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Title: Earth Features and Their Meaning

Author: William Herbert Hobbs

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EARTH FEATURES AND THEIR MEANING

THE MACMILLAN COMPANY
NEW YORK · BOSTON · CHICAGO DALLAS · SAN FRANCISCO

MACMILLAN & CO., Limited
LONDON · BOMBAY · CALCUTTA MELBOURNE

THE MACMILLAN CO. OF CANADA, Ltd.
TORONTO

Plate 1.

Mount Balfour and the Balfour Glacier in the Selkirks.

EARTH FEATURES AND
THEIR MEANING

AN INTRODUCTION TO GEOLOGY

FOR THE STUDENT AND THE GENERAL
READER

BY
WILLIAM HERBERT HOBBS
PROFESSOR OF GEOLOGY IN THE UNIVERSITY OF MICHIGAN
AUTHOR OF “EARTHQUAKES. AN INTRODUCTION TO
SEISMIC GEOLOGY”; “CHARACTERISTICS OF
EXISTING GLACIERS”; ETC.

New York THE MACMILLAN COMPANY 1921

All rights reserved

Copyright, 1912, By THE MACMILLAN COMPANY.

Norwood Press J. S. Cushing Co.—Berwick & Smith Co. Norwood, Mass., U.S.A.

TO THE MEMORY
OF
GEORGE HUNTINGTON WILLIAMS

PREFACE
The series of readings contained in the present volume give in
somewhat expanded form the substance of a course of illustrated lectures
which has now for several years been delivered each semester at the
University of Michigan. The keynote of the course may be found in the
dominant characteristics of the different earth features and the geological
processes which have been betrayed in the shaping of them. Such a
geological examination of landscape is replete with fascinating revelations,
and it lends to the study of Nature a deep meaning which cannot but
enhance the enjoyment of her varied aspects.
That there is a real place for such a cultural study of geology within the
University is believed to be shown by the increasing number of students
who have elected the work. Even more than in former years the American
travels afar by car or steamship, and the earth’s surface features in all their
manifold diversity are thus one after the other unrolled before him. The
thousands who each year cross the Atlantic to roam over European
countries may by historical, literary, or artistic studies prepare themselves to
derive an exquisite pleasure as they visit places identified with past
achievement of one form or another. Yet the Channel coast, the gorge of the
Rhine, the glaciers of Switzerland, and the wild scenery of Norway or
Scotland have each their fascinating story to tell of a history far more
remote and varied. To read this history, the runic characters in which it is
written must first of all be mastered; for in every landscape there are strong
individual lines of character such as the pen artist would skillfully extract
for an outline sketch. Such character profiles are often many times repeated
in each landscape, and in them we have a key to the historical record.
An object of the present readings has thus been to enable the student to
himself pick out in each landscape these more significant lines and so read
directly from Nature. In the landscapes which have been represented, the

aim has been to draw as far as possible upon localities well known to
travelers and likely to be visited, either because of their historical interest or
their purely scenic attractions. It should thus be possible for a tourist in
America or Europe to pursue his landscape studies whenever he sets out
upon his travels. The better to aid him in this endeavor, some suggestions
concerning the itinerary of journeys have been supplied in an appendix.
Regarded as a textbook of geology, the present work offers some
departures from existing examples. Though it has been customary to
combine in a single text historical with dynamical and structural geology, a
tendency has already become apparent to treat the historical division apart
from the others. Again, a desire to treat the science of geology
comprehensively has led some authors into including so many subjects as to
render their texts unnecessarily encyclopedic and correspondingly
uninteresting to the general reader. It is the author’s belief that there is a real
need for a book which may be read intelligently by the general public, and
it must be recognized that the beginner in the subject cannot cover the entire
field by a single course of readings. The present work has, therefore, been
prepared with a view to selecting for study those dominant geological
processes which are best illustrated by features in northern North America
and Europe. It is this desire to illustrate the readings by travels afield, which
accounts for the prominence given to the subject of glaciation; for the larger
number of colleges and universities in both America and Europe are
surrounded by the heavy accumulations that have resulted from former
glaciations.
Emphasis has also been placed upon the dependence of the dominant
geological processes of any region upon existing climatic conditions, a fact
to which too little attention has generally been given. This explains the
rather full treatment of desert regions, of which, in our own country
particularly, much may be illustrated upon the transcontinental railway
journeys.
More than in most texts the attempt has here been made to teach directly
through the eye with the efficient aid of apt illustrations intimately
interwoven with the text. For such success as has been reached in this
endeavor, the author is greatly indebted to two students of the University of
Michigan,—Mr. James H. Meier, who has prepared the line drawings of
landscapes, and Mr. Hugh M. Pierce, who has draughted the diagrams.
Though credit has in most cases been given where illustrations have been

made from another’s photographs, yet especial mention should here be
made of the debt to Dr. H. W. Fairbanks of Berkeley, California, whose
beautiful and instructive photographs are reproduced upon many a page.
As given at the University of Michigan, the lectures reflected in the
present volume are supplemented by excursions and by so much laboratory
practice as is necessary to become familiar with the more common minerals
and rocks, and to read intelligently the usual topographical and geological
maps. In the appendices the means for carrying out such studies, in part
with newly devised apparatus, have been indicated.
The scope of the book precludes the possibility of furnishing the reader
with the sources for the body of fact and theory which is presented,
although much may be inferred from the names which appear beneath the
illustrations, and more definite knowledge will be found in the references to
literature supplied at the ends of chapters. A large amount of original and
unpublished material is for a similar reason unlabeled, and it has been left
for the professional geologist to detect these new strands which have been
drawn into the web.
WILLIAM HERBERT HOBBS.

Ann Arbor, Michigan,
October 25, 1911.

CONTENTS

CHAPTER I
The Compilation of Earth History
PAGE
The sources of the history—Subdivisions of geology—The study of earth features and
their significance—Tabular recapitulation—Geological processes not universal—
Change, and not stability, the order of nature—Observational geology versus
speculative philosophy—The scientific attitude and temper—The value of the
hypothesis—Heading references 1

CHAPTER II
The Figure of the Earth
The lithosphere and its envelopes—The evolution of ideas concerning the earth’s figure
—The oblateness of the earth—The arrangement of oceans and continents—The
figure toward which the earth is tending—Astronomical versus geodetic
observations—Changes of figure during contraction of a spherical body—The
earlier figures of the earth—The continents and oceans at the close of the Paleozoic
era—The flooded portions of the present continents—The floors of the hydrosphere
and atmosphere—Reading references 8

CHAPTER III
The Nature of the Materials in the Lithosphere

The rigid quality of our planet—Probable composition of the earth’s core—The earth a
magnet—The chemical constitution of the earth’s surface shell—The essential
nature of crystals—The lithosphere a complex of interlocking crystals—Some
properties of natural crystals, minerals—The alterations of minerals—Reading
references 20

CHAPTER IV
The Rocks of the Earth’s Surface Shell
The processes by which rocks are formed—The marks of origin—The metamorphic
rocks—Characteristic textures of the igneous rocks—The classification of rocks—
Subdivisions of the sedimentary rocks—The different deposits of ocean, lake, and
river—Special marks of littoral deposits—The order of deposition during a
transgression of the sea—The basins of deposition of earlier ages—The deposits of
the deep sea—Reading references 30

CHAPTER V
Contortions of the Strata within the Zone of Flow
The zones of fracture and flow—Experiments which illustrate the fracture and flow of
solid bodies—The arches and troughs of the folded strata—The elements of folds
—The shapes of rock folds—The overthrust fold—Restoration of mutilated folds—
The geological map and section—Measurement of the thickness of formations—
The detection of plunging folds—The meaning of an unconformity—Reading
references 40

CHAPTER VI
The Architecture of the Fractured Superstructure
The system of the fractures—The space intervals of joints—The displacements upon
joints: faults—Methods of detecting faults—The base of the geological map—The
field map and the areal geological map—Laboratory models for study of geological
maps—The method of preparing the map—Fold vs. fault topography—Reading
references 55

CHAPTER VII
The Interrupted Character of Earth Movements:
Earthquakes and Seaquakes

Nature of earthquake shocks—Seaquakes and seismic sea waves—The grander and the
lesser earth movements—Changes in the earth’s surface during earthquakes: faults
and fissures—The measure of displacement—Contraction of the earth’s surface
during earthquakes—The plan of an earthquake fault—The block movements of
the disturbed district—The earth blocks adjusted during the Alaskan earthquake of
1899 67

CHAPTER VIII
The Interrupted Character of Earth Movements:
Earthquakes and Seaquakes (concluded)
Experimental demonstration of earth movements—Derangement of water flow by earth
movement—Sand or mud cones and craterlets—The earth’s zones of heavy
earthquake—The special lines of heavy shock—Seismotectonic lines—The heavy
shocks above loose foundations—Construction in earthquake regions—Reading
references 81

CHAPTER IX
The Rise of Molten Rock to the Earth’s Surface; Volcanic
Mountains of Exudation
Prevalent misconceptions about volcanoes—Early views concerning volcanic mountains
—The birth of volcanoes—Active and extinct volcanoes—The earth’s volcano
belts—Arrangement of volcanic vents along fissures, and especially at their
intersections—The so-called fissure eruptions—The composition and the properties
of lava—The three main types of volcanic mountain—The lava dome—The
basaltic lava domes of Hawaii—Lava movements within the caldron of Kilauea—
The draining of the lava caldrons—The outflow of the lava floods 94

CHAPTER X
The Rise of Molten Rock to the Earth’s Surface; Volcanic
Mountains of Ejected Materials

The mechanics of crater explosions—Grander volcanic eruptions of cinder cones—The
eruption of Volcano in 1888—The eruption of Taal volcano on January 30, 1911—
The materials and the structure of cinder cones—The profile lines of cinder cones
—The composite cone—The caldera of composite cones—The eruption of
Vesuvius in 1906—The sequence of events within the chimney—The spine of Pelé
—The aftermath of mud flows—The dissection of volcanoes—The formation of
lava reservoirs—Character profiles—Reading references 115

CHAPTER XI
The Attack of the Weather
The two contrasted processes of weathering—The rôle of the percolating water—
Mechanical results of decomposition: spheroidal weathering—Exfoliation or
scaling—Dome structure in granite masses—The prying work of frost—Talus—
Soil flow in the continued presence of thaw water—The splitting wedges of roots
and trees—The rock mantle and its shield in the mat of vegetation—Reading
references 149

CHAPTER XII
The Life Histories of Rivers
The intricate pattern of river etchings—The motive power of rivers—Old land and new
land—The earlier aspects of rivers—The meshes of the river network—The upper
and lower reaches of a river contrasted—The balance between degradation and
aggradation—The accordance of tributary valleys—The grading of the flood plain
—The cycles of stream meanders—The cut-off of the meander—Meander scars—
River terraces—The delta of the river—The levee—The sections of delta deposits 158

CHAPTER XIII
Earth Features shaped by Running Water
The newly incised upland and its sharp salients—The stage of adolescence—The
maturely dissected upland—The Hogarthian line of beauty—The final product of
river sculpture: the peneplain—The river cross sections of successive stages—The
entrenchment of meanders with renewed uplift—The valley of the rejuvenated river
—The arrest of stream erosion by the more resistant rocks—The capture of one
river by another—Water and wind gaps—Character profiles—Reading references 169

CHAPTER XIV

The Travels of the Underground Water
The descent within the unsaturated zone—The trunk channels of descending water—
The caverns of limestones—Swallow holes and limestone sinks—The sinter
deposits—The growth of stalactites—Formation of stalagmites—The Karst and its
features—A desert from the destruction of forests—The ponore and the polje—The
return of the water to the surface—Artesian wells—Hot springs and geysers—The
deposition of siliceous sinter by plant growth—Reading references 180

CHAPTER XV
Sun and Wind in the Lands of Infrequent Rains
The law of the desert—The self-registering gauge of past climates—Some
characteristics of the desert waste—Dry weathering: the red and brown desert
varnish—The mechanical breakdown of the desert rocks—The natural sand blast—
The dust carried out of the desert 197

CHAPTER XVI
The Features in Desert Landscapes
The wandering dunes—The forms of dunes—The cloudburst in the desert—The zone of
the dwindling river—Erosion in and about the desert—Characteristic features of
the arid lands—The war of dune and oasis—The origin of the high plains which
front the Rocky Mountains—Character profiles—Reading references 209

CHAPTER XVII
Repeating Patterns in the Earth Relief
The weathering processes under control of the fracture system—The fracture control of
the drainage lines—The repeating pattern in drainage networks—The dividing lines
of the relief patterns: lineaments—The composite repeating patterns of the higher
orders—Reading references 223

CHAPTER XVIII
The Forms carved and molded by Waves
The motion of a water wave—Free waves and breakers—Effect of the breaking wave 231
upon a steep, rocky shore: the notched cliff—Coves, sea arches, and stacks—The
cut rock terrace—The cut and built terrace on a steep shore of loose materials—

The work of the shore current—The sand beach—The shingle beach—Bar, spit,
and barrier—The land-tied island—A barrier series—Character profiles—Reading
references

CHAPTER XIX
Coast Records of the Rise or Fall of the Land
The characters in which the record has been preserved—Even coast line the mark of
uplift—A ragged coast line the mark of subsidence—Slow uplift of the coasts; the
coastal plain and cuesta—The sudden uplifts of the coast—The upraised cliff—The
uplifted barrier beach—Coast terraces—The sunk or embayed coast—Submerged
river channels—Records of an oscillation of movement—Simultaneous contrary
movements upon a coast—The contrasted islands of San Clemente and Santa
Catalina—The Blue Grotto of Capri—Character profiles—Reading references 245

CHAPTER XX
The Glaciers of Mountain and Continent
Conditions essential to glaciation—The snow-line—Importance of mountain barriers in
initiating glaciers—Sensitiveness of glaciers to temperature changes—The cycle of
glaciation—The advancing hemicycle—Continental and mountain glaciers
contrasted—The nourishment of glaciers—The upper and lower cloud zones of the
atmosphere 261

CHAPTER XXI
The Continental Glaciers of Polar Regions
The inland ice of Greenland—The mountain rampart and its portals—The marginal rock
islands—Rock fragments which travel with the ice—The grinding mill beneath the
ice—The lifting of the grinding tools and their incorporation within the ice—
Melting upon the glacier margins in Greenland—The marginal moraines—The
outwash plain or apron—The continental glacier of Antarctica—Nourishment of
continental glaciers—The glacier broom—Field and pack ice—The drift of the
pack—The Antarctic shelf ice—Icebergs and snowbergs and the manner of their
birth—Reading references 271

CHAPTER XXII
The Continental Glaciers of the “Ice Age”

Earlier cycles of glaciation—Contrast of the glaciated and nonglaciated regions—The
“driftless area”—Characteristics of the glaciated regions—The glacier gravings—
Younger records over older: the glacier palimpsest—The dispersion of the drift—
The diamonds of the drift—Tabulated comparison of the glaciated and nonglaciated
regions—Unassorted and assorted drift—Features into which the drift is molded—
Marginal or “kettle” moraines—Outwash plains—Pitted plains and interlobate
moraines—Eskers—Drumlins—The shelf ice of the ice age—Character profiles 297

CHAPTER XXIII
Glacial Lakes which marked the Decline of the Last Ice
Age
Interference of glaciers with drainage—Temporary lakes due to ice blocking—The
“parallel roads” of the Scottish glens—The glacial Lake Agassiz—Episodes of the
glacial lake history within the St. Lawrence Valley—The crescentic lakes of the
earlier stages—The early Lake Maumee—The later Lake Maumee—Lakes Arkona
and Whittlesey—Lake Warren—Lakes Iroquois and Algonquin—The Nipissing
Great Lakes—Summary of lake stages—Permanent changes of drainage effected
by the glacier—Glacial Lake Ojibway in the Hudson’s Bay drainage basin—
Reading references 320

CHAPTER XXIV
The Uptilt of the Land at the Close of the Ice Age
The response of the earth’s shell to its ice mantle—The abandoned strands as they
appear to-day—The records of uplift about Mackinac Island—The present
inclinations of the uplifted strands—The hinge lines of uptilt—Future
consequences of the continued uptilt within the lake region—Gilbert’s prophecy of
a future outlet of the Great Lakes to the Mississippi—Geological evidences of
continued uplift—Drowning of southwestern shores of Lakes Superior and Erie—
Reading references 340

CHAPTER XXV
Niagara Falls a Clock of Recent Geological Time

Features in and about the Niagara gorge—The drilling of the gorge—The present rate of
recession—Future extinction of the American Fall—The captured Canadian Fall at
Wintergreen Flats—The Whirlpool Basin excavated from the St. David’s gorge—
The shaping of the Lewiston Escarpment—Episodes of Niagara’s history and their
correlation with those of the glacial lakes—Time measures of the Niagara clock—
The horologe of late glacial time in Scandinavia—Reading references 352

CHAPTER XXVI
Land Sculpture by Mountain Glaciers
Contrasted sculpturing of continental and mountain glaciers—Wind distribution of the
snow which falls in mountains—The niches which form on snowdrift sites—The
augmented snowdrift moves down the valley: birth of the glacier—The excavation
of the glacial amphitheater or cirque—Life history of the cirque—Grooved and
fretted uplands—The features carved above the glacier—The features shaped
beneath the glacier—The cascade stairway in glacier-carved valleys—The
character profiles which result from sculpture by mountain glaciers—The sculpture
accomplished by ice caps—The Norwegian tind or beehive mountain—Reading
references 367

CHAPTER XXVII
Successive Glacier Types of a Waning Glaciation
Transition from the ice cap to the mountain glacier—The piedmont glacier—The
expanded-foot glacier—The dendritic glacier—The radiating glacier—The
horseshoe glacier—The inherited-basin glacier—Summary of types of mountain
glacier—Reading references 383

CHAPTER XXVIII
The Glacier’s Surface Features and the Deposits upon its
Bed
The glacier flow—Crevasses and séracs—Bodies given up by the Glacier des Bossons
—The moraines—Selective melting upon the glacier surface—Glacier drainage—
Deposits within the vacated valley—Marks of the earlier occupation of mountains
by glaciers—Reading references 390

CHAPTER XXIX

A Study of Lake Basins
Fresh water and saline lakes—Newland lakes—Basin-range lakes—Rift-valley lakes—
Earthquake lakes—Crater lakes—Coulée lakes—Morainal lakes—Pit lakes—Glint
or colk lakes—Ice-dam lakes—Glacier-lobe lakes—Rock-basin lakes—Valley
moraine lakes—Landslide lakes—Border lakes—Ox-bow lakes—Saucer lakes—
Crescentic levee lakes—Raft lakes—Side-delta lakes—Delta lakes—Barrier lakes
—Dune lakes—Sink lakes—Karst lakes: poljen—Playa lakes—Salines—Alluvial-
dam lakes—Résumé—Reading references 401

CHAPTER XXX
The Ephemeral Existence of Lakes
Lakes as settling basins—Drawing off of water by erosion of outlet—The pulling in of
headlands and the cutting off of bays—Lake extinction by peat growth—Extinction
of lakes in desert regions—The rôle of lakes in the economy of nature—Ice
ramparts on lake shores—Reading references 426

CHAPTER XXXI
The Origin and the Forms of Mountains
A mountain defined—The festoons of mountain arcs—Theories of origin of the
mountain arcs—The Atlantic and Pacific coasts contrasted—The block type of
mountain—Mountains of outflow or upheap—Domed mountains of uplift;
laccolites—Mountains carved from plateaus—The climatic conditions of the
mountain sculpture—The effect of the resistant stratum—The mark of the rift in the
eroded mountains—Reading references 435

APPENDICES
A. The quick determination of the common minerals 449
B. Short descriptions of some common rocks 462
C. The preparation of topographical maps 467
D. Laboratory models for study in the interpretation of geological maps 472
E. Suggested itineraries for pilgrimages to study earth features 475
Index 489

LIST OF PLATES
PLATE
1. Mount Balfour and the Balfour Glacier in the Selkirks Frontispiece
FACING PAGE

2. A. Layers compressed in experiments and showing the effect of a
competent
layer in the process of folding 44
B. Experimental production of a series of parallel thrusts within
closely folded strata 44
C. Apparatus to illustrate shearing action within the overturned limb
of a fold 44
3. A. An earthquake fault opened in Formosa in 1906 with vertical and
lateral displacements combined 72
B. Earthquake faults opened in Alaska in 1889 on which vertical
slices of the earth’s shell have undergone individual
adjustments 72
4. A. Experimental tank to illustrate the earth movements which are
manifested in earthquakes. The sections of the earth’s shell
are here represented before adjustment has taken place 82
B. The same apparatus after a sudden adjustment 82
C. Model to illustrate a block displacement in rocks which are
intersected by master joints 82
5. A. Once wooded region in China now reduced to desert through
deforestation 156
B. “Bad Lands” in the Colorado Desert 156
6. A. Barren Karst landscape near the famous Adelsberg grottoes 188
B. Surface of a limestone ledge where joints have been widened
through solution 188
7. A. Ranges of dunes upon the margin of the Colorado Desert 210
B. Sand dunes encroaching upon the oasis of Oued Souf, Algeria 210

8. A. The granite needles of Harney Peak in the Black Hills of South
Dakota 216
B. Castellated erosion chimneys in El Cobra Cañon, New Mexico 216
9. Map of the High Plains at the eastern front of the Rocky Mountains 220
10. A. View in Spitzbergen to illustrate the disintegration of rock under
the control of joints 228
B. Composite pattern of the joint structures within recent alluvial
deposits of the Syrian Desert 228
11. A. Ripple markings within an ancient sandstone 232
B. Wave breaking as it approaches the shore 232
12. A. V-shaped cañon cut in an upland recently elevated from the sea,
San Clemente Island, California 256
B. A “hogback” at the base of the Bighorn Mountains, Wyoming 256
13. A. Precipitous front of the Bryant Glacier outlet of the Greenland
inland ice 272
B. Lateral stream beside the Benedict Glacier outlet, Greenland 272
14. View of the margin of the Antarctic continental glacier in Kaiser
Wilhelm Land 282
15. A. An Antarctic ice foot with boat party landing 290
B. A near view of the front of the Great Ross Barrier, Antarctica 290
16. A. Incised topography within the “driftless area” 300
B. Built-up topography within the glaciated region 300
17. A. Soled glacial bowlders which show differently directed striæ upon
the same facet 306
B. Perched bowlder upon a striated ledge of different rock type,
Bronx Park, New York 306
C. Characteristic knob and basin surface of a moraine 306
18. A. Fretted upland of the Alps seen from the summit of Mount Blanc 372
B. Model of the Malaspina Glacier and the fretted upland above it 372
19. A. Contour map of a grooved upland, Bighorn Mountains, Wyoming 372
B. Contour map of a fretted upland, Philipsburg Quadrangle,
Montana 372
20. Map of the surface modeled by mountain glaciers in the Sierra Nevadas
of California 376
21. A. View of the Harvard Glacier, Alaska, showing the characteristic
terraces 394
B. The terminal moraine at the foot of a mountain glacier 394

22. A. Model of the vicinity of Chicago, showing the position of the
outlet of the former Lake Chicago 400
B. Map of Yosemite Falls and its earlier site near Eagle Peak 400
23. A. View of the American Fall at Niagara, showing the accumulation
of blocks beneath 414
B. Crystal Lake, a landslide lake in Colorado 414
24. A. Apparatus for exercise in the preparation of topographic maps 468
B. The same apparatus in use for testing the contours of a map 468
C. Modeling apparatus in use 468

ILLUSTRATIONS IN THE TEXT
FIG. PAGE
1. Diagram to show the measure of the earth’s surface irregularities 11
2. Map to show the reciprocal relation of areas of land and sea 11
3. The tetrahedral form toward which the earth is tending 12
4. A truncated tetrahedron to show the reciprocal relation of projection and
depression upon the surface 13
5. Approximations to earlier and present figures of the earth 15
6. Diagrams for comparison of coasts upon an upright and upon an inverted
tetrahedron 17
7. The continents, including submerged portions 18
8. Diagram to indicate the altitude of different parts of the lithosphere surface 18
9. Diagram to show how the terrestrial rocks grade into the meteorites 22
10. Comparison of a crystalline with an amorphous substance 24
11. “Light figure” seen upon etched surface of calcite 25
12. Battered sand grains which have developed crystal faces 26
13. Unassimilated grains of quartz within a garnet crystal 28
14. New minerals developed about the core of an augite crystal 28
15. A common rim of new mineral developed by reaction where earlier minerals
come into contact 28
16. Laminated structure of a sedimentary rock 30
17. Characteristic textures of igneous rocks 33
18. Diagram to show the order of sediments laid down during a transgression of the
sea 37
19. Fractures produced by compression of a block of molder’s wax 41
20. Apparatus to illustrate the folding of strata 41
21. Diagrams of fold types 42

22. Diagrams to illustrate crustal shortening 42
23. Anticlinal and synclinal folds 43
24. Diagrams to illustrate the shapes of rock folds 44
25. Secondary and tertiary flexures superimposed upon the primary ones 44
26. A bent stratum to illustrate tension and compression upon opposite sides 45
27. A geological section with truncated arches restored 47
28. Diagram to illustrate the nature of strike and dip 47
29. Diagram to show the use of T symbols for strike and dip observation 48
30. Diagram to show how the thickness of a formation is determined 49
31. A plunging anticline 50
32. A plunging syncline 50
33. An unconformity upon the coast of California 51
34. Series of diagrams to illustrate the episodes involved in the production of an
angular unconformity 52
35. Types of deceptive or erosional unconformities 53
36. A set of master joints in shale 55
37. Diagram to show the manner of replacement of one set of joints by another 56
38. Diagram to show the different combinations of joint series 56
39. View of the shore in West Greenland 57
40. View in Iceland which shows joint intervals of more than one order 57
41. Faulted blocks of basalt near Woodbury, Connecticut 58
42. A fault in previously disturbed strata 59
43. Diagram to show the effect of erosion upon a fault 60
44. A fault plane exhibiting drag 60
45. Map to show how a fault may be indicated by abrupt changes in strike and dip 61
46. A series of parallel faults revealed by offsets 61
47. Field map prepared from the laboratory table 64
48. Areal geological map based upon the field map 64
49. A portion of the ruins of Messina 67
50. Ruins of the Carnegie Palace of Peace at Cartaga, Costa Rica 68
51. Overturned bowlders from Assam earthquake of 1897 69

52. Post sunk into ground during Charleston earthquake 69
53. Map showing localities where shocks have been reported at sea off Cape
Mendocino, California 70
54. Effect of seismic water wave in Japan 70
55. A fault of vertical displacement 71
56. Escarpment produced by an earthquake fault in India 72
57. A fault of lateral displacement 72
58. Fence parted and displaced by lateral displacement on fault during California
earthquake 72
59. Fault with vertical and lateral displacements combined 72
60. Diagram to show how small faults may be masked at the earth’s surface 73
61. “Mole hill” effect above buried earthquake fault 73
62. Post-glacial earthquake faults 74
63. Earthquake cracks in Colorado desert 74
64. Railway tracks broken or buckled at time of earthquake 75
65. Railroad bridge in Japan damaged by earthquake 75
66. Diagrams to show contraction of earth’s crust during an earthquake 76
67. Map of the Chedrang fault of India 76
68. Displacements along earthquake fault in Alaska 77
69. Abrupt change in direction of throw upon an earthquake fault 77
70. Map of faults in the Owens Valley, California, formed during earthquake of
1872 78
71. Marquetry of the rock floor in the Tonopah district, Nevada 79
72. Map of Alaskan coast to show adjustments of level during an earthquake 79
73. An Alaskan shore elevated seventeen feet during the earthquake of 1899 80
74. Partially submerged forest from depression of shore in Alaska during
earthquake 80
75. Effect of settlement of the shore at Port Royal during earthquake of 1907 80
76. Diagrams to illustrate the draining of lakes during earthquakes 83
77. Diagram to illustrate the derangements of water flow during an earthquake 84
78. Mud cones aligned upon an earthquake fissure in Servia 84
79. Craterlet formed near Charleston, South Carolina, during the earthquake of
1886 85

80. Cross section of a craterlet 85
81. Map of the island of Ischia to show the concentration of earthquake shocks 87
82. A line of earth fracture revealed in the plan of the relief 87
83. Seismotectonic lines of the West Indies 88
84. Device to illustrate the different effects of earthquakes in firm rock and in loose
materials 88
85. House wrecked in San Francisco earthquake 90
86. Building wrecked in California earthquake by roof and upper floor battering
down the upper walls 91
87. Breached volcanic cone in New Zealand showing the bending down of the
strata near the vent 96
88. View of the new Camiguin volcano formed in 1871 in the Philippines 97
89. Map to show the belts of active volcanoes 98
90. A portion of the “fire girdle” of the Pacific 98
91. Volcanic cones formed in 1783 above the Skaptár fissure in Iceland 99
92. Diagrams to illustrate the location of volcanic vents upon fissure lines 100
93. Outline map showing the arrangement of volcanic vents upon the island of Java 100

94. Map showing the migration of volcanoes along a fissure 101
95. Basaltic plateau of the northwestern United States due to fissure eruptions of
lava 102
96. Lava plains about the Snake River in Idaho 102
97. Characteristic profiles of lava volcanoes 103
98. A driblet cone 104
99. Leffingwell Crater, a cinder cone in the Owens Valley, California 104
100. Map of Hawaii and its lava volcanoes 106
101. Section through Mauna Loa and Kilauea 106
102. Schematic diagram to illustrate the moving platform in the crater of Kilauea 107
103. View of the open lava lake of Halemaumau 108
104. Map to show the manner of outflow of the lava from Kilauea in the eruption of
1840 109
105. Lava of Matavanu flowing down to the sea during the eruption of 1906 110
106. Lava stream discharging into the sea from a lava tunnel 111
107. Diagrammatic representation of the structure of lava volcanoes as a result of
the draining of frozen lava streams 112

108. Diagram to show the formation of mesas by outflow of lava in valleys and
subsequent erosion 112
109. Surface of lava of the Pahoehoe type 113
110. Three successive views to show the growth of the island of Savaii, from lava
outflow in 1906 113
111. View of the volcano of Stromboli showing the excentric position of the crater 116
112. Diagrams to illustrate the eruptions within the crater of Stromboli 117
113. Map of Volcano in the Æolian Islands 118
114. “Bread-crust” lava projectile from the eruption of Volcano in 1888 119
115. “Cauliflower cloud” of steam and ash rising above the cinder cone of Volcano 120
116. Eruption of Taal volcano in 1911 seen from a distance of six miles 120
117. The thick mud veneer upon the island of Taal (after a photograph by Deniston) 121
118. A pear-shaped lava projectile 121
119. Artificial production of a cinder cone 122
120. Diagram to show the contrast between a lava dome and a cinder cone 123
121. Mayon volcano on the island of Luzon, Philippine Islands 123
122. A series of breached cinder cones due to migration of the eruption along a
fissure 124
123. The mouth upon the inner cone of Mount Vesuvius from which flowed the lava
of 1872 124
124. A row of parasitic cones raised above a fissure opened on the flanks of Etna in
1892 125
125. View of Etna, showing the parasitic cones upon its flanks 125
126. Sketch map of Etna to show the areas covered by lava and tuff respectively 126
127. Panum crater showing the caldera 126
128. View of Mount Vesuvius before the eruption of 1906 127
129. Sketches of the summit of the Vesuvian cone to bring out the changes in its
outline 128
130. Night view of Vesuvius from Naples before the outbreak of 1906, showing a
small lava stream descending the central cone 129
131. Scoriaceous lava encroaching upon the tracks of the Vesuvian railway 130
132. Map of Vesuvius, showing the position of the lava mouths opened upon its
flanks during the eruption of 1906 131
133. The ash curtain over Vesuvius lifting and disclosing the outlines of the
mountain 132
134. The central cone of Vesuvius as it appeared after the eruption of 1906 132

135. A sunken road upon Vesuvius filled with indrifted ash 133
136. View of Vesuvius from the southwest during the waning stages of the eruption 133
137. The main lava stream advancing upon Boscotrecase 133
138. A pine snapped off by the lava and carried forward upon its surface 133
139. Lava front pushing over and running around a wall in its path 134
140. One of the ruined villas in Boscotrecase 134
141. Three diagrams to illustrate the sequence of events during the cone-building
and crater-producing periods 135
142. The spine of Pelé rising above the chimney of the volcano after the eruption of
1902 136
143. Successive outlines of the Pelé spine 137
144. Corrugated surface of the Vesuvian cone due to the mud flows which followed
the eruption of 1906 138
145. View of the Kammerbühl near Eger in Bohemia 139
146. Volcanic plug exposed by natural dissection of a volcanic cone in Colorado 140
147. A dike cutting beds of tuff in a partly dissected volcano of southwestern
Colorado 140
148. Map and general view of St. Paul’s rocks, a volcanic cone dissected by waves 141
149. Dissection by explosion of Little Bandai-san in 1888 141
150. The half-submerged volcano of Krakatoa before and after the eruption of 1883 142
151. The cicatrice of the Banat 142
152. Diagram to illustrate a probable cause of formation of lava reservoirs and the
connection with volcanoes upon the surface 143
153. Effect of relief of load upon rocks by arching of a competent formation 144
154. Character profiles connected with volcanoes 146
155. Diagrams to show the effect of decomposition in producing spheroidal
bowlders 150
156. Spheroidal weathering of an igneous rock 151
157. Dome structure in granite mass 152
158. Talus slope beneath a cliff 153
159. Striped ground from soil flow 154
160. Pavement of horizontal surface due to soil flow 154
161. Tree roots prying rock apart on fissure 154
162. Bowlder split by a growing tree 155

