of the planet. We
know that some volcanic eruptions occur under the sea, and doubtless vast
numbers more take place there of which we know nothing. But the
conditions under which these submarine discharges are effected, the
behaviour of the outflowing lava under a body of oceanic water, and the
part played by fragmentary materials in the explosions, can only be
surmised. Now and then a submarine volcano pushes its summit above the
sea-level, and allows its operations to be seen, but in so doing it becomes

practically a terrestrial volcano, and the peculiar submarine phenomena are
still effectually concealed from observation.
The volcanic records of former geological periods, however, are in large
measure those of eruptions under the sea. In studying them we are
permitted, as it were, to explore the sea-bottom. We can trace how sheets of
coral and groves of crinoids were buried under showers of ashes and stones,
and how the ooze and silt of the sea-floor were overspread with streams of
lava. We are thus, in some degree, enabled to realize what must now happen
over many parts of the bed of the existing ocean.
The geologist who undertakes an investigation into the history of volcanic
action within the area of the British Isles during past time, with a view to
the better comprehension of this department of terrestrial physics, finds
himself in a situation of peculiar advantage. Probably no region on the face
of the globe is better fitted than these islands to furnish a large and varied
body of evidence regarding the progress of volcanic energy in former ages.
This special fitness may be traced to four causes—1st, The remarkable
completeness of the geological record in Britain; 2nd, The geographical
position of the region on the oceanic border of a continent; 3rd, The
singularly ample development to be found there of volcanic rocks
belonging to a long succession of geological ages; and 4th, The extent to
which this full chronicle of volcanic activity has been laid bare by
denudation.
1. In the first place, the geological record of Britain is singularly complete.
It has often been remarked how largely all the great periods of geological
time are represented within the narrow confines of these islands. The gaps
in the chronicle are comparatively few, and for the most part are not of great
moment.
Thanks to the restricted area of the country and to the large number of
observers, this remarkably full record of geological history has been studied
with a minute care which has hardly been equalled in any other country.
The detailed succession of all the formations has been so fully determined
in Britain that the very names first applied here to them and to their
subdivisions have in large measure passed into the familiar language of
geology all over the globe. Every definite platform in the stratigraphical
series has been more or less fully worked out. A basis has thus been laid for

referring each incident in the geological history of the region to its proper
relative date.
2. In the second place, the geographical position of Britain gives it a notable
advantage in regard to the manifestations of volcanic energy. Rising from
the margin of a great ocean-basin and extending along the edge of a
continent, these islands have lain on that critical border-zone of the
terrestrial surface, where volcanic action is apt to be most vigorous and
continuous. It has long been remarked that volcanoes are generally placed
not far from the sea. From the earliest geological periods the site of Britain,
even when submerged below the sea, has never lain far from the land which
supplied the vast accumulations of sediment that went to form the
Palæozoic and later formations, while, on the other hand, it frequently
formed part of the land of former geological periods. It was thus most
favourably situated as a theatre for both terrestrial and submarine volcanic
activity.
3. In the third place, this advantageous geographical position is found to
have been attended with an altogether remarkable abundance and
persistence of volcanic eruptions. No tract of equal size yet known on the
face of the globe furnishes so ample a record of volcanic activity from the
earliest geological periods down into Tertiary time. Every degree of energy
may be signalized in that record, from colossal eruptions which piled up
thousands of feet of rock down to the feeblest discharge of dust and stones.
Every known type of volcano is represented—great central cones like Etna
or Vesuvius, scattered groups of small cones like the puys of France, and
fissure- or dyke-eruptions like those of recent times in Iceland.
Moreover, the accurate manner in which the stratigraphy of the country has
been established permits each successive era in the long volcanic history to
be precisely determined, and allows us to follow the whole progress of that
history stage by stage, from the beginning to the end.
These characteristics may be instructively represented on a map, such as
that which accompanies the present volume (Map I.). The reader will there
observe how repeatedly volcanic eruptions have taken place, not merely
within the general area of the British Isles, but even within the same limited
region of that area. The broad midland valley of Scotland has been
especially the theatre for their display. From the early part of the Lower

Silurian period, through the ages of the Old Red Sandstone, Carboniferous
and Permian systems, hundreds of volcanic vents were active in that region,
while in long subsequent time there came the fissure-eruptions of the
Tertiary series.
4. In the fourth place, the geological revolutions of successive ages have
made this long volcanic chronicle fully accessible to observation. Had the
lavas and ashes of one period remained buried under the sedimentary
accumulations of the next, their story would have been lost to us. We should
only have been able to decipher the latest records which might happen to lie
on the surface. Fortunately for the progress of geology, the endless
vicissitudes of a continental border have brought up the very oldest rocks
once more to the surface. All the later formations of the earth's crust have
likewise been upraised and exposed to denudation during long cycles of
time. In this manner, the rocky framework of the country has been laid bare,
and each successive chapter of its geological history may be satisfactorily
deciphered. The singularly complete volcanic chronicle, after being
entombed under younger deposits, has been broken up and raised once
more into view. The active vents of former periods have been dissected,
submarine streams of lava have been uncovered, sheets of ashes that fell
over the sea-bottom have been laid bare. The progress of denudation is
specially favoured in such a variable and moist climate as that of Britain,
and thus by the co-operation of underground and meteoric causes the
marvellous volcanic records of this country have been laid open in minutest
detail.
There is yet another respect in which the volcanic geology of Britain
possesses a special value. Popular imagination has long been prone to see
signs of volcanic action in the more prominent rocky features of landscape.
A bold crag, a deep and precipitous ravine, a chasm in the side of a
mountain, have been unhesitatingly set down as proof of volcanic
disturbance. Many a cauldron-shaped recess, like the corries of Scotland or
the cwms of Wales, has been cited as an actual crater, with its encircling
walls still standing almost complete.
The relics of former volcanoes in this country furnish ample proofs to
dispel these common misconceptions. They show that not a single crater
anywhere remains, save where it has been buried under lava; that no trace
of the original cones has survived, except in a few doubtful cases where

they may have been preserved under subsequent accumulations of material;
that in the rugged tracts, where volcanic action has been thought to have
been most rife, there may be not a vestige of it, while, on the other hand,
where the uneducated eye would never suspect the presence of any remnant
of volcanic energy, lavas and ashes may abound. We are thus presented with
some of the most impressive contrasts in geological history, while, at the
same time, this momentous lesson is borne in upon the mind, that the
existing inequalities in the configuration of a landscape are generally due
far less to the influence of subterranean force than to the action of the
superficial agents which are ceaselessly carving the face of the land. Those
rocks which from their hardness or structure are best able to withstand that
destruction rise into prominence, while the softer material around them is
worn away. Volcanic rocks are no exception to this rule, as the geological
structure of Britain amply proves.
In the following chapters, forming Book I. of this work, I propose to begin
by offering some general remarks regarding the nature and causes of
volcanic action, so far as these are known to us. I shall then proceed to
consider the character of the evidence that may be expected to be met with
respecting the former prevalence of that action at any particular locality
where volcanic disturbances have long since ceased. The most telling
evidence of old volcanoes is naturally to be found in the materials which
they have left behind them, and the reader's attention will be asked to the
special characteristics of these materials, in so far as they give evidence of
former volcanic activity.
As has been already remarked, many of the most prominent phenomena of a
modern volcano are only of transient importance. The earthquakes and
tremors, and the constant disengagement of steam and gases, that play so
conspicuous a part in an eruption, may leave no sensible record behind
them. But even the cones of ashes and lava, which are piled up into
mountainous masses, have no true permanence: they are liable to ceaseless
erosion by the meteoric agencies of waste, and every stage in their
degradation may be traced. In successive examples we can follow them as
they are cut down to the very core, until in the end they are entirely effaced.
We may well, therefore, ask at the outset by what more enduring records we
may hope to detect the traces of former volcanic action. The following
introductory chapters will be devoted to an attempt to answer this question.

I shall try to show the nature and relative importance of the records of
ancient volcanoes; how these records, generally so fragmentary, may be
pieced together so as to be made to furnish the history which they contain;
how their relative chronology may be established; how their testimony may
be supplemented in such wise that the position of long vanished seas, lands,
rivers, and lakes may be ascertained; and how, after ages of geological
revolution, volcanic rocks that have lain long buried under the surface now
influence the scenery of the regions where they have once more been
exposed to view.
From this groundwork of ascertained fact and reasonable inference, we
shall enter in Book II. upon the story of the old volcanoes of the British
Isles. It is usual to treat geological history in chronological order, beginning
with the earliest ages. And this method, as on the whole the most
convenient, will be adopted in the present work. At the same time, the plan
so persistently followed by Lyell, of working backward from the present
into the past, has some distinct advantages. The volcanic records of the later
ages are much simpler and clearer than those of older times, and the student
may, in some respects, profitably study the history of the Tertiary eruptions
before he proceeds to make himself acquainted with the scantier chronicles
of the eruptions of the Palæozoic periods. But as I wish to follow the
gradual evolution of volcanic phenomena, and to show how volcanic energy
has varied, waxing and waning through successive vast intervals of time, I
will adhere to the chronological sequence.

CHAPTER II
The Nature and Causes of Volcanic Action—Modern Volcanoes.
A volcano is a conical or dome-shaped hill or mountain, consisting of
materials which have been erupted from an orifice leading down from the
surface into the heated interior of the earth. Among modern and recent
volcanoes three types may be recognized. In the first and most familiar of
these, the lavas and ashes ejected from the central vent have gathered
around it by successive eruptions, until they have built up a central cone
like those of Etna and Vesuvius. As this cone grows in height and diameter,
lateral or parasitic cones are formed on its flanks, and may become
themselves the chief actively erupting vents. This type of volcano, which
has been so long well known from its Mediterranean examples, was until
recently believed by geologists to be the normal, or indeed the only, phase
of volcanic energy on the face of the earth.
A modification of this type is to be found in a few regions where
fragmentary discharges are small in amount and where the eruptions are
almost wholly confined to the emission of tolerably liquid lava. A vast
dome with gently sloping declivities may in this way be formed, as in the
Sandwich Islands and in certain parts of Iceland.
The second type of volcano is at the present day extensively developed only
in Iceland, but in Tertiary time it appears to have had a wide range over the
globe, for stupendous memorials of it are preserved in North-Western
Europe, in Western America, and in India. It is distinguished by the
formation of numerous parallel fissures from which the lava gushes forth,
either with or without the formation of small cinder-cones along the lines of
the chasms.
The third type is distinguished by the formation of groups of cinder-cones
or lava-domes, which from their admirable development in Central France
have received the name of Puys. From these vents considerable streams of
lava have sometimes been discharged.

Without entering here into a detailed inquiry regarding the nature and
causes of Volcanic Action, we may with advantage consider briefly the two
main factors on which this action appears to depend.
1. Much uncertainty still exists as to the condition and composition of the
earth's interior. The wide distribution of volcanoes over the globe, together
with the general similarity of materials brought by them up to the surface,
formerly led to the belief that our planet consists of a central mass of
molten rock enclosed within a comparatively thin solid crust. Physical
arguments, however, have since demonstrated that the earth, with such a
structure, would have undergone great tidal deformation, but that in actual
fact it has a greater rigidity than if it were made of solid glass or steel.
From all the evidence obtainable it is certain that the temperature of the
earth's interior must be high. The rate of increase of this temperature
downward from the surface differs from place to place; but an increase is
always observed. At a depth of a few miles, every known substance must be
much hotter than its melting point at the surface. But at the great pressures
within the earth, actual liquefaction is no doubt prevented, and the nucleus
remains solid, though at a temperature at which, but for the pressure, it
would be like so much molten iron.
Any cause which will diminish the pressure may allow the intensely hot
material within the globe to pass into the liquid state. There is one known
cause which will bring about this result. The downward increment of
temperature proves that our planet is continually losing heat. As the outer
crust is comparatively cool, and does not become sensibly hotter by the
uprise of heat from within, the hot nucleus must cool faster than the crust is
doing. Now cooling involves contraction. The hot interior is contracting
faster than the cooler shell which encloses it, and that shell is thus forced to
subside. In its descent it has to adjust itself to a constantly diminishing
diameter. It can do so only by plication or by rupture.
When the terrestrial crust, under the strain of contraction, is compressed
into folds, the relief thus obtained is not distributed uniformly over the
whole surface of the planet. From an early geological period it appears to
have followed certain lines. How these came to be at first determined we
cannot tell. But it is certain that they have served again and again, during
successive periods of terrestrial readjustment. These lines of relief coincide,

on the whole, with the axes of our continents. The land-areas of the globe
may be regarded as owing their existence above sea-level to this result of
terrestrial contraction. The crust underneath them has been repeatedly
wrinkled, fractured and thrust upward by the vast oceanic subsidence
around them. The long mountain-chains are thus, so to speak, the crests of
the waves into which the crust has from time to time been thrown.
Again, the great lines of fracture in the crust of the earth probably lie in
large measure within the land-areas, or at least parallel with their axes and
close to their borders. Where the disposition of the chief ruptures and of the
predominant plications can be examined, these leading structural features
are found to be, on the whole, coincident. In the British Islands, for
instance, the prevalent trend of the axes of folding from early Palæozoic to
Tertiary time has been from south-west to north-east. How profoundly this
direction of earth-movement has affected the structure of the region is
shown by any ordinary map, in the long hill-ranges of the land and in the
long inlets of the sea. A geological map makes the dependence of the
scenery upon the building of the rocks still more striking. Not only have
these rocks been plicated into endless foldings, the axes of which traverse
the British Islands with a north-easterly trend: they have likewise been
dislocated by many gigantic ruptures, which tend on the whole to follow the
same direction. The line of the Great Glen, the southern front of the
Highlands, and the northern boundary of the Southern Uplands of Scotland,
are conspicuous examples of the position and effect of some of the greater
fractures in the structure of this country.
The ridging up of any part of the terrestrial crust will afford some relief
from pressure to the parts of the interior immediately underneath. If, as is
probable, the material of the earth's interior is at the melting point proper
for the pressure at each depth, then any diminution of the pressure may
allow the intensely heated substance to pass into the liquid state. It would
be along the lines of terrestrial uplift that this relief would be given. It is
there that active volcanoes are found. The molten material is forced upward
under these upraised ridges by the subsidence of the surrounding regions.
And where by rupture of the crust this material can make its way to the
surface, we may conceive that it will be ejected as lava or as stones and
ashes.

Viewed in a broad way, such appears to be the mechanism involved in the
formation and distribution of volcanoes over the surface of the earth. But
obviously this explanation only carries us so far in the elucidation of
volcanic action. If the molten magma flowed out merely in virtue of the
influence of terrestrial contraction, it might do so for the most part
tranquilly, though it would probably be affected by occasional sudden
snaps, as the crust yielded to accumulations of pressure. Human experience
has no record of the actual elevation of a mountain-chain. We may believe
that if such an event were to happen suddenly or rapidly, it would be
attended with gigantic catastrophes over the surface of the globe. We can
hardly conceive what would be the scale of a volcanic eruption attending
upon so colossal a disturbance of the terrestrial crust. But the eruptions
which have taken place within the memory of man have been the
accompaniments of no such disturbance. Although they have been many in
number and sometimes powerful in effect, they have seldom been attended
with any marked displacement of the surrounding parts of the terrestrial
crust. Contraction is, of course, continuously and regularly in progress, and
we may suppose that the consequent subsidence, though it results in
intermittent wrinkling and uplifting of the terrestrial ridges, may also be
more or less persistent in the regions lying outside these ridges. There will
thus be a constant pressure of the molten magma into the roots of
volcanoes, and a persistent tendency for the magma to issue at the surface at
every available rent or orifice. The energy and duration of outflow, if they
depended wholly upon the effects of contraction, would thus vary with the
rate of subsidence of the sinking areas, probably assuming generally a
feeble development, but sometimes bursting into fountains of molten rock
hundreds of feet in height, like those observed from time to time in Hawaii.
2. The actual phenomena of volcanic eruptions, however, show that a
source of explosive energy is almost always associated with them, and that
while the transference of the subterranean molten magma towards the
volcanic vents may be referred to the results of terrestrial contraction, the
violent discharge of materials from those vents must be assigned to some
kind of energy stored up in the substance of the earth's interior.
The deep-seated magma from which lavas ascend contains various vapours
and gases which, under the enormous pressure within and beneath the
terrestrial crust, are absorbed or dissolved in it. So great is the tension of

these gaseous constituents, that when from any cause the pressure on the
magma is suddenly relieved, they are liberated with explosive violence.
A volcanic paroxysm is thus immediately the effect of the rapid escape of
these imprisoned gases and vapours. With such energy does the explosion
sometimes take place, that the ascending column of molten lava is blown
into the finest impalpable dust, which may load the air around a volcano for
many days before it falls to the ground, or may be borne in the upper
regions of the atmosphere round the globe.
The proportion of dissolved gases varies in different lavas, while the lavas
themselves differ in the degree of their liquidity. Some flow out tranquilly
like molten iron, others issue in a pasty condition and rapidly congeal into
scoriæ and clinkers. Thus within the magma itself the amount of explosive
energy is far from being always the same.
It is to the co-operation of these two causes—terrestrial contraction and its
effects on the one hand, and the tension of absorbed gases and vapours the
other—that the phenomena of volcanoes appear to be mainly due. There is
no reason to believe that modern volcanoes differ in any essential respect
from those of past ages in the earth's history. It might, indeed, have been
anticipated that the general energy of the planet would manifest itself in far
more stupendous volcanic eruptions in early times than those of the modern
period. But there is certainly no geological evidence in favour of such a
difference. One of the objects of the present work is to trace the continuity
of volcanic phenomena back to the very earliest epochs, and to show that,
so far as the geological records go, the interior of the planet has reacted on
its exterior in the same way and with the same results.
We may now proceed to inquire how far volcanoes leave behind them
evidence of their existence. I shall devote the next two or three chapters to a
consideration of the proofs of volcanic action furnished by the very nature
of the materials brought up from the interior of the earth, by the
arrangement of these materials at the surface, by the existence of the actual
funnels or ducts from which they were discharged above ground, and by the
disposition of the masses of rock which, at various depths below the
surface, have been injected into and have solidified within the terrestrial
crust.

CHAPTER III
Ancient Volcanoes: Proofs of their existence derived from the
Nature of the Rocks erupted from the Earth's Interior. A.
Materials erupted at the Surface—Extrusive Series. i. Lavas,
their General Characters. Volcanic Cycles. ii. Volcanic
Agglomerates, Breccias and Tuffs.
The materials brought by volcanic action from the earth's interior have
certain common characters which distinguish them from other constituents
of the terrestrial crust. Hence the occurrence of these materials on any part
of the earth's surface affords convincing proofs of former volcanic
eruptions, even where all outward trace of actual volcanoes may have been
effaced from the topographical features of the ground.
Volcanic products may be classed in two divisions—1st, Those which have
been ejected at the surface of the earth, or the Extrusive series; and 2nd,
Those which have been injected into the terrestrial crust at a greater or less
distance below the surface, and which are known as the Intrusive series.
Extrusive rocks may be further classified in two great groups—(i.) The
Lavas, or those which have been poured out in a molten condition at the
surface; and (ii.) The Fragmental Materials, including all kinds of
pyroclastic detritus discharged from volcanic vents.
Taking first the Extrusive volcanic rocks, we may in the present chapter
consider those characters in them which are of most practical value in the
investigation of the volcanic phenomena of former geological periods.

i. lavas

The term Lava is a convenient and comprehensive designation for all those
volcanic products which have flowed out in a molten condition. They differ
from each other in composition and structure, but their variations are
comprised within tolerably definite limits.

As regards their composition they are commonly classed in three divisions
—1st, The Acid lavas, in which the proportion of silicic acid ranges from a
little below 70 per cent upwards; 2nd, The Intermediate lavas, wherein the
percentage of silica may vary from 55 to near 70; and 3rd, The Basic lavas,
where the acid constituent ranges from 55 per cent downwards. Sometimes
the most basic kinds are distinguished as a fourth group under the name of
Ultrabasic, in which the percentage of silica may fall below 40.
The structures of lavas, however, furnish their most easily appreciated
characteristics. Four of these structures deserve more particular attention:
1st, Cellular, vesicular or pumiceous structure; 2nd, The presence of glass,
or some result of the devitrification of an original glass; 3rd, Flow-structure;
and 4th, The arrangement of the rocks in sheets or beds, with columnar and
other structures.

Fig. 1.—Vesicular structure, Lava from Ascension Island, slightly
less than natural size.

1. The cellular, vesicular, scoriaceous or pumiceous structure of
volcanic rocks (Fig. 1) could only have arisen in molten masses from the
expansion of imprisoned vapours or gases, and is thus of crucial importance

in deciding the once liquid condition of the rocks which display it. The
vesicles may be of microscopic minuteness, but are generally quite visible
to the naked eye, and are often large and conspicuous. Sometimes these
cavities have been subsequently filled up with calcite, quartz, agate, zeolites
or other mineral deposition. As the kernels thus produced are frequently
flattened or almond-shaped (amygdales), owing to elongation of the steam-
holes by movement of the lava before its consolidation, the rocks containing
them are said to be amygdaloidal.
This structure, though eminently characteristic of superficial lavas, is not
always by itself sufficient to distinguish them from the intrusive rocks.
Examples will be given in later chapters where dykes, sills and other masses
of injected igneous material are conspicuously cellular in some parts. But, in
such cases, the cavities are generally comparatively small, usually spherical
or approximately so, tolerably uniform in size and distribution, and,
especially when they occur in dykes, distributed more particularly along
certain lines or bands, sometimes with considerable regularity (see Figs. 90,
91, and 236).
Among the superficial lavas, however, such regularity is rarely to be seen.
Now and then, indeed, a lava, which is not on the whole cellular, may be
found to have rows of vesicles arranged parallel to its under or upper
surface, or it may have acquired a peculiar banded structure from the
arrangement of its vesicles in parallel layers along the direction of flow. The
last-named peculiarity is widely distributed among the Tertiary lavas of
North-Western Europe, and gives to their weathered surfaces a deceptive
resemblance to tuffs or other stratified rocks (see Figs. 260, 310 and 311). It
will be more particularly referred to a few pages further on. In general,
however, we may say that the steam-cavities of lavas are quite irregular in
size, shape and distribution, sometimes increasing to such relative
proportions as to occupy most of the bulk of the rock, and in other places
disappearing, so as to leave the lava tolerably compact. When a lava
presents an irregularly vesicular character, like that of the slags of an iron-
furnace, it is said to be slaggy. When its upper surface is rugged and full of
steam-vesicles of all sizes up to large cavernous spaces, it is said to be
scoriaceous, and fragments of such a rock ejected from a volcanic vent are
spoken of as scoriæ.

Attention to the flattening of the steam-vesicles in cellular lavas, which has
just been alluded to as the result of the onward movement of the still molten
mass, may show, by the trend and grouping of these elongated cavities, the
probable direction of the flow of the lava before it came to rest. Sometimes
the vesicles have been drawn out and flattened to such a degree that the rock
has acquired in consequence a fissile structure. In other instances, the
vesicles have been originally formed as long parallel and even branching
tubes, like the burrows of Annelids or the borings of Teredo. Some
remarkable examples of this exceptional structure have been obtained from
the Tertiary plateau-basalts of the Western Isles, of which an example is
represented in Fig. 2.
In many cases the vesicles extend through the whole thickness of a lava.
Frequently they may be found most developed towards the top and bottom;
the central portion of the sheet being compact, while the top and bottom are
rugged, cavernous or scoriaceous.
Though originally the vesicles and cavernous spaces, blown open by the
expansion of the vapours dissolved in molten lava, remained empty on the
consolidation of the rock, they have generally been subsequently filled up
by the deposit within them of mineral substances carried in aqueous
solution. The minerals thus introduced are such as might have been derived
from the removal of their constituent ingredients by the solvent action of
water on the surrounding rock. And as amygdaloids are generally more
decayed than the non-vesicular lavas, it has been generally believed that the
abstraction of mineral material and its re-deposit within the steam-vesicles
have been due to the influence of meteoric water, which at atmospheric
temperatures and pressures has slowly percolated from the surface through
the cellular lava, long after the latter had consolidated and cooled, and even
after volcanic energy at the locality had entirely ceased.

Fig. 2.—Elongation and branching of steam-vesicles in
a lava, Kilninian, Isle of Mull, a little less than natural
size.

Examples, however, are now accumulating which certainly prove that, in
some cases, the vesicles were filled up during the volcanic period. Among
the Tertiary basalt-plateaux of the Inner Hebrides, for instance, it can be
shown that the lavas were already amygdaloidal before the protrusion of the
gabbros and granophyres which mark later stages of the same continuous
volcanic history, and even before the outpouring of much of the basalt of
these plateaux. Not improbably the mineral secretions were largely due to
the influence of hot volcanic vapours during the eruption of the basalts. This
subject will be again referred to in the description of the Tertiary volcanic
series.
Vesicular structure is more commonly and perfectly developed among the
lavas which are basic and intermediate in composition than among those
which are acid.
While the existence of a highly vesicular or scoriaceous structure may
generally be taken as proof that the rock displaying it flowed out at the
surface as a lava, other evidence pointing to the same conclusion may often
be gathered from the rocks with which the supposed lava is associated.
Where, for example, a scoriaceous lava is covered with stratified deposits
which contain pieces of that lava, we may be confident that the rock is an
interstratified or contemporaneous sheet. It has been erupted after the

deposition of the strata on which it rests, and before that of the strata which
cover it and contain pieces of it. In such a case, the geological date of the
eruption could be precisely defined. Illustrations of this reasoning will be
given in Chapter iv., and in the account of the volcanic series of
Carboniferous age in Central Scotland, where a basic lava can sometimes be
proved to be a true flow and not an intrusive sill by the fact that portions of
its upper slaggy surface are enclosed in overlying sandstone, shale or
limestone.
2. The presence of glass, or of some result of the devitrification of an
original glass, is an indication that the rock which exhibits it has once been
in a state of fusion. Even where no trace of the original vitreous condition
may remain, stages in its devitrification, that is, in its conversion into a
stony or lithoid condition, may be traceable. Thus what are called
spherulitic and perlitic structures (which will be immediately described),
either visible to the naked eye or only observable with the aid of the
microscope, afford evidence of the consolidation and conversion of a glassy
into a lithoid substance.
Striking evidence of the former glassy, and therefore molten, condition of
many rocks now lithoid is to be gained by the examination of thin slices of
them under the microscope. Not only are vestiges of the original glass
recognizable, but the whole progress of devitrification may be followed into
a crystalline structure. The primitive crystallites or microlites of different
minerals may be seen to have grouped themselves together into more or less
perfect crystals, while scattered crystals of earlier consolidation have been
partially dissolved in and corroded by the molten glass. These and other
characteristics of once fused rocks have to a considerable extent been
imitated artificially by MM. Fouqué and Michel Lévy, who have fused the
constituent minerals in the proper proportions.
Since traces of glass or of its representative devitrified structures are so
abundantly discoverable in lavas, we may infer the original condition of
most lavas to have been vitreous. Where, for instance, the outer selvages of
a basic dyke or sill are coated with a layer of black glass which rapidly
passes into a fine-grained crystalline basalt, and then again into a more
largely crystalline or doleritic texture in the centre, there can be no
hesitation in believing that glassy coating to be due to the sudden chilling
and consolidation of the lava injected between the cool rocks that enclose it.

The part that solidified first may be regarded as probably representing the
condition of the whole body of lava at the time of intrusion. The lithoid or
crystalline portion between the two vitreous outer layers shows the
condition which the molten rock finally assumed as it cooled more slowly.
Some lavas, such as obsidians and pitchstones, have consolidated in the
glassy form. More usually, however, a lithoid structure has been developed,
the original glass being only discoverable by the microscope, and often not
even by its aid. Two varieties of devitrification may be observed among
lavas, which, though not marked off from each other by any sharp lines, are
on the whole distinctive of the two great groups of acid and basic rocks.

Fig. 3.—Microlites of the
Pitchstone of Arran
(magnified 70 diameters).

(1) Among the acid rocks, what is called the Felsitic type of devitrification
is characteristic. Thus, obsidians pass by intermediate stages from a clear
transparent or translucent glass into a dull flinty or horny mass. When thin
slices of these transitional forms are examined under the microscope,
minute hairs and fibres or trichites, which may be observed even in the most
perfectly glassy rocks, are seen to increase in number until they entirely take
the place of the glass. Microlites of definite minerals may likewise be
observed, together with indefinite granules, and the rock finally becomes a
rhyolite, felsite or allied variety (Fig. 3).
At the same time it should be observed that, even in the vitreous condition
of a lava, definite crystals of an early consolidation were generally already
present. Felspars and quartz, usually in large porphyritic forms, may be seen
in the glass, often so corroded as to indicate that they were in course of
being dissolved in the magma at the time of the cooling and solidification of

the mass. In obsidians and pitchstones such relics of an earlier or derived
series of crystallized minerals may often be recognized, while in felsites and
quartz-porphyries they are equally prominent. Where large dispersed
crystals form a prominent characteristic in a rock they give rise to what is
termed the Porphyritic structure.

Fig. 4.—Perlitic structure in
Fig. 5.—Spherulitic structure
Felsitic Glass, Isle of Mull
(magnified).
(magnified).

Accompanying the passage of glass into stone, various structures make their
appearance, sometimes distinctly visible to the naked eye, at other times
only perceptible with the aid of the microscope. One of these structures,
known as Perlitic (Fig. 4), consists in the formation of minute curved or
straight cracks between which the vitreous or felsitic substance, during its
contraction in cooling, assumed a finely globular form.
Another structure, termed Spherulitic (Fig. 5), shows the development of
globules or spherules which may range from grains of microscopic
minuteness up to balls two inches or more in diameter. These not
infrequently present a well-formed internal fibrous radiation, which gives a
black cross between crossed Nicol prisms. Spherulites are more especially
developed along the margins of intrusive rocks, and may be found in dykes,
sills and bosses (see Figs. 375 and 377). Where the injected mass is not
thick it may be spherulitic to the very centre, as can be seen among the
felsitic and granophyric dykes of Skye.
Some felsitic lavas possess a peculiar nodular structure, which was
developed during the process of consolidation. So marked does this
arrangement sometimes become that the rocks which display it have
actually been mistaken for conglomerates. It is well exhibited among the

Lower Silurian lavas of Snowdon, the Upper Silurian lavas of Dingle, and
the Lower Old Red Sandstone lavas near Killarney.

Fig. 6.—Micropegmatitic or
Granophyric structure in Fig. 7.—Ophitic structure in
Granophyre, Mull Dolerite, Gortacloghan, Co.
(magnified). Derry (magnified).

A marked structure among some intrusive rocks, especially of an acid
composition, is that called Micropegmatitic or Granophyric. It consists in a
minute intergrowth of two component minerals, especially quartz and
felspar, and is more especially characteristic of certain granitic or granitoid
rocks which have consolidated at some distance from the surface and occur
as bosses, sills and dykes. It is also met with, however, in some basic sills.
Examples of all these and other structures will occur in the course of the
following description of British volcanic rocks.
(2) The second type of devitrification, conspicuous in rocks of more basic
composition, is marked by a more complete development of crystallization.
Among basic, as among acid rocks, there are proofs of the consolidation of
definite minerals at more than one period. Where the molten material has
suddenly cooled into a black glass, porphyritic felspars or other minerals are
often to be seen which were already floating in the magma in its molten
condition. During devitrification, however, other felspars of a later period of
generation made their appearance, but they are generally distinguishable
from their predecessors. Probably most basic and intermediate rocks, when
poured out at the surface as lavas, were no longer mere vitreous material,
but had already advanced to various stages of progress towards a stony
condition. These stages are still to some extent traceable by the aid of the
microscope.

Microlites of the component minerals are first developed, which, if the
process of aggregation is not arrested, build up more or less perfect crystals
or crystalline grains of the minerals. Eventually the glass may be so
completely devitrified by the development of its constituent minerals as to
be wholly used up, the rock then becoming entirely crystalline, or to survive
only in scanty interstitial spaces. In the family of the basalts and dolerites
the gradual transition from a true glass into a holocrystalline compound may
be followed with admirable clearness. The component minerals have
sometimes crystallized in their own distinct crystallographic forms
(idiomorphic); in other cases, though thoroughly crystalline, they have
assumed externally different irregular shapes, fitting into each other without
their Proper geometric boundaries (allotriomorphic).
A specially characteristic feature of many basic rocks is the presence of
what is termed an Ophitic structure (Fig. 7). Thus the component crystals of
pyroxene occur as large plates separated and penetrated by small needles
and crystals of felspar. The portions of pyroxene, divided by the enclosed
felspar, are seen under the microscope to be in optical continuity, and to
have crystallized round the already formed felspar. This structure is never
found in metamorphic crystalline rocks. It has been reproduced artificially
from fusion by Messrs. Fouqué and Michel Lévy.
The name Variolitic is applied to another structure of basic rocks (Fig. 8), in
which, especially towards the margin of eruptive masses, abundant
spheroidal aggregates have been developed from the size of a millet-seed to
that of a walnut, imbedded in a fine-grained or compact greenish matrix into
which the kernels seem to shade off. These kernels consist of silicates
arranged either radially or in concentric zones.
3. Flow-structure is an arrangement of the crystals, vesicles, spherulites, or
devitrification-streaks in bands or lines, which sweep round any enclosed
object, such as a porphyritic crystal or detached spherulite, and represent the
curving flow of a mobile or viscous mass. Admirable examples of this
structure may often be observed in old lavas, as well as in dykes and sills,
the streaky lines of flow being marked as distinctly as the lines of foam that
curve round the boulders projecting from the surface of a mountain-brook.
Flow-structure is most perfectly developed among the obsidians, rhyolites,
felsites and other acid rocks, of which it may be said to be a frequently

conspicuous character (Fig. 9). In these rocks it is revealed by the parallel
arrangement of the minute hair-like bodies and crystals, or by alternate
layers of glassy and lithoid material. The streaky lines thus developed are
sometimes almost as thin and parallel as the leaves of a book. But they
generally show interruptions and curvatures, and may be seen to bend round
larger enclosed crystals, or to gather into eddy-like curves, in such a manner
as vividly to portray the flow of a viscous substance. These lines represent
on a minute scale the same flow-structure which may be traced in large
sheets among the lavas. The porphyritic crystals and the spherulites are also
drawn out in rows in the same general direction. Sometimes, indeed, the
spherulites have been so symmetrically grouped in parallel rows that they
appear as rod-like aggregates which extend along the margin of a dyke.

Fig. 8.—Variolitic or Orbicular structure,
Napoleonite, Corsica (nat. size).

Among lavas of more basic composition flow-structure is not so often well
displayed. It most frequently shows itself by the orientation of porphyritic
felspars or of lines of steam-vesicles. Occasionally, however, sheets of
basalt may be found in which a distinct streakiness has been developed
owing to variations in the differentiation of the original molten magma. A
remarkable and widespread occurrence of such a structure is met with
among the Tertiary basalt-plateaux of the Inner Hebrides and the Faroe
Islands. In the lower parts of these thick accumulations of successive lava-
sheets, a banded character is so marked as to give the rocks the aspect of
truly stratified deposits. The observer, indeed, can hardly undeceive himself
as to their real nature until he examines them closely. As a full description
of this structure will be given in a later chapter, it may suffice to state here

that the banding arises from two causes. In some cellular lavas, the vesicles
are arranged in layers which lie parallel with the upper and under surfaces
of the sheets. These layers either project as ribs or recede into depressions
along the outcrop, and thus impart a distinctly stratified aspect to the rock.
More frequently, however, the banded structure is produced by the
alternation of different varieties of texture, and even of composition, in the
same sheet of basalt. Lenticular seams of olivine-basalt may be found
intercalated in a more largely crystalline dolerite. These differences appear
to point to considerable variations in the constitution of the magma from
which the lavas issued—variations which already existed before the
discharge of these lavas, and which showed themselves in the successive
outflow of basaltic and doleritic material during the eruption of what was
really, as regards its appearance at the surface, one continuous stream of
molten rock. It is impossible to account for such variations in the same sheet
of lava by any process of differentiation in the melted material during its
outflow and cooling. Analogous variations occur among the basic sills and
bosses of the Tertiary volcanic series of Britain. These, as will be more fully
discussed in later chapters, indicate a considerable amount of heterogeneity
in the deep-seated magma from which the intrusive sheets and bosses were
supplied (see vol. ii. pp. 329, 342).

Fig. 9.—Flow-structure in Rhyolite, Antrim, slightly reduced.

It is a common error to assume that flow-structure is a distinctive character
of lavas that have flowed out at the surface. In reality some of the most
perfect examples of the structure occur in dykes and sills, both among acid
and basic rocks. Innumerable instances might be quoted from the British
Isles in support of this statement.
Although, in the vast majority of cases, the presence of flow-structure may
be confidently assumed to indicate a former molten condition of the rock in
which it occurs, it is not an absolutely reliable test for an igneous rock.
Experiment has shown that under enormous pressure even solid metals may
be made to flow into cavities prepared for their reception. Under the vast
compression to which the earth's crust is subjected during terrestrial
contraction, the most obdurate rocks are crushed into fragments varying
from large blocks to the finest powder. This comminuted material is driven
along in the direction of thrust, and when it comes to rest presents a
streakiness, with curving lines of flow round the larger fragments, closely
simulating the structure of many rhyolites and obsidians. It is only by
attention to the local surroundings that such deceptive resemblances can be
assigned to their true cause.

Fig. 10.—Lumpy, irregular trachytic Lava-streams
(Carboniferous), East Linton, Haddingtonshire.

4. The disposition of lavas in sheets or beds is the result of successive
outflows of molten rock. Such sheets may range from only a yard or two to
several hundred feet in thickness. As a rule, though with many exceptions,
the basic lavas, such as the basalts, appear in thinner beds than the acid
forms. This difference is well brought out if we compare, for instance, the
massive rhyolites or felsites of North Wales with the thin sheets of basalt in
Antrim and the Inner Hebrides. The regularity of the bedded character is
likewise more definite among the basic than among the acid rocks, and this
contrast also is strikingly illustrated by the two series of rocks just referred
to. The rhyolites and felsites, sometimes also the trachytes and andesites,
assume lumpy, irregular forms, and some little care may be required to trace
their upper and under surfaces, and to ascertain that they really do form
continuous sheets, though varying much in thickness from place to place
(Fig. 10). Like modern acid lavas, they seem to have flowed out in a pasty
condition, and to have been heaped up round the vents in the form of
domes, or with an irregular hummocky or mounded surface. The basalts,
and dolerites, and sometimes the andesites, have issued in a more fluid
condition, and have spread out in sheets of more uniform thickness, as may
be instructively seen in the sea-cliffs of Antrim, Mull, Skye, and the Faroe
Islands, where the horizontal or gently-inclined flows of basalt lie upon
each other in even parallel beds traceable for considerable distances along
the face of the precipices (Figs. 11, 265, and 286). The andesites of the Old
Red Sandstone (Figs. 99, 100) and Carboniferous series (Figs. 107, 108,
111, 112, 113, 123) in Scotland likewise form terraced hills.
The length of a lava-stream may vary within wide limits. Sometimes an
outflow of lava has not reached the base of the cone from the side of which
it issued, like the obsidian stream on the flanks of the little cone of the
island of Volcano. In other cases, the molten rock has flowed for forty or
fifty miles, like the copious Icelandic lava-floods of 1783. In the basalt-
plateaux of the Inner Hebrides a single sheet may sometimes be traced for
several miles.

Fig. 11.—View at the entrance of the Svinofjord, Faroe Islands, illustrating the
terraced forms assumed by basic lavas. The island on the left is Borö, that in the
centre Viderö, and that on the right Svinö.

Some lavas, more especially among the basic series, assume in cooling a
Columnar structure, of which two types may be noticed. In one of these the
columns pass with regularity and parallelism from the top to the bottom of a
bed (Figs. 171, 225). The basalt in which Fingal's Cave, in the isle of Staffa,
has been hollowed out may be taken as a characteristic example (Fig. 266a).
Not infrequently the columns are curved, as at the well-known Clam-shell
Cave of Staffa. In the other type, the columns or prisms are not persistent,
but die out into each other and have a wavy, irregular shape, somewhat like
prisms of starch. These two types may occur in successive sheets of basalt,
or may even pass into each other. At Staffa the regularly columnar bed is
immediately overlain with one of the starch-like character. The columnar
structure in either case is a contraction phenomenon, produced during the
cooling and shrinking of the lava. But it is difficult to say what special
conditions in the lava were required for its production, or why it should
sometimes have assumed the regular, at others the irregular form. It may be
found not only in superficial lavas but in equal perfection in some dykes and
intrusive sills or injections, as among the Tertiary volcanic rocks of the
island of Canna (Figs. 307 and 308).

The precipitation of a lava-stream into a lake or the sea may cause the outer
crust of the rock to break up with violence, so that the still molten material
inside may rush into the water. Some basic lavas on flowing into water or
into a watery silt have assumed a remarkable spheroidal sack-like or pillow-
like structure, the spheroids being sometimes pressed into shapes like piles
of sacks. A good instance of this structure occurs in a basalt at Acicastello in
Sicily.[1] A similar appearance will be described in a later chapter as
peculiarly characteristic of certain Lower Silurian lavas associated with
radiolarian cherts in Britain and in other countries (Fig. 12).
[1] See Prof. G. Platania in Dr. Johnston-Lavis' South Italian Volcanoes, Naples
(1891), p. 41 and plate xii.

Fig. 12.—Sack-like or pillow-form structure of basic
lavas (Lower Silurian), Bennan Head, Ballantrae,
Ayrshire.

It probably seldom happens that a solitary sheet of lava occurs among non-
volcanic sedimentary strata, with no other indication around it of former
volcanic activity. Such an isolated record does not seem to have been met
with in the remarkably ample volcanic register of the British Isles. The
outpouring of molten rock has generally been accompanied with the
ejection of fragmentary materials. Hence among the memorials of volcanic
eruptions, while intercalated lavas are generally associated with sheets of
tuff, bands of tuff may not infrequently be encountered in a sedimentary

series without any lava. Instances in illustration of these statements may be
culled from the British Palæozoic formations back even into the Cambrian
system.
A characteristic feature of some interest in connection with the flow of lava
is the effect produced by it on the underlying rocks. If these are not firmly
compacted they may be ploughed up or even dislocated. Thus the tuffs of
the Velay have sometimes been plicated, inverted, and fractured by the
movement of a flowing current of basalt.[2] The great heat of the lava has
frequently induced considerable alteration upon the underlying rocks.
Induration is the most common result, often accompanied with a reddening
of the altered substance. Occasionally a beautifully prismatic structure has
been developed in the soft material immediately beneath a basalt, as in
ferruginous clay near the village of Esplot in the Velay, in which the close-
set columns are 50 centimetres long and 4 to 5 centimetres in diameter.[3]
Changes of this nature, however, are more frequent among sills than among
superficial lavas. Many examples of them may be gathered from the
Scottish Carboniferous districts.
[2] M. Boule, Bull. Cart. Géol. France, No. 28, tom. iv. (1892), p. 235.

[3] M. Boule. Op. cit. p. 234.

Variations of structure in single lava-sheets.—From what has been said
above in regard to certain kinds of flow-structure among basic rocks, it will
be evident that some considerable range of chemical, but more particularly
of mineralogical, composition may be sometimes observed even within the
same sheet of lava. Such differences, it is true, are more frequent among
intrusive rocks, especially thick sills and large bosses. But they have been
met with in so many instances among superficial lavas as to show that they
are the results of some general law, which probably has a wide application
among the surface-products of volcanic action. Scrope expressed the
opinion that in the focus of a volcano there may be a kind of filtration of the
constituents of a molten mass, the heavier minerals sinking through the
lighter, so that the upper portions of the mass will become more felspathic
and the lower parts more augitic and ferruginous.[4]
[4] Volcanoes, p. 125.

Leopold von Buch found that in some of the highly glassy lavas of the
Canary Islands the felspar increases towards the bottom of the mass,

becoming so abundant as almost to exclude the matrix, and giving rise to a
compound that might be mistaken for a primitive rock.[5]
[5] Description Physique des Isles Canaries (1836), p. 190.

Darwin observed that in a grey basalt filling up the hollow of an old crater
in James Island, one of the Galapagos group, the felspar crystals became
much more abundant in the lower scoriaceous part, and he discussed the
question of the descent of crystals by virtue of their specific gravity through
a still molten lava.[6]
[6] Geological Observations on Volcanic Islands (1844), p. 117.

Mr. Clarence King during a visit to Hawaii found that in every case where
he broke newly-congealed streamlets of lava, "the bottom of the flow was
thickly crowded with triclinic felspars and augites, while the whole upper
part of the stream was of nearly pure isotropic and acid glass."[7] This
subject will be again referred to when we come to discuss the characters of
intrusive sills and bosses, for it is among them that the most marked
petrographical variations may be observed. Examples will be cited both
from the intrusive and extrusive volcanic groups of Britain.
[7] U.S. Geol. Exploration of the Fortieth Parallel, vol. i. (1878), p. 716.