163. Rock mantle beneath soil and vegetable mat 155
164. Diagram to show the varying thickness of mantle rock upon the different
portions of a hill surface 156
165. Gullies from earliest stage of a river’s life 160
166. Partially dissected upland 160
167. Longitudinal sections of upper portion of a river valley 161
168. Map and sections of a stream meander 163
169. Tree undermined on the outer bank of a meander 164
170. Diagrams to show the successive positions of stream meanders 164
171. An ox-bow lake in the flood plain of a river 165
172. Schematic representation of a series of river terraces 165
173. “Bird-foot” delta of the Mississippi River 167
174. Diagrams to show the nature of delta deposits as exhibited in sections 168
175. Gorge of the River Rhine near St. Goars 169
176. Valley with rounded shoulders characteristic of the stage of adolescence 170
177. View of a maturely dissected upland 170
178. Hogarth’s line of beauty 171
179. View of the oldland of New England, with Mount Monadnock rising in the
distance 171
180. Comparison of the cross sections of river valleys of different stages 172
181. The Beavertail Bend of the Yakima River 173
182. A rejuvenated river valley 174
183. Plan of a river narrows 174
184. Successive diagrams to illustrate the origin of “trellis drainage” 175
185. Sketch maps to show the earlier and present drainage near Harper’s Ferry 176
186. Section to illustrate the history of Snickers Gap 177
187. Character profiles of landscapes shaped by stream erosion in humid climates 177
188. Diagram to show the seasonal range in the position of the water table 180
189. Diagram to show the effect of an impervious layer upon the descending water 181
190. Sketch map to illustrate corrosion of limestone along two series of vertical
joints 181
191. Diagram to show the relation of limestone caverns to the river system of the
district 182

192. Plan of a portion of Mammoth Cave, Kentucky 183
193. Trees and shrubs growing upon the bottoms of limestone sinks 183
194. Diagrams to show the manner of formation of stalactites and stalagmites 185
195. Sinter formations in the Luray caverns 186
196. Map of the dolines of the Karst region 187
197. Cross section of a doline formed by inbreak 187
198. Sharp Karren of the Ifenplatte 188
199. The Zirknitz seasonal lake 189
200. Fissure springs arranged at intersections of rock fractures 190
201. Schematic diagrams to illustrate the different types of artesian wells 191
202. Cross section of Geysir, Iceland 192
203. Apparatus for simulating geyser action 193
204. Cone of siliceous sinter about the Lone Star Geyser 194
205. Former shore lines in the Great Basin 198
206. Map of the former Lake Bonneville 199
207. Borax deposits in Death Valley, California 201
208. Hollowed forms of weathered granite in a desert of Central Asia 201
209. Hollow hewn blocks in a wall in the Wadi Guerraui 202
210. Smooth granite domes shaped by exfoliation 203
211. Granite blocks rent by diffission 204
212. “Mushroom Rock” from a desert in Wyoming 205
213. Windkanten shaped by sand blast in the desert 205
214. The “stone lattice” of the desert 206
215. Shadow erosion in the desert 206
216. Cliffs in loess with characteristic vertical jointing 207
217. A cañon in loess worn by traffic and wind 207
218. Diagrams to illustrate the effects of obstructions in arresting wind-driven sand 209
219. Sand accumulating on either side of a firm and impenetrable obstruction 210
220. Successive diagrams to illustrate the history of the town of Kunzen upon the
Kurische Nehrung 210
221. View of desert barchans 211

222. Diagrams to show the relationships of dunes to sand supply and wind direction 211
223. Ideal section showing the rising mountain wall about a desert and the
neighboring slope 212
224. Dry delta at the foot of a range upon the borders of a desert 213
225. Map of distributaries of streams which issue at the western base of the Sierra
Nevadas 213
226. A group of “demoiselles” in the “bad lands” 214
227. Amphitheater at the head of the Wadi Beni Sur 215
228. Mesa and outlier in the Leucite Hills of Wyoming 216
229. Flat-bottomed basin separating dunes 216
230. Billowy surface of the salt crust on the central sink of the desert of Lop 217
231. Schematic diagram to show the zones of deposition in their order from the
margin to the center of a desert 217
232. Mounds upon the site of the buried city of Nippur 218
233. Exhumed structures in the buried city of Nippur 218
234. Section across the High Plains 219
235. Section across the lenticular threads of alluvial deposits of the High Plains 220
236. Distributaries of the foot hills superimposed upon an earlier series 220
237. Character profiles in the landscapes of arid lands 220
238. Rain sculpturing under control by joints 224
239. Sagging of limestone above joints 224
240. Map of the joint-controlled Abisko Cañon in Northern Lapland 225
241. Map of the gorge of the Zambesi River below Victoria Falls 225
242. Controlled drainage network of the Shepaug River in Connecticut 226
243. A river network of repeating rectangular pattern 226
244. Squared mountain masses which reveal a distribution of joints in block patterns
of different orders 228
245. Island groups of the Lofoten Archipelago 229
246. Diagrams to illustrate the composite profiles of the islands on the Norwegian
coast 229
247. Diagram to show the nature of the motions within a free water wave 231
248. Diagram to illustrate the transformation of a free wave into a breaker 232
249. Notched rock cliff and fallen blocks 233

250. A wave-cut chasm under control by joints 233
251. Grand Arch upon one of the Apostle Islands in Lake Superior 234
252. Stack near the shore of Lake Superior 234
253. The Marble Islands, stacks in a lake of the southern Andes 235
254. Squared stacks revealing the position of the joint planes on which they were
carved 235
255. Ideal section cut by waves upon a steep rocky shore 236
256. Map showing the outlines of the island of Heligoland at different stages in its
history 236
257. Ideal section carved by waves upon a steep shore of loose materials 237
258. Sloping cliff and boulder pavement at Scituate, Massachusetts 237
259. Map to show the nature of the shore current and the forms which are molded by
it 238
260. Crescent-shaped beach in the lee of a headland 239
261. Cross section of a beach pebble 239
262. A storm beach on the northeast shore of Green Bay 240
263. Spit of shingle on Au Train Island, Lake Superior 240
264. Barrier beach in front of a lagoon 241
265. Cross section of a barrier beach with lagoon in its rear 242
266. Cross section of a series of barriers and an outer bar 242
267. A barrier series and an outer bar on Lake Mendota at Madison, Wisconsin 242
268. Series of barriers at the western end of Lake Superior 243
269. Character profiles resulting from wave action upon shores 243
270. The even shore line of a raised coast 246
271. The ragged coast line produced by subsidence 246
272. Portion of the Atlantic coastal plain at the base of the oldland 246
273. Ideal form of cuestas and intermediate lowlands carved from a coastal plain 247
274. Uplifted sea cave on the coast of California 248
275. Double-notched cliff near Cape Tiro, Celebes 248
276. Uplifted stacks on the coast of California 249
277. Uplifted shingle beach across the entrance to a former bay upon the coast of
California 250
278. Raised beach terraces near Elie, Fife, Scotland 250

279. Uplifted sea cliffs and terraces on the Alaskan coast 250
280. Diagrams to show how excessive sinking upon the sea floor will cause the
shore to migrate landward 251
281. A drowned river mouth or estuary upon a coastal plain 251
282. Archipelago of steep rocky islets due to submergence 252
283. The submerged Hudsonian channel which continues the Hudson River across
the continental shelf 252
284. Marine clay deposits near the mouths of the Maine rivers which preserve a
record of earlier subsidence and later elevation 253
285. View of the three standing columns of the Temple of Jupiter Serapis, at
Pozzuoli 254
286. Three successive views to set forth the recent oscillations of level on the
northern shore of the Bay of Naples 255
287. Relief map of San Clemente Island, California 256
288. Relief map of Santa Catalina Island, California 257
289. Cross section of the Blue Grotto, on the island of Capri 258
290. Character profiles of coast elevation and subsidence 259
291. Map showing the distribution of existing glaciers and the two important wind
poles of the earth 263
292. An Alaskan glacier spreading out at the foot of the range which nourishes it 264
293. Surface of a glacier whose upper layers spread with but slight restraint from
retaining walls 265
294. Section through a mountain glacier 267
295. Profile across the largest of the Icelandic ice caps 267
296. Ideal section across a continental glacier 267
297. View of the Eyriks Jökull, an ice cap of Iceland 268
298. The zones of the lower atmosphere as revealed by recent kite and balloon
exploration 269
299. Map of Greenland, showing the area of inland ice and the routes of explorers 271
300. Profile in natural proportions across the southern end of the continental glacier
of Greenland 272
301. Map of a glacier tongue with dimple above 273
302. Edge of the Greenland inland ice, showing the nunataks diminishing in size
toward the interior 274
303. Moat surrounding a nunatak in Victoria Land 274
304. A glacier pavement of Permo-Carboniferous age in South Africa 276

305. Diagrams to illustrate the manner of formation of scape colks 277
306. Marginal moraine now forming at the edge of the continental glacier of
Greenland 279
307. Small lake between the ice front and a moraine which it has recently built 279
308. View of a drained lake bottom between the ice front and an abandoned moraine 280
309. Diagrams to show the manner of formation and the structure of an outwash
plain and fosse 280
310. Map of the ice masses of Victoria Land, Antarctica 282
311. Sections across the inland ice and the shelf ice of Antarctica 283
312. Diagram to show the nature of the fixed glacial anticyclone above continental
glaciers 284
313. Snow deltas about the margins of a glacier tongue in Greenland 285
314. View of the sea ice of the Arctic region 286
315. Map of the north polar regions, showing the area of drift ice and the tracks of
the Jeannette and the Fram 288
316. The shelf ice of Coats Land with surrounding pack ice 290
317. Tidewater cliff on a glacier tongue from which icebergs are born 290
318. A Greenlandic iceberg after a long journey in warm latitudes 291
319. Diagram showing one way in which northern icebergs are born from the glacier
tongue 291
320. A northern iceberg surrounded by sea ice 292
321. Tabular Antarctic iceberg separating from the shelf ice 293
322. Map of the globe, showing the areas covered by continental glaciers during the
“ice age” 297
323. Glaciated granite bowlder weathered out of a moraine of Permo-Carboniferous
age, South Australia 298
324. Map to show the glaciated and nonglaciated regions of North America 298
325. Map of the glaciated and nonglaciated areas of northern Europe 299
326. An unstable erosion remnant characteristic of the “driftless area” 300
327. Diagram showing the manner in which a continental glacier obliterates existing
valleys 301
328. Lake and marsh district in northern Wisconsin 302
329. Cross section in natural proportion of the latest North American continental
glacier 303
330. Diagram showing the earlier and the later glacier records together upon the
same limestone surface 304

331. Map to show the outcroppings of peculiar rock types in the region of the Great
Lakes, and some localities where “drift copper” has been collected 305
332. Map of the “bowlder train” from Iron Hill, Rhode Island 306
333. Shapes and approximate natural sizes of some of the diamonds from the Great
Lakes region 307
334. Glacial map of a portion of the Great Lakes region 308
335. Section in coarse till 310
336. Sketch map of portions of Michigan, Ohio, and Indiana, showing the
distribution of moraines 312
337. Map of the vicinity of Devil’s Lake, Wisconsin, partly covered by the
continental glacier 313
338. Moraine with outwash apron in front 313
339. Fosse between an outwash plain and a moraine 314
340. View along an esker in southern Maine 315
341. Outline map of moraines and eskers in Finland 315
342. Sketch maps showing the relationships of drumlins and eskers 316
343. View of a drumlin, showing an opening in the till 317
344. Outline map of the front of the Green Bay lobe to show the relationships of
drumlins, moraines, outwash plains, and ground moraine 317
345. Character profiles referable to continental glacier 318
346. View of the flood plain of the ancient Illinois River near Peoria 320
347. Broadly terraced valleys which mark the floods that once issued from the
continental glacier of North America 321
348. Border drainage about the retreating ice front south of Lake Erie 321
349. The “parallel roads” of Glen Roy in the Scottish Highlands 322
350. Map of Glen Roy and neighboring valleys of the Scottish Highlands 322
351. Three successive diagrams to set forth the late glacial lake history of the
Scottish glens 324
352. Harvesting time on the fertile floor of the glacial Lake Agassiz 325
353. Map of Lake Agassiz 325
354. Map showing some of the beaches of Lake Agassiz and its outlet 326
355. Narrows of the Warren River where it passed between jaws of granite and
gneiss 327
356. Map of the valley of the Warren River near Minneapolis 327
357. Portion of the Herman beach on the shore of the former Lake Agassiz 328

358. Map of the continental glacier of North America when it covered the entire St.
Lawrence basin 329
359. Outline map of the early Lake Maumee 330
360. Map to show the first stages of the ice-dammed lakes within the St. Lawrence
basin 330
361. Outline map of the later Lake Maumee and its outlet 332
362. Outline map of lakes Whittlesey and Saginaw 333
363. Map of the glacial Lake Warren 333
364. Map of the glacial Lake Algonquin 334
365. Outline map of the Nipissing Great Lakes 335
366. Probable preglacial drainage of the upper Ohio region 337
367. Diagrams to illustrate the episodes in the recent history of a Connecticut river 338
368. The notched rock headland of Boyer Bluff on Lake Michigan 341
369. View of Mackinac Island from the direction of St. Ignace 342
370. The “Sugar Loaf”, a stack of Lake Algonquin upon Mackinac Island 342
371. Beach ridges in series on Mackinac Island 343
372. Notched stack of the Nipissing Great Lakes at St. Ignace 343
373. Series of diagrams to illustrate the evolution of ideas concerning the uplift of
the lake region since the Ice Age 344
374. Map of the Great Lakes region to show the isobases and hinge lines of uptilt 345
375. Series of diagrams to indicate the nature of the recovery of the crust by uplift
when unloaded of an ice mantle 346
376. Portion of the Inner Sandusky Bay, for comparison of the shore line of 1820
with that of to-day 350
377. Ideal cross section of the Niagara Gorge to show the marginal terrace 353
378. View of the bed of the Niagara River above the cataract where water has been
drained off 353
379. View of the Falls of St. Anthony in 1851 354
380. Ideal section to show the nature of the drilling process beneath the cataract 355
381. Plan and section of the gorge, showing how the depth is proportional to the
width 355
382. Comparative views of the Canadian Falls in 1827 and 1895 356
383. Map to show the recession of the Canadian Fall 357
384. Comparison of the present with the future falls 358

385. Bird’s-eye view of the captured Canadian Fall at Wintergreen Flats 358
386. Map of the Whirlpool Basin 360
387. Map of the cuestas which have played so important a part in fixing the
boundaries of the lake basins 361
388. Bird’s-eye view of the cuestas south of Lakes Ontario and Erie 362
389. Sketch map of the greater portion of the Niagara Gorge to illustrate Niagara
history 363
390. Snowdrift hollowing its bed by nivation 368
391. Amphitheater formed upon a drift site in northern Lapland 369
392. The marginal crevasse on the highest margin of a glacier 370
393. Niches and cirques in the Bighorn Mountains of Wyoming 371
394. Subordinate cirques in the amphitheater on the west face of the Wannehorn 371
395. “Biscuit cutting” effect of glacial sculpture in the Uinta Mountains of Wyoming 372

396. Diagram to show the cause of the hyperbolic curve of cols 372
397. A col in the Selkirks 373
398. Diagrams to illustrate the formation of comb ridges, cols, and horns 374
399. The U-shaped Kern Valley in the Sierra Nevadas of California 375
400. Glaciated valley wall, showing the sharp line which separates the abraded from
the undermined rock surface 375
401. View of the Vale of Chamonix from the séracs of the Glacier des Bossons 376
402. Map of an area near the continental divide in Colorado 377
403. Gorge of the Albula River in the Engadine cut through a rock bar 378
404. Idealistic sketch, showing glaciated and nonglaciated side valleys 378
405. Character profiles sculptured by mountain glaciers 379
406. Flat dome shaped under the margin of a Norwegian ice cap 379
407. Two views which illustrate successive stages in the shaping of tinds 380
408. Schematic diagram to bring out the relationships of the various types of
mountain glaciers 383
409. Map of the Malaspina Glacier of Alaska 384
410. Map of the Baltoro Glacier of the Himalayas 385
411. View of the Triest Glacier, a hanging glacieret 385
412. Map of the Harriman Fjord Glacier of Alaska 386
413. Map of the Rotmoos Glacier, a radiating glacier of Switzerland 386

414. Outline map of the Asulkan Glacier in the Selkirks, a horseshoe glacier 387
415. Outline map of the Illecillewaet Glacier of the Selkirks, an inherited-basin
glacier 388
416. Diagram to illustrate the surface flow of glaciers 390
417. Diagram to show the transformation of crevasses into séracs 391
418. View of the Glacier des Bossons, showing the position of accidents to Alpinists 392
419. Lines of flow upon the surface of the Hintereisferner Glacier in the Alps 393
420. Lateral and medial moraines of the Mer de Glace and its tributaries 393
421. Ideal cross section of a mountain glacier 394
422. Diagrams to illustrate the melting effects upon glacier ice of rock fragments of
different sizes 394
423. Small glacier table upon the Great Aletsch Glacier 395
424. Effects of differential melting and subsequent refreezing upon a glacier surface 396
425. Dirt cone with its casing in part removed 396
426. Schematic diagram to show the manner of formation of glacier cornices 397
427. Superglacial stream upon the Great Aletsch Glacier 398
428. Ideal form of the surface left on the site of a piedmont glacier apron 399
429. Map of the site of the earlier piedmont glacier of the Upper Rhine 399
430. Diagram and map to bring out the characteristics of newland lakes 402
431. View of the Warner Lakes, Oregon 402
432. Schematic diagram to illustrate the characteristics of basin-range lakes 403
433. Schematic diagram of rift-valley lakes and the valley of the Jordan 403
434. Map of the rift-valley lakes of East Central Africa 404
435. Earthquake lakes formed in 1811 in the flood plain of the Lower Mississippi 404
436. View of a crater lake in Costa Rica 405
437. Diagrams to illustrate the characteristics of crater lakes 406
438. View of Snag Lake, a coulée lake in California 406
439. Diagrams to illustrate the characteristics of morainal lakes 407
440. Diagram to show the manner of formation of pit lakes 408
441. Diagrams to illustrate the characteristics of pit lakes 408
442. Diagram to show the manner of formation of glint lakes 409
443. Map of a series of glint lakes on the boundary of Sweden and Norway 409

444. Map of ice-dam lakes near the Norwegian boundary of Sweden 410
445. Wave-cut terrace of a former ice-dam lake in Sweden 410
446. View of the Márjelen Lake from the summit of the Eggishorn 411
447. Diagrams to illustrate the arrangement and the characters of rock-basin lakes 412
448. Convict Lake, a valley-moraine lake of California 413
449. Lake basins produced by successive slides from the steep walls of a glaciated
mountain valley 414
450. Lake Garda, a border lake upon the site of a piedmont apron 414
451. Diagrams to bring out the characteristics of ox-bow lakes 415
452. Diagrammatic section to illustrate the formation of saucer-like basins between
the levees of streams on a flood plain 415
453. Saucer lakes upon the bed of the former river Warren 416
454. Levee lakes developed in series within meanders in a delta plain 417
455. Raft lakes along the banks of the Red River in Arkansas and Louisiana 418
456. Map of the Swiss lakes Thun and Brienz 419
457. Delta lakes formed at the mouth of the Mississippi 419
458. Delta lakes at the margin of the Nile delta 420
459. Diagrams to illustrate the characteristics of barrier lakes 420
460. Dune lakes on the coast of France 421
461. Sink lakes in Florida, with a schematic diagram to illustrate the manner of their
formation 421
462. Map of the Arve and the Upper Rhone 426
463. View of the Arve and the Rhone at their junction 427
464. A village in Switzerland built upon a strath at the head of Lake Poschiavo 428
465. View of the floating bog and surrounding zones of vegetation in a small glacial
lake 429
466. Diagram to show how small lakes are transformed into peat bogs 430
467. Map to show the anomalous position of the delta in Lake St. Clair 431
468. A bowlder wall upon the shore of a small lake 432
469. Diagrams to show the effect of ice shove in producing ice ramparts upon the
shores of lakes 433
470. Various forms of ice ramparts 433
471. Map of Lake Mendota, showing the position of the ridge which forms from ice
expansion and the ice ramparts upon the shores 434

472. The great multiple mountain arc of Sewestan, British India 436
473. Diagrams to illustrate the theories of origin of mountain arcs 437
474. Festoons of mountain arcs about the borders of the Pacific Ocean 438
475. The interrupted Armorican Mountains common to western Europe and eastern
North America 438
476. A zone of diverse displacement in the western United States 439
477. Section of an East African block mountain 439
478. Tilted crust blocks in the Queantoweap valley 440
479. View of the laccolite of the Carriso Mountain 441
480. Map of laccolitic mountains 441
481. Ideal sections of laccolite and bysmalite 442
482. The gabled façade largely developed in desert landscapes 443
483. Balloon view of the Mythen in Switzerland 444
484. The battlement type of erosion mountain 445
485. Symmetrically formed low islands repeated in ranks upon Temagami Lake,
Ontario 445
486. Forms of crystals of a number of minerals 454
487. Forms of crystals of a number of minerals 457
488. A student’s contour map 469
489. Models to represent outcrops of rock 472
490. Special laboratory table set with a problem in geological mapping which is
solved in Figs. 47 and 48 472
491. Three field maps to be used as suggestions in arranging laboratory table for
problems in the preparation of areal geological maps 473
492. Sketch map of Western Scotland and the Inner Hebrides to show location of
some points of special geological interest 481
493. Outline map of a geological pilgrimage across the continent of Europe 483

EXPLANATORY LIST OF ABBREVIATIONS
FOR JOURNAL NAMES IN READING
REFERENCES

Am. Geol.: American Geologist.
Am. Jour. Sci.: American Journal of Science, New Haven.
Ann. de Géogr.: Annales de Géographie, Paris.
Ann. Rept. Geol. and Geogr. Surv. Ter.: Annual Report of the Geological
and Geographical Survey of the Territories (Hayden), Washington.
Ann. Rept. Geol. and Nat. Hist. Surv. Minn.: Annual Report of the
Geological and Natural History Survey of Minnesota, Minneapolis.
Ann. Rept. Mich. Geol. Surv.: Annual Report of the Michigan Geological
Survey, Lansing.
Ann. Rept. U. S. Geol. Surv.: Annual Report of the United States
Geological Survey, Washington.
Bull. Am. Geogr. Soc.: Bulletin of the American Geographical Society, New
York.
Bull. Earthq. Inv. Com. Japan: Bulletin of the Earthquake Investigation
Committee of Japan, Tokyo.
Bull. Geogr. Soc. Philadelphia: Bulletin of the Geographical Society of
Philadelphia.
Bull. Geol. Soc. Am.: Bulletin of the Geological Society of America.

Bull. Mus. Comp. Zoöl.: Bulletin of the Museum of Comparative Zoölogy,
Harvard College, Cambridge.
Bull. N. Y. State Mus.: Bulletin of the New York State Museum, Albany.
Bull. Soc. Belge d’Astronomie: Bulletin de la Société Belge d’Astronomie,
Brussels.
Bull. Soc. Belge Géol.: Bulletin de la Société Belge de Géologie, Brussels.
Bull. Soc. Sc. Nat. Neuchâtel: Bulletin de la Société des Sciences Naturelles
de Neuchâtel.
Bull. Univ. Calif. Dept. Geol.: Bulletin of the University of California,
Department of Geology, Berkeley.
Bull. U. S. Geol. Surv.: Bulletin of the United States Geological Survey,
Washington.
Bull. Wis. Geol. and Nat. Hist. Surv.: Bulletin of the Wisconsin Geological
and Natural History Survey, Madison.
C. R. Cong. Géol. Intern.: Comptes Rendus de la Congrès Géologique
Internationale.
Dept. of Mines, Geol. Surv. Branch, Canada: Department of Mines,
Geological Survey Branch, Canada.
Geogr. Abh.: Geographische Abhandlungen.
Geogr. Jour.: Geographical Journal, London.
Geol. Folio U. S. Geol. Surv.: Geological Folio of the United States
Geological Survey.
Geol. Mag.: Geological Magazine, London (sections designated by
decades).
Jour. Am. Geogr. Soc.: Journal of the American Geographical Society, New
York.
Jour. Coll. Sci. Imp. Univ. Tokyo: Journal of the College of Science of the
Imperial University, Tokyo, Japan.

Jour. Geol.: Journal of Geology, Chicago.
Jour. Sch. Geogr.: Journal of School Geography.
Livret Guide Cong. Géol. Intern.: Livret Guide Congrès Géologique
Internationale.
Mem. Geol. Surv. India: Memoirs of the Geological Survey of India,
Calcutta.
Mitt. Geogr. Ges. Hamb.: Mitteilungen der Geographische Gesellschaft,
Hamburg.
Mon. U. S. Geol. Surv.: Monograph of the United States Geological Survey,
Washington.
Nat. Geogr. Mag.: National Geographic Magazine, Washington.
Nat. Geogr. Mon.: National Geographic Monographs, American Book
Company, New York.
Naturw. Wochenschr.: Naturwissenschaftliche Wochenschrift.
Pet. Mitt.: Petermanns Mittheilungen aus Justus Perthes’ Geographischer
Anstalt, Gotha.
Pet. Mitt., Ergänzungsh. or Erg.: Petermanns Mittheilungen, Gotha
(Ergänzungsheft or Supplementary Paper).
Phil. Jour. Sci.: Philippine Journal of Science, Manila.
Phil. Trans.: Philosophical Transactions of the Royal Society, London.
Proc. Am. Acad. Arts and Sci.: Proceedings of the American Academy of
Arts and Sciences.
Proc. Am. Assoc. Adv. Sci.: Proceedings of the American Association for
the Advancement of Science.
Proc. Am. Phil. Soc.: Proceedings of the American Philosophical Society,
Philadelphia.

Proc. Bost. Soc. Nat. Hist.: Proceedings of the Boston Society of Natural
History, Boston.
Proc. Ind. Acad. Sci.: Proceedings of the Indiana Academy of Science.
Proc. Linn. Soc. New South Wales: Proceedings of the Linnean Society of
New South Wales.
Proc. Ohio State Acad. Sci.: Proceedings of the Ohio State Academy of
Science.
Prof. Pap. U. S. Geol. Surv.: Professional Paper of the United States
Geological Survey, Washington.
Pub. Carneg. Inst.: Publication of the Carnegie Institution of Washington.
Pub. Mich. Geol. and Biol. Surv.: Publication of the Michigan Geological
and Biological Survey, Lansing.
Quart. Jour. Geol. Soc. Lond.: Quarterly Journal of the Geological Society,
London.
Rept. Brit. Assoc. Adv. Sci.: Report of the British Association for the
Advancement of Science.
Rept. Geol. Surv. Mich.: Report of the Geological Survey of Michigan,
Lansing.
Rept. Mich. Acad. Sci.: Report of the Michigan Academy of Science,
Lansing.
Rept. Nat. Conserv. Com.: Report of the National Conservation
Commission, Washington.
Rept. Smithson. Inst.: Report of the Smithsonian Institution, Washington.
Sci. Bull. Brooklyn Inst. Arts and Sci.: Science Bulletin of the Brooklyn
Institute of Arts and Sciences.
Scot. Geogr. Mag.: Scottish Geographic Magazine, Edinburgh.
Smith. Cont. to Knowl.: Smithsonian Contributions to Knowledge,
Washington.

Tech. Quart.: Technology Quarterly of the Massachusetts Institute of
Technology, Boston.
Trans. Am. Inst. Min. Eng.: Transactions of the American Institute of
Mining Engineers, New York.
Trans. Roy. Dublin Soc.: Transactions of the Royal Dublin Society.
Trans. Seis. Soc. Japan: Transactions of the Seismological Society of Japan,
Tokyo.
Trans. Wis. Acad. Sci.: Transactions of the Wisconsin Academy of
Sciences, Arts, and Letters, Madison.
U. S. Geogr. and Geol. Surv. Rocky Mt. Region: United States
Geographical and Geological Survey of the Rocky Mountain Region
(Powell), Washington.
Zeit. d. Gesell. f. Erdk. z. Berlin: Zeitschrift der Gesellschaft für Erdkunde
zu Berlin.
Zeit. f. Gletscherk: Zeitschrift für Gletscherkunde, Berlin.

EARTH FEATURES AND THEIR
MEANING

CHAPTER I

THE COMPILATION OF EARTH HISTORY

The sources of the history.—The science which deals with the chapters
of earth history that antedate the earliest human writings is geology. The
pages of the record are the layers of rock which make up the outer shell of
our world. Here as in old manuscripts pages are sometimes found to be
missing, and on others the writing is largely effaced so as to be indistinct or
even illegible. An intelligent interpretation of this record requires a
knowledge of the materials and the structure of the earth, as well as a proper
conception of the agencies which have caused change and so developed the
history. These agencies in operation are physical and chemical processes,
and so the sciences of physics and chemistry are fundamental in any
extended study of geology. Not only is geology, so to speak, founded upon
chemistry and physics, but its field overlaps that of many other important
sciences. The earliest earth history has to do with the form, size, and
physical condition of a minor planet in the solar system. The earliest portion
of the story belongs therefore to astronomy, and no sharp line can be drawn
to separate this chapter from those later ones which are more clearly within
the domain of geology.
Subdivisions of geology.—The terms “cosmic geology” and
“astronomic geology” have sometimes been used to cover the astronomy of
the earth planet. The later earth history develops, among other things, the
varied forms of animal and vegetable life which have had a definite order of
appearance. Their study is to a large extent zoölogy and botany, though here
considered from an essentially different viewpoint. This subdivision of our
science is called paleontological geology or paleontology, which in
common usage includes the plant as well as the animal world, or what is

sometimes called paleobotany. In order to fix the order of events in
geological history, these biological studies are necessary, for the pages of
the record have many of them been misplaced as a result of the vicissitudes
of earth history, and the remains of life in the rock layers supply a
pagination from which it is possible to correctly rearrange the misplaced
pages. As compiled into a consecutive history of the earth since life
appeared upon it, we have the division of historical geology; though this
differs but little from stratigraphical geology, the emphasis in the case of the
former being placed on the history itself and in the latter upon the
arrangement of events—the pagination of the record.
So far as they are known to us, the materials of which the earth is
composed are minerals grouped into various characteristic aggregates
known as rocks. Here the science is founded upon mineralogy as well as
chemistry, and a study of the rock materials of the earth is designated
petrographical geology or petrography. The various rocks which enter into
the composition of the earth’s outer shell—the only portion known to us
from direct observation—are built into it in an architecture which, when
carefully studied, discloses important events in the earth’s history. The
division of the science which is concerned with earth architecture is
geotectonic or structural geology.
The study of earth features and their significance.—The features
upon the surface of the earth have all their deep significance, and if
properly understood, a flood of light is thrown, not only upon present
conditions, but upon many chapters of the earth’s earlier history. Here the
relation of our study to topography and geography is very close, so that the
lines of separation are but ill defined. The terms “physiographical geology”,
“physiography”, and “geomorphology” are concerned with the
configuration of the earth’s surface—its physiognomy—and with the
genesis of its individual surface features. It is this genetical side of
physiography which separates it from topography and lends it an absorbing
interest, though it causes it to largely overlap the division of dynamical
geology or the study of geological processes. In fact, the difference between
dynamical geology and physiography is largely one of emphasis, the stress
being laid upon the processes in the former and upon the resultant features
in the latter.
Under dynamical geology are included important subdivisions, such as
seismic geology, or the study of earthquakes, and vulcanology, or the study

of volcanoes. Another large subject, glacial geology, belongs within the
broad frontier common to both dynamical geology and physiography. A
relatively new subdivision of geological science is orientational geology,
which is concerned with the trend of earth features, and is closely related
both to physiography and to dynamical and structural geology.
Tabular recapitulation.—In a slightly different arrangement from the
above order of mention, the subdivisions of geology are as follows:—

Subdivisions of Geology

Petrographical Geology. Materials of the earth.
Architecture of the earth’s
Geotectonic Geology.
outer shell.
Dynamical Geology. Earth processes.
Seismic Geology—earthquakes. Vulcanology—
volcanoes. Glacial Geology—glaciers, etc.
Earth physiognomy and its
Physiographical Geology.
genesis.
The arrangement and the
Orientational Geology.
trend of earth features.

In one way or another all of the above subdivisions of geology are in
some way concerned in the genesis of earth physiognomy, and they must
therefore be given consideration in a work which is devoted to a study of
the meaning of earth features. The compiled record of the rocks is, however,
something quite apart and without pertinence to the present work. As
already indicated its subdivisions are:—

Astronomic Geology. Planetary history of the earth.
Statigraphic Geology. The pagination of earth records.
Historical Geology. The compiled record and its interpretation.
Paleontological Geology. The evolution of life upon the earth.

In every attempt at systematic arrangement difficulties are encountered,
usually because no one consideration can be used throughout as the basis of
classification. Such terms as “economic geology” and “mining geology”
have either a pedagogical or a commercial significance, and so would
hardly fit into the system which we have outlined.

Geological processes not universal.—It is inevitable that the geology
of regions which are easily accessible for study should have absorbed the
larger measure of attention; but it should not be forgotten that geology is
concerned with the history of the entire world, and that perspective will be
lost and erroneous conclusions drawn if local conditions are kept too often
before the eyes. To illustrate by a single instance, the best studied regions of
the globe are those in which fairly abundant precipitation in the form of rain
has fitted the land for easy conditions of life, and has thus permitted the
development of a high civilization. In degree, and to some extent also in
kind, geologic processes are markedly different within those widely
extended regions which, because either arid or cold, have been but ill fitted
for human habitation. Yet in the historical development of the earth, those
geologic processes which obtain in desert or polar regions are none the less
important because less often and less carefully observed.
Change, and not stability, the order of nature.—Man is ever prone to
emphasize the importance of apparent facts to the disadvantage of those less
clearly revealed though equally potent. The ancient notion of the terra
firma, the safe and solid ground, arose because of its contrast with the far
more mobile bodies of water; but this illusion is quickly dispelled with the
sudden quaking of the ground. Experience has clearly shown that, both
upon and beneath the earth’s surface, chemical and physical changes are
going on, subject to but little interruption. “The hills rock-ribbed and
ancient as the sun” is a poetical metaphor; for the Himalayas, the loftiest
mountains upon the globe, were, to speak in geological terms, raised from
the sea but yesterday. Even to-day they are pushing up their heads, only to
be relentlessly planed down through the action of the atmosphere, of ice,
and of running water. Even more than has generally been supposed, the
earth suffers change. Often within the space of a few seconds, to the
accompaniment of a heavy earthquake, many square miles of territory are
bodily uplifted, while neighboring areas may be relatively depressed. Thus
change, and not stability, is the order of nature.
Observational geology versus speculative philosophy.—There
appears to be a more or less prevalent notion that the views which are held
by scientists in one generation are abandoned by those of the next; and this
is apt to lead to the belief that little is really known and that much is largely
guessed. Some ground there undoubtedly is for such skepticism, though
much of it may be accounted for by a general failure among scientists, as

well as others, to clearly differentiate that which is essentially speculative
from what is based broadly upon observed facts. Even with extended
observation, the possibility of explaining the facts in more than one way is
not excluded; but the line is nevertheless a broad one which separates this
entire field of observation from what is essentially speculative philosophy.
To illustrate: the mechanics of the action which goes on within volcanic
craters is now fairly well understood as a result of many and extended
observations, and it is little likely that future generations of geologists will
discredit the main conclusions which have been reached. The cause of the
rise of the lava to the earth’s surface is, on the other hand, much less clearly
demonstrated, and the views which are held express rather the differing
opinions than any clear deductions from observation. Again, and similarly,
the physical history of the great continental glaciers of the so-called “ice
age” is far more thoroughly known than that of any existing glacier of the
same type; but the cause of the climatic changes which brought on the
glaciation is still largely a matter for speculation.
In the present work, the attempt will be, so far as possible, to give an
exposition of geologic processes and the earth features which result from
them, with hints only at those ultimate causes which lie hidden in the
background.
The scientific attitude and temper.—The student of science should
make it his aim, not only clearly to separate in his studies the proximate
from the ultimate causes of observed phenomena, but he should keep his
mind always open for reaching individual conclusions. No doctrines should
be accepted finally upon faith merely, but subject rather to his own
reasoning processes. This should not be interpreted to mean that concerning
matters of which he knows little or nothing he should not pay respect to the
recognized authorities; but his acceptance of any theory should be subject to
review so soon as his own horizon has been sufficiently enlarged. False
theories could hardly have endured so long in the past, had not too great
respect been given to authorities, and individual reasoning processes been
held too long in subjection.
The value of the hypothesis.—Because all the facts necessary for a full
interpretation of observed phenomena are not at one’s hand, this should not
be made to stand in the way of provisional explanations. If science is to
advance, the use of hypothesis is absolutely essential; but the particular
hypothesis adopted should be regarded as temporary and as indicating a line

of observation or of experimentation which is to be followed in testing it.
Thus regarded with an open mind, inadequate hypotheses are eventually
found to be untenable, whereas correct explanations of the facts by the same
process are confirmed. Most hypotheses of science are but partially correct,
for we now “see through a glass darkly”; but even so, if properly tested, the
false elements in the hypothesis are one after the other eliminated as the
embodied truth is confirmed and enlarged. Thus “working hypothesis”
passes into theory and becomes an integral part of science.

Reading References for Chapter I

The most comprehensive of general geological texts written in English
is Chamberlin and Salisbury’s “Geology” in three volumes (Henry Holt,
1904-1906), the first volume of which is devoted exclusively to geological
processes and their results. An abridged one-volume edition of the work
intended for use as a college text was issued in 1906 (College Geology,
Henry Holt). Other standard texts are:—

Sir Archibald Geikie. Text-book of Geology, 4th ed. 2 vols. London, 1902,
pp. 1472.
W. B. Scott. An Introduction to Geology. 2d ed. Macmillan, 1907, pp. 816.
J. D. Dana. Manual of Geology. New edition. American Book Company,
1895, pp. 1087.
Joseph LeConte. Elements of Geology. (Revised by Fairchild.) Appleton,
1905, pp. 667.

A very valuable guide to the recent literature of dynamical and
structural geology is Branner’s “Syllabus of a Course of Lectures on
Elementary Geology” (Stanford University, 1908).
On the relation of geology to landscape, a number of interesting books
have been written:—

James Geikie. Earth Sculpture or the Origin of Land-Forms. New York and
London, 1896, pp. 397.

John E. Marr. The Scientific Study of Scenery. Methuen, London, 1900,
pp. 368.
Sir A. Geikie. The Scenery of Scotland. 3d ed. Macmillan, London, 1901,
pp. 540.
Sir John Lubbock. The Scenery of Switzerland and the Causes to which it is
Due. Macmillan, London, 1896, pp. 480.
Lord Avebury. The Scenery of England. Macmillan, London, 1902, pp.
534.
Sir A. Geikie. Landscape in History, and Other Essays. Macmillan, London,
1905, pp. 352.
N. S. Shaler. Aspects of the Earth. Scribners, New York, 1889, pp. 344.
G. de La Noe et Emm. de Margerie. Les Formes du Terrain, Service
Géographique de l’Armée. Paris, 1888, pp. 205, pls. 48.
W. M. Davis. Practical Exercises in Physical Geography, with
Accompanying Atlas. Ginn and Co., Boston, 1908, pp. 148, pls. 45.
John Muir. The Mountains of California. Unwin, London, 1894, pp. 381.

Upon the use and interpretation of topographic maps in illustration of
characteristic earth features, the following are recommended:—

R. D. Salisbury and W. W. Atwood. The Interpretation of Topographic
Maps, Prof. Pap., 60 U.S. Geol. Surv., pp. 84, pls. 170.
D. W. Johnson and F. E. Matthes. The Relation of Geology to Topography,
in Breed and Hosmer’s Principles and Practice of Surveying, vol. 2.
Wiley, New York, 1908.
Général Berthaut. Topologie, Étude du Terrain, Service Géographique de
l’Armée. Paris, 1909, 2 vols., pp. 330 and 674, pls. 265.

The United States Geological Survey issues free of charge a list of 100
topographic atlas sheets which illustrate the more important physiographic
types. In his “Traité de Géographie Physique”, Professor E. de Martonne

has given at the end of each chapter the important foreign maps which
illustrate the physiographic types there described.
“The Principles of Geology”, by Sir Charles Lyell, published first in
three volumes, appeared in the years 1830-1833, and may be said to mark
the beginning of modern geology. Later reduced to two volumes, an
eleventh edition of the work was issued in 1872 (Appleton) and may be
profitably read and studied to-day by all students of geology. Those familiar
with the German language will derive both pleasure and profit from a
perusal of Neumayr’s “Erdgeschichte” (2d ed. revised by Uhlig. Leipzig
and Vienna, 2 vols., 1895-1897), and especially the first volume,
“Allgemeine Geologie.” A recent French work to be recommended is
Haug’s “Traité de Géologie” (Paris, 1907).
Some texts of physical geography may well be consulted, especially
Emm. de Martonne’s “Traité de Géographie Physique.” Colin, Paris, 1909,
pp. 910, pls. 48, and figs. 396.

Note. An explanatory list of abbreviations used in the reading
references follows the List of Illustrations.

CHAPTER II

THE FIGURE OF THE EARTH

The lithosphere and its envelopes.—The stony part of the earth is
known as the lithosphere, of which only a thin surface shell is known to us
from direct observation. The relatively unknown central portion, or “core”,
is sometimes referred to as the centrosphere. Inclosing the lithosphere is a
water envelope, the hydrosphere, which comprises the oceans and inland
bodies of water, and has a mass 1/4540 that of the lithosphere. If uniformly
distributed, the hydrosphere would cover the lithosphere to the depth of
about two miles, instead of being collected in basins as it now is. Though
apparently not continuous, if we take into account the zone of underground
water upon the continents, the hydrosphere may properly be considered as a
continuous film about the lithosphere. It is a fact of much significance that
all the ocean basins are connected, so that the levels are adjusted to furnish
a common record of deposits over the entire surface that is sea-covered.
Enveloping the hydrosphere is the gaseous envelope, the atmosphere,
with a mass 1/1200000 that of the lithosphere. The atmosphere is a mixture of
the gases oxygen and nitrogen in parts by volume of one of the former to
four of the latter, with a relatively small percentage of carbon dioxide.
Locally, and at special seasons, the atmosphere may be charged with
relatively large percentages of water vapor; and we shall see that both the
carbon dioxide and the vapor contents are of the utmost importance in
geological processes and in the influence upon climate. Unlike the water
which composes the hydrosphere, the gases of the atmosphere are
compressible. Forced down by the weight of superincumbent gas, the layers
of the atmosphere at the level of the sea sustain a pressure of about fifteen

pounds to the square inch; but this pressure steadily decreases in ascending
to higher levels. From direct instrumental observation, the air has now been
investigated to a height of more than twelve miles from the earth’s surface.
The evolution of ideas concerning the earth’s figure.—The ideas
which in all ages have been promulgated concerning the figure of the earth
have been many and varied. Though among them are not wanting the purely
speculative and fantastic, it will be interesting to pass in review such
theories as have grown directly out of observation.
The ancient Hebrews and the Babylonians were dwellers of the desert,
and in the mountains which bounded their horizon they saw the confines of
the earth. Pushing at last westward beyond the mountains, they found the
Mediterranean, and thus arrived at the view that the earth was a disk with a
rim of mountains which was floated upon water. The rare but violent
rainfalls to which they were accustomed—the desert cloudburst—further
led them to the belief that the mountain rim was continued upward in a
dome or firmament of transparent crystal upon which the heavenly bodies
were hung and from which out of “windows of heaven” the water “which is
above the earth” was poured out upon the earth’s surface. Fantastic as this
theory may seem to-day, it was founded upon observation, and it well
illustrates the dangers of reasoning from observation within too limited a
field.
As soon as men began to sail the sea, it was noticed that the water
surface is convex, for the masts of ships were found to remain visible long
after their hulls had disappeared below the horizon. It is difficult to say how
soon the idea of the earth’s rotundity was acquired, but it is certainly of
great antiquity. The Dominican monk Vincentius of Beauvais, in a work
completed in 1244, declared that the surfaces of the earth and the sea were
both spherical. The poet Dante made it clear that these surfaces were one,
and in his famous address upon “The Water and the Land”, which was
delivered in Verona on the 20th of January, 1320, he added a statement that
the continents rise higher than the ocean. His explanation of this was that
the continents are pulled up by the attraction of the fixed stars after the
manner of attraction of magnets, thus giving an early hint of the force of
gravitation.
The earth’s rotundity may be said to have been first proven when
Magellan’s ships in 1521 had accomplished the circumnavigation of the

globe. Circumnavigation, soon after again carried out by Sir Francis Drake,
proved that the earth is a closed body bounded by curving surfaces in part
enveloped by the oceans and everywhere by the atmosphere. The great
discovery of Copernicus in 1530 that the earth, like Venus, Mars, and the
other planets, revolves about the sun as a part of a system, left little room
for doubt that the figure of the earth was essentially that of a sphere.
The oblateness of the earth.—Every schoolboy is to-day familiar with
the fact that the earth departs from a perfect spherical figure by being
flattened at the ends of its axis of rotation. The polar diameter is usually
given as 1/299 shorter than the equatorial one. This oblateness of the
spheroid was proven by geodesists when they came to compare the lengths
of measured degrees of arc upon meridians in high and in low latitudes.
The oblateness of the geoid is
well understood from accepted
hypotheses to be the result of the
once more rapid rotation of the planet
when its materials were more plastic,
and hence more responsive to
deformation. An elastic hoop rotating
rapidly about an axis in its plane
appears to the eye as a solid, and
becomes flattened at the ends of its
axis in proportion as the velocity of
rotation is increased. Like the earth,
the other planets in the solar system
are similarly oblate and by amounts
dependent on the relative velocities
of rotation.
The departure of the geoid from
the spherical surface, owing to its Fig. 1.—Diagrams to afford a correct
impression of the measure of the
oblateness, is so small that in the
inequalities upon the earth’s surface
figures which we shall use for compared to the earth’s radius. The shell
illustration it would be less than the represented in b is 1/100 of the earth’s
thickness of a line. Since it is well radius, and in a this zone is magnified
recognized and not important in our for comparison with surface inequalities.
present consideration, we shall for

the time being speak of the figure of the earth in terms of departures from a
standard spherical surface.
The arrangement of oceans and continents.—There are other
departures from a spherical surface than the oblateness just referred to, and
these departures, while not large, are believed to be full of significance.
Lest the reader should gain a wrong impression of their magnitude, it may
be well to introduce a diagram drawn to scale and representing prominent
elevations and depressions of the earth (Fig. 1).
Wrong impressions concerning the figure of the lithosphere are
sometimes gained because its depressions are obliterated by the oceans. The
oceans are, indeed, useful to us in showing where the depressions are
located, but the figure of the earth which we are considering is the naked
surface of the rock. In a broad way, the earth’s shape will be given by the
arrangement of the oceans and the continents. As soon as we take up the
study of this arrangement, we find that quite significant facts of distribution
are disclosed.
One of the most significant facts
involved in the distribution of land and
sea, is a concentration of the land areas
within the northern and the seas within
the southern hemisphere. The
noteworthy exception is the occurrence
of the great and high Antarctic
Fig. 2.—Map on Mercator’s projection to continent centered near the earth’s
show the reciprocal relation of the land
and sea areas (after Gregory and Arldt).
south pole; and there are extensions of
the northern continent as narrowing
land masses to the southward of the equator. Hardly less significant than the
existence of land and water hemispheres is the reciprocal or antipodal
distribution of land and sea (Fig. 2). A third fact of significance is a
dovetailing together of sea and land along an east-and-west direction. While
the seas are generally A-shaped and narrow northward, the land masses are
V-shaped and narrow southward, but this occurs mainly in the southern
hemisphere. Lastly, there is some indication of a belt of sea dividing the
land masses into northern and southern portions along the course of a great
circle which makes a small angle with the earth’s equator. Thus the western

continent is nearly divided by a mediterranean sea,—the Caribbean,—and
the eastern is in part so divided by the separation of Europe from Africa.
The figure toward which the
earth is tending.—Thus far in our
discussion of the earth’s figure we
have been guided entirely by the
present distribution of land and water.
There are, however, depressions upon
the surface of the land, in some cases
extending below the level of the sea,
which are not to-day occupied by
Fig. 3.—The form toward which the figure water. By far the most notable of these
of the earth is tending, a tetrahedron
with symmetrically truncated angles. is the great Caspian Depression, which
with its extension divides central and
eastern Asia upon the east from Africa and Europe upon the west. This
depression was quite recently occupied by the sea, and when added to the
present ocean basins to indicate depressions of the lithosphere, it shows that
the earth’s figure departs from the standard spheroid in the direction of the
form represented in Fig. 3. This form approximates to a tetrahedron, a
figure bounded by four equal triangular faces, here with symmetrically
truncated angles. Of all regular figures with plane surfaces the tetrahedron
has the smallest volume for a given surface, and it presents moreover a
reciprocal relation of projection to depression. Every line passing through
its center thus finds the surface nearer than the average distance upon one
side and correspondingly farther upon the other (Fig. 4).
Astronomical versus geodetic observations.—Confirmation of the
conclusions arrived at from the arrangement of oceans and continents has
been secured in other fields. It was pointed out that the earth’s oblateness
was proven by comparison of the measured degrees of latitude upon the
earth’s surface in lower and higher latitudes, the degree being found longer
as the pole is approached. Any variation from the spherical surface must
obviously increase the size of the measured degree of latitude in proportion
to the departure from the standard form, and so the tetrahedral figure with
one of its angles at the south pole will require that the degrees of latitude be
longer in the southern than they are in the northern hemisphere. This has
been found by measurement to be the case, and the result is further
confirmed by pendulum studies upon the distribution of the earth’s

attraction or gravity. If less of the mass of the earth is concentrated in the
southern hemisphere, its attraction as measured in vibrations of the
pendulum should be correspondingly smaller.
Other confirmations of the
tetrahedral figure of the earth have
been derived from a comparison of
astronomical data, which assume the
earth to be a perfect spheroid, with
geodetic measurements, which are
based upon direct measurements.
Thus the arc measured in an east-and-
west direction across Europe revealed
a different curvature near the angle of
the tetrahedral figure from what was
found farther to the eastward. Fig. 4.—A truncated tetrahedron, showing
Changes of figure during how the depression upon one side of the
center is balanced by the opposite
contraction of a spherical body.—If projection.
we inquire why the earth in cooling
should tend to approach the tetrahedral figure, an answer is easily found.
When formed, the earth appears to have been a but slightly oblate spheroid,
or practically a sphere—the shape which of all incloses the most space for a
given surface. Cooled and solidified at the surface to the temperature of the
surrounding air, and the core still hot and continuing to lose heat, the core
must continue to contract though the outer shell is no longer able to do so.
The superficial area being thus maintained constant while the volume
continues to diminish, the figure must change from the initial one of
greatest bulk to others of smaller volume, and ultimately, if the process
should continue indefinitely, to the tetrahedron, which of all regular figures
has the minimum volume for a given surface.
That a contracting sphere does indeed pass through such a series of
changes has been shown by the behavior of contracting soap bubbles and of
rubber balloons, as well as by experiments upon the exhaustion of air
contained in hollow metal spheres of only moderate strength. In all these
instances, the ultimate form produced indicates an indenting of four sides of
the sphere which have the positions of the faces of a tetrahedron. The late
Professor Prinz of Brussels secured some extremely interesting results in

which he obtained intermediate forms with six angles, but unfortunately
these studies were not prepared for publication at the time of his death.
The earth’s departure from the spheroid in the direction of the modified
tetrahedron is, as we have seen, no hypothesis, but observed fact revealed in
(1) the concentration of the land about a central ocean in the northern
hemisphere; in (2) the antipodal relation of the land to the water areas, and
in (3) the threefold subdivision of the surface into north and south belts by
the two greater oceans and the Caspian Depression.
The earlier figures of the earth.—The manner in which continent and
ocean are dovetailed into each other in an east-and-west direction has been
generally adduced as additional evidence for the tetrahedral figure as above
described. Closer examination shows that instead of being in harmony with
this figure, it indicates a departure from it, and, as we shall see, a significant
departure which undoubtedly has its origin in the earlier history of the
planet. The mediterranean seas of both the eastern and the western
hemispheres likewise interfere with the perfection of the tetrahedral figure
and require an explanation.
Let us then examine in outline the past history of the world with
reference especially to the evolution of the continents and to the times and
the manners of surface change. It is now well known that there have been
three major periods of great deformation of the earth’s shell. The first of
these of which we have record came at the end of the first great era of
geologic history, the so-called Eozoic era; a second great transformation
came at the close of the second or Paleozoic era; and a third began at the
end of the next or Mesozoic era, an adjustment which is apparently
continuing to-day. Each of these great surface deformations was
accompanied by great volcanic eruptions of which we have the evidence in
the lavas remaining for our inspection, and each was followed by the
formation of great glaciers which spread over large areas of the existing
continents.
Before the earliest of these great changes, the earth appears to have
approximated in its figure somewhat closely to the ideal spheroid, for it was
everywhere enveloped in the hydrosphere as a universal ocean. Toward the
close of this period came the adjustments which brought the lithosphere to
protrude through the hydrosphere in shield-like continents whose
distribution, as shown by the rocks of this period, is of great significance.

Within the northern hemisphere rose three land shields spaced at nearly
equal intervals and at nearly equal distances from the northern pole. One of
these was centered where now is Hudson Bay, another about the present
Baltic Sea, and the relics of the third are found in northeastern Siberia.
These earliest continents have been referred to as the Laurentian, Baltic,
and Angara shields. Within the southern hemisphere shields appear to have
developed in somewhat similar grouping, namely, in South America, in
South Africa, and in Australia (Figs. 3 and 5).

Fig. 5.—Approximations to earlier and present figures of the earth.

These coigns or angles of a form into which the earlier spheroid of the
earth was being transformed have persisted through the greater part of
subsequent geologic time, and have been enlarged by the growth of
sediments about them as well as by the later elevation and wrinkling of
these deposits into marginal mountain ranges.
The continents and oceans which arose at the close of the Paleozoic
era.—At the close of the second great era in the recorded history of the
earth, the now somewhat enlarged continents were profoundly altered
during a series of convulsive movements within the surface shell of the
lithosphere. When these convulsions were over, there was a new disposition
of land and sea, but one quite different from the present arrangement.
Instead of being extended in north-south belts, as they are at present, the
continents stretched out in broad east-west zones, one in the northern and
the other in the southern hemisphere. To the broad southern continent of
which so little now remains, the name “Gondwana Land” has been given,
and to the sea which divided the northern from the southern continent the
name “Ocean of Tethys.” The northern continent stretched across the site of
the present Atlantic Ocean as the “North Atlantis”, its northern shore to the
westward being somewhat farther south than the present northern coast of

North America, since life forms migrated in the northern ocean from the site
of Behring Sea to that of the present North Atlantic.
This arrangement of land and water during the middle period of the
earth’s recorded history, when considered with reference both to its earlier
and to its later evolution, may perhaps be best accounted for by the
assumption that the lithosphere was then shaped like Fig. 5 (middle). In this
figure two truncated tetrahedrons are joined in a common plane of contact
which may be described as the twin plane. This medial depression upon the
lithosphere was occupied by the intercontinental sea, the Ocean of Tethys.
Near the close of this second great era of the earth’s continental history,
crustal convulsions, which were perhaps the most remarkable in the entire
record, resulted in the almost complete disappearance of the southern
continent and a concentration of the land within the northern hemisphere as
a somewhat interrupted belt surrounding a central polar ocean (Figs. 3 and
5).
Upon the assumption of twin tetrahedrons in the intermediate era of
continental evolution, both the Ocean of Tethys of that time and its present
remnants, the Caribbean and Mediterranean seas, are accounted for. The V-
shaped continent extensions and the A-shaped oceans of the southern
hemisphere (Fig. 2) may likewise be considered as relics of the now largely
submerged tetrahedron of the southern hemisphere, since this had its apex
to the northward (Fig. 6).
Thus we see that the lithosphere
can scarcely be regarded as a perfect
spheroid, since in the course of
geologic ages it has undergone
successive departures from this
original form. In its present state it
has been described as tetrahedral,
though we must keep in mind that the
sharp angles of that figure are deeply
truncated. The soundings first by Fig. 6.—Diagrams for comparison of shore
Nansen and more recently by Peary lines upon tetrahedrons which have an
in the Arctic basin, far to the north of angle, the first at the south and the
second at the north.
the continental border, showed that
this depression is characterized by profound depths, and so have afforded

confirmation of the tetrahedral figure. To match this depression at the
northern extremity of the earth’s axis, a high continent reaching to
elevations in excess of 10,000 feet has been penetrated by Sir Ernest
Shackleton at the opposite extremity of this polar diameter. Considering its
size and its elevation, the Antarctic continent with its glacier mantle is the
largest protuberance upon the surface of the lithosphere.
In our study of the departures of the earth from the standard spheroidal
surface, we might even go a step farther and show how the tetrahedron,
which best represents the symmetry of the present figure, is somewhat
deformed by a flattening perpendicular to the Pacific Ocean. To draw
attention to this flattening of the earth, it has sometimes been described as
“potato-shaped”, since the outline perpendicular to this face is imperfectly
heart-shaped or like a flattened “peg top.”
The flooded portions of the present
continents.—We are accustomed to
think of the continents as ending at the
shores of the oceans. If, however, we are
to regard them as platforms which rise
from the ocean depressions, their
margins should be considerably
Fig. 7.—The continents with submerged extended, for a submerged shelf now
portions added (after Gilbert).
practically surrounds all the continents to
a nearly uniform depth of 100 fathoms or 600 feet. The oceans thus more
than fill their basins and may be thought of as spilling over upon the
continents. In Fig. 7, the submerged portions of the continents have been
joined to those usually represented, thus adding about 10,000,000 square
miles to their area, and giving them one third, instead of one fourth, of the
lithosphere surface.

Fig. 8.—Diagram to indicate the altitude of different parts of the
lithosphere surface.

The floors of the hydrosphere and atmosphere.—The highest
altitudes upon the continents and the profoundest deeps of the ocean are
each removed about 30,000 feet, or nearly 6 miles, from the level of the sea.
In comparison with the entire surface of the lithosphere, these extremes of
elevation represent such small areas as to be almost inappreciable. Only
about 1/80 of the lithosphere surface rises more than 6000 feet above sea
level, and about the same proportion lies deeper than 18,000 feet below the
same datum plane (Fig. 8). Almost the entire area of the lithosphere is
included either in the so-called continental plateau or platform, in the
oceanic platform, or in the slope which separates the two. The continental
platform includes the continental shelf above referred to, and represents
about one third of the entire area of the planet. This platform has a range of
elevation from 6000 feet above to 600 feet below sea level and has an
average altitude of about 2300 feet. The oceanic platform slopes more
steeply, ranges in depth from 12,000 to 18,000 feet below sea level, and
comprises about one half the lithosphere surface. The remaining portion of
the surface, something less than one eighth of all, is included in the steep
slopes between the two platforms, between 600 and 12,000 feet below sea.
The two platforms and the slope between them must not, however, be
thought of as continuous features upon the surface, but merely as
representing the average elevations of portions of the lithosphere.

Reading References for Chapter II

On the evolution of ideas concerning the earth’s figure:—

Suess. The Face of the Earth (Clarendon Press, 1906), vol. 2, Chapter 1.
v. Zittel. History of Geology and Paleontology (Walter Scott, London,
1901), Chapters 1-2.

The departure of the spheroid toward the tetrahedron:—
W. Lowthian Green. Vestiges of the Molten Globe, Part 1. London, 1875.
(Now a rare work, but it contains the original statement of the idea.)
J. W. Gregory. The Plan of the Earth and Its Causes, Geogr. Jour., vol. 13,
1899, pp. 225-251 (the best general statement of the arguments for a
tetrahedral form).
W. Prinz. L’échelle reduite des expériences géologiques, Bull. Soc. Belge
d’Astronomie, 1899.
B. K. Emerson. The Tetrahedral Earth and Zone of the Intercontinental
Seas, Bull. Geol. Soc. Am., vol. 11, 1911, pp. 61-106, pls. 9-14.
M. P. Rudski. Physik der Erde (Tauchnitz, Leipzig, 1911), Chapters 1-3 (the
best discussion of the geoid from the purely mathematical standpoint,
so far as the spheroid is concerned).

The earlier figures of the earth:—
Th. Arldt. Die Entwicklung der Kontinente und ihrer Lebewelt.
Engelmann, Leipzig, 1907. (Contains a valuable series of map plates,
showing the probable boundaries of the continents in the different
geological periods).

CHAPTER III

THE NATURE OF THE MATERIALS IN THE LITHOSPHERE

The rigid quality of our planet.—For a long time it was supposed that
the solid earth constituted a crust only which was floated upon a liquid
interior. This notion was clearly an outgrowth of the then generally accepted
Laplacian hypothesis of the origin of the universe, which assumed fluid
interiors for the planets, the crust being suggested by the winter crust of
frozen water upon the surface of our inland lakes. To-day the nebular
hypothesis in the Laplacian form is fast giving place to quite different
conceptions, in which solid particles, and not gaseous ones, are conceived
to have built up the lithosphere. The analogy with frozen water has likewise
been abandoned with the discovery that frozen rock, instead of floating,
sinks in its molten equivalent.
Yet even more cogent arguments have been brought forward to show
that whatever may be the state of aggregation within the earth’s core—and
it may be different from any now known to us—it nevertheless has many of
the properties recognized as belonging to solid and rigid bodies.
Provisionally, therefore, we may regard the earth’s core as rigid and
essentially solid. It was long ago pointed out by the late Lord Kelvin that if
our lithosphere were not more rigid than a ball of glass of the same size, it
would be constantly passing through periodic six-hourly distortions of great
amplitude in response to the varying attractions of the moon. An equally
striking argument emanating from the same high authority is furnished by
the well-known egg-spinning demonstration. For illustration, Kelvin was
accustomed to take two eggs, one boiled and the other raw, and attempt to
spin them upon their ends. For the boiled, and essentially solid, egg this is
easily accomplished, but internal friction of the liquid contents of the raw

egg quickly stops any rotary motion which is imparted to it. Upon the same
grounds it is argued that had the earth’s interior possessed the properties of
a liquid, rotation must long since have ceased.
A stronger proof of earth rigidity than either of these has been lately
furnished by the instrumental study of earthquakes. With the delicate
apparatus which is now installed for the purpose, heavy earthquakes may be
sensed which have occurred anywhere upon the earth’s surface, the earth
movement sending its own message by the shortest route through the core
of the earth to the observing station. A heavy shock which occurs in New
Zealand is recorded in England, almost diametrically opposite, in about
twenty-one minutes after its occurrence. The laws of wave propagation and
their relation to the properties of the transmitting medium are well known,
and in order to explain such extraordinary velocity it is necessary to assume
that for such impulses the earth’s interior is much more rigid than the finest
tool steel.
Probable composition of the earth’s core.—In deriving views
concerning the nature of the earth’s interior we are greatly aided by
astronomical studies. The common origin long ago indicated for the planets
of the solar system and the sun has been confirmed by the analysis of light
with the aid of the spectroscope. It has thus been found that the same
chemical elements which we find in the earth are present also in the sun and
in the other stellar bodies. Again, the group of planets of the solar system
which are nearest to the sun—Mercury, Venus, the Earth, and Mars—have
each a high density, all except Mars, the most distant, having specific
gravities very closely 5½, that of Mars being about 4. This average specific
gravity is also that of the solid bodies, the so-called meteorites, which reach
the surface of our planet from the surrounding space. Yet though the earth
as a whole is thus found to have a specific gravity five and a half times that
of water, its surface shell has an average density of less than half this value,
or 2.7.
The study of meteorites has given us a possible clew to the nature of the
earth’s interior; for when both terrestrial and celestial rock types are
classified and placed in orderly arrangement, it is found that the chemical
elements which compose the two groups are identical, and that these are
united according to the same physical and chemical laws. No new element
has been discovered in the one group that has not been found in the other,
and though some compounds of these elements, the minerals, occur in the

earth’s crust that have not been found in meteorites, and though some occur
in meteorites which are not known from the earth, yet of those which are
common to both bodies there is agreement, even to the minor details (Fig.
9). It is found, however, that the commonest of the minerals in the earth’s
shell are absent from meteorites, as the commoner constituents of
meteorites are wanting in the earth’s crust. This observation would go far to
show that we may in the two cases be examining different portions of quite
similar bodies; and this view is strikingly confirmed when the rocks of the
two groups are arranged in the order of their densities (Fig. 9).

Fig. 9.—Diagram to show how terrestrial rocks grade into those of the
meteorites. 1, oxygen; 2, silicon; 3, aluminium; 4, alkali metals; 5, alkaline
earth metals; 6, iron, nickel, cobalt, etc.; a, granites and rhyolites; b,
syenites and trachytes; c, diorites and andesites; d, gabbros and basalts; e,
ultra-basic rocks; f, basic inclosures in basalt, etc.; g, iron basalts of west
Greenland; h, iron masses of Ovifak, west Greenland; a’-d’, meteorites in
order of density (after Judd).

In a broad way, density, structure, and chemical composition are all
similarly involved in the gradations illustrated by the diagram; and it is
significant that while there are terrestrial rocks not represented by
meteorites, the densest and the most unusual of the terrestrial rocks are
chemically almost identical with the less dense of the celestial bodies.

The earth a magnet.—The denser, and likewise the more common, of
the meteorite rocks—the so-called meteoric irons—are composed almost
entirely of the elements iron, nickel, and cobalt. Such aggregates are not
known as yet from terrestrial sources, although transitional types appear to
exist upon the island of Disco off the west coast of Greenland. If it were
possible to explore the earth’s interior, would such combinations of the iron
minerals be encountered? Apart from the surprising velocity of transmission
of earthquake waves, the strongest argument for an iron core to the
lithosphere is found in the magnetic property of the earth as a whole. The
only magnetic elements known to us are those of the heavy meteorites—
iron, nickel, and cobalt,—and the earth is, as we know, a great magnet
whose northern pole in British America and whose southern pole in
Antarctica have at last been visited by Amundsen and David, respectively.
The specific gravity of iron is 7.15, and those of nickel and cobalt, which in
the meteorites are present in relatively small amounts, are 7.8 and 7.5,
respectively. Considering that the surface shell of the earth has a specific
gravity of 2.7, these values must be regarded as agreeing well with the
determined density of the earth (5.6) and the other planets of its group
(Mercury 5.7, Venus 5.4, Mars 4).
The chemical constitution of the earth’s surface shell.—The number
of the so-called chemical elements which enter into the earth’s composition
is more than eighty, but few of these figure as important constituents of the
portion known to us. Nearly one half of the mass of this shell is oxygen, and
more than a quarter is silicon. The remaining quarter is largely made up of
aluminium, iron, calcium, magnesium, and the alkalies sodium and
potassium, in the order named. These eight constituent elements are thus the
only ones which play any important rôle in the composition of the earth’s
surface shell. They are not found there in the free condition, but combined
in the definite proportions characteristic of chemical compounds, and as
such they are known as minerals.
The essential nature of crystals.—A crystal we are accustomed to
think of as something transparent bounded by sharp edges and angles, our
ideas having been obtained largely from the gem minerals. This outward
symmetry of form is, however, but an expression of a power which resides,
so to speak, in the heart or soul of the crystal individual—it has its own
structural make-up, its individuality. No more correct estimates of the
comparison of crystal individualities would be obtained by the study of

outward forms alone of two minerals than would be gained by a judgment
of persons from the cut of their clothing. Too often this outward dress tells
only of the conditions by which both men and crystals have been
surrounded, and but little of the power inherent in the individual. Many a
battered mineral fragment with little beauty to recommend it, when placed
under suitable conditions for its development, has grown into a marvel of
beauty. Few minerals are so mean that they have not within them this
inherent power of individuality which lifts them above the world of the
amorphous and shapeless.
Just as the real nature of a person is
first disclosed by his behavior under
trying circumstances, so of a crystal it
is its conduct under stress of one sort
or another which brings out its real
character. By way of illustration let us
prepare a sphere from the mineral
quartz—it matters not whether we
destroy the beautiful outlines of the
crystal or employ a battered fragment
—and then prepare a sphere of similar
size and shape from a noncrystalline or
amorphous substance like glass. If now
these two spheres be introduced into a
bath of oil and raised to a higher
temperature, the glass globe undergoes
an enlargement without change of its
Fig. 10.—Comparison of a crystalline with
an amorphous substance when expanded form; but the crystal ball reveals its
by heat and when attacked by acid. individuality by expanding into a
spheroid in which each new dimension
is nicely adjusted to this more complex figure (Fig. 10).
We may, instead of submitting the two balls to the “trial by fire”, allow
each to be attacked by the powerful reagent, hydrofluoric acid. The
common glass under the attack of the acid remains as it was before, a
sphere, but with shrunken dimensions. The crystal, on the other hand, is
able to control the action of the solvent, and in so doing its individuality is
again revealed in a beautifully etched figure having many curving outlines
—it is as though the crystal had possessed a soul which under this trial has

been revealed. This glimpse into the nature of the crystal, so as to reveal its
structural beauty, is still more surprising when the crystal is taken from the
acid in the early stages of the action and held close beneath the eye. Now
the little etchings upon the surface display each the individuality of the
substance, and joining with their neighbors they send out a beautifully
symmetrical and entirely characteristic picture (Fig. 11).
The lithosphere a complex of
interlocking crystals.—To the layman the
crystal is something rare and expensive, to
be obtained from a jeweler or to be seen
displayed in the show cases of the great
museums. Yet the one most striking quality
of the lithosphere which separates it from
the hydrosphere and the atmosphere is its
crystalline structure,—a structure
belonging also to the meteorite, and with Fig. 11.—“Light figure” seen upon
little doubt to all the planets of the earth an etched surface of a crystal of
calcite (after Goldschmidt and
group. A snowflake caught during its fall Wright).
from the sky reveals all the delicate tracery
of crystal boundary; collected from a thick layer lying upon the ground, it
appears as an intricate aggregate of broken fragments more or less firmly
cemented together. And so it is of the lithosphere, for the myriads of
individuals are either the ruins of former crystals, or they are grown
together in such a manner that crystal facets had no opportunity to develop.
Such mineral individuals as once possessed the crystal form may have
been broken and their surfaces ground away by mutual attrition under the
rhythmic beating of the waves upon a shore or in the continuous rolling of
pebbles on a stream bed, until as battered relics they are piled away together
in a bed of sand. Yet no amount of such rough handling is sufficient to
destroy the crystal individuality, and if they are now surrounded with
conditions which are suitable for their growth, their individual nature again
becomes revealed in new crystal outlines. Many of our sandstones when
turned in the bright sunlight send out flashes of light to rival a bank of snow
in early spring. These bright flashes proceed from the facets of minute
crystals formed about each rounded grain of the sand, and if we examine
them under a lens, we may note the beauty of line formed with such

exactness that the most delicate instruments can detect no difference
between the similar angles of neighboring crystals (Fig. 12).