Volcanic cycles.—Closely related to the problem of the range of structure
and composition in a single mass of lava is another problem presented by
the remarkable sequence of different types of lava which are erupted within
a given district during a single volcanic period. Nearly thirty years ago
Baron von Richthofen drew attention to the sequence of volcanic materials
erupted within the same geographical area. He showed, more especially
from observations in Western America, that a definite order of appearance in
the successive species of lava could be established, the earliest eruptions
consisting of materials of an intermediate or average composition, and those
of subsequent outflows becoming on the whole progressively more acid, but
finishing by an abrupt transition to a basic type. His sequence was as
follows: 1. Propylite; 2. Andesite; 3. Trachyte; 4. Rhyolite; 5. Basalt.[8] This
generalisation has been found to hold good over wide regions of the Old
World as well as the New. It is not, however, of universal application.[9]
Examples are not uncommon of an actual alternation of acid and basic lavas
from the same, or at least from adjacent vents. Such an alternation occurs

among the Tertiary eruptions of Central France and among those of Old Red
Sandstone age in Scotland.
[8] Trans. Acad. California, 1868. Prof. Iddings' Journ. Geol., vol. i. (1893), p. 606.

[9] See Prof. Brögger, "Die Eruptivgesteine des Kristianiagebietes," part ii. (1895), p.
175; Zeitsch. Kryst. und Mineral, vol. xvi. (1890) p. 83. This author would, from this
point of view, draw a distinction between rocks which have consolidated deep within
the earth and those which have flowed out at the surface, since he thinks that we are
not justified in applying our experience of the order of sequence in the one series to
the other. Yet there can be no doubt that in many old volcanic districts the masses that
may be presumed to have consolidated at a great depth have been in unbroken
connection with masses that reached the surface. These latter, as Prof. Iddings has
urged, furnish a much larger body of evidence than the intrusive sheets and bosses.

The range of variation in the nature of the eruptive rocks during the whole
of a volcanic period in any district may be termed "a volcanic cycle." In
Britain, where the records of many volcanic periods have been preserved, a
number of such cycles may be studied. In this way the evolution of the
subterranean magma during one geological age may be compared with that
of another. It will be one of the objects of the following chapters to trace out
this evolution in each period where the requisite materials for the purpose
are available. We shall find that back to Archæan time a number of distinct
cycles may be observed, differing in many respects from each other, but
agreeing in the general order of development of the successive eruptions.
Leaving these British examples for future consideration, it may be useful to
cite here a few from the large series now collected from the European
continent and North America.[10]
[10] Prof. M. Bertrand in a suggestive paper published in 1888 dealt with the general
order of appearance of eruptive rocks in different provinces of Europe. But the
materials then at his command probably did not warrant him in offering more than a
sketch of the subject, Bull. Soc. Geol., France, xvi. p. 573. In the same volume there
is a paper by M. Le Verrier, who announces his opinion that the eruption of the basic
rocks takes place in times of terrestrial calm, while that of the acid rocks occurs in
periods of great disturbance, op. cit. p. 498. Compare also Prof. Brögger, Die
Eruptivgesteine des Kristianiagebietes, ii. p. 169.

Among the older rocks of the European continent, Prof. Brögger has shown
that in the Christiania district the eruptive rocks which traverse the
Cambrian and Silurian formations began with the outburst of basic material
such as melaphyre, augite-porphyrite, and gabbro-diabase, having from
about 44 to about 52 per cent of silica. These were followed by rocks with a
silica-percentage ranging from about 50 to 61, including some characteristic
Norwegian rocks, like the rhomben-porphyry. The acidity continued to

increase, for in the next series of eruptions the silica-percentage rose to
between 60 and 67, the characteristic rock being a quartz-syenite. Then
came deep-seated protrusions of highly acid rocks, varieties of granite,
containing from 68 to 75 per cent of silica. The youngest eruptive masses in
the district show a complete change of character. They are basic dykes
(proterobase, diabase, etc.).[11]
[11] Eruptivgest. Kristianiageb., 1895.

The same author institutes a comparison between the post-Silurian eruptive
series of Christiania and that of the Triassic system in the Tyrol, and
believes that the two cycles closely agree.[12]
[12] Op. cit. He supposes in each case the pre-existence of a parent magma from
which the eruptive series started and which had a silica-percentage of about 64 or 65.
In this difficult subject it is of the utmost importance to accumulate fact before
proceeding to speculation.

During Tertiary time in Central France more than one cycle may be made
out in distinct districts. Thus in the Velay, during the Miocene Period,
volcanic activity began with the outpouring of basalts, followed
successively by trachytes, labradorites and augitic andesites, phonolites and
basalts. The Pliocene eruptions showed a reversion to the intermediate types
of augitic andesites and trachytes, followed by abundant basalts, which
continued to be poured forth in Pleistocene time.[13]
[13] M. Boule, "Description Géologique du Velay," Bull. Carte. Géol. France, 1892.
This author draws special attention to the evidence for the alternation of basic and
more acid material in the Tertiary eruptions of Central France.

Further north, in Auvergne, where the eruptions come down to a later
period, the volcanic sequence appears to have been first a somewhat acid
group of lavas (trachytes or domites), followed by a series of basalts, then
by andesites and labradorites, the latest outflows again consisting of basalts.
[14]

[14] M. Michel Lévy, Bull. Soc. Géol. France, 1890, p. 704.

Not less striking is the succession of lavas in the Yellowstone region, as
described by Mr. Iddings. The first eruptions consisted of andesites. These
were followed by abundant discharges of basalt, succeeded by later
outflows of andesite, and of basalt like that previously erupted. After a
period of extensive erosion, occupying a prolonged interval of time,
volcanic energy was renewed by the eruption of a vast flood of rhyolite,

after which came a feebler outflow of basalt that brought the cycle to a
close, though geysers and fumaroles show that the volcanic fires are not yet
entirely extinguished below.[15]
[15] Journal of Geology, Chicago, i. (1893) p. 606. See also this author's excellent
monograph on "Electric Peak and Sepulchre Mountain," 12th Ann. Rep. U.S. Geol.
Survey (1890-91), and Mr. H. W. Turner on "The Succession of Tertiary Volcanic
Rocks in the Sierra Nevada of North America," 14th Ann. Rep. U.S. Geol. Survey
(1892-93), p. 493.

But not only is there evidence of a remarkable evolution or succession or
erupted material within the volcanic cycle of a single geological period. One
of the objects of the present work is to bring forward proofs that such cycles
have succeeded each other again and again, at widely separated intervals,
within the same region. After the completion of a cycle and the relapse of
volcanic energy into repose, there has been a renewal of the previous
condition of the subterranean magma, giving rise ultimately to a similar
succession of erupted materials.
If we are at a loss to account for the changes in the sequence of lavas during
a single volcanic cycle, our difficulties are increased when we find that in
some way the magma is restored each time to somewhat the same initial
condition. Analogies may be traced between the differentiation which has
taken place within a plutonic intrusive boss or sill and the sequence of lavas
in volcanic cycles. It can be shown that though the original magma that
supplied the intrusive mass may be supposed to have had a fairly uniform
composition deep down in its reservoir, differentiation set in long before the
intrusive mass consolidated, the more basic constituents travelling outwards
to the margin and leaving the central parts more acid. If some such process
takes place within a lava-reservoir, it may account for a sequence of
variations in composition. But this would not meet all the difficulties of the
case, nor explain the determining cause of the separation of the constituents
within the reservoir of molten rock, whether arising from temperature,
specific gravity, or other influence. This subject will be further considered
in connection with intrusive Bosses.
Another fact which may be regarded as now well established is the
persistence of composition and structure in the lavas of all ages.
Notwithstanding the oft-repeated cycles in the character of the magma, the
materials erupted to the surface, whether acid or basic, have retained with
wonderful uniformity the same fundamental characteristics. No part of the

contribution of British geology to the elucidation of the history of volcanic
action is of more importance than the evidence which it furnishes for this
persistent sameness of the subterranean magma. An artificial line has
sometimes been drawn between the volcanic products of Tertiary time and
those of earlier ages. But a careful study of the eruptive rocks of Britain
shows that no such line of division is based upon any fundamental
differences.
The lavas of Palæozoic time have of course been far longer exposed to
alterations of every kind than those of the Tertiary periods, and certain
superficial distinctions may be made between them. But when these
accidental differences are eliminated, we find that the oldest known lavas
exhibit the same types of structure and composition that are familiar in
those of Tertiary and recent volcanoes. Many illustrations of this statement
will be furnished in later chapters. As a particularly striking instance, I may
cite here the most ancient and most modern lavas of the Grand Cañon of the
Colorado. Mr. Walcott and Mr. Iddings have shown that in the Lower
Cambrian, or possibly pre-Cambrian, formations of that great gorge, certain
basic lavas were contemporaneously interstratified, which, in spite of their
vast antiquity, are only slightly different from the modern basalts that have
been poured over the surrounding plateau.[16]
[16] 14th Annual Report U.S. Geol. Survey (1892-93).

The chief lavas found in Britain.—Of the lavas which have been poured
out at the surface within the region of the British Isles, the following
varieties are of most frequent occurrence. In the acid series are Rhyolites
and Felsites, but the vitreous forms are probably all intrusive. The
intermediate series is represented by Trachytes and Andesites (Porphyrites),
which range from a glassy to a holocrystalline structure. The basic series
includes Dolerites, Diabases, Basalts, Limburgites (or Magma-basalts,
containing little or no felspar), and Picrites or other varieties of Peridotites.
The intrusive rocks display a greater variety of composition and structure.

ii. volcanic agglomerates, breccias and tuffs

The coarser fragmentary materials thrown from volcanic vents are known as
Agglomerates where they show no definite arrangement, and especially
where they actually fill up the old funnels of discharge. When they have

accumulated in sheets or strata of angular detritus outside an active vent
they are termed Breccias, or if their component stones have been water-
worn, Conglomerates. The finer ejected materials may be comprehended
under the general name of Tuffs.
Although these various forms of pyroclastic detritus consist as a rule of
thoroughly volcanic material, they may include fragments of non-volcanic
rocks. This is especially the case among those which represent the earliest
explosions of a volcano. The first efforts to establish an eruptive vent lead to
the shattering of the terrestrial crust, and the consequent discharge of
abundant debris of that crust. The breccias or agglomerates thus produced
may contain, indeed, little or no truly volcanic material, but may be made
up of fragments of granite, gneiss, sandstone, limestone, shale, or whatever
may happen to be the rocks through which the eruptive orifice has been
drilled. If the first explosions exhausted the energy of the vent, it may
happen that the only discharges from it consisted merely of non-volcanic
debris. Examples of this kind have been cited from various old volcanic
districts. A striking case occurs at Sepulchre Mountain in the Yellowstone
Park, where the lower breccias, the product of the earliest explosions of the
Electric Peak volcano, and attaining a thickness of 500 feet, are full of
pieces of the Archæan rocks which underlie the younger formations of that
district.[17] These non-volcanic stones do not occur among the breccias
higher up. Obviously, however, though most abundant at first, pieces of the
underlying rocks may reappear in subsequent discharges, wherever by the
energy of explosion, fragments are broken from the walls of a volcanic
chimney and hurled out of the crater. Illustrations of these features will be
given in the account of the British Carboniferous, Permian and Tertiary
volcanic rocks.
[17] Prof. J. P. Iddings, 12th Ann. Rep. U.S. Geol. Survey (1890-91), p. 634.

It will be obvious that where the component materials of such fragmentary
accumulations consist entirely of non-volcanic rocks, great caution must be
exercised in attributing them to volcanic agency. Two sources of error in
such cases may here be pointed out. In the first place, where angular detritus
has been precipitated into still water, as, for instance, from a crag or rocky
declivity into a lake, a very coarse and tumultuous kind of breccia may be
formed. It is conceivable that, in course of time, such a breccia may be
buried under ordinary sediments, and may thereby be preserved, while all

trace of its parent precipice may have disappeared. The breccia might
resemble some true volcanic agglomerates, but the resemblance would be
entirely deceptive.
In the second place, notice must be taken of the frequent results of
movements within the terrestrial crust, whereby rocks have not only been
ruptured but, as already pointed out, have been crushed into fragments. In
this way, important masses of breccia or conglomerate have been formed,
sometimes running for a number of miles and attaining a breadth of several
hundred feet. The stones, often in huge blocks, have been derived from the
surrounding rocks, and while sometimes angular, are sometimes well-
rounded. They are imbedded in a finer matrix of the same material, and may
be scattered promiscuously through the mass, in such a way as to present
the closest resemblance to true volcanic breccia. Where the crushed material
has included ancient igneous rocks it might deceive even an experienced
geologist. Indeed, some rocks which have been mapped and described as
volcanic tuffs or agglomerates are now known to be only examples of
"crush-conglomerates."[18]
[18] For an account of "crush-conglomerates," see Mr. Lamplugh's paper on those of
the Isle of Man, Quart. Journ. Geol. Soc., li. (1895), p. 563. Mr. M'Henry has pointed
to probable cases of mistake of this kind in Ireland, Nature, 5th March 1896. A.
Geikie, Geol. Mag. November 1896.

Not only have vast quantities of detritus of non-volcanic rocks been shot
forth from volcanic vents, but sometimes enormous solid masses of rock
have been brought up by ascending lavas or have been ejected by explosive
vapours. Every visitor to the puys of Auvergne will remember the great
cliff-like prominence of granite and mica-schist which, as described long
ago by Scrope, has been carried up by the trachyte of the Puy Chopine, and
forms one of the summits of the dome (Fig. 344). The same phenomenon is
observable at the Puy de Montchar, where large blocks of granite have been
transported from the underlying platform. Abich has described some
remarkable examples in the region of Erzeroum. The huge crater of
Palandokän, 9687 feet above the sea contains, in cliff-like projections from
its walls as well as scattered over its uneven bottom, great masses of
marmorised limestone and alabaster, associated with pieces of the green
chloritic schists, serpentines and gabbros of the underlying non-volcanic
platform. These rocks, which form an integral part of the structure of the
crater, have been carried up by masses of trachydoleritic, andesitic and

quartz-trachytic lavas.[19] Examples will be given in a later chapter showing
how gigantic blocks of mica-schist and other rocks have been carried many
hundred feet upwards and buried among sheets of lava or masses of
agglomerates during the Tertiary volcanic period in Britain (Fig. 262).
[19] Abich, Geologie des Armenischen Hochlandes (Part ii., western half), 1882, p.
76.

In the vast majority of cases, the fragmentary substances ejected by ancient
volcanic explosions, like those of the present day, have consisted wholly or
mainly of material which existed in a molten condition within the earth, and
which has been violently expelled to the surface. Such ejected detritus
varies from the finest impalpable dust or powder up to huge masses of solid
rock. These various materials may come from more than one source. Where
a volcanic orifice is blown out through already solidified lavas belonging to
previous eruptions, the fragments of these lavas may accumulate within or
around the vent, and be gradually consolidated into agglomerate or breccia.
Again, explosions within the funnel may break up lava-crusts that have
there formed over the cooling upper surface of the column of molten rock.
Or the uprising lava in the chimney may be spurted out in lumps of slag and
bombs, or may be violently blown out in the form of minute lapilli, or of
extremely fine dust and ashes.
Although in theory these several varieties of origin may be discriminated, it
is hardly possible always to distinguish them among the products of ancient
volcanic action. In the great majority of cases old tuffs, having been
originally deposited in water, have undergone a good deal of decomposition,
and such early alteration has been aggravated by the subsequent influence of
percolating meteoric water.
Where disintegration has not proceeded too far, the finer particles of tuffs
may be seen to consist of minute angular pieces of altered glass, or of
microlites or crystals, or of some vitreous or semi-vitreous substance, in
which such microlites and crystals are enclosed. It has already been stated
that the occurrence of glass, or of any substance which has resulted from the
devitrification of glass, may be taken as good evidence of former volcanic
activity.
Most commonly, especially in the case of tuffs of high antiquity, like those
associated with the Palæozoic formations, the fresh glassy and microlitic

constituents, so conspicuous in modern volcanic ashes, are hardly to be
recognised. The finer dust which no doubt contained these characteristic
substances has generally passed into dull, earthy, granular, or structureless
material, though here and there, among basic tuffs, remaining as palagonite.
In the midst of this decayed matrix, the lapilli of disrupted lavas may
endure, but it may be difficult or impossible to decide whether they were
derived from the breaking up of older lavas by explosion, or from the
blowing out of the lava-crusts within the funnel.
The cellular structure, which we have seen to be a markedly volcanic
peculiarity among the lavas, is not less so in their fragments among the
agglomerates, breccias and tuffs; indeed it may be said to be eminently
characteristic of them. The vesicles in the lapilli, bombs, and blocks are
sometimes of large size, as in masses of ejected slag, but they range down to
microscopic minuteness. The lapilli of many old tuffs are sometimes so
crowded with such minute pores, as to show that they were originally true
pumice.
The composition of tuffs must obviously depend upon that of the lavas from
which they were derived. But their frequently decayed condition makes it
less easy, in their case, to draw definite boundary-lines between varieties. In
a group of acid lavas, the tuffs may be expected to be also acid, while
among intermediate or basic lavas, the tuffs will generally be found to
correspond. There are, however, exceptions to this general rule. As will be
afterwards described in detail, abundant felsitic tuffs may be seen among
the andesitic lavas of Lower Old Red Sandstone age in Scotland, and
rhyolitic tuffs occur also among the Tertiary basalts of Antrim.
As a rule, basic and intermediate tuffs, like the lavas from which they have
been derived, are rather more prone to decomposition than the acid
varieties. One of their most characteristic features is the presence in them of
lapilli of a minutely vesicular pumice, which will be more particularly
described in connection with volcanic necks, of which it is a characteristic
constituent. Occasional detached crystals of volcanic minerals, either entire
or broken, may be detected in them, though perhaps less frequently than in
the agglomerates. The earthy matrix is generally greenish in colour, varying
into shades of brick-red, purple and brown.

The acid tuffs are, on the whole, paler in colour than the others, sometimes
indeed they are white or pale grey, but graduate into tones of hæmatitic red
or brown, the varying ferruginous tints being indicative of stages in the
oxidation of the iron-bearing constituent minerals. Small rounded lapilli or
angular fragments of felsite or rhyolite may be noticed among them,
sometimes exhibiting the most perfect flow-structure. As typical examples
of such tuffs, I may refer to those of the Pentland Hills, near Edinburgh, and
those that lie between the two groups of basalt in Antrim.

Fig. 13.—Alternations of
coarser and finer Tuff.

Thrown out promiscuously from active vents, the materials that form tuffs
arrange themselves, on the whole, according to relative size over the surface
on which they come to rest, the largest being generally grouped nearest to
the focus of discharge, and the finest extending farthest from it. As the
volcanoes of which records have been preserved among the geological
formations were chiefly subaqueous, the fragmentary substances, as they
fell into water, would naturally be to some extent spread out by the action of
currents or waves. They would thus tend to take a more or less distinctly
stratified arrangement. Moreover, as during an eruption there might be
successive paroxysms of violence in the discharges, coarser and finer
detritus would successively fall over the same spot. In this way, rapid
alternations of texture would arise (Fig. 13). A little experience will enable
the observer to distinguish between such truly volcanic variations and those
of ordinary sedimentation, where, for instance, layers of gravel and sand
repeatedly alternate. Besides the volcanic nature of the fragments and their
non-water-worn forms, he will notice that here and there the larger blocks
may be placed on end—a position the reverse of that usual in the disposal of
aqueous sediments, but one which is not infrequently assumed by ejected

stones, even when they fall through some little depth of water. Further, the
occurrence of large pieces of lava, scattered at random through deposits of
fine tuff, would lead him to recognize the tumultuous discharges of a
volcanic focus, rather than the sorting and sifting action of moving water.
Admirable illustrations of these various characteristics may be gathered in
endless number from the Palæozoic volcanic chronicles of Britain. I may
especially cite the basin of the Firth of Forth as a classical region for the
study of Carboniferous examples.

Fig. 14.—Alternations of Tuff (t,
t,) with non-volcanic sediment
(l, l).

When the conditions of modern volcanic eruptions are considered, it will be
seen that where ejected ashes and stones fall into water, they will there
mingle with any ordinary sediment that may be in course of deposition at
the time. There will thus be a blending of volcanic and non-volcanic
detritus, and the transition between the two may be so insensible that no
hard line of demarcation can be drawn. Such intermingling has continually
taken place during past ages. One of the first lessons learnt by the geologist,
who begins the study of ancient volcanic records, is the necessity of
recognizing this gradation of material, and likewise the frequently recurring
alternations of true tuff with shale, sandstone, limestone or other entirely
non-volcanic detritus (Fig. 14). He soon perceives that such facts as these
furnish him with some of the most striking proofs of the reality and progress
of former eruptions. The intermingling of much ordinary detritus with the
volcanic material may be regarded as indicative either of comparatively
feeble activity, or at least of considerable distance from the focus of
discharge. It is sometimes possible to trace such intermixtures through
gradually augmenting proportions of volcanic dust and stones, until the

deposit becomes wholly volcanic in composition, and so coarse in texture as
to indicate the proximity of the eruptive vent. On the other hand, the gradual
decrease of the volcanic ejections can be followed in the upward sequence
of a series of stratified deposits, until the whole material becomes entirely
non-volcanic.
The occurrence of thin partings of tuff between ordinary sedimentary strata
points to occasional intermittent eruptions of ashes or stones, the vigour and
duration of each eruptive interval being roughly indicated by the thickness
and coarseness of the volcanic detritus. The pauses in the volcanic activity
allowed the deposit of ordinary sediment to proceed unchecked. The nature
of such non-volcanic intercalations gives a clue to the physical conditions of
sedimentation at the time, while their thickness affords some indication of
the relative duration of the periods of volcanic repose.
A little reflection will convince the observer that in such a section as that
represented in Fig. 14 the volcanic intercalations must be regarded as a mere
local accident. Evidently the normal conditions of sedimentation at the time
these strata were accumulated are indicated by the limestone bands (l, l).
Had there been no volcanic eruptions, a continuous mass of limestone
would have been deposited, but this continuity was from time to time
interrupted by the explosions that gave rise to the intercalated bands of tuff
(t, t).
The application of these rules of geological evidence will be best
understood from actual examples of their use. Many illustrations of them
will be subsequently given, more especially from the volcanic records of the
Carboniferous period.
One of the most interesting peculiarities of interstratified tuffs is the not
infrequent occurrence of the remains of plants and animals imbedded in
them. Such remains would have been entombed in the ordinary sediment
had there been no volcanic eruptions, and their presence in the tuffs is
another convincing proof of contemporaneous volcanic action during the
deposition of a sedimentary series. But they may be made to furnish much
more information as to the chronology of the eruptions and the physical
geography of the localities where the volcanoes were active, as will be set
forth farther on.

Tuffs, as already remarked, frequently occur without any accompaniment of
lava, although lavas seldom appear without some tuff. We thus learn that in
the past, as at present, discharges of fragmentary materials alone were more
common than the outflow of lava by itself. The relative proportions of the
lavas and tuffs in a volcanic series vary indefinitely. In the Tertiary basalt-
plateaux of Britain, the lavas succeed each other, sheet above sheet, for
hundreds of feet, with few and trifling fragmental intercalations. Among the
Carboniferous volcanic ejections, on the other hand, many solitary or
successive bands of tuff may be observed without any visible sheets of lava.
Viewed broadly, however, in their general distribution during geological
time, the two great groups of volcanic material may be regarded as having
generally appeared together. In all the great volcanic series, from the base of
the Palæozoic systems up to the Tertiary plateaux, lavas and tuffs are found
associated, much as they are among the ejections of modern volcanoes.
They often alternate, and thus furnish evidence as to oscillations of energy
at the eruptive vents.
Now and then, by the explosions from a volcano at the present day, a single
stone may be ejected at such an angle and with such force as to fall to the
ground at a long distance from the vent. In like manner, among the volcanic
records of former periods, we may occasionally come upon a single block of
lava imbedded among tuffs or even in non-volcanic strata. Where such a
stone has fallen upon soft sediment, it can be seen to have sunk into it,
pressing down the layers beneath it, and having the subsequently deposited
layers heaped over it. An ejected block of this nature is represented among
the tuffs shown in Fig. 13. Another instance from a group of non-volcanic
sediments is given in Fig. 15, and is selected from a number of illustrations
of this interesting feature which have been observed among the Lower
Carboniferous formations of the basin of the Firth of Forth. A solitary block,
imbedded in a series of strata, would not, of course, by itself afford a
demonstration of volcanic activity. There are various ways in which such
stones may be transported and dropped over a muddy water-bottom. They
may, for example, be floated off attached to sea-weeds, or wrapped round
by the roots of trees. But where a block of basalt or other volcanic rock has
obviously descended with such force as to crush down the deposits on
which it fell, and when lavas and tuffs are known to exist in the vicinity,
there can be little hesitation in regarding such a block as having been

ejected from a neighbouring vent, either during an explosion of exceptional
violence or with an unusually low angle of projection.

Fig. 15.—Ejected block of Basalt which has fallen among
Carboniferous shales and limestones, shore, Pettycur,
Fife.

In conclusion, reference may conveniently be made here to another variety
of fragmental volcanic materials which cannot always be satisfactorily
distinguished from true tuffs, although arising from a thoroughly non-
volcanic agency. Where a mass of lava has been exposed to denudation, as,
for instance, when a volcanic island has been formed in a lake or in the sea,
the detritus worn away from it may be spread out like any other kind of
sediment. Though derived from the degradation of lava, such a mechanical
deposit is not properly a tuff, nor can it even be included among strictly
volcanic formations. It may be called a volcanic conglomerate, rhyolitic
conglomerate, diabase sandstone, felsitic shale, or by any other name that
will adequately denote its composition and texture. But it may not afford
proof of strictly contemporaneous volcanic activity. All that we are entitled
to infer from such a deposit is that, at the time when it was laid down,
volcanic rocks of a certain kind were exposed at the surface and were
undergoing degradation. But the date of their original eruption may have
been long previous to that of the formation of the detrital deposit from their
waste.
Nevertheless, it is sometimes possible to make sure that the conglomerate or
sandstone, though wholly due to the mechanical destruction of already
erupted lavas, was in a general sense contemporaneous with the volcanoes
that gave forth these lavas. The detrital formation may be traced perhaps up
to the lavas from which it was derived, and may be found to be intercalated
in the same sedimentary series with which they are associated. Or it may
contain large bombs and slags, such as most probably came either directly
from explosions or from the washing down of cinder-cones or other
contemporaneously existing volcanic heaps. Examples of such intercalated

conglomerates will be given from the Lower Old Red Sandstone of Central
Scotland and from the Tertiary volcanic plateaux of the Inner Hebrides.

CHAPTER IV
Materials erupted at the Surface—Extrusive Series—continued.
iii. Types of old Volcanoes—1. The Vesuvian Type; 2. The
Plateau or Fissure Type; 3. The Puy Type. iv. Determination
of the relative Geological Dates of ancient Volcanoes. v.
How the Physical Geography associated with ancient
Volcanoes is ascertained.
Having now taken note of the various materials ejected to the surface from
volcanic orifices, we may pass to the consideration of these orifices
themselves, with the view of ascertaining under what various conditions
volcanic action has taken place in the geological past. We have seen that
modern and not long extinct volcanoes may be grouped into three types,
and a study of the records of ancient volcanoes shows that the same types
may be recognized in the eruptions of former ages. The following chapters
will supply many illustrations of each type from the geological history of
the British Isles. In dealing with these illustrations, however, we must ever
bear in mind the all-powerful influence of denudation. We ought not to
expect to meet with the original forms of the volcanoes. Some little
reflection and experience may be required before we can realize under what
aspect we may hope to recognize ancient and much-denuded volcanoes. It
may therefore be of advantage to consider here, in a broad way, which of
the original characters are most permanent, and should be looked for as
mementoes of ancient volcanoes after long ages of denudation.

iii. types of old volcanoes

The three forms of ancient volcanoes now to be discussed are—1st, the
Vesuvian type; 2nd, the Plateau or Fissure type; and 3rd, the Puy type.
1. The Vesuvian Type.—In this kind of volcano, lavas and fragmental
ejections are discharged from a central vent, which is gradually built up by
successive eruptions of these materials. As the cone increases in size,
parasitic cones appear on its sides, and in the energy and completeness of
their phenomena become true volcanoes, almost rivalling their parent

mountain. Streams of lava descend upon the lower grounds, while showers
of dust and ashes are spread far and wide over the surrounding country.
If a transverse section could be made of a modern Vesuvian cone, the
volcanic pile would be found to consist of alternations of lavas and tuffs,
thickest at the centre, and thinning away in all directions. At some distance
from the crater, these volcanic materials might be seen to include layers of
soil and remains of land-vegetation, marking pauses between the eruptions,
during which soil accumulated and plants sprang up upon it. Where the
lavas and ashes had made their way into sheets of fresh water or into the
sea, they would probably be found interstratified with layers of ordinary
sediment containing remains of the animal or vegetable life of the time.

Fig. 16.—Effects of denudation on a Vesuvian cone.

Conceive now the effects of prolonged denudation upon such a pile of
volcanic rocks. The cone will eventually be worn down, the crater will
disappear, and the only relics of the eruptive orifice may be the top of the
central lava-column and of any fragmental materials that solidified within
the vent (Fig. 16). The waste will, on the whole, be greater at the cone than
on the more level areas beyond. It might, in course of time, reach the
original surface of the ground on which the volcano built up its heap of
ejected material. The central lava-plug might thus be left as an isolated
eminence rising from a platform of older non-volcanic rocks, and the
distance between it and the dwindling sheets of lava and tuff which came
out of it would then be continually increased as their outcrop receded under
constant degradation.
This piece of volcanic history is diagrammatically illustrated in Fig. 16. The
original forms of the central volcano and of its parasitic cones are suggested
by the dotted lines in the upper half of the Figure. All this upper portion has
been removed by denudation, and the present surface of the ground is

shown by the uppermost continuous line. The general structure of the
volcanic pile is indicated underneath that line—the lenticular sheets of lava
and tuff (l, l), the dykes (d, d), and the lavas (p, p) and agglomerates (a, a)
of the central vent and of the subordinate cones.
The waste, though greatest on the higher ground of the great cone, would
not stop there. It would extend over the flatter area around the volcano.
Streams flowing over the plain would cut their way down through the lavas
and tuffs, eroding ravines in them, and leaving them in detached and ever
diminishing outliers on the crests of the intervening ridges. We can easily
picture a time when the last of these relics would have been worn away, and
when every vestige of the volcanic ejections would have been removed,
save the lava-column marking the site of the former vent.
Every stage in this process of effacement may be recognized in actual
progress among the extinct volcanoes of the earth's surface. Probably
nowhere may the phenomena be more conveniently and impressively
studied than among the volcanic districts of Central France. On the one
hand, we meet there with cinder-cones so perfect that it is hard to believe
them to have been silent ever since the beginnings of history. On the other
hand, we see solitary cones of agglomerate or of lava, which have been left
isolated, while their once overlying and encircling sheets of ejected material
have been so extensively worn away as to remain merely in scattered
patches capping the neighbouring hills. Valleys many hundreds of feet in
depth have been cut by the rivers through the volcanic sheets and the
underlying Tertiary strata and granite since the older eruptions ceased. And
yet these eruptions belong to a period which, in a geological sense, is quite
recent. It is not difficult to contemplate a future time, geologically not very
remote, when in the valley of the Loire not a trace will remain of the
wonderfully varied and interesting volcanic chronicle of that district, save
the plugs that will mark the positions of the former active vents.
In the British Islands, ancient volcanoes of the Vesuvian type are well
represented among the Palæozoic systems of strata. Their preservation has
been largely due to the fact that they made their appearance in areas that
were undergoing slow subsidence. Their piles of erupted lava and ashes
were chiefly heaped up over the sea-floor, and were buried under the sand,
silt and ooze that gathered there. Thus covered up, they were protected from
denudation. It is only in much later geological ages that, owing to upheaval,

gradual degradation of the surface, and removal of their overlying cover of
stratified formations, they have at last been exposed to waste. The process
of disinterment may be observed in many different stages of progress. In
some localities, only the tops of the sheets of lava and tuff have begun to
show themselves; in others, everything is gone except the indestructible
lava-plug.
These inequalities of denudation arise not only from variations in the
durability of volcanic rocks, but still more from the relative position of
these rocks in the terrestrial crust, and the geological period at which, in the
course of the general lowering of the surface, they have been laid bare. Not
only are volcanic rocks of many different ages, and lie, therefore, on many
successive platforms within the crust of the earth: their places have been
still further dependent upon changes in the arrangement of that crust.
Fracture, upheaval, depression, curvature, unconformable deposition of
strata, have contributed to protect some portions, while leaving others
exposed to attack. Hence it happens that the volcanic record varies greatly
in its fulness of detail from one geological system or one district to another.
Some chapters have been recorded with the most surprising minuteness, so
that the events which they reveal can be realized as vividly as those of a
modern volcano. Others, again, are meagre and fragmentary, because the
chronicle is still for the most part buried underground, or because it has
been so long exposed at the surface that only fragments of it now remain
there.
In the descriptions which will subsequently be given of ancient British
volcanoes of the Vesuvian type, it will be shown that at many successive
periods during Palæozoic time, and at many distinct centres, lavas and tuffs
have been piled up to a depth of frequently more than 5000 feet—that is to
say, higher than the height of Vesuvius. Sometimes the vent from which
these materials were ejected can be recognized. In other places, it is still
buried under later formations, or has been so denuded as to be represented
now merely by the column of molten or fragmental rock that finally
solidified in it. Examples will be quoted of such ancient vents, measuring
not less than two miles in diameter, with subsidiary "necks" on their flanks,
like the parasitic cones on Etna.
I shall show that while the ejected volcanic products have accumulated in
greatest depth close to the vent that discharged them, they die away as they

recede from it, sometimes so rapidly that a volcanic pile which is 7000 feet
thick around its source may entirely thin out and disappear in a distance of
not more than ten or twelve miles. I shall point out how, as the lavas and
tuffs are followed outwards from their centre, they not only get thinner, but
are increasingly interstratified among the sedimentary deposits with which
they were coeval, and that in this way their limits, their age, and the
geographical conditions under which they were accumulated can be
satisfactorily fixed.
As illustrations of the Vesuvian type in the volcanic history of Britain, I
may refer to the great Lower Silurian volcanoes of Cader Idris, Arenig,
Snowdon and the Lake District, and to the Old Red Sandstone volcanoes of
Central Scotland.
2. The Plateau or Fissure type is, among modern volcanoes, best developed
in Iceland, as will be more fully detailed in Chapter xl. In that island, during
a volcanic eruption, the ground is rent open into long parallel fissures, only
a few feet or yards in width, but traceable sometimes for many miles, and
descending to an unknown depth into the interior. From these fissures lava
issues—in some cases flowing out tranquilly in broad streams from either
side, in other cases issuing with the discharge of slags and blocks of lava
which are piled up into small cones set closely together along the line of the
rent. It was from a fissure of this kind that the great eruption of 1783 took
place—the most stupendous outpouring of lava within historic time.
By successive discharges of lava from fissures, or from vents opening on
lines of fissure, wide plains may be covered with a floor of rock hundreds
or thousands of feet in thickness, made up of horizontal beds. The original
topography, which might have been undulating and varied, is completely
buried under a vast level lava-desert.
The rivers which drained the country before the beginning of the volcanic
history will have their channels filled up, and will be driven to seek new
courses across the lava-fields. Again and again, as fresh eruptions take
place, these streams will be compelled to shift their line of flow, each river-
bed being in turn sealed up in lava, with all its gravels, silts and drift-wood.
But the rain will continue to fall, and the drainage to seek its way seaward.
When the last eruption ceases, and the rivers are at length left undisturbed
at their task of erosion, they will carve that lava-floor into deep gorges or

open valleys. Where they flow between the lavas and the slopes against
which these ended, they will cut back the volcanic pile, until in course of
time the lavas will present a bold mural escarpment to the land that once
formed their limit. The volcanic plain will become a plateau, ending off in
this vertical wall and deeply trenched by the streams that wind across it.
And if the denudation is continued long enough, the plateau will be reduced
to detached hills, separated by deep and wide valleys.

Fig. 17.—Section to illustrate the structure of the Plateau type.

This geological history is illustrated by the diagram in Fig. 17. The stippled
ground underneath (x, x) represents the original undulating surface of the
country on which the plateau eruptions were poured out. The lavas of these
eruptions are shown by the horizontal lines to have entirely buried the
heights and hollows of the old land, and to have risen up to the upper dotted
line, which may be taken to mark the limit reached by the accumulation of
volcanic material. The dark lines (d, d) which come up through the bedded
lavas indicate the dykes with their connected vents. Denudation has since
stripped off the upper part of the volcanic series down to the uppermost
continuous black line which represents the existing surface of the ground.
The level sheets of lava have been deeply trenched, and in one instance the
valley has not only been cut through the volcanic pile, but has been partly
eroded out of the older rocks below. To the right and left, the lavas end off
abruptly in great escarpments.
The succession of events here depicted has occurred more than once in
Britain. The Plateau type is chiefly developed in this country among the
great Tertiary basalt districts of Antrim and the Inner Hebrides, which
reappear in the Faroe Islands, and again still farther north in Iceland. But it
also occurs among the volcanic rocks of the Old Red Sandstone and
Carboniferous periods.
As compared with the other volcanic types, that of the Plateaux is
distinguished by the wide extent of surface which its rocks cover, by the
great preponderance of lavas over tuffs, and by the regularity and

persistence of the individual sheets of rock. In Britain, the plateau-lavas are
even still often approximately horizontal, and lie piled on each other in
tolerably regular beds to a thickness of 1000, and in one place to more than
3000 feet. They form wide level or gently undulating tablelands, which rise
in bold escarpments from the surrounding country and have been deeply
carved into valleys. The sides of their cliffs and slopes are marked by
parallel lines of terrace, arising from the outcrop of successive sheets of
lava (Figs. 11, 265).
With the Tertiary basalt-plateaux are connected thousands of dykes, that
follow each other along nearly parallel lines in a general north-westerly
direction, and mark the position of fissures up which the molten lava
ascended. Occasional necks of agglomerate or basalt indicate the sites of
some of the eruptive vents.
The Carboniferous volcanic plateaux have been more extensively denuded
than those of Tertiary age, so that a large number of their vents have been
laid bare. In general these vents are of comparatively small size, though
larger than those of the Carboniferous Puys. In some districts, abundant
dykes traverse the rocks on which the plateaux rest, though the fissures
seem to have been less numerous than in Tertiary time.
3. The Puy type, as before remarked, takes its name from the well-known
puys, or volcanic cones, of Central France. Volcanoes of this type form
conical hills, generally of small size, consisting usually of fragmental
materials, sometimes of lava. Where a cone is partially effaced by a second,
and even by a third, successive slight shiftings of the vent are to be inferred
(see Figs. 29 and 214). In many cases, no lava has issued from such cones,
nor were the ashes and cinders dispersed far from the vent. Hence, in the
progress of denudation, cones of this kind are easily effaced.
From the uniformity of composition of their materials, the simplicity and
regularity of their forms, and their small size, it may be inferred that many
of these cones were the products of single eruptions. They may conceivably
have been thrown up in a few days, or even in a single day. The history of
Monte Nuovo, in the Bay of Naples, which was formed within twenty-four
hours in the year 1538, is a memorable example of the rapidity with which a
cone more than 400 feet high may be thrown up at some distance from a
central vent.

The smallest independent volcanoes are included in the Puy type. In many
instances the diameter of the funnel has not exceeded a few yards; the
largest examples of the type seldom exceed 1000 feet in width.
Where lavas have been discharged, as well as ashes and stones, a more
vigorous activity is indicated than where merely cones of tuff were formed.
The lavas may come from more than one side of a cone, and may flow in
opposite directions for a distance of several miles. It is observable that
considerable streams of lava have issued from the base of a cinder-cone
without disturbing it. The molten rock has found a passage between the
loose materials and the surface on which they rest,[20] though, in some
cases, the cone may have been thrown up after the emission of the lava.
[20] M. Boule, Bull. Carte Géol. France, No. 28, tome iv. p. 232.

In the history of a puy there is commonly a first discharge of fragmentary
material; then lava may flow out, followed by a final discharge of loose
stones and ashes. Hence the products of such a vent group themselves into
three layers—two of breccia separated by an intervening sheet of lava.[21]
[21] M. Boule, Bull. Carte Géol. France, No. 28, tome iv.

Great changes are wrought on puys and their connected lavas and tuffs
during the progress of denudation. The cones are eventually destroyed, and
only a stump of agglomerate or lava is left to mark its place. The connection
of a lava-stream with its parent vent may likewise be effaced, and the lava
itself may be reduced to merely a few separate patches, perhaps capping a
ridge, while the surrounding ground has been hollowed into valleys. If the
waste continues long enough, even these outliers will disappear, and
nothing but the neck or stump of the little volcano will remain.

Fig. 18.—Diagram illustrating the structure and denudation of Puys.

The accompanying diagram (Fig. 18) may help to make these changes more
intelligible. The upper dotted lines show the original forms of three puys
with the covering of loose materials discharged by them over the
surrounding ground. The lower shaded portion represents the surface as left

by denudation, and a section of the three vents beneath that surface. The
whole of the cones and craters has here been swept away, and only the
volcanic "neck" is in each case left. In the vent to the right, the material that
fills it up is a coarse agglomerate, which projects as a rounded dome above
the surrounding country. The central pipe is filled with fragmentary
materials, through which molten rock has risen, giving off dykes and veins.
In the vent to the left hand, only lava is seen to occupy the orifice,
representing the column of molten rock which solidified there and brought
the activity of this little volcano to an end. It will be observed that in each
of these volcanic hills the present outlines are very far from being those of
the original volcano, and that the eminence projects because of its greater
resistance to the forces of denudation that have not only removed the
superficial volcanic material, but have made some progress in lowering the
level of the ground on which that material was accumulated.
The typical area for the study of Puys is the extraordinarily interesting
volcanic region of Central France. There the volcanic cones are clustered in
irregular groups, sometimes so close as to be touching each other;
elsewhere separated by intervals of several miles. They may be traced in all
stages of decay, from the most perfect cones and craters to the isolated
eminence that marks the site of a once active chimney. Their lavas, too, may
be seen as detached fragments of plateaux, many hundred feet above the
valleys that have been excavated since they flowed.[22]
[22] See Desmarest's classic map and his papers in Mem. Acad. Roy. Sciences, Paris,
1774, 1779; Journ. de Physique, 1779; Scrope's Geology of Central France, 1827,
and Extinct Volcanoes of Central France, 1858; Lecoq's Époques Géologiques de
l'Auvergne, 1867; M. Michel Lévy, Bull. Soc. Géol. France, 1890, p. 688; M. Boule,
Bull. Carte Géol. France, No. 28, tome iv. 1892.