Fig. 12.—Battered sand grains which have taken on a new lease of life and
have developed a crystal form. a, a single grain grown into an individual
crystal; b, a parallel growth about a single grain; c, growth of neighboring
grains until they have mutually interfered and so destroyed the crystal
facets—the common condition within the mass of a rock (after Irving and
Van Hise).

This individual nature of the crystal is believed to reside in a
symmetrical grouping of the chemical molecules of the substance into
larger and so-called “crystal molecules.” The crystal quality belongs to the
chemical elements and to their compounds in the solid condition, but not to
ordinary mixtures of them.
Some properties of natural crystals, minerals.—No two mineral
species appear in crystals of the same appearance, any more than two
animal species have been given the same form; and so minerals may be
recognized by the individual peculiarities of their crystals. Yet for the
reason that crystals have so generally been prevented from developing or
retaining their characteristic faces, in the vast number of instances it is the
behavior, and not the appearance, of the mineral substance which is made
use of for identification.
When a mineral is broken under the blow of a hammer, instead of
yielding an irregular fracture, like that of glass, it generally tends to part
along one or more directions so as to leave plane surfaces. This property of
cleavage is strikingly illustrated for a single direction in the mineral mica,
for two directions in feldspar, and for three directions in calcite or Iceland
spar. Other properties of minerals, such as hardness, specific gravity, luster,

color, fusibility, etc., are all made use of in rough determinations of the
minerals. Far more delicate methods depend upon the behavior of minerals
when observed in polarized light, and such behavior is the basis of those
branches of geological science known as optical mineralogy and as
microscopical petrography. An outline description of some of the common
minerals and the means for identifying them will be found in appendix A.
The alterations of minerals.—By far the larger number of minerals
have been formed in the cooling and consequent consolidation of molten
rock material such as during a volcanic eruption reaches the earth’s surface
as lava. Beginning their growth at many points within the viscous mass, the
individual crystals eventually may grow together and so prevent a
development of their crystal faces.
Another class of minerals are deposited from solution in water within
the cavities and fissures of the rocks; and if this process ceases before the
cavities have been completely closed, the minerals are found projecting
from the walls in a beautiful lining of crystal—the Krystallkeller or “crystal
cellar.” It is from such pockets or veins within the rocks that the valuable
ores are obtained, as are the crystals which are displayed in our mineral
cabinets.
There is, however, a third process by which
minerals are formed, and minerals of this class are
produced within the solid rock as a product of the
alteration of preëxisting minerals. Under the
enormous pressures of the rocks deep below the
earth’s surface, they are as permeable to the
percolating waters as is a sponge at the surface.
Under these conditions certain minerals are
Fig. 13.—Crystal of garnet dissolved and their material redeposited after
developed in a schist traveling in the solution, or solution and
with grains of quartz redeposition of mineral matter may go on together
included because not
within the mass of the same rock. One new mineral
assimilated.
may have been produced from the dissolved
materials of a number of earlier species, or several new minerals may be the
result of the alteration of a preëxisting mineral with a more complex
chemical structure. Where the new mineral has been formed “in place”, it
has sometimes been able to utilize the materials of all the minerals which
before existed there, or it may have been obliged to inclose within itself

those earlier constituents which it could not assimilate in its own structure
(Fig. 13).
At other times a crystal which is imbedded in
rock has been attacked upon its surface by the
percolating solutions, and the dissolved materials
have been deposited in place as a crown of new
minerals which steadily widens its zone until the
center is reached and the original crystal has been
entirely transformed (Fig. 14). It is sometimes
possible to say that the action by which these
changes have been brought about has involved a
nice adjustment of supply of the chemical Fig. 14.—A crystal of
constituents necessary to the formation of the new augite within the mass
mineral or minerals. In rocks which are aggregates of a rock altered in part
of several mineral species, a newly formed to form a rim of the
minerals hornblende
mineral may appear only at the common margin of and magnetite. Note the
certain of these species, thus showing that they original outline of the
supply those chemical elements which were augite crystal.
necessary to the formation of the new substance
(Fig. 15). Thus it is seen that below the earth’s surface chemical reactions
are constantly going on, and the earlier rocks are thus locally being
transformed into others of a different mineral constitution.
Near the earth’s surface the carbon
dioxide and the moisture which are present
in the atmosphere are constantly changing
the exposed portions of the lithosphere into
carbonates, hydrates, and oxides. These
compounds are more soluble than are the
minerals out of which they were formed, and
they are also more bulky and so tend to
crack off from the parent mass on which
they were formed. As we are to see, for both
Fig. 15.—A new mineral
(hornblende) forming as an of these reasons the surface rocks of the
intermediate “reaction rim” lithosphere succumb to this attack from the
between the mineral having atmosphere.
irregular fractures (olivine) and
In connection with those wrinklings of
the surface shell of the lithosphere from

which mountains result, the underlying rocks
the dusty white mineral (lime-
soda feldspar).
are subjected to great strains, and even
where no visible partings are produced, the rocks are deformed so that
individual minerals may be bent into crescent-shaped or S-shaped forms, or
they are parted into one or more fragments which remain imbedded within
the rock.

Reading References for Chapter III

Theories of origin of the earth:—

Thomson and Tait. Natural Philosophy. 2d ed. Cambridge, 1883, pp. 422.
T. C. Chamberlin. Chamberlin and Salisbury’s Geology, vol. 2, pp. 1-81.

Rigidity of the earth:—

Lord Kelvin. The Internal Condition of the Earth as to Temperature,
Fluidity, and Rigidity, Popular Lectures and Addresses, vol. 2, pp. 299-
318; Review of evidence regarding the physical condition of the earth,
ibid., pp. 238-272.
Hobbs. Earthquakes (Appleton, New York, 1907), Chapters xvi and xvii.

Composition of the earth’s core and shell:—

O. C. Farrington. The Preterrestrial History of Meteorites, Jour. Geol., vol.
9, 1901, pp. 623-236.
E. S. Dana. Minerals and How to Study Them (a book for beginners in
mineralogy). Wiley, New York, 1895.

On the nature of crystals:—
Victor Goldschmidt. Ueber das Wesen der Krystalle, Ostwalds Annalen
der Naturphilosophie, vol. 9, 1909-1910, pp. 120-139, 368-419.

CHAPTER IV

THE ROCKS OF THE EARTH’S SURFACE SHELL

The processes by which rocks are formed.—Rocks may be formed in
any one of several ways. When a portion of the molten lithosphere, so-
called magma, cools and consolidates, the product is igneous rock. Either
igneous or other rock may become disintegrated at the earth’s surface, and
after more or less extended travel, either in the air, in water, or in ice, be
laid down as a sediment. Such sediments, whether cemented into a coherent
mass or not, are described as sedimentary or clastic rocks. If the fluid from
which they were deposited was the atmosphere, they are known as
subaërial or eolian sediments; but if water, they are known as subaqueous
deposits. Still another class are ice-deposited and are known as glacial
deposits.
But, as we have learned, rocks may
undergo transformations through
mineral alteration, in which case they
are known as metamorphic rocks.
When these changes consist chiefly in
the production of more soluble
minerals at the surface, accompanied
Fig. 16.—Laminated structure of
by thorough disintegration, due to the
sedimentary rock, Western Kansas (after direct attack of the atmosphere, the
a photograph by E. S. Tucker). resulting rocks are called residual
rocks.
The marks of origin.—Each of the three great classes of rocks, the
igneous, sedimentary, and metamorphic, is characterized by both coarser

and finer structures, in the examination of which they may be identified.
The igneous rocks having been produced from magmas, which are
essentially homogeneous, are usually without definite directional structures
due to an arrangement of their constituents, and are said to have a massive
structure. Sedimentary rocks, upon the other hand, have been formed by an
assorting process, the larger and heavier fragments having been laid down
when there was high velocity of either wind or water current, and the
smaller and lighter fragments during intermediate periods. They are
therefore more or less banded, and are said to have a bedded or laminated
structure (Fig. 16).
Again, igneous rocks, being due to a process of crystallization, are
composed of mineral individuals which are bounded either by crystal planes
or by irregular surfaces along which neighboring crystals have interfered
with each other; but in either case the grains possess sharply angular
boundaries. Quite different has been the result of the attrition between
grains in the transportation and deposition of sediments, for it is
characteristic of the sedimentary rocks that their constituent grains are well
rounded. Eolian sediments have usually more perfectly rounded grains than
subaqueous deposits.
Glacial deposits, if laid down directly by the ice, are unstratified,
relatively coarse, and contain pebbles which are both faceted and striated.
Such deposits are described as till or tillite. If glacier-derived material is
taken up by the streams of thaw water and is by them redeposited, the
sediments are assorted or stratified, and they are described as fluvio-glacial
deposits.
The metamorphic rocks.—Both the coarser structures and the finer
textures of the metamorphic rocks are intermediate between those of the
igneous and the sedimentary classes. A metamorphosed sedimentary rock,
in proportion to its alteration, loses the perfect lamination and the rounded
grain which were its distinguishing characters; while an igneous rock takes
on in the process an imperfect banding, and the sharp angles of its
constituent grains become rounded off by a sort of peripheral crushing or
granulation. Metamorphic rocks are therefore characterized by an
imperfectly banded structure described as schistosity or gneiss banding, and
the constituent grains may be either angular or rounded. If the
metamorphism has not been too intense or too long continued, it is
generally possible to determine, particularly with the aid of the polarizing

microscope, whether the original rock from which it was derived was of
igneous or of sedimentary origin. There are, however, many examples
which have defied a reliable verdict concerning their origin.
Characteristic textures of the igneous rocks.—In addition to the
massiveness of their general aspect and the angular boundaries of their
constituents, there are many additional textures which are characteristic of
the igneous rocks. While those that have consolidated below the earth’s
surface, the intrusive rocks, are notably compact, the magmas which arrive
at the surface of the lithosphere before their consolidation reveal special
structures dependent either upon the expansion of steam and other gases
within them, or upon the conditions of flow over the earth’s surface.
Magmas which thus reach the surface of the earth are described as lavas,
and the rocks produced by their consolidation are extrusive or volcanic
rocks. The steam included in the lava expands into bubbles or vesicles
which may be large or small, few or many. According to the number and the
size of these cavities, the rock is said to have a vesicular, scoriaceous, or
pumiceous texture.
Most lavas, when they arrive at the earth’s surface, contain crystals
which are more or less disseminated throughout the molten mass. The
tourist who visits Mount Vesuvius at the time of a light eruption may thrust
his staff into the stream of lava and extract a portion of the viscous
substance in which are seen beautiful white crystals of the mineral leucite,
each bounded by twenty-four crystal faces. It is clear that these crystals
must have developed by a slow growth within the magma while it was still
below the surface, and when the inclosing lava has consolidated, these
earlier crystals lie scattered within a groundmass of glassy or minutely
crystalline material. This scattering of crystals belonging to an earlier
generation within a groundmass due to later consolidation is thus an
indication of interruption in the process of crystallization, and the texture
which results is described as porphyritic (Fig. 17 b). Should the lava arrive
at the surface before any crystals have been generated and consolidate
rapidly as a rock glass, its texture is described as glassy (Fig. 17 c).
When the crystals of the earlier generation are numerous and needle-
like in form, as is very often the case, they arrange themselves “end on”
during the rock flow, so that when consolidation has occurred, the rock has
a kind of puckered lamination which is the characteristic of the fluxion or
flow texture. This texture has sometimes been confused with the lamination

of the sedimentary rocks, so that wrong conclusions have been reached
regarding origin. At other times the same needle-like crystals within the
lava have grouped themselves radially to form rounded nodules called
spherulites. Such nodules give to the rock a spherulitic texture, which is
nowhere better displayed than in the beautiful glassy lavas of Obsidian Cliff
in the Yellowstone National Park.

Fig. 17.—Characteristic textures of igneous rocks. a, granitic texture
characteristic of the deep-seated intrusive rocks; b, porphyritic texture
characteristic of the extrusive and of the near-surface intrusive rocks; c,
glassy texture of an extrusive rock.

Those intrusive rocks which consolidate deep below the earth’s surface,
part with their heat but slowly, and so the process of crystallization is
continued without interruption. Starting from many centers, the crystals
continue to grow until they mutually intersect in an interlocking complex
known as the granitic texture (Fig. 17 a).
Classification of rocks.—In tabular form rocks may thus be classified
as follows:—
Intrusive. Granitic or porphyritic texture.
Igneous. Massive and with sharply Extrusive. Glassy or porphyritic texture; often also with
angular grains. vesicular, scoriaceous, pumiceous, fluxion, or spherulitic
textures.
Subaërial. Sands and loess.
Subaqueous. (See below.)
Sedimentary. Laminate and with Glacial. Coarse, unstratified deposits with faceted pebbles.
rounded grains. Till and tillite.
Fluvio-glacial. Stratified sands and gravels with “worked
over” glacial characters.

Metamorphic. Schistose and with Metamorphic proper. Due to below surface changes.
grains either angular or rounded. Residual. Disintegrated at or near surface.

Subdivisions of the sedimentary rocks.—While the eolian sediments
are all the product of a purely mechanical process of lifting, transportation,
and deposition of rock particles, this is not always the case with the
subaqueous sediments, since water has the power of dissolving mineral
substance, as it has also of furnishing a home for animal and vegetable life.
Deposited materials which have been in solution in water are described as
chemical deposits, and those which have played a part in the life process as
organic deposits. The organic deposits from vegetable sources are peat and
the coals, while limestones and marls are the chief depositories of the
remains of the animal life of the water. The tabular classification of the
sediments is as follows:—

Classification of Sediments.
Subaqueous Conglomerate, sandstone and shale.
Deposited by water.
Subaërial or Eolian Sandstone and loess.
Deposited by wind.
Mechanical
Glacial Till and tillite.
Deposited by ice.
Fluvio-glacial Sands and gravels.
Glacier-water deposits.
Calcareous tufa Deposited in springs and rivers.
Chemical Oölitic limestone Deposited at the mouths of rivers between high and low
tide.
Formed of plant remains. Peats and coals.
Organic Formed of animal Limestones and marls.
remains.
Winds are under favorable conditions capable of transporting both dust
and sand, but not the larger rock fragments. The dust deposits are found
accumulating outside the borders of deserts as the so-called loess (Fig. 216),
though the sand is never carried beyond the desert border, near which it
collects in wide belts of ridges described as dunes. When this sand has been
cemented into a coherent mass, it is known as eolian sandstone. A section of
the appendix (B) is devoted to an outline description of some of the
commoner rock types.

The different deposits of ocean, lake, and river.—Of the subaqueous
sediments, there are three distinct types resulting: (1) from sedimentation in
rivers, the fluviatile deposits; (2) from sedimentation in lakes, the lacustrine
deposits; and (3) from sedimentation in the ocean, marine deposits. Again,
the widest range of character is displayed by the deposits which are laid
down in the different parts of the course of a stream. Near the source of a
river, coarse river gravels may be found; in the middle course the finer silts;
and in the mouth or delta region, where the deposits enter the sea or a lake,
there is found an assortment of silts and clays. Except within the delta
region, where the area of deposition begins to broaden, the deposits of
rivers are stretched out in long and relatively narrow zones, and are so
distinguished from the far more important lacustrine and marine deposits.
Lakes and oceans have this in common that both are bodies of standing
as contrasted with flowing water; and both are subject to the periodical
rhythmic motions and alongshore currents due to the waves raised by the
wind. About their margins, the deposits of lake and ocean are thus in large
part wrested by the waves from the neighboring land. Their distribution is
always such that the coarsest materials are laid down nearest to the shore,
and the deposits become ever finer in the direction of deeper water.
Relatively far from shore may be found the finest sands and muds or
calcareous deposits, while near the shore are sands, and, finally, along the
beach, beds of beach pebbles or shingle. When cemented into coherent
rocks, these deposits become shales or limestones, sandstones, and
conglomerates, respectively.
As regards the limestones, their origin is involved in considerable
uncertainty. Some, like the shell limestone or coquina of the Florida coast,
are an aggregation of remains of mollusks which live near the border of the
sea. Other limestones are deposited directly from carbonate of lime in
solution in the water. A deposit of this nature is forming in southern Florida,
both as a flocculent calcareous mud and as crystals of lime carbonate upon
a limestone surface. Again, there is the reef limestone which is built up of
the stony parts of the coral animal, and, lastly, the calcareous ooze of the
deep-sea deposits.
The marine sediments which are derived from the continents, the so-
called terrigenous deposits, are found only upon the continental shelf and
upon the continental slope just outside it. Of these terrigenous deposits, it is
customary to distinguish: (1) littoral or alongshore deposits, which are laid

down between high and low tide levels; (2) shoal water deposits, which are
found between low-water mark and the edge of the continental shelf; and
(3) aktian or offshore deposits, which are found upon the continental slope.
The littoral and shoal water deposits are mainly gravels and sands, while the
offshore deposits are principally muds or lime deposits.
Special marks of littoral deposits.—The marks of ripples are often left
in the sand of a beach, and may be preserved in the sandstone which results
from the cementation of such deposits (pl. 11 A). Very similar markings are,
however, quite characteristic of the surface of wind-blown sand. For the
reason that deposits are subject to many vicissitudes in their subsequent
history, so that they sometimes stand at steep angles or are even overturned,
it is important to observe the curves of sand ripples so as to distinguish the
upper from the lower surface.
In the finer sands and muds of sheltered tidal flats may be preserved the
impressions from raindrops or of the feet of animals which have wandered
over the flat during an ebb tide. When the tide is at flood, new material is
laid down upon the surface and the impressions are filled, but though
hardened into rock, these surfaces are those upon which the rock is easily
parted, and so the impressions are preserved. In the sandstones of the
Connecticut valley there has been preserved a quite remarkable record in
the footprints of animals belonging to extinct species, which at the time
these deposits were laid down must have been abundant upon the
neighboring shores.
Between the tides muds may dry out and crack in intersecting lines like
the walls of a honeycomb, and when the cracks have been filled at high
tide, a structure is produced which may later be recognized and is usually
referred to as “mud-crack” structure. This structure is of special service in
distinguishing marine deposits from the subaërial or continental deposits.
A variation in the direction of winds of successive storms may be
responsible for the piling up of the beach sand in a peculiar “plunge and
flow” or “cross-bedded” structure, a structure which is extremely common
in littoral deposits, though simulated in rocks of eolian origin.
The order of deposition during a transgression of the sea.—Many
shore lines of the continents are almost constantly migrating either
landward or seaward. When the shore line advances over the land, the coast
is sinking, and marine deposits will be formed directly above what was

recently the “dry land.” Such an invasion of the land by the sea, due to a
subsidence of the coast, is called a transgression of the sea, or simply a
transgression. Though at any moment the littoral, shoal water, and offshore
deposits are each being laid down in a particular zone, it is evident that each
must advance in turn in the direction of the shore and so be deposited above
the zones nearer shore. Thus there comes to be a definite series of
continuous beds, one above the other, provided only that the process is
continued (Fig. 18). At the very bottom of this series there will usually be
found a thin bed of pebbly beach materials, which later will harden into the
so-called basal conglomerate. If the size of the pebbles is such as to make
possible an identification, it will generally be found that these represent the
ruins of the rock over which the sea has advanced upon the land.

Fig. 18.—Diagram to show the order of the sediments laid down during a
transgression of the sea.

Next in order above the basal conglomerate, will follow the coarser and
then the finer sands, upon which in turn will be laid down the offshore
sediments—the muds and the lime deposits. Later, when cemented together,
these become in order, coarser and finer sandstones, shales, and limestones.
The order of superposition, reading from the bottom to the top, thus gives
the order of decreasing age of the formations.
A subsequent uplift of the coast will be accompanied by a recession of
the sea, and when later dissected by nature for our inspection, the order of
superposition and the individual character of each of the deposits may be
studied at leisure. From such studies it has been found that along with the
inorganic deposits there are often found the remains of life in the hard parts
of such invertebrate animals as the mollusks and the crustacea. These so-
called fossils represent animals which were gradually developed from
simpler to more and more complex forms; and they thus serve the purpose
of successive page numbers in arranging the order of disturbed strata, at the
same time that they supply the most secure foundation upon which rests the
great doctrine of evolution.

The basins of earlier ages.—It was the great Viennese geologist,
Professor Suess, who first pointed out that in mountain regions there are
found the thickest and the most complete series of the marine deposits;
whereas outside these provinces the formations are separated by wide gaps
representing periods when no deposits were laid down because the sea had
retired from the region. The completeness of the series of deposits in the
mountain districts can only be interpreted to mean that where these but
lately formed mountains rise to-day, were for long preceding ages the
basins for deposition of terrigenous sediments. It would seem that the
lithosphere in its adjustment had selected these earlier sea basins with their
heavy layers of sediment for zones of special uplift.
The deposits of the deep sea.—Outside the continental slope, whose
base marks the limit of the terrigenous deposits, lies the deeper sea, for the
most part a series of broad plains, but varied by more profound steep-walled
basins, the so-called “deeps” of the ocean. As shown by the dredgings of the
Challenger expedition and others of more recent date, the deposits upon the
ocean floor are of a wholly different character from those which are derived
from the continents. Except in the great deeps, or between depths of five
hundred and fifteen hundred fathoms, these deposits are the so-called
“ooze”, composed of the calcareous or chitinous parts of algæ and of
minute animal organisms. The pelagic or surface waters of the ocean are, as
it were, a great meadow of these plant forms, upon which the minute
crustacea, such as globigerina, foraminifera, and the pteropods, feed in
countless myriads. The hard parts of both plant and animal organisms
descend to the bottom and there form the ooze in which are sometimes
found the ear bones of whales and the teeth of sharks.
In the deeps of the ocean, none of these vegetable or animal deposits are
being laid down, but only the so-called “red clay”, which is believed to
represent decomposed volcanic material deposited by the winds as fine dust
on the surface of the ocean, or the product of submarine volcanic eruption.
From the absence of the ooze in these profound depths, the conclusion is
forced upon us that the hard parts of the minute organisms are dissolved
while falling through three or four miles of the ocean water.

Reading References for Chapter IV

J. S. Diller. The Educational Series of Rock Specimens collected and
distributed by the United States Geological Survey, Bull. 150 U. S.
Geol. Surv., 1898, pp. 1-400.
L. V. Pirsson. Rocks and Rock Minerals. Wiley, New York, 1908.
Sir John Murray. Deep-sea Deposits, Reports of the Challenger
expedition, Chapter iii.
L. W. Collet. Les dépôts marins. Doin, Paris, 1907 (Encyclopédie
Scientifique).

CHAPTER V

CONTORTIONS OF THE STRATA WITHIN THE ZONE OF
FLOW

The zones of fracture and flow.—It is easy to think of the atmosphere
and the hydrosphere as each sustaining at any point the load of the
superincumbent material. At the sea level the weight of air upon each
square inch of surface is about fifteen pounds, whereas upon the floor of the
hydrosphere in the more profound deeps the load upon the square inch must
be measured in tons. Near the lithosphere surface the rocks support by their
strength the load of rock above them, but at greater depths they are unable
to do this, for the load bears upon each portion of the rock with a pressure
equivalent to the weight of a rock column which extends upward to the
surface. The average specific gravity of rock is 2.7, and it is thus easy to
calculate the length of the inch square column which has a weight
equivalent to the crushing strength of any given rock. At the depth
represented by the length of such a column, rocks cannot yield to pressure
by fracture, for the opening of a crack implies that the rock upon either side
is strong enough to prevent the walls from closing. At this depth, rock must
therefore yield to pressure not by fracture, as it would at the surface, but by
flow after the manner of a liquid; and so the zone below this critical level is
referred to as the zone of flow.
In contrast, the near-surface zone is called the zone of fracture. But
different rocks possess different strengths, and these are subject to
modifications from other conditions, such, for example, as the proximity of
an uncooled magma. The zone of flow is therefore joined to the zone of
fracture, not upon a definite surface, but in an intermediate zone described
as the zone of fracture and flow.
Experiments which illustrate the fracture and flow of solid bodies.
—A prismatic block prepared from stiff molders’ wax, if crushed between
the jaws of a testing machine, yields a system of intersecting fractures

which are perpendicular to the free
surfaces of the block and take two
directions each inclined by half of a right
angle to the direction of compression (Fig.
19). This experiment may illustrate the
manner in which fractures are produced by
the compression within the zone of fracture
of the lithosphere, as its core continues to
contract.
To reproduce the conditions within the
zone of flow, it will be necessary to load
the lateral surfaces of the block instead of
leaving them unconstrained as in the
above-described experiment. The
experiment is best devised as in Fig. 20.
Here a series of layers having varying
degrees of rigidity is prepared from
beeswax as a base, either stiffened by
admixture of varying proportions of plaster
of Paris, or weakened by the use of Venice
turpentine. Such a series of layers may Fig. 19.—Two intersecting parallel
represent rocks of as widely different series of fractures produced upon
characters as limestone and shale. The load each free surface of a prismatic
block of stiff molders’ wax when
which is to represent superincumbent rock broken by compression from the
is supplied in the experiment by a deep ends (after Daubrée and Tresca).
layer of shot.

Fig. 20.—Apparatus to illustrate the folding of strata within the zone of
flow (after Willis).

When compression is applied to the layers from the ends, these
normally solid materials, instead of fracturing, are bent into a series of
folds. The stiffer, or more competent, layers are found to be less contorted
than are the weaker layers, particularly if the latter have been protected
under an arch of the more competent layer (pl. 2 A).
The arches and troughs of the folded strata.—Every series of folds is
made up of alternating arches and troughs. The arches of the strata the
geologist calls anticlines or anticlinal folds, and the troughs he calls
synclines or synclinal folds (Fig. 21). When a stratum is merely dropped in
a bend to a lower level without producing a complete arch or a complete
trough, this half fold is termed a monocline.
Any flexuring of the strata implies
a reduction of their surface area, or,
considering a single section, a
shortening. If the arches and troughs
are low and broad, the deformation of
Fig. 21.—Diagrams representing a, an the strata is slight, the shortening is
anticline; b, a syncline; and c, a
monocline.
comparatively small, and the folds are
described as open (Fig. 22 b). If they
be relatively both high and narrow, the deformation is considerable, a larger
amount of crustal shortening has gone on, and the folds are described as
close (Fig. 22 c). This closing up of the folds may continue until their sides
have practically the same slope, in which case they are said to be isoclinal
(Fig. 22 d).
The elements of folds.—Folds must
always be thought of as having extension
in each of the three dimensions of space
(Fig. 23), and not as properly included
within a single plane like the cross sections
which we so often use in illustration. A
fold may be conceived of as divided into
equal parts by a plane which passes along
the middle of either the arch or the trough,
and is called the axial plane. The line in
which this plane intersects the arch or the Fig. 22.—A comparison of folds to
trough is the axis, which may be called the express increasing degrees of
crustal shortening or progressive

crestline in an anticline, and the troughline deformation within the zone of
flow: a, stratum before folding; b,
in a syncline. open folds; c, close folds; d,
In the case of many open folds the axis isoclinal folds.
is practically horizontal, but in more
complexly folded regions this is seldom true. The departure of the axis from
the horizontal is called the pitch, and folds of this type are described as
pitching folds or plunging folds. The axis is in reality in these cases thrown
into a series of undulations or “longitudinal folds”, and hence pitch will
vary along the axis.

Fig. 23.—Anticlinal and synclinal folds in strata (after Willis).

The shapes of rock folds.—By the axial plane each fold is divided into
two parts which are called its limbs, which may have either the same or
different average inclinations. To describe now the shapes of rock folds and
not the degree of compression of the district, some additional terms are
necessary. Anticlines or synclines whose limbs have about the same
inclinations are known as upright or symmetrical folds. The axial plane of
the symmetrical fold is vertical (Fig. 24). If this plane is inclined to the
vertical, the folds are unsymmetrical. So soon as the steeper of the two
limbs has passed the vertical position and inclines in the same direction as
the flatter limb, the fold is said to be overturned. The departure from
symmetry may go so far that the axial plane of the fold lies at a very flat

angle, and the fold is then said to be
recumbent. The observant traveler by train
along any of the routes which enter the Alps
may from his car window find illustrations
of most of these types of rock folds, as he
may also, though generally less easily, in
passing through the Appalachian Mountains.
In
regions
which
have
been
closely
folded
the
larger Fig. 25.—Secondary and tertiary
flexures flexures superimposed upon the primary
ones.
Fig. 24.—Diagrams to illustrate the of the
different shapes of rock folds. strata
may be found with folds of a smaller order
of magnitude superimposed upon them, and these in turn may show
crumplings of still lower orders. It has been found that the folds of the
smaller orders of magnitude possess the shapes of the larger flexures, and
much is therefore to be learned from their careful study (Fig. 25). It is also
quite generally discovered that parallel planes of ready parting, which are
described as rock cleavage, take their course parallel to the axial plane
within each minor fold. As was long ago shown by the pioneer British
geologists, these planes of cleavage are essentially parallel and follow the
fold axes throughout large areas.

Plate 2.

A. Layers compressed in experiments and showing the effect of a
competent layer in the process of folding (after Willis).

B. Experimental production of a series of parallel thrusts within closely
folded strata (after Willis).

C. Apparatus to illustrate shearing action within the overturned limb of a
fold.

The overthrust fold.—Whenever a stratum is bent, there is a tendency
for its particles to be separated upon the convex side of the bend, at the
same time that those upon the concave side are crowded closer together—

there is tension in the former case and compression in the latter (Fig. 26).
Within an unsymmetrical or an overturned fold, the peculiar distortions in
the different sections of the stratum are less simple and are best illustrated
by pl. 2 C. This apparatus shows two similar piles of paper sheets, upon the
edges of each of which a series of circles has been drawn. When now one of
the piles is bent into an unsymmetrical fold, it is seen that through an
accommodation by the paper sheets sliding each over its neighbor large
distortions of the circles have occurred. In that steeper limb which with
closer folding will be overturned the circles have been drawn out into long
and narrow ellipses, and this indicates that those rock particles which before
the bending were included in the circle have been moved past each other in
the manner of the blades of a pair of shears. Such extreme “shearing” action
is thus localized in the underturned limb of the fold, and a time must come
with continuation of the compression when the fold will rupture at this
critical place along a plane parallel to the longest axis of the ellipses or
nearly parallel to the axial plane of the anticline. Such structures probably
occur in the zone of combined fracture and flow, up into which the beds are
forced in cases of close compression. Relief thus being found upon this
plane of fracture, the upper portion of the fold will now ride over the lower,
and the displacement is described as a thrust or overthrust.
In the long series of experiments
conducted by Mr. Bailey Willis of the
United States Geological Survey, all
the stages between the overturned fold
Fig. 26.—A bent stratum to illustrate and the overthrust fold were
tension upon the convex and reproduced. Where a series of folds
compression upon the concave side was closely compressed, a parallel
(after Van Hise).
series of thrusts developed (pl. 2 B), so
that a series of slices cutting across neighboring strata was slid in
succession, each over the other, like the scales upon a fish or the shingles
upon a roof. Quite remarkable structures of this kind have been discovered
in rocks of such closely folded districts as the Northwest Highlands of
Scotland, where the overriding is measured in miles. Near the thrust planes
the rocks show a crushing of the grains, and the planes themselves are
sometimes corrugated and polished by the movement.
Restoration of mutilated folds.—Since flexuring of the rocks takes
place within the zone of flow at a distance of several miles below the

earth’s surface, it is quite obvious that the results of the process can be
studied only after some thousands of feet of superincumbent strata have
been removed. We are a little later to see by what processes this lowering of
the surface is accomplished, but for the present it may be sufficient to
accept the fact, realizing that before foldings in the strata can reach the
surface, they must have passed through the upper zone of fracture.
It might perhaps be supposed that the anticlines would appear as the
mountains upon the surface, and occasionally this is true; as, for example,
in the folded Jura Mountains of western Europe. More generally, the
mountains have a synclinal structure and the valleys an anticlinal one; but
as no general rule can be applied, it is necessary to make a restoration of the
truncated folds in each district before their character can be known.
The geological map and section.—The earth’s surface is in most
regions in large part covered with soil or with other incoherent rock
material, so that over considerable areas the hard rocks are hidden from
view. Each locality at which the rock is found at the earth’s surface “in
place” is described as an outcropping or exposure. In a study of the region
each such exposure must be examined to determine the nature of the rock,
especially for the purpose of correlation with neighboring exposures, and,
in addition, both the probable direction in which it is continued along the
surface—the strike—and the inclination of its beds—the dip. If the
outcroppings are sufficiently numerous, and rock type, strike and dip, may
all be determined, the folds of the district may be restored with almost as
much accuracy as though their curves were everywhere exposed to view. A
cross section through the surface which represents the observed outcrops
with their inclinations and the assumed intermediate strata in their probable
attitudes is described as a geological section (Fig. 27). A map upon which
the data have been entered in their correct locations, either with or without
assumptions concerning the covered areas, is known as a geological map.

Fig. 27.—A geological section based upon observations at outcrops, but
with the truncated arches restored.

If the axes of folds are absolutely horizontal, and the surface of the earth
be represented as a plain, the lines of intersection of the truncated strata
with the ground, or with any horizontal surface, will give the directions of
continuation of the individual strata. This strike direction is usually
determined at each exposure by use of a compass provided with a spirit
level. When that edge of the leveled compass which is parallel to the north-
south line upon the dial is held against the sloping rock stratum, the angle of
strike is measured in degrees by the compass needle. If the cardinal
directions have been placed in their correct positions upon the compass dial,
the needle will point to the northwest when the strike is northeast, and vice
versa (Fig. 28 a). Upon the geologist’s compass it is therefore customary to
reverse the initials which represent the east and west directions, in order
that the correct strike may be read directly from the dial (Fig. 28 b).

Fig. 28.—Diagram to illustrate the manner of determining the strike of rock
beds at an outcropping. a, a compass which has the cardinal directions in
their natural positions; b, a compass with the east and west initials
reversed upon the dial; c, home-made clinometer in position to determine
the dip.

By the dip is meant the inclination of the stratum at any exposure, and
this must obviously be measured in a vertical plane along the steepest line
in the bedding plane. The dip angle is always referred to a horizontal plane,
and hence vertical beds have a dip of 90°. The device for measuring this
angle of dip, the clinometer, is merely a simple pendulum which serves as
an indicator and is centered at the corner of a graduated quadrant. A home-
made variety is easily constructed from a square piece of board and an
attached paper quadrant (Fig. 28 c), but the geologist’s compass is always
provided with a clinometer attachment to the dial.
Since the strike is the intersection
of the bedding plane with a horizontal
surface, and the dip is the intersection
with that particular vertical plane
which gives the steepest inclination,
the strike and dip are perpendicular to
each other. To represent them upon
maps, it is more or less customary to
use the so-called T symbols, the top of
Fig. 29.—Diagram to show the use of T
symbols to indicate the dip and strike of the T giving the direction of the strike
outcroppings. and the shank that of the dip. If
meridians are drawn upon the map, the
direction or attitude of the T can be found by the use of a simple protractor;
and when entered upon the map, the exact angle of the strike may be
supplied by a figure near the top of the T, and the dip angle by a figure at
the end of the shank. It is the custom, also, to make the length of the shank
inversely proportional to the steepness of the dip, so that in a broad way the
attitudes of the strata may be taken in at a glance (Fig. 29). It is further of
advantage to make the top of the T a double line, so that some symbol or
color may show the correlations of the different exposures. To illustrate, in
Fig. 29, the symbol marked a represents an outcrop of limestone, the strike
of which is 50° east of north (N. 50° E.), and the dip of which is 45°
southeast. In the same figure b represents a shale outcrop in horizontal beds,
which have in consequence a universal strike and a dip of 0°. An exposure
of limestone in vertical beds which strike N. 60° E. is shown at c, etc.
Measurement of the thickness of formations.—When formations still
lie in horizontal beds, we may sometimes learn their thickness directly

either from the depth of borings to
the underlying rock, or by
measurements upon steep cañon
walls. If the beds stand vertically, the
matter is exceedingly simple, for in
this case the thickness is the width of
the outcrops of the formation
between the beds which bound it Fig. 30.—Diagram to show how the
thickness of a formation may be
upon either side. In the general case, obtained from the angle of the dip and
in which the beds are neither the width of the exposures.
horizontal nor vertical, the thickness
must be obtained indirectly from the width of the exposures and the angle
of the dip. The factor by which the exposure width must be multiplied is
known as the sine of the dip angle (Fig. 30), which is given with sufficient
accuracy for most purposes in the following table. It is obvious that in order
to obtain the full thickness of a formation it is necessary to measure from
the contact with the adjacent formation upon the one side to a similar
contact with the nearest formation upon the other.

Natural Sines

0° .00 35° .57 70° .94
5° .09 40° .64 75° .97
10° .17 45° .71 80° .98
15° .26 50° .77 85° 1.00
20° .34 55° .82 90° 1.00
25° .42 60° .87
30° .50 65° .91

Fig. 31.—Combined surface and sectional views of a plunging anticline
(after Willis).

Fig. 32.—Combined surface and sectional views of a plunging syncline
(after Willis).

The detection of plunging folds.—When the axis of a fold is
horizontal, its outcrops upon a plain will continue to have the same strike
until the formation comes to an end. Upon a generally level surface,
therefore, any regular progressive variation in the strike direction is an
indication that the folds have a plunging or pitching character. Many serious

mistakes of interpretation have been made because of a failure to recognize
this evidence of plunging folds. The way in which the strikes are
progressively modified will be made clear by the diagrams of Figs. 31 and
32, the first representing a pitching anticline and the second a pitching
syncline. In both these reciprocal cases the strikes of the beds undergo the
same changes, and the dip directions serve to distinguish which of the two
structures is present in a given case. There is, however, one further
difference in that the hard layers of the plunging anticline, where they
disappear below the surface in the axis, will present a domed surface
sloping forward like the back of a whale as it rises above the surface of the
sea. Plunging folds in series will thus appear in the topography as a series of
sharply zigzagging ranges at those localities where the harder layers
intersect the surface. Such features are encountered in eastern Pennsylvania,
where the hard formations of the Appalachian Mountain system plunge
northeastward under the later formations. The pitch of the larger fold is
often disclosed by that of the minor puckerings superimposed upon it.

Fig. 33.—Unconformity between a lower and an upper series of beds upon
the coast of California. Note how the hard layer stands in relief upon the
connecting surface (after Fairbanks).

The meaning of an unconformity.—The rock beds, which are
deposited one above the other during a transgression of the sea, are usually
parallel and thus represent a continuous process of deposition. Such beds

are said to be conformable. Where, upon the other hand, two series of
deposits which are not parallel to each other are separated by a break, they
are said to form unconformable series, and the break or surface of junction
is an unconformity (Fig. 33).
Here it is evident that the sediments which compose the lower series of
beds have been folded in the zone of flow, though the upper series has
evidently escaped this vicissitude. Furthermore, the surface which delimits
the lower series from the upper is somewhat irregular and shows a hard
layer standing in relief, as it would if it had opposed greater resistance to
the attacks of the atmosphere upon it.
In reality, an unconformity between
formations must be interpreted to mean
that the lower series is not only older
than the upper, as shown by the order
of superposition, but that the time of its
deposition was separated from that of
the upper by a hiatus in which
important changes took place in the
lower series. The stages or episodes in
the history of the beds represented in
Fig. 34.—Series of diagrams to illustrate in
Fig. 33 may be read as follows (see
succession the episodes involved in the Fig. 34 a-e):—
historical development of an angular (a) Deposition of the lower series
unconformity. The vertical arrows
indicate direction of movement of the during a transgression of the sea.
land, and the horizontal arrows the (b) Continued subsidence and
direction of shore migration. burial of the lower series beneath
overlying sediments, and flexuring in the zone of flow.
(c) Elevation of the combined deposits to and far above sea level and
removal by erosion of vast thicknesses of the upper sediments.
(d) A new subsidence of the truncated lower series and deposition of the
upper series across its eroded surface.
(e) A new elevation of the double series to its present position above sea
level.
From this succession of episodes it is seen that a break of this kind
between two series of deposits involves a double oscillation of subsidence
followed by elevation—a large depression followed by a large elevation, a

smaller subsidence followed by elevation.
The time interval which must have been
represented by these repeated operations is
so vast as at first to stagger the mind in
contemplating it. When, as in this instance,
the dips of the lower series of beds differ
from those of the upper, we have to do
with an angular unconformity. It may be,
however, that the lower series was not so
far depressed as to enter the zone of flow,
and that its beds meet those of the upper
series with apparent conformity. Such an
unconformity is often extremely difficult to
recognize, and it is described as a
deceptive or erosional unconformity. Fig. 35.—Types of deceptive or
With a deceptive unconformity the erosional unconformities.
clew to its real nature is usually some fact
which indicates that the lower series of sediments had been raised above the
level of the sea before the upper series was deposited upon it. This may be
apparent either in the irregularity of the surface on which the two series are
joined, in some evidence of the action of waves such as would be furnished
by a basal conglomerate in the upper series, or some indication of different
resistance of different rocks of the lower series to attacks of the atmosphere
upon them (Figs. 33 and 35 a-c).
In most cases, at least, the lowest member of the upper series will be a
different type of rock from the uppermost member of the lower series,
hence the frequent occurrence of the discordant cross bedding in sandstone
should not deceive even the novice into the assumption of an unconformity.

Reading References to Chapter V

The zones of fracture and flow:—

C. R. Van Hise. Principles of North American Precambrian Geology, 16th
Ann. Rept. U.S. Geol. Surv., 1895, Pt. I, pp. 581-603.

Bailey Willis. Mechanics of Appalachian Structure, 13th Ann. Rept. U.S.
Geol. Surv., 1893, Pt. II, pp. 217-253.
A. Daubrée. Études Synthétiques de Géologie Expérimentale. Paris, 1879,
pp. 306-328, pl. II.
W. Prinz. Quelques remarques générales à propos de l’essai de carte
tectonique de la belgique, etc., Bull. Soc. Belge Geol., vol. 18, 1904, p.
143, pl. V.

Analysis of folds:—

Van Hise and Willis as above; de Margerie et Heim; Les dislocations de
l’écorce terrestre (in French and German languages). Zurich, 1888.

Geological maps:—

Wm. H. Hobbs. The Mapping of the Crystalline Schists, Jour. Geol., vol. 10,
1902, pp. 780-792, 858-890.

CHAPTER VI

THE ARCHITECTURE OF THE FRACTURED
SUPERSTRUCTURE

The system of the fractures.—
In referring to experiments made
upon the fracture of solid blocks
under compression (p. 41), it was
shown that two series of parallel
fractures develop perpendicular to
each free surface of the block, and
that these series are each of them
inclined by half of a right angle to
the direction of compression, and
thus perpendicular to each other. The
fragments into which a block with
one free surface would thus tend to
be divided should be square prisms
perpendicular to the free surface. It
would be interesting, if it were
Fig. 36.—A set of master joints developed in
shale upon the shores of Cayuga Lake near
practicable, to learn from experiment
Ithaca, New York (after U. S. G. S.). how these prisms would be further
fractured by a continuation of the
compression. From mechanical considerations involving the resolution of
forces with reference to the ready-formed fractures, it seems probable that
the next series of fractures to form would bisect the angles of the first
double series or set. Wherever rocks are found exposed in their original
attitudes, they are, in fact, seen to be intersected by two parallel series of

fractures which are perpendicular
to the earth’s surface and to each
other and are described as joints. In
many cases more than two series of
such fractures are found, yet even
in these cases two more perfectly
developed series are prominent and
almost exactly perpendicular to
each other as well as to the earth’s
surface. This omnipresent double
series or set of joints is the well-
known set of master joints, and
very often it is found developed
practically alone (Fig. 36). Over
large areas, the direction of the set Fig. 37.—Diagram to show how sets of
master joints differing in direction by half
of master joints may remain a right angle may abruptly replace each
practically constant, or this set may other.

quite suddenly give place to a similar
set which is, however, turned through
half a right angle from the first (Fig.
37). Not infrequently two such sets
of master joints are found together
bisecting each other’s angles within
the same rocks, and to them are
sometimes added additional though
less perfect series of joint planes.
Studied throughout a
considerable district, the various
series which make up these two sets
of master joints may be seen locally
developed in different combinations
as well as in association with
Fig. 38.—Diagram to show the different additional fissure planes which are
combinations of the series composing two not easily reduced to any simple law
double sets of master joints, and in a, a, a
additional disorderly fractures.
of arrangement (Fig. 38 a, a, a).
Only rarely are regular joint series

observed which do not stand perpendicular to the original attitude of the
rock beds. In a few localities, however, rectangular joint sets have been
discovered which divide the rock into prisms parallel to the earth’s surface
and with the joint series inclined to it each by half a right angle. Where the
rock beds have been much disturbed, the complex of joints may be such as
to defy all attempts at orderly arrangement.

Fig. 39.—View on the shore at Holstensborg, West Greenland, to show the
subequal spacing of the joints (after Kornerup).

The space intervals of joints.—
The same kind of subequal spacing
which characterizes the fractures near
the surface of the block in Daubrée’s
experiment (Fig. 19, p. 41) is found
simulated by the rock joints (Fig. 39).
Such unit intervals between fractures
may be grouped together into larger
units which are separated by fractures
of unusual perfection. We may think FigIceland
. 40.—View of an exposed hillside in
upon which the snow collected in
of such larger space units as having crannies along the joints brings out to
the smaller ones superimposed upon advantage both the larger and the
them (Fig. 40). smaller intervals of the joint system
(after Thoroddsen).
The displacements upon joints
—faults.—In the vast majority of cases, the joint fractures when carefully
examined betray no evidence of any appreciable movement of the two walls
upon each other. Generally the rock layers are seen to cross the joints

without apparent displacement. Joints are therefore planes of disjunction
only, and not planes of displacement.

Fig. 41.—Faulted blocks of basalt divided by joints near Woodbury,
Connecticut. To show the structure of the rock, some of the foliage has
been removed in preparing the sketch from a photograph.

Within many districts, however, a displacement may be seen to have
occurred upon certain of the joint planes, and these are then described as
faults. Such displacements of necessity imply a differential movement of
sections or blocks of the earth’s crust, the so-called orographic blocks,
which are bounded by the joint planes and play individual rôles in the
movement. A simple case of such displacements in rocks intersected by a
single set of master joints is represented in the model of plate 4 C. The most
prominent fault represented by this model runs lengthwise through the
middle, and the displacement which is measured upon it not only varies
between wide limits, but is marked by abrupt changes at the margins of the
larger blocks. This vertical displacement upon the fault is called its throw.
Though not illustrated by the model, horizontal displacements may likewise
occur, and these will be more fully discussed when the subject of
earthquakes is considered in the following chapter. An actual example of
blocks displaced by vertical adjustment is represented in Fig. 41, a simple
type of faulting which has taken place in rocks but slightly disturbed from
their original attitude, but intersected by a relatively simple system of
master joints. In those regions where the beds have been folded and perhaps
overthrust before their elevation into the zone of fracture, and which are
further intersected by disorderly fissure planes, the results are far more
complex. In such cases the planes of individual displacement may not be
vertical, though they are generally steeper than 45°. For their description it
is necessary to make use of additional technical terms (Fig. 42). The
inclination of a sloping fault plane measured against the vertical is called

the hade of the fault. The total displacement is measured along the plane of
the fault from a point upon one limb to the point from which it was
separated in the other. The additional terms are made sufficiently clear by
the diagram.
Methods of detecting faults.—The
first effect of a fault is usually to produce a
crack at the surface of the earth; and,
provided there is a vertical displacement or
throw, an escarpment which rises upon the
upthrown side of the fault. In general it
may be said that escarpments which appear Fig. 42.—A fault in previously
at the earth’s surface as plane surfaces disturbed strata. AB,
probably represent planes of fracture, displacement; AC, throw; BD,
stratigraphic throw; BC, heave;
though not necessarily planes of faulting. angle CAB, hade.
In many cases the actual displacements lie
buried under loose rock débris near to and paralleling the escarpment, and
in some cases as a result of the erosional processes working upon
alternately hard and soft layers of rock, the escarpment may later appear
upon the downthrown side or limb of the fault (Fig. 43). As an illustration
of a fault escarpment, the façade of El Capitan and many other rock faces of
the Yosemite valley may be instanced.
When we have further studied the
erosional processes at the earth’s
surface, it will be appreciated that
faults tend to quickly bury
themselves from sight, whereas fold
structures will long remain in
evidence. Many faults will thus be
overlooked, and too great weight is
likely to be ascribed to the folds in
accounting for the existing attitudes
and positions of the rock masses.
Faults must therefore be sought out if
mistakes of interpretation are to be
avoided.
The most satisfactory evidence of
a fault is the discovery of a rock bed

Fig. 43.—Diagrams to show how an which may be easily identified, and
escarpment originally on the upthrown
side of the fault may, through erosion,
which is actually seen displaced on a
appear upon the downthrown side. plane of fracture which intersects it
(Fig. 42, p. 59). When such an
easily recognizable layer is not to
be found, the plane of displacement
may perhaps be discovered as a
narrow zone composed of angular
fragments of the rock cemented
together by minerals which form
out of solution in water. Such a
fractured rock zone which follows
a plane of faulting is a fault
breccia. If the fault breccia, or vein
rock, is much stronger than the
rock on either side, it may
eventually stand in relief at the
surface like a dike or wall. At other
times the displacement produces FigThe . 44.—A fault plane exhibiting “drag.”
opening is artificial (after Scott).
little fracture of the walls, but they
slide over each other in such a manner as to yield either a smoothly
corrugated or an evenly polished surface which is described as
“slickensides.” It may be, however, that during the movement either one or
both of the walls have “dragged”, and so are curled back in the immediate
neighborhood of the fault plane (Fig. 44).
When, as is quite generally the case, the actual plane of displacement of
a fault is not open to inspection, the movement may be proven by the
observation of abrupt, as contrasted with gradual, changes in the strikes and
dips of neighboring exposures (Fig. 45); or by noting that some easily
recognized formation has been sharply offset in its outcrops (Fig. 46).
There are in addition many indications rather than proofs of the
presence of faults, which must be taken account of in every general study of
the geology of a district. Thus the outcrops of all neighboring formations
may terminate abruptly upon a straight line which intersects all alike. Deep-
seated fissure springs may be aligned in a striking manner, and so indicate
the course of a prominent fracture, though not necessarily of a fault. Much

the same may be said of the dikes
of cooled magma which have been
injected along preëxisting fractures.
The base of the geological
map.—Modern topographic maps
form an important part of the
library of the serious student of
physiography; they are the
gazetteer of this branch of science.
Every civilized nation has to-day
either completed a topographic
atlas of its territory, or it is
vigorously prosecuting a survey to
furnish maps which represent the
relief with some detail, and
publishing the results in the form of
an atlas of quadrangles. Thus a
relief map will erelong be Fig. 45.—Map to show how a fault may be
indicated in abrupt changes of the strike
obtainable of any part of the and dip of neighboring exposures.

civilized world, and may be
purchased in separate sections.
Nowhere is this work being taken up
with greater vigor than in the United
States, where a vast domain
representing every type of
topographic peculiarity is being
attacked from many centers. Here
and elsewhere the relief of the land is
Fig. 46.—A series of parallel faults indicated
being expressed by so-called
by successive offsets in the course of an
easily recognizable rock formation. contours or lines of equal altitude
upon the earth’s surface. It is as
though a series of horizontal planes, separated by uniform intervals of 20 or
40 or 100 feet, had been made to intersect the surface, and the intersection
curves, after consecutive numeration, had been dropped into a single plane
for printing.

Where the slopes are steep, the contour lines in the topographic map
will appear crowded together and so produce a deep shade upon the map;
whereas with relatively flat surfaces white patches will stand out
prominently upon the map. More and more the topographic map is coming
into use, and for the student of nature in particular it is important to acquire
facility in interpreting the relief from the topographic map. To further this
end, a special model has been devised, and its use is described in appendix
C. Usually before any satisfactory geological map can be prepared, a
contoured topographic map of the district to be studied must be available.
The field map and the areal geological map.—As the atlas of
topographic maps is the physiographic gazetteer, so geological maps
together constitute the reference dictionary of descriptive geology. Not only
are topographic maps of many districts now generally available, but more
and more it has become the policy of governments to supply geological
maps in the same quadrangle form which is the unit of the topographic map.
The geological map is, however, a complex of so many conventional
symbols, that without some practical experience in the actual preparation of
one, it is exceedingly difficult for the student to comprehend its
significance. A modern geological map is usually a rectangular sheet printed
in color, upon which are many irregular areas of individual hue joined to
each other like the parts of a child’s picture puzzle.
The colored areas upon the geological map are each supposed to
indicate where a certain rock type or formation lies immediately below the
surface, and this distribution represents the best judgment of the geologist
who, after a study of the district, has prepared the map. Unfortunately the
conventions in use are such that his observation and his theory have been
hopelessly intermingled in the finished product. Armed with the geological
map, the student who visits the district finds spread out before him, it may
be, a landscape of hill and valley, of green forest and brown farming land,
which is as different as may be from the colored puzzle which he holds in
his hand. Hidden under the farm vegetation or masked by the woods are
scattered outcroppings of rock which have been the basis of the geologist’s
judgment in preparing the map. Experience shows that in order to bridge the
wide gap between the geology in the landscape and the patches of color
upon the map something more than mere examination of the colored sheet
is necessary. We shall therefore describe, with the aid of laboratory models,
the various stages necessary to the preparation of a geological map, and

every student should be advised to follow this by practical study of some
small area where rocks are found in outcrop.
Though the published areal geological map represents both fact and
theory, the map maker retains an unpublished field map or map of
observations, upon which the final map has been based. This field map
shows the location of each outcrop that has been studied, with a record of
the kind of rock and of such observations as strike, dip, and pitch. Our task
will therefore be to prepare: (1) a field map; (2) an areal geological map;
and (3) some typical geological sections.
Laboratory models for the study of geological maps.—In order to
represent in the laboratory the disposition of rock outcrops in the field,
special laboratory tables are prepared with removable covers and with fixed
tops, which are divided into squares numbered like the township sections of
the national domain (Fig. 47). To represent the rock outcrops, blocks are
prepared which may be fixed in any desired position by fitting a pin into a
small augur hole bored through the table. The outcrop blocks for the
sedimentary rock types are so constructed as to show the strike and dip of
the beds. (See Appendix D.)

Fig. 47.—Field map prepared from a laboratory table.

The method of preparing the map.—To prepare the map, use is made
of a geological compass with clinometer attachment, a protractor, and a map
base divided into sections like the top of the table, and on the scale of one
inch to the foot. Each exposure represented upon the table is “visited” and
then located upon the base map in its proper position and attitude. The
result is the field map (Fig. 47), which thus represents the facts only, unless

there have been uncertainties in the correlation of exposures or in
determining the position of the bedding plane.

Fig. 48.—Areal geological map constructed from the field map of Fig. 47,
with two selected geological sections.

To prepare the areal geological map from the field map, it is first
necessary to fix the boundaries which separate formations at the surface;
and now perhaps for the first time it is realized how large an element of
uncertainty may enter if the exposures were widely separated. It is clear that
no two persons will draw these lines in the same positions throughout,
though certain portions of them—where the facts are more nearly adequate
—may correspond. In Fig. 48 is represented the areal geological map
constructed from the field map, with the doubtful area at one side left blank.
Some conclusions from this map may now be profitably considered.
The complexly folded sandstone formation at the left of the map appears as
the oldest member represented, since its area has been cut through by the
intrusive granite which does not intrude other formations, and is
unconformably overlaid by the limestone and its basal layer of
conglomerate. The limestone in turn is unconformably overlaid by the
merely tilted sandstone beds at the right of the map. These three
sedimentary formations clearly represent decreasing amounts of close
folding, from which it is clear that each earlier formation has passed
through an episode not shared by that of next younger age. Of the other
intrusive rocks, the dike of porphyry is younger than all the other
formations, with the possible exception of the upper sandstone. Offsetting

of the formations has disclosed the course of a fault, and from its relations
to the dikes we may learn that of these the porphyry is younger and the
basalt older than the date of the faulting.
The dashed lines upon the map (AB and CD) have been selected as
appropriate lines along which to construct geological sections (Fig. 48,
below map), and from these sections the exposed thicknesses of the
different formations may be calculated. In one instance only, that of the
conglomerate, can we be sure that this exposed thickness measures the
entire formation.
Fold versus fault topography.—The more resistant or “stronger” rock
beds, as regards attacks of the atmosphere, in the course of time come to
stand in relief, separated by depressions which overlie the “weaker”
formations. Simple open folds which are not plunging exercise an influence
upon topography by producing generally long and straight ridges. More
complex flexures, since they generally plunge, make themselves apparent
by features which in the map are represented by curves. Fracture structures,
and especially block displacements, are differentiated from these curving
features by the dominance of straight or nearly rectilinear lines upon the
map. The effect of erosion is to reduce the asperity of features and to mold
them with flowing curves. The fracture structures are for this reason much
more likely to be overlooked, and if they are not to elude the observer, they
must be sought out with care. Fold and fracture structures may both be
revealed upon the same map.

Reading References to Chapter VI

Joint systems:—

John Phillips. Observations made in the Neighborhood of Ferrybridge in
the Years 1826-1828, Phil. Mag., 2d ser., vol. 4, 1828, pp. 401-409;
Illustrations of the geology of Yorkshire, Pt. II, The Limestone District.
London, 1836, pp. 90-98.
Samuel Haughton. On the Physical Structure of the Old Red Sandstone of
the County of Waterford, considered with reference to cleavage, joint
surfaces, and faults, Trans. Roy. Soc. London, vol. 148, 1858, pp. 333-
348.

W. C. Brögger. Spaltenverwerfungen in der Gegend Langesund-Skien, Nyt
Magazin for Naturvidernskaberne, vol. 28, 1884, pp. 253-419.
Wm. H. Hobbs. The Newark System of the Pomperaug Valley, Connecticut,
21st Ann. Rept. U. S. Geol. Surv., Pt. III, 1901, pp. 85-143.

Geological map:—

Wm. H. Hobbs. The Interpretation of Geological Maps, School Science and
Mathematics, vol. 9, 1909, pp. 644-653.

CHAPTER VII

THE INTERRUPTED CHARACTER OF EARTH
MOVEMENTS: EARTHQUAKES AND SEAQUAKES

Nature of earthquake shocks.—Man’s belief in the stability of Mother
Earth—the terra firma—is so inbred in his nature that even a light shock of
earthquake brings a rude awakening. The terror which it inspires is no doubt
largely to be explained by this disillusionment from the most fundamental
of his beliefs. Were he better advised, the long periods of quiet which
separate earthquakes, and not the lighter shocks which follow all grander
disturbances, would occasion him concern.

Fig. 49.—View of a portion of the ruins of Messina after the earthquake of
December 28, 1908.

Earthquakes are the sensible manifestations of changes in level or of
lateral adjustments of portions of the continents, and the seismic
disturbances upon the sea—seaquakes and seismic sea waves—relate to
similar changes upon the floor of the ocean.
During the grander or catastrophic earthquakes, the changes are indeed
terrifying, and have usually been accompanied by losses to life and
property, which are only to be compared with those of great conflagrations
or of inundations on thickly populated plains. The conflagration has all too
frequently been an aftermath of the great historic earthquakes. The
earthquake of December 28, 1908, in southern Italy, destroyed almost the
entire population of a great city, and left of its massive buildings only a
confused heap of rubble (Fig. 49). Two years later a heavy earthquake
resulted in great damage to cities in Costa Rica (Fig. 50), while two years
earlier our own country was first really awakened to the danger in which it
stands from these convulsive earth throes; though, as we shall see, these
dangers can be largely met through proper methods of construction.

Fig. 50.—Ruins of the Carnegie Palace of Peace at Cartago, Costa Rica,
destroyed when almost completed by the great earthquake of May 4, 1910
(after a photograph by Rear-Admiral Singer, U.S.N.).

Earthquakes are usually preceded for a brief instant by subterranean
rumblings whose intensity appears to bear no relation to the shocks which
follow. The ground then rocks in wavelike motions, which, if of large
amplitude, may induce nausea, prevent animals from keeping upon their
feet, and wreck all structures not specially adapted to withstand them.
Heavy bodies are sometimes thrown up from the ground (Fig. 51), and at

other times similar heavy masses are, apparently because of their inertia,
more deeply imbedded in the earth. Thus gravestones and heavy stone posts
are often sunk more deeply in the ground and are surrounded by a hollow
and perhaps by small open cracks in the surface (Fig. 52). When bodies are
thrown upward, it would imply that a quick upward movement of the
ground had been suddenly arrested, while the burial of heavy bodies in the
earth is probably due to a movement which begins suddenly and is less
abruptly terminated.

Fig. 51.—Bowlders thrown into the air and overturned during the Assam
earthquake of 1897 (after R. D. Oldham).

Seaquakes and seismic sea waves.—
Upon the ocean the quakes which emanate
from the sea floor are felt on shipboard as
sudden joltings which produce the
impression that the ship has struck upon a Fig. 52.—Heavy post sunk deeper
shoal, though in most instances there is no into the ground during the
Charleston earthquake of August
visible commotion in the water. The 31, 1886 (after Dutton).
distribution of these shocks, as indicated
either by the experiences of neighboring ships at the time of a particular
shock, or by the records of vessels which at different times have sailed over
an area of frequent seismic disturbance, appears to be limited to narrow
zones or lines (Fig. 53). The same tendency of under-sea disturbances to be

localized upon definite straight lines has been often illustrated by the
behavior of deep-sea cables which are laid in proximity to one another and
which have been known to part simultaneously at points ranged upon a
straight line.
Far grander disturbances upon the
floor of the ocean have been revealed
by the great sea waves—the so-called
“tidal waves”, properly referred to as
tsunamis—which recur in those sea
districts which adjoin the special
earthquake zones upon the continents
(p. 86). The forerunner of such a sea
wave approaching the shore is usually
Fig. 53.—Map showing the localities at a sudden withdrawal of the water so as
which shocks have been reported at sea to lay bare a portion of the bottom, but
off Cape Mendocino, California. this is well-recognized to be the
premonition of a gigantic oncoming
wave which sweeps all before it and is only halted when it has rolled over
all the low-lying country and encountered a mountain wall. Such seismic
waves have been especially common upon the Pacific shore of South
America and upon the Japanese littoral (Fig. 54). These waves proceed
from above the great deeps upon the ocean bottom, and clearly result from
the grander earth movements to which these depressions owe their
exceptional depth. The withdrawal of the water from neighboring shores
may be presumed to be connected with a descent of the floor of the
depression and the consequent drawing-in of the ocean surface above. The
later high wave would thus represent the dispersion of the mountain of
water which is raised by the meeting of the waters from the different sides
of the depression.

Fig. 54.—Effect of a seismic water wave at Kamaishi, Japan, in 1896 (after
E. R. Scidmore).

The grander and the lesser earth
movements.—Upon the land the grander
and so-called catastrophic earthquakes are
usually the accompaniment of important
changes in the surface of the ground that will
be discussed in later sections. Those shocks
which do little damage to structures produce
no visible changes in the earth’s surface, Fig. 55.—A fault of vertical
except, it may be, to shake down some displacement.
water-soaked masses of earth upon the
steeper slopes. Still other movements, and these too slight to be felt even in
the night when the animal world is at rest, may yet be distinguished by their
sounds, the unmistakable rumblings which are characteristic alike of the
heaviest and the lightest of earthquake shocks.
Changes in the earth’s surface during
earthquakes—faults and fissures.—Each
of the grander among historic earthquakes
has been accompanied by noteworthy
changes in the configuration of the earth’s
surface within the district where the shocks
Fig. 56.—Escarpment produced by were most intense. A section of the ground is
an earthquake fault of vertical usually found to have moved with reference
displacement which cut across the to another upon the other side of a vertical
Chedrang River and thus
produced a waterfall, Assam
plane which is usually to be seen; we have

here to do with the actual making of a fault
earthquake of 1897 (after R. D.
Oldham).
or displacement such as we find the fossil
examples of within the rocks. The displacement, or throw, upon the fault
plane may be either upward or downward or laterally in one direction or the
other, or these movements may be combined. A movement of adjacent
sections of the ground upward or downward with reference to each other
(Fig. 55) has been often observed, notably at Midori after the great Japanese
earthquake of 1891, and in the Chedrang valley of Assam after the
earthquake of 1897 (Fig. 56).

A
lateral
throw,
unaccom
Fig. 57.—A fault of lateral panied Fig. 58.—Fence parted and
displacement. displaced fifteen feet by a
by
transverse fault formed during
appreciable vertical displacement (Fig. 57), the California earthquake of 1906
is especially well illustrated by the fault in (after W. B. Scott).
California which was formed during the
earthquake of 1906 (Fig. 58). A
combination of the two types of
displacement in one (Fig. 59) is
exemplified by the Baishiko fault of
Formosa at the place shown in plate 3 A.
The measure of displacement.—To
afford some measure of the displacements
which have been observed upon
earthquake faults, it may be stated that the
maximum vertical throw measured upon
the fault in the Neo valley of Japan (1891) Figlateral
. 59.—Fault with vertical and
displacements combined.
was 18 feet, in the Chedrang valley of
Assam (1897) 35 feet, and of the Alaskan coast (1899) 47 feet. Large
sections of land were bodily uplifted in these cases within the space of a
few seconds, or at most a few minutes, by the amounts given. The largest
recorded lateral displacement measured upon an earthquake fault is about
21 feet upon the California rift after the earthquake of 1906; though an

amount only slightly less than this is indicated in the shifting of roads and
arroyas dating from the earthquake of 1872 in the Owens valley, California.
Fault lines once established are planes of special weakness and become
later the seat of repeated movements of the same kind.

Plate 3.

A. An earthquake fault opened in Formosa in 1906, with vertical and
lateral displacements combined (after Omori).

B. Earthquake faults opened in Alaska in 1889, on which vertical slices of
the earth’s shell have undergone individual adjustments (after Tarr and
Martin).