Another well-known region of modern Puys is that of the Eifel, where the
cones and craters are often so fresh that it is difficult to believe them to be
prehistoric.[23] One of the most remarkable denuded puy-regions in Europe
covers a wide territory in the Swabian Alps of Würtemberg. No fewer than
125 denuded necks filled with tuff, agglomerate and basalt have there been
mapped and described. They are of higher antiquity than the Upper
Miocene strata, and have thus probably been exposed to prolonged
denudation. In external aspect and internal structure they present the closest
parallel to the Carboniferous and Permian "necks" of Britain described in
Books VI. and VII. of the present work.[24]

Among the Palæozoic volcanoes of Britain many admirable illustrations of
the Puy type are to be found. Their cones are almost always entirely gone,
though traces of them occasionally appear. The "necks" that show the
position of the vents are in some districts crowded together as thickly as
those of Auvergne. During the Carboniferous and Permian periods in
Central Scotland, clusters of such little volcanoes must have risen among
shallow lagoons and inland sheets of water, casting out their ashes and
pouring forth their little streams of lava over the water-bottom around them
and then dying out. As these eruptions took place in a region that was
gradually subsiding, the cones and their ejected ashes and lavas were one by
one submerged, the looser materials being washed down and spread out
among the silt, sand or mud, and enveloping the remains of any plants or
animals that might be strewn over the floor of the lake or sea. Hence the
Puys of Palæozoic time in Britain have been preserved with extraordinary
fulness of detail. They have been dissected by denudation, both among the
hills of the interior and along the margin of the sea. Their structure can thus
be in some respects made out even more satisfactorily than that of the much
younger and more perfect cones of Central France.
[23] The Eifel district has been fully described by Hibbert, Von Dechen, and other
writers. Von Dechen's little handbooks to the Eifel and Siebengebirge are useful
guides.
[24] These Würtemberg vents have been elaborately described and discussed by
Professor W. Branco of Tübingen in his Schwabens 125 Vulkan-Embryonen und
deren tufferfülte Ausbruchsröhren, das grösste Gebiet chemaliger Maare auf der
Erde, Stuttgart, 1894.

iv. determination of the relative geological dates of ancient
volcanoes

In themselves, accumulations of volcanic materials do not furnish any exact
or reliable evidence of the geological period in which they were erupted.
The lavas of the early Palæozoic ages may, indeed, on careful examination,
be distinguished from those of Tertiary date, but, as we have seen, the
difference is rather due to the effects of age and gradual alteration than to
any inherent fundamental distinction between them. In all essential
particulars of composition and internal structure, the lavas of the Cambrian
or Silurian period resemble those of Tertiary and modern volcanoes. The
igneous magmas which supply volcanic vents thus appear to have been very

much what they are now from early geological epochs. At least no
important difference, according to relative age, has yet been satisfactorily
established among them.
But although the rocks themselves afford no precise or trustworthy clue to
their date, yet where they have been intercalated contemporaneously among
fossiliferous stratified formations, of which the geological horizon can be
determined from included organic remains, it is easy to assign them to their
exact place in geological chronology. A determination of this kind is only an
application of the general principle on which the sequence of the geological
record is defined. A few illustrations will suffice to make this point quite
obvious.
Among the volcanic tuffs in the upper part of Snowdon various fossils
occur, which are identical with those found in the well-known Bala
Limestone. As the accepted reading of such evidence, we conclude that
these tuffs must therefore be of the same geological age as that limestone.
Now the position of this seam of rock has been well established as a
definite horizon in the series of Lower Silurian formations. And we
consequently without hesitation place the eruptions of the Snowdon volcano
on that same platform, and speak of them as belonging to the Bala division
of the Lower Silurian period.
Again, in West Lothian the tuffs and lavas ejected from many scattered puys
were interstratified among shales and limestones in which the characteristic
fossils of the Carboniferous Limestone are abundant. There cannot,
therefore, be any doubt that these eruptions were much younger than those
of Snowdon, and that they took place at the time when the Carboniferous
Limestone was being deposited. We thus speak of them as belonging to
volcanoes which were active in that early part of the Carboniferous period
to which the thick Mountain Limestone of Ireland and Derbyshire belongs.
As yet another illustration of the determination of geological age, an
example from the plateau-type of eruption may be given. The great basalt-
plateaux of Antrim and the Inner Hebrides are built up of lavas that lie
unconformably on the Chalk. They are thus proved to be later than the
Cretaceous system, and this deduction would hold true even if no organic
remains were found associated with the volcanic rocks. But here and there,
intercalated between the basalts, lie layers of shale, limestone and tuff

containing well-preserved remains of plants which are recognizable as older
Tertiary forms of vegetation. This fossil evidence definitely places the date
of the eruptions in older Tertiary time.
It is clear that, proceeding on this basis of reasoning, we may arrange the
successive volcanic eruptions of any given district, make out their order of
sequence in time, and thus obtain materials for a consecutive history of
them. Or, proceeding from that district into other regions, we may compare
its volcanic phenomena with theirs, determine the relative dates of their
respective eruptions, and in this way compile a wider history of volcanic
action in past time. It is on these principles that the general and detailed
chronology of the volcanic rocks of the British Isles has been worked out,
and that the following chapters have been arranged.

v. how the physical geography associated with ancient
volcanoes is ascertained

While the materials erupted from old volcanic vents tell plainly enough
their subterranean origin, they may leave us quite in the dark as to the
conditions under which they were thrown out at the surface. Yet a careful
examination of the strata associated with them may throw much light on the
circumstances in which the eruptions took place. Many of the results of
such examination will be given in subsequent chapters. I will here submit
illustrations of how four different phases of physical geography during
former volcanic eruptions may be satisfactorily determined.

Fig. 19.—Section
illustrating submarine
eruptions; alternations of
lavas and tuffs with
limestones and shales
full of marine
organisms.

1. Submarine Eruptions.—As by far the largest accessible part of the crust
of the earth consists of old marine sediments, it is natural that the volcanic
records preserved in that crust should be mainly those of submarine
eruptions. That many lavas during the geological past were poured out upon
the sea-bottom is plainly shown by the thick beds of marine organisms
which they have overspread and which lie above them (Fig. 19). In Central
Scotland, for example, sheets of basalt have flowed over a sea-bottom on
which thick groves of crinoids, bunches of coral and crowds of sea-shells
were living. Not less striking is the evidence supplied by bands of tuff.
Around Limerick, for instance, the thick Carboniferous Limestone is
interrupted by many thin layers of tuff marking intervals when showers of
volcanic dust fell over the sea-bottom, killing off the organisms that lived
there. But the limestone that overlies these volcanic intercalations is again
crowded with fossils, proving that the crinoids, corals and shells once more
spread over the place and flourished as abundantly as ever above the tuff.
The accompanying diagram (Fig. 19) illustrates these statements. At the
bottom a thick mass of limestone (l) full of crinoids, corals, brachiopods
and other marine organisms bears witness to a long time of repose, when

the clear sea-water teemed with life. At last a volcanic explosion took place,
which threw out the first seam of tuff (t). But this was only a transient
interruption, for the accumulation of calcareous sediment was immediately
resumed, and the next band of limestone was laid down. Thereafter a more
prolonged or vigorous eruption ejected a larger mass of dust and stones,
which fell over the bottom and prevented the continuation of the limestone.
But that the sea still abounded in life is shown by the numerous organisms
imbedded in the second stratified band of tuff. At last an access of volcanic
vigour gave vent to a stream of slaggy lava, which rolled over the sea-
bottom and solidified in the thick sheet of amydaloidal basalt marked B.
This outflow was followed by a further discharge of ashes and stones,
which, from their absence of stratification, may be supposed to have been
the result of a single explosion, or at least to have fallen too rapidly for the
marine currents to rearrange them in layers. When the water cleared, the
abundant sea-creatures returned, and from their crowded remains limestone
once more gathered over the bottom. Yet the volcanic history had not then
reached its close, for again there came a discharge of ashes, followed by the
outpouring of a second lava, which consolidated as a sheet of columnar
basalt (B').
It is not necessary, in order to prove the eruptions to have been submarine,
that organic remains should be found in the tuffs or between them. If the
volcanic ejections are intercalated among strata which elsewhere can be
proved to be marine, their discharge must obviously have taken place under
the sea. The vent that discharged them may have raised its head above the
sea-level, but its lavas and tuffs were spread out over the adjoining sea-
floor.
2. Lacustrine Eruptions.—The same line of evidence furnishes proof that
some volcanoes arose in inland sheets of water. If their products are
interstratified among sandstones, gravels and shell-marls, wherein the
remains of land-plants, insects and lacustrine shells, are preserved, we may
be confident that the eruptions took place in or near to some lake-basin. The
older lavas and tuffs of Central France supply an instructive example of
such an association. In Britain, the abundant and extensive outpouring of
lavas and tuffs during the time of the Lower Old Red Sandstone probably
occurred in large lakes. Among the sediments of these bodies of water,
interstratified between the volcanic sheets, remains of land-plants are

abundant, together with, here and there, those of myriapods washed down
from the woodlands, and of many forms of ganoid fishes.

Fig. 20.—Diagram illustrating volcanic
eruptions on a river-plain.

3. Fluviatile Eruptions.—Volcanoes have sometimes arisen on river-plains
or on the edges of valleys and gorges, and have poured out their lavas and
discharged their ashes over the channels or alluvial lands of the streams.
Volcanic materials, usurping the water-channels, bury or are interstratified
with fluviatile sand or shingle, containing perhaps remains of the vegetation
or animal life of the surrounding land. There may thus be a constant shifting
of the river-courses, and a consequent deposit of fluviatile sediment at many
successive levels among the lavas and tuffs. In Fig. 20 some of these
changes are indicated in a series of bedded lavas (l). The lower part of the
diagram shows the dying out of a bed of river gravel (g) against the sloping
end of a lava-stream, and the sealing up of this intercalation by a fresh
outpouring of lava. Higher up in the diagram a section is shown of a gully
or ravine which has been cut out of the lavas by a stream, and has become
choked up with water-worn detritus. Subsequent outflows of lava have
rolled over this channel and sealed it up. Examples of such intercalations of
lava with old river deposits, and of the burying of water-courses, will be
cited in the account of the Tertiary volcanic plateaux of Britain in Chapter
xxxviii.
4. Terrestrial Eruptions.—That volcanoes in former times broke out on land
as well as in water may readily be expected. But it is obvious that the proofs
of a terrestrial origin may not be always easy to obtain, for every land-
surface is exposed to denudation; and thus the relics of the eruptions of one
age may be effaced by the winds, rains, frosts and rivers of the next. In
assigning any volcanic group to a terrestrial origin, we may be guided partly
by negative evidence, such as the absence of all trace of marine organisms
in any of the sedimentary layers associated with the group. But such

evidence standing by itself would not be satisfactory or sufficient. If,
however, between the sheets of lava there occur occasional depressions,
filled with hardened sediment full of land-plants, with possibly traces of
insects and other terrestrial organisms, we may with some confidence infer
that these silted-up hollows represent pools or lakes that gathered on the
surface of the lava-sheets, and into which the vegetation of the surrounding
ground was blown or washed. Rain falling on the rugged surface of a lava-
field would naturally gather into pools and lakes, as the bottoms of the
hollows became "puddled" by the gradual decay of the rock and the
washing of fine silt into the crevices of the lava.

Fig. 21.—Diagram illustrating volcanic
eruptions on a land-surface.

Again, it may be expected that prolonged exposure to the air would give
rise to disintegration of the lava and to the consequent formation of soil.
Terrestrial vegetation would naturally spring up on such soil; trees might
take root upon it. Hence, if another lava-flood deluged the surface, the soil
and its vegetable mantle would be entombed under the molten rock.
These geological changes are represented diagrammatically in Fig. 21. Two
hollows among the lavas are there shown to have been filled with silt,
including successive layers of vegetation now converted into coal. One of
the soils (s) is marked between the lavas, and the charred stump of a tree
with its roots still in another layer of soil higher up is seen to have been
engulphed in the overlying sheet of melted rock.
Admirable illustrations of this succession of events are to be encountered
among the great Tertiary basaltic plateaux which cover so large an area in
the north-west of Europe. Not only has no trace of any marine organism
been found among their interstratified sedimentary layers, but they have
yielded a terrestrial flora which is preserved in hollows between the
successive sheets of basalt. A full account of these rocks will be given in
Book VIII.

CHAPTER V
Underground Phases of Volcanic Action. B. Materials injected or
consolidated beneath the Surface—Intrusive Series: I. Vents
of Eruption—i. Necks of Fragmentary Materials; ii. Necks of
Lava-form Materials; iii. Distribution of Vents in relation to
Geological Structure-Lines; iv. Metamorphism in and around
Volcanic Cones, Solfataric Action; v. Inward Dip of Rocks
towards Necks; vi. Influence of contemporaneous
Denudation upon Volcanic Cones; vii. Stages in the History
of old Volcanic Vents.
In our profound ignorance of the nature of the earth's interior, we know as
yet nothing certain regarding the condition and distribution there of those
molten materials which form the prime visible source of volcanic energy.
By the study of volcanoes and their products we learn that the fused
substances are not everywhere precisely the same and do not remain
absolutely uniform, even in the same volcanic region. But in what manner
and from what causes these variations arise is still unknown. We are further
aware that the molten magma, under a centre of volcanic disturbance,
manifests from time to time energetic movements which culminate in
eruptions at the surface. But what may be the exciting cause of these
movements, to what depth they descend, and over what extent of superficies
they spread, are matters regarding which nothing better than conjecture can
as yet be offered. It is true that, in some cases, a magma of fairly uniform
composition has been erupted over a vast tract of the earth's surface, and
must have had a correspondingly wide extent within the terrestrial crust.
Thus in the case of the older Tertiary volcanic eruptions of North-Western
Europe, basalt of practically the same composition was discharged from
thousands of fissures and vents distributed from the south of Antrim
northward beyond the Inner Hebrides, through the chain of the Faroe
Islands and over the whole breadth of Iceland. Under the British Isles alone,
the subterranean reservoirs of molten lavas must have been at least 40,000
square miles in united area. If they stretched continuously northwards
below the Faroe Islands and Iceland, as is highly probable, that is, for 600

miles further, their total extent may have been comparable to such a region
as Scandinavia.
Was this vast underground body of lava part of a universal liquid mass
within the globe, or was it rather of the nature of one or more lakes or large
vesicles within the crust? We can only offer speculation for answer. On the
other hand, there seems to be good proof that in some districts, both now
and in former geological periods, such differences exist between the
materials ejected from vents not far distant from each other as to show the
existence of more limited distinct reservoirs of liquid rock underneath.
Some of the questions here asked will be further dealt with in later pages in
connection with such geological evidence as can be produced regarding
them. But it will be found that at every step in the endeavour to ascertain
the origin of volcanic phenomena difficulties present themselves which are
now and may long remain insoluble.

I. Vents of Eruption
It is a general belief that the first stage in the formation of a volcano of the
Vesuvian type by the efforts of subterranean energy is the rending of the
terrestrial crust in a line of fissure. Some of the most remarkable groups of
active volcanoes on the face of the globe are certainly placed in rows, as if
they had risen along some such great rents. The actual fissure, however, is
not there seen, and its existence is only a matter of probable inference.
Undoubtedly the effect of successive eruptions must be to conceal the
fissure, even if it ever revealed itself at the surface.
What is supposed to have marked the initial step in the formation of a great
volcano is occasionally repeated in the subsequent history of the mountain.
During the convulsive shocks that precede and accompany an eruption, the
sides of the cone, and even sometimes part of the ground beyond, are rent
open, occasionally for a distance of several miles, and on the fissures thus
formed minor volcanoes are built up.
It is in Iceland, as already stated, that the phenomena of fissures are best
displayed. There the great deserts of lava are from time to time dislocated
by new lines of rent, which ascend up to the surface and stretch for

horizontal distances of many miles. From these long narrow chasms lava
flows out to either side; while cones of slag and scoriæ usually form upon
them. This interesting eruptive phase will be more fully described in the
chapters dealing with the Tertiary volcanic rocks of Britain.
There can be no doubt, however, that in a vast number of volcanic vents of
all geological periods no trace can be discovered of their connection with
any fissure in the earth's crust. Such fissures may indeed exist underneath,
and may have served as passages for the ascent of lava to within a greater or
less distance from the surface. But it is certain that volcanic energy has the
power of blowing out an opening for itself through the upper part of the
crust without the existence of any visible fissure there. What may be the
limits of depth at which this mode of communication with the outer air is
possible we do not yet know. They must obviously vary greatly according
to the structure of the terrestrial crust on the one hand, and the amount and
persistence of volcanic energy on the other. We may suppose that where a
fissure terminates upward under a great depth of overlying rock, the internal
magma may rise up to the end of the rent, and even be injected laterally into
the surrounding parts of the crust, but may be unable to complete the
formation of a volcano by opening a passage to the surface. But where the
thickness of rock above the end of the fissure is not too great, the expansive
energy of the vapours absorbed in the magma may overcome the resistance
of that cover, and blow out an orifice by which the volcanic materials can
reach the surface. In the formation of new cones within the historic period
at a distance from any central volcano, the existence of an open fissure at
the surface has not been generally observed. When, for example, Monte
Nuovo was formed, it rose close to the shore among fields and gardens, but
without the appearance of any rent from which its materials were
discharged.
That in innumerable instances during the geological past, similar vents have
been opened without the aid of fissures that reached the surface, will be
made clear from the evidence to be drawn from the volcanic history of the
British Isles. So abundant, indeed, are these instances that they may be
taken as proving that, at least in the Puy type of volcanoes, the actual vents
have generally been blown out by explosions rather than by the ascent of
fissures to the open air.

In cases where, as in Iceland, fissures open at the surface and discharge lava
there, the channel of ascent is the open space between the severed walls of
the rent. Within this space the lava will eventually cool and solidify as a
dyke. It is obvious that a comparatively small amount of denudation will
suffice to remove all trace of the connection of such a dyke with the stream
of lava that issued from it. Among the thousands of dykes belonging to the
Tertiary period in the British Islands, it is probable that many may have
served as lines of escape for the basalt at the surface. But it is now
apparently impossible to distinguish between those which had such a
communication with the outer air and those that ended upward within the
crust of the earth. The structure of dykes will be subsequently discussed
among the subterranean intrusions of volcanic material.
In an ordinary volcanic orifice the ground-plan is usually irregularly
circular or elliptical. If that portion of the crust of the earth through which
the vent is drilled should be of uniform structure, and would thus yield
equally to the effects of the volcanic energy, we might anticipate that the
ascent and explosion of successive globular masses of highly heated
vapours would give rise to a cylindrical pipe. But in truth the rocks of the
terrestrial crust vary greatly in structure; while the direction and force of
volcanic explosions are liable to change. Hence considerable irregularities
of ground-plan are to be looked for among vents.
Some of these irregularities are depicted in Fig. 22, which represents the
ground plan of some vents from the Carboniferous volcanic districts of
Scotland. They are all drawn on the same scale. Other examples will be
cited in later chapters from the same and other parts of the British Isles.
Some of the most marked departures from the normal and simple type of
vent occur where two orifices have been opened close to each other, or
where the same vent has shifted its position (Figs. 29, 125, 205, and 214).
Curiously irregular or elongated forms may thus arise in the resultant
"necks" now visible at the surface. Many striking examples of these features
may be seen among the Carboniferous and Permian volcanoes to be
afterwards described. Occasionally where an open fissure has served as a
vent it has given rise to a long dyke-like mass (No. 1 in Fig. 22).

Fig. 22.—Ground-plans of some Volcanic
vents from the Carboniferous districts
of Scotland.
1. Linhope Burn, near Mosspaul,
Roxburghshire; the shaded parts are
intrusions of trachytic material. 2.
Hazelside Hill, two miles W. from
Newcastleton, Roxburghshire. 3. St.
Magdalen's, Linlithgow. 4. South-west
side of Coom's Fell (see Fig. 174). 5.
Neck on Greatmoor, Roxburghshire. 6.
Pester Hill, Tarras Water. 7. Head of
Routing Burn, S.E. side of Hartsgarth
Fell, Liddesdale. 8. Hartsgarth Flow,
Liddesdale.

The size of a volcanic vent may vary indefinitely from a diameter of not
more than a yard or two up to one or two or more miles. As a rule, the
smaller the vents the more numerously are they crowded together. In the
case of large central volcanoes like Etna, where many subsidiary vents,
some of them forming not inconsiderable hills, may spring up along the
sides of the parent cone, denudation will ultimately remove all the material
that was heaped up on the surface, and leave the stumps or necks of the
parasitic vents in groups around the central funnel.
Each volcanic chimney, by which vapours, ashes or lava are discharged at
the surface, may be conceived to descend in a more or less nearly vertical
direction until it reaches the surface of the lava whence the eruptions
proceed. After the cessation of volcanic activity, this pipe will be left filled

up with the last material discharged, which will usually take the form of a
rudely cylindrical column reaching from the bottom of the crater down to
the lava-reservoir. It will be obvious that no matter how great may be the
denudation of the volcano, or how extensive may be the removal of the
various materials discharged over the surrounding ground, the pipe or
funnel with its column of solid rock must still remain. No amount of waste
of the surface of the land can efface that column. Successively lower and
yet lower levels may be laid bare in it, but the column itself goes still
further down. It will continue to make its appearance at the surface until its
roots are laid bare in the lava of the subterranean magma. Hence, of all the
relics of volcanic action, the filled-up chimney of the eruptive vent is the
most enduring. Save where it may have been of the less deep-seated nature
of a "hornito" upon a lava-stream, we may regard it as practically
permanent. The full meaning of these statements will be best understood
from a consideration of the numerous illustrations to be afterwards given.
The stumps of volcanic columns of this nature, after prolonged denudation,
generally project above the surrounding ground as rounded or conical
eminences known as "Necks" (Fig. 23. See also Figs. 52, 82, 102, 109, 123,
133, 144, 178, 192, 195, 203, 204, 209, 294, 298, 306 and 310). Their
outlines, however, vary with the nature of their component materials. The
softer rocks, such as tuffs and agglomerates, are apt to assume the form of
smooth domes or cones, while the harder and especially the crystalline
rocks rise into irregular, craggy hills. Occasionally, indeed, it may happen
that a neck makes no prominence on the surface of the ground, and its
existence may only be discoverable by a careful examination of the
geological structure of the locality. Now and then an old vent will be found
not to form a hill, but to sink into a hollow. Such variations, however, have
little or no reference to original volcanic contours in the history of the
localities which display them. They arise mainly from the differing
hardness and structure of the materials that have filled the vents, and the
consequent diversity in the amount of resistance which they have offered to
the progress of denudation.

Fig. 23.—View of an old volcanic "Neck" (The Knock, Largs, Ayrshire, a vent of
Lower Carboniferous age).

The materials now found in volcanic funnels are of two kinds: 1st,
Fragmentary, derived from volcanic explosions; and 2nd, Lava-form,
arising from the ascent and consolidation of molten rock within the funnel.

i. Necks of Fragmentary Materials

By far the most satisfactory evidence of a former volcanic orifice is
furnished by a neck of fragmentary materials. Where "bosses" of crystalline
rock rise to the surface and assume the outward form of necks, we cannot
always be certain that they may not have been produced by subterranean
intrusions that never effected any connection with the surface. In other
words, such bosses may not mark volcanic orifices at all, though they may
have been part of the underground protrusions of volcanoes in their
neighbourhood. But where the chimney has been filled with debris, there
can be no doubt that it truly marks the site of a once active volcano. The
fragmentary material is an eloquent memorial of the volcanic explosions
that drilled the vent, kept it open, and finally filled it up. These explosions
could not have taken place unless the elastic vapours which caused them
had found an escape from the pressure under which they lay within the crust
of the earth. Now and then, indeed, where the outpouring of lava or some
other cause has left cavernous spaces within the crust, there may
conceivably be some feeble explosion there, and some trifling accumulation
of fragmentary materials. But we may regard it as practically certain that
the mass of tumultuous detritus now found in volcanic necks could not have

been formed unless where a free passage had been opened from the molten
magma underneath to the outer surface of the planet.
Considerable diversity may be observed in the nature and arrangement of
the fragmentary materials in volcanic necks. The chief varieties may be
arranged in four groups: (1) Necks of non-volcanic detritus; (2) Necks of
volcanic agglomerate or tuff; (3) Necks of agglomerate or tuff with a central
plug of lava; and (4) Necks of agglomerate or tuff with veins, dykes or
some lateral irregular mass of lava.
(1) Necks of non-volcanic Detritus.—During the first convulsive efforts of a
volcanic focus to find a vent at the surface, the explosions that eventually
form the orifice do so by blowing out in fragments the solid rocks of the
exterior of the terrestrial crust. Of the detritus thus produced, shot up the
funnel and discharged into the air, part may gather round the mouth of the
opening and build up there a cone with an enclosed crater, while part will
fall back into the chimney, either to accumulate there, should the explosions
cease, or to be thrown out again, should they continue. In the feeblest or
most transient kinds of volcanic energy, the explosive vapours may escape
without any accompanying ascent of the molten magma to the surface, and
even without any sensible discharge of volcanic "ashes" from that magma.
In such cases, as I have already pointed out, the detritus of the non-volcanic
rocks, whatever they may be, through which volcanic energy has made an
opening, accumulate in the pipe and eventually consolidate there. Examples
of this nature will be adduced in later chapters from the volcanic districts of
Britain.
Where only non-volcanic materials fill up a vent we may reasonably infer
that the eruptions were comparatively feeble, never advancing beyond the
initial stage when elastic vapours made their escape with explosive
violence, but did not lead to the outflow of lava or the discharge of ashes. In
the great majority of necks, however, traces of the earliest eruptions have
been destroyed by subsequent explosions, and the uprise of thoroughly
volcanic fragments. Yet even among these fragments, occasional blocks
may be detected which have been detached from the rocks forming the
walls of the funnel.
The general name of Agglomerate, as already stated, is given to all
accumulations of coarse, usually unstratified, detritus in volcanic funnels,

irrespective of the lithological nature of the materials. For further and more
precise designation, when an agglomerate is mainly made up of fragments
of one particular rock, the name of that rock may be prefixed as sandstone-
agglomerate, granite-agglomerate, basalt-agglomerate, trachyte-
agglomerate. Volcanic agglomerate is a useful general term that may
include all the coarser detritus ejected by volcanic action.
Where volcanic explosions have been of sufficient violence or long
continuance, the upper part of the funnel may be left empty, and on the
cessation of volcanic activity, may be filled with water and become a lake.
The ejected detritus left round the edge of the orifice sometimes hardly
forms any wall, the crater-bottom being but little below the level of the
surrounding ground. Explosion-lakes are not infrequent in Central France
and the Eifel (Maare). A more gigantic illustration is afforded by the
perfectly circular crater of Coon Butte in Arizona, about 4000 feet in
diameter and 600 feet deep. It has been blown out in limestone, the debris
of which forms a rampart 200 feet high around it. Examples will afterwards
be cited from the Tertiary volcanic plateaux of North-Western Europe.
Vents may also be formed by an engulphment or subsidence of the material,
like that which has taken place at the great lava cauldron of Hawaii, still an
active volcano. The picturesque Crater Lake of Oregon is an admirable
instance of this structure.
(2) Necks of Agglomerate or Tuff.—In the vast majority of cases, the
explosions that clear out a funnel through the rocks of the upper part of the
crust do not end by merely blowing out these rocks in fragments. The
elastic vapours that escape from the molten lava underneath are usually
followed by an uprise of the lava within the pipe. Relieved from the
enormous pressure under which it had before lain, the lava as it ascends is
kept in ebullition, or may be torn into bombs which are sent whirling up
into the air, or may even be blown into the finest dust by the sudden
expansion of the imprisoned steam. If its ascent is arrested within the vent,
and a crust is formed on the upper surface of the lava-column, this
congealed crust may be disrupted and thrown out in scattered pieces by
successive explosions, but may re-form again and again.

Fig. 24.—Section of neck of
agglomerate, rising through
sandstones and shales.

In many vents, both in recent and in ancient times, volcanic progress has
never advanced beyond this early stage of the ejection of stones and dust.
The column of lava, though rising near enough to the surface to supply by
its ebullition abundant pyroclastic detritus, coarse and fine, has not flowed
out above ground, nor even ascended to the top of the funnel. It may have
formed, at the surface, cones of stones and cinders with enclosed craters.
But thereafter the eruptions have ceased. The vents, filled up with the
fragmentary ejected material, have given passage only to hot vapours and
gases. As these gradually ceased, the volcanoes have become finally extinct.
Denudation has attacked their sides and crests. If submerged in the sea or a
lake, the cones have been washed down, and their materials have been
strewn over the bottom of the water. If standing on the land, they have been
gradually levelled, until perhaps only the projecting knob or neck of
solidified rubbish in each funnel has remained to mark its site. The buried
column of compacted fragmentary material will survive as the only
memorial of the eruptions (Fig. 24. For views of necks formed of
agglomerate or tuff see Figs. 23, 82, 102, 123, 144, 178, 192, 203, 204, 209,
210, 212, 216).
The volcanic agglomerates of such vents sometimes include, among their
non-volcanic materials, pieces of rock which bear evidence of having been
subjected to considerable heat (see vol. ii. p. 78). Carbonaceous shales, for
instance, have had their volatile constituents driven off, limestones have
been converted into marble, and a general induration or "baking" may be
perceptible. In other cases, however, the fragments exhibit no sensible
alteration. Fossiliferous limestones and shales often retain their organic
remains so unchanged that specimens taken out of the agglomerate cannot
be distinguished from those gathered from the strata lying in situ outside.
Some stones have evidently been derived from a deeper part of the
chimney, where they have been exposed to a higher temperature than

others, or they may have been lain longer within the influence of hot
ascending vapours.
The volcanic materials in agglomerate range in size from the finest dust to
blocks several yards in length, with occasionally even much larger masses.
The proportions of dust to stones vary indefinitely, the finer material
sometimes merely filling in the interstices between the stones, at other
times forming a considerable part of the whole mass.
The stones of an agglomerate may be angular or subangular, but are more
usually somewhat rounded. Many of them are obviously pieces that have
been broken from already solid rock and have had their edges rounded by
attrition, probably by knocking against each other and the walls of the
chimney as they were hurled up and fell back again. Their frequently
angular shapes negative the supposition that they could have been produced
by the discharge of spurts of still liquid lava. As already stated, they have
probably been in large measure derived from the violent disruption of the
solidified cake or crust on the top of the column of lava in the pipe. Many
of them may have been broken off from the layer of congealed lava that
partially coated the rough walls of the funnel after successive uprises of the
molten material. Among them may be observed many large and small
blocks that appear to have been derived from the disruption of true lava-
streams, as if beds of lava had been pierced in the formation of the vent, or
as if those that congealed on the slopes of the cone had been broken up by
subsequent explosions. These fragments of lava are sometimes strongly
amygdaloidal. A characteristic feature, indeed, of the blocks of volcanic
material in the agglomerates is their frequent cellular structure. Many of
them may be described as rough slags or scoriæ. These have generally come
from the spongy crust or upper part of the lava where the imprisoned steam,
relieved from pressure, is able to expand and gather into vesicles.
Less frequently evidence is obtainable that the blocks were partially or
wholly molten at the time of expulsion. Sometimes, for example, a mass
which presents on one side such a broken face as to indicate that it came
from already solidified material, will show on the other that its steam-
vesicles have been pulled out in such a way as to conform to the rounded
surface of the block. This elongation could only take place in lava that was
not yet wholly consolidated. It seems to indicate that such blocks were
derived from a thin hardened crust lying upon still molten material, and that

they carried up parts of that material with them. As each stone went
whirling up the funnel into the open air, its melted part would be drawn
round the gyrating mass, and would rapidly cool there.
In other cases, we encounter true volcanic bombs, that is, rounded or bomb-
shaped blocks of lava, with their vesicles elongated all round them and
conforming to their spherical shape. Sometimes such blocks are singularly
vesicular in the centre, with a more close-grained crust on the outside. Their
rapid centrifugal motion during flight would allow of the greater expansion
of the dissolved steam in the central part of each mass, while the outer parts
would be quickly chilled, and would assume a more compact texture.
Bombs of this kind are met with among ancient volcanic products, and, like
those of modern volcanoes, have obviously been produced by the ejection
of spurts or gobbets of lava from the surface of a mass in a state of violent
ebullition. Occasionally they are hollow inside, the rotation in these cases
having probably been exceptionally rapid.
Passing from the larger blocks to the smaller fragments, we notice the great
abundance of nut-like subangular or rounded pieces of lava in the
agglomerates. These include lumps of fine grain not specially vesicular, and
probably derived from the disruption of solidified rock. But in many
agglomerates, especially those associated with the outpouring of basalts or
other basic lavas (as those of Carboniferous and Tertiary age described in
later chapters), they comprise also vast numbers of very finely cellular
material or pumice. These pumiceous lapilli have been already alluded to as
ingredients of the stratified tuffs. But they are still more characteristic of the
necks, and reach there a larger size, ranging from the finest grains up to
lumps as large as a hen's egg, or even larger.
The peculiar distinctions of this ejected pumice are the extreme minuteness
of its vesicles, their remarkable abundance, their prevalent spherical forms,
and the thinness of the walls which separate them. In these respects they
present a marked contrast to the large irregularly-shaped steam-cavities of
the outflowing lavas, or even of the scoriæ in the agglomerates.
This characteristic minutely vesicular pumice is basic in composition.
Where not too much decayed, it may be recognized as a basic glass. Thus
among the remarkable agglomerates which fill up the Pliocene or
Pleistocene vents of the Velay, the fragments consist of a dark very basic

glass, which encloses such a multitude of minute steam-cavities that, when
seen under the microscope, they are found to be separated from each other
by walls so thin that the slice looks like a pattern of delicate lace.[25] In
necks of earlier date, such as those of older Tertiary, and still more of
Palæozoic, time, the glass has generally been altered into some palagonitic
material.
[25] M. Boule, Bull. Cart. Géol. France, No. 28, tome iv. (1892) p. 193.

This finely pumiceous substance appears to be peculiar to the vents and to
the deposits of tuff immediately derived from them. It is not found, so far as
I know, among any of the superficial lavas, and, of course, would not be
looked for among intrusive rocks. It was evidently a special product of the
volcanic chimney, as distinguished from the mass of the magma below. We
may perhaps regards it as in some way due to a process of quiet simmering
within the vent, when the continual passage of ascending vapours kept the
molten lava there in ebullition, and gave it its special frothy or finely
pumiceous character.
The compacted dust, sand or gravelly detritus found in necks, and
comprised under the general name of Tuff, consists partly of the finer
particles produced during the violent disruption of already solidified rocks,
partly of the detritus arising from the friction and impact of stones
ascending and descending above an active vent during times of eruption,
and partly of the extremely light dust or ash into which molten lava may be
blown by violent volcanic explosions. In old volcanic necks, where the
rocks have long been subjected to the influence of percolating meteoric
water, it is not perhaps possible to discriminate, except in a rough way, the
products from these three sources. The more minutely comminuted material
has generally undergone considerable alteration, so that under the
microscope it seldom reveals any distinctive structures. Here and there in a
slide, traces may occasionally be detected of loose volcanic microlites,
though more usually these can only be found in lapilli of altered glass or
finely pumiceous lava.
The composition of the detritus in a neck of agglomerate or tuff has almost
always a close relation to that of any lavas which may have been emitted
from that vent. If the lavas have been of an acid character, such as rhyolites,
felsites or obsidians, the pyroclastic materials will almost always be found

to be also acid. Where, on the other hand, the lavas have been intermediate
or basic, so also will be the tuffs and agglomerates. Occasionally, however,
as has already been pointed out, from the same or closely adjoining vents
lavas of very different chemical composition have been successively
erupted. Felsites or rhyolites have alternated with diabases, basalts or
andesites. In such cases, a commingling of acid and basic detritus may be
observed, as, for example, among the volcanoes of the Old Red Sandstone.
It has even happened sometimes that such a mixture of material has taken
place when only one class of lavas has been poured out at the surface, as in
the agglomerates that fill vents among the basalts of the Inner Hebrides. But
we may be sure that, though not discharged at the surface, the lavas of
which pieces are found in the tuffs must have risen high enough in the vents
to be actually blown out in a fragmentary form. The occurrence of felsitic
fragments among the otherwise basic agglomerates of Mull and Skye will
be described in subsequent pages, likewise the intercalation of rhyolitic
detritus between the basalts of Antrim. A similar association occurs among
the modern vents of Iceland.
Among the contents of the tuffs and agglomerates that occupy old volcanic
vents, some are occasionally to be observed of which the source is not
easily conjectured. Detached crystals of various minerals sometimes occur
abundantly which were certainly not formed in situ, but must have been
ejected as loose lapilli with the other volcanic detritus. Where these crystals
belong to minerals that enter into the composition of the lavas of the district
in which they are found, they may be regarded as having probably been
derived from the explosion of such lavas in the vents, the molten magma
being blown into dust, and its already formed crystals being liberated and
expelled as separate grains. But it seems to be extremely rare to find any
neighbouring lava in which the minerals in question are so largely and so
perfectly crystallized as they are in these loose crystals of the neck. The
beautifully complete crystals of augite found in the old tuffs of Vesuvius
and on the flanks of Stromboli may be paralleled among Palæozoic tuffs
and agglomerates in Britain. Thus the necks belonging to the Arenig and
Llandeilo volcanoes of southern Scotland are sometimes crowded with
augite, varying from minute seed-like grains up to perfectly formed crystals
as large as hazel nuts. The conditions under which such well-shaped
idiomorphic minerals were formed were probably different from those that
governed the cooling and consolidation of the ordinary lavas.

But besides the minerals that may be claimed as belonging to the volcanic
series of a district, others occur not infrequently in some tuff-necks, the
origin of which is extremely puzzling. Such are the large felspars, micas,
garnets and the various gems that have been obtained from necks. The large
size of some of these crystals and their frequently perfect crystallographic
forms negative the idea that they can, as a rule, be derived from the
destruction of any known rocks, though they may sometimes be
conceivably the residue left after the solution of the other constituents of a
rock by the underground magma, like the large residual felspars enclosed in
some dykes. The crystals in question, however, seem rather to point to some
chemical processes still unknown, which, in the depths of a volcanic focus,
under conditions of pressure and temperature which we may speculate
about but can perhaps hardly ever imitate in our laboratories, lead to the
elaboration of the diamond, garnet, sahlite, smaragdite, zircon and other
minerals.[26] Examples of such foreign or deep-seated crystals will be
described from the probably Permian necks of Central Scotland.
[26] For lists of the minerals found in the diamond-bearing necks of Kimberley, see
M. Boutan in Frémy's Encyclopédie Chimique (1886), vol. ii. p. 168; Dr. M. Bauer's
Edelsteinkunde (1895), p. 223.

Whatsoever may be the source and nature of the fragmentary materials that
fill old volcanic vents, they present, as a general rule, no definite
arrangement in the necks. Blocks of all sizes are scattered promiscuously
through the agglomerate, just as they fell back into the chimney and came
to rest there. The larger masses are placed at all angles, or stand on end, and
are sometimes especially conspicuous in the centre of a neck, though more
usually dispersed through the whole. Such a thoroughly tumultuous
accumulation is precisely what might be expected where explosions have
taken place in still liquid and in already consolidated lavas, and where the
materials, violently discharged to the surface, have fallen back and come
finally to rest in the chimney of the volcano.
Nevertheless, this absence of arrangement sometimes gives place to a
stratification which becomes more distinct in proportion as the material of
the vent passes from coarse agglomerate into fine tuff. It is possible that the
existence and development of this structure depend on the depth at which
the materials accumulate in the funnel. We may conceive, for instance, that
in the lower parts of the chimney, the stones and dust, tumultuously falling

and rebounding from projections of the rugged walls, will hardly be likely
to show much trace of arrangement, though even there, if the explosions
continue to keep an open though diminishing passage in the vent,
alternations of coarser and finer layers, marking varying phases of
eruptivity, may be formed in the gradually heightening pile of agglomerate.
Rude indications of some such alternations may sometimes be detected in
what are otherwise quite unstratified necks.
In the upper part of a volcanic funnel, however, close to and even within the
crater, the conditions are not so unfavourable to the production of a
stratified arrangement. As the pipe is filled up, and the activity of eruption
lessens, explosions may occur only from the very middle of the orifice. The
debris that falls back into the vent will gather most thickly round the walls,
whence it will slide down to the central, still eruptive hole. It will thus
assume a stratified arrangement, the successive layers lying at the steepest
angles of repose, or from 30° to 35°, and dipping down in an inverted
conical disposition towards the centre. If the process should continue long
enough, the crater itself may be partially or completely filled up with
detritus (Fig. 25).
Of this gradual infilling of a volcanic chimney with stratified agglomerate
and tuff, examples belonging to different geological periods will be cited in
subsequent chapters. I may here especially allude to one of the most
recently observed and best marked illustrations, which occurs on the west
side of Stromö, in the Faroe Islands (see Figs. 310, 311, 312). A neck has
there been filled up with coarse agglomerate, which is rudely stratified, the
layers dipping steeply into the centre, where the tumultuous assemblage of
large blocks no doubt points to the final choking up of the diminished
orifice of explosion. The walls of the neck are nearly vertical, and consist of
the bedded basaltic lavas through which the vent has been opened. They
terminate upward in a conical expansion, evidently the old crater, which has
subsequently been filled up by the inroads of several lava-streams from
adjacent vents. It is here manifest that the bedded agglomerate belongs to
the uppermost part of the volcanic funnel.

Fig. 25.—Neck filled with stratified tuff. A. ground plan; B. transverse section.

Where vents have been filled up with tuff rather than with agglomerate, the
stratified structure is best developed. Alternations of coarser and finer
detritus give rise to more or less definite layers, which, though inconstant
and irregular, serve to impart a distinctly stratified character to the mass.
Where there has been no subsequent disturbance within a vent, these layers
show the same inward dip towards the centre just referred to, at the ordinary
angles of repose. Now and then, where a neck with this structure has been
laid bare on a beach, its denuded cross-section presents a series of
concentric rings of strata from the walls towards the centre. Good
illustrations of these features are supplied by the probably Permian necks of
eastern Fife (Figs. 25 A and 217).[27]
[27] See also the sections of vents on the west coast of Stromö Faroes, above referred
to.

It has frequently happened, however, that, owing to subsidence of the
materials filling up the vents or to later volcanic disturbances, the
compacted tuffs have been broken up and thrown into various positions,
large masses being even placed on end. Among the Carboniferous and
Permian necks of Central Scotland such dislocated and vertical tuffs are of
common occurrence (see Figs. 145, 218). If, as is probable, we are justified
in regarding the stratified parts of necks as indicative of the uppermost parts
of volcanic funnels, not far from the surface, the importance of this
inference will be best understood when the Carboniferous and Permian
volcanoes are described.

(3) Necks with a central Lava-plug.—Some vents of agglomerate or tuff are
pierced by a plug of lava, as may be instructively seen in many of the
Carboniferous and Permian necks of the centre and south of Scotland (Fig.
26; compare also Figs. 148, 174, 207, and 226). Where this structure shows
itself, the contrast in hardness and durability between the more destructible
fragmentary material and the solid resisting lava leads to a topographical
distinction in the outer forms of necks. The smooth declivities of the friable
tuffs are crowned or interrupted by more craggy features, which mark the
position of the harder intrusive rock.

Fig. 26.—Section of neck of
agglomerate (a a) with plug of lava
(b).

The plug, like the pipe up which it has risen, is in general irregularly
circular in ground-plan. It may be conceived to be a column of rock,
descending to an unknown depth into the interior, with a casing of
pyroclastic debris surrounding it. It may vary considerably in the proportion
which its cross-section bears to that of the surrounding fragmental material.
Sometimes it does not occupy more than a small part of the whole, often
appearing in the centre. In other cases, it more than equals all the rest of the
material in the vent, while instances may be noted where only occasional
patches of tuff or agglomerate are visible between the lava-plug and the
wall of the pipe. From these we naturally pass to the second type of vent,
where no fragmentary material is to be seen, but where the chimney is now
entirely filled with some massive once-molten rock.
A neck with a lava-plug probably contains the records of two stages in
volcanic progress, the first of which, indicated by the tuff or agglomerate,
was confined to the discharge of fragmentary materials; while the second,
shown by the lava-plug, belonged to the time when, after the earlier
explosions, lava ascended in the vent and solidified there, thus bringing the
eruptions from that particular orifice to an end. Where a small central

column of lava rises through the tuff, we may suppose that the funnel had
been mainly choked up by the accumulation in it of ejected detritus, which
was compacted to a solid mass adhering to the wall of the funnel, but
leaving a central orifice to be kept open by the gradually waning energy of
the volcano. By a final effort that impelled molten rock up that duct and
allowed it to consolidate there, the operations of the vent were brought to a
close.
Where, on the other hand, only occasional strips of tuff or agglomerate are
to be found between the lava-plug and the wall of the pipe, the last uprise of
lava may be supposed to have been preceded by more vigorous explosions
which cleared the throat of the volcano, driving out the accumulated
detritus and leaving only scattered patches adhering to the sides of the
funnel.
There is, no doubt, some downward limit to the production of fragmentary
material, and if we could lay bare successive levels in the chimney of a
volcano we should find the agglomerate eventually replaced entirely by
lava.
The materials of the lava-plugs vary widely in composition. Sometimes
they are remarkably basic, and present rocks of the picrite or limburgite
type; in other cases they are thoroughly acid rocks such as felsite and
granophyre. Many intermediate varieties may be found between these
extremes. It is noteworthy that, in districts where the lavas erupted to the
surface have been andesitic or basaltic, the material which has finally
solidified in the vents is often more acid in composition, trachytic rocks
being specially frequent.

Fig. 27.—Section of agglomerate neck (a
a) with dykes and veins (b b).