The greater number of earthquake
faults are found in the loose rock
cover which so generally mantles the
firmer rock basement, and it is almost
certain that the throws within the
solid rock are considerably larger
than those which are here measured
at the surface, owing to the
adjustments which so readily take
place in the looser materials. Those
lighter shocks of earthquake which
are accompanied by no visible
displacements at the surface do,
however, in some instances affect in
Fig. 60.—Diagram to show how small
a measure the flow of water upon the faults in the rock basement may be
surface, and thus indicate that small masked at the surface through
changes of surface level have adjustments within the loose rock
mantle.
occurred without breaks sufficiently
sharp to be perceived (Fig. 60). Intermediate between the steep escarpment

and the masked displacement just described is the so-called “mole-hill”
effect,—a rounded and variously cracked slope or ridge above the position
of a buried fault (Fig. 61).
The escarpments due to earthquake
faults in loose materials at the earth’s
surface can obviously retain their
steepness for a few years or decades at
the most; for because of their
verticality they must gradually
disappear in rounded slopes under the
action of the elements. Smaller
displacements within a rock which
rapidly disintegrates under the action
of frost and sun will likewise before
Fig. 61.—Diagram to show the appearance long be effaced. In those exceptional
of a “mole hill” above a buried
instances where a resistant rock type
earthquake fault (after Kotô).
has had all altered upper layers planed
away until a fresh and hard surface is exposed, and has further been
protected from the frost and sun beneath a thin layer of soil, its original
surface may be retained unaltered for many centuries. Upon such a surface
the lightest of sensible shocks, or even the smaller earth movements which
are not perceived at the time, may leave an almost indelible record. Such
records particularly show that the movements which they register occur
upon the planes of jointing within the rock, and that these ready formed
cracks have probably been the seats of repeated and cumulative adjustments
(Fig. 62).

Contraction of the earth’s surface
during earthquakes.—The wide
variations in the amount of the lateral
displacement upon earthquake faults, like
those opened in California in 1906, show
that at the time of a heavy earthquake
Fig. 62.—Post-glacial earthquake faults there must be large local changes in the
of small but cumulative density of the surface materials. Literally,
displacement, eastern New York
(after Woodworth). thousands of fissures may appear in the
lowlands, many of them no doubt a

secondary effect of the shaking, but
others, like the quebradas of the
southern Andes or the “earthquake
cracks” in the Colorado desert (Fig. 63),
may have a deeper-seated origin. Many
facts go to show, however, that though
local expansion does occur in some
localities, a surface contraction is a far
more general consequence of earth
movement. In civilized countries of high
industrial development, where lines of
metal of one kind or another run for
long distances beneath or upon the
surface of the ground, such general Fig. 63.—Earthquake cracks in
contraction of the surface may be easily Colorado desert (after a photograph
by Sauerven).
proven. Comparatively seldom are lines
of metal pulled apart in such a way as to show an expansion of the surface;
whereas bucklings and kinkings of the lines appear in many places to prove
that the area within which they are found has, as a whole, been reduced.

Fig. 64.—Diagrams to show how railway tracks are either broken or
buckled locally within the district visited by an earthquake.

Fig. 65.—The Biwajima railroad bridge in Japan after the earthquake of
1891 (after Milne and Burton).

Water pipes laid in the ground at a
depth of some feet may be bowed up
into an arch which appears above the
surface; lines of curbing are raised into
broken arches, and the tracks of
Fig. 66.—Diagrams to show how the
railways are thrown into local loops
compression of a district and its
consequent contraction during an and kinks which imply a very
earthquake may close up the joint spaces considerable local contraction of the
within the rock basement and surface (Fig. 64). With unvarying
concentrate the contraction of the
overlying mantle where this is partially
regularity railway or other bridges
cut through and so weakened in the which cross rivers or ravines, if the
valley sections. structures are seriously damaged,
indicate that the river banks have
drawn nearer together at the time of the disturbance. In such cases,
whenever the bridge girder has remained in place upon its abutments, these
have either been broken or back-tilted as a whole in such a manner as to
indicate an approach of the foundations which was prevented at the top by
the stiffness of the girder (Fig. 65).

Fig. 67.—Map of the Chedrang fault which made its appearance during the
Assam earthquake of 1897. The figures give the amounts of the local

vertical displacement measured in feet (after R. D. Oldham).

The simplest explanation of such an approach of the banks at the sides
of the valleys cut in loose surface material is to be found in a general
closing up of the joint spaces within the underlying rock, and an adjustment
of the mantle upon the floor mainly in the valley sections (Fig. 66).
The plan of an earthquake fault.—In
our consideration of earthquake faults we
have thus far given our attention to the
displacement as viewed at a single locality
only. Such displacements are, however,
continued for many miles, and sometimes
for hundreds of miles; and when now we
examine a map or plan of such a line of
faulting, new facts of large significance
make their appearance. This may be well
illustrated by a study of the plan of the
Chedrang fault which appeared at the time
of the Assam earthquake of 1897 (Fig. 67).
From this map it will be noticed that the
upward or downward displacement upon
the perpendicular plane of the fault is not
uniform, but is subject to large and sudden
changes. Thus in order the measurements
in feet are 32, 0, 18, 35, 0, 8, 25, 12, 8, 2,
0. The fault formed in 1899 upon the
shores of Russell Fjord in Alaska (Fig. 68)
reveals similar sudden changes of throw,
only that here the direction of the
movement is often reversed; or, otherwise Fig. 68.—Map giving the
expressed, the upthrow is suddenly displacements in feet measured
along an earthquake fault formed
transferred from one side of the fault to the in Alaska in 1899 (after Tarr and
other. Such abrupt changes in the direction Martin).
of the displacement have been observed
upon many earthquake faults, and a particularly striking one is represented
in Fig. 69.

The block movements of the disturbed
district.—The displacements upon
earthquake faults are thus seen to be
subdivided into sections, each of which
differs from its neighbors upon either side
and is sharply separated from them, at least
Fig. 69.—Abrupt change in the
direction of throw upon an
in many instances. These points of abrupt
earthquake fault which was change of displacement are, in many cases at
formed in the Owens valley, least, the intersection points with transverse
California, in 1872. The observer faults (Fig. 69). Such points of abrupt
looks directly along the course of
the fault from the left foreground change in the degree or in the direction of
to the cliff beyond and to the left the displacement may be, when looked at
of the impounded water (after a from above, abrupt turning points in the
photograph by W. D. Johnson).
direction of extension of the fault, whose
course upon the map appears as a zigzag line made up of straight sections
connected by sharp elbows (Fig. 70).
Such a grouping of surface faults as are represented upon the map is
evidence that the area of the earth’s shell, which is included, has at the time
of the earthquake been subject to adjustments as a series of separate units or
blocks, certain of the boundaries of which are the fault lines represented.
The changes in displacement measured upon the larger faults make it clear
that the observed faults can represent but a fraction of the total number of
lines of displacement, the others being masked by variations in the
compactness of the loose mantling deposits. Could we but have this mantle
removed, we should doubtless find a rock floor separated into parts like an
ancient Pompeiian pavement, the individual blocks in which have been
thrown, some upward and some downward, by varying amounts. Less than
a hundred miles away to the eastward from the Owens Valley, a portion of
this pavement has been uncovered in the extensive operations of the
Tonapah Mining District, so that there we may study in all its detail the
elaborate pattern of earth marquetry (Fig. 71) which for the floor of the
Owens valley is as yet denied us.
The earth blocks adjusted during the Alaskan earthquake of 1899.
—For a study of the adjustments which take place between neighboring
earth blocks during a great earthquake, the recent Alaskan disturbance has
offered the advantage that the most affected district was upon the seacoast,
where changes of level could be referred to the datum of the sea’s surface.

He
re
a
gre
at
isla
nd
an
d
lar
ge
sec
tio
ns Fig. 71.—Marquetry of the rock floor of
the Tonapah Mining District, Nevada
of (after Spurr).
the
neighboring shore underwent movements
both as a whole in large blocks and in
adjustments of their subordinate parts
among themselves (Fig. 72). Some
sections of the coast were here elevated
by as much as 47 feet, while neighboring
sections were uplifted by smaller amounts
(Fig. 73), and certain smaller sections
were even dropped below the level of the
Fig. 70.—Map of the faults within an
area of the Owens valley, California, sea.
formed in part during the The amount of such subsidence is,
earthquake of 1872, and in part due
to early disturbances, In the western
however, difficult to ascertain, for the
portions the displacements cut across reason that the former shore features are
firm rock and alluvial deposits alike now covered with water and thus
without deviation of direction (after a removed from observation. In favorable
map by W. D. Johnson).
localities the minimum amount of
submergence may sometimes be measured upon forest trees which are now
flooded with sea water. In Fig. 74 a portion of the coast is represented
where the beach sand is now extended back into the spruce forest, a
distance of a hundred feet or more, and where sedgy beach grass is growing
among trees whose roots are now laved in salt water. At the front of this

forest the
great
storm
waves
overturn
the trees
and pile
the Fig. 73.—View on Haencke Island,
Disenchantment Bay, Alaska,
wreckag revealing the shore that rose
e in front seventeen feet above the sea
of those during the earthquake of 1899,
and was found with barnacles
that still still clinging to the rock (after
remain Tarr and Martin).
standing.

Fig. 72.—Map of a portion of the
Alaskan coast to show the
Upon the glaciated rock surfaces of
adjustments in level during the the Alaskan coast, exceptionally favorable
earthquake of 1899 (after Tarr and opportunities are found for study of the
Martin). intrica
te
patter
n of
the
earth
mosai
Fig. 74.—Partially submerged forest c Fig. 75.—Settlement of a section of the
upon the shore of Knight Island, which shore at Port Royal, Jamaica, during
Alaska, due to the sinking of a the earthquake of January 14, 1907,
section of the coast during the is adjacent to a similar but larger
earthquake of 1899 (after Tarr and under settlement of the near shore during
Martin). adjust the earthquake of 1692 (after a
ment at the time of an earthquake. Upon photograph by Brown).
Gannett Nunatak the surface was found
divided by parallel faults into distinct slices which individually underwent
small changes of level (plate 3 B).

CHAPTER VIII

THE INTERRUPTED CHARACTER OF EARTH
MOVEMENTS: EARTHQUAKES AND SEAQUAKES
(Concluded)

Experimental demonstration of earth movements.—The study of the
Alaskan earthquake of 1899 showed that during this adjustment within the
earth’s shell some of the local blocks moved upward and by larger amounts
than their neighbors, and that still others were actually depressed so that the
sea flowed over them. It must be evident that such differential vertical
movements of neighboring blocks at the earth’s surface can only take place
if lateral transfers of material are made beneath it. From under those strips
of coast land which were depressed, material must have been moved so as
to fill the void which would otherwise have formed beneath the sections
that were uplifted. If we take into consideration much larger fractions upon
the surface of our planet, we are taught by the great seaquakes which are
now registered upon earthquake instruments at distant stations that large
downward movements are to-day in progress beneath the sea much more
than sufficient to compensate all extensions of the earth’s surface within
those districts where the land is rising in mountains. From under the
offshore deeps of the ocean to beneath the growing mountains upon the
shore, a transfer of earth material must be assumed to take place when
disturbances are registered.
Within the time interval that separates the sudden adjustments of the
surface which are manifested in earthquakes, the condition of strain which
brings them about is steadily accumulating, due, as we generally assume, to
earth contraction through loss of its heat. It seems probable that the

resistance to an immediate adjustment is found in the rigidity of the shell
because of the compression to which it is subjected. To illustrate: a row of
blocks well fitted to each other may be held firmly as a bridge between the
jaws of a vice, because so soon as each block starts to fall a large resistance
from friction upon its surface is called into existence, a force which
increases with the degree of compression.
It is thus possible upon this assumption crudely to demonstrate the
adjustment of earth blocks by the simple device represented in plate 4 A.
The construction of this experimental tank is so simple that little
explanation is necessary. Wooden blocks of different heights are supported
in water within a tank having a glass front, and are kept in a strained
condition at other than their natural positions of flotation by the
compression of a simple vice at the top. Held firmly in this position, they
may thus represent the neighboring blocks within the earth’s outer shell
which are supported upon relatively yielding materials beneath, and
prevented from at once adjusting themselves to their natural positions
through the compression to which they are subjected. Held as they now are,
the water near the ends of the tank is forced up beneath the blocks to higher
than its natural level, and thus tends to flow from both ends toward the
center. Such a movement would permit the end blocks to drop and force the
middle ones to rise. The end blocks are, let us say, the sections of Alaskan
coast line which sunk during the earthquake, as the center blocks are the
sections which rose the full measure of 47 feet. Upon a larger scale the end
blocks may equally well be considered as the floor of the great deeps off the
Alaskan coast, whose sinking at the time of the earthquake was the cause of
the great sea wave. Upon this assumption the center blocks would represent
the Alaskan coast regarded as a whole, which underwent a general uplift.
Though we may not, in our experiment, vary the tendency to adjustment
by any contractional changes in either the water or the blocks, we may
reduce the compression of the vice, which leads to the same general result.
As the compression of the vice is slowly relaxed, a point is at last reached at
which friction upon the block surfaces is no longer sufficient to prevent an
adjustment taking place, and this now suddenly occurs with the result
shown in plate 4 B. In the case of the earth blocks, this sudden adjustment is
accompanied by mass movements of the ground separated by faults, and
these movements produce successional vibrations that are particularly large
near the block margins, and other frictional vibrations of such small

measure as to be generally appreciated by sounds only. The jolt of the
adjustments has thrown some blocks beyond their natural position of rest,
and these sink and rise subsequently in order to readjust themselves with
lighter vibrations, which may be repeated and continued for some time. In
the case of the earth these later adjustments are the so-called aftershocks
which usually continue throughout a considerable period following every
great earthquake. Gradually they fall off in intensity and frequency until
they can no longer be felt, and are thereafter continued for a time as
rumblings only.

Plate 4.

A. Experimental tank to illustrate the earth movements which are manifested
in earthquakes. The sections of the earth’s shell are here represented
before adjustment has taken place.

B. The same apparatus after a sudden adjustment.

C. Model to illustrate a block displacement in rocks which are intersected
by master joints.

Derangement of water flow by earth movement.—The water which
supported the blocks in our experiment has represented the more mobile
portion of the earth’s substance beneath its outer zone of fracture. The
surface water layers in the tank may, however, be considered in a different
way, since their behavior is remarkably like that of the water within and
upon the earth’s surface during an earth adjustment. At the instant when
adjustment takes place in the tank, water frequently spurts upward from the
cracks between the sinking end blocks; and if in place of one of the higher
center blocks we insert one whose top is below the level of the water in the
tank, a “lake” will be formed above it. When the adjustment occurs, this
lake is immediately drained by outflow of the water at its bottom along one
of the cracks between the blocks (Fig. 76).
Such derangements of water flow
as have been illustrated by the
experiment are among the
commonest of the phenomena which
accompany earthquakes. Lakes and
swamp lands have during
earthquakes been suddenly drained,
fountains of water have been seen to
shoot up from the surface and have
played for some minutes or hours
before their sudden disappearance in Fig. 76.—Diagrams to illustrate the
a sucking down of the water with draining of lakes during earthquakes.

later readjustment. During the great earthquake of the lower Mississippi
valley in 1811, known as the New Madrid earthquake, the earlier Lake
Eulalie was completely drained, and upon the now exposed bed there
appeared parallel fissures on which were ranged funnel-like openings down
which the water had been sucked. In other sections of the affected region
the water shot up in sheets along fissures to the tops of high trees. Areas
where such spurting up of the water has been observed have in most cases
been shown to correspond to areas of depression, and such areas have
sometimes been left flooded with water. During the Indian earthquake of
1819 an area of some 200 square miles suddenly sank and was transformed
into a lake.

Fig. 77.—Diagram to illustrate-the derangements of flow of water at the time
of an earthquake; water issuing at the surface over downthrown rocks, and
being sucked down in upthrown blocks.

Fig. 78.—Mud cones aligned upon a fissure opened at Moraza, Servia,
during the earthquake of April 4, 1904 (after Michailovitch).

Sand or mud cones and craterlets.—From a very moderate depth
below the surface to that of several miles, all pore spaces and all larger

openings within the rock are completely filled with water, the “trunk lines”
of whose circulation is by way of the joints or along the bedding planes of
the rocks. The principal reservoirs, so to speak, of this water inclosed within
the rock are the porous sand formations. When, now, during an earthquake a
block of the earth’s shell is suddenly sunk and as suddenly arrested in its
downward movement, the effect is to compress the porous layers and so
force the contained water upward along the joints to the surface, carrying
with it large quantities of the sand (Fig. 77).

Fig. 79.—One of the many craterlets formed near Charleston, South
Carolina, during the earthquake of August 31, 1886. The opening is twenty
feet across, and the leaves about it are encased in sand as were those upon
the branches of the overhanging trees to a height of some twenty feet (after
Dutton).

Ejected at the surface this water
appears in fountains usually arranged
in line over joints, or even in
continuous sheets, and the sand
collecting about the jets builds up lines
of sand or mud cones sometimes
described as “mud volcanoes” (Fig.
78). The amount of sand thus poured Fig. 80.—Cross section of a craterlet to
out is sometimes so great that blankets show the trumpet-like form of the sand
column.
of quicksand are spread over large
sections of the country. Most frequently, however, the sand is not built

above the general level of the surface, but forms a series of craterlets which
are largely shaped as the water is sucked down at the time of the
readjustment with which the play of such earthquake fountains is
terminated (Fig. 79). Subsequent excavations made about such craterlets
have shown them to have the form of a trumpet, and that in the sand which
so largely fills them there are generally found scales of mica and such light
bodies as would be picked out from the heterogeneous materials of the sand
layers and carried upward in the rush of water to the surface (Fig. 80).
The earth’s zones of heavy earthquake.—Since earthquakes give
notice of a change of level of the ground, the special danger zones from this
source are the growing mountain systems which are usually found near the
borders of the sea. Such lines of mountains are to-day rising where for long
periods in the past were the basins of deposition of former seas. They thus
represent the zones upon the earth’s surface which are the most unstable—
which in the recent period have undergone the greatest changes of level.
By far the most unstable belt upon the
earth’s surface is the rim surrounding the
Pacific Ocean, within which margin it has
been estimated that about 54 per cent of
the recorded shocks of earthquake have
occurred. Next in importance for seismic
instability is the zone which borders both
the Mediterranean Sea and the Caribbean
—the American Mediterranean—and is
extended across central Asia through the
Himalayas into Malaysia. Both zones FigIschia . 81.—Map of the island of
to show how the shocks of
approximate to great circles upon the recent earthquakes have been
earth’s surface and intersect each other at concentrated at the crossing point
an angle of about 67°. It has been of two fractures (after Mercalli
and Johnston-Lavis).
estimated that about 95 per cent of the
recorded continental earthquakes have emanated from these belts.
The special lines of heavy shock.—Within any earthquake district the
shocks are not felt with equal severity at all places, but there are, on the
contrary, definite lines which the disturbance seems to search out for special
damage. From their relations to the relief of the land these lines would
appear to be lines of fracture upon the boundaries of those sections of the
crust that play individual rôles in the block adjustment which takes place.

More or less masked as these lines are beneath the
rounded curves of the landscape, they are given an
altogether unenviable prominence with each
succeeding earthquake. At such times we may think
of the earth’s surface as specially sensitized for
laying bare its hidden structure, as is the sensitized
plate under the magical influence of the X rays.
When, at the time of an earthquake, blocks are
adjusted with reference to their neighbors, the
movements of oscillation are greatest in those
marginal portions of direct contact. Corners of
blocks—the intersecting points of the important
faults—should for the same reason be shaken with a
double violence, and this assumption appears to be
confirmed by observation. Upon the island of
Ischia, off the Bay of Naples, the shocks from recent
earthquakes have been strangely concentrated near
Fig. 82.—A line of earth the town of Casamicciola, which was last destroyed
fracture indicated in the in 1883. This unfortunate city lies at the crossing
plan of the relief, which point of important fractures whose course upon the
may at any time become
the seat of movement island is marked by numerous springs and suffioni
and resultant shock. (Fig. 81).
Seismotectonic lines.—The lines of important
earth fractures, as will be more clearly shown in the sequel (p. 227), are
often indicated with some clearness by straight lines in the plan of the
surface relief (Fig. 82). Lines of this nature are easily made out upon the
map of the West Indies, and if we represent upon it by circles of different
diameters the combined intensities of the recorded earthquakes in the
various cities, it appears that the heavily shaken localities are ranged upon
lines stamped out in the relief, with the most severely damaged places at
their intersections (Fig. 83). These lines of exceptional instability are known
as seismotectonic lines—earthquake structure lines.

Fig. 83.—Seismotectonic lines of the West Indies.

The heavy shocks above loose foundations.—It is characteristic of
faults that they soon bury themselves from sight under loose materials, and
are thus made difficult of inspection. The escarpment which is the direct
consequence of a vertical displacement upon a fault tends to migrate from
the place of its formation, rounding the surface as it does so and burying the
fault line beneath its deposits (Fig. 43, p. 60).
This is not, however, the sole reason why loose foundations should be
places of special danger at the time of earth shocks, for the reason that
earthquake waves are sent out in all directions from the surfaces of
displacement through the medium of the underlying rock. These waves
travel within the firm rock for considerable distances with only a gradual
dissipation of their energy, but with their entry into the loose surface
deposits their energy is quickly used up in local vibrations of large
amplitude, and hence destructive to buildings.
The essential difference between firm
rock and such loose materials as are found
upon a river bottom or in the “made land”
about our cities may be illustrated by the
simple device which is represented in Fig.
84. Two similar metal pans are suspended
from a firm support by bands of steel and
Fig. 84.—Device to illustrate the “elastic” braid of similar size and shape, and
different effects upon the carry each a small block of wood standing
transmission and the character of
shocks which are produced by
upon its end. Similar light blows are now
firm rock and by loose materials. administered directly to the pans with the
effect of upsetting that block which is
supported by the loose braid because of the large range or amplitude of
movement that is imparted to the pan. The “elastic” braid, because of these
large vibrations of which it is susceptible, may represent the loose materials

when an earthquake wave passes into them. In the case of the steel support,
the energy of the blow, instead of being dissipated in local swingings of the
pan, is to a large extent transmitted through the elastic metal to materials
beyond. The steel thus resembles in its high elasticity the firmer rock
basement, which receives and transmits the earthquake shocks, but except
when ruptured in a fault is subject to vibrations of small amplitude only.
Construction in earthquake regions.—Wherever earthquakes have
been felt, they are certain to occur again; and wherever mountains are
growing or changes of level are in progress, there no record of past
earthquakes is required in order to forecast the future seismic history.
Although the future earthquakes may be predicted, the time of their coming
is, fortunately or unfortunately, still hidden from us. If one’s lot is to be cast
in an earthquake country, the only sane course to pursue is to build with due
regard to future contingencies.
The danger, from destructive fires may to-day be largely met by
methods of construction which levy an additional burden of cost. Though
the danger from seismic disturbances can hardly be met as fully as that from
fire, yet it is true that buildings may be so constructed as to withstand all
save those heaviest shocks in the immediate vicinity of the lines of large
displacement. Here, also, a considerable additional expense is involved in
the method of construction, in the case of residences particularly.
From what has been said, it is obvious that much of the danger from
earthquakes can be met by a choice of site away from lines of important
fracture and from areas of relatively loose foundation. The choice of
building materials is next of importance. Those buildings which succumb to
earthquakes are in most cases racked or shaken apart, and thus they become
a prey to their own inherent properties of inertia. Each part of a structure
may be regarded as a weight which is balanced upon a stiff rod and pivoted
upon the ground. When shocks arrive, each part tends to be thrown into
vibration after the manner of an inverted pendulum. In proportion,
therefore, as the weights are large and rest upon long supports, the danger
of overthrow and of tearing apart is increased. In general, structures are best
constructed of light materials whose weight is concentrated near the ground.
Masonry structures, and especially high ones, are, therefore, the least suited
for resisting earthquakes, of which the late complete destruction of the city
of Messina is a grewsome reminder. Despite repeated warnings in the past,
the buildings of that stricken city were generally constructed of heavy

rubble, which in addition had been poorly cemented (Fig. 49, p. 67). Such
structures are usually first ruptured at the edges and corners, since here the
vibrations which tend to tear the building asunder are resisted by no
supports and are reënforced from neighboring walls.

Fig. 85.—House wrecked in San Francisco earthquake of 1906 because the
floors and partitions were not securely fastened to the walls (after R. L.
Humphrey).

An advantage of the first importance is evidently secured if the rods of
the pendulum, of which the building is conceived to be composed, have
sufficient elasticity to be considerably distorted without rupture and to again
recover their original position. This is the supreme advantage of structural
steel for all large buildings, which is coupled, however, with the
disadvantage that the riveted fastenings are apt to be quickly sheered off
under the vibrations. Large and high buildings, when sufficiently elastic,
have fortunately the property of destroying the earth waves by interference
before they have traveled above the lower stories.
For large structures in which wood cannot be used, strongly reënforced
concrete is well adapted, for it has in general the same advantages as steel
with somewhat reduced elasticity, but with a more effective binding
together of the parts. This requirement of thorough bracing and tying
together of the several parts of a building causes it to vibrate, not as many
pendulums, but as one body. If met, it removes largely the danger from
racking strains, and for small structures particularly it is the requirement
which is most easily complied with. For such buildings it is therefore
necessary that the framework should be built in a close network with every

joint firmly braced and with all parts securely tied together. Especial
attention should be given to the fastenings of floor and partition ends. The
house shown in Fig. 85 could not have been subjected to heavy shocks, for
though the walls are thrown down, the floors and partitions have been left
near their original positions.

Fig. 86.—Building wrecked at San Mateo, California, during the late
earthquake. The heavy roof and upper floor, acting as a unit, have battered
down the upper walls (after J. C. Branner).

This tendency of the walls, floors, partitions, and roof to act as
individual units in the vibration, is one that must be reckoned with and be
met by specially effective bracing and tying at the junctions. Otherwise
these larger parts of the structure may act like battering rams to throw over
the walls or portions of them (Fig. 86).

Reading References for Chapters VII and VIII

General works:—
John Milne. Seismology. London, 1898, pp. 320.
C. E. Dutton. Earthquakes in the Light of the New Seismology. Putnam,
New York, 1904, pp. 314.
A. Sieberg. Handbuch der Erdbebenkunde. Braunschweig, 1904, pp. 362.

Count F. de Montessus de Ballore. Les Tremblements de Terre,
Géographie Séismologique. Paris, 1906, pp. 475; La Science
Séismologique. Paris, 1907, pp. 579.
William Herbert Hobbs. Earthquakes, an Introduction to Seismic Geology.
Appleton, New York, 1907, pp. 336.
C. G. Knott. The Physics of Earthquake Phenomena. Clarendon Press,
Oxford, 1908, pp. 283.
E. Rudolph. Ueber Submarine Erdbeben und Eruptionen, Beiträge zur
Geophysik, vol. 1, 1887, pp. 133-365; vol. 2, 1895, pp. 537-666; vol.
3, 1898, pp. 273-536.

Descriptive reports of some important earthquakes:—
C. E. Dutton. The Charleston Earthquake of August 31, 1886, 9th Ann.
Rept. U. S. Geol. Surv., 1889, pp. 203-528.
B. Kotô. On the Cause of the Great Earthquake in Central Japan, 1891,
Jour. Coll. Sci. Imp. Univ., Tokyo, Japan, vol. 5, 1893, pp. 295-353,
pls. 28-35.
John Milne and W. K. Burton. The Great Earthquake of Central Japan.
1891, pp. 10, pls. 30.
R. D. Oldham. Report on the Great Earthquake of 12th June, 1897, Mem.
Geol. Surv. India. Vol. 29, 1899, pp. 379, pls. 42.
A. C. Lawson, and others. The California Earthquake of April 18, 1906,
Report of the State Earthquake Investigation Commission, three quarto
vols. (Carnegie Institution of Washington); many plates and figures.
Italian Photographic Society, Messina and Reggio before and after the
Earthquake of December 28, 1908 (an interesting collection of
pictures). Florence, 1909.
R. S. Tarr and L. Martin. Recent Changes of Level in the Yakutat Bay
Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, pls.
12-23.

William Herbert Hobbs. The Earthquake of 1872 in the Owens Valley,
California, Beiträge zur Geophysik, vol. 10, 1910, pp. 352-385, pls,
10-23.

Faults in connection with earthquakes:—
William H. Hobbs. On Some Principles of Seismic Geology, Beiträge zur
Geophysik, vol. 8, 1907, Chapters iv-v.

Expansion or contraction of the earth’s surface during earthquakes:—
William H. Hobbs. A Study of the Damage to Bridges during Earthquakes,
Jour. Geol., vol. 16, 1908, pp. 636-653; The Evolution and the Outlook
of Seismic Geology, Proc. Am. Phil. Soc., vol. 48, 1909, pp. 27-29.

Earthquake construction:—
John Milne. Construction in Earthquake Countries, Trans. Seis. Soc., Japan,
vol. 14, 1889-1890, pp. 1-246.
F. de Montessus de Ballore. L’art de bâtir dans les pays à tremblements de
terre (34th Congress of French Architects), L’Architecture, 193 Année,
1906, pp. 1-31.
Gilbert, Humphrey, Sewell, and Soulé. The San Francisco Earthquake and
Fire of April 18, 1906, and their Effects on Structures and Structural
Materials, Bull. 324, U. S. Geol. Surv., 1907, pp. 1-170, pls. 1-57.
William H. Hobbs. Construction in Earthquake Countries, The Engineering
Magazine, vol. 37, 1909, pp. 1-19.
Lewis Alden Estes. Earthquake-proof Construction, a discussion of the
effects of earthquakes on building construction with special reference
to structures of reënforced concrete, published by Trussed Concrete
Steel Company. Detroit, 1911, pp. 46.

CHAPTER IX

THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE

VOLCANIC MOUNTAINS OF EXUDATION

Prevalent misconceptions about volcanoes.—The more or less
common impression that a volcano is a “burning mountain” or a “smoking
mountain” has been much fostered by the school texts in physical
geography in use during an earlier period. The best introduction to a
discussion of volcanoes is, therefore, a disillusionment from this notion. Far
from being burning or smoking, there is normally no combustion whatever
in connection with a volcanic eruption. The unsophisticated tourist who,
looking out from Naples, sees the steam cap which overhangs the Vesuvian
crater tinged with brown, easily receives the impression that the material of
the cloud is smoke. Even more at night, when a bright glow is reflected to
his eye and soon fades away, only to again glow brightly after a few
moments have passed, is it difficult to remove the impression that one is
watching an intermittent combustion within the crater. The cloud which
floats away from the crest of the mountain is in reality composed of steam
with which is admixed a larger or smaller proportion of fine rock powder
which gives to the cloud its brownish tone. The glow observed at night is
only a reflection from molten lava within the crater, and the variation of its
brightness is explained by the alternating rise and fall of the lava surface by
a process presently to be explained.
Not only is there no combustion in connection with volcanic eruptions,
but so far as the volcano is a mountain it is a product of its own action. The
grandest of volcanic eruptions have produced no mountains whatever, but
only vast plains or plateaus of consolidated molten rock, and every volcanic

mountain at some time in its history has risen out of a relatively level
surface.
When the traditional notions about volcanoes grew up, it was supposed
that the solid earth was merely a “crust” enveloping still molten material.
As has already been pointed out in an earlier chapter, this view is no longer
tenable, for we now know that the condition of matter within the earth’s
interior, while perhaps not directly comparable to any that is known, yet has
properties most resembling known matter in a solid state; it is much more
rigid than the best tool steel. While there must be reservoirs of molten rock
beneath active volcanoes, it is none the less clear that they are small, local,
and temporary. This is shown by the comparative study of volcanic outlets
within any circumscribed district.
It is perhaps not easy to frame a definition of a volcano, but its essential
part, instead of being a mountain, is rather a vent or channel which opens
up connection between a subsurface reservoir of molten rock and the
surface of the earth. An eruption occurs whenever there is a rise of this
material, together with more or less steam and admixed gases, to the
surface. Such molten rock arriving at the surface is designated lava. The
changes in pressure upon this material during its elevation induce secondary
phenomena as the surface is approached, and these manifestations are often
most awe inspiring. While often locally destructive, the geological
importance of such phenomena is by reason of their terrifying aspect likely
to be greatly exaggerated.
Early views concerning volcanic mountains.—As already pointed out,
a volcano at its birth is not a mountain at all, but only, so to speak, a shaft or
channel of communication between the surface and a subterranean reservoir
of molten rock. By bringing this melted rock to the surface there is built up
a local elevation which may be designated a mountain, except where the
volume of the material is so large and is spread to such distances as to
produce a plain (see fissure eruptions below).
In the early history of geology it was the view of the great German
geologist von Buch and his friend and colleague von Humboldt, that a
volcanic mountain was produced in much the same manner as is a blister
upon the body. The fluids which push up the cuticle in the blister were here
replaced by fluid rock which elevated the sedimentary rock layers at the
surface into a dome or mound which was open at the top—the so-called

crater. This “elevation-crater” theory of volcanoes long held the stage in
geological science, although it ignored the very patent fact that the layers on
the flanks of volcanic cones are not of sedimentary rock at all, but, on the
contrary, of the volcanic materials which are brought up to the surface
during the eruption. The observational phase of science was, however,
dawning, and the English geologists Scrope and Lyell were able to show by
study of volcanic mountains that the mound about the volcanic vent was
due to the accumulation of once molten rock which had been either exuded
or ejected. Making use of data derived from New Zealand, Scrope showed
that, instead of being elevated during the formation of a volcanic mountain,
the sedimentary strata of the vicinity may be depressed near the volcanic
vent (Fig. 87).
The birth of volcanoes.—To
confirm the impression that the
formation of the volcanic mountain is
in reality a secondary phenomenon
connected with eruptions, we may cite
Fig. 87.—Breached volcanic cone near the observed birth of a number of
Auckland, New Zealand, showing the volcanoes. On the 20th of September,
bending down of the sedimentary strata
in the neighborhood of the vent (after
1538, a new volcano, since known as
Heaphy and Scrope). Monte Nuovo (new mountain), rose on
the border of the ancient Lake
Lucrinus to the westward of Naples. This small mountain attained a height
of 440 feet, and is still to be seen on the shore of the bay of Naples. From
Mexico have been recorded the births of several new volcanoes: Jorullo in
1759, Pochutla in 1870, and in 1881 a new volcano in the Ajusco
Mountains about midway between the Gulf of Mexico and the Pacific
Ocean. The latest of new volcanoes is that raised in Japan on November 9,
1910, in connection with the eruption of Usu-san. This “New Mountain”
reached an elevation of 690 feet.
As described by von Humboldt,
Jorullo rose in the night of the 28th of
September, 1759, from a fissure which
opened in a broad plain at a point 35
miles distant from any then existing
Fig. 88.—View of the new Camiguin
volcano from the sea. It was formed in
volcano. The most remarkable of new
1871 over a nearly level plain. The town volcanoes rose in 1871 on the island of

of Catarman appears at the right nearCamiguin northward from Mindanao
the shore (after an unpublished
photograph by Professor Dean C.
in the Philippine archipelago. This
Worcester). mountain was visited by the
Challenger expedition in 1875, and
was first ascended and studied thirty years later by a party under the
leadership of Professor Dean C. Worcester, the Secretary of the Interior of
the Philippine Islands, to whom the writer is indebted for this description
and the accompanying illustration of this largest and most interesting of
new-born volcanoes. As in the case of Jorullo, the eruption began with the
formation of a fissure in a level plain, some 400 yards distant from the town
of Catarman (Fig. 88). The eruption continued for four years, at the end of
which time the height of the summit was estimated by the Challenger
expedition to be 1900 feet. At the time of the first ascent in 1905, the height
was determined by aneroid as 1750 feet, with sharp rock pinnacles
projecting some 50 or 75 feet higher.
Active and extinct volcanoes.—The terms “active” and “extinct” have
come into more or less common use to describe respectively those
volcanoes which show signs of eruptive activity, and those which are not at
the time active. The term “dormant” is applied to volcanoes recently active
and supposed to be in a doubtfully extinct condition. From a well-known
volcano in the vicinity of Naples, volcanoes which no longer erupt lava or
cinder, but show gaseous emanations (fumeroles) are said to be in the
solfatara condition, or to show solfataric activity.
Experience shows that the term “extinct”, while useful, must always be
interpreted to mean apparently extinct. This may be illustrated by the
history of Mount Vesuvius, which before the Christian era was forested in
the crater and showed no signs of activity; and in fact it is known that for
several centuries no eruption of the volcano had taken place. Following a
premonitory earthquake felt in the year 63, the mountain burst out in grand
explosive eruption in 79 A.D. This eruption profoundly altered the aspect of
the mountain and buried the cities of Pompeii, Stabeii, and Herculaneum
from sight. Once more, this time during the middle ages, for nearly five
centuries (1139 to 1631) there was complete inactivity, if we except a light
ash eruption in the year 1500. During this period of rest the crater was again
forested, but the repose was suddenly terminated by one of the grandest
eruptions in the mountain’s history.