(4) Necks with Dykes, Veins, or irregular intrusions of Lava.—While the
presence of a central plug of lava in a neck of fragmental material may

indicate that the vent was still to some extent open, there is another
structure which seems to point to the ascent of lava after the funnel has
been choked up. Numerous instances have been observed where lava has
been forced upward through rents in a mass of tuff or agglomerate, and has
solidified there in the form of dykes or veins (Fig. 27). Illustrations of this
structure abound among the Carboniferous and Permian necks of Britain.
Here, again, though on a less marked scale, the contrast in the amount and
character of the weathering of the two groups of rock gives rise to
corresponding topographical features, which are especially observable in
cliffs and coast-sections, where the dykes and veins project out of the tuffs
as dark prominent walls (Figs. 135, 149, 166, 168, 219, 221, 222).
These intrusive injections are generally irregular in their forms, the lava
having evidently been driven through a mass of material which, not having
yet consolidated sufficiently to acquire a jointed structure, afforded few
dominant lines of division along which it could ascend. Now and then,
however, sharply defined dykes or veins, which at a distance look like dark
ribbons, may be seen running vertically or at a high angle, and with a
straight or wavy course, through the fine compacted tuff of a vent.
Frequently the injected material has found its readiest line of ascent along
the walls of the funnel, between the tuff and the surrounding rocks.
Occasionally it has made its way into rents in these rocks, as well as into
the body of the neck.
It is worthy of remark in passing that complete consolidation of the
fragmentary material does not appear to be always requisite in order to
allow of the formation of such fissures as are needed for the production of
dykes. A singularly interesting illustration of this fact may be seen on the
northern crest of the outer crater of the Puy Pariou in Auvergne. A dyke of
andesite 8 or 10 feet broad may there be traced running for a distance of
about 300 yards through the loose material of the cone. The rock is highly
vesicular, and the vesicles have been elongated in the direction of the course
of the dyke so as to impart a somewhat fissile structure to the mass.
There can be little doubt that the dykes and veins which traverse necks of
agglomerate belong to one of the closing phases in the history of the vents
in which they occur. They could only have been injected after the pipes had
been so choked up that explosions had almost or entirely ceased, and
eruptions had consequently become nearly or quite impossible. They show,

however, that volcanic energy still continued to manifest itself by impelling
the molten magma into these extinct funnels, while at the same time it may
have been actively discharging materials from other still open vents in the
same neighbourhood.
With regard to the composition of these dykes and veins, it may be
remarked that in a district of acid lavas they may be expected to be felsitic
or rhyolitic, sometimes granophyric. Where, on the other hand, the lavas
poured out at the surface have been intermediate or basic, the veins in the
necks may be andesites, basalts or other still more basic compounds. But it
is observable, as in the case of the lava-plugs, that the injections into the
necks may be much more acid than any of the superficial lavas. The advent
of acid material in the later part of a volcano's history has been already
alluded to, and many examples of it will be given in this work.
After all explosions and eruptions have ceased, heated vapours may still for
a long period continue to make their way upward through the loose spongy
detritus filling up the vent. The ascent of such vapours, and more
particularly of steam, may induce considerable metamorphism of the
agglomerate, as is more particularly noticed at p. 71.

ii. Necks of Lava-form Material

The second type of neck is that in which the volcanic pipe has been entirely
filled up with some massive or crystalline rock. As already remarked, it is
not always possible to be certain that bosses of rock, having the external
form of necks of this kind, mark the sites of actual volcanic orifices.
Eruptive material that has never reached the surface, but has been injected
into the crust of the earth, has sometimes solidified there in forms which,
when subsequently exposed by denudation, present a deceptive resemblance
to true volcanic necks. Each example must be examined by itself, and its
probable origin must be determined by a consideration of all the
circumstances connected with it. Where other evidence exists of volcanic
activity, such, for instance, as the presence of bedded tuffs or intercalated
sheets of lava, the occurrence of neck-like eminences or bosses of felsite,
andesite, dolerite, basalt or other eruptive rock, would furnish a
presumption that these marked the sites of some of the active vents of the
period to which the tuffs and lavas belonged.

If a neck-like eminence of this kind were found to possess a circular or
elliptical ground-plan, and to descend vertically like a huge pillar into the
crust of the earth; if the surrounding rocks were bent down towards it and
altered in the manner which I shall afterwards describe in detail; if,
moreover, the material composing the eminence were ascertained to be
closely related petrographically to some parts of the surrounding volcanic
series, it might with some confidence be set down as marking the place of
one of the active vents from which that series was ejected.
The chief contrast in external form between this type of neck and that
formed of fragmentary material arises from differences in the relative
durability of their component substance. The various kinds of lava-form
rock found in necks are, as a whole, much harder and more indestructible
than agglomerates and tuffs. Consequently bosses of them are apt to stand
out more prominently. They mount into higher points, present steeper
declivities, and are scarped into more rugged crags. But essentially they are
characterized by similar conical outlines, and by rising in the same solitary
and abrupt way from lower ground around them (see Figs. 109, 133, and
195, 294).

Fig. 28.—Section of neck filled with massive
rock.

Various joint-structures may be observed in these necks. In some cases
there is a tendency to separate into joints parallel to the bounding walls, and
occasionally this arrangement goes so far that the rock has acquired a fissile
structure as if it were composed of vertical strata. In other instances, the
rock shows a columnar structure, the columns diverging from the outer
margin, or curving inwards, or displaying various irregular groupings. More
usually, however, this jointing is so indefinite that no satisfactory

connection can be traced between it and the walls of the orifice in which the
rock has solidified.
Some of the most remarkable examples of necks ever figured and described
are those to which attention was called by Captain Dutton as displayed in
the Zuni plateau of New Mexico, where, amid wide denuded sheets of
basalt, numerous prominent crags mark the sites of eruptive vents. The
basalt of these eminences is columnar, the columns standing or lying in all
sorts of attitudes, and in most cases curved.[28] In the Upper Velay, in
Central France, numerous conspicuous domes and cones of phonolite rise
amidst the much-worn basalt-plateau of that region (Fig. 345). Many
instances will be cited in later chapters from the British Isles.
[28] U.S. Geol. Survey, 6th Annual Report, 1884-85, p. 172.

iii. Distribution of Vents in Relation to Geological Structure-lines

Where the positions of true volcanic necks can be accurately determined, it
is interesting to study their distribution and their relation to the main lines
of geological structure around them. Sometimes a distinct linear
arrangement can be detected in their grouping. Those of the Lower Old Red
Sandstone of Central Scotland, for instance, can be followed in lines for
distances of many miles (Map No. III). Yet when we try to trace the
connection of such an arrangement with any known great lines of
dislocation in the terrestrial crust, we can seldom establish it satisfactorily.
In the case of the Scottish Old Red Sandstone just cited, it is obvious that
the vents were opened along a broad belt of subsidence between the
mountains of crystalline schist on the north, and those of convoluted
Silurian strata on the south, either margin of that belt being subsequently, if
not then, defined by lines of powerful fault. No vents have risen along these
faults, nor has any relation been detected between the sites of the volcanic
foci and dislocations in the area of ancient depression.
Indeed, it may be asserted of the vents of Britain that they are usually
entirely independent of any faults that traverse at least the upper visible part
of the earth's crust. They sometimes rise close to such lines of fracture
without touching them, but they are equally well developed where no
fractures are to be found. Now and then one of them may be observed rising

along a line of fault, but such a coincidence could hardly fail occasionally
to happen. From the evidence in the British Isles, it is quite certain that if
volcanic vents have, as is possible, risen preferably along lines of fissure in
the terrestrial crust, these lines are seldom those of the visible superficial
faults, but must lie much deeper, and are not generally prolonged upward to
the surface. The frequent recurrence of volcanic outbursts at successive
geological periods from the same or adjacent vents seems to point to the
existence of lines or points of weakness deep down in the crust, within
reach of the internal molten magma, but far beneath the horizon of the
stratified formations at the surface, with their more superficial
displacements.
While sometimes running in lines, old volcanic vents of the Vesuvian and
Puy types often occur also in scattered groups. Two or three may be found
together within an area of a few hundred yards. Then may come an interval
where none, or possibly only a solitary individual, may appear. And beyond
that space may rise another sporadic group. These features are well
exhibited by the Carboniferous and Permian series of Scotland, to the
account of which the reader is referred.
A large neck may have a number of smaller ones placed around it, just as a
modern Vesuvian cone has smaller parasitic cones upon its flanks. An
instructive example of this arrangement is to be seen at the great vent of the
Braid Hills belonging to the Lower Old Red Sandstone and described in
Chapter xx. Other instances may be cited from the Carboniferous and
Permian volcanic series (see Figs. 90, 148, 213).
Not infrequently the irregularities in the ground-plan of a neck, as already
remarked, may be accounted for on the supposition that they mark the site
of more than one vent. Sometimes, indeed, it is possible to demonstrate the
existence of two or even more vents which have been successively opened
nearly on the same spot. The first orifice having become choked up, another
has broken out a little to one side, which in turn ceasing to be effective from
the same or some other cause, has been succeeded by a third (Fig. 29). The
three cones and craters of the little island of Volcanello supply a singularly
perfect recent instance of this structure (Fig. 214). Here the funnel has twice
shifted its position, each cone becoming successively smaller and partially
effacing that which preceded it. In Auvergne, the Puy de Pariou has long
been celebrated as an example of a fresh cinder-cone partially effacing an

earlier one. In the much denuded Palæozoic volcanic tracts of Britain,
where the cones have long since disappeared and only the stumps of the
volcanic cylinders are left, many illustrations occur of a similar
displacement of the funnel, especially among the volcanoes of the
Carboniferous system.
Among the irregularities of necks that may indicate a connection with lines
of fissure, reference may be made here to dykes or dyke-like masses of
agglomerate which are sometimes to be seen among the volcanic districts of
Britain. In these cases the fragmentary materials, instead of lying in a more
or less cylindrical pipe, appear to fill up a long fissure. We may suppose
that the explosions which produced them did actually occur in fissures
instead of in ordinary vents. The remarkable Icelandic fissures with their
long rows of cinder cones are doubtless, at least in their upper parts, largely
filled up with slag and scoriæ. Some illustrations of this structure will be
given in the account of the Carboniferous volcanic rocks of Scotland (see
No. 1 in Fig. 22).

Fig. 29.—Successive shiftings of vents giving rise to double or triple cones. A,
ground-plan; B, vertical section.

There is yet another consideration in regard to the form and size of necks
which deserves attention. Where the actual margin of a neck and its line of
vertical junction with the rocks through which it has been drilled can be
seen, there is no room for dispute as to the diameter of the original funnel,
which must have been that of the actual neck. But in many cases it is
impossible to observe the boundary; not merely because of superficial soil
or drift, but occasionally because the volcanic detritus extends beyond the
actual limits of the funnel. In such cases the necks have retained some
portion of the original volcanic cone which accumulated on the surface
around the eruptive vent. It may even chance that what appears to be a large
neck would be considerably reduced in diameter, and might be shown to
include more than one pipe if all this outer casing could be removed from it.

In Fig. 30, for example, a section is given of a neck (n) from which on the
right-hand side all the cone and surrounding tuffs (t) have been removed by
denudation, the original form of the volcano being suggested by the dotted
lines. On the left side, however, the tuffs which were interstratified with the
contemporaneous sediments are still connected with the neck, denudation
not having yet severed them from it. The overlying strata (l, l) which
originally overspread the extinct volcano have been bent into an anticline,
and the neck of the vent has thus been laid bare by the removal of the crest
of the arch.

Fig. 30.—Section to show the connection of a neck with a cone and
surrounding bedded tuffs.

The instances where a structure of this kind is concealed are probably fewer
in number in proportion to their antiquity. But among Tertiary cones they
may perhaps not be so rare. The possibility of their occurrence should be
kept in view during the investigation of extinct volcanoes. The term Neck
ought not properly to be applied to such degraded volcanic cones. The true
neck still remains preserved in the inside of them. As illustrative of the
structure here referred to, I may cite the example of the Saline Hill (Fig.
148) and of Largo Law (Fig. 226), both in Fife.

iv. Metamorphism in and around Volcanic Vents—Solfataric
Action

The prolonged ascent of hot vapours, stones, dust and lava, in the funnel of
a volcano must necessarily affect the rocks through which the funnel has
been driven. We may therefore expect some signs of alteration in the
material forming the walls of a volcanic neck. The nature of the
metamorphism will no doubt depend, in the first place, on the character and
duration of the agents producing it, and in the second, on the susceptibility
of the rocks to undergo change. Mere heat will indurate rocks, baking

sandstone, for instance, into quartzite, and shales into porcellanite. But
there will almost invariably be causes of alteration other than mere high
temperature. Water-vapour, for instance, has probably always been one of
the most abundant and most powerful of them. The copious evolution of
steam from volcanoes is one of their most characteristic features at the
present day, and that it was equally so in past time seems to be put beyond
question by the constantly recurring vesicular structure in ancient lavas and
in the lapilli and ejected blocks of old agglomerates and tuffs. Direct
experiment has demonstrated, in the hands of various skilful observers,
from the time of Sir James Hall to that of Professor Daubrée, how
powerfully rocks are acted upon when exposed to superheated vapour of
water under great pressure. But the steam of volcanoes often contains other
vapours or mineralizing agents dissolved in it, which increase its
metamorphic influence. The mineral acids, for instance, must exert a
powerful effect in corroding most minerals and rocks. At the Solfatara of
Naples and at other volcanic orifices in different parts of Italy, considerable
alteration is seen to be due to this cause.
Bearing these well-known facts in mind, we may be prepared to find
various proofs of metamorphism around and within old volcanic vents. The
surrounding rocks are generally much hardened immediately contiguous to
a neck, whether its materials be fragmental or massive. Sandstones, for
example, are often markedly bleached, acquire the vitreous lustre and
texture of quartzite, lose their usual fissility, break irregularly into angular
blocks, and on an exposed surface project above the level of the unaltered
parts beyond. Shales are baked into a kind of porcelain-like substance.
Coal-seams are entirely destroyed for economic purposes, having been
burnt into a kind of cinder or fused into a blistered slag-like mass.
Limestones likewise lose their usual bluish-grey tint, become white and
hard, and assume the saccaroid texture of marble.
The distance to which this metamorphism extends from the wall is, among
the exposed necks in Britain, smaller than might be anticipated. Thus I have
seldom been able to trace it among those of Carboniferous or Permian age
for more than 15 or 20 yards in ordinary arenaceous and argillaceous strata,
even where every detail of a neck and its surroundings has been laid bare in
plan upon a beach. The alteration seems to reach furthest in carbonaceous
seams, such as coals.

It is evident that the element of time must enter into the question of the
amount of metamorphism produced in the terrestrial crust immediately
surrounding a volcanic pipe. A volcano, of which the eruptions begin and
end within an interval of a few days or hours, cannot be expected to have
had much metamorphic influence on the rocks through which its vent was
opened. On the other hand, around a funnel which served for many
centuries as a channel for the escape of hot vapours, ashes or lava to the
surface, there could hardly fail to be a considerable amount of alteration.
The absence or comparatively slight development of metamorphism at the
Carboniferous and Permian necks of Scotland may perhaps be regarded as
some indication that these volcanoes were generally short-lived. On the
other hand, more extensive alteration may be taken as pointing to a longer
continuance of eruptive vigour.
The same causes which have induced metamorphism in the rocks
surrounding a volcanic vent might obviously effect it also among the
fragmentary materials by which the vent may have been filled up. When the
eruptions ceased and the funnel was left choked with volcanic debris, hot
vapours and gases would no doubt still continue for a time to find their way
upward through the loose or partially compacted mass. In their ascent they
would permeate this material, and in the end produce in it a series of
changes similar to, and possibly even more pronounced than, those
traceable in the walls of the vent. Instances of this kind of metamorphism
will be cited in the following chapters (see in particular p. 404).

v. Inward Dip of Docks towards Necks

One concluding observation requires to be made regarding the relation of
old volcanic necks to the rocks which immediately surround them. Where a
vent has been opened through massive rocks, such as granite, felsite,
andesite or basalt, it is generally difficult or impossible to determine
whether there has been any displacement of these rocks, beyond the
disruption of them caused by the explosions that blew out the orifice. But
where the pipe has been drilled through stratified rocks, especially when
these still lie nearly flat, the planes of stratification usually supply a ready
test and measure of any such movement. Investigation of the volcanic rocks
of Britain has shown me that where any displacement can be detected at a

neck, it is almost invariably in a downward direction. The strata
immediately around the vent tend to dip towards it, whatever may be their
prevalent inclination in the ground beyond (Fig. 24). This is the reverse of
the position which might have been expected. It is so frequent, however,
that it appears to indicate a general tendency to subsidence at the sites of
volcanic vents. After copious eruptions, large cavernous spaces may
conceivably be left at the roots of volcanoes, and the materials that have
filled the vents, losing support underneath, will tend to gravitate
downwards, and if firmly welded to their surrounding walls may drag these
irregularly down with them. Examples of such sagging structures are
abundantly to be seen among the dissected vents of the Carboniferous and
Permian volcanic series of Scotland.

vi. Influence of Contemporaneous Denudation upon Volcanic
Cones

It must be remembered that former vents, except those of the later
geological periods, are revealed at the surface now only after extensive
denudation. As a rule, the volcanoes that formed them appeared and
continued in eruption during periods of general subsidence, and were one
by one submerged and buried beneath subaqueous deposits. We can
conceive that, while a volcanic cone was sinking under water, it might be
seriously altered in form and height by waves and currents. If it consisted of
loose ashes and stones, it might be entirely levelled, and its material might
be strewn over the floor of the sea or lake in which it stood. But, as has
been already pointed out, the destruction of the cone would still leave the
choked-up pipe or funnel from which the materials of that cone had been
ejected. Though, during the subsidence, every outward vestige of the actual
volcano might disappear, yet the agglomerate or lava that solidified in the
funnel underneath would remain. And if these materials had risen some way
within the cone or crater, or if they reached at least a higher level in the
funnel than the surrounding water-bottom or land-surface, the destruction of
the cone might leave a projecting knob or neck to be surrounded and
covered by the accumulating sediments of the time. It is thus evident that
the levelling of a cone of loose ashes during gradual subsidence, and the
deposition of a contemporary series of sedimentary deposits, might give rise

to a true neck, which would be coeval with the geological period of the
volcano itself.
In practice it is extremely difficult to decide how far any now visible neck
may have been reduced to the condition of a mere stump or core of a
volcano before being buried under the stratified accumulations of its time.
In every case the existence of the neck is a proof of denudation, and
perhaps, in most cases, the chief amount of that denudation is to be ascribed
not to the era of the original volcano, but to the comparatively recent
interval that has elapsed since, in the progress of degradation, the volcanic
rocks, after being long buried within the crust, were once more laid bare by
the continuous waste and lowering of the level of the land.

vii. Stages in the History of old Volcanic Vents

Let us now try to follow the successive stages in the history of a volcano
after its fires had quite burnt out, and when, slowly sinking in the waters of
the sea or lake wherein it had burst forth, it was buried under an ever-
growing accumulation of sedimentary material. The sand, mud, calcareous
ooze, shell-banks, or whatever may have been the sediment that was
gathering there, gradually crept over the submerged cone or neck, and
would no doubt be more or less mixed with any volcanic detritus which
waves or currents could stir up. If the cone escaped being levelled, or if it
left a projecting neck, this subaqueous feature would be entombed and
preserved beneath these detrital deposits. Hundreds or thousands of feet of
strata might be laid down over the site of the volcano, which would then
remain hidden and preserved for an indefinite period, until in the course of
geological revolutions it might once again be brought to the surface.
These successive changes involve no theory or supposition. They must
obviously have taken place again and again in past time. That they actually
did occur is demonstrated by many examples in the British Isles. I need
only refer here to the interesting cases brought to light by mining operations
in the Dairy coal-fields of Ayrshire, which are more fully described in
Chapter xxvii. (p. 433). In that district a number of cones of tuff, one of
which is 700 feet in height, have been met with in the course of boring and
mining for ironstone and coal. The well-known mineral seams of the coal-

field can be followed up to and over these hidden hills of volcanic tuff
which in the progress of denudation have not yet been laid bare (Fig. 146).
The subsidence which carried down the water-bottom and allowed the
volcanic vents to be entombed in sedimentary deposits may have been in
most cases tolerably equable, so that at any given point these deposits
would be sensibly horizontal. But subsequent terrestrial disturbances might
seriously affect this regularity. The sedimentary formations, piled above
each other to a great depth, and acquiring solidity by compression, might be
thrown into folds, dislocated, upheaved or depressed. The buried volcanic
funnels would, of course, share in the effects of these disturbances, and
eventually might be so squeezed and broken as to be with difficulty
recognizable. It is possible that some of the extreme stages of such
subterranean commotions are revealed among the "Dalradian" rocks of
Scotland. Certain green schists which were evidently originally sediments,
and probably tuffs, are associated with numerous sills and bosses of
eruptive material. The way in which these various rocks are grouped
together strikingly suggests a series of volcanic products, some of the
crushed bosses recalling the forms of true necks in younger formations. But
they have been so enormously compressed and sheared that the very lavas
which originally were massive amorphous crystalline rocks have passed
into fissile hornblende-schists.

Fig. 31.—Diagram illustrating the gradual emergence of buried volcanic cones
through the influence of prolonged denudation.

Among the Palæozoic systems of Britain, however, where considerable
fracture and displacement have taken place, examples of successive stages
in the reappearance of buried volcanic cones and necks may be gathered in
abundance. As an illustrative diagram of the process of revelation by the

gradual denudation of an upheaved tract of country, Fig. 31 may be referred
to (compare also Fig. 147).
Here three volcanic vents are represented in different stages of re-
emergence. In the first (A) we see a cone and funnel which, after having
been buried under sedimentary deposits (s, s,) have been tilted up by
subterranean movements. The overlying strata have been brought within the
influence of denudation, and their exposed basset edges along the present
surface of the land (g, g) bear witness to the loss which they have suffered.
Already, in the progress of degradation, a portion of the volcanic materials
which, ejected from that vent, were interstratified with the
contemporaneous sediments of the surrounding sea-floor, has been exposed
at t. A geologist coming to that volcanic intercalation would be sure that it
pointed to the existence of some volcanic vent in the neighbourhood, but
without further evidence he would be unable to tell whether it lay to right or
left, whether it was now at the surface or lay still buried under cover of the
stratified deposits which were laid down upon it.
In the second or central example (B) we have a pipe and cone which have
been similarly disturbed. But in this case denudation has proceeded so far as
to reveal the cone and even to cut away a portion of it, as shown by the
dotted lines to the right hand. Owing, however, to the general inclination of
the rocks towards the left, that side of the cone, together with the tuffs or
lavas connected with it, still lies buried and protected under cover of the
sedimentary formations (s, s).
The third example (C) shows a much more advanced stage of destruction.
Here the whole of the cone has been worn away. All the lavas and tuffs
which were ejected from it towards the right have likewise disappeared, and
strata older than the eruptions of this vent now come to the surface there. To
the left, however, a little portion of its lavas still remains at l, though all the
intervening volcanic material has been removed. That solitary fragment of
the outpourings of this volcano once extended further to the left hand, but
the occurrence of the large dislocation (f) has carried this extension for
down below the surface. The vent in this instance, owing to its position, has
suffered more from denudation than the other two. Yet, judged by the size
of its neck, it was probably larger than either of them, and threw out a more
extensive pile of volcanic material. Its funnel has been filled with
agglomerate (a), through which a central plug of lava (p) has ascended, and

into which dykes or veins (d, d), the last efforts of eruption, have been
injected.
This diagram will serve to illustrate the fact already so often insisted on,
that although denudation may entirely remove a volcanic cone, and also all
the lavas and tuffs which issued from it, the actual filled-up pipe cannot be
so effaced, but is practically permanent.

CHAPTER VI
Underground Phases of Volcanic Action—continued. II.
Subterranean Movements of the Magma: i. Dykes and Veins;
ii. Sills and Laccolites; iii. Bosses (Stocks, Culots)—
Conditions that govern the Intrusion of Molten Rock within
the Terrestrial Crust.

II. Subterranean Movements of the Magma
In the foregoing pages attention has been more specially directed to those
aspects of volcanic energy which reveal themselves above ground and in
eruptive vents. We have now to consider the various ways in which the
molten magma is injected into the crust of the earth.
Such injection must obviously take place during the expulsion of volcanic
materials to the surface. If the explosive violence of an eruption, or the
concomitant movements of the earth's crust, should lead to ruptures among
the subterranean rocks, the molten magma will be forced into these rents. It
is evident that this may happen either with or without any discharge of lava
at the surface. It may be either entirely a plutonic, that is, a deep-seated
phenomenon, or it may be part of a truly volcanic series of events.
It is clear that, by the study of old volcanoes that have had their structure
laid bare by denudation, we may hope to obtain fresh light in regard to
some of the more deeply-seated features of volcanic energy, which in a
modern volcano are entirely concealed from view. A little reflection will
convince us that the conditions for consolidation within the crust are so
different from those at the surface that we may expect them to make
themselves visible in the internal characters of the rocks.
An essential distinction between underground propulsions of molten rock
and superficial outflows of the same material lies in the fact that while the
latter are free to take any shape which the form and slope of the ground may
permit, the subterranean injections, like metal poured into a mould, are

always bounded by the walls of the aperture into which they are thrust.
According, therefore, to the shape of this aperture a convenient
classification of such intrusions may be made. Where the molten material
has risen up vertical fissures or irregular cracks, it has solidified as Dykes
and Veins. Where it has been thrust between the divisional planes either of
stratified or unstratified rocks, so as to form beds, these are conveniently
known as Sills, Laccolites or Intrusive Sheets. Where it has taken the form
of large cylindrical masses, which, ascending through the crust, appear at
the surface in rounded, elliptical or irregularly-shaped eminences, these are
called Bosses (Stocks, Culots).
Further contrasts between the superficial and subterranean consolidation of
molten material are to be found in the respective textures and minute
structures of the rocks. The deep-seated intrusions are commonly
characterized by a general and markedly greater coarseness of
crystallization than is possessed by lavas poured out at the surface. This
difference of texture, obviously in great measure the result of slower
cooling, shows itself in acid, intermediate, and basic magmas. A lava which
at the surface has cooled as a fine-grained, compact black basalt, in which
neither with the naked eye nor with the lens can the constituent minerals be
distinctly determined, may conceivably be represented at the roots of its
parent volcano by a coarse-textured gabbro, in which the felspars and
pyroxenes may have grown into crystals or crystalline aggregates an inch or
more in length. Mr. Iddings has pointed out that the various porphyrites
which form the dykes and sills of Electric Peak are connected with a central
boss of coarsely crystalline diorite.[29] Examples of the same relation from
different volcanic centres in Britain will be cited in later chapters.
[29] 12th Ann. Rep. U.S. Geol. Survey (1890-91), p. 595.

This greater coarseness of texture is shown by microscopic examination to
be accompanied by other notable differences. In particular, the glassy
residuum, or its devitrified representatives, which may be so frequently
detected among the crystals of outflowing lavas, is less often traceable in
the body of subterranean intrusive rocks, though it may sometimes be
noticed at their outer margins where they have been rapidly chilled by
contact with the cool upper part of the crust into which they have been
impelled. Various minerals, the constituents of which exist in the original
magma, but which may be hardly or not all recognisable in the superficial

lavas, have had leisure to crystallize out in the deep-seated intrusions and
appear sometimes among the components of the general body of the rock,
or as well-terminated crystals in its drusy cavities.
Considerable though the variations may be between the petrographical
characters of the intrusive and extrusive rocks of a given district and of the
same eruptive period, they appear generally to lie within such limits as to
suggest a genetic relation between the whole series. Conditions of
temperature and pressure, and the retention or escape of the absorbed
vapours which play so large a part in volcanic activity, must exercise great
influence on the crystallization of constituent minerals, and on the
consolidation and ultimate texture of the rocks. Slow cooling under great
pressure and with the mineralizing vapours still largely retained seems to be
pre-eminently favourable for the production of a holocrystalline texture in
deep-seated portions of the magma, while rapid cooling under merely
atmospheric pressure and with a continuous disengagement of vapours,
appears to be required for the finer grain, more glassy structure, and more
vesicular character of lavas poured out at the surface.
Besides these differences, however, there is evidence of a migration of the
constituent minerals in the body of large intrusive masses before
consolidation. In particular, the heavier and more basic constituents travel
towards the cooling margin, leaving the central portions more acid. This
subject will be more fully considered in connection with the internal
constitution of Bosses, and some British examples will then be cited.
Reference, however, may here be made to one of the most exhaustive and
instructive studies of the relations of the subterranean and superficial
erupted rocks of an old volcano, which will be found in the monograph by
Mr. Iddings on Electric Peak and Sepulchre Mountain in the Yellowstone
Park of Western America. From the data there obtainable he draws the
deduction that one parent magma, retaining the same chemical composition,
may result in the ultimate production of rocks strikingly different from each
other in structure and mineralogical constitution, yet chemically identical.
Electric Peak includes the central funnel filled up with coarsely crystalline
diorite, and having a connected series of sills and dykes of various
porphyrites. Sepulchre Mountain, separated from its neighbouring eminence
by a fault of 4000 feet, displays some of the superficial discharges from the
vent—coarse breccias with andesite-lavas. These rocks are not chemically

distinguishable from the intrusive series, but the lavas are, on the whole,
more glassy, while the materials of the bosses, sills and dykes are more
crystalline. The latter display much more visible quartz and biotite.[30]
[30] 12th Ann. Rep. U.S. Geol. Survey, 1890-91. As already stated, the eruptions of
this volcanic centre became progressively more acid, and this change appears to be
exhibited by the extrusive lavas as well as by the intrusive rocks.

By practice in the field, supplemented by investigation with the aid of the
microscope, a geologist acquires a power of discriminating with fair
accuracy, even in hand specimens, the superficial from the subterranean
igneous rocks of an old volcanic district.
Denudation, while laying bare the underground mechanism of an ancient
volcano, has not always revealed the evidence of the actual structural
relations of the rocks, or has first exposed and then destroyed it. Sometimes
a mass of eruptive rock has been worn down and left in such an isolated
condition that its connection with the rest of the volcanic network cannot be
determined. So far as its position goes, it might perhaps be either a remnant
of a lava-stream or the projecting part of some deeper-seated protrusion.
But its texture and internal structure will often enable a confident opinion to
be expressed regarding the true relations of such a solitary mass.

i. Dykes and Veins

For the study of these manifestations of volcanic energy, the British Isles
may be regarded as a typical region. It was thence that the word "dyke"
passed into geological literature. Thousands of examples of both dykes and
veins may be seen from the Outer Hebrides southwards across the length
and breadth of the southern half of Scotland, far into the north of England
and towards the centre of Ireland. They may be found cutting the crests of
the mountains and extending as reefs below the level of the sea. They are
thus exposed in every conceivable divergence of position and in endless
varieties of enclosing rock. Moreover, they can be shown to represent a vast
range of geological time. One system of them belongs to some remote part
of the Archæan periods, another is as young as the older Tertiary ages.

Fig. 32.—Dyke, Vein and Sill. The dyke (d) rises
along a small fault among sandstones, shales, and
ironstones (sh), and gives off a vein (v) and an
intrusive sheet or sill (b).

Full details regarding these interesting relics of volcanic activity will be
given in later chapters, especially in Chapters xxxiv. and xxxv. It may
suffice here to note that each of the three types of old volcanoes above
described has, in Britain, its accompaniment of dykes and veins. The
plateaux, however, present by far the most abundant and varied
development of them. The dykes of this series are characterized not only by
their prodigious numbers in and around some of the plateaux, but by the
long distances to which they may be traced beyond these limits. They are
chiefly found in connection with the Tertiary basalt-plateaux, though the
Carboniferous andesite-plateaux present a feebler display of them. The
Tertiary dykes are pre-eminently distinguished by their persistent rectilinear
lines, sometimes for distances of many miles, and their general north-
westerly direction. They form a vast system extending over an area of some
40,000 square miles. Throughout that wide region their persistence of
direction and of petrographical characters point to the former existence of
one or more reservoirs of an andesitic and basaltic magma underneath the
northern half of Britain, and to the rupture of the crust overlying this
subterranean reservoir by thousands of parallel fissures. They thus
constitute perhaps the most astonishing feature in the volcanic history of
Tertiary time.
The dykes and veins connected with the puys are mainly to be found at or
close to the vents. Not infrequently they traverse the agglomerates of the
necks, and are sometimes to be traced to a central pipe or core of basalt.

The larger cones are likewise intersected with similar vertical, inclined or
tortuously irregular walls of intruded lava. Occasionally a radiate
arrangement may be observed in such cases, like that noticeable at some
modern volcanoes, the dykes diverging from the eruptive centre.
Many dykes exist regarding which there is no evidence to connect them
with any actual volcanic rocks. They have been injected into fissures, but
whether this took place during volcanic paroxysms, or owing to some
subterranean movements which never culminated in any eruption, cannot be
decided.
The question of the age of dykes, like that of intrusive masses of all kinds,
is often difficult or impossible to decide. A dyke must of course be younger
than the rocks which it traverses, and a limit to its antiquity is thus easily
fixed. But we cannot always affirm that because a dyke stops short of a
particular rock, or series of rocks, it is older than these. The Hett Dyke, in
the north of England, rises through the Coal-measures, but stops at the
Magnesian Limestone; yet this cessation does not necessarily imply that the
dyke was in place before the deposition of that limestone. The structure
may have arisen from the dyke-fissure having ended at the bottom of the
limestone. Where dykes rise up to the base of an unconformable formation
without in any single case entering it, and where fragments of them are
enclosed in that formation, they must be of higher antiquity, and must have
been laid bare by extensive denudation before the unconformable strata
were deposited upon them. The great system of dykes in the Lewisian
Gneiss of the north-west of Scotland is in this way proved to be much more
ancient than the Torridon Sandstones under which it passes (Figs. 35, 36).
Where two dykes cross each other, it is sometimes not difficult to decide
upon their relative antiquity. In intrusive rocks, the finest-grained parts are
those which lie nearest the outer margin, where the molten material was
rapidly chilled by coming in contact with cool surfaces of rock. Such
"chilled margins" of closer grain are common characteristics of dykes.
Wherever a dyke carries its chilled margin across another dyke, it must be
the younger of the two, and wherever such a margin is interrupted by
another dyke, it must belong to the older.
As a rule, the uprise of molten material in a fissure has so effectually sealed
it up that in the subsequent disturbances of the terrestrial crust the fissure

has not been reopened, though others may have been produced near it, or
across it. Sometimes, however, the enormous tension to which the crust was
exposed opened the fissure once more, sometimes even splitting a dyke
along its centre, and a new ascent of molten rock took place within the rent.
Hence double or treble or compound dykes have been produced. The
second or later infillings are generally somewhat different from the original
dyke. Occasionally, indeed, they present a strong contrast to it. Thus, among
the dykes of Skye examples occur where the centre is occupied by an acid
granophyre, while the sides are occupied by dykes of basalt. Instances of
this compound type of dyke will be given in the account of the Tertiary
volcanic rocks of Britain.
It is obvious that in a wide fissure the central portion may remain molten
for some time after the sides have consolidated. If the fissure served as a
channel for the ascent of lava to the surface, it is conceivable that the
central still fluid part might be driven out and be replaced by other material
from below, and that this later material might differ considerably in
composition from that which first filled the opening. Such, according to Mr.
Iddings, has been the probable history of some of the dykes at the old
volcano of Electric Peak.[31] But we can hardly suppose that this
explanation of compound dykes can have any wide application. It could
only hold good of broad fissures having an outlet, and is probably
inadmissible in the case of the numerous compound dykes not more than 10
or 15 feet in diameter, where the several bands of rock are sharply marked
off from each other. The abrupt demarcation of the materials in these dykes,
their closer texture along their mutual boundaries, the indications of
solution of the older parts of the group by the younger, and of injection of
the latter into the former, show that they belong to separate and
unconnected intrusions. These questions will be again referred to in the
account of the British Tertiary dykes (Chapter xxxv. vol. ii. p. 159).
[31] 12th Ann. Rep. U.S. Geol. Survey (1890-91), p. 587.

Another kind of compound dyke has arisen from the manner in which the
original fissure has been produced. While, in general, the dislocation has
taken the form of a single rectilinear rent, which on opening has left two
clean-cut walls, cases occur where the rupture has followed several parallel
lines, and the magma on rising into the rents appears as two or more vertical
sheets or dykes, separated by intervening partitions of the surrounding rock.

Examples of this structure are not infrequent among the Tertiary dykes of
Scotland. One of these may be noticed rising through the cliffs of Lewisian
gneiss on the east coast of the island of Lewis, south of Stornoway. One of
the most extraordinary instances of the same structure yet observed is that
described by Professor A. C. Lawson from the Laurentian rocks at the
mouth of White Gravel River, on the N.E. coast of Lake Superior. In a
breadth of only about 14 feet no less than 28 vertically intrusive sheets or
dykes of diabase, from 1 inch to 6½ inches broad, rise through the granite,
which is thus split into 27 thin sheets. The diabase undoubtedly cuts the
granite, some of the sheets actually anastomosing and sending veins into the
older rock.[32]
[32] American Geologist (1894), p. 293.

From the evidence supplied by the modern eruptions of Iceland, it is evident
that gaping fissures, which are filled by ascending lava and thereby
converted into dykes, in many instances serve as channels by which molten
rock escapes to the surface. It would be interesting if any test could be
discovered whereby those dykes could be distinguished which had ever
established a connection with the outer air. If the lava continued to ascend
in the fissures, and to pour out in superficial streams for a long time, the
rocks on either side would be likely to undergo considerably more
metamorphism than where there was only one rapid injection of the magma,
which would soon cool. Possibly in the much greater alteration of the same
rocks by some dykes than by others, a sign of such a connection with the
surface may survive. This subject will be again referred to in the account of
the Tertiary dykes of Britain in Book VIII., where the whole of the
phenomena of this phase of volcanic action will be fully discussed (see vol.
ii. p. 163).

ii. Sills and Laccolites

The word "sill," derived from a remarkable sheet of eruptive rock in the
north of England, known as the Great Whin Sill (Chapter xxix.), is now
applied as a convenient general term to masses of intrusive material, which
have been injected between such divisional planes as those of stratification,
and which now appear as sheets or beds (Fig. 33). These masses are
likewise called Intrusive Sheets, and where the injected material has

accumulated in large blister-like expansions, these are known as Laccolites
(Fig. 34).

Fig. 33.—Section of Sill or Intrusive Sheet.

Sills vary from only an inch or two up to 500 feet or more in thickness.
Lying, as they frequently do, parallel with strata above and below them,
they resemble in some respects true lava-sheets erupted contemporaneously
with the series of sediments among which they are intercalated. And,
indeed, cases occur in which it is hardly possible to decide whether to
regard a given mass as a sill or as a superficial lava. In general, however,
sills exhibit the coarser texture above referred to as specially characteristic
of subterranean eruptive masses. Moreover they are usually, though not
always, free from the vesicular and amygdaloidal structures of true surface-
lavas. Their under and upper surfaces, unlike the more scoriaceous parts of
lavas, are commonly much closer in grain than the general body of the
mass; in other words, they possess chilled borders, the result of more rapid
consolidation by contact with cooler rock. Again, instead of conforming to
the stratification of the formations among which they lie, as truly
interstratified lavas do, they may be seen to break across the bedding and
pursue their course on a higher or lower platform. The strata that overlie
them, instead of enclosing pieces of them and wrapping round irregularities
on their surface, as in the case of contemporaneously erupted lava-sheets,
are usually indurated, sometimes even considerably altered, while in many
cases they are invaded by veins from the eruptive sheet, or portions of them
are involved in it, and are then much hardened or metamorphosed.
The petrographical character of the sills in a volcanic district depends
primarily on the constitution of the parent magma, whence both they and
the outflowing lavas have issued. Where the lavas are rhyolites or felsites
the sills are acid, where basalts have been erupted the sills are basic, though
there has often been a tendency towards the appearance of more acid
material, such as trachyte. As we have seen, considerable differences in

petrographical characters may arise between the intrusive and extrusive
offshoots from the same parent magma during the course of a volcanic
cycle. This question will be more appropriately discussed together with the
leading characters of Bosses.
Between the upper and under surface of a thick sill considerable
petrographical variation may sometimes be observed, especially where the
rock is of basic constitution. Differences both of texture and even to some
extent of composition can be detected. Sometimes what have been called
"segregation veins" traverse the mass, consisting of the same minerals as
the general body of the rock, but in larger crystals and in somewhat
different proportions. That these veins belong to the period of original
consolidation appears to be shown by the absence of fine-grained, chilled
margins, and by the way in which the component crystals of the veins are
interlocked with those of the body of the rock. Other veins of finer grain
and more acid composition probably belong to a later phase of
consolidation, when, after the separation and crystallization of the more
basic minerals, the more acid mother liquor that remained was, in
consequence of terrestrial movements, injected into cracks in the now
solidified, though still highly heated, rock. Examples of these features will
be cited from various geological formations in the following chapters.
Reference has already been made to the difference occasionally perceptible
between the constitution of the upper and that of the under portions of
superficial lavas. A similar variation is sometimes strongly marked among
sills, especially those of a basic character, the felspars remaining most
abundant above, while the olivines and augites preponderate below. Mr.
Iddings has observed some excellent illustrations of this character in the
great series of sills connected with the volcanic pipe of Electric Peak in the
Yellowstone country.[33] Some examples of the same structure will
subsequently be cited from the Carboniferous volcanic series of Central
Scotland.
[33] "Electric Peak and Sepulchre Mountain," 12th Ann. Rep. U.S. Geol. Survey
(1890-91), p. 584.

The greatest extreme of difference which I have observed in the
petrographical characters of any group of sills is that displayed by the
Tertiary gabbros of Skye. These rocks occur as sheets interposed among the
bedded basalts, and injected between each other in such a manner as to

form thick piles of rudely stratified sills. They possess a remarkable banded
structure, due to the aggregation of their component minerals in distinct
layers, some of which are dark in colour, from the abundance of their iron-
ore, pyroxene and olivine; while others are light-coloured, from the
predominance of their felspar. From the manner in which the component
minerals of one band interlace with those of the contiguous bands, it is quite
certain that the structure is not due to successive injections of material
among already consolidated rocks, but belongs to the original conditions of
expulsion of the gabbro as a whole. It seems to indicate that the magma
which supplied the sills was at the time of its extrusion heterogeneous in
composition, and that the banding arises from the simultaneous or rapidly
successive protrusion of different portions of this variously-constituted
magma. The details of the structure will be described in the general account
to be given of the Tertiary volcanic rocks (Chapters xliii. and xliv.).
Besides such visible differences in the composition of sills, others much
less obtrusive may occasionally be detected with the aid of microscopic or
chemical research. The outer parts of some sills are thus discovered to be
more basic or more acid than the inner portions. Or evidence may be
obtained pointing to the probable melting down of surrounding rocks by the
erupted magma, with a consequent local change in the chemical and
mineralogical constitution of the mass.
In regard to their position in the geological structure of an old volcanic
district I may here remark that sills, seldom entirely absent, are more
especially developed either among the rocks through which the volcano has
driven its vent, or about the base of the erupted lavas and tuffs. Many
illustrations of this distribution will be described from the various volcanic
areas of Britain belonging to Palæozoic and Tertiary time. At the base of the
great Cambrian and Lower Silurian volcanic series of Merionethshire, sills
are admirably developed, while among the basaltic eruptions which closed
the long volcanic record in the north of Ireland and the Inner Hebrides, they
play a notable part.
From the frequent place which sills take at the base of a volcanic series, it
may be inferred that they generally belong to a late phase in the history of
an eruptive episode or cycle, when the orifices of discharge had become
choked up, and when the volcanic energy found an easier passage laterally
between the strata underneath the volcanic pile or between the sheets of that

pile itself, than upward through the ever-increasing thickness of ejected
material.
While there is an obvious relation between most sills and some eruptive
centre in their neighbourhood, cases occur in which no trace of any
contemporaneous volcano can be found, but where the intrusive sheet
remains as the sole evidence of the movements of the subterranean magma.
The Great Whin Sill, one of the most extensive intrusive sheets in the
British Isles, is an instance of this kind. Though this large mass of injected
material can be traced for a distance of about 80 miles, and though the strata
beneath and above it are well exposed in innumerable sections, no evidence
has yet been detected to show that it was connected with any vent that
formed a volcano at the surface (see vol. ii. p. 2). The absence of this
evidence may, of course, arise from the failure of denudation to uncover the
site of the vent, which may possibly still remain buried under the
Carboniferous strata that overlie the sill towards the south-east. But it may
be due to the non-existence of any such vent. We can quite conceive that
volcanic energy should sometimes have failed to complete the formation of
an actual volcano. Aided by subterranean movements, it might have been
potent enough to disrupt the lower parts of the terrestrial crust, to propel the
molten magma into fissures, even to inject it for many miles between the
planes of stratification, which would be lines of least resistance, and yet in
default of available rents, might have been unable to force its way through
the upper layers and so reach the surface. Examples of such incompleted
volcanoes are perhaps to be recognized among solitary sills, which not
infrequently present themselves in the geological structure of Britain. But
the positive decision of this question is almost always frustrated by the
imperfection of the evidence, and the consequent possibility that a
connected vent may still lie concealed under overlying strata.
Besides the more usual intrusions of molten material in the form of sheets
of which the vertical thickness bears but a small proportion to the horizontal
extent, there occur also large and thick cakes of intruded material in which
the vertical thickness may approach, or perhaps even surpass, the horizontal
diameter. These dome-shaped or irregular expansions form a connecting
link between ordinary sills and the bosses to be subsequently described.
They have received the name of Laccolites from Mr. G. K. Gilbert, who
worked out this peculiar type of structure in the case of the Henry

Mountains in southern Utah[34] (Fig. 34). The same type has since been
found distributed over Arizona and Colorado, and it has been recognized as
essentially that of many eruptive masses or bosses in all parts of the world.
[34] "Geology of the Henry Mountains," U.S. Geog. and Geol. Survey of the Rocky
Mountain Region, 1877. For a review of the whole subject of laccolites in Western
America see a paper by Mr. Whitman Cross, in the 14th Annual Report of the
Director of the U.S. Geological Survey, 1892-93 (pub. 1895), p. 157.

Fig. 34.—Ideal section of three Laccolites. (After Mr.
Gilbert.)

In Western America, owing in large measure to the previously undisturbed
condition of the sedimentary formations, the relations of the injected
igneous material to these formations can be satisfactorily ascertained. The
geological structure of the various isolated laccolites thus clearly presented,
helps to explain the structure of other intrusive bodies which, having been
injected among plicated and dislocated rocks, do not so readily admit of
interpretation.
In Colorado, Utah and Arizona the eruptive magma, usually a porphyrite,
diorite or quartz-porphyry, has risen in one or more pipes, and has then
intruded itself laterally between the planes of the sedimentary formations
which, over the centre of intrusion, have been pushed upward into a vast
dome-shaped or blister-like elevation. The horizon on which this lateral and
vertical expansion of the intruded material took place would seem to have
lain several thousand feet below the surface. It ranges from the Cambrian to
the Tertiary formations. Subsequent denudation has cut down the upraised
mantle of sedimentary layers, and has revealed more or less of the igneous
rock underneath, which is thus allowed to protrude and to be affected by
atmospheric erosion. In this way, wide plains of horizontal or gently
undulating Secondary and Tertiary strata have been diversified by the
appearance of cones, detached or in groups, which have become more

peaked and varied in outline in proportion as their original sedimentary
covering has been removed from them. The largest of the laccolitic masses
in the Henry Mountains is about 7000 feet deep and about 4 miles in
diameter. Less than one-half of the cover of overarching strata has been
removed, and denudation has cut deeply into the remaining part.
That the type of structure, so well exhibited among the Henry Mountains,
has not been more abundantly recognized elsewhere probably arises from
the fact not that it is rare, but that the conditions for its development are
seldom so favourable as in Western America. Obviously where stratified
rocks have been much disturbed, they cease to furnish definite or regular
platforms for the reception of eruptive material, and to afford convenient
datum-lines for estimating what was probably the shape of the intruded
magma. We may believe that the effect of the propulsion of eruptive
material is usually to upheave the overlying crust, and thus to give rise to a
laccolitic form of intrusion. The upheaval relatively to the surrounding
country will be apt to be practically permanent, the intruded body of rock
being welded to the surrounding formations, and forming in this way a solid
and resisting core directly united by pipes or funnels with the great magma-
reservoir underneath. On the other hand, where the molten rock, instead of
consolidating underground, has been copiously discharged at the surface, its
emission must tend towards the production of cavernous spaces within the
crust. The falling in of the roofs of such caverns will give rise to shocks of
earthquakes. Subsequent uprisings of the magma may fill these spaces up,
and when the rock has solidified in the form of laccolites or bosses, it may
effectually put an end there to further eruptions.
Some contact metamorphism may be observed along the upper and under
surfaces of large sills. The rocks over the American laccolites have
sometimes been highly altered. But as the change is the same in kind as that
attendant upon Bosses, though generally less in degree, it will be considered
with these intrusive masses. The problems in terrestrial physics suggested
by the intrusion of such thick and persistent masses of eruptive material as
those which form sills and laccolites will likewise be discussed in
connection with the mechanism of the remaining intrusive masses which
have now to be described.

iii. Bosses (Stocks, Culots)

The term Boss has been applied to masses of intrusive rock which form at
the surface rounded, craggy or variously-shaped eminences, having a
circular, elliptical or irregular ground-plan, and descending into the
terrestrial crust with vertical or steeply-inclined sides (Fig. 28). Sometimes
they can be seen to have pushed the surrounding rocks aside. In other places
they seem to occupy the place of these rocks through which, as it were, an
opening has been punched for the reception of the intrusive material.
Occasionally, more especially in the case of large bosses, like those in
which granite so frequently appears, the eruptive mass may be observed to
rise here and there in detached knobs through the surrounding rocks, or to
enclose patches of these, in such a manner as to indicate that the large body
of eruptive material terminates upward in a very irregular surface, of which
only the more prominent parts project through the cake of overlying rocks.
In true bosses, unlike sills or laccolites, we do not get to any bottom on
which the eruptive material rests. Laccolites, indeed, may be regarded as
intermediate between the typical sill and the typical boss. The difference
between a laccolite and a boss lies in the fact that the body of the laccolite
does not descend into an unknown depth in the crust, but lies upon a
platform on which it has accumulated, the magma having ascended by one
or more ducts, which generally bear but a small proportion in area to the
mass of the laccolite. The boss, on the other hand, is not known to lie on
any horizon, nor to proceed from smaller ducts underneath, but plunges as a
great pillar or irregular mass, which may frequently be noticed to widen
downwards into the crust. There can be no doubt, however, that many
masses of eruptive rock, which, according to the definition here given,
should be called bosses, would be found to be truly laccolites if their
structure below ground could be ascertained. It is obvious that our failure to
find any platform on which the body of a boss lies, may arise merely from
denudation having been as yet insufficient to lay such a platform bare. It is
hardly probable that a boss several miles in diameter should descend as a
column of that magnitude to the magma-reservoir from which its material
came. More probably it has been supplied through one or more smaller
ducts. The large boss now visible at the surface may thus be really a
laccolitic expansion on one or more horizons. M. Michel Lévy lays stress
on the general widening of granitic bosses as they descend into the crust.[35]
While his observations are supported by many illustrations from all parts of
the globe, and are probably true of the deeper-seated masses of granite, it is

no less true that numerous examples have been met with where a granite
boss is sharply marked off from the rocks which it has invaded and on
which it may be seen to lie. Apart from the cases where granite seems to
form part of a vast internal, once molten mass, into which its encircling
gneisses seem to graduate, there are others in which this rock, as now
visible, has been injected into the crust as a boss or as a laccolite. Instances
will be described in later chapters where such bosses have risen through
Cambrian, Silurian, Devonian and Carboniferous formations. It may be said
that between such granitic intrusions and volcanic operations no connection
can be traced. But reasons will be brought forward in later chapters to
regard some of the granitic bosses as parts of the mechanism of Palæozoic
volcanoes. It will also be shown that among the intrusive rocks of the
Tertiary volcanic series of Britain there occur bosses of truly granophyric
and granitic material. Hence, though mainly what is called a "plutonic"
rock, granite has made its appearance among the subterranean protrusions
of volcanoes.
[35] M. Michel Lévy, Bull. Carte Géol. France, No. 35, tome v. (1893), p. 32. The
view stated in the text is also that adopted by Prof. Brögger with reference to the
granite of the Christiania district. "Die Eruptivgesteine des Kristianiagebietes."

It is no doubt true that many intrusive masses, which must be included
under the general name of bosses, have probably had no connection
whatever with volcanic action properly so called. They are plutonic
injections, that is, portions of the subterranean magma which have been
intruded into the terrestrial crust during its periods of disturbance, and have
not been accompanied with any superficial discharges, which are essential
in truly volcanic energy. It has been proposed to draw a distinction between
such deep-seated intrusions and those which represent volcanic funnels.[36]
If this were always practicable it would certainly be desirable. But the
distinction is not one that can in every case be satisfactorily drawn. Even in
regard to granitic bosses, which may generally be assumed to be plutonic in
origin, the British examples just referred to have in all likelihood been
connected with undoubted volcanic outbursts. Without, therefore,
attempting here to separate the obviously volcanic necks of eruptive
material from the probably plutonic bosses, I propose to describe briefly the
general characters of bosses considered as a group of intrusive rocks,
together with the phenomena which accompany them, and the conditions
under which they may have been injected.

[36] M. Michel Lévy, Bull. Carte Géol. France, No. 35, tome v. (1893).

Bosses, whether of plutonic or volcanic origin, are frequently not merely
single masses of eruptive rock, but are accompanied with a system of dykes
and veins, some of which can be traced directly into the parent-mass, while
others traverse it as well as the surrounding rocks. Hence the history of a
boss may be considerably more complex than the external form of the mass
might suggest.
The petrographical characters of bosses link them with the other
underground injections of igneous material, more especially with sills and
laccolites. Indeed, on mere lithological grounds no satisfactory line could
be drawn between these various forms of intrusive rocks. The larger the
mass the more coarsely crystalline it may be expected to be. But the whole
range of structure, texture and composition, from those of the narrowest
vein to those of the widest boss, constitutes one connected series of
gradations.
Acid, intermediate and basic rocks are abundantly displayed among the
bosses. Huge masses of granite, granophyre, quartz-porphyry, felsite or
rhyolite, represent the acid series. Intermediate varieties consist of trachyte,
phonolite, diorite, andesite or other rock. The basic bosses include varieties
of gabbro, dolerite, basalt, picrite, and other compounds.
In a boss of large size, a considerable range of texture, composition and
structure may often be observed. The rock is generally much coarser in
grain than that of thin sills or dykes. Sometimes it exhibits a finer texture
along the margin than in the centre, though this variation is not usually so
marked as in sills and dykes. The rapidly-chilled and therefore more close-
textured selvage seems to have been developed much more fully in small
than in large masses of eruptive material. The latter, cooling more slowly,
allowed even their marginal parts to retain their heat, and sometimes
perhaps even their molten condition, longer than small injections. Some
influence must also have been exercised by the temperature of the rocks
into which the eruptive material was intruded. Where this temperature was
high, as in deep-seated parts of the crust, it would allow the intrusive
magma to cool more slowly, and thus to assume a more coarsely crystalline
condition. The absence of a close grain round the margins of granitic bosses
may be due to this cause.

But a much more important distinction may be traced between the central
and marginal parts of some large bosses and thick sills. I have already
alluded to the fact that while the middle of a large intrusive mass may be
decidedly acid, taking even the form of granite, the outer borders are
sometimes found to be much more basic, passing into such a rock as
gabbro, or even into some ultra-basic compound. Between these extremes
of composition no sharp division is sometimes discoverable, such as might
have been expected had the one rock been intruded into the other. The
differences graduate so insensibly into each other as to suggest that
originally the whole mass of the rock formed one continuous body of
eruptive material. It is possible that in some cases the magma itself was
heterogeneous at the time of intrusion.[37] But the frequency of the
distribution of the basic ingredients towards the outer margin, and the acid
towards the centre, points rather to a process of differentiation among the
constituents of the boss before consolidation. In some instances the
differentiation would appear to have taken place before crystallization to
any great extent had set in, because the minerals ultimately developed in the
central parts differ from those at the sides. In other cases, the transference
of material would seem to have been in progress after the component
minerals had crystallized out of the magma, for they are the same
throughout the whole intrusive mass, but differ in relative proportions from
centre to circumference.[38]
[37] The Tertiary gabbros of the Inner Hebrides have already been cited, and will be
more fully described in a later chapter as exhibiting the heterogeneousness of an
eruptive magma.
[38] See Messrs. Dakyns and Teall, Quart. Journ. Geol. Soc. xlviii. (1892), p. 104;
Prof. Brögger, op. cit. 1. (1894), p. 15; Mr. A. Harker, op. cit. p. 320; Prof. Iddings,
Journ. Geol. Chicago, i. (1893), p. 833; Bull. Phil. Soc. Washington, ii. (1890), p.
191; 1892, p. 89.

As illustrations of these features I may cite two good examples, one from
Scotland and one from England. The mass of Garabol Hill, in the Loch
Lomond district, consists mainly of granite, occupying an area of about
12½ square miles. Messrs. Dakyns and Teall have shown that while the
central portions consist of granite, the south-eastern margin affords a
remarkable series of intermediate rocks, such as hornblende-biotite-granite,
tonalite (quartz-mica-diorite), diorite and augite-diorite, which lead us
outwards into highly basic compounds, including wehrlites (olivine-diallage

rocks), picrites (olivine-augite rocks), serpentine (possibly representing
dunites, saxonites, and lherzolites), and a peculiar rock consisting
essentially of enstatite, diallage, brown hornblende and biotite. The authors
regard the whole of these widely different rocks as the products of one
original magma, the more basic marginal area having consolidated first as
peridotites, followed by diorites, tonalites and granites in the order of
increasing acidity. The most acid rock in the whole series consists of felspar
and quartz, is almost devoid of ferro-magnesian minerals, and occurs in
narrow veins in the granite and tonalite. It indicates that after the
segregation and consolidation of the whole boss, ruptures occurred which
were filled in by the ascent of the very latest and most acid remaining
portion of still fluid magma.[39]
[39] Messrs. Dakyns and Teall, Quart. Journ. Geol. Soc. xlviii. (1892), p. 104.

The case of Carrock Fell in Cumberland has been described by Mr. A.
Harker, who has ascertained that the gabbro of this boss has in its central
portions a specific gravity of less than 2·85 and a silica-percentage
sometimes as high as 59·46, whilst its marginal zone gives a specific
gravity above 2·95 and a silica-percentage as low as 32·50. The migration
of the heavy iron ores towards the margin is readily apparent to the naked
eye, and is well established by chemical analysis, the oxides of iron
amounting in the centre to 6·24 (Fe2O3 3·60, FeO 2·64), and at the margin
to 25·54 (Fe2O3 8·44, FeO 17·10).[40] Neither in this instance nor in that of
Garabol Hill has any evidence been noticed which would suggest that the
basic and acid rocks belong to different periods of intrusion. They pass so
insensibly into each other as to form in each case one graduated mass.
[40] Mr. A. Harker, op. cit. p. 320.

From these and other examples which have been observed, it is difficult to
escape the conclusion that the differences between the basic margin and the
acid centre are due to some process of segregation or differentiation while
the mass was still in a liquid condition, and its constituents could pass from
one part of the boss to another. According to Professor Brögger, it may be
stated as a general law that differentiation sets in during consolidation, and
is determined by, and dependent on, the laws of crystallization in a magma,
in so far as the compounds which, on given conditions, would first
crystallize out, diffuse themselves towards the cooling margin so as to

produce in the contact-stratum a peculiar chemical composition in the still
liquid material before crystallization takes place.[41]
[41] This general conclusion is stated by Professor Brögger from his investigation of
the rocks of Gran, Quart. Journ. Geol. Soc. l. (1894), p. 36.

If during the process of differentiation, and before consolidation, injections
of the magma occur, they may be expected to differ in character according
to the portion of the magma from which they are derived. Professor Brögger
believes that among the basic eruptive rocks of Gran in the Christiania
district, one and the same magma has in the bosses solidified as olivine-
gabbro-diabases, and in the dykes as camptonites, bostonites, pyroxenites,
hornblendites, and more acid augite-diorites.[42]
[42] Quart. Journ. Geol. Soc. l. (1894), p. 35.

Various opinions have been propounded as to the cause or causes of this so-
called differentiation, but none of them are entirely satisfactory. We must
await the results of further exploration in the field and of continued research
in the laboratory.
What appears to have taken place within a subterranean molten magma
which has been propelled into the earth's crust as a boss or laccolite, with or
without a connected system of dykes, may possibly be made to throw some
light on the remarkable changes in the characters of lavas successively
erupted from the same vent during the continuance of a volcanic cycle.
Whether or not any such process of differentiation can be proved to take
place within a subterranean volcanic reservoir, the sequence of erupted
lavas bears a curious resemblance to the order in which the constituents of
some large bosses succeed each other from margin to centre. The earliest
lavas may be of an intermediate or even basic character, but they generally
tend to become more acid. Nevertheless alternations of basic and acid lavas
which have been noted in various districts would seem to show that if there
be a process of differentiation in the magma-basins, it is not regular and
continuous, but liable to interruption and renewal. The return to basic
eruptions, which so often marks the close of a volcanic cycle, is likewise
not easily explicable on the supposition of continuous differentiation.
Where no sensible evidence of differentiation is traceable in the general
body of a large intrusive mass, indications that some such process has there
been in progress are perhaps supplied by the more acid dykes or veins, and

the so-called "segregation veins," which have been already alluded to as
traversing large intrusive masses. Though these portions differ to a greater
or less extent in texture and composition from the main substance of the
boss, the differences are not such as to prevent us from regarding them as
really parts of the same parent magma. The veins, which are more acid than
the rock that they traverse, may be regarded as having emanated from some
central or deeper-seated part of a boss, which still remained fluid after the
marginal or upper portion had consolidated sufficiently far to be capable of
being rent open during subterranean disturbance. But that the mass, though
coherent enough to be fissured, still remained at a high temperature, may be
inferred from the general absence of chilled edges to these veins. The
evidence of differentiation supplied by "segregation veins" has been
referred to in the case of Sills.
The study of the petrographical variations in the constitution of large
eruptive bosses has a twofold interest for the geologist. In the first place, it
affords him material for an investigation of the changes which a volcanic
magma undergoes during its eruption and consolidation, and thereby
provides him with some data for an elucidation of the cause of the sequence
of erupted products during a volcanic cycle. In the second place, it yields to
him some interesting analogies with the structures of ancient gneisses, and
thus helps towards the comprehension of the origin and history of these
profoundly difficult but deeply fascinating rocks.
Bosses, like sills, occur in the midst of volcanic sheets, and also as solitary
protrusions. Where they rise amidst interstratified lavas and tuffs they may
often be recognized as occupying the position of volcanic vents. They are
then necks, and their characters in this connection have already been given.
Where, however, as so frequently happens, they appear among rocks in
which no trace of any contemporaneous volcanic material is to be detected,
their relation to former volcanic activity remains uncertain.
Of this doubtful nature some of the most notable examples are supplied by
the great granitic bosses which occur so frequently among the older
Palæozoic rocks of Britain. The age of these can sometimes be
approximately fixed, and is then found to correspond more or less closely
with some volcanic episode. Thus the granite-bosses of Galloway, in the
south of Scotland, disrupt Upper Silurian strata, but are older than the
Upper Old Sandstone. Hence they probably belong to the period of the

Lower Old Red Sandstone, which was eminently characterized by the
vigour and long continuance of its volcanoes. The granite of Arran and of
the Mourne Mountains can be shown by one line of reasoning to be younger
than surrounding Carboniferous formations, by other arguments to be
probably later than the Permian period, and by a review of the whole
evidence to form almost certainly part of the volcanic history of Tertiary
time.
But even where it can be shown that the uprise of a huge boss of eruptive
material was geologically contemporaneous with energetic volcanic action,
this coincidence may not warrant the conclusion that the boss therefore
marks one of the volcanic centres of activity. Each example must be judged
by itself. There have, doubtless, been many cases of the intrusion of molten
material in bosses, as well as in sills, without the establishment of any
connection with the surface. Such incompleted volcanoes have been
revealed by denudation after the removal of a great thickness of
superincumbent rock. The evidence which would have decided the question
to what extent any of them became true volcanic vents has thus been
destroyed. We can only reason tentatively from a careful collation of all the
facts that are now recoverable. Illustrations of this kind of reasoning will be
fully given in subsequent chapters.
It has been supposed that a test for the discrimination of a subterranean
protrusion from a true volcanic chimney may be found in the condition of
the surrounding rocks, which in the case of the prolonged flow of molten
matter up a vent would be likely to undergo far more metamorphism than
would be the case in the injection of a single eruptive mass.[43] But, as has
been already pointed out, no special or excessive metamorphism of the
encircling rocks is noticeable around many vents. There is certainly no
more alteration contiguous to numerous true necks than around bosses,
which there is no reason to suppose ever communicated directly with the
surface, and which were probably the result of a single intrusion. We must
always remember that the denudation which has revealed these bosses has
generally removed the evidence of their upward termination and of their
possible connection with any volcanic ejections. Many of them may mark
the sites of true vents from which only single eruptions took place. The
opening of a volcanic vent does not necessarily imply a prolonged ascent of
volcanic material. In a vast number of cases the original eruption was the

first and last effort of the volcano, so that in such circumstances there seems
no more reason for much alteration of the walls of the chimney than for the
metamorphism of the rocks round a boss, laccolite, sill or dyke.
[43] See, for example, Mr. Harker, Quart. Journ. Geol. Soc. l. (1894), p. 329.

The metamorphism produced by intrusions of molten material upon the
rocks with which they have come in contact has long been studied. Its
amount varies so greatly in different cases that the conditions on which it
has specially depended are not easily determined. Three factors have
obviously been of great importance—first, the bulk of the intruded material;
secondly, the chemical composition and lithological texture and structure of
the rocks affected; and thirdly, the constitution and temperature of the
invading magma.
1. It is clear that a huge boss of eruptive material will be likely to effect
much more alteration of the surrounding rocks than a small boss, sill or
dyke. Its initial temperature will probably be higher at the time of its
assuming its final place than that of the same material after it has found its
way into the narrower space of a thin sill or dyke. It will likewise take much
longer to cool. Hence the influence of its heat and its vapours will continue
to act long after those of the dyke or sill have ceased to manifest
themselves.
2. It is equally evident that much of the resultant metamorphism will
depend on the susceptibility of the rocks to change. An obdurate material
such as pure quartz-sand, for example, will resist further alteration than
mere hardening into quartzite. Shales and mudstones may be indurated into
cherty substances of various textures. Limestones and dolomites, on the
other hand, may become entirely crystalline, and may even have new
minerals, such as garnet, tremolite, pyroxene, etc., developed in them.
Hence in comparing the amount of metamorphism attendant on two
separate bosses we must always take into account the nature of the rocks in
which it has been induced.
3. But perhaps the most effective cause of variation in the nature and
amount of contact metamorphism has been the constitution of the eruptive
magma. A broad distinction may be drawn between the alteration produced
by basic and by acid rocks. The intrusion of basic material has often
produced singularly little change, even when the eruptive mass has been of

considerable size. The greatest amount of alteration is to be found where the
basic boss has caught up and enveloped portions of the surrounding rocks.
Thus where the gabbro of Carrock Fell has invaded the basic Lower
Silurian lavas of the Lake District, the enveloped portions of the latter show
considerable modification. Their groundmass becomes darker and more
lustrous, the felspars assume a clearer appearance and lose some of their
conspicuous inclusions, the pyroxenic constituents are converted into pale
amphibole, and the glassy base disappears. At the actual line of contact the
felspars of the lavas have become disengaged from their original matrix,
which seems to have been dissolved and absorbed in the gabbro-magma.
Brown mica has been exceptionally developed in the altered lava. At the
same time, a change is noticeable in the character of the gabbro itself near
the contact. Brown mica is there to be seen, though not a constituent of the
rock elsewhere. The eruptive material has incorporated the basic
groundmass of the lavas, leaving the felspars undissolved.[44]
[44] Mr. Harker, Quart. Journ. Geol. Soc. vol. l. (1894), p. 331.

Much more serious are the changes produced by intrusions of acid material,
though here again the metamorphism varies within wide limits, being
sometimes hardly perceptible, and in other cases advancing so far as to
convert mere sedimentary material into thoroughly crystalline rocks. Small
sills and dykes of felsite and granophyre may produce very slight change
even upon shales and limestones, as may be seen among the eruptive rocks
of Skye and Raasay. Large bosses of granophyre, and still more of granite,
have been accompanied with the most extensive metamorphism. Round
these eruptive masses every gradation may be traced among sandy and
argillaceous sediments, until they pass into crystalline mica-schists, which
do not appear to be distinguishable from rocks of Archæan age. Admirable
examples of this extreme alteration may be observed around the great
granite bosses of Galloway.[45] Again, among calcareous rocks a transition
may be traced from dull grey ordinary fossiliferous limestones and
dolomites into pure white crystalline marbles, full of crystals of tremolite,
zoisite, garnet and other minerals. The alteration of the fossiliferous
Cambrian limestones of Strath in Skye by the intrusive bosses of Tertiary
granite well illustrates this change.[46]
[45] See Explanation to Sheet 9 of the Geological Survey of Scotland, p. 22; Prof.
Bonney and Mr. Allport, Proc. Roy. Soc. xvi. (1889); Miss Gardiner, Quart. Journ.

Geol. Soc. vol. xlvi. (1890), p. 569.
[46] Macculloch, Trans. Geol. Soc. vol. iii. (1816), p. 1; Description of the Western
Isles, vol. i. p. 322. See also Quart. Journ. Geol. Soc. vol. xiv. (1857), p. 1; and vol.
xliv. (1888), p. 62.

Without entering further here into the wide subject of contact
metamorphism, to which a large literature has now been devoted, we may
note the effects which have been produced in the eruptive material itself by
its contact with the surrounding rocks. Not only have these rocks been
altered, but very considerable modifications have likewise taken place in
the active agent of the change.
Sometimes the alteration of the invading material has been effected without
any sensible absorption of the mineral constituents of the rocks invaded.
This appears to be the case in those instances where sheets of basalt,
intruded among coals or highly carbonaceous shales, have lost their
compact crystalline character and have become mere clays. In the coal-
fields of Britain, where many examples of this change have been noted, the
igneous material is known as "white trap." The iron oxides have been in
great part removed, or, together with the lime of the component minerals,
have been converted into carbonates. Traces of the original felspar crystals
may still be detected, but the groundmass has been changed into a dull,
earthy, friable and decomposed substance.
Nearly always, however, the alteration of the intrusive magma has resulted
from the incorporation of portions of the surrounding rocks. Reference has
been made above to the alteration of the Carrock Fell gabbro by the
absorption of some of the basic lavas around it. But still more remarkable is
the change produced in some acid rocks by the incorporation of basic
material into their substance. Professor Sollas has described in great detail a
remarkable instance of this effect in the probably Tertiary eruptive rocks of
the Carlingford district in the north-east of Ireland. He has ascertained that
the eruptive gabbro of that district is older than the granite, for it is
traversed by granophyre dykes which enclose pieces of it. The granophyre
dykes, on the other hand, often show a lithoidal or chilled margin, which is
not visible in the gabbro. He believes that the gabbro is not only older than
the acid protrusions, but was already completely solid, traversed by
contraction-joints, and probably fractured by earth-movements, before the
injection of the granophyric material, which at the time of its intrusion was

in a state of extreme fluidity, for it has found its way into the minutest
cracks and crevices. He has especially studied the alteration produced by
the granophyre upon the enclosed pieces of basic rock. The diallage,
isolated from the other constituents of the gabbro, may commonly be seen
to have broken up into numerous granules, like the augite grains of basalt,
while in some cases biotite and hornblende have been developed with the
concomitant excretion of magnetite. The acid rock itself has undergone
considerable modification owing to the incorporation of basic material into
its substance. Professor Sollas distinguishes the following varieties of the
rock:—Biotite-granophyre, biotite-amphibole-granophyre, augite-
[47]
granophyre, diallage-amphibole-augite-granophyre.
[47] Trans. Roy. Irish Acad. xxx. (1894), part xii. p. 477.

Similar phenomena have been described by Mr. Harker as occurring where
granophyre has invaded the gabbro of Carrock Fell.[48] The same observer
has more recently detected some interesting examples furnished by
injections of Tertiary granophyre in the agglomerates of Skye. The acid
rock is roughly estimated by him to have taken up about one-fourth of its
bulk of gabbro fragments. He has investigated the minute structure of the
rock thus constituted, and has been able to recognize the augite of the
original gabbro, in various stages of alteration and completely isolated, the
other minerals having been dissolved in the acid magma.[49]
[48] Quart. Journ. Geol. Soc. li. (1895), p. 183.

[49] Op. cit. lii. The metamorphism produced upon fragments of different kinds of
foreign material enclosed within various igneous rocks has in recent years been
studied in great detail by Professor Lacroix—Les Enclaves des Roches Volcaniques,
Macon, 1893.

It is not easy to comprehend the conditions under which large masses of
molten material have been injected into the crust of the earth. The two main
factors in volcanic action—terrestrial contraction and the energy of the
vapours in the magma—have no doubt played the chief part in the process.
But the relative share of each and the way in which the enormous load of
overlying rock has been overcome are not readily intelligible.
Let us first consider for a moment the pressure of the superincumbent crust
under which the injection in many cases took place. The Whin Sill of
England may serve as a good illustration of the difficulties of the problem.
This notable mass of intrusive rock has been forced between the

stratification planes of the Carboniferous Limestone series in one, or
sometimes more than one, sheet. It stretches for a horizontal distance of not
less than 80 miles with an average thickness of between 80 and 100 feet.
From the area over which it can be traced its total extent underground must
be at least 400 square miles (see Chapter xxix.).
In any single section the Whin Sill might be supposed to be a truly
interstratified sheet, so evenly does it seem to be intercalated between the
sedimentary strata. But here and there it diverges upward or downward in
such a way as to prove it to be really a vast injected sheet. The age of the
injection cannot be precisely fixed. It must be later than the Carboniferous
Limestone. There is no trace of any stratigraphical break in the
Carboniferous system of the region traversed by the sill. If the injection
took place during the Carboniferous period, it does not appear to have been
attended with any local disturbance, such as we might suppose would have
been likely to accompany the extravasation of so enormous a mass of
igneous material. If the date of injection be assigned to the next volcanic
episode in the geological history of Britain—that of the Permian period—it
will follow that the Whin Sill was intruded into its present position under
the superincumbent weight of the whole of the Carboniferous system higher
than the platform followed by the injected rock. The overlying body of
strata would thus exceed 5000 feet in thickness, or in round numbers would
amount at least to an English mile. The pressure of this mass of
superincumbent material, at the depth at which the injected magma was
forced between the strata, must have been so gigantic that it is difficult to
believe that the energy of the magma would have been able to achieve of
itself so stupendous a task as the formation of the Great Whin Sill.
The volume of injected material is likewise deserving of special attention.
Many sills exceed 300 or 400 feet in thickness; and some laccolites must
enormously surpass these limits. The intrusion of so vast a body of new
material into the terrestrial crust will necessitate either a corresponding
elevation of that part of the crust overlying the injected magma or a
subsidence of that part underlying it, or some combination of both
movements. It is conceivable that, where the body of protruded magma was
large and the thickness of overlying crust was small, the expansive force of
the vapours under high tension in the molten rock may have sufficed for the
uplift. This result will be most likely to be effected around a volcanic

chimney where the magma has the least amount of overlying load, and
encounters that relief from pressure which enables it to become a powerful
agent in terrestrial physics.
But in the case of the larger bodies of injected rock, especially where they
do not seem to have been accompanied by the opening of any volcanic
vents, the propulsion of the igneous material into the crust has probably
been effected as a consequence of disturbance of the terrestrial crust. When
the strain of contraction leads to the pushing upward of the terrestrial areas
intervening between wide regions of subsidence, even though the
differential movement may be slight, the isogeotherms undergo
deformation. The intensely hot nucleus is squeezed upward, and if in the
process of compression ruptures take place in the crust, and cavities in it are
consequently opened, the magma will at once be forced into them. Such
ruptures may be expected to take place along lines of weakness. Rocks will
split along their stratification-planes, and the tendency to separation along
these lines may be aided by the readiness of the energetic magma to find its
way into and to enlarge every available opening. Hence we may expect that,
besides vertical fractures, leading to the production of dykes and bosses,
there will often be horizontal thrusts and ruptures, which will give rise to
the formation of sills.
There is still another feature of terrestrial contraction which may help us to
follow the behaviour of the magma within the crust. Plication of the crust is
one of the most characteristic results of the contracting strain. Where a great
series of sedimentary formations has been violently compressed so that its
component strata have been thrown into rapid folds and squeezed into a
vertical position, the portion of the crust thus treated may possibly be on the
whole strengthened against the uprise of molten material through it. But the
folding is often accompanied with dislocation. Not only are the rocks
thrown into endless plications, but portions of them are ruptured and even
driven horizontally over other parts. Such greatly disturbed areas of the
crust are not infrequently found to have been plentifully injected with
igneous rocks in the form of dykes, veins, sills, laccolites and bosses.
The elevation of a mountain-chain is known to be accompanied with a
diminution of density in the crust underneath. Mr. O. Fisher has suggested
that along such lines of terrestrial uplift there may be a double bulge in the
crust, one portion rising to form the upheaved land and the other sinking

down into the hot nucleus. If the lighter descending crust were there melted
it might form a magma ready to be poured out as lava on the opening of any
vent. The lava thus ejected would be of the lighter kinds. It has been
remarked as certainly a curious fact that the lavas which issue from high
mountain ranges are generally much more acid than the heavy basic lavas
which are so characteristic of volcanoes close to the level of the sea.
But even where no actual mountain-chain is formed, there are gentle
undulations of the crust which no doubt also affect the isogeotherms. If any
series of disturbances should give rise to a double system of such
undulations, one crossing the other, there would be limited dome-shaped
elevations at the intersections of these waves, and if at the same time actual
rupture of the crust should take place, the magma might find its way
upward under such domes and give rise to the formation of laccolitic
intrusions. Cessation of the earth-movements might allow the intruded
material slowly to solidify without ever making an opening to the surface
and forming a volcano. Doubtless many sills, laccolites and bosses
represent such early or arrested stages in volcanic history.
Propelled into the crust at a high temperature, and endowed with great
energy from the tension of its absorbed vapours and gases, the magma will
avail itself of every rent which may be opened in the surrounding crust, and
where it has succeeded in reaching the surface, its own explosive violence
may enable it to rupture the crust still further, and open for itself many new
passages. Thus an eruptive laccolite or boss is often fringed with veins,
dykes and sills which proceed from its mass into the rocks around.
The question how far an ascending mass of magma can melt down its walls
is one to which no definite answer can yet be given. Recent observations
show that where the difference in the silica percentage between the magma
and the rock attacked is great, there may be considerable dissolution of
material from this cause. Allusion has already been made to Mr. Harker's
computation that some of the acid granophyres of Skye have melted down
about a fourth of their bulk of the basic gabbros. If such a reaction should
take place between the magma of a boss, sill or laccolite and the rocks
among which it has been intruded, great changes might result in the
composition of the intruded rock. We are not yet, however, in possession of
evidence to indicate that absorption of this kind really takes place on an
extensive scale within the earth's crust. If it did occur to a large extent, we

should expect much greater varieties in the composition of eruptive rocks
than usually occur, and also some observable relation between the
composition of the igneous material and that of the rocks into which it has
been injected. But enough is not yet known of this subject to warrant any
decided opinion regarding it.

CHAPTER VII
Influence of Volcanic Rocks on the Scenery of the Land—Effects of
Denudation.
As considerable popular misapprehension exists respecting the part which
volcanism has played in the evolution of the existing topography of the
earth's surface, and as the British Isles, from their varied geological
structure, offer special facilities for the discussion of this subject, it may not
be out of place to devote a final section of the present Introduction to a
consideration of the real topographical influence of volcanic action.
With modern, and especially with active, volcanoes we need not here
concern ourselves. Their topographical forms are well known, and give rise
to no difficulty. The lofty cones of the Vesuvian type, with their widespread
lavas and ashes, their vast craters and their abundant parasitic volcanoes;
the crowded, but generally diminutive, cones and domes of the puy type, so
well displayed in Auvergne, the Eifel and the Bay of Naples; and the vast
lava deserts of the plateaux, so characteristically developed in Iceland and
Western America, illustrate the various ways in which volcanic energy
directly changes the contours of a terrestrial surface.
But the circumstances are altered when we deal with the topographical
influence of long extinct volcanoes. Other agencies then come into play,
and some caution may be needed in the effort to disentangle the elements of
the complicated problem, and to assign to each contributing cause its own
proper effect.
Reference has already been made to the continuous denudation of volcanic
hills from the time that they are first erupted. But the comparative rapidity
of the waste and the remarkable topographical changes which it involves
can hardly be adequately realized without the inspection of an actual
example. A visit to the back of Monte Somma, already alluded to, will teach
the observer, far more vividly than books can do, how a volcanic cone is
affected by daily meteoric changes. The sides of such a cone may remain
tolerably uniform slopes so long as they are always being renewed by
deposits from fresh eruptions. But when the volcanic activity ceases, and

the declivities undergo no such reparation, they are rapidly channelled by
the descent of rain-water, until the furrows grow by degrees wide and deep
ravines, with only narrow and continually-diminishing crests between them.
If unchecked by any fresh discharge of volcanic material, the degradation
will at last have removed the whole cone.
It is thus obvious that purely volcanic topography, that is, the terrestrial
scenery due directly to the eruption of materials from within the earth, can
never become in a geological sense very old. It can only endure so long as it
is continually renewed by fresh eruptions, or where it is carried down by
subsidence under water and is there buried under a cover of protecting
sediments. When, therefore, we meet with volcanic rocks of ancient date
exposed at the surface, we may be quite certain that their present contours
are not those of the original volcano, but have been brought about by the
processes of denudation.
It is true that, in the general erosion of the surface of the land, volcanic
rocks of ancient date sometimes rise into wonderfully craggy heights,
including, perhaps, cones and deep crater-like hollows, which to popular
imagination betoken contours left by now extinguished volcanic fires.
Examples of such scenery are familiar in various parts of Britain; but the
resemblance to recent volcanic topography is deceptive. There are, indeed,
a few hills wherein the progress of denudation seems not as yet to have
entirely removed the lavas and tuffs that gathered round the original vents.
Some of the tuff-cones of eastern Fife, for example, present cases of this
kind. Again, the great granophyre domes and cones of the Tertiary volcanic
series of the Inner Hebrides, though they have undoubtedly been
extensively denuded, may possibly retain contours that do not greatly differ
from those which these protruded bosses originally assumed under the mass
of rock which has been removed from them. Nevertheless, putting such
doubtful exceptions aside, we may confidently affirm that hills composed of
ancient volcanic material give no clue to the forms of the original
volcanoes.
It can hardly be too often repeated that the fundamental law in the universal
decay and sculpture of the land is that the waste is proportioned to the
resistance offered to it: the softer rocks are worn down with comparative
rapidity, while the harder varieties are left projecting above them. As a
general rule, volcanic rocks are more durable than those among which they

are interstratified, and hence project above them, but this is not always the
case. No universal rule can, indeed, be laid down with regard to the relative
durability of any rocks. While, therefore, topographic contours afford a
valuable indication of the nature and disposition of the rocks below the
surface, they cannot be relied upon as in all circumstances an infallible
guide in this respect. No better proof can be offered of the caution that is
needed in tracing such contours back to their origin than is furnished by the
old volcanic rocks of Britain. These eruptive masses, consisting usually of
durable materials and ranging through a vast cycle of geological time,
usually rise into prominent features and thus support the general law. But
they include also many easily eroded members, which, instead of forming
eminences, are worn into hollows. They include, in short, every type of
scenery, from featureless plains and rolling lowlands to craggy and spiry
mountains.
The first point, then, which is established in an investigation of the
topographical influence of old volcanic rocks is that their prevailing
prominence arises from relative durability amidst universal degradation.
When we proceed further to inquire why they vary so much from each other
in different places, and how their complicated details of feature have been
elaborated, we soon learn that such local peculiarities have arisen mainly
from variations in the internal structure and grouping of the rocks
themselves.
Here again the general law of sculpture comes into play. The local features
have depended upon the comparative resistance offered to the sculpturing
agents by the different portions of a volcanic series. Each distinct variety of
rock possesses its own characteristic internal structure. The lines along
which atmospheric disintegration will most effectually carry on its carving
work are thus already traced in the very substance and architecture of the
rock itself. Each rock consequently yields in its own way to the processes of
disintegration, and thus contributes its own distinctive share to
topographical feature.
Among the massive rocks abundant examples of such special types of
weathering may be cited, from the acid and basic series, and from
superficial lavas as well as from intrusive bosses and sills. Acid bosses,
such as those of granite, granophyre and quartz-porphyry, tend to weather
into blocks and finally into sand, and as this tendency is somewhat

uniformly distributed through the rocks, they are apt to assume rounded,
dome-shaped or conical forms which, at a distance, may seem to have
smooth declivities, but on examination are generally found to be covered
with a slowly-descending sheet of disintegrated blocks and debris (Fig.
346). When less prone to decay, and especially where traversed by a
strongly-defined system of vertical joints, they may shoot up into tower-like
heights, with prominent spires and obelisks. Basic bosses, when their
materials decay somewhat rapidly, give rise to analogous topographical
forms, though the more fertile soils which they produce generally lead to
their being clothed with vegetation. Where they consist of an obdurate rock,
much jointed and fissured, like the gabbro of the Inner Hebrides, they form
exceedingly rugged mountains, terminating upward in serrated crests and
groups of aiguilles (Figs. 331, 333).
Acid lavas that have been superficially erupted weather into irregularly
craggy hills, like the flanks of Snowdon. Those of intermediate
composition, where they have accumulated in thick masses, are apt to
weather into conical forms, as may be seen among the Cheviot, Pentland
and Garleton Hills (Figs. 109, 110, 133); but where they have been poured
out in successive thin sheets they have built up undulating plateaux with
terraced sides, as among the Ayrshire and Campsie Fells and the hills of
Lorne (Figs. 99, 107). Basic lavas have issued in comparatively thin sheets,
frequently columnar or slaggy, forming flat-topped hills and terraced
escarpments, such as are typically developed among the Tertiary basalt-
plateaux of the Inner Hebrides and the Faroe Islands (Figs. 11, 265, 283,
284, 286).
One of the most frequent causes of local peculiarities of topography among
old volcanic rocks is the intercalation of very distinct varieties of material
in the same volcanic series. Where, for instance, lavas and tuffs alternate,
great inequalities of surface may be produced. The tuffs, being generally
more friable, decay faster and give rise to hollows, while the lavas, being
more durable, project in bold ridges or rise into mural escarpments (Fig.
265). Again, where dykes weather more readily than the rocks which they
traverse, they originate deep narrow clefts, while where they weather more
slowly than the rocks around them, they project as dark ribs. Thus in Skye
some dykes which rise through the obdurate gabbro are marked by chasms
which reach up even to the highest crests of the mountains (Fig. 333), while

of those which run in the pale crumbling granophyre, some stand up as
black walls that can be followed with the eye across the ridges even from a
long distance.
Many further illustrations of these principles might be cited here from the
old volcanic districts of Britain. But they will present themselves
successively in later chapters. For my present purpose it is enough to show
that the scenery of these districts is not directly due to volcanic action, but
is the immediate result of denudation acting upon volcanic rocks, modified
and directed by their geological structure.
It may, however, be useful, in concluding the discussion of this subject, to
cite some typical volcanic regions in the British Isles as illustrations of the
relations between geology and topography, which, besides impressing the
main lesson here enforced, may serve also to show some of the striking
contrasts which geology reveals between the present and former conditions
of the surface of the globe. Among these contrasts none are more singular
than those offered by tracts where volcanic action has once been rife, and
where the picture of ancient geography presented in the rocks differs so
widely from the scenery of the same places to-day as to appeal vividly to
the imagination.
The first district to which I may refer where ancient volcanic rocks are well
developed is that of Devonshire. The story of the Devonian volcanoes will
be told in some detail in later chapters, when it will be shown that the
eruptions were again and again renewed during a long course of ages. Yet,
abundant as the intercalated lavas and tuffs are, they can hardly be said to
have had any marked effect on the scenery, though here and there a harder
or larger mass of diabase rises into a prominent knoll or isolated hill. When
the amount of volcanic material in this region is considered, we may feel
some surprise at the trifling influence which it has exerted in the general
denudation of the surface.
To one who wanders over the rich champaign of southern Devonshire, and
surveys from some higher prominence the undulating tree-crowned ridges
that slope down into orchard-filled hollows, and the green uplands that
sweep in successive waves of verdure to the distant blue tors of Dartmoor,
the scene appears as a type of all that is most peaceful, varied and fertile in
English landscape. In the trim luxuriance that meets the eye on every side,

the hand of man is apparent, though from many a point of vantage no sound
may be heard for a time to show that he himself is anywhere near us. Yet
ever and anon from the deep lanes, hidden out of sight under their canopy
of foliage, there will come the creak of the groaning waggon and the crack
of the waggoner's whip, as evidence that there are roads and human traffic
through this bosky silent country.
Amid so much quiet beauty, where every feature seems to be eloquent of
long generations of undisturbed repose, it must surely stir the imagination
to be told that underneath these orchards, meadows and woodlands lie the
mouldering remnants of once active and long-lived volcanoes. Yet we have
only to descend into one of the deep lanes to find the crumbling lavas and
ashes of the old eruptions. The landscape has, in truth, been carved out of
these volcanic rocks, and their decomposition has furnished the rich loam
that nourishes so luxuriant a vegetation.
Not less impressive is the contrast presented between the present and
former condition of the broad pastoral uplands of the south of Scotland.
Nowhere in the British Islands can the feeling of mere loneliness be more
perfectly experienced than among these elevated tracts of bare moorland.
They have nothing of the grandeur of outline peculiar to mountain tracts.
Sometimes, for miles around one of their conspicuous summits, we may see
no projecting knob or pinnacle. The rocks have been gently rounded off into
broad featureless hills, which sink into winding valleys, each with its thread
of streamlet and its farms along the bottom, and its scattered remnants of
birch-wood or alder-copse along its slopes and dingles. Across miles of
heathy pasture and moorland, on the summits of this great tableland, we
may perchance see no sign of man or his handiwork, though the bleating of
the sheep and the far-off barking of the collie tell that we are here within the
quiet domain of the south-country shepherd.
In this pastoral territory, also, though they hardly affect the scenery,
volcanic rocks come to the surface where the foldings of the earth's crust
have brought up the oldest formations. Their appearance extends over so
wide an area as to show that a large part of these uplands lies on a deeply-
buried volcanic floor. A whole series of submarine volcanoes, extending
over an area of many hundreds of square miles, and still in great part
overlain with the accumulated sands and silts of the sea-bottom, now
hardened into stone, underlies these quiet hills and lonely valleys.