Fig. 89.—Map showing the location of the belts of active volcanoes.

The earth’s volcano belts.—The distribution of volcanoes is not
uniform, but, on the contrary, volcanic vents appear in definite zones or
belts, either upon the margins of the continents or included within the
oceanic areas (Fig. 89). The most important of these belts girdles the Pacific
Ocean, and is represented either by chains or by more widely spaced
volcanic mountains throughout the Cordilleran Mountain system of South
and Central America and Mexico, by the volcanoes of the Coast and
Cascade ranges of North America, the festooned volcanic chain of the
Aleutian Islands, and the similar island arcs off the eastern coast of the
Eurasian continent. The belt is further continued through the islands of
Malaysia to New Zealand, and on the Pacific’s southern margin are found
the volcanoes of Victoria Land, King Edward Land, and West Antarctica.

Fig. 90.—A portion of the “fire girdle” of the Pacific, showing the relation
of the chains of volcanic mountains to the deeps of the neighboring ocean
floor.

This volcano girdle is by no means a perfect one, for in addition to the
principal festoons of the western border there are many secondary ones, and

still other arcs are found well toward the center of the oceanic area. Another
broad belt of volcanoes borders the Mediterranean Sea, and is extended
westward into the Atlantic Ocean. Narrower belts are found in both the
northern and southern portions of the Atlantic Ocean, on the margins of the
Caribbean Sea, etc. The fact of greatest significance in the distribution
seems to be that bands of active volcanoes are to be found wherever
mountain ranges are paralleled by deeps on the neighboring ocean floor
(Fig. 90). As has been already pointed out in the chapter upon earthquakes,
it is just such places as these which are the seat of earthquakes; these are
zones of the earth’s crust which are undergoing the most rapid changes of
level at the present time. Thus the rise of the land in mountains is
proceeding simultaneously with the sinking of the sea floor to form the
neighboring deeps.

Fig. 91.—Volcanic cones formed in 1783 above the Skaptár fissure in
Iceland (after Helland).

Arrangement of volcanic vents along
fissures and especially at their
intersections.—Within those districts in
which volcanoes are widely separated from
their neighbors, the law of their arrangement
Fig. 92.—Diagrams to illustrate the is difficult to decipher, but the view that
location of volcanic vents upon volcanic vents are aligned over fissures is
fissure lines, a, openings caused now supported by so much evidence that
by lateral movement of fissure
illustrations may be supplied from many
walls; b, openings formed at
fissure intersections. regions. An exceptionally perfect line of
small cones is found along the Skaptár cleft
in Iceland, upon which stands the large volcano of Laki. This fissure
reopened in 1783, and great volumes of lava were exuded. Over the cleft
there was left a long line of volcanic cones (Fig. 91). There are in Iceland
two dominating series of parallel fissures of the same character which take
their directions respectively northeast-southwest and north-south. Many
such fissures are traceable at the surface as deep and nearly straight clefts or
gjás, usually a few yards in width, but extending for many miles. The

Eldgjá has a length of more than 18 English miles and a depth varying from
400 to 600 feet. On some of these fissures no lava has risen to the surface,
whereas others have at numerous points exuded molten rock. Sometimes
one end only of a fissure, the more widely gaping portion, has supplied the
conduits for the molten lava. This is well illustrated by the cratered
monticules raised by the common ant over the cracks which separate the
blocks of cement sidewalk, the hillocks being located where the most
favorable channel was found for the elevation of the materials.

Fig. 93.—Outline map of the eastern portion of the island of Java, displaying
the arrangement of volcanic vents in alignment upon fissures with the
larger mountains at fissure intersections (after Verbeek).

Those places upon fissures which become lava conduits appear to be the
ones where the cleft gapes widest so as to furnish the widest channel.
Wherever a differential lateral movement of the walls has occurred,
openings will be found in the neighborhood of each minor variation from a
straight line (Fig. 92 a). Wherever there are two or more series of fissures,
and this would appear to be the normal condition, places favorable for lava
conduits occur at fissure intersections. Within such veritable volcano
gardens as are to be found in Malaysia, the law of volcano distribution
became apparent so soon as accurate maps had been prepared. Thus the
outline map of a portion of the island of Java (Fig. 93) shows us that while
the volcanoes of the island present at first sight a more or less irregular
band or zone, there are a number of fissures intersecting in a network, and
that the volcanoes are aligned upon the fissures with the larger cones
located at the intersections. So also in Iceland, the great eruption of Askja in
1875 occurred at the intersection of two lines of fissure.

Outside these closely packed volcanic regions, similar though less
marked networks are indicated; as, for example, in and near the Gulf of
Guinea. If now, instead of reducing the scale of our volcano maps, we
increase it, the same law of distribution is no less clearly brought out. The
monticules or small volcanic cones which form upon the flanks of larger
volcanic mountains are likewise built up over fissures which on numerous
occasions have been observed to open and the cones to form upon them.
Still further reducing now the
area of our studies and considering
for the moment the “frozen” surface
of the boiling lava within the caldron
of Kilauea, this when observed at
night reveals in great perfection the
sudden formation of fissures in the
crust with the appearance of Fig. 94.—Map of the Puy Pariou in the
miniature volcanoes rising Auvergne of central France. The seat of
eruption has migrated along the fissure
successively at more or less regular upon which the earlier cone had been
intervals along them. built up (after Scrope).
It not infrequently happens that
after a volcanic vent has become established above some conduit in a
fissure, the conduit migrates along the fissure, thus establishing a new cone
with more or less complete destruction of the old one (Fig. 94).
The so-called fissure eruptions.—The grandest of all volcanic
eruptions have been those in which the entire length and breadth of the
fissures have been the passageway for the upwelling lava. Such grander
eruptions have been for the most part prehistoric, and in later geologic
history have occurred chiefly in India, in Abyssinia, in northwestern
Europe, and in the northwestern United States. In western India the
singularly horizontal plateaus of basaltic lava, the Dekkan traps, cover some
200,000 square miles and are more than a mile in depth. The underlying
basement where it appears about the margins of the basalt is in many places
intersected by dikes or fissure fillings of the same material. No cones or
definite vents have been found.
The larger portion of the northwestern British Isles would appear to
have been at one time similarly blanketed by nearly horizontal beds of
basaltic lava, which beds extended northwestward across the sea through

the Orkney and Faroe islands to Iceland.
Remnants of this vast plateau are to-day
found in all the island groups as well as in
large areas of northeastern Ireland, and
fissure fillings of the same material occur
throughout large areas of the British Isles. In
many cases these dikes represent once
Fig. 95.—Basaltic plateau of the
molten rock which may never have
northwestern United States due communicated with the surface at the time
to fissure eruptions of lava. of the lava outpouring, yet they well
illustrate what we might expect to find if the
basalt sheets of Iceland or Ireland were to be removed.
The floods of basaltic lava which in the northwestern United States have
yielded the barren plateau of the Cascade Mountains (Fig. 95) would appear
to offer another example of fissure eruption, though cones appear upon the
surface and perhaps indicate the position of lava outlets during the later
phases of the eruptive period. The barrenness and desolation of these lava
plains is suggested by Fig. 96.

Fig. 96.—Lava plains about the Snake River in Idaho.

Though the greater effusions of lava have occurred in prehistoric times,
and the manner of extrusion has necessarily been largely inferred from the
immense volume of the exuded materials and the existence of basaltic dikes
in neighboring regions, yet in Iceland we are able to observe the connection
between the dikes and the lava outflows. Professor Thoroddsen has stated
that in the great basaltic plateau of Iceland, lava has welled out quietly from
the whole length of fissures and often on both sides without giving rise to

the formation of cones. At three wider portions of the great Eld cleft, lava
welled out quietly without the formation of cones, though here in the
southern prolongation of the fissure, where it was narrower, a row of low
slag cones appeared. Where the lava outwellings occurred, an area of 270
square miles was flooded.

Fig. 97.—Characteristic profiles of lava volcanoes. 1, basaltic lava
mountain; 2, mountain of siliceous lava (after Judd).

The composition and the properties of lava.—In our study of igneous
rocks (Chapter IV) it was learned that they are composed for the most part
of silicate minerals, and that in their chemical composition they represent
various proportions of silica, alumina, iron, magnesia, lime, potash, and
soda. The more abundant of these constituents is silica, which varies from
35 to 70 per cent of the whole. Whenever the content of silica is relatively
low,—basic or basaltic lava,—the cooled rock is dark in color and relatively
heavy. It melts at a relatively low temperature, and is in consequence
relatively fluid at the temperatures which lavas usually have on reaching the
earth’s surface. Furthermore, from being more fluid, the water which is
nearly always present in large quantity within the lava more readily makes
its escape upon reaching the surface. Eruptions of such lava are for this
reason without the violent aspects which belong to extrusions of more
siliceous (more “acidic”) lavas. For the same reason, also, basaltic lava
flows more freely and can spread much farther before it has cooled
sufficiently to consolidate. This is equivalent to saying that its surface will
assume a flatter angle of slope, which in the case of basaltic lava seldom
exceeds ten degrees and may be less than one degree (Fig. 97).
Siliceous lavas, on the other hand, are, when consolidated, relatively
light both in color and weight and melt at relatively high temperatures.

They are, therefore, usually but partly fused
and of a viscous consistency when they arrive
at the earth’s surface. Because of this viscosity
they offer much resistance to the liberation of
the contained water, which therefore is
released only to the accompaniment of more
or less violent explosions. The lava is blown
into the air and usually falls as consolidated
Fig. 98.—A driblet cone (after J. fragments of various degrees of coarseness.
D. Dana).
It must
not, however, be assumed that the
temperature of lava is always the same
when it arrives at the surface, and hence it
may happen that a siliceous lava is exuded
at so high a temperature that it behaves like
a normal basaltic lava. On the other hand, Figcrater, . 99.—View of Leffingwell
a cinder cone in the Owens
basaltic lavas may be extruded at unusually valley, California (after an
low temperatures, in which case their unpublished photograph by W. D.
behavior may resemble that of the normal Johnson).
siliceous lavas. If, however, as is generally
the case, the energy of explosion of a basaltic lava is relatively small, any
ejected portions of the liquid lava travel to a moderate height only in the air,
so that on falling they are still sufficiently pasty to adhere to rock surfaces
and thus build up the remarkably steep cones and spines known as “spatter
cones” or “driblet cones” (Fig. 98). When, on the other hand, the energy of
explosion is great, as is normally the case with siliceous lavas, the portions
of ejected lava have been fully consolidated before their fall to the surface,
so that they build up the same type of accumulation as would sand falling in
the same manner. The structures which they form are known as tuff, cinder,
or ash cones (Fig. 99).
Whenever the contained water passes off from siliceous lavas without
violent explosions, the lava may flow from the vent, but in contrast to
basaltic lavas it travels a short distance only before consolidating. The
resulting mountain is in consequence proportionately high and steep (Fig.
97). Eruptions characterized by violent explosions accompanied by a fall of
cinder are described as explosive eruptions. Those which are relatively

quiet, and in which the chief product is in the form of streams of flowing
lava, are spoken of as convulsive eruptions.
The three main types of volcanic mountain.—If the eruptions at a
volcanic vent are exclusively of the explosive type, the material of the
mountain which results is throughout tuff or cinder, and the volcano is
described as a cinder cone. If, on the other hand, the vent at every eruption
exudes lava, a mountain of solid rock results which is a lava dome. It is,
however, the exception for a volcano which has a long history to manifest
but a single kind of eruption. At one time exuding lava comparatively
quietly, at another the violence with which the steam is liberated yields only
cinder, and the mountain is a composite of the two materials and is known
as a composite volcanic cone.
The lava dome.—When successive lava flows come from a crater, the
structure which results has the form of a more or less perfect dome. If the
lava be of the basaltic or fluid type, the slopes are flat, seldom making an
angle of as much as ten degrees with the horizon and flatter toward the
summit (Fig. 101, p. 106). If of siliceous or viscous lava, on the other hand,
the slopes are correspondingly steep and in some cases precipitous. To this
latter class belong some of the Kuppen of Germany, the puys of central
France, and the mamelons of the Island of Bourbon.
The basaltic lava domes of
Hawaii.—At the “crossroads of the
Pacific” rises a double line of lava
volcanoes which reach from 20,000 to
30,000 feet above the floor of the
ocean, some of them among the
grandest volcanic mountains that are
known. More than half the height and a
much larger proportion of the bulk of
the largest of these are hidden beneath
the ocean’s surface. The two great
active vents are Mokuaweoweo (on
Fig. 100.—Map of Hawaii and the lava
Mauna Loa) and Kilauea, distinct
volcanoes of Mokuaweoweo (Mauna volcanoes notwithstanding the fact that
Loa) and Kilauea (after the government their lava extravasations have been
map by Alexander). merged in a single mass. The rim of
the crater of Mauna Loa is at an elevation of 13,675 feet above the sea,

whereas that of Kilauea is less than 4000 feet and appears to rest upon the
flank of the larger mountain (Figs. 100 and 101). Although one crater is but
20 miles distant from the other and nearly 10,000 feet lower, their eruptions
have apparently been unsympathetic. Nowhere have still active lava
mountains been subjected to such frequent observations extending
throughout a long period, and the dynamics of their eruptions are fairly well
understood. To put this before the reader, it will be best to consider both
mountains, for though they have much in common, the observations from
one are strangely complementary to those of the other. The lower crater
being easily accessible, Kilauea has been often visited, and there exists a
long series of more or less consecutive observations upon it, which have
been assembled and studied by Dana and Hitchcock. The place of outflow
of the Kilauea lavas has not generally been visible, whereas Mokuaweoweo
has slopes rising nearly 14,000 feet above the sea and displays the records
of outflow of many eruptions, some of which were accompanied by the
grandest of volcanic phenomena.

Fig. 101.—Section through Mauna Loa and Kilauea.

Lava movements within the caldron of Kilauea.—The craters of
these mountains are the largest of active ones, each being in excess of seven
miles in circumference. In shape they are irregularly elliptical and consist of
a series of steps or terraces descending to a pit at the bottom, in which are
open lakes of boiling lava. Enough is known of the history of Kilauea to
state that the steep cliffs bounding the terraces are fault walls produced by
inbreak of a frozen lava surface. The cliff below the so-called “black ledge”
was produced by the falling in of the frozen lava surface at the time of the
outflow of 1840, the lava issuing upon the eastern flank of the mountain
and pouring into the sea near Nanawale. Since that date the floor of the pit
below the level of this ledge has been essentially a movable platform of
frozen lava of unknown and doubtless variable thickness which has risen
and descended like the floor of an elevator car between its guiding ways
(Fig. 102). The floor has, however, never been complete, for one or more
open lakes are always to be seen, that of Halemaumau located near the

southwestern margin having been much the most persistent. Within the
open lakes the boiling lava is apparently white hot at the depth of but a few
inches below the surface, and in the overturnings of the mass these hotter
portions are brought to the surface and appear as white streaks marking the
redder surface portions. From time to time the surface freezes over, then
cracks open and erupt at favored points along the fissures, sending up jets
and fountains of lava, the material of which falls in pasty fragments that
build up driblet cones. Small fluid clots are shot out, carrying a threadlike
line of lava glass behind them, the well-known “Pelé’s hair.” Sometimes the
open lakes build up congealed walls, rising above the general level of the
pit, and from their rim the lava spills over in cascades to spread out upon
the frozen floor, thus increasing its thickness from above (Fig. 103). At
other times a great dome of lava has been pushed up from the pit of
Halemaumau under a frozen shell, the molten lava shining red through
cracks in its surface and exuding so as to heal each widely opened fissure as
it forms.
At intervals of from a few years to nine
or ten years the crater has been periodically
drained, at which times the moving
platform of frozen lava has sunk more or
less rapidly to levels far below the black Fig. 102.—Schematic diagram to
illustrate the moving platform of
ledge and from 900 to 1700 feet below the frozen lava which rises and falls
crater rim. Following this descent a slow in the crater of Kilauea.
progressive rise is inaugurated, which has
sometimes gone on at a rate of more than a hundred feet per year, though it
is usually much slower than this. When the platform has reached a height
varying from 700 to 350 feet below the crater rim, another sudden
settlement occurs which again carries the pit floor downward a distance of
from 300 to 700 feet.

Fig. 103.—View of the open lava lake of Halemaumau within the crater of
Kilauea, the molten lava shown cascading over the raised lava walls on to
the floor of the pit (after Pavlow).

The draining of the lava caldrons.—The changes which go on within
the crater of Mokuaweoweo, though less studied than those of Kilauea,
appear to be in some respects different. Here every eruption seems to be
preceded by a more or less rapid influx of melted lava to the pit of the
crater, this phenomenon being observed from a distance as a brilliant light
above the crater—the reflection of the glow from overhanging vapor
clouds. The uprising of the lava has often been accompanied by the
formation of high lava fountains upon the surface, and the molten lava
sometimes appears in fissures near the crater rim at levels well above the
lava surface within the pit.
Although in many cases the lava which has thus flooded the crater has
suddenly drained away without again becoming visible, it is probable that
in such cases an outlet has been found to some submarine exit, since under-
ocean discharge effects have been observed in connection with eruptions of
each of the volcanoes.
Inasmuch as no earthquakes are felt in connection with such outflows as
have been described, it is probable that the hot lava fuses a passageway for
itself into some open channel underneath the flanks of the mountain. Such a
course is well illustrated by the outflow of Kilauea in 1840, when, it will be
remembered, occurred the great down-plunge of the crater that yielded the
pit below the black ledge. At this time the lava first made its appearance

upon the flanks of the mountain at
the bottom of a small pit or inbreak
crater which opened five miles
southeast of the main crater of
Kilauea (Fig. 104). Within this new
crater the lava rose, and small
ejections soon followed from fissures
formed in its neighborhood. Some
time after, the lava sank in the first Fig. 104.—Map showing the manner of
new crater, only to reappear outflow of lava from Kilauea during the
successively at other small openings eruption of 1840. The outflowing lava
made its appearance successively at the
(Fig. 104, B, C, m, n) and finally to points A, B, C, m, n, and finally at a
issue in volume at a point eleven point below n, from whence it issued in
miles from the shore and flow volume and flowed down to the sea at
thereafter upon the surface of the Nanawale (after J. D. Dana).

mountain until it had reached the sea.
Only the slightest earth tremors were felt, and as no rumblings were heard,
it is evident that the lava fused its way along a buried channel largely open
at the time (see below, p. 112).
In a majority of the eruptions of Mokuaweoweo, when the outflowing
lavas have become visible, the molten rock has apparently fused its way out
to the surface of the mountain at points from 1000 to 3000 feet below the
bottom of the crater, and this discharge has corresponded in time to the
lowering of the lava surface within the crater. There are, however, three
instances upon record in which the lava issued from definite rents which
were formed upon the mountain flanks at comparatively low levels. In
contrast to the formation of fused outlets, these ruptures of a portion of the
mountain’s flank were always accompanied by vigorous local earthquakes
of short duration. In one instance (the eruption of 1851) such a rent
appeared under the same conditions but at an elevation of 12,500 feet, or
near the level of the lava in the crater.
The outflow of the lava floods.—In order to properly comprehend
these and many otherwise puzzling phenomena connected with volcanoes,
it is necessary to keep ever in mind the quite remarkable heat-insulating
property of congealed lava. So soon as a thin crust has formed upon the
surface of molten rock, the heat of the underlying fluid mass is given off

with extreme slowness, so that lava streams no longer connected with their
internal lava reservoirs may remain molten for decades.

Fig. 105.—Lava of Matavanu upon the Island of Savaii flowing down to the
sea during the eruption of 1906. The course may be followed by the jets of
steam escaping from the surface down to the great steam cloud which rises
where the fluid lava discharges into the sea (after H. I. Jensen).

We have seen that for Mokuaweoweo and Kilauea, lava either quietly
melts its way to the surface at the time of outflow, or else produces a rent
for its egress to the accompaniment of vigorous local earthquakes. In either
case if the lava issues at a point far below the crater, gigantic lava fountains
arise at the point of outflow, the fluid rock shooting up to heights which
range from 250 to 600 or more feet above the surface. A certain proportion
of this fluid lava is sufficiently cooled to consolidate while traveling in the
air, and falling, it builds up a cinder cone which is left as a location
monument for the place of discharge. From this outlet the molten lava
begins its journey down the slope of the mountain, and quickly freezes over
to produce a tunnel, beneath the roof of which the fluid lava flows with
comparatively slow further loss of heat. Save for occasional steam jets
issuing from its surface, it may give little indication of its presence until it
has reached the sea (Fig. 105).

Fig. 106.—Lava stream discharging into the sea from beneath the frozen
roof of a lava tunnel. Eruption of Matavanu on Savaii in 1906 (after
Sapper).

If sufficient in volume and the shore be not too distant, the stream of
lava arrives at the sea, where, discharging from the mouth of its tunnel, it
throws up vast volumes of steam and induces ebullition of the water over a
wide area (Fig. 106). Professor Dana, who visited Hawaii a few months
only after the great outflow of 1840, states that the lava, upon reaching the
ocean, was shivered like melted glass and thrown up in millions of particles
which darkened the sky and fell like hail over the surrounding country. The
light was so bright that at a distance of forty miles fine print could be read
at midnight.
Protected from any extensive
consolidation by its congealed cover,
the lava within a stream may all drain
Fig. 107.—Diagrammatic representation of away, leaving behind an empty lava
the structure of the flanks of lava tunnel, which in the case of the
volcanoes as a result of the draining of
frozen lava streams. Hawaiian volcanoes sometimes has its
roof hung with beautiful lava
stalactites and its floor studded with thin lava spines. Later lava outflows
over the same or neighboring courses bury such tunnels beneath others of
similar nature, giving to the mountain flanks an elongated cellular structure

illustrated schematically in Fig. 107. These buried channels may in the
future be again utilized for outflows similar in character to that of Kilauea
in 1840.
While the formation of lava
stalactites of such perfection and
beauty is peculiar to the Hawaiian
lava tunnels, the formation of the Fig. 108.—Diagram to show the manner of
tunnel in connection with lava formation of mesas or table mountains
by the outflow of lava in valleys and the
outflow is the rule wherever a subsequent more rapid erosion of the
dissipation at the end has permitted intervening ridges. R, earlier river
of drainage. A few hours only after valley; R’R’, later valleys.
the flow has begun, the frozen
surface has usually a thickness of a few inches, and this cover may be
walked over with the lava still molten below. At first in part supported by
the molten lava, the tunnel roof sometimes caves in so soon as drainage has
occurred.

Fig. 109.—Surface of lava of the Pahoehoe type.

Wherever basaltic lava has spread out in valleys on the surface of more
easily eroded material, either cinder or sedimentary formations, the softer
intervening ridges are first carried away by the eroding agencies, leaving
the lava as cappings upon residual elevations. Thus are derived a type of
table mountain or mesa of the sort well illustrated upon the western slopes
of the Sierra Nevadas in California (Fig. 108).
The surface which flowing lava assumes, while subject to considerable
variation, may yet be classified into two rather distinct types. On the one
hand there is the billowy surface in which ellipsoidal or kidney-shaped

masses, each with dimensions of
from one to several feet, lie merged
in one another, not unlike an irregular
collection of sofa pillows. This type
of lava has become known as the
Pahoehoe, from the Hawaiian
occurrence (Fig. 109). A variation
from this type is the “corded” or
“ropy” lava, the surface of which
much resembles rope as it is coiled
along the deck of a vessel, the coils
being here the lines of scum or
scoriæ arranged in this manner by the
currents at the surface of the stream
(Fig. 123, p. 124). A quite different
type is the block lava (Aa type) Fig. 110.—Three successive views to
which usually has a ragged illustrate the growth of the Island of
Savaii from the outflow of lava at
scoriaceous surface and consists of Matavanu in the year 1906. a, near the
more or less separate fragments of beginning of the outflow; b, some weeks
cooled lava (Fig. 131, p. 130). later than a; c, some weeks later than b
(after H. I. Jensen).
Wherever lava flows into the sea
in quantity, it extends the margin of the shore, often by considerable areas.
The outflow of Kilauea in 1840 extended the shore of Hawaii outward for
the distance of a quarter of a mile, and a more recent illustration of such
extension of land masses is furnished by Fig. 110.

CHAPTER X

THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE

VOLCANIC MOUNTAINS OF EJECTED MATERIALS

The mechanics of crater explosions.—If we now turn from the lava
volcano to the active cinder cone, we encounter an entire change of scene.
In place of the quiet flow and convulsive movements of the molten lava, we
here meet with repeated explosions of greater or less violence. If we are to
profitably study the manner of the explosions, considering the volcanic vent
as a great experimental apparatus, it would be well to select for our purpose
a volcano which is in a not too violent mood. The well-known cinder cone
of Stromboli in the Eolian group of islands north of Sicily has, with short
and unimportant interruptions, remained in a state of light explosive activity
since the beginning of the Christian era. Rising as it does some three
thousand feet directly out of the Mediterranean, and displaying by day a
white steam cap and an intermittent glow by night, its summit can be seen
for a distance of a hundred miles at sea and it has justly been called the
“Lighthouse of the Mediterranean.” The “flash” interval of this beacon may
vary from one to twenty minutes, and it may show, furthermore,
considerable variation of intensity.
For the reason that the crater of the mountain is located at one side and
at a considerable distance below the actual summit, the opportunity here
afforded of looking into the crater is most favorable whenever the direction
of the wind is such as to push aside the overhanging steam cloud (Fig. 111).
Long ago the Italian vulcanologist Spallanzani undertook to make
observations from above the crater, and many others since his day have
profited by his example.

Within the crater of the volcano there is seen a lava surface lightly
frozen over and traversed by many cracks from which vapor jets are
issuing. Here, as in the Kilauea crater, there are open pools of boiling lava.
From some of these, lava is seen welling out to overflow the frozen surface;
from others, steam is ejected in puffs as though from the stack of a
locomotive. Within others lava is seen heaving up and down in violent
ebullition, and at intervals a great bubble of steam is ejected with explosive
violence, carrying up with it a considerable quantity of the still molten lava,
together with its scumlike surface, to fall outside the crater and rattle down
the mountain’s slope into the sea. Following this explosion the lava surface
in the pool is lowered and the agitation is renewed, to culminate after the
further lapse of a few minutes in a second explosion of the same nature. The
rise of the lava which precedes the ejection appears at night as a brighter
reflection or glow from the overhanging steam cloud—the flash seen by the
mariner from his vessel.
What is going on within the crater
of Stromboli we may perhaps best
illustrate by the boiling of a stiff
porridge over a hot fire. Any one who
has made corn mush over a hot camp
fire is fully aware that in proportion as
the mush becomes thicker by the
Fig. 111.—The volcano of Stromboli,
addition of the meal, it is necessary to
showing the excentric position of the stir the mass with redoubled vigor if
crater (after a sketch by Judd). anything is to be retained within the
kettle. The thickening of the mush
increases its viscosity to such an extent that the steam which is generated
within it is unable to make its escape unless aided by openings continually
made for it by the stirring spoon. If the stirring motion be stopped for a
moment, the steam expands to form great bubbles which soon eject the
pasty mass from the kettle.
For the crater of Stromboli this process is illustrated by the series of
diagrams in Fig. 112. As the lava rises toward the surface, presumably as a
result of convectional currents within the chimney of the volcano, the
contained steam is relieved from pressure, so that at some depth below the
surface it begins to separate out in minute vesicles or bubbles, which,
expanding as they rise, acquire a rapidly accelerating velocity. Soon they

flow together with a quite sudden increase of their expansive energy, and
now shooting upward with further accelerated velocity, a layer of liquid
lava with its cover of scum is raised on the surface of a gigantic bubble and
thrown high into the air. Cooled during their flight, the quickly congealed
lava masses become the tuff or volcanic ash which is the material of the
cinder cone.

Fig. 112.—Diagrams to illustrate the nature of eruptions within the crater
of Stromboli.

Grander volcanic eruptions of cinder cones.—Most cinder and
composite cones, in the intervals between their grander eruptions, if not
entirely quiescent, lapse into a period, of light activity during which their
crater eruptions appear to be in all essential respects like the habitual
explosions within the Strombolian crater. This phase of activity is,
therefore, described as Strombolian. By contrast, the occasional grander
eruptions which have punctuated the history of all larger volcanoes are
described in the language of Mercalli as Vulcanian eruptions, from the best
studied example.
Just what it is that at intervals brings on the grander Vulcanian outburst
within a volcano is not known with certainty; but it is important to note that
there is an approach to periodicity in the grander eruptions. It is generally
possible to distinguish eruptions of at least two orders of intensity greater
than the Strombolian phase; a grander one, the examples of which may be
separated by centuries, and one or more orders of relatively moderate

intensity which recur at intervals perhaps of decades, their time intervals
subdividing the larger periods marked off by the eruptions of the first order.
The eruption of Volcano in 1888.—In
the Eolian Islands to the north of Sicily was
located the mythical forge of Vulcan. From
this locality has come our word “volcano”,
and both the island and the mountain bear no
other name to-day (Fig. 113). There is in the
structure of the island the record of a
somewhat complex volcanic history, but the
form of the large central cinder cone was,
according to Scrope, acquired during the
eruption of 1786, at which time the crater is
reported to have vomited ash for a period of
fifteen days. Passing after this eruption into
Fig. 113.—Map of Volcano in the
the solfatara condition, with the exception of
Eolian group of islands. The a light eruption in 1873, the volcano
smaller craters partially dissected remained quiet until 1886. So active had
by the waves belong to Vulcanello been the fumeroles within the crater during
(after Judd).
the latter part of this period that an extensive
plant had been established there for the collection especially of boracic
acid. In 1886 occurred a slight eruption, sufficient to clear out the bottom of
the crater, though not seriously to disturb the English planter whose
vineyards and fig orchards were in the valley or atrio near the point d upon
the map (Fig. 113), nearly a mile from the crater rim. On the 3d of August,
1888, came the opening discharge of an eruption, which, while not of the
first order of magnitude, was yet the greatest in more than a century of the
mountain’s history, and may serve us to illustrate the Vulcanian phase of
activity within a cinder cone. During the day, to the accompaniment of
explosions of considerable violence, projectiles fell outside the crater rim
and rolled down the steep slopes toward the atrio. These explosions were
repeated at intervals of from twenty to thirty minutes, each beginning in a
great upward rush of steam and ash, accompanied by a low rumbling sound.
During the following night the eruptions increased in violence, and the
anxious planter remained on watch in his villa a mile from the crater.
Falling asleep toward morning, he was rudely awakened by a rain of
projectiles falling upon his roof. Hastily snatching up his two children he

ran toward the door just as a red hot projectile, some two feet in diameter,
descended through the roof, ceiling, and floor of the drawing room, setting
fire to the building. A second projectile similar to the first was smashed into
fragments at his feet as he was emerging from the house, burning one of the
children. Making his escape to Vulcanello at the extremity of the island, the
remainder of the night and the following day, until rescue came from Lipari,
were spent just beyond the range of the falling masses.
When the writer visited the island
some months later, the eruption was
still so vigorous that the crater could
not be reached. The ruined villa,
smashed and charred, stood with its
walls half buried in ash and lapilli,
among which were partly smashed Fig. 114.—“Bread-crust” lava projectile
pumiceous lava projectiles. The from the eruption of Volcano in 1888
entire atrio about the mountain lay (after Mercalli).
buried in cinder to the depth of
several feet and was strewn with projectiles which varied in size from a
man’s fist to several feet in diameter (Fig. 114). The larger of these
exhibited the peculiar “bread-crust” surface and had generally been
smashed by the force of their fall after the manner of a pumpkin which has
been thrown hard against the ground. One of these projectiles fully three
feet in diameter was found at the distance of a mile and a half from the
crater. Though diminished considerably in intensity, the rhythmic
explosions within the crater still recurred at intervals varying from four
minutes to half an hour, and were accompanied by a dull roar easily heard at
Lipari on a neighboring island six miles away. Simultaneously, a dark cloud
of “smoke”, the peculiar “cauliflower cloud” or pino mounted for a couple
of miles above the crater (Fig. 115), and the rise was succeeded by a rain of
small lava fragments or lapilli outside the crater rim.
There seems to be no good reason to doubt that Vulcanian cinder
eruptions of this type differ chiefly in magnitude from the rhythmic
explosion within the crater of Stromboli, if we except the elevation of a
considerable quantity of accessory and older tuff which is derived from the
inner walls of the crater and carried upward into the air together with the
pasty cakes of fresh lava derived from the chimney. It is this accessory
material which gives to the pino its dark or even black appearance.