A contrast of another type meets us in the broad midland valley of Scotland.
Around the city of Edinburgh, for instance, the landscape is diversified by
many hills and crags which show where harder rocks project from amidst
the sediments of the Carboniferous system. On some of these crags the forts
of the early races, the towers of Celt and Saxon, and the feudal castles of
the middle ages were successively planted, and round their base clustered
for protection the cots of the peasants and the earliest homesteads of the
future city. Beneath these crags many of the most notable events in the
stormy annals of the country were transacted. Under their shadow, and not
without inspiration from their local form and colour, literature, art and
science have arisen and flourished. Nowhere, in short, within the compass
of the British Isles has the political and intellectual progress of the people
been more plainly affected by the environment than in this central district of
Scotland.
When now we inquire into the origin and history of the topography which
has so influenced the population around it, we find that its prominences are
relics of ancient volcanoes. The feudal towers are based on sills and dykes
and necks. The fields and gardens, monuments and roadways, overlie sheets
of lava or beds of volcanic ashes. Not only is every conspicuous eminence
immediately around of volcanic origin, but even the ranges of blue hills that
close in the distant view to south and north and east and west are mainly
built up of lavas and tuffs. The eruptions of which these heights are
memorials belong to a vast range of geological ages, the latest of them
having passed away long before the advent of man. But they have left their
traces deeply engraven in the rocky framework of the landscape. While
human history, stormy or peaceful, has been slowly evolving itself during
the progress of the centuries in these fertile lowlands, the crags and heights
have remained as memorials of an earlier history when Central Scotland
continued for many ages to be the theatre of vigorous volcanic activity.
As a final illustration of the influence of volcanic rocks in scenery, and of
the contrast between their origin and their present condition, I may cite the
more prominent groups of hills in the Inner Hebrides. In the singularly
varied landscapes of that region three distinct types of topography attract
the eye of the traveller. These are best combined and most fully developed
in the island of Skye. Throughout the northern half of that picturesque
island, the ground rises into a rolling tableland, deeply penetrated by arms

of the sea, into which it slopes in green declivities, while along its outer
borders it plunges in ranges of precipice into the Atlantic. Everywhere, alike
on the cliffs and the inland slopes, long parallel lines of rock-terrace meet
the eye. These mount one above another from the shores up to the flat tops
of the highest hills, presenting level or gently-inclined bars of dark crag that
rise above slopes of debris, green sward and bracken. It is these parallel,
sharply-defined bars of rock, with their intervening strips of verdure, that
give its distinctive character to the scenery of northern Skye. On hillside
after hillside and in valley after valley, they reappear with the same almost
artificial monotony. And far beyond the limits of Skye they are repeated in
one island after another, all down the chain of the Inner Hebrides.
In striking contrast to this scenery, and abruptly bounding it on the south,
rise the Red Hills of Skye—a singular group of connected cones. Alike in
form and in colour, these hills stand apart from everything around them.
The verdure of the northern terraced tableland here entirely disappears. The
slopes are sheets of angular debris,—huge blocks of naked stone and trails
of sand, amidst which hardly any vegetation finds a footing. The decay of
the rock gives it a pale yellowish-grey hue, which after rain deepens into
russet, so that in favourable lights these strange cones gleam with a warm
glow as if they, in some special way, could catch and reflect the radiance of
the sky.
Immediately to the west of these pale smooth-sloped cones, the dark mass
of the Cuillin Hills completes the interruption of the northern tableland. In
almost every topographical feature these hills present a contrast to the other
two kinds of scenery. Their forms are more rugged than those of any other
hill-group in Britain (Fig. 331). Every declivity among them is an irregular
pile of crags, every crest is notched like a saw, every peak is sharpened into
a pinnacle. Instead of being buried under vast sheets of their own debris,
these hills show everywhere their naked rock, which seems to brave the
elements as few other rocks can do. Unlike the pale Red Hills, they are
dark, almost black in tone, though when canopied with cloud they assume a
hue of deepest violet.
Each of these three distinct types of topography owes its existence to the
way in which a special kind of volcanic rock yields to the influences of
denudation. The terraced tableland of the north is built up of hundreds of
sheets of basaltic lava, each of the long level ledges of brown rock marking

the outcrop of one or more of these once molten streams. The black rugged
mass of the Cuillin Hills consists of a vast protruded body of eruptive
material, which, in the form of endless sills and bosses of gabbro and
dolerite, has invaded the basalt-plateau, and has now been revealed by the
gradual removal of the portion of that plateau which it upraised. The pale
cones and domes of the Red Hills mark the place of one of the last
protrusions in the volcanic history of Britain—that of large masses of an
acid magma, which broke through the basalt-plateau and also disrupted the
earlier gabbro.
In no part of North-Western Europe has volcanic activity left more varied
and abundant records of its operations than in these three contiguous tracts
of Skye. It is interesting therefore to note the striking contrast between the
former and the present landscapes of the region. The lavas of the basaltic
tableland crumble into a rich loam, that in the mild moist climate of the
Hebrides supports a greener verdure than any of the other rocks around will
yield. The uplands have accordingly become pasture-grounds for herds of
sheep and cattle. The strips of lowland along the valleys and in the recesses
of the coast-line furnish the chief tracts of arable land in the island, and are
thus the main centres of the crofter population. The bays and creeks of the
much-indented shores form natural harbours, which in former days attracted
the Norse sea-rovers, and supplied them with sites for their settlements.
Norse names still linger on headland and inlet, but the spirit of adventure
has passed away, and a few poor fishing-boats, here and there drawn up on
the beach, are usually the only token that the islanders make any attempt to
gather the harvest of the sea.
The mountain groups which so abruptly bound the basalt-plateau on the
south, and present in their topographical features such distinctive scenery,
comprise a region too lofty, too rugged and too barren for human
occupation. The black Cuillins and the pale Red Hills are solitudes left to
the few wild creatures that have not yet been exterminated. The corries are
the home of the red deer. The gabbro cliffs are haunts of the eagle and the
raven. Where patches of soil have gathered in the crannies of the gabbro,
alpine plants find their home. In the chasms left by the decay of the dykes
between the vertical walls of their fissures, the winter snows linger into
summer, and conceal with their thick drifts the mouldering surface of the
once molten rock beneath them. On every side and at every turn a mute

appeal is made to the imagination by the strange contrasts between the quiet
restfulness of to-day, when the sculpture-tools of nature are each busily
carving the features of the landscape, and the tumult of the time when the
rocks, now so silent, were erupted.

The general discussion of the subject of Volcanism in this Introduction will,
I hope, have prepared the reader who has no special geological training for
entering upon the more detailed descriptions in the rest of this treatise. As
already stated, the chronological order of arrangement will be followed.
Beginning with the records of the earliest ages, we shall follow the story of
volcanic action down to the end of the latest eruptions.
Each great geological system will be taken as a whole, representing a long
period of time, and its volcanic evolution will be traced from the beginning
of the period to the close. Some variety of treatment is necessarily entailed
by the wide range in the nature and amount of the evidence for the volcanic
history of different ages. But where practicable, an outline will first be
given of what can be gathered respecting the physical geography of each
geological period in Britain. In the description which will then follow of the
volcanic phenomena, an account of the general characters of the erupted
rocks will precede the more detailed narrative of the history of the volcanic
eruptions in the several regions where they took place. References to the
published literature of each formation will be given in the first part of each
section, or will be introduced in subsequent pages, as may be found most
convenient.

BOOK II
VOLCANIC ACTION IN PRE-CAMBRIAN TIME

CHAPTER VIII
PRE-CAMBRIAN VOLCANOES

The Beginnings of Geological History—Difficulties in fixing on a
generally-applicable Terminology—i. The Lewisian
(Archæan) Gneiss; ii. The Dalradian or Younger Schists of
Scotland; iii. The Gneisses and Schists of Anglesey; iv. The
Uriconian Volcanoes; v. The Malvern Volcano; vi. The
Charnwood Forest Volcano.
The early geological history of this globe, like the early history of mankind,
must be drawn from records at once scanty and hardly decipherable.
Exposed to the long series of revolutions which the surface of the planet has
undergone, these records, never perhaps complete at the first, have been in
large measure obliterated. Even where they still exist, their meaning is often
so doubtful that, in trying to interpret it, we find little solid footing, and feel
ourselves to be groping, as it were, in the dimness of mythological legend,
rather than working in the light of trustworthy and intelligible chronicles.
These primeval records have been more particularly the objects of sedulous
study during the last twenty years all over Europe and in North America. A
certain amount of progress in their decipherment has been made. But the
problems they still present for solution are numerous and obscure.
Fortunately, with many of these problems the subject of the present treatise
is not immediately connected. We need only concern ourselves with those
which are related to the history of primeval volcanic activity.
To the earliest and least definite division of the geological annals various
names have been applied. Some writers, believing that this period preceded
the first appearance of plants or animals upon the globe, have named it
Azoic—the lifeless age of geological history. But the absence of any
hitherto detected trace of organic existence among the oldest known rocks
cannot be held to prove that these rocks were formed before the advent of
living things on the surface of the earth. The chance discovery of a single
fossil, which might at any moment be made, would show the name "Azoic"
to be a misnomer. Other geologists, believing that, as a matter of fact,
organic structures of low types do actually occur in them, have called these

old rocks "Eozoic," to denote that they were deposited during the dawn of
life upon our planet. But the supposed organisms have not been everywhere
accepted as evidence of former life. By many able observers they are
regarded as mere mineral aggregates. Another term, "Archæan," has been
proposed for the primeval ages of geological history, which are recorded in
rocks that carry us as far as may ever be possible towards the beginnings of
that history.
In choosing some general term to include the oldest known parts of the
earth's crust, geologists are apt unconsciously to assume that the rocks thus
classed together represent a definite section of geological time, comparable,
for instance, to that denoted by one of the Palæozoic systems. Yet it is
obvious that, under one of these general terms of convenient classification,
a most multifarious series of rocks may be included, representing not one
but possibly many, and widely separated, periods of geological history.
In many countries the oldest sedimentary accumulations, whether
fossiliferous or not, are underlain by a series of crystalline rocks, which
consist in great part of coarse massive gneisses and other schists. All over
the world these rocks present a singular sameness of structure and
composition. What might be found below them no man can say. They are in
each country the oldest rocks of which anything is yet known, and
whatsoever may be our theory of their origin, we must, at least for the
present, start from them as the fundamental platform of the terrestrial crust.
But though crystalline rocks of this persistent character are widely
distributed, both in the Old World and in the New, they in themselves
furnish no means of determining their precise geological age. No method
has yet been devised whereby the oldest gneiss of one country can be
shown to be the true stratigraphical equivalent of the oldest gneiss of
another. Palæontology is here of no avail, and Petrology has not yet
provided us with such a genetic scheme as will enable us to make use of
minerals and rock-structures, as we do of fossils, in the determination of
geological horizons. All that can be positively affirmed regarding the
stratigraphical relations of the rocks in question is that they are vastly more
ancient than the oldest sedimentary and fossiliferous formations in each
country where they are found. The "Lewisian" gneiss of the north-west of
Scotland, the "Urgneiss" of Central Europe, and the "Laurentian" gneiss of
Canada occupy similar stratigraphical positions, and present a close

resemblance in lithological characters. We may conveniently class them
under one common name to denote this general relationship. But we have,
as yet, no means of determining how far they belong to one continuous
period of geological history. They may really be of vastly different degrees
of antiquity.
From the very nature of the case, any name by which we may choose to
designate such ancient rocks cannot possess the precise stratigraphical value
of the terms applied to the fossiliferous formations. Yet the convenience of
possessing such a general descriptive epithet is obvious.
Until much more knowledge of the subject has been gained, any
terminology which may be proposed must be regarded as more or less
provisional. The comprehensive term "pre-Cambrian" may be usefully
adopted as a general designation for all rocks older than the base of the
Cambrian system, irrespective of their nature and origin. Already it is well
known that under this term a vast series of rocks, igneous and sedimentary,
is included. In some regions several successive formations, or systems of
formations, may be recognized in this series. But until some method has
been devised for determining the stratigraphical relations of these
formations in different regions, it would seem safest not to attempt to
introduce general names for universal adoption, but to let the sequence of
rocks in each distinct geological province be expressed by a local
terminology. This caution is more especially desirable in the case of
sedimentary deposits. We may surmise as to the equivalence of the rocks
called Huronian, Torridonian and Longmyndian, but whilst so much is mere
conjecture, it is certainly injudicious to transfer the local names of one
province to the rocks of another.
The only relaxation of this general precaution which I think may at present
be made is the adoption of a common name for the oldest type of gneisses.
The term "Archæan" has been applied to these rocks, and if it is used
simply to express a common petrographical type, occupying the lowest
horizon in the stratigraphical series of a country, it has obvious advantages.
But I would still retain the local names as subordinate terms to mark the
local characteristics of the Archæan rocks of each province. Thus the
"Laurentian" rocks of Canada and the "Lewisian" rocks of Scotland are
widely-separated representatives of the peculiar stratigraphical series which
is known as Archæan.

The pre-Cambrian rocks of Britain include several distinct systems or
groups. How far those of even one part of this comparatively limited region
are the proper equivalents of those of another and distant part is a problem
still unsolved. Hence each distinct area, with its own type of rocks, will
here be treated by itself. The following rock-types will be described: i. The
Lewisian (Archæan) Gneiss; ii. The Younger (Dalradian) Schists of
Scotland; iii. The Gneisses and Schists of Anglesey; iv. The Uriconian
Group; v. The Malvern Group; vi. The Charnwood Forest Group (see Map
I.).

i. the lewisian (archaæn) gneiss

The British Isles are singularly fortunate in possessing an admirable
development of pre-Cambrian rocks. These ancient masses rise up in
various parts of the islands, but the region where they are most extensively
displayed, and where their stratigraphical position and sequence are most
clearly shown, lies in the north-west of Scotland.[50] In that territory they
form the whole chain of the Outer Hebrides, and likewise extend as an
irregular selvage along the western margin of the counties of Sutherland
and Ross. The lowest known platform of the fossiliferous formations has
there been discovered and has been traced for a distance of more than 100
miles. From this definite horizon, the high antiquity of all that lies below it
is impressively demonstrated. The accompanying diagram (Fig. 35) will
explain the general relations of the various geological formations of the
region.
[50] These rocks have been the subject of much discussion, but geologists are now
agreed as to their succession and structure. A full summary of the literature of the
controversy regarding them will be found in the Quarterly Journal of the Geological
Society, vol. xliv. (1888), p. 378.

In certain dark shales (b) which occupy a well-defined and readily-traceable
position among the rocks of Sutherland and Ross, numerous specimens of
the trilobite genus Olenellus, together with other fossils, have been found.
By common consent among geologists, the zone of rock in which this genus
appears is taken as the lowest stage of the Cambrian system. In Britain it
marks the oldest known group of fossiliferous strata—the platform on
which the whole of the Palæozoic systems rest.

Fig. 35.—Diagram illustrating the stratigraphical relations of the pre-Cambrian
and Cambrian rocks of the North-west Highlands of Scotland.
c, Durness Limestones, with Upper Cambrian and perhaps Lower Silurian
fossils, 1500 feet, top nowhere seen. b, Serpulite grit and "fucoid" shales, 70 to
80 feet, containing the Olenellus-zone. a, Quartzite, with abundant annelid
tubes, about 600 feet. II. Red Sandstones and Conglomerates, sometimes 8000
feet or more (Torridonian). I. Gneiss with dykes, etc. (Lewisian).

From the definite geological epoch indicated by this platform, we can go
backward into pre-Cambrian time, and realize in some measure how
prodigious must be the antiquity of the successive groups of rock which
emerge from beneath the base of the Palæozoic systems. Nowhere is this
antiquity more impressively proclaimed than in the north-west of Scotland.
From below the Olenellus-zone with its underlying sheets of quartzite (a), a
thick group of dull red sandstones and conglomerates (II.) rises into a series
of detached conical or pyramidal mountains, which form one of the most
characteristic features in the scenery of that region. As this detrital
formation is well developed around Loch Torridon, it has been termed
Torridonian. It attains a thickness of at least 8000 or 10,000 feet, and is
traceable all the way from the extreme northern headlands of Sutherland to
the southern cliffs of the island of Rum.
In judging of the chronological significance of the geological structure of
the north-west of Scotland, we are first impressed by the stratigraphical
break between the base of the Cambrian system and the Torridonian
deposits below. This break is so complete that here and there the thick
intervening mass of sandstones and conglomerates has been nearly or
wholly removed by denudation before the lowest Cambrian strata were laid
down. Such a discordance marks the passage of a protracted interval of
time.
Again, when the composition of the Torridonian rocks is considered, further
striking evidence is obtained of the lapse of long periods. The sandstones,
conglomerates and shales of this pre-Cambrian system present no evidence

of cataclysmal action. On the contrary, they bear testimony that they were
accumulated much in the same way and at the same rate as the subsequent
Palæozoic systems. In that primeval period, as now, sand and silt were
spread out under lakes and seas, were ripple-marked by the agitation of the
water, and were gradually buried under other layers of similar sediment.
The accumulation of 10,000 feet of such gradually-assorted detritus must
have demanded a long series of ages. Here, then, in the internal structure of
the Torridonian rocks, there is proof that in passing across them, from their
summit to their base, we make another vast stride backward into the early
past of geological history.
But when attention is directed to the relations of the Torridonian strata to
the rocks beneath them, a still more striking proof of an enormously
protracted period of time is obtained. Between the two series of formations
lies one of the most marked stratigraphical breaks in the geological
structure of the British Isles. There is absolutely nothing in common
between them, save that the conglomerates and sandstones have been
largely made out of the waste of the underlying gneiss. The denudation of
the crystalline rocks before the deposition of any of the Torridonian
sediments must have been prolonged and gigantic. The more, indeed, we
study the gneiss, the more do we feel impressed by the evidence for the
lapse of a vast interval of time, here unrecorded in rock, between the last
terrestrial movements indicated by the gneiss and the earliest of the
Torridonian sediments.
In this manner, reasoning backward from the horizon of the Olenellus-zone,
we are enabled to form some conception of the vastness of the antiquity of
the fundamental rocks of the North-west Highlands. The nature and origin
of these rocks acquire a special interest from a consideration of their age.
They contain the chronicles of the very beginnings of geological history, in
so far as this history is contained in the crust of the earth. No part of the
geological record is so obscure as this earliest chapter, but we need not here
enter further into its difficulties than may be necessary for the purpose of
understanding what light it can be made to throw on the earliest
manifestations of volcanic action.
Under the term Lewisian Gneiss (I. in Fig. 35) a series of rocks is
comprised which differ from each other in composition, structure and age,
though most of them possess such crystalline and generally foliated

characters as may be conveniently included under the designation of gneiss.
The complexity of these ancient crystalline masses was not recognized at
the time when Murchison called them the "Fundamental" or "Lewisian"
gneiss. It is only since the Geological Survey began to study and map them
in full detail that their true nature and history have begun to be understood.
[51]

[51] See the Report of this Survey work by Messrs. Peach, Horne, Gunn, Clough,
Cadell and Hinxman, Quart. Journ. Geol. Soc. vol. xliv. (1888), pp. 378-441; and
Annual Reports of Director-General of the Geological Survey in the Report of The
Science and Art Department for 1894, p. 279, and 1895, p. 17 of reprint. The general
area of the gneiss is shown in Map I.

The researches of the Survey have shown the so-called Lewisian gneiss to
comprise the following five groups of rock: 1. A group of various more or
less banded and foliated rocks which form together the oldest and chief part
of the gneiss (Fundamental complex); 2. Highly basic dykes cutting the first
group; 3. Dykes and sills of dolerite, epidiorite and hornblende-schist; 4. A
few dykes of peculiar composition; 5. Gneissose granite and pegmatite.
The first of these groups, forming the main body of the gneiss, has been
critically studied on the mainland from Cape Wrath to Skye. But its
development in the Outer Hebrides has not yet been worked out, although
the name "Lewisian" was actually taken from that chain of islands. So far as
at present known, however, the gneiss of the Hebrides repeats the essential
characters of that of the mainland.
Mr. Teall, as the result of a careful investigation in the field and with the
microscope, has ascertained that on the mainland between Skye and Cape
Wrath the rocks of the "fundamental complex" are essentially composed of
olivine, hypersthene, augite (including diallage), hornblende, biotite,
plagioclase, orthoclase, microcline and quartz. He has further observed that
these minerals are associated together in the same manner as in peridotites,
gabbros, diorites and granites. Treating the rocks in accordance with their
composition and partly with their structure, but excluding theoretical
considerations, he has arranged them in the following five subdivisions:—
1. Rocks composed of ferro-magnesian minerals, without felspar
or quartz—Pyroxenites, Hornblendites.
2. Rocks in which pyroxenes are the dominating ferro-magnesian
constituents, felspar always being present, sometimes quartz:

A, Without quartz, Hypersthene-augite-rocks (pyroxene
granulites; rocks of the Baltimore-gabbro type) and augite-
rocks (gabbros); B, With quartz, Augite-gneiss.
3. Rocks in which hornblende is the prevalent ferro-magnesian
constituent: A, Without quartz, or containing it only in small
quantity; rocks basic in composition: (a) massive or only
slightly foliated (Amphibolites, as epidote-amphibolite,
zoisite-amphibolite, garnet-amphibolite); (b) foliated
(Hornblende-schist). B, With quartz; rocks intermediate or
acid in composition: (a) with compact hornblende and a
granular structure (Hornblende-gneiss proper); (b) with
hornblende occurring in fibrous or other aggregates; (c) with
compact hornblende and a more or less granulitic structure
(Granulitic hornblende-gneiss).
4. Rocks in which biotite is the predominant ferro-magnesian
constituent; felspar and quartz both present: (a) Biotite
occurring as independent plates or in aggregates of two or
three large individuals (Biotite-gneiss); (b) Biotite occurring
in aggregates of numerous small individuals (rare type); (c)
Biotite occurring as independent plates in a granulitic
structure.
5. Rocks in which muscovite and biotite are present, together with
felspar and quartz—Muscovite-biotite-gneiss. These, though
not forming a well-defined natural group, are placed together
for purposes of description. They are all foliated, some
having the aspect of mica-schists, others being typical
augen-gneisses, or light grey gneisses with abundant
oligoclase and inclusions of microlitic epidote.
The rocks of each of these types are usually restricted to relatively small
areas, and they succeed each other with much irregularity all the way from
Skye to Cape Wrath. Their chemical and mineralogical composition proves
them to have decided affinities with the plutonic igneous masses of the
earth's crust.
The only exceptions to this prevalent igneous type occur in the districts of
Gairloch and Loch Carron, where the gneiss appears to be associated with a

group of mica-schists, graphitic-schists, quartzites and siliceous granulites,
limestones, dolomites, chlorite-schists and other schists. That these are
altered sedimentary formations can hardly be doubted. What their precise
relations to the fundamental complex of the gneiss may be has not yet been
satisfactorily determined. They are certainly far older than the Torridon
sandstone which covers them unconformably. Possibly they may represent a
sedimentary formation still more ancient than the gneiss.
Save these obscure relics of a pre-Torridonian system of strata, the gneiss
never presents any structure which suggests the alteration of clastic
constituents. Everywhere its mineral composition points to a connection
with the subterranean intrusions of different igneous magmas, while the
manner in which its different rock-groups are associated together, and the
internal structure of some of them, still further link it with phenomena
which will be described in succeeding chapters as parts of the records of
volcanic action.
An interesting feature of the fundamental complex, as bearing on the origin
of the gneiss, is to be found in the occurrence of bosses and bands which
are either non-foliated or foliated only in a slight degree. These
comparatively structureless portions present much of the character of bosses
or sills of true eruptive rocks. They occur in various parts of Sutherland and
Ross. Their external margins are not well defined, and they pass insensibly
into the ordinary gneiss, the dark basic massive rocks shading off into
coarse basic gneisses, and the pegmatites of quartz and felspar which
traverse them merging into bands of grey quartzose gneiss.
So far, therefore, as present knowledge goes, the main body or fundamental
complex of the Lewisian gneiss in the North-west Highlands of Scotland
consists of what may have been originally a mass of various eruptive rocks.
It has subsequently undergone a succession of deformations from enormous
stresses within the terrestrial crust, which have been investigated with great
care by the Geological Survey. But it presents structures which, in spite of
the abundant proofs of great mechanical deformation, are yet, I venture to
think, original, or at least belong to the time of igneous protrusion before
deformation took place. The alternation of rocks of different petrographical
constitution suggests a succession of extravasations of eruptive materials,
though it may not be always possible now to determine the order in which
these followed each other. In the feebly foliated or massive bands and

bosses there is a parallel arrangement of their constituent minerals or of fine
and coarse crystalline layers which recalls sometimes very strikingly the
flow-structure of rhyolites and other lavas. This resemblance was strongly
insisted on by Poulett Scrope, who believed that the laminar structure of
such rocks as gneiss and mica-schist was best explained by the supposition
of the flow of a granitic magma under great pressure within the earth's
crust.[52]
[52] Volcanoes, pp. 140, 283, 299.

The conviction that these parallel structures do, in some cases, really
represent traces of movements in the original unconsolidated igneous
masses, not yet wholly effaced by later mechanical stresses, has been
greatly strengthened in my mind by a recent study of the structures of
various eruptive bosses, especially those of gabbro in the Tertiary volcanic
series of the Inner Hebrides. The banded structure, the separation of the
constituent minerals into distinct layers or zones, the alternation of
markedly basic with more acid layers, and the puckering and plication of
those bands, can be seen as perfectly among the Tertiary gabbro bosses of
Skye as in the Lewisian gneiss (see Figs. 336 and 337). It cannot be
contended that such structures in the gabbro are due to any subsequent
terrestrial disturbance and consequent deformation. They must be accepted
as part of the original structure of the molten magma.[53] It seems to me,
therefore, highly probable that the parallel banding in the uncrushed cores
of the Lewisian gneiss reveals to us some of the movements of the original
magma at the time of its extrusion and before it underwent those great
mechanical stresses which have so largely contributed to the production of
many of its most characteristic structures.
[53] See A. Geikie and J. J. H. Teall, Quart. Journ. Geol. Soc. vol. 1. (1894), p. 645.

While the material of the oldest gneiss presents many affinities to plutonic
rocks of much younger date, a wide region of mere speculation opens out
when we try to picture the conditions under which this material was
accumulated. Some geologists have boldly advanced the doctrine that the
Archæan gneisses represent the earliest crust that consolidated upon the
surface of the globe. But these rocks offer no points of resemblance to the
ordinary aspect of superficial volcanic ejections. On the contrary, the
coarsely-crystalline condition even of those portions of the gneiss which
seem most nearly to represent original structure, the absence of anything

like scoriæ or fragmental bands of any kind, and the resemblances which
may be traced between parts of the gneiss and intrusive bosses of igneous
rock compel us to seek the nearest analogies to the original gneiss in deep-
seated masses of eruptive material. It is difficult to conceive that any rocks
approaching in character to the gabbros, picrites, granulites and other
coarsely-crystalline portions of the old gneiss could have consolidated at or
near the surface.
When the larger area of gneiss forming the chain of the Outer Hebrides is
studied, we may obtain additional information regarding the probable origin
and the earliest structures of the fundamental complex of the Lewisian
gneiss. In particular, we may look for some unfoliated cores of a more acid
character, and perhaps for evidence which will show that both acid and
basic materials were successively protruded. We may even entertain a faint
hope that some trace may be discovered of superficial or truly volcanic
products connected with the bosses which recall those of later date and
obviously eruptive nature. But up to the present time no indication of any
such superficial accompaniments has been detected. If any portions of the
old gneiss represent the deeper parts of columns of molten rock that flowed
out at the surface as lava, with discharges of fragmentary materials, all this
superincumbent material, at least in the regions which have been studied in
detail, had disappeared entirely before the deposition of the very oldest part
of the Torridonian rocks, unless some trace of it may remain among the
pebbles of the Torridonian conglomerates, to which reference will be
immediately made.
So far, then, as the evidence now available allows a conclusion to be drawn,
the Lewisian gneiss reveals to us a primeval group of eruptive rocks
presenting a strong resemblance to some which in later formations are
connected, as underground continuations, with bedded lavas and tuffs that
were erupted at the surface; and although no proof has yet been obtained of
true volcanic ejections associated with the fundamental complex, the rocks
seem to be most readily understood if we regard them as having
consolidated from igneous fusion at some depth, and we may plausibly
infer that they may have been actually connected with the discharge of
volcanic materials at the surface. The graphite-schists, mica-schists, and
limestones of the Gairloch and Loch Carron may thus be surviving
fragments of the stratified crust into which these deep-seated masses were

intruded, and through which any volcanic eruptions that were connected
with them had to make their way.
The limited areas occupied by the several varieties of rock in the
fundamental complex suggests the successive protrusion of different
magmas, or of different portions from one gradually changing magma. Mr.
Teall has ascertained that whenever in this series of rocks the relative ages
of two petrographical types can be clearly ascertained, the more basic is
older than the more acid.
But besides all the complexity arising from original diversity of area,
structure and composition among the successive intrusions, a further
intricacy has been produced by the subsequent terrestrial disturbances,
which on a gigantic scale affected the north-west of Europe after the
formation of the fundamental complex of the old gneiss, but long before the
Torridonian period. By a series of terrestrial stresses that came as precursors
of those which in later geological times worked such great changes among
the rocks of the Scottish Highlands, the original bosses and sheets of the
gneiss were compressed, plicated, fractured and rolled out, acquiring in this
process a crumpled, foliated structure. Whether or not these disturbances
were accompanied by any manifestations of superficial volcanic action has
not yet been determined. But we know that they were followed by a
succession of dyke-eruptions, to which, for extent and variety, there is no
parallel in the geological structure of Britain, save in the remarkable
assemblage of dykes belonging to the Tertiary volcanic period[54] (Fig. 36).
[54] Quart. Journ. Geol. Soc. vol. xliv. (1888), p. 389 et seq.

Click on image to view larger. Walker & Boutall sc.
Fig. 36.—Map of a portion of the Lewisian gneiss of Ross-shire.
Taken from Sheet 107 of the Geological Survey of Scotland on the
scale of one inch to a mile. The white ground (A) marks the general
body of the Lewisian gneiss. This is traversed by dykes of dolerite
(B), which are cut by later dykes of highly basic material (peridotite,
picrite, etc., P). The gneiss and its system of dykes is overlain
unconformably by the nearly horizontal Torridon Sandstone (t),
which is injected by sheets of oligoclase-porphyry (F).

For the production of these dykes a series of fissures was first opened
through the fundamental complex of the gneiss, having a general trend from
E.S.E. to W.N.W., running in parallel lines for many miles, and so close
together in some places that fifteen or twenty of them occurred within a
horizontal space of one mile. The fissures were probably not all formed at
the same time; at all events, the molten materials that rose in them exhibit
distinct evidence of a succession of upwellings from the igneous magma
below.
Considered simply from the petrographical point of view, the materials that
have filled the fissures have been arranged by Mr. Teall in the following
groups: 1. Ultrabasic dykes, sometimes massive (peridotites), sometimes
foliated (talcose schists containing carbonates and sometimes gedrite); 2.
Basic dykes which where massive take the forms of dolerite and epidiorite,
and where foliated appear as hornblende-schist, the same dyke often
presenting the three conditions of dolerite, epidiorite and hornblende-schist;
3. Dykes of peculiar composition, comprising microcline-mica rocks and

biotite-diorite with macro-poikilitic plagioclase; 4. Granites and gneissose
granites (biotite-granite with microcline); 5. Pegmatites (microcline-quartz
rocks with a variable amount of oligoclase or albite).[55]
[55] Annual Report of Geological Survey for 1895, p. 18 of reprint.

Distinct evidence of a succession of eruptions can be made out among these
rocks. By far the largest proportion of the dykes consists of basic materials.
The oldest and most abundant of them are of plagioclase-augite rocks,
which, where uncrushed, differ in no essential feature of structure or
composition from the dolerites and basalts of more modern periods, though
they have been plentifully changed into epidiorite and hornblende-schist.[56]
They present, too, most of the broad features that characterize the dykes of
later times—the central more coarsely-crystalline portion, the marginal
band of finer grain, passing occasionally into what was probably a basic
glass, and the transverse jointing. They belong to more than one period of
emission, for they cross each other. They vary in width up to nearly 200
feet, and sometimes run with singular persistence completely across the
whole breadth of the strip of gneiss in the west of Sutherland and Ross.
Dozens of dykes have been followed by the Geological Survey for distances
of ten or twelve miles.
[56] See Mr. Teall, Quart. Journ. Geol. Soc. vol. xli. (1885), p. 133.

Later in time, and much less abundant, are certain highly basic dykes—
peridotites with schistose modifications—which cut across the dolerites in a
more nearly east-and-west direction. There are likewise occasional dykes of
peculiar composition, which, as above stated, have been distinguished by
Mr. Teall as microcline-mica rocks and biotite-diorite.
Last of all comes a group of thoroughly acid rocks—varieties of granite and
pegmatite—which form intrusive sheets and dykes. The granites contain
biotite with microcline, and are sometimes gneissose. The pegmatites are
microcline-quartz rocks with a variable amount of oligoclase or albite.
These dykes coincide in direction with the basalts and dolerites, but they are
apt to run together into belts of granite and pegmatite, sometimes 1500 feet
broad.
Up to the present time no evidence has been found of any superficial
outpouring of material in connection with this remarkable series of dykes in
the Lewisian gneiss. That they may have been concomitant with true

volcanic eruptions may be plausibly inferred from the close analogy which,
in spite of their antiquity and the metamorphism they have undergone, they
still present to the system of dykes that forms a part of the great Tertiary
volcanic series of Antrim and the Inner Hebrides. The close-set fissures
running in a W.N.W direction, the abundant uprise into these fissures of
basic igneous rocks, followed by a later and more feeble extravasation of
acid material, are features which in a singular manner anticipate the
volcanic phenomena of Tertiary time.
There can be no question as to the high antiquity of these dykes. They were
already in place before the advent of those extraordinary vertical lines of
shearing which have so greatly affected both the gneiss and the dykes; and
these movements, in turn, had long been accomplished before the Torridon
Sandstone was laid down, for the dykes, with their abundant deformation,
run up to and pass beneath the sandstone which buries them and all the
rocks with which they are associated. Though later than the original
fundamental complex, the dykes have become so integral and essential a
part of the gneiss as it now exists that they must be unhesitatingly grouped
with it.
With so wide an extension of the subterranean relics of volcanic energy, it is
surely not too much to hope that somewhere there may have been
preserved, and may still be discovered, proofs that these eruptive rocks
opened a connection with the surface, and that we may thus recognize
vestiges of the superficial products of actual Archæan volcanoes. Among
the pebbles in the conglomerates of the Torridon Sandstone there occur,
indeed, fragments of felsites which possess great interest from the
perfection with which they retain some of the characteristic features of
younger lavas. Mr. Teall has described their minute structure. They are dark,
purplish, compact rocks, consisting of a spherulitic micropegmatitic,
micropoikilitic or microcrystalline groundmass, in which are imbedded
porphyritic crystals or crystal-groups of felspar, often oligoclase. These
spherulitic rocks occasionally show traces of perlitic structure. They bear a
striking resemblance to some of the Uriconian felsites of Shropshire,
pebbles from which occur in the Longmynd rocks.[57] These fragments
suggest the existence of volcanic materials at the surface when the Torridon
Sandstone was deposited. Possibly they may represent some vanished
Lewisian lavas. But the time between the uprise of the dykes and the

formation of the Torridonian series was vast enough for the advent of many
successive volcanic episodes. The pebbles may therefore be the relics of
eruptions that took place long after the period of the dykes.
[57] Annual Report of Geological Survey for 1895, p. 21 of reprint.

Among the Torridonian strata no undoubted trace of any contemporaneous
volcanic eruptions has been met with.[58] The only relics of volcanic rocks
in this enormous accumulation of sediments are the pebbles just referred to,
which may be referable to a time long anterior to the very oldest parts of the
Torridonian series.
[58] The supposed tuff referred to in Quart. Journ. Geol. Soc. vol. xlviii. (1892), p.
168, is probably not of truly volcanic origin.

That Archæan time witnessed volcanic eruptions on a considerable scale,
and with great variety of petrographical material, has recently been shown
in detail by Mr. Otto Nordenskjöld from a study of the rocks of Småland in
Sweden. He has described a series of acid outbursts, including masses of
rhyolite and dacite, together with agglomerates and tuffs, likewise basic
eruptions, with dioritic rocks, augite-porphyrite and breccia. He refers these
rocks to the same age as most of the Scandinavian gneisses, and remarks
that though they have undergone much mechanical deformation and
metamorphism, they have yet here and there retained some of their
distinctive volcanic structures, such as the spherulitic.[59] When the large
area of Lewisian gneiss forming the chain of the Outer Hebrides is
investigated it may possibly supply examples of a similar series of ancient
volcanic masses.
[59] "Über Archæische Ergussgesteine aus Småland," Sveriges Geol. Undersökn, No.
135 (1894).

ii. the dalradian or younger schists of scotland

We now come to one of the great gaps in the geological record. The
Lewisian gneiss affords us glimpses of probable volcanic activity at the
very beginning of geological history. An enormous lapse of time, apparently
unrepresented in Britain by any geological record, must be marked by the
unconformability between the gneiss and the Torridon Sandstone. Another
prodigious interval is undoubtedly shown by the Torridonian series. Neither
this thick accumulation of sediment nor the Cambrian formations, which to

a depth of some 2000 feet overlie the Torridon Sandstone, have yielded any
evidence of true superficial eruptions, though they are traversed by
numerous dykes, sills and bosses. The age of these intrusive masses cannot
be precisely fixed; a large proportion of them is certainly older than the
great terrestrial displacements and concurrent metamorphism of the North-
West Highlands.
While from the Lewisian gneiss upward to the highest visible Cambrian
platform in Sutherland, no vestige of contemporaneous volcanic rocks is to
be seen, the continuity of the geological record is abruptly broken at the top
of the Durness Limestone. By a series of the most stupendous dislocations,
the rocks of the terrestrial crust have there been displaced to such a degree
that portions have been thrust westward for a horizontal distance of
sometimes as much as ten miles, while they have been so crushed and
sheared as to have often lost entirely their original structures, and to have
passed into the crystalline and foliated condition of schists. Portions of the
floor of Lewisian gneiss, and large masses of the Torridon Sandstone, which
had been buried under the Cambrian sediments, have been torn up and
driven over the Durness Limestone and quartzite.
Though much care has been bestowed by the officers of the Geological
Survey on the investigation of the complicated mass of material which,
pushed over the Cambrian strata, forms the mountainous ground that lies to
the east of a line drawn from Loch Eribol, in the north of Sutherland, to the
south-east of Skye, some uncertainty still exists as to the age and history of
the rocks of that region. For the purposes of this work, therefore, the rest of
the country eastwards to the line of the Great Glen—that remarkable valley
which cuts Scotland in two—may be left out of account.
To the east of the Great Glen the Scottish Highlands display a vast
succession of crystalline schists, the true stratigraphical relations of which
to the Lewisian gneiss have still to be determined, but which, taken as a
whole, no one now seriously doubts must be greatly younger than that
ancient rock. Murchison first suggested that the quartzites and limestones
found in this newer series are the equivalents of those of the North-West.
This identification may yet be shown to be correct, but must be regarded as
still unproved. Traces of fossils (annelid-pipes) have been found in some of
the quartzites, but they afford little or no help in determining the horizons of
the rocks. In Donegal, where similar quartzites, limestones and schists are

well developed, obscure indications of organic remains (corals and
graptolites) have likewise been detected, but they also fail to supply any
satisfactory basis for stratigraphical comparison.
Essentially the schists of the Scottish Highlands east of the Great Glen
consist of altered sedimentary rocks. Besides quartzites and limestones,
there occur thick masses of clay-slate and other slates and schists, with
bands of graphitic schist, greywacke, pebbly grit, quartzite, boulder-beds
and conglomerates. Among rocks that have been so disturbed and foliated it
is necessarily difficult to determine the true order of succession. In the
Central Highlands, however, a certain definite sequence has been found to
continue as far as the ground has yet been mapped. Were the rocks always
severely contorted, broken and placed at high angles, this sequence might
be deceptive, and leave still uncertain the original order of deposition of the
whole series. But over many square miles the angles of inclination are low,
and the successive bands may be traced from hill to hill, across strath and
glen, forming escarpments along the slopes and outliers on the summits,
precisely as gently-undulating beds of sandstone and limestone may be seen
to do in the dales of Yorkshire. It is difficult to resist the belief, though it
may, perhaps, be premature to conclude, that this obvious and persistent
order of succession really marks the original sequence of deposition. In
Donegal also a definite arrangement of the rock-groups has been
ascertained which, when followed across the country, gives the key to its
geological structure.[60]
[60] Geol. Survey Memoirs: Geology of N.W. Donegal, 1891.

In the order of succession which has been recognized during the progress of
the Geological Survey through the Central and Southern Highlands, it is
hard in many places to determine whether the sequence that can be
recognized is in an upward or downward direction. Two bands of limestone,
which appear to retain their relative positions across Scotland for a distance
of some 230 miles, may afford a solution of this difficulty, and if, as is
probable, they are to be identified with the similar limestones of Donegal,
Mayo and Galway, their assistance will thus be available across a tract of
more than 400 miles. What is regarded as the lower zone of limestone is
particularly well seen about Loch Tay; what is believed to be the upper is
typically displayed in the heart of Perthshire, about Blair-Athol.

From under the Loch Tay Limestone a great thickness of mica-schists,
"green schists," schistose grits and conglomerates, slates and greywackes,
emerges up to the border of the Highlands. Above that calcareous band
thick masses of mica-schist and sericite-schist are succeeded by a well-
marked zone of quartzite, which forms the mountains of Ben-y-Glo and
Schihallion, and stretches south-westward across Argyllshire into Islay and
Jura. The second or Blair-Athol Limestone lies next to this quartzite. If the
limestones are identical with those of Donegal, Mayo and Galway, the
quartzites may doubtless be also regarded as continued in those of the same
Irish counties, where they form some of the most conspicuous features in
the scenery, since they rise into such conspicuous mountains as Erigal,
Slieve League, Nephin, and the twelve Bins of Connemara.
The age of this vast system of altered rocks has still to be determined. It is
possible that they may include some parts of the Torridonian series, or even
here and there a wedge of the Lewisian gneiss driven into position by
gigantic disruptions, like those of the North-West Highlands. But there can
be no doubt that the schists, quartzites and limestones form an assemblage
of metamorphosed sedimentary strata which differs much in variety of
petrographical character, as well as in thickness, from the Torridonian
sandstone, and which has not been identified as the equivalent of any
known Palæozoic system or group of formations in Britain. It may
conceivably embrace the Cambrian series of the North-West Highlands, and
also the sedimentary deposits that succeeded the Durness Limestone, of
which no recognizable vestige remains in Sutherland or Ross.
That the metamorphic rocks east of the line of the Great Glen are at least
older than the Arenig formation of the Lower Silurian system may be
inferred from an interesting discovery recently made by the officers of the
Geological Survey. A narrow strip of rocks has been found which, from
their remarkable petrographical characters, their order of sequence and their
scanty fossil contents (Radiolaria), are with some confidence identified
with a peculiar assemblage of rocks on the Arenig horizon of the Silurian
system in the Southern Uplands of Scotland, to which fuller reference will
be made in Chapter xii. This strip or wedge of probably Lower Silurian
strata intervenes between the Highland schists and the Old Red Sandstone
in Kincardineshire, Forfarshire and Dumbartonshire. It has been recognized
also, occupying a similar position, in Tyrone in Ireland. The schists in some

places retain their foliated character up to the abrupt line of junction with
the presumably Lower Silurian strata, while in other districts, as at
Aberfoyle, they have been so little affected that it is hardly possible to draw
a line between the Highland rocks and those of this border-zone, which
indeed are there perhaps more metamorphosed than the Highland grits to
the north of them. The metamorphism of the schists may have been mainly
effected before the final disturbances that wedged in this strip of Silurian
strata along the Highland border, though some amount of crushing and
schist-making seems to have accompanied these disturbances. No trace of
any similar strip of Palæozoic rocks has ever been detected among the folds
of the schists further into the Highlands. But some of the Highland rocks in
the region of Loch Awe lose their metamorphosed character, and pass into
sedimentary strata which, so far as petrographical characters are concerned,
might well be Palæozoic.
Until some clue is found to the age of the Younger or Eastern schists,
quartzites and limestones of the Highlands, it is desirable to have some
short convenient adjective to distinguish them. As a provisional term for
them I have proposed the term "Dalradian," from Dalriada, the name of the
old Celtic kingdom of the north of Ireland and south-west of Scotland.[61]
[61] Presidential Address to Geological Society, 1891, p. 39.

The special feature for which this Dalradian series is cited in the present
volume is the evidence it furnishes of powerful and extensive volcanic
action. In a series of rocks so greatly dislocated, crumpled and
metamorphosed, we cannot look for the usual clear proofs of
contemporaneous eruptions. Nevertheless all over the Scottish Highlands,
from the far coast of Aberdeenshire to the Mull of Cantyre, and across the
west of Ireland from the headlands of Donegal into Galway, there occurs
abundant evidence of the existence of rocks which, though now forming an
integral part of the schists, can be paralleled with masses of undoubtedly
volcanic origin.

Fig. 37.—Section showing the position of Sills in the mica-schist series
between Loch Tay and Amulree.
a, Mica-schist; b, b, Sills.

Intercalated in the vast pile of altered sediments lie numerous sheets of
epidiorite and hornblende-schist, which were erupted as molten materials,
not improbably as varieties of diabase-lava. Most of these sheets are
doubtless intrusive "sills," for they can be observed to break across from
one horizon to another. But some of them may possibly be
contemporaneous lava-streams. A sheet may sometimes be followed for
many miles, occupying the same stratigraphical platform. Thus a band of
sills may be traced from the coast of Banffshire to near Ben Ledi, a distance
of more than 100 miles. Among the hornblendic sills of this band some
occur on a number of horizons between the group of Ben Voirlich grits and
the Ben-y-Glo quartzite. One of the most marked of these is a sheet,
sometimes 200 feet thick, which underlies the Loch Tay Limestone.
Another interesting group in the same great band has been mapped by the
Geological Survey on the hills between Loch Tay and Amulree, some of
them being traceable for several miles among the mica-schists with which
they alternate (Fig. 37).
In Argyllshire also, between Loch Tarbert and Loch Awe, and along the
eastern coasts of the islands of Islay and Jura, an abundant series of sheets
of epidiorite, amphibolite and hornblende-schist runs with the prevalent
strike of the schists, grits and limestones of that region. Similar rocks
reappear in a like position in Donegal, where, as in Scotland, the frequency
of the occurrence of these eruptive rocks on the horizons of the limestones
is worthy of remark. The persistence, number and aggregate thickness of
the sills in this great band mark it out as the most extensive series of
intrusive sheets in the British Isles.
In addition to the sills there occur also bosses of similar material, which in
their form and their obvious relation to the sheets recall the structure of
volcanic necks. They consist of hornblendic rocks, like the sills, but are
usually tolerably massive, and show much less trace of superinduced
foliation.
Besides the obviously eruptive masses there is another abundant group of
rocks which, I believe, furnishes important evidence as to contemporaneous
volcanic action during the accumulation of the Dalradian series. Throughout
the Central and South-Western Highlands certain zones of "green schist"
have long occupied the attention of the officers of the Geological Survey.
They occur more especially on two horizons between the Loch Tay

Limestone and a much lower series of grits and fine conglomerates, which
run through the Trossachs and form the craggy ridges of Ben Ledi, Ben
Voirlich and other mountains near the Highland border. In the lower group
of "green schists," thick hornblendic sills begin to make their appearance,
increasing in number upwards. The upper group of "green schists" lies
between two bands of garnetiferous mica-schist, above the higher of which
comes the Loch Tay Limestone. The peculiar greenish tint and
corresponding mineral constituents of these schists, however, are likewise
found diffused through higher parts of the series.
So much do the "green schists" vary in structure and composition that no
single definition of them is always applicable. At one extreme are dull green
chlorite-schists, passing into a "potstone," which, like that of Trondhjem,
can be cut into blocks for architectural purposes.[62] At the other extreme lie
grits and quartzites, with a slight admixture of the same greenish-coloured
constituent. Between these limits almost every stage may be met with, the
proportion of chlorite or hornblende and of granular or pebbly quartz
varying continually, not only vertically, but even in the extension of the
same bed. The quartz-pebbles are sometimes opalescent, and occasionally
larger than peas. An average specimen from one of the zones of "green
schists" is found, on closer examination, to be a thoroughly schistose rock,
composed of a matrix of granular quartz, through which acicular
hornblende and biotite crystals, or actinolite and chlorite, are ranged along
the planes of foliation.
[62] From such a rock, which crosses the upper part of Loch Fyne, the Duke of
Argyll's residence at Inveraray has been built.

That these rocks are essentially of detrital origin admits of no doubt. They
differ, however, from the other sedimentary members of the Dalradian
series in the persistence and abundance of the magnesian silicates diffused
through them. The idea which they suggested to my mind some years ago
was that the green colouring-matter represents fine basic volcanic dust,
which was showered out during the accumulation of ordinary quartzose,
argillaceous and calcareous sediments, and that, under the influence of the
metamorphism which has so greatly affected all the rocks of the region, the
original pyroxenes and felspars suffered the usual conversion into
hornblendes, chlorites and micas. This view has occurred also to my
colleagues on the Survey, and is now generally adopted by them.

Not only are these "green schists" traceable all through the Central and
South-Western Highlands, rocks of similar character, and not improbably on
the same horizons, reappear in the north-west of Ireland, and run thence
south-westward as far as the Dalradian rocks extend. If we are justified in
regarding them as metamorphosed tuffs and ashy sediments, they mark a
widespread and long-continued volcanic period during the time when the
later half of the Dalradian series was deposited.
Besides the extensive development of basic sills which, though probably in
great part later than the "green schists," may belong to the same prolonged
period of subterranean activity, numerous acid protrusions are to be
observed in the Dalradian series of Scotland and Ireland. That these masses
were erupted at several widely-separated intervals is well shown by their
relation to the schists among which they occur. Some of the great bosses
and sills of granite were undoubtedly injected before the metamorphism of
the schists was completed, for they have shared in the foliation of the
region. Others have certainly appeared after the metamorphism was
complete, for they show no trace of having suffered from its effects. Thus
some of the vast tracts of newer granite in the Grampian chain, which cover
many square miles of ground, must be among the newest rocks of that area.
They have recently been found by Mr. G. Barrow, of the Geological Survey,
to send veins into the belt of probably Lower Silurian strata which flanks
the Highland schists. They are thus later than the Arenig period. Not
impossibly they may be referable to the great granite intrusions which
formed so striking a feature in the history of the Lower Old Red Sandstone.

iii. the gneisses and schists of anglesey

In the island of Anglesey an interesting series of schists and quartzites
presents many points of resemblance to the Dalradian or younger schists of
the Highlands. At present the geologist possesses no means of determining
whether these Welsh rocks are the equivalents of the Scottish in
stratigraphical position, but their remarkable similarity justifies a brief
allusion to them in this place. Much controversy has arisen regarding the
geology of Anglesey, but into this dispute it is not necessary for my present
purpose to enter.[63] I will content myself with expressing what seems to
me, after several traverses, to be the geological structure of the ground.

[63] The literature of Anglesey geology is now somewhat voluminous, but I may refer
to the following as the chief authorities. The island is mapped in Sheet 78 of the
Geological Survey of England and Wales, and its structure is illustrated in Horizontal
Sections, Sheet 40. A full account of its various formations and of their relations to
each other is given in vol. iii. of the Memoirs of the Geological Survey, "The
Geology of North Wales," by Sir A. C. Ramsay, 2nd edit. 1881. The subject has been
discussed by Professor Hughes, Quart. Journ. Geol. Soc. vols. xxxiv. (1878) p. 137,
xxxv. (1879) p. 682, xxxvi. (1880) p. 237, xxxviii. (1882) p. 16; Brit. Assoc. Rep.
(1881) pp. 643, 644; Proc. Camb. Phil. Soc. vol. iii. pp. 67, 89, 341; by Professor
Bonney, Quart. Journ. Geol. Soc. vol. xxxv. (1879) pp. 300, 321; Geol. Mag. (1880)
p. 125; by Dr. H. Hicks, Quart. Journ. Geol. Soc. vols. xxxiv. (1878) p. 147, xxxv.
(1879) p. 295; Geol. Mag. (1879) pp. 433, 528 (1893) p. 548; by Dr. C. Callaway,
Quart. Journ. Geol. Soc. vols. xxxvii. (1881) p. 210, xl. (1884) p. 567; and by the
Rev. J. F. Blake, Quart. Journ. Geol. Soc. vol. xliv. (1888) p. 463. Further references
to the work of these observers in Anglesey are given in Chapter xiii. p. 220 et seq.
The Pre-Cambrian areas of Anglesey are shown in Map II.

There are two groups of rocks in Anglesey to which a pre-Cambrian age
may with probability be assigned. In the heart of the island lies a core of
gneiss which, if petrographical characters may be taken as a guide, must
certainly be looked upon as Archæan. In visiting that district with my
colleague Mr. Teall I was much astonished to find there so striking a
counterpart to portions of the Lewisian gneiss of the north-west of
Sutherland and Ross. The very external features of the ground recall the
peculiar hummocky surface which so persistently characterizes the areas of
this rock throughout the north-west of Scotland. If the geologist could be
suddenly transported from the rounded rocky knolls of Sutherland, Ross-
shire or the Hebrides to those in the middle of Anglesey, south of
Llanerchymedd, he would hardly be aware of the change, save in the
greater verdure of the hollows, which has resulted from a more advanced
state of decomposition of the rocks at the surface, as well as from a better
climate and agriculture.
When we examine these rocky hummocks in detail we find them to consist
of coarse gneisses, the foliation of which has a prevalent dip to N.N.W.
Some portions abound in dark hornblende and garnets, others are rich in
brown mica, the folia being coarsely crystalline and rudely banded, as in the
more massive gneisses of Sutherland. Abundant veins of coarse pegmatite
may here and there be seen, with pinkish and white felspars and milky
quartz. Occasionally the gneiss is traversed by bands of a dark greenish-
grey rock, which remind one of the dykes of the north-west of Scotland.
There are other rocks, some of them probably intrusive and of later date, to

be seen in the same area; but they require more detailed study than they
have yet received.
The relation of this core of gneiss and its associated rocks to the second
group of pre-Cambrian rocks has not hitherto been satisfactorily
ascertained. The core may conceivably be an eruptive boss in that group,
and may have acquired its foliation during the movements that produced the
foliation of the surrounding schists. But it seems more probable that the
gneiss is much older than these schists, though it would undoubtedly
participate in the effects of the mechanical movements which gave rise to
their deformation, cleavage and foliation.
The second group of rocks occupies a large area in the west and in the
centre and south of Anglesey. The schists of which it consists are obviously
in the main a clastic series. One of their most conspicuous members is
quartzite, which, besides occurring sporadically all over the island, forms
the prominent mass of Holyhead Mountain. There are likewise flaggy
chloritic schists, green and purple phyllites or slates, and bands of grit,
while parts of the so-called "grey gneiss" consist of pebbly sandstones that
have acquired a crystalline structure. That some order of sequence among
these various strata may yet be worked out is not impossible, but the task
will be one of no ordinary difficulty, for the plications and fractures are
numerous, and much of the surface of the ground is obscured by the spread
of Palæozoic formations and superficial deposits.
These Anglesey schists are so obviously an altered sedimentary series that it
is not surprising that they should have been regarded as metamorphosed
Cambrian strata. All that can be positively affirmed regarding their age is
that they are not only older than the lowest fossiliferous rocks around them
—that is, than Arenig or even Tremadoc strata—but that they had already
acquired their present metamorphic character before these strata were laid
down unconformably upon them. There is no actual proof that they include
no altered Cambrian rocks. But when we consider their distinctly crystalline
structure, and the absence of such a structure from any portion of the
Cambrian areas of the mainland; when, moreover, we reflect that the
metamorphism which has affected them is of the regional type, and can
hardly have been restricted to merely the limited area of Anglesey; we must
agree with those observers who, in spite of the absence of positive proof of
their true geological horizon, have regarded these rocks as of much higher

antiquity than the Cambrian strata of the neighbourhood. No one familiar
with the Dalradian rocks of Scotland and Ireland can fail to be struck with
the close resemblance which these younger Anglesey schists bear to them,
down even into the minutest details. Petrographically they are precisely the
counterparts of the quartzites and schists of Perthshire and Donegal, and a
further connection may be established of a palæontological kind. The upper
part of the Holyhead quartzite was found by Mr. B. N. Peach and myself in
the autumn of the year 1890 to be at one place crowded with annelid-pipes,
and I subsequently found the same to be the case with some of the flaggy
quartzites near the South Stack.

Fig. 38.—Sketch of crushed basic igneous rock among the
schists, E. side of Porth-tywyn-mawr, E. side of
Holyhead Straits.

For the purpose of the inquiry which forms the theme of this work, the
feature of greatest interest about these younger schists of Anglesey is the
association of igneous rocks with them. They include bands of dark basic
material, the less crushed parts of which resemble the diabases of later
formations, while the sheared portions pass into epidiorites and true
hornblende-schists. As in other regions where eruptive rocks have been
crushed down and changed into the schistose modification, it is frequently
possible to see groups of uncrushed cores round which, under severe
mechanical stresses, the rock has undergone this conversion. Lines of
movement through the body of the rock may be detected by bands of schist,
the gradation from the solid core to the hornblende-schist being quite
gradual. The accompanying figure (Fig. 38) represents a portion of one of
these crushed basic igneous rocks on the east side of Holyhead Straits.

As in the Dalradian series of the Highlands, many, perhaps most, of these
igneous bands are probably intrusive sills, but others may be intercalated
contemporaneous sheets. They occur across the whole breadth of the island
from the Menai Strait to the shores of Holyhead.
Besides these undoubtedly igneous rocks, the green chloritic slates of
Anglesey deserve notice. They are well-bedded strata, consisting of
alternations of foliated fine grit or sandstone, with layers more largely made
up of schistose chlorite. The gritty bands sometimes contain pebbles of blue
quartz, and evidently represent original layers of sandy sediment, but with
an admixture of chloritic material. The manner in which this green chloritic
constituent is diffused through the whole succession of strata, and likewise
aggregated into bands with comparatively little quartzose sediment, reminds
one of the "green schists" of the Central Highlands and Donegal, and
suggests a similar explanation. Taken in connection with the associated
basic igneous rocks, these chloritic schists seem to me to represent a thick
group of volcanic tuffs and interstratified sandy and clayey layers. If this
inference is well founded, and if we are justified in grouping these Anglesey
rocks with the Dalradian schists of Scotland and Ireland, a striking picture
is presented to the mind of the wide extent and persistent activity of the
volcanoes of that primeval period in Britain.[64]
[64] Mr. E. Greenly, late of the Geological Survey of Scotland, has recently
established himself on the Menai Strait for the purpose of working out in detail the
geological structure of this interesting and complicated region. We may therefore
hope that some of the still unsolved problems presented by the rocks of Anglesey
will before long be satisfactorily explained.

iv. the uriconian volcanoes

Along the eastern borders of Wales a ridge of ancient rocks, much broken
by faults and presenting several striking unconformabilities, has long been
classic ground in geology from the descriptions and illustrations of
Murchison's Silurian System.[65] The main outlines of the structure of that
district, first admirably worked out by this great pioneer, were delineated on
the maps and sections of the Geological Survey, wherein it was shown that
in the Longmynd an enormously thick group of stratified rocks, which,
though unfossiliferous, were referred to the Cambrian system, rose in the
very heart of the country; that to the east of these rocks lay strata of

Caradoc or Bala age; that by a great hiatus in the stratigraphy the Upper
Silurian series transgressively wrapped round everything below it; that yet
again the Coal-measures crept over all these various Palæozoic formations,
followed once more unconformably by Permian and Triassic deposits.[66]
Besides all this evidence of extraordinary and repeated terrestrial
movement, it was found that the region was traversed by some of the most
powerful dislocations in this country, while to complete the picture of
disturbance, many protrusions of igneous rocks were recognized.
[65] See especially chap. xix. vol. i. p. 225.

[66] The area is embraced in Sheet 61 of the Geological Survey, and is illustrated by
Nos. 33 and 36 of the sheets of Horizontal Sections. In the early editions of the
Survey maps the "felspathic traps" and the "greenstones" of the Wrekin district were
distinguished by separate colours, but unfortunately this useful and so far correct
discrimination was given up in subsequent editions, where all the acid and basic
rocks are merged into one.

In a territory so complicated, though it had been sedulously and skilfully
explored, there could hardly fail to remain features of structure which had
escaped the notice of the first observers. In particular, the igneous rocks had
been dealt with only in a general way, and they consequently offered a
favourable field for more detailed study; while by a more searching
examination of some of the rocks for fossils, important corrections of the
earlier work might yet be made.
A notable step towards a revision of the received opinions regarding the
igneous rocks of this region was taken by Mr. Allport, who showed that the
so-called "greenstone" included masses of devitrified spherulitic pitchstones
and perlites, together with indurated volcanic breccias, agglomerates and
ashes.[67] Subsequently Professor Bonney described more fully the
petrographical characters of the Wrekin igneous rocks, confirming and
extending the observations of Mr. Allport.[68]
[67] Quart. Journ. Geol. Soc. vol. xxxiii. (1877) p. 449.

[68] Op. cit. vol. xxxv. (1879) p. 662; vol. xxxviii. (1882) p. 124.

But the correction of the prevalent error as to the geological age of these
rocks was due to Dr. Callaway, who, after spending much time and labour
in ascertaining, by a careful search for fossils, the position of the
superincumbent rocks (wherein he discovered Cambrian organisms), and in
a detailed investigation of the structure and relationships of the igneous

masses themselves, was led to regard them as part of an ancient pre-
Cambrian ridge; and he proposed for the volcanic group the name of
Uriconian, from the name of the former Roman town which stood not far to
the west of them.[69] He has shown how essentially volcanic this ancient
series of rocks is, how seldom they present any clearly-marked evidence of
stratification, and how small is the proportion of sedimentary material
associated with them.[70]
[69] Quart. Journ. Geol. Soc. vols. xxx. (1874) p. 196, xxxiv. (1878) p. 754, xxxv.
(1879) p. 643, xlii. (1886) p. 481. For a criticism of Dr. Callaway's views as to the
order of succession among the rocks of this district, see Prof. Blake, op. cit. vol. xlvi.
(1890) p. 386, and Dr. Callaway's reply, vol. xlvii. (1891) p. 109.
[70] Op. cit. vol. xlvii. (1891) p. 123.

Subsequently Professor Lapworth, by his discovery of the Olenellus-fauna,
marking the lowest known fossiliferous Cambrian zone in the Wrekin
district, and his recognition of Cambrian fossils under the Coal-measures of
Warwickshire, supplied valuable evidence for the discussion of the
geological position of the older rocks of the Midlands. He has mapped in
minute detail the rocks of the Wrekin, and has exhausted all the evidence
that is at present obtainable on the subject. But unfortunately the
publication of his researches is still delayed.[71]
[71] Geol. Mag. (1882) p. 563, (1886) p. 319, (1887) p. 78, (1888) p. 484; and a joint
paper with Mr. W. W. Watts on the Geology of South Shropshire, Proc. Geol. Assoc.
vol. xiii. (1894) pp. 302, 335.

It is now recognized that the core of the ancient ridge, extending from near
Wellington through the Wrekin, Caer Caradoc and other hills, until it sinks
beneath the Upper Silurian formations, is formed of igneous rocks that
consist partly of lavas, partly of volcanic breccias and fine tuffs. The lavas
are thoroughly acid rocks of the felsitic or rhyolitic type. One of them,
about 100 feet thick, which forms a prominent feature on the flanks and
crest of Caer Caradoc, shows abundant finely-banded flow-structure, often
curved or on end, while its bottom and upper parts are strongly
amygdaloidal, the cavities being occasionally pulled out in the direction of
flow and lined with quartz or chalcedony. Some of the detached areas of
eruptive rocks show the beautiful spherulitic and perlitic structures first
noticed in this region by Mr. Allport. More recently the structures of these
acid rocks have been described by Mr. F. Rutley.[72]

[72] Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 540. Mr. Rutley more particularly
describes those of Caradoc Hill.

The breccias and tuffs appear to consist mainly of felsitic material. In the
coarser varieties, fragments of finely-banded felsite may be noticed, while
the finer kinds pass into a kind of hornstone (hälleflinta), which in hand-
specimens could hardly be distinguished from close-grained felsite. In some
places, these pyroclastic rocks are well stratified, but elsewhere no
satisfactory bedding can be recognized in them. Various other rocks, which
are probably intrusive, occur in the ridge. At either end of the Wrekin there
is a mass of pink microgranite, while at Caer Caradoc numerous sheets of
"greenstone," intercalated in the fine tuffs, sweep across the hill. Mr. Rutley
has published an account of these basic rocks, which he classes as
"melaphyres," or altered forms of basalt or andesite.[73] That at least some
of them are intrusive is manifest by the way in which they ramify through
the surrounding strata. But others are so strongly amygdaloidal and slaggy
that they may possibly be true interbedded lavas, though there may be some
hesitation in admitting that such basic outflows could be erupted in the
midst of thoroughly acid ejections.[74] Leaving these doubtful flows out of
account, we have here a group of undoubted volcanic rocks represented by
acid lavas and pyroclastic materials, by intrusive bosses of acid rocks, and
by younger basic sills. The general lithological characters of these masses
and the sequence of their appearance thus strongly resemble those of
subsequent Palæozoic volcanic episodes.
[73] Op. cit. p. 534.

[74] This difficulty, however, need not be in itself insuperable, as is evident from the
remarkable alternation of basic and acid lavas and tuffs in the Cambrian volcanic
group of St. David's and in the Old Red Sandstone series of the Pentland Hills.

The geological age of this volcanic group is a question of much interest and
importance in regard to the history of volcanism in this country. An inferior
limit to the antiquity of the group can at once be fixed by the fact that, as
originally pointed out by Dr. Callaway, the quartzite which overlies the
volcanic rocks passes under a limestone containing Cambrian fossils in
which Professor Lapworth has since recognized Olenellus, Paradoxides and
other Lower Cambrian forms. The eruptions, therefore, must be at least as
old as the earlier part of the Cambrian period. But it is affirmed that the
quartzite rests with a complete unconformability on the volcanic rocks. If
this be so, then the epoch of eruption must be shifted much farther back.

Fig. 39.—Section across the Uriconian series of Caer Caradoc.
S3, Upper Silurian; S2, Bala group; S1, Arenig group; C, Cambrian; L,
Longmyndian; u, Uriconian; f f, faults.

The evidence adduced in favour of this great break appears to me to be
threefold. In the first place, the quartzite contains fragments of the volcanic
rocks. I do not think much stress can be laid on this fact. When I visited the
ground, what struck me most in the composition of the quartzite was its
singularly pure quartzose character, and the comparative scarcity of felsite-
pebbles in it. Any deposit laid down conformably upon the top of the
breccias and tuffs might obviously contain some of these materials, while, if
laid down unconformably, it might reasonably be expected to be full of
them. In the second place, this quartzite is alleged to pass transgressively
across the edges of successive sheets of the volcanic group, and thus to
have a quite discordant dip and strike. I failed to find satisfactory evidence
of this unconformability in the northern part of the district. But in the Caer
Caradoc area the quartzite does appear to steal across the outcrops of the
older rocks, which plunge at nearly right angles in an opposite direction. In
the third place, the felsitic volcanic group is believed by Professor
Lapworth to pass upwards into the Longmynd rocks. Obviously, if this
group lies at the very bottom of the vast Longmynd series, the discordance
between it and the quartzite must be enormous, and the date of the volcanic
eruptions must be placed vastly farther back in geological antiquity. Though
the evidence does not seem to me to amount to clear proof, I am disposed,
in the meantime, to accept it as affording the most probable solution of the
difficulties presented by the structure of the ground.
The sequence of the rocks around Caer Caradoc is partly concealed by
surface accumulations, but if these could be cleared away the structure of
the ground would be, according to Messrs. Lapworth and Watts, as shown
in Fig. 39.[75]
[75] Proc. Geol. Assoc. vol. xiii. (1894), pp. 314, 315.

If, then, this volcanic group underlies the whole of the Longmynd series,
and if, as it now appears, that series is older than the Olenellus-zone of the
Lower Cambrian rocks, we can hardly include the volcanic rocks of the

Wrekin and Caer Caradoc in the Cambrian system. They must belong to a
still older geological formation, and I think we cannot do better than adopt
for them Dr. Callaway's name, Uriconian.
There are still, however, many problems to be solved before the geological
history of that region is completely understood. The rocks of the Longmynd
must be more fully worked out. It is improbable that strata which look so
likely to yield fossils should for ever prove barren. The lower half at least
may be hopefully searched, although the upper massive reddish sandstones
and conglomerates offer less prospect of success. On the west side of the
Longmynd, above Pontesbury, there occurs a small area of volcanic rocks
like those of the Wrekin district, including a well-marked nodular felsite
and fine tuffs. These rocks have been regarded by Dr. Callaway as another
axis of the Uriconian series. It is very difficult, however, by any
combination of geological structures, to bring up a portion of the very
bottom of the Longmynd series and place it apparently at the top. This is a
feat which a detailed study of the region, and the detection of
unconformabilities in the Longmynd, may possibly accomplish. In the
meantime, however, I would venture to suggest whether it is not more
probable that we have here a detached area of much younger volcanic
rocks, like those which, in various districts, may be included in the
Cambrian system, and which will be referred to in some detail in
subsequent pages.

v. the malvern volcano

Regarding the age and origin of the oldest rocks of the Malvern Hills some
controversy has arisen, and no general agreement has yet been reached.[76]
It is clear that the core of crystalline rocks which is overlain unconformably
by the Hollybush Sandstone must be older than the Upper Cambrian rocks.
There is no good evidence of any stratigraphical break in the Cambrian
system of England or Wales, and it may be reasonably inferred that the
break seen at the base of the Hollybush Sandstones indicates that the rocks
underneath that horizon are pre-Cambrian. Some portions of these certainly
very ancient rocks are gneisses or schists; others have been described as
"felsites," and have been regarded as passing into schists, and as the original
material from which portions of the foliated series of the range have been

produced by mechanical deformation. Not improbably the whole series of
rocks is of igneous origin, but has been subsequently rendered more or less
schistose.
[76] There is no room here for a full bibliography of the geological literature devoted
to this locality. In the monograph by J. Phillips in vol. ii. part i. of the Memoirs of the
Geological Survey, a list of writings is given up to the time of its publication in 1848.
Since that year many additional papers have appeared. I may especially refer to H. B.
Holl, Quart. Journ. Geol. Soc. xxi. (1865) p. 72; J. H. Timins, op. cit. xxii. (1867);
Mr. F. Rutley, op. cit. xliii. (1887) p. 481; Dr. Callaway, op. cit. xliii. (1887) p. 525,
xlv. (1889) p. 475, xlix. (1893) p. 398; Prof. Green, op. cit. li. (1895) p. 1; Mr. H. D.
Acland, Geol. Mag. 1894, p. 48.

There is one area where the rocks have escaped metamorphism, and where
they present some of the well-known features of ancient volcanic materials.
This tract was first indicated by Dr. H. B. Holl as one occupied by "altered
primordial rocks and post-primordial trap." Its evidently igneous materials
have been examined and described by different observers, among whom Dr.
Callaway has contributed some detailed papers on the subject. More
recently Professor Green, who had the advantage of sections exposed in the
excavations for the construction of a reservoir for supplying water to Great
Malvern, came to the conclusion that the rocks consist mainly of felsites,
having many of the characters of rhyolites. With these are associated felsitic
tuffs, while bands of dolerite, probably intrusive, form likewise part of the
series. So far as the somewhat meagre evidence allows an opinion to be
formed, there appears to be an alternation of felsites, lavas and tuffs placed
in a more or less vertical position, striking in a northerly direction, and
traversed by several sheets of intrusive dolerite.
No junction has been found between these unfoliated volcanic rocks and the
schists that form the core of the range. Judging merely from their present
relative condition, one would naturally infer that the volcanic rocks must be
the younger of the two groups. But, as Professor Green has pointed out, it is
conceivable that the latter may have locally escaped crushing, and yet be of
the same age as the felsites and epidiorites of the neighbouring Raggedstone
Hill, which have been in part considerably affected by mechanical
movements.[77]
[77] Op. cit. p. 7. The metamorphism of the igneous rocks of the Malvern Hills into
schists has been especially investigated by Dr. Callaway.

For our present inquiry it is perhaps sufficient to take note that in the heart
of the Malvern Hills there lies a remnant of a volcanic district, probably of
pre-Cambrian age, the rocks of which had been raised up into a vertical
position so as to form islets or reefs in the sea in which the Upper Cambrian
strata (Hollybush Sandstone and Upper Lingula shales) were deposited.
Until some more precise evidence is obtained as to the geological age of
these rocks it may be convenient to place them provisionally with the
volcanic Uriconian series.

vi. the charnwood forest volcano

In the heart of England the great Triassic plain is diversified by the uprise
through it of the peaks and crests of an old Triassic land-surface, which are
embraced in the district known as Charnwood Forest. These scattered
eminences consist of materials not only immensely older than the Trias, but
once doubtless buried under thousands of feet of Palæozoic strata. They had
been laid bare by denudation and carved into picturesque crags and
pinnacles before the New Red Sandstone was deposited around and above
them.
To these vestiges of an early Mesozoic land, still half buried among Triassic
strata, a peculiar interest attaches from the obviously high antiquity of their
rocks and their uprise in the very centre of the island. Various opinions have
been expressed as to the age of their component rocks. When they were
mapped by the Geological Survey they were recognized to be as old as any
group of rocks then known, and they were accordingly placed in the
Cambrian system. More recent research has suggested that they may be still
more ancient, and may be regarded as pre-Cambrian.
The rocks of Charnwood Forest have been the subject of an exhaustive
research by the Rev. E. Hill and Professor Bonney, to whom most of our
knowledge regarding them is due. These observers first pointed out the truly
volcanic nature of the coarse clastic rocks of the district. They have traced
their relations in the field, and have likewise described their structure and
composition as shown by the microscope. Subsequently the district has
been re-mapped on the scale of six inches to a mile by Mr. Fox Strangways
for the Geological Survey, while Mr. W. W. Watts, another member of the
Survey, has studied the petrography of the ground, and has traced the

boundaries of the several rock-groups so far as these can be determined.
Confirming generally the stratigraphical arrangement sketched by Messrs.
Hill and Bonney, Mr. Watts has proposed the following classification of the
rocks:—[78]

7. Groby and Swithland slates.
6. Hanging Rocks conglomerate The Brand series.
and Bradgate quartzite.
5. Woodhouse beds (ashy grits).

4. Slate-agglomerate of Roecliffe. The Maplewell series
(volcanic tuffs
3. Hornstone beds of Beacon Hill. and agglomerates).
2. Felsitic agglomerate of Benscliffe.
1. Quartzose, felspathic and felsitic grits. The Blackbrook series.
[78] Annual Report of Director-General of the Geological Survey, in the Report of
Science and Art Department for 1895.

Under any computation or measurement, the total thickness of detrital
material in this series of formations must amount to several thousand feet.
The chief interest centres in the middle series, which consists largely of
fragmental volcanic rocks, with intercalations of slate and grit. As was first
shown by Mr. Hill and Professor Bonney, these volcanic materials vary
from exceedingly coarse agglomerates to fine, ashy or felspathic slates. In
most cases distinct bedding can be recognized in them, but more
particularly in the fine-grained material. Yet even among the massive
agglomerates a tendency may be seen towards an orientation of the blocks
with their long axes parallel. That this arrangement is not entirely due to the
effects of cleavage may be inferred from the many exceptions to it, which
would hardly have occurred had such powerful cleavage affected the whole
district, as would be needed to rearrange the large blocks in the
agglomerates. Besides, the coarser parts often intercalate with fine
felspathic grits, which distinctly mark the stratification of the whole.
The remarkably coarse breccia of Benscliffe is mainly made up of blocks of
quartz-porphyry, felsite or rhyolite, with slate fragments. The Roecliffe

agglomerate, another extraordinarily coarse rock, consists of slate
fragments imbedded in an andesitic matrix, some of the blocks of slate
being six feet long. The finer tuffs have been ascertained to consist of
felsitic or andesitic detritus, sometimes forming exceedingly compact flinty
rocks or hornstones.
In this thick accumulation of detrital rocks we are presented with a series of
alternations of coarser and finer pyroclastic material, interstratified among
green, grey and purple slates and grits, which probably represent the non-
volcanic sediments of the time of eruption. The succession of strata bears
witness to a long series of eruptions of varying intensity, but culminating at
two distinct periods in the discharge of huge blocks of rock (Benscliffe and
Roecliffe agglomerates).
After some search I have been unable to detect a single vesicular fragment
among the stones in the breccias and tuffs, and Messrs. Hill and Bonney
were not more successful. Not a trace of anything in the least degree
scoriaceous is anywhere to be found. The paste in which the blocks lie
consists of such fine material as would result from the trituration of felsite
and slate. It contains many broken crystals of felspar, with grains of clear
quartz. A gradation can be traced from the coarser into the finer bands of
volcanic and non-volcanic material, fine slates being also interleaved with
highly-felspathic partings of grit.
Having looked with some care for a trace of a true volcanic neck in the
district, I have not seen anything that could be unhesitatingly so designated.
Even in the north-western part of the district, where the breccias are
coarsest, and there is least trace of ordinary sediments, some signs of
bedding can usually be detected in the position of the imbedded stones and
the partings of finer tuff. Both the coarser and finer detritus suggest the kind
of material discharged from vents before the uprise of any lava. The entire
absence of scoriaceous fragments is noteworthy, and the abundance of slate
blocks rather points to the early eruptions of a volcanic focus. Possibly,
while the chief centre of eruption lay towards the north-west, numerous
vents may have been opened all over the district, discharging abundant
showers of dust and stones, but seldom or never culminating in the actual
outpouring of lava.

No indubitable lava-sheet has, in my judgment, been yet recognized in
Charnwood Forest. Various opinions have been expressed as to some of the
more compact close-grained rocks, and even the verdicts of the same
observers have varied from time to time, the rocks once considered as
felsites being afterwards regarded as tuffs, and subsequently placed with the
felsites or andesites after all. It is not necessary for my present purpose to
enter into these questions, which are rather of local interest. I will only say
that, in my opinion, the rocks of Sharpley, Peldar, and Bardon Hill are
massive rocks, as they have finally been classed by Messrs. Hill and
Bonney. But I cannot look upon them as lavas, at least I have seen no
evidence to lead me to believe that they were ever erupted at the surface. I
have fully considered the arguments of Mr. Hill and Professor Bonney on
this point.[79] There can, I think, be no doubt of the close association of
these felsitic rocks and the breccias, but the structure of the rocks in the
field seems to me to be decidedly in favour of the view expressed above.
The microscope affords no assistance in the question.[80] The doubtful rocks
seem to me rather to be intrusive masses which have been protruded into
the volcanic sedimentary series among which they rise. They are acid, fine-
grained, porphyritic rocks, which would formerly have been included under
the general name of felsites or quartz-porphyries. Their coarse porphyritic
parts rapidly pass into close-grained felsitic material. Many of the blocks in
the breccias are precisely like parts of these rocks. It might hence be
asserted that these fragmental deposits are later than the eruptive bosses. At
least it is obvious that rocks of the same type as those of Sharpley, Peldar,
and Bardon Hill must have been disrupted to produce the coarse breccias.
[79] Quart. Journ. Geol. Soc. xlvii. (1891), pp. 80-88.

[80] See Messrs. Hill and Bonney, op. cit. xxxiv. (1878), p. 211.

Later eruptive rocks, consisting of masses of syenite and granite, with still
younger dykes of dolerite, andesite, diorite and felsite, have successively
made their appearance, and add to the diversity of the igneous phenomena
of this district.
The question of the age of this isolated volcanic series is one of much
interest, but of great perplexity. Though a resemblance may be admitted to
exist between some of the slates and parts of the Cambrian system of North
Wales, the difference between the Charnwood rocks and the undoubted
Cambrian series of Warwickshire, only thirteen miles to the south-west, is

such as to indicate that the former are probably older than the latter. While
the Charnwood rocks have been intensely cleaved and crushed, those of
Warwickshire have undergone no such change. The argillaceous strata in
the one region have been converted into slates, in the other they remain
mere shales. Though cleavage is sometimes irregularly developed, its rapid
disappearance in so short a distance as the interval between Charnwood
Forest and Nuneaton seems most explicable if we suppose that the rocks at
the more easterly locality were cleaved before those towards the west were
deposited. If this inference be well grounded the pre-Cambrian age of the
Charnwood volcanoes would be established. But the argument is not
conclusive. No fossils of any kind have yet been found in any of the old
rocks of Charnwood.[81] Merely lithological resemblances or differences are
all that can be used as a guide to the geological age of these masses. Mr.
Watts has suggested that possibly the quartzite of Bradgate (No. 6 of the
Charnwood groups) may be the equivalent of the quartzite which in
Shropshire and Warwickshire forms the base of the sedimentary Cambrian
formations. If that correlation could be established, the volcanic series
below the quartzite in Charnwood might be regarded as representing the
Uriconian volcanic series of Shropshire.
[81] Since this page was in type, Professor Lapworth has found a worm-burrow low
down in the Brand Series, and one or two additional examples have since been
obtained by Mr. J. Rhodes of the Geological Survey. These are the first undoubted
organisms from the Charnwood Forest rocks. Mr. Watts, Geol. Mag. 1896, p. 487.

BOOK III
THE CAMBRIAN VOLCANOES

CHAPTER IX
CHARACTERISTICS OF THE CAMBRIAN SYSTEM IN BRITAIN

The Physical Geography of the Cambrian Period—The Pioneers of Palæozoic Geology
in Britain—Work of the Geological Survey in Wales—Subdivisions of the
Cambrian System in Britain.
In leaving the investigation of the pre-Cambrian rocks and entering upon that of the Palæozoic
systems, that is, the great series of sedimentary formations which include the earliest records of
organized life upon the surface of the globe, the geologist feels much as the historian when,
quitting the domain of legend and tradition, he can tread firmly in the region of documentary
evidence. From the bottom of the Cambrian system upward through the long series of geological
formations, the chronicle, though often sadly incomplete, is usually clear and legible. As we follow
the lowest fossiliferous strata across a territory, we recognize that they bear witness to the same
processes of denudation and deposition which have been going on uninterruptedly on the face of
the globe ever since. The beds of conglomerate represent the gravels and shingles of old coast-
lines and river-beds. The sandstones recall the familiar features of sandy sea-bottoms not far from
land. The shales bear witness to the fall of fine sediment in stiller water, such as now takes place in
the deeper parts of seas and lakes. Notwithstanding their vast antiquity, the strata themselves
exhibit no exceptional peculiarities of origin. They seem to be just such familiar deposits as are
gathering under fitting conditions at the present time.
Some writers have speculated on the far greater intensity of all geological activities in the early
times of the planet's history. But if we may interpret the record of the stratified formations by the
phenomena of to-day, there is for these speculations no confirmation in the sedimentation of the
oldest stratified deposits. It is of course quite intelligible, if not probable, that many geological
forces may have been more vigorous in primeval times than they afterwards became. But of the
gigantic tides, prodigious denudation and violent huddling together of the waste of the earth's
surface, which have been postulated for the early Palæozoic ages, there is assuredly nowhere any
indication among the stratified formations. In those vast orderly repositories, layer succeeds layer
among thinly-laminated shales, as gently and equably as the fine silt of each tide sinks to-day over
the floor of a sheltered estuary. At the primeval period of which these sediments are the memorial,
the waters receded from flat shores and left tracts of mud bare to the sky, precisely as they do still.
Then as now, the sun shone and dried such mud-flats, covering their surfaces with a network of
cracks; the rain fell in heavy drops, that left their imprints on the drying mud; and the next tide
rose so gently as to overflow these records of sunshine and shower without effacing them, but
spreading over them a fresh film of sediment, to be succeeded by other slowly-accumulating
layers, under which they have lain preserved during the long cycles of geological history.
That organized creatures had already appeared upon the earth's surface before the beginning of the
Cambrian period cannot be doubted. The animal remains in the lowest Cambrian strata are far
from being the simple forms which might be expected to indicate the first start of animal life upon
the surface of the earth. On the contrary, though they are comparatively scanty in types, and often
rare or absent throughout a thick mass of sedimentary deposits, they show beyond dispute that,
when they flourished, invertebrate life had already reached such a stage of advancement and
differentiation that various leading types had appeared which have descended, in some cases with
generic identity, down to our own day. There must have been a long pedigree to these organisms of

the oldest known fossiliferous rocks. And somewhere on the earth's surface we may yet hope to
find the remains of their progenitors in pre-Cambrian deposits.
The researches of many explorers in Europe and North America have brought to light an
interesting series of organic remains from the Cambrian system. Of the plants of the time hardly
any traces have survived, save some markings which have been referred to sea-weeds. The earliest
known sponges and corals occur in this system, likewise the ancestors of the graptolites, which
played so prominent a part in the life of the next or Silurian period. There were already
representatives of crinoids and star-fishes, besides examples of the extinct group of cystideans.
Sea-worms crawled over the muddy and sandy sea-bottom, for they have left their trails and
burrows in the hardened sediments. Molluscs had by this time appeared in their four great
divisions of Brachiopods, Lamellibranchs, Gasteropods and Cephalopods, though the forms yet
discovered among Cambrian rocks are comparatively few. The most abundant and characteristic
inhabitants of the Cambrian seas were the trilobites, of which many genera have been disinterred
from the strata. In the lowest fossiliferous Cambrian group the trilobitic genus Olenellus, already
referred to, is the characteristic form. Higher up Paradoxides is predominant, while towards the
top of the system the most characteristic genus is Olenus.
From the organic remains which have been preserved, we may legitimately infer the existence of
others which have entirely disappeared. There seems no reason to doubt that the leading grades of
invertebrate life which are wanting in the known Cambrian fauna were really represented in the
Cambrian seas. The chance discovery of a band of limestone may any day entirely alter our
knowledge as to the relative proportions of the several divisions of the animal kingdom in the
earliest Palæozoic rocks. Sand is rather adverse to the preservation of a varied representation of the
organisms of the overlying sea-water. Mud is generally favourable, but calcareous accumulations
are greatly more so, and they usually consist almost entirely of organic remains. Thus in the
Cambrian series of the north-west of Scotland the quartzites that form the lower group, though
sometimes crowded with worm-burrows, contain hardly any other sign of organisms. The
overlying shales, besides their abundant worm-castings, have yielded perfect specimens of
Olenellus and other fossils. But in the uppermost group, consisting of limestones, every particle of
the sediment appears to have passed through the intestines of worms, and as it gathered on the sea-
bottom it enclosed and has preserved a varied and abundant assemblage of organisms, including
trilobites, gasteropods and a number of cephalopods. While in the Cambrian rocks of Europe
calcareous bands are comparatively rare, in those of North America they are not infrequent. Hence
it is largely from American deposits that our knowledge of the Cambrian fauna has been derived.
Not a vestige of any vertebrate organism has yet been detected among the earlier Palæozoic
sediments. So far as we know, there were no fishes in the Cambrian seas. The highest organisms
then existing were chambered shells, a once abundant and singularly varied class, of which the
living Nautilus is now the sole representative.
In trying to realize the general geographical conditions of Cambrian time, the geologist finds
himself entirely without any evidence as to the character of the terrestrial vegetation. We can
hardly doubt that the land was clothed with plants, probably including lycopods and ferns, possibly
even cycads and conifers. But no remains of this flora have yet been recovered. Nor have any
traces of land-animals been detected. All that we yet know of the life of the period has been
gleaned from marine sediments, which show that the invertebrate population by which the sea was
then tenanted embraced some of the leading types of structure that have survived through all the
long vista of geological time down to our own day.

Some of the shore-lines of the Cambrian waters may still be traced, and it is possible to say where
the land of the time stood and where lay the sea. In the British area the largest relic of Cambrian
land is found in the far north-west of Scotland. Formed partly of the Lewisian Gneiss and partly of
the Torridon Sandstone, it takes in the whole chain of the Outer Hebrides and likewise part of the
present western seaboard of Sutherland and Ross. Along the margin of that northern land the white
sand was laid down which now gleams in sheets of snow-like quartzite on most of the higher
mountains from Cape Wrath to Skye. The sea lay to the east and, so far as we know, may have
stretched across the rest of Scotland, and the north and centre of England. Another vestige of the
land of this ancient era occurs in Anglesey. There, and likewise over scattered tracts in the
Midlands, and in the south-west of England, the geologist seems to descry the last relics of islets
that rose out of the Cambrian sea, and are now surrounded with its hardened sediments.
While such was the general aspect of the region of the British Isles during Cambrian time, volcanic
action manifested itself at various localities over the area, breaking out on the sea-bottom, and
pouring forth sheets of lava and showers of ashes, which mingled with the sand and silt that were
settling there at the time. In the northern or Scottish tract no trace of this subterranean activity has
been found; but in the English Midlands and over much of Wales abundant evidence has been
obtained to show that in those districts the Cambrian period was marked by frequent and
prolonged eruptions.
As its name denotes, the Cambrian system is typically developed in Wales. It was there that
Sedgwick first worked out the stratigraphical relations of its ancient sediments, and that Murchison
demonstrated the succession of organic remains contained in them, applying to them the principles
of classification laid down by William Smith in regard to the Secondary formations. It was there
too that some of the earliest and most memorable achievements were made in the investigation of
ancient volcanic rocks. Sedgwick and Murchison, besides the admirable work which they
accomplished in establishing the stratigraphy of the older Palæozoic formations, clearly
recognized that among these formations there were preserved the records of contemporaneous
submarine eruptions. Sedgwick showed that the mountainous masses of eruptive rock in North
Wales were really lavas and ashes, which had been discharged over the sea-floor at the time when
the ancient sediments of that region were deposited, while Murchison established the same fact by
numerous observations in the east and south of Wales, and in the bordering English counties. De la
Beche had found similar evidence among the "grauwacke" rocks of Devonshire.[82]
[82] For early researches on the older Palæozoic volcanic rocks of Britain, see Sedgwick, Proc. Geol. Soc. vols.
ii. (1838) pp. 678, 679, iii. (1841) p. 548, iv. (1843) p. 215; Quart. Journ. Geol. Soc. vols. i. (1845) pp. 8-17,
iii. (1847) p. 134. Murchison, Proc. Geol. Soc. vol. ii. (1833-34) p. 85; Silurian System (1839) pp. 225, 258,
268, 287, 317, 324, 401; Siluria, 4th edit. (1867) p. 76 et seq. De la Beche, Mem. Geol. Survey, vol. i. (1846)
pp. 29-36. A. C. Ramsay in the Maps and Horizontal Sections of Wales published by the Geological Survey;
also Descriptive Catalogue of the Rock-Specimens in the Museum of Practical Geology, 1st edit. (1858), 2nd
edit. (1859), 3rd edit. (1862); "The Geology of North Wales," forming vol. iii. of Memoirs of the Geological
Survey, 1st edit. (1866), 2nd edit. (1881).

Following in the track thus opened up by these great masters, the officers of the Geological Survey
were enabled to unravel, as had never before been attempted, the complicated structure of the old
volcanic regions of Wales. At the outset of the following discussion I wish to express my
admiration of the labours of the early pioneers who thus laid for us the foundation of our
knowledge of volcanic action in the Palæozoic periods. To De la Beche and his associates in the
Survey a special measure of gratitude is due from all who have followed in their steps and profited
by their work. When we consider the condition of geological science, and especially of the
department of petrography, in this country at the time when these early and detailed investigations
were carried on, when we remember the imperfection of much of the topography on the old one-

inch Ordnance maps (which were the only maps then available), when we call to mind the rugged
and lofty nature of the ground where some of the most complicated geological structures are
displayed, we must admit that at the period when these maps and sections were produced they
could not have been better done; nay, that as in some important respects they were distinctly in
advance of their time, their publication marked an era in the progress of structural, and especially
of volcanic, geology. The separation of lavas and tuffs over hundreds of square miles in a
mountainous region, the discrimination of intrusive sheets and eruptive bosses, the determination
of successive stratigraphical zones of volcanic activity among some of the oldest fossiliferous
formations, were achievements which will ever place the names of Ramsay, Selwyn, Jukes and
their associates high in the bede-roll of geological science. No one ever thinks now of making a
geological excursion into Wales without carrying with him the sheets of the Geological Survey
map. These form his guide and handbook, and furnish him with the basis of information from
which he starts in his own researches.
But science does not stand still. The most perfect geological map that can be made to-day will be
capable of improvement thirty or forty years hence. The maps of the Geological Survey are no
exception to this rule. In criticizing and correcting them, however, let us judge them not by the
standard of knowledge which we have now reached, but by that of the time when they were
prepared. It is easy to criticize; it is not so easy to recognize how much we owe to the very work
which we pronounce to be imperfect.
The ancient volcanoes of Wales, thanks mainly to the admirable labours of my former friend and
chief, Sir Andrew C. Ramsay, have taken a familiar place in geological literature. But a good deal
has been learnt regarding them since he mapped and wrote. The volcanic history, as he viewed it,
began in the Arenig period. The progress of subsequent inquiry, however, has shown that there are
volcanic rocks in Wales of much older date. I shall show that the Cambrian period, both in South
and North Wales, was eminently volcanic.
Much controversy having arisen as to the respective limits and nomenclature of the older
Palæozoic rocks, let me state, at the outset of the inquiry into the volcanic eruptions of Cambrian
time, that under the term "Cambrian" I class all the known Palæozoic rocks which lie below the
bottom of what is termed the Arenig group. It was maintained by Sir Andrew Ramsay and his
colleagues on the Geological Survey that on the mainland of Wales no base is ever found to the
Cambrian system. More recently certain conglomerates have been fixed upon as the true Cambrian
base, both in South and North Wales, and endeavours have been made to trace an unconformability
at that line, all rocks below it being treated as pre-Cambrian. But conglomerates do not necessarily
mark a stratigraphical discordance, and in South Wales there is no trace of any unconformability
between the strata above and below the supposed line of break.[83] Professor Bonney has shown
that in North Wales several zones of conglomerate have been erroneously identified as the
supposed basal platform of the Cambrian series, and more recently Mr. Blake has pointed out that
some of these conglomerates are unquestionably Lower Silurian.
[83] See a discussion of this subject in Quart. Journ. Geol. Soc. vol. xxxix. (1883), p. 305.

My own examination so far confirms the conclusions arrived at by these observers. Like my
predecessors in the Geological Survey, however, I have been unable to detect anywhere in
Caernarvonshire or Merionethshire a base to the Cambrian system, and I am compelled to agree
with them in regarding as Cambrian (partly even as Lower Silurian) all the rocks from Bangor to
Llanllyfni, which have more recently been classed as pre-Cambrian. But though thus supporting
their general stratigraphy, I am bound to acknowledge that they failed to recognize the existence of
a great volcanic series below the Arenig horizon. The existence of this series, noticed by

Sedgwick, was first definitely stated by Professor Hughes,[84] and his statements have been
confirmed and extended by subsequent observers, notably by Professor Bonney and Mr. Blake.
The Cambrian period is thus proved to have been perhaps even more continuously volcanic than
the Lower Silurian period was in Wales.
[84] Proc. Camb. Phil. Soc. vol. iii. (1877), p. 89. The Cambrian volcanic areas of North Wales are represented
in Map II.

The following table shows the subdivisions of the Cambrian system now recognized in Britain:—

Wales. Western England. N.W. Scotland.
(Ranging up to 12,000 feet or more.) (About 3000 feet.) (About 2000 feet.)
Upper or Tremadoc Slates Shineton Shales Limestones, about 1500 feet
Olenus Lingula Flags (Dictyograptus or thick, divisible into seven
Zones. (Lingulella, Dictyonema, Olenus, groups (Archæocyathus,
Olenus, etc. etc.). Maclurea, Ophileta,
Middle or Conglomerates and Murchisonia, Orthoceras,
Menevian group
Paradoxides limestones (Comley), and vast quantities of annelid
(Paradoxides).
Zones. with Paradoxides, etc. castings).
Shales ("fucoid beds"), with
Harlech and Llanberis Thin quartzite passing up
Olenellus, Salterella, etc.
Lower or group with into green flags, grits,
Quartzites with annelid
Olenellus basement volcanic shales and sandstone
burrows. The base of the
Zones. rocks; bottom not (Comley Sandstone),
series lies unconformably on
seen. containing Olenellus.
pre-Cambrian rocks.

CHAPTER X
THE CAMBRIAN VOLCANOES OF SOUTH WALES

In the southern part of the Principality of Wales a remarkably varied display of British
Cambrian volcanic rocks has been preserved. The district around St. David's has the
distinction of being the first in which volcanic rocks of such high antiquity were
recognized. As far back as the year 1842, Ramsay found that "felspathic volcanic ash"
was associated with other proofs of igneous action, and this fact was recorded by him on
the published Horizontal Sections of the Geological Survey. Unfortunately he afterwards
came to regard the rocks as "altered Cambrian," thus following certain hypothetical views
which, as will be further alluded to in the sequel, he had adopted in explanation of the
phenomena in Caernarvonshire and in Anglesey. The volcanic nature of these ancient
materials was subsequently rediscovered by Dr. Hicks, who has devoted much time and
labour to their study. Distinguishing the volcanic series of St. David's by the name
"Pebidian," he has contended that it forms a pre-Cambrian system separated by an
unconformability from the base of the Cambrian formations. He likewise endeavoured to
show that an older system of rhyolitic lavas, felsitic breccias and hälleflintas could be
distinguished, which he termed "Arvonian"; and more ancient still, a core of granitoid or
gneissic rocks, which he separated under the name of "Dimetian." My own investigation
of the ground thoroughly convinced me that there are no pre-Cambrian rocks at St.
David's; that the "Arvonian" and "Dimetian" series are merely intrusive rocks (quartz-
porphyry, granite, etc.) which have invaded the volcanic series; and that the "Pebidian,"
instead of being a pre-Cambrian formation on which the Cambrian base rests
unconformably, is a group of volcanic rocks into which the Cambrian strata pass down
conformably, and which in the St. David's district constitutes the lowest group of the
Cambrian system.[85]
[85] For Dr. Hicks' views, see especially his papers in the Quart. Journ. Geol. Soc. vols. xxxi. xxxiii.
xxxiv. xl. My criticism of them will be found in op. cit. vol. xxxix. (1883), subsequently in the main
confirmed by Prof. Lloyd Morgan, op. cit. xlvi. p. 241. See also Prof. Blake, op. cit. xl. (1884). Dr.
Hicks in his more recent papers has merely reiterated his previously published opinions.

Click on image to view larger Walker & Boutall sc.
Fig. 40.—Map of the volcanic district of St. David's.

The volcanic geology of St. David's possesses a special interest inasmuch as it embraces a
tolerably full development of various features which characterize the volcanic groups of
later Palæozoic systems. Though the rocks are chiefly tuffs, they include also sheets of
lava, as well as sills, dykes and bosses. They show a remarkable range in chemical
composition from quite basic to highly acid materials. They present the amplest proofs of
having been erupted and spread out over the sea-bottom, and they likewise afford clear
evidence of alternation with the ordinary non-volcanic sediment of the time to which they
belong. In these respects they are particularly noteworthy, for they prove that in the
earliest Palæozoic ages the essential features of volcanic action were already as well
developed as in any subsequent epoch of geological history.
The volcanic group of St. David's attains a visible thickness of about 1800 feet. Its upper
part graduates upward into purple and green Lower Cambrian sandstones. The base of the
group is not seen owing to the plicated structure of the district. Hence the total thickness
of volcanic material cannot be determined, neither can we tell on what it rests, whether on
a still lower sedimentary series or on some platform of pre-Cambrian rocks.
The structure of the group, notwithstanding all that has been written about it, has never
yet been adequately worked out. The unfortunate and barren controversy about supposed
pre-Cambrian rocks at St. David's has tended to obscure the real importance of these
rocks as the oldest well-preserved record of volcanic action in Britain. They deserve to be
carefully surveyed on maps of a large scale, in the same detailed manner as has been so
successfully applied to the elucidation of younger volcanic tracts. Until such detailed
investigation is made, any account of them which is given can be little more than a
general outline of the subject. The following description is the result of my examination

of the ground in company with my colleague Mr. B. N. Peach, and afterwards with the
late Mr. W. Topley.[86] A few additional observations, from the subsequent exploration of
Professor Lloyd Morgan,[87] are incorporated in the narrative.
[86] Quart. Journ. Geol. Soc. vol. xxxix. (1883), p. 294 et seq. While the essential parts of the
investigation are given in the following pages, I would refer the reader to this paper for details not
transferred to the present volume.
[87] Op. cit. vol. xlvi. (1890), p. 241.

The geologist who traces these St. David's rocks in the field cannot fail to be struck with
their general resemblance to volcanic masses of later Palæozoic date. Many of the lavas
and tuffs are in outward characters quite indistinguishable from those of the Lower Old
Red Sandstone and Carboniferous systems of Britain. So many points of detail may be
observed to be common to the Palæozoic eruptive rocks all over the country from the
Cambrian to the Permian periods as to indicate that volcanic phenomena must have
recurred under much the same conditions throughout Palæozoic time.
By far the larger part of the Cambrian volcanic group of St. David's consists of bedded
tuffs, though a few lavas are interstratified in it, particularly towards the top. The whole
has subsequently been invaded by acid protrusions, and lastly by basic dykes.
1. Bedded Tuffs and Lavas.—The tuffs, which are the predominant members of the
volcanic group, present many varieties of colour, from dark purple, through tints of brick-
red and lilac, to pale pink, yellow and creamy white, but not unfrequently assume various
shades of dull green. They vary likewise in texture from somewhat coarse breccias or
agglomerates, through many gradations, into fine silky schists in which the tuffaceous
character is almost lost. Generally they are distinctly granular, presenting to the naked eye
abundant angular and subangular lapilli, among which broken crystals of a white,
somewhat kaolinized, felspar and fragments of fine-grained felsite are often conspicuous.
The greater part of the tuffs, particularly the purple, red and dark-green varieties, which
constitute so large a proportion of the whole, has been derived from the explosion of basic
rocks similar in character to the diabases now found associated with them. On the other
hand, the paler varieties, both in the form of fine tuffs and of breccias, have probably
resulted mainly from the destruction of more siliceous lavas, probably felsites (rhyolites)
or other acid rocks.
That many of the tuffs are due to the destruction of diabase-lavas may be surmised from
their close general external resemblance to these rocks, and from the way in which they
are associated with the contemporaneous sheets of diabase. Some of the dull dark-purple
tuffs might almost at first sight be mistaken for truly eruptive rocks. The analyses of two
typical examples of these basic tuffs (Nos. I. and II.), and one (No. III.) of an intermediate
variety containing an admixture of acid fragments, are given in the subjoined table.

H2O and
SiO2 Al2O3 Fe2O3 FeO MnO CaO MgO K2O Na2O Loss on Total. Specific
Gravity.
Ignition.
I. 51·25 20·41 3·02 3·91 0·21 4·53 7·22 2·93 1·82 5·02 100·32 2·84

II. 48·11 13·30 3·70 8·10 1·43 8·48 9·51 1·57 1·96 4·21 100·37 2·92
III. 61·54 16·30 4·40 3·66 0·32 3·08 2·99 1·62 2·81 2·99 99·71 ...

I. Purplish-red shaly tuff from below olivine-diabase, Crag Rhosson. Analysis by Mr. J.
S. Grant Wilson.
II. Dull purple and green tuff from the lowest group of tuffs between Pen-maen-melyn
and Pen-y-foel. Analysis by Mr. Wilson.
III. Greenish shaly finely granular tuff, from road-side, north of Board Schools, St.
David's. Analysis by Prof. A. Renard of Ghent.

Although the majority of the tuffs are more or less basic, they frequently contain evidence
in the form of small felsitic lapilli that acid lavas were present in the eruptive vents, while
the pale tuffs show that at the time of their discharge it was these acid lavas and not the
diabases that were blown out by the explosions. Appended are three analyses of the acid
tuffs (Nos. IV. V. and VI.).

H2O and
Specific
SiO2 Al2O3 Fe2O3 FeO MnO CaO MgO K2O Na2O Loss on Total.
Gravity.
Ignition.
IV. 80·59 11·29 0·28 1·41 trace 0·52 0·95 2·98 0·72 1·96 100·70 2·55
V. 73·42 12·09 0·91 3·13 0·25 2·94 1·12 1·67 3·88 1·28 100·69 2·74
VI. 72·63 16·23 2·70 0·48 ... 0·18 1·36 3·35 0·15 3·00 100·12 ...

IV. Greenish felsitic breccia, Clegyr Hill; angular fragments of various felsites in a
greenish base. Analysis by Mr. J. S. Grant Wilson.
V. Grey granular felsitic tuff, Bridge over Allan River north from St. David's Board
Schools. Analysis by Mr. Wilson.
VI. Pale pinkish-white, finely schistose tuff—a characteristic sample of the "Porth-lisky
schists." Analysis by Prof. Renard.

Many varieties of texture can be traced among the tuffs, from coarse breccias or
agglomerates, with blocks a yard or more in length, to fine schistose mudstones or
sericitic schists. One of the most remarkable of the finer kinds, found near Pen-y-foel, is
externally dirty-green, compact and tolerably homogeneous, but with distinct evidence of
its clastic character. Under the microscope it is found to be composed mainly of lapilli of
a peculiar rock, which is characterized by the abundance and freshness of its plagioclase
(an unusual feature in the volcanic group of St. David's); by the large, well-defined
crystals (one of which measured 0·022 inch by 0·0125 inch) of augite; by large crystals
replaced by green decomposition-products, but having the external form of olivine; by the
absence or scantiness of any base or groundmass; and, in many of the lapilli, by the
abundance of spherical cells, either empty or filled up as amygdales with decomposition-
products. These spherical vapour-vesicles, so characteristic of the basic or palagonitic
lapilli in many Palæozoic volcanic vents, were found in one fragment, where they were

particularly abundant, to range from a minimum of 0·0008 inch to a maximum of 0·0033
inch, with a mean of about 0·0018. The rock from which these lapilli have been derived
comes nearest to one of the diabases from the same part of the district (which will
afterwards be referred to), but shows a closer approach to basalt rocks.
Another interesting tuff is that of which the analysis (No. II.) has been given. It occurs not
far from the horizon of the rock just described. Under a low power, it is seen to be
composed mainly of fragments of diabase like the rocks of Rhosson and Clegyr Foig.
These fragments are subangular, or irregular in shape, and vary considerably in size. They
are sometimes finely cellular—the cavities, as in the case just referred to, being spherical.
The plagioclase crystals in the diabase-lapilli are everywhere conspicuous; so also is the
augite, which occurs in larger forms than in the rock of Rhosson or Clegyr Foig. Next in
abundance to these basic fragments are rounded or subangular pieces of felsite. These
weather out in conspicuous grey rough projections on the exposed face of the rock; under
the microscope they are seen to consist of fine granular felsite, which shows a
groundmass remaining dark between crossed nicols, but with luminous points and
filaments, and an occasional spherulite giving the usual cross in polarized light. Lapilli of
an older tuff may here and there be detected. A few angular and subangular grains of
quartz are scattered through the rock. The lapilli are bound together by a finely-granular
dirty-green substance.
As a typical illustration of the minute structure of the felsitic tuffs, I may refer to the rock
No. V. of the foregoing analyses. It is composed mainly of fragments of various felsites,
many of which show good flow-structure. Large, and usually broken, crystals of
orthoclase are dispersed among the other ingredients. Here and there a fragment of
diabase may be detected; but I could find no trace of pieces of the peculiar
microcrystalline spherulitic quartz-porphyries of St. David's. There is but little that could
be called matrix cementing the lapilli together. The presence of fragments of diabase may
possibly reduce the proportion of silica and increase that of magnesia, as compared with
what would otherwise have been present in the rock.
Some of the tuffs appear to have been a kind of volcanic mud. A specimen of this nature
collected from the road-side section, north of the Board School, presents a finely-granular
paste enclosing abundant angular and subangular lapilli of diabase, a smaller proportion
of felsite (sometimes displaying perfect flow-structure), broken plagioclase crystals, and a
greenish micaceous mineral which has been subsequently developed out of the matrix
between the lapilli.
Though they lie in the sedimentary series above the main volcanic group, I may refer here
to certain thin bands of tuff at Castell, on account of their interest in relation to the true
Cambrian age of the volcanic group. They are not quite so fresh as the tuff that occurs in
thicker masses, but their volcanic origin is readily observable. One band appears to be
made up of the debris of some basic rock, like the diabase of the district, through which
detached plagioclase crystals are scattered. The lapilli are subangular; and around their
border a granular deposit of hæmatite has taken place, giving a red colour to the rock.
Another band presents small angular lapilli, almost entirely composed of a substance

which to the naked eye, or with a lens, is dull, white and clay-like, easily scratched, and
slightly unctuous to the touch. Under the microscope, with a low power, it becomes pale
greyish-green and transparent, and is seen to consist in large part of altered felspar
crystals, partially kaolinized and partially changed into white mica and calcite. These
scattered crystals are true volcanic lapilli, and have not been derived from the mechanical
waste of any pre-existing volcanic rock. In the tuffs interstratified with the conglomerate,
at the quarry above Porth-clais, though much decomposed, crystals of plagioclase can
likewise still be traced. These strata are also true tuffs, and not mere detritus due to
mechanical degradation (see Fig. 41).
The general result of the study of the microscopic structure of the Cambrian tuffs of St.
David's may be briefly summed up as follows:—
1. These pyroclastic deposits are almost wholly composed of fragments of eruptive rocks,
sometimes rounded, but usually angular or subangular. In the more granular varieties very
little matrix is present; it consists of fine debris of the same materials. No detached
microlites have been noted, such as are common among modern volcanic ashes; but there
are abundant ejected crystals. In these respects the Cambrian tuffs resemble those of the
other Palæozoic systems. A mingling of grains of quartz-sand may indicate the
intermixture of ordinary with volcanic sediment.
2. They may be divided into two groups—one composed mainly of fragments of diabase
or other similar basic rocks, the other of felsite. The former group has doubtless been
derived from the explosion of such rocks as the diabase-sheets of the district. The felsitic
tuffs have not been observed to contain any fragments of the microcrystalline quartz-
porphyries of St. David's. They have been derived from true fine-grained felsites or
rhyolites. There are various intermediate varieties of tuff, due to the mingling in various
proportions of the two kinds of debris.
3. They are marked by the presence of some characteristic features of the volcanic vents
of later Palæozoic time, and in particular by presenting the following peculiarities: (a)
lapilli of a minutely-cellular pumice with spherical cells; (b) lapilli with well-developed
flow-structure; (c) lapilli consisting of a pale green serpentinous substance resembling
altered palagonite and probably originally glass; (d) lapilli derived from the destruction of
older tuffs; and (e) lapilli consisting of ejected crystals, especially of felspars, sometimes
entire, often broken.
4. They frequently show that they have undergone metamorphism, by the development of
a pale greenish micaceous mineral between the lapilli, the change advancing until the fine
tuffs occasionally pass into fine silky schists.
In my study of the St. David's district, I was unable to observe any evidence that the basic
and siliceous tuffs characterize two distinct periods of volcanicity. From the foregoing
analyses it appears that some of the oldest visible tuffs which are seen between Pen-maen-
melyn and Pen-y-foel contain only 48·11 per cent of silica; while a specimen from Porth-
lisky yielded 72·63 per cent of that ingredient. Specimens taken even from adjacent beds

show great differences in the percentage of silica, as may be seen in the analyses Nos. III.
and V.
This alternation of basic and siliceous fragmental materials has its parallel in the
neighbouring eruptive rocks, some of which are olivine-diabases containing only 45 per
cent of silica, while others are highly siliceous quartz-porphyries. But all the siliceous
eruptive rocks, so far as I have been able to discover, are intrusive, and belong, I believe,
to a later period than that of the volcanic group; in no single instance do they appear to me
to be true superficial lava-flows. On the other hand, the basic eruptive rocks occur both as
contemporaneous sheets and as intrusive masses. The presence of both siliceous and basic
lavas in the Cambrian volcanic reservoirs, however, is proved by the character of the tuffs.
It would appear from the evidence at present known, that while the basic lavas were most
abundant in the vents during the volcanic period recorded by the rocks of St. David's,
furnishing the material for most of the fragmental eruptions, and issuing in occasional
superficial streams of molten rock, the siliceous lavas did not flow forth at the surface,
though their debris was copiously discharged in the form of dust and lapilli.
The rise of both basic and acid lavas at different periods in the same or adjoining vents, so
familiar in recent volcanic phenomena, thus appears to have also characterized some of
the oldest examples of volcanic action. An interesting parallel may be traced between the
succession of events at St. David's and that which occurred in the volcanic group of the
Lower Old Red Sandstone of the Pentland Hills, near Edinburgh, of which a detailed
account will be given in Chapter xx. of this volume. It is also worthy of remark that in the
latest of the volcanic episodes in British geology a remarkable similarity to the St. David's
volcanic group may be observed. Some of the older Tertiary agglomerates are full of
pieces of acid rocks (felsites, rhyolites or granophyres), while the lavas poured out at the
surface were mainly basalts.
In the volcanic group of St. David's the tuffs contain evidence that ordinary sedimentation
was not entirely interrupted by the volcanic discharges. Thus, in the Allan valley, west
from the Cathedral, one of the schistose tuffs is full of well-rounded pebbles of white
quartz. Occasional shaly bands indicate the deposit of mud with the tuffs.
Excluding the granites and porphyries (which are described at p. 155), two kinds of
eruptive rocks are associated with the volcanic group. One of these is certainly intrusive
and of late date, viz. dykes and veins of diabase, to be afterwards referred to. The other
kind occurs in long parallel sheets, some of which, if not all, are true contemporaneous
lava-streams, erupted at intervals during the accumulation of the volcanic group. They
form prominent crags to the west of St. David's, such as Clegyr Foig, Rhosson, and the
rocky ground rising from the eastern shores of Ramsey Sound. Their dip and strike
coincide with those of the tuffs above and below them. It is possible that some of these
sheets may be intrusive sills intercalated along the bedding of the tuffs; and in one or two
cases I have observed indications of what, on further and more careful exploration, may
prove to be disruption across the bedding.
But it is the interbedded sheets that possess the chief interest as superficial lava-streams of
such venerable antiquity. They present many of the ordinary features of true lava-flows. In

particular a slaggy structure may be detected at the bottom of a sheet, the vesicles being
here and there lengthened in the direction of flow. Some of the sheets are in part
amygdaloidal. The alternation of these sheets with tuffs, evidently derived from lavas of
similar character, is another argument in favour of their contemporaneous date. One of the
best localities for studying these features lies between Clegyr Foig and the coast, west of
Rhosson.
The eruptive rocks thicken towards the south-west, as if the main vents had lain in that
direction. There are doubtless intrusive as well as contemporaneously interbedded masses
in the rough ground between Pen-maen-melyn and Treginnis. To separate these out would
be a most interesting and beautiful piece of mapping for any competent geologist in
possession of a good map on a sufficiently large scale.
The interbedded lavas, so far as I have had an opportunity of studying them, appear to
present remarkable uniformity of petrographical characters. Megascopically they are dull,
fine-grained to compact, sparingly porphyritic, ranging in colour from an epidote-green to
dull blackish-green and dark chocolate-brown. Some of them are finely porphyritic from
the presence of small glistening surfaces which present the colour and metallic lustre of
hæmatite and yield its characteristic streak. Obviously basic rocks, they present, as I have
said, a close external resemblance to many of the lavas of the Lower Old Red Sandstone
and Carboniferous districts of Scotland. From their chemical composition and
microscopic structure they may be most appropriately ranged among the diabases. The
analyses of two of the most conspicuous diabases of this class in the district, those of
Rhosson (VII.) and Clegyr Foig (VIII.), by Mr. J. S. Grant Wilson, are shown in the
following table:—

H2O
SiO2 Al2O3 Fe2O3 FeO MnO CaO MgO K2O Na2O and Insoluble Total. Specific
Loss on Residue. Gravity.
Ignition.
VII. 45·92 18·16 1·18 9·27 0·19 7·19 10·07 1·78 2·12 4·22 0·04 100·14 2·96
VIII. 45·38 16·62 4·06 8·63 0·14 8·19 9·41 0·71 2·20 4·34 0·08 99·76 2·99

The two rocks here analyzed, likewise that from the crag south of Castell and that from
the cliffs at the southern end of the promontory between Ramsey Sound and Pen-y-foel,
show under the microscope a general similarity of composition and structure. They
present a variable quantity of a base, which under a ⅕ objective is resolved into ill-
defined coalescent globulites and fibre-like bodies, that remain dark when rotated
between crossed nicols. In some varieties, as in part of Rhosson Crag, the base is nearly
lost in the crowd of crystalline constituents; in others, as in the crag south of Castell, it
forms a large part of the whole mass, and may be seen in distinct spaces free from any
crystalline particles. Through this base are diffused, in vast numbers, irregularly-shaped
grains of augite, seldom showing idiomorphic forms. These grains, or granules, may
perhaps average about 0·003 inch in diameter. Plagioclase is generally hardly to be
recognized, though here and there a crystal with characteristic twinning may be detected

in the base. Magnetite occurs abundantly—its minute octahedra, with their peculiar colour
and lustre, being apparent with reflected light on the fresher specimens, though apt to be
lost as diffused ferruginous blotches in the more decomposed varieties. But perhaps the
most remarkable ingredient is olivine. Red hæmatitic crystals are visible, even to the
naked eye, dispersed through the groundmass of the rocks. With a lens these may be
observed to be orthorhombic in form, and to be evidently pseudomorphs after some
mineral which has been converted chiefly into hæmatite. Such red pseudomorphs are
common in Carboniferous and Old Red Sandstone lavas, where in some cases they appear
to be after hornblende, and in others after augite, but occasionally are suggestive of
olivine, though with no trace of the original substance of that mineral. In the lava
associated with the tuffs at the south end of the promontory between Ramsey Sound and
Pen-y-foel, however, certain large, well-developed pseudomorphs are undoubtedly after
olivine. They have the characteristic contour of that mineral and its peculiar transverse
curved and irregular fractures. The average length of these pseudomorphs was found,
from the measurement of six examples, to be 0·023 inch, the largest being 0·034, and the
smallest 0·014. Seen by transmitted light they present a structureless pale-green material
nearly inert in polarized light, round the borders and across fissures in which an opaque
substance has been developed, as serpentine and magnetite have been grouped in the
familiar alteration of olivine. The opaque material is bright brick-red in reflected light,
and is evidently now chiefly oxidized into hæmatite. Every stage may be traced, from
orthorhombic forms with the incipient development of transverse fissures filled with iron-
oxide, to others of distorted shapes in which the ferruginous matter occupies the whole, or
nearly the whole, of the mould of the original crystal.
The rocks now described differ from the Palæozoic andesites or "porphyrites," with which
I am acquainted, in their more basic composition, in the less abundance of their
microscopic base, in the comparatively inconspicuous development of their felspars of
later consolidation, in the absence of large porphyritic felspars of earlier consolidation, in
the extraordinary prominence of the granular augite, and in the presence of olivine. In
composition and structure they are essentially forms of olivine-diabase.

Fig. 41.—Section showing
the interstratification of
tuff and conglomerate
above Lower Mill, St.
David's.

Above the volcanic group of St. David's lies a band of quartz-conglomerate which has
been taken by Dr. Hicks to mark the base of the Cambrian system. This rock, though

mainly composed of quartz and quartzite, contains fragments of the underlying volcanic
rocks. But that it does not mark any decided break in the sedimentation, much less the
violent unconformability and vast interval of time which it has been erroneously supposed
to do, is well illustrated by the occurrence of bands of tuff, as well as diffused volcanic
dust, in the conglomerate and also in the green and red shales and sandstones which
conformably overlie it. An example of this intercalation of volcanic material is
represented in Fig. 41. On the left side vertical layers of fine reddish tuff (a) are
succeeded by a band of quartz conglomerate (b) of the usual character. Parallel to this
conglomerate comes a band, about six inches thick, of fine tuff (c), followed by ashy
sandstone (d), which graduates into conglomerate (e). No more complete evidence could
be desired of the perfect inosculation of the conglomerate with the volcanic group. On the
coast at Nun's Chapel similar evidence presents itself. The conglomerate there contains
some thin seams of tuff, and is intercalated between a series of dull green agglomerates
and tuffs and some greenish shales and sandstones with layers of tuff.
Not less striking is the evidence of the contemporaneous eruption of fine volcanic dust in
the overlying shales and sandstones.[88] Some of the red shales are full of this material,
which here and there is gathered into the thin seams or ribs of which the microscopic
characters have already been described. This diffused volcanic detritus marks, no doubt,
the enfeebled discharges of fine dust towards the close of the volcanic episode in the
Lower Cambrian period at St. David's. It would be difficult to find an instance of a more
perfect transition from a series of thoroughly volcanic masses into a series of ordinary
mechanical sediments.
[88] These are a portion of Dr. Hicks' "Caerfai group" in the Lower Cambrian series. They have
yielded Lower Cambrian fossils.

2. Intrusive Granite and Quartz-Porphyry.—The heart of the volcanic group is pierced by
a mass of granite which also cuts the conglomerate and overlying shales and sandstones
on the east side. The age of this intrusive boss cannot be more definitely fixed than by
saying that it must be later than the volcanic group. This rock has been the subject of a
remarkable amount of description, and has been dignified by being actually elevated into
a distinct Archæan "formation" composed of "highly crystalline gneissic rocks," with
"bands of limestone, hornblende, chlorite, and micaceous schists."[89] Into this somewhat
dreary chapter of English geological literature it is fortunately not necessary to enter here.
I will only say that the rock is unquestionably a granite, with no essential differences from
many other bosses regarding which there has been no controversy. It is a holocrystalline
rock with a thoroughly granitic texture, and composed of the ordinary minerals of granite
—quartz, orthoclase and plagioclase, among which a green chloritic mineral, more or less
plentiful, probably represents original hornblende, biotite or augite. Sometimes the quartz
and felspar in the body of the rock show a micropegmatitic arrangement, and the same
structure occurs in veins that traverse it. This structure gives the rock some resemblance
to the Tertiary granites and granophyres of Ireland and Scotland.
[89] See the papers cited on p. 145 and my discussion of the relations of this granite in Quart. Journ.
Geol. Soc. vol. xxxix.; also Prof. Lloyd Morgan, op. cit. vol. xlvi. (1890).

This granite has undergone a good deal of decomposition, for its felspars are turbid, and
its original ferro-magnesian constituents are always replaced by green chloritic
aggregates, while epidote is also present. The rock tends to become finer in grain towards
the margin, and then sometimes assumes a more decidedly pegmatitic structure, like
graphic granite. At the northern end of the granite ridge, a gradation can be traced from
the ordinary texture through increasingly fine-grained varieties until we pass into a
microcrystalline spherulitic porphyry. After a careful examination of the ground I satisfied
myself that the spherulitic quartz-porphyries, which form a conspicuous feature in the
geology of St. David's, are really offshoots from this granitic core.[90]
[90] These apophyses from the granite constitute the "Arvonian" formation of Dr. Hicks' pre-
Cambrian series of St. David's.

These spherulitic rocks have been fully described.[91] They consist of a base composed of
a microcrystalline aggregate of quartz and orthoclase, which is distributed between the
spherulites. These have been developed in remarkable beauty and perfection. While the
microcrystalline structure is everywhere recognizable, the spherulites occasionally
disappear. But their absence is merely local, and they may be found both in large dykes
and narrow veins. A further porphyritic structure is given to the rocks by the presence in
them of abundant quartz, which takes the form of conspicuous rounded blebs or worn
crystals sometimes distinctly dihexihedral, but with somewhat blunted angles. Porphyritic
plagioclase is also common. Flow-structure is occasionally traceable. Some parts of these
rocks where the porphyritic elements are locally absent might be cursorily classed as
felsites; but they all possess a microcrystalline and not a felsitic base. They cannot be
confounded with the true felsites of which fragments occur in the tuffs.
[91] See, for example, J. Davies, Quart. Journ. Geol. Soc. vol. xxiv. p. 164, xxxv. p. 203; also the
paper already referred to, op. cit. xxxix. p. 315; and Mr. Teall's British Petrography, p. 334.

In addition to the parallelism that may be traced between the earliest Palæozoic
agglomerates and those of the youngest volcanic series of Britain, a close analogy may
also be noticed between the acid intrusive rocks of the two widely-separated periods. In
both cases we have a granitic core sending out apophyses which assume a spherulitic
structure and traverse earlier volcanic products of the district.
These spherulitic quartz-porphyries of St. David's occur as bosses, dykes (elvans) or
veins, cutting through all horizons of the volcanic group, and in one case apparently, if not
actually, reaching the quartz conglomerate. One of the best exposures of this intrusive
character may be seen in the cliff below Nun's Chapel, where the elvan runs along the
face of the cliff through the uppermost zone of the volcanic group, cutting the strata
somewhat irregularly. Apparently in connection with this dyke, a network of intrusions of
decomposed quartz-porphyry may be observed in the shales along the face of the cliff
immediately below Nun's Chapel. On the whole, the intruded material has forced its way
along the bedding-planes of the shales, but has also broken across them, sending out
finger-like branches.
3. Diabase Dykes and Sills.—The latest rocks of the St. David's district are dykes and
intrusive sheets of diabase, which traverse all the other formations. The dykes are

specially abundant in the granite. One or two may be detected in almost every artificial
opening which has been made in that rock; while on the coast-section they are here and
there profusely abundant. They are likewise frequent in the quartz-porphyries, and occur
also in the volcanic group as well as in the sandstones and shales above the conglomerate,
but become fewer in number as they recede from the granite centre.[92]
[92] The occurrence of these dykes is paralleled by that of the similar intrusions in the quartz-felsite
of Llyn Padarn to be afterwards described.

In external characters, the rock composing these dykes and sheets may be described as
usually a dull dirty-green or yellowish-brown mass, to which the old name of "wacke"
might appropriately be given. It exhibits the texture and mode of weathering of the more
distinctly crystalline members of the basalt family. It is occasionally amygdaloidal or
cellular, the kernels or cavities being arranged parallel with the sides of the dyke. Here
and there a rudely prismatic structure extends between the walls.
The microscopic structure of this rock has been described by Professor Judd, Mr. Davies
and Mr. Tawney. It is a diabase, but more allied in structure to true basalt than the olivine-
diabase of the volcanic group. It especially differs from the older rock in the abundance
and freshness of its felspars, in the comparative scarcity of its augite, and in the absence
of olivine. The magnesian silicates are very generally replaced by green decomposition-
products diffused through the mass. An occasional crystal of hornblende, recognizable by
its cleavages and dichroism, may be detected. Some of the diabase dykes present excellent
examples of flow-structure. A beautiful instance occurs in a dyke that cuts the shales, in a
small cove to the east of Nun's Chapel. The shale and eruptive rock are in contact; and the
small acicular prisms of felspar, besides ranging themselves in line parallel to the side of
the dyke, stream round the larger felspar crystals.
Some of the dykes or veins are only three inches broad. They send out fingers, and
sometimes break abruptly across from one line to another. They appear generally to have
followed the lines of joint in the granite, as Mr. Tawney has observed;[93] consequently
they must be posterior to the development of the system of jointing in that rock.
[93] Proc. Nat. Hist. Soc. Bristol, vol. ii. part ii. (1879), p. 115.

Besides the abundant dykes, there has been a more limited extrusion of the same material
in sheets parallel (or approximately so) to the bedding of the sandstones and shales. These
sheets are well displayed at St. John's Point, where evidence of their being intrusive, and
not truly bedded, may be seen along the fine cliffs which have been cut by the waves on
this part of the coast-line.
The sedimentary series which overlies the volcanic group of St. David's, and contains the
fossils of the lower part of the Cambrian system, gradually loses all trace of volcanic
material, as its members are followed upward in stratigraphical order.[94] We thus learn
that the eruptions of this district came to an end in an early part of the Cambrian period.
But as we shall see in the following pages, volcanic activity was subsequently renewed at
no great distance in the next or Silurian period.

[94] Dr. Hicks has noted the occurrence of "volcanic tuff" in the Lower Lingula Flags of Porth-y-
Rhaw, a little to the east of St. David's (Quart. Journ. Geol. Soc. vol. xx. 1864, p. 240). This
intercalation is marked as a "dyke" in the MS. notes of Sir A. C. Ramsay on a copy of the Geological
Survey map of the district.

CHAPTER XI
THE CAMBRIAN VOLCANOES OF NORTH WALES, THE
MALVERN HILLS AND WARWICKSHIRE

NORTH WALES
The Cambrian volcanic rocks in the northern part of the Welsh Principality
have their main development in Caernarvonshire. Southwards from that
tract, though the Lower Cambrian strata form a vast pile of sedimentary
material in the Harlech anticline, which is estimated by the Geological
Survey to be from 6000 to 7000 feet thick, they have yielded no trace of
any contemporaneous volcanic rocks.[95] The purple slates that rise along
the centre of the anticline dip below the grits and conglomerates on either
side without disclosing a glimpse of the base of the system. This enormous
accumulation of sedimentary deposits seems to diminish in thickness as it is
traced northwards, for towards the Menai Strait it does not reach more than
a fourth part of the depth which it is said to display in the Harlech anticline.
[96]
In the Pass of Llanberis the series of grits that overlies the purple slates
is estimated to be about 1300 feet thick.[97] This gradual thinning away of
the Cambrian series towards the north was, in the opinion of Sir Andrew
Ramsay, accompanied by an increasing metamorphism of the lower
portions of the system. In his view, the long ridge of quartz-porphyry which
crosses the lower end of Llyn Padarn represents the result of the extreme
alteration of the stratified rocks. He believed that he could trace an
insensible passage from the slates, grits and conglomerates into the
porphyry, and he was led to the "conviction that the solid porphyry itself is
nothing but the result of the alteration of the stratified masses carried a
stage further than the stage of porcellanite, into the condition of that kind of
absolute fusion that in many other regions seems to have resulted in the
formation of granites, syenites and other rocks, commonly called intrusive."
[98]
Certain structural lines in the quartz-porphyry he looked upon as
indicating "traces of stratification in a rock, the original felspathic and

quartzose material of which has been metamorphosed into true porphyry."
[99]
In conformity with these ideas, the remarkable felspathic strata which
lie nearest the porphyry were regarded as metamorphosed Cambrian rocks,
and where similar rocks reappear over a large area near Bangor they were
coloured on the map with the same tint and lettering as were used for the
so-called "altered Cambrian" of Anglesey.
[95] Mem. Geol. Surv. vol. iii. 2nd edit. "Geology of North Wales," p. 21. It is
possible that this thickness has been somewhat overestimated. Dr. Hicks (Geol. Mag.
1880, p. 519) has referred to certain "highly felsitic rocks, for the most part a
metamorphic series of schists, alternating with harder felsitic bands, probably
originally felsitic ashes," lying at the bottom of the whole pile, and he has claimed
them as pre-Cambrian. But I have not found any evidence of such rocks, nor any
trace of igneous materials save dykes and sills, acid and basic, such as are indicated
on the Survey map.
[96] Ibid. p. 24.

[97] Ibid. p. 173.

[98] Mem. Geol. Surv. vol. iii. 2nd edit. p. 173.

[99] Ibid. p. 174.

No one who has examined this Caernarvonshire ground can have failed to
find the sections which doubtless led my predecessor to form the
convictions to which he gave expression in the passages I have just quoted.
It is easy to see how these sections, wherein it is certainly difficult to draw a
sharp line between the igneous rock and the clastic materials derived from
it, would be welcomed as appearing to offer confirmation of the ideas
concerning metamorphism which were then in vogue. There cannot,
however, be any doubt that my friend was mistaken in his interpretation of
the structure of that part of the country. It is to me a subject of keen regret
that in his later years, when the subject was revived, he was no longer able
to re-examine this ground himself, for no one would have confessed more
frankly his error, and done more ample justice to those who, coming after
him, have been able in some parts to correct his work.
The quartz-porphyry, felsite or rhyolite of Llyn Padarn, as well as that of
Llandeiniolen, is not a metamorphic but an eruptive rock, as has been
demonstrated by Professors Hughes and Bonney. There is no true passage
of the sedimentary rocks into it; on the contrary, the conglomerates which
abut against it are in great part made out of its fragments, so that it was
already in existence before these Cambrian strata were deposited upon it.

These conclusions must be regarded as wholly indisputable. But most of the
critics of the work of the Geological Survey have proceeded to certain
further deductions. They have maintained that the presence of fragments of
the porphyry in the overlying conglomerate marks an unconformability
between the two rocks, that the conglomerate shows the base of the
Cambrian system, and that the porphyry is therefore pre-Cambrian.
These assertions and inferences do not seem to me to be warranted. They
have, in my judgment, been disproved by Mr. Blake,[100] who shows that
there is no break in the Cambrian series, that the various porphyries and
their accompaniments are parts of that series, and that there is no certain
proof of the existence of any pre-Cambrian rocks in the whole district.[101]
[100] In an excellent memoir read before the Geological Society in 1888, with the
main conclusions of which I agree.
[101] Quart. Journ. Geol. Soc. vol. xliv. p. 271. For subsequent papers by Mr. Blake,
see op. cit. vols. xlviii. (1892) p. 243, xlix. (1893) p. 441.

That the igneous rocks of the Llyn Padarn area mark a volcanic period has
been recognized by most writers since Professor Bonney pointed out the
flow-structure of the quartz-porphyry, and other proofs of active volcanic
eruptions have been traced by him, as well as by Professor Hughes and Mr.
Blake, in the stratified rocks which stretch north-eastwards to Bangor. The
extent and persistence of these ancient volcanic phenomena, and their
probable connection with the remarkable northward attenuation of the
Cambrian sedimentary rocks, deserve special attention.
It is generally agreed that the rocks variously termed quartz-porphyries,
felsites or rhyolites form the oldest members of this volcanic series.[102]
They come to the surface in two long ridges, one running from Caernarvon
to near Bangor, the other from Llanllyfni to Ann's Chapel, at the mouth of
Nant Francon (Map. II.). Whether the materials of these two ridges are parts
of one originally continuous sheet or group of sheets, or, if different
protrusions, whether they belong to the same geological horizon, or
whether, as Mr. Blake believes, they are distinct masses, separated by a
considerable thickness of detrital material, cannot in the present state of our
knowledge be positively decided. It seems to me probable that they are
connected underground, as a continuous platform beneath the overlying
pyroclastic materials.