The
eruption of
Taal
volcano on
January
30, 1911.—
The recent
Fig. 115.—Peculiar “cauliflower eruption of
cloud” or pino composed of steam the cinder
and ash, rising above the cinder
cone of Volcano during the
cone known
waning phases of the explosive as Taal
eruption of 1888 (after a volcano is
photograph by B. Hobson). of interest,
not only because so fresh in mind, but
because two neighboring vents erupted Fig. 116.—Double explosive
simultaneously with explosions of nearly eruption of Taal volcano on the
morning of January 30, 1911.
equal violence (Fig. 116). This Philippine
volcano lies near the center of a lake some fifteen miles in diameter and
about fifty miles south of the city of Manila. After a period of rest extending
over one hundred and fifty years, the symptoms of the coming eruption
developed rapidly, and on the morning of January 30 grand explosions of
steam and ash occurred simultaneously in the neighboring craters, and the
condensed moisture brought down the ash in an avalanche of scalding mud
which buried the entire island. Almost the entire population of the island,
numbering several hundreds, was literally buried in the blistering mud (Fig.
117); and the gases from the explosions carried to the distant shores of the
lake added to this number many hundred victims.
The shocks which accompanied the explosions raised a great wave upon
the surface of the lake, which, advancing upon the shores, washed away
structures for a distance of nearly a half mile.
The materials and the structure of cinder cones.—Obviously the
materials which compose cinder cones are the cooled lava fragments of
various degrees of coarseness which have been ejected from the crater. If
larger than a finger joint, such fragments are referred to as volcanic
projectiles, or, incorrectly, as “volcanic bombs.” Of the larger masses it is
often true that the force of expulsion has not been applied opposite the
center of mass of the body. Thus it follows that they undergo complex

whirling motions during their
flight, and being still semiliquid,
they develop curious pear-shaped
or less regular forms (Fig. 118).
When crystals have already
separated out in the lava before its
rise in the chimney of the volcano,
the surrounding fluid lava may be
blown to finely divided volcanic
dust which floats away upon the
wind, thus leaving the crystals
intact to descend as a crystal rain
about the crater. Such a shower
occurred in connection with the
eruption of Etna in 1669, and the
black augite crystals may to-day be
gathered by the handful from the
slopes of the Monti Rossi (Fig. 125, Fig. 117.—The thick mud veneer upon the
p. 125). island of Taal (after a photograph by
Deniston).

The term lapilli, or sometimes
rapilli, is applied to the ejected lava
fragments when of the average size
of a finger joint. This is the material
which still partially covers the
unexhumed portions of the city of
Pompeii. Volcanic sand, ash, and
dust are terms applied in order to
increasingly fine particles of the
Fig. 118.—A pear-shaped lava projectile.
ejected lava. The finest material, the
volcanic dust, is often carried for hundreds and sometimes even for
thousands of miles from the crater in the high-level currents of the
atmosphere. Inasmuch as this material is deposited far from the crater and
in layers more or less horizontal, such material plays a small rôle in the
formation of the cinder cone. The coarser sands and ash, on the other hand,
are the materials from which the cinder cone is largely constructed.

The manner of formation and the
structure of cinder cones may be
illustrated by use of a simple
laboratory apparatus (Fig. 119).
Through an opening in a board, first
white and then colored sand is sent up
in a light current of air or gas supplied
from suitable apparatus. The
alternating layers of the sand form in
the attitudes shown; that is to say,
dipping inward or toward the chimney
of the volcano at all points within the
crater rim, and outward or away from
Fig. 119.—Artificial production of the
structure of a cinder cone with use of
it at all points outside (Fig. 119). If the
colored sands carried up in alternation experiment is carried so far that at its
by a current of air (after G. Linck). termination sand slides down the crater
walls into the chimney below, the
inward dipping layers will be truncated, or even removed entirely, as shown
in Fig. 119 b.

Fig. 120.—Diagram to show the contrast between a lava dome and a cinder
cone. AAA, cinder cone; BabC, lava dome; DE, line of low cinder cones
above a fissure (after Thoroddsen).

The profile lines of cinder cones.—The shapes of cinder cones are
notably different from those of lava mountains. While the latter are domes,
the mountains constructed of cinder are conical and have curves of profile
that are concave upward instead of convex (Fig. 120). In the earlier stages
of its growth the cinder cone has a crater which in proportion to the height
of the mountain is relatively broad (Fig. 99, p. 104).
Speaking broadly, the diameter of the crater is a measure of the violence
of the explosions within the chimney. A single series of short and violent
explosive eruptions builds a low and broad cinder cone. A long-continued

succession of moderately violent
explosions, on the other hand, builds
a high cone with crater diameter
small if compared with the
mountain’s altitude, and the profile
afforded is a remarkably beautiful
sweeping curve (Fig. 121). Toward
the summit of such a cone the loose
materials of which it is composed are
Fig. 121.—Mayon volcano on the island of
at as steep an angle as they can lie, Luzon, P.I. A remarkably perfect high
the so-called angle of repose of the cinder cone.
material; whereas lower down the
flatter slopes have been determined by the distribution of the cinder during
its fall from the air. When one makes the ascent of such a mountain, he
encounters continually steeper grades, with the most difficult slope just
below the crest.
The composite cone.—The life histories
of volcanoes are generally so varied that
lava domes and the pure types of cinder
Fig. 122.—A series of breached cones are less common than volcanoes in
cinder cones where the place of
eruption has migrated along the
which paroxysmal eruptions have alternated
underlying fissure. The Puys with explosions, and where, therefore, the
Noir, Solas, and La Vache in the structure of the mountain represents a
Mont Dore Province of central composite of lava and cinder. Such
France (after Scrope).
composite cones possess a skeleton of solid
rock upon which have been built up alternate sloping layers of cinder and
lava. In most respects such cones stand in an intermediate position between
lava domes and cinder cones.
Regarded as a retaining wall for the lava which mounts in the chimney,
the cinder cone is obviously the weakest of all. Should lava rise in a cinder
cone without an explosion occurring, the cone is at once broken through
upon one side by the outwelling of the lava near the base. Thus arises the
characteristic breached cone of horseshoe form (Fig. 122).
Quite in contrast with the weak cinder cone is the lava dome with its
rock walls and relatively flat slopes. Considered as a retaining wall for lava
it is much the strongest type of volcanic mountain, and it is likely that the
hydrostatic pressure of the lava within the crater would seldom suffice to

rupture the
walls, were
it not that
the molten
rock first
fuses its
Fig. 124.—A row of parasitic cones way into
raised above a fissure which was old stream
opened upon the flanks of Mount tunnels
Etna during the eruption of 1892
(after De Lorenzo). buried
under the
mountain slopes (see ante, p. 112).
Composite cones have a strength as
retaining walls for lava which is Fig. 123.—The bocca or mouth
intermediate between that of the other upon the inner cone of Mount
types. Their Vulcanian eruptions of the Vesuvius from which flowed the
lava stream of 1872. This lava
convulsive type are initiated by the stream appears in the foreground
formation of a rent or fissure upon the with its characteristic “ropy”
mountain flanks at elevations well above surface.
the base, the opening of the fissure being
generally accompanied by a local earthquake of greater or less violence.
From one or more such fissures the lava issues usually with sufficient
violence at the place of outflow to build up over it either an enlarged type of
driblet cone, referred to as a “mouth”, or bocca[1] (Fig. 123), or one or more
cinder cones which from their position upon the flanks of the larger volcano
are referred to as parasitic cones (Fig. 124). The lava of Vesuvius more
frequently yields bocchi at the place of outflow, whereas the flanks of Etna
are pimpled with great numbers of parasitic cinder cones, each the
monument to some earlier eruption (Fig. 125).

Fig. 125.—View looking toward the summit of Etna from a position upon the
southern flank near the village of Nicolosi. The two breached parasitic
cones seen behind this village are the Monti Rossi which were thrown up in
1669 and from which flowed the lava which overran Catania (after a
photograph by Sommer).

It is generally the case that a single eruption makes but a relatively
small contribution to the bulk of the mountain. From each new cone or
bocca there proceeds a stream of lava spread in a relatively narrow stream
extending down the slopes (Fig. 126).
The caldera of composite cones.—
Because of the varied episodes in the history
of composite cones, they lack the regular
lines characteristic of the two simpler types.
The larger number of the more important
composite cones have been built up within
an outer crater of relatively large diameter,
the Somma cone or caldera, which
surrounds them like a gigantic ruff or collar.
Fig. 126.—Sketch map of Etna, This caldera is clearly in most cases at least
showing the individual surface the relic of an earlier explosive crater, after
lava streams (in black) and the
tuff covered surface (stippled). which successive eruptions of lesser
violence have built a more sharply conical
structure. This can only be interpreted to mean that most larger and long-
active volcanoes have been born in the grandest throes of their life history,
and that a larger or smaller lateral migration of the vent has been
responsible for the partial destruction of the explosion crater. Upon
Vesuvius we find the crescent-like rim of Monte Somma; on Etna it is the

Val del Bove, etc. It is this caldera of composite cones which gave rise to
the theory of the “elevation crater” of von Buch (see ante, p. 95, and Fig.
127).

Fig. 127.—Panum crater, showing the caldera and the later interior cones
(after Russell).

The eruption of Vesuvius in 1906.—The volcano Vesuvius rises on the
shores of the beautiful bay of Naples only about ten miles distant from the
city of Naples. The mountain consists of the remnant of an earlier broad-
mouthed explosion crater, the Monte Somma, and an inner, more conical
elevation, the Monte Vesuvio. Before the eruption of 1906 this central cone
was sharply conical and rose to a height of about 4300 feet above the
surface of the bay, or above the highest point of the ancient caldera. The
base of this inner cone is at an elevation of something less than half that of
the entire mass, and is separated from the encircling ring wall of the old
crater by the atrio, to which corresponds in height a perceptible shelf or
piano upon the slope toward the bay of Naples (Fig. 128).
An active composite cone like
that of Vesuvius is for the greater part
of the time in the Strombolian
condition; that is to say, light crater
explosions continue with varying
intensity and interval, except when
the mountain has been excited to the
periodic Vulcanian outbreaks with Fig. 128.—View of Mount Vesuvius as it
appeared from the Bay of Naples shortly
which its history has been before the eruption of 1906. The horn to
punctuated. The Strombolian the left is Monte Somma.
explosions have sufficient violence to

eject small fragments of hot lava, which, falling about the crater, slowly
built up a rather sharp cone. The period of Strombolian activity has,
therefore, been called the cone-producing period. Just before each new
outbreak of the Vulcanian type, the altitude of the mountain has, therefore,
reached a maximum, and since the larger explosive eruptions remove
portions of this cone at the same time that they increase the dimensions of
the crater, the Vulcanian stage in contrast to the other has been called the
crater-producing period. In this period, then, the material ejected during the
explosions does not consist solely of fresh lava cakes, but in part of the
older débris derived from the crater walls, whence it is avalanched upon the
chimney after each larger explosion. The overhanging cloud, which during
the Strombolian period has consisted largely of steam and is noticeably
white, now assumes a darker tone, the “smoke” which characterizes the
Vulcanian eruption.
On several historical occasions the cone of
Vesuvius has been lowered by several hundred feet,
the greatest of relatively recent truncations having
occurred in 1822 and in 1906. Between Vulcanian
eruptions the Strombolian activity is by no means
uniform, and so the upward growth of the cone is
subject to lesser interruptions and truncations (Fig.
129).
The Vesuvian eruption of 1906 has been selected
as a type of the larger Vulcanian eruption of
composite cones, because it combined the explosive
and paroxysmal elements, and because it has been
observed and studied with greater thoroughness than
any other. The latest previous eruption of the
Vulcanian order had occurred in 1872. Some two
years later the period of active cone building began
and proceeded with such rapidity that by 1880 the
new cone began to appear above the rim of the
crater of 1872. From this time on occasional light
eruptions interrupted the upbuilding process, and as
the repairs were not in all cases completed before a
new interruption, a nest of cones, each smaller than
the last, arose in series like the outdrawn sections of

Fig. 129.—A series of an old-time spyglass. At one time no less than five
consecutive sketches of
the summit of the
concentric craters were to be seen.
Vesuvian cone, showing For a brief period in the fall of 1904 Vesuvius
the modifications in its
had been in almost absolute repose, but soon
outline (after Sir
William Hamilton).
thereafter the Strombolian crater explosions were
resumed. On May 25, 1905, a small stream of lava
began to issue from a fissure high up upon the central cone, and from this
time on the lava continued to flow down to the valley or atrio, separating
the inner cone from the caldera remnant of Monte Somma. Seen in the
night, this stream of lava appeared from Naples like a red hot wire laid
against the mountain’s side (Fig. 130). With gradual augmentation of
Strombolian explosions and increase in volume of the flowing lava stream,
the same condition continued until the first days of April in 1906. The
flowing lava had then overrun the tracks of the mountain railway and
accumulated in considerable quantity within the atrio (Fig. 131).

Fig. 130.—Night view of Vesuvius from Naples before the
outbreak of 1906. A small lava stream is seen
descending from a high point upon the central cone
(after Mercalli).

On the morning of April 4, a preliminary stage of the eruption was
inaugurated by the opening of a new radial fissure about 500 feet below the
summit of the cone (Fig. 132 a), and by early afternoon the cone-destroying
stage began with the rise of a dark “cauliflower cloud” or pino to replace
the lighter colored steam cloud. The cone was beginning to fall into the
crater, and old lava débris was mingled in the ejections with the lava clots
blown from the still fluid material within the chimney. From now on short
and snappy lightning flashes played about the black cloud, giving out a
sharp staccato “tack-a-tack.” The volume and density of the cloud and the
intensity of the crater explosions continued to increase until the culmination
on April 7. On April 5 at midnight a new lava mouth appeared upon the
same fissure which had opened near the summit, but now some 300 feet
lower (Fig. 132 b). The lava now welled out in larger volume corresponding
to its greater head, and the stream which for ten months had been flowing
from the highest outlet upon the cone now ceased to flow. The next
morning, April 6, at about 8 o’clock, lava broke out at several points some
distance east of the opening b, and evidently upon another fissure transverse
to the first (Fig. 132 c). The lava surface within the chimney must still have
remained near its old level,—effective draining had not yet begun,—since
early upon the following morning a small outflow began nearly at the top of
the cone upon the opposite side and at least a thousand feet higher.

Fig. 131.—Scoriaceous lava encroaching upon the tracks of the Vesuvian
railway (after a photograph by Sommer).

The culmination of the eruption
came in the evening of April 7, when,
to the accompaniment of light
earthquakes felt as far as Naples, lava
issued for the first time in great
volume from a mouth more than
halfway down the mountain side
(Fig. 132 f), and thus began the
drainage of the chimney. At about the
same time with loud detonations a
huge black cloud rose above the
crater in connection with heavy
explosions, and a rain of cinder was
general in the region about the
mountain but especially within the
northeast quadrant. Those who were
so fortunate as to be in Pompeii had a Fig. 132.—Map of Vesuvius, showing the
clear view of the mountain’s summit position and order of formation of the
where red hot masses of lava were lava mouths upon its flanks during the
thrown far into the air. The direction eruption of 1906 (after Johnston-Lavis).
of these projections was reported to
have been not directly upward, but inclined toward the northeast quadrant
of the mountain; but since with a northeast surface wind the heaviest

deposit of ash and dust should have been upon the southwestern quadrant of
the mountain, it is evident that the material was carried upward until it
reached the contrary upper currents of the atmosphere, to be by them
distributed.

Fig. 133.—The ash curtain which had overhung Vesuvius lifting and
disclosing the outlines of the mountain on April 10, 1911 (after De
Lorenzo).

When the heavy curtain of ash,
which now for a number of succeeding
days overhung all the circum-Vesuvian
country, began to lift (Fig. 133), it was
seen that the summit of the cone had
been truncated an average of some 500
feet (Fig. 134). All the slopes and
Fig. 134.—The central cone of Vesuvius as much of the surrounding country had
it appeared after the eruption of 1906, the aspect of being buried beneath a
but with the earlier profile indicated.
The truncation represents a lowering of cocoa-colored snow of a depth to the
the summit by some five hundred feet, northeastward of several feet, where it
with corresponding increase in the had drifted into all the hollow ways so
diameter of the crater (after Johnston-
as almost to efface them (Fig. 135).
Lavis).
More than thrice as heavy as water, the
weak roof timbers of the houses at the base of the mountain gave way
beneath the added load upon them, thus making many victims. Inasmuch,
however, as the ash-fall partakes of the same general characters as in
eruptions from cinder cones, we may here give our attention especially to

the streams of lava which issued upon the opposite flank of the mountain
(Fig. 136).

Th
e
main
lava
Fig. 135.—A sunken road filled with stream
indrifted cocoa-colored ash from the desce
Vesuvian eruption of 1906.
nded
the first steep slopes with the velocity of
a mile in twenty-five minutes, about the
strolling speed of a pedestrian, but this
Fig. 136.—View of Vesuvius taken from
rate was gradually reduced as the stream the southwest during the waning
advanced farther from the mouth. stages of the eruption of 1906. In the
Taking advantage of each depression of middle distance may be discerned the
the surface, the black stream advanced several lava mouths aligned upon a
fissure, and the courses of the
slowly but relentlessly toward the cities streams which descend from them. In
at the southwest base of the mountain. the foreground is the main lava
With a motion not unlike that of a heap stream with scoriaceous surface
(after W. Prinz).
of coal falling over itself down a slope,
the block lava advances without burning
the objects in its path (Fig. 137).

Th
e
beauti
Fig. 137.—The main lava stream of
ful
1906 advancing upon the village of pines
Fig. 138.—An Italian pine snapped off
Boscotrecase. are by the lava and carried forward
merel upon its surface as a passenger (after
y charred where snapped off and are Haug).

carried forward upon the surface of the stream (Fig. 138). When a real
obstruction, such as a bridge or a villa, is encountered, the stream is at first
halted, but the rear crowding upon the van, unless a passage is found at the
side, the lava front rises higher and higher until by its weight the
obstruction is forced to give way (Figs. 139 and 140).
The sequence of events within
the chimney.—The thorough study
of this Vesuvian eruption has placed
us in a position to infer with some
confidence in our conclusions the
sequence of events within the
chimney and crater of the volcano,
both before and during the eruption.
Fig. 139.—Lava front both pushing over and Anticipating some conclusions
running around a wall which lies athwart derived from the observed dissection
its course (after Johnston-Lavis). of volcanoes, which will be
discussed below, it may be stated
that what might be termed the core
of the composite cone—the
chimney—is a more or less
cylindrical plug of cooled lava
which during the active period of
the vent has an interior bore of
probably variable caliber. This plug
in its lower section appears in solid
black in all the diagrams of Fig. Fig. 140.—One of the villas in Boscotrecase
141. During the cone-building which was ruined by the Vesuvian lava
flow of 1906. The fragments of masonry
period (Fig. 141 a and b) the plug from the ruined walls traveled upon the
is obviously built upward along lava current, where they sometimes
with the cone, for lava often flows became incased in lava.
out at a level a few hundred feet
only below the crater rim. By what process this chimney building goes on is
not well understood, though some light is thrown upon it by the post-
eruption stage of Mont Pelé in 1902-1903 (see below).
Both the older and newer sections of this plug or chimney are furnished
some support against the outward pressure of the contained lava by the
surrounding wall of tuff; and they are, therefore, in a condition not unlike

that of the inner barrel of a great gun over
which sleeves of metal have been shrunk so
as to give support against bursting pressures.
On the other hand, when not sustaining the
hydrostatic pressure of the liquid lava
within, the chimney would tend to be
crushed in by the pressure of the
surrounding tuff. Its strength to withstand
bursting pressures is dependent not alone
upon the thickness of its rock walls, but also
upon its internal diameter or caliber. A steam
cylinder of given thickness of wall, as is
well known, can resist bursting pressures in
proportion as its internal diameter is small.
So in the volcanic chimney, any tendency to
remelt from within the chimney walls must
weaken them in a twofold ratio.
We are yet without accurate temperature
observations upon the lava in volcanic
chimneys, but it seems almost certain that
these temperatures rise as the Vulcanian
Fig. 141.—Three diagrams to stage is approaching, and such elevation of
illustrate the sequence of events
within the crater of a composite temperature must be followed by a greater or
cone during the cone-building less re-fusion of the chimney walls. The
and crater-producing periods. a sequence of events during the late Vesuvian
and b, two successive stages of the
eruption is, then, naturally explained by
cone building or Strombolian
period; c, enlargement of the progressive re-fusion and consequent
crater, truncation of the cone, and weakening of the chimney walls, thus
destruction of the upper chimney permitting a radial fissure to open near the
during the relatively brief crater-
producing or Vulcanian period.
top and gradually extend downwards. Thus
at first small and high outlets were opened
insufficient to drain the chimney, but later, on April 7, after this fissure had
been much extended and a new and larger one had opened at a lower level,
the draining began and the surface of lava commenced rapidly to sink.

Fig. 142.—The spine of Pelé rising above the chimney of the volcano after
the eruption of 1902 (after Hovey).

When the rapid sinking of the lava surface occurred, the lower lava
layers were almost immediately relieved of pressure, thus causing a sudden
expansion of the contained steam and resulting in grand crater explosions.
The partially refused and fissured upper chimney, now unable to withstand
the inward pressure of the surrounding tuff walls, since outward pressures
no longer existed, crushed in and contributed its materials and those of the
surrounding tuff to the fragments of fresh lava rising in volume in the grand
explosions (Fig. 141 c). In outline, then, these seem to be the conditions
which are indicated by the sequence of observed events in connection with
the late Vesuvian outbreak.

Fig. 143.—Outlines of the Pelé spine upon successive dates. The full line
represents its outline on December 26, 1902; the dotted-dashed line is a
profile of January 3, 1902; while the dotted line is that of January 9, 1903.
The dark line is a fissure (after E. O. Hovey).

The spine of Pelé.—The disastrous eruption of Mont Pelé upon
Martinique in the year 1902 is of importance in connection with the
interesting problem of the upward growth of volcanic chimneys during the
cone-building period of a volcano. After the conclusion of this great
Vulcanian eruption, a spine of lava grew upward from the chimney of the
main crater until it had reached an elevation of more then a thousand feet
above its base, a figure of the same order of magnitude as the probable
height of the upper section of the Vesuvian chimney previous to the
eruption of 1906. The Pelé spine (Fig. 142) did not grow at a uniform rate,
but was subject to smaller or larger truncations, but for a period of 18 days
the upward growth was at the rate of about 41 feet per day. Later, the mass
split upon a vertical plane revealing a concave inner surface, and was
somewhat rapidly reduced in altitude to 600 feet (Fig. 143), only to rise
again to its full height of about 1000 feet some three months later.
While apparently unique as an observed phenomenon, and not free from
uncertainty as to its interpretation, the growth of this obelisk has at least
shown us that a mass of rock can push its way up above the chimney of an
active volcano even when there are no walls of tuff about it to sustain its
outward pressures.

The aftermath of mud flows.—
When the late Vulcanian explosions of
Vesuvius had come to an end, all
slopes of the mountain, but especially
the higher ones, were buried in thick
deposits of the cocoa-colored ash,
Fig. 144.—Corrugated surface of the
Vesuvian cone after the mud flows which included in which were larger and
followed the eruption in 1906 (after smaller projectiles. As this material is
Johnston-Lavis). extremely porous, it greedily sucks up
the water which falls during the first
succeeding rains. When nearly saturated, it begins to descend the slopes of
the mountain and soon develops a velocity quite in contrast with that of the
slow-moving lava. The upper slopes are thus denuded, while the fields and
even the houses about the base are invaded by these torrents of mud (lava
d’acqua). Inasmuch as these mud flows are the inevitable aftermath of all
grander explosive eruptions, the Italian government has of late spent large
sums of money in the construction of dikes intended to arrest their progress
in the future. It was streams of this sort that buried the city of Herculaneum
after the explosive eruption of 79 A.D.
After the mud flows have occurred, the Vesuvian cone, like all similar
volcanic cones under the same conditions, is found with deep radial
corrugations (Fig. 144), such as were long ago described as “barrancoes”
and supposed to support the “elevation crater” theory of volcano formation.
The dissection of volcanoes.—To the uninitiated it might appear a
hopeless undertaking to attempt to learn by observation the internal
structure of a volcano, and especially of a complex volcano of the
composite type. The earliest successful attempt appears to have been made
by Count Caspar von Sternberg in order to prove the correctness of the
theory of his friend, the poet Goethe. Goethe had claimed that a little hill in
the vicinity of Eger, on the borders of Bohemia, was an extinct volcano,
though the foremost geologist of the time the famous Werner, had
promulgated the doctrine that this hill, in common with others of similar
aspect, originated in the combustion of a bed of coal. The elevation in
question, which is known as the Kammerbühl, consists mainly of cinder,
and Goethe had maintained that if a tunnel were to be driven horizontally
into the mountain from one of its slopes, a core or plug of lava would be
encountered beneath the summit. The excavations, which were completed

in 1837, fully verified the poet’s view, for a lava plug was found to occupy
the center of the mass and to connect with a small lava stream upon the side
of the hill (Fig. 145).
It is not, however, to such
expensive projects that reference is
here made, but rather to processes
which are continually going on in
nature, and on a far grander scale.
The most important dissecting agent
for our purpose is running water,
which is continually paring down the Fig. 145.—The Kammerbühl near Eger,
showing the tunnel completed in 1837
earth’s surface and disclosing its which proved the volcanic nature of the
buried structures. How much more mountain (after Judd).
convincing than any results of
artificial excavation, as evidence of the internal structure of a volcano, is
the monument represented in Fig. 146, since here the lava plug stands in
relief like a gigantic thumb still surrounded by a remnant of cinder deposits.
Such exposed chimneys of former volcanoes are found in many regions,
and have become known as volcanic necks, pipes, or plugs.

Fig. 146.—Volcanic plug exposed by natural dissection of a volcanic cone
in Colorado (U. S. G. S.).

Not infrequently the beds of tuff
composing the flanks of the volcano,
upon dissection by the same process,
bring to light walls of cooled lava
standing in relief (Fig. 147)—the
filling of the fissure which gave outlet
to the flanks of the mountain at the
time of the eruption. Study of exposed
dikes formed in connection with recent
eruptions of Vesuvius has shown that
in many instances they are still hollow,
the lava having drained from them
before complete consolidation.
Another agent which is effective in
Fig. 147.—A dike cutting beds of tuff in a uncovering the buried structures of
partly dissected volcano of southwestern volcanoes is the action of waves on
Colorado (after Howe, U. S. G. S.).
shores. Always a relatively vigorous
erosive agency, the softer structures of volcanic cones are removed with
especial facility by this agent. On the shores of the island of Volcano, the
little cone of Vulcanello has been nearly half carried away by the waves, so
as to reveal with especial perfection the structure of the cinder beds as well
as the internal rock skeleton of the mass. Here the characteristic dips of lava
streams, intercalated as they now are between tuff deposits and the lava
which consolidated in fissures, are both revealed.
In mid-Atlantic a quite perfect crater, the St.
Paul’s Rocks, has been cut nearly in half so as to
produce a natural harbor (Fig. 148).
In still other instances we may thank the
volcano itself for opening up the interior of the
mountain for our inspection. The eruption in 1888
of the Japanese volcano of Bandai-san, by
removing a considerable part of the ancient cone,
has afforded us a section completely through the
mountain. The summit and one side of the small
Bandai was carried completely away, and there Fig. 148.—Map and
was substituted a yawning crater eccentric to the general view of St.
former mountain and having its highest wall no

less than 1500 feet in height (Fig. 149). In two Paul’s Rocks, a volcanic
cone dissected by waves.
hours from the first warning of the explosion the
catastrophe was complete and the eruption over.
The eruption of Krakatoa in 1883,
probably the grandest observed
volcanic explosion in historic times,
left a volcanic cone divided almost in
Fig. 149.—Dissection by explosion of Little
Bandai-san in 1888 (after Sekiya).
half and open to inspection (Fig. 150).
Rakata, Danan, and Perbuatan had
before constituted a line of cones built up round individual craters
subsequent to the partial destruction of an earlier caldera, portions of which
were still existent in the islands Verlaten and Lang. By the eruption of 1883
all the exposed parts and considerable submerged portions of the two
smaller cones were entirely destroyed, and the larger one, known as Rakata,
was divided just outside the plug so as to leave a precipitous wall rising
directly from the sea and showing lava streams in alternation with
somewhat thicker tuff layers, the whole knit together by numerous lava
dikes.
In order to carry our dissecting
process down to levels below the base
of the volcanic mountain, it is usually
necessary to inspect the results of
Fig. 150.—The half-submerged volcano of erosion by running water. Here the
Krakatoa in the Sunda Straits before plug or chimney, instead of being
and after the eruption of 1883 (after
Verbeek). surrounded by tuff, is inclosed by the
country rock of the region, which is
commonly a sedimentary formation. Such exposed lower sections of
volcanic chimneys are numerous along the northwestern shores of the
British Isles. Where aligned upon a dislocation or noteworthy fissure in the
rocks, the group of plugs has been referred to as a scar or cicatrice (Fig.
151). Associated with the plugs of the cicatrice are not infrequently dikes,
or, it may be, sheets of lava extended between layers of sediment and
known as sills.
If we are able to continue the dissection process to still greater depths,
we encounter at last igneous rock having a texture known as granitic and
indicating that the process of consolidation was not only exceedingly slow
but also uninterrupted. This rock is found in masses of larger dimensions,

and though generally of more or less
irregular form, no one dimension is of
a different order of magnitude from the
others. Such masses are commonly
described as bosses, or, if especially
large, as batholites (Fig. 152). Fig. 151.—The cicatrice of the Banat
Wherever the rock beds appear as (after Suess).
though they had been forced up by the
upward pressure of the igneous mass, the latter takes the form of a
mushroom and has been described as a laccolite (Figs. 479-481, pp. 441-
442). Evidence seems, however, to accumulate that in the greater number of
cases the molten rock has fused its way upward, in part assimilating and in
part inclosing the rock which it encountered. This process of upward fusion
has been likened to the progress of a red hot iron burning its way through a
board.
The formation of lava reservoirs.—The discarding of the earlier
notion that the earth has a liquid interior makes it proper in discussing the
subject of volcanoes to at least touch upon the origin of the molten rock
material. As already pointed out, such reservoirs as exist must be local and
temporary, or it would be difficult to see how the existing condition of earth
rigidity could be maintained. From the rate at which rock temperatures rise,
at increasing depths below the surface, it is clear that all rocks would be
melted at very moderate depths only, if they were not kept in a solid state
by the prodigious loads which they sustain. Any relief from this load should
at once result in fusion of the rock.
Now the restriction of active volcanoes to those zones of the earth’s
surface within which mountains are rising, and where in consequence
earthquakes are felt, has furnished us at least a clew to the origin of the
lava. Regarded as a structure capable of sustaining a load, the competency
of an arch is something quite remarkable, so that the arching up of strong
rock formations into anticlines within the upper layers of the zone of flow,
or of combined fracture and flow, would be sufficient to remove the load
from relatively weak underlying beds, which in consequence would be
fused and form local reservoirs of lava (Figs. 152 and 153).
It has been further quite generally observed that lines of volcanoes, in so
far as they betray any relation in position to neighboring mountain ranges,
tend to appear upon the rear or flatter limb of unsymmetrical arches, or

where local tension would favor the
opening of channels toward the
surface. Moreover, wherever recent
block movements of surface portions
of the earth’s shell have been
disclosed in the neighborhood of
volcanoes, the latter appear to be
connected with downthrown blocks,
as though the lava had, so to speak,
been squeezed out from beneath the
depressed block or blocks.
W
e must Fig. 152.—Diagram to illustrate a probable
not, cause of formation of lava reservoirs,
howev and to show the connection between such
reservoirs and the volcanoes at the
er, surface.
forget
Fig. 153.—Result of experiment
that the igneous rocks are greatly restricted
with layers of composition to
illustrate the effect of relief of in the range of their chemical composition.
load upon rocks by arching of No igneous rock type is known which could
competent formation (after be formed by the fusion of any of the
Willis).
carbonate rocks such as limestone or
dolomite, or of the more siliceous rocks, such as sandstone or quartzite.
There remains only the argillaceous class of sediments, the shales and
slates, and so soon as we examine the composition of these rocks we are
struck by the remarkable resemblance to that of the class of igneous rocks.
For purposes of comparison there is given below the composite or average
constitution of igneous rocks in parallel column, with the average attained
by combining the analyses of 56 slates and shales, the latter recalculated
with water excluded:

Average Igneous Rock
Average Shale
(Clark) (Washington)
SiO2 61.25 61.69 63.34
Al2O3 15.81 15.94 15.56

3.61 } 2.65 } } 7.89
Fe2O3 2.70 1.88 4.41
6.31 4.53
FeO 3.48

MgO 4.47 4.90 3.54
CaO 5.03 5.02 3.33
Na2O 3.64 4.09 1.29
K2O 2.87 3.35 3.52
TiO2 .62 .48 .53
100.00 100.00 100.00

This close resemblance is probably of deep significance, for the reason
that shales and slates are structurally the weakest of all rocks and for the
further reason that they rather generally directly underlie the carbonate
rocks, which are by contrast the strongest (see ante, p. 37). For these
reasons shales and slates are the only rocks which are likely to be fused by
relief from load through the formation of anticlinal arches within the earth’s
zone of flow. If this view is well founded, lavas and other igneous rocks are
in large part fused argillaceous sediments formed in connection with the
process of folding, or are refused rocks of igneous origin and similar
composition.
Character profiles.—The character profiles of features connected in
their origin with volcanoes are particularly easy to recognize, and in a few
cases in which they might be confused with others of a different origin, an
examination of the materials of the features should lead to a definitive
judgment.
The lava plains which result from massive outflows of basalt might
perhaps strictly be regarded as lack of feature, so great may be their
continuous extent. Wherever definite vents exist, a broad flat dome is the
usual result of the extravasation of a basaltic lava. The puys of France and
many of the Kuppen of Germany, being formed from less fluid lava, have
afforded profiles with relatively small radius of curvature.
In its youthful stage, the cinder cone usually presents a broad summit
sag and relatively short side slopes, whereas the cone of later stages is apt to
present long sweeping and upwardly concave curves with both the gradient
and the radius of curvature increasing rapidly toward the summit. In
contrast, too, with the earlier stage, the crest is relatively small. A marked
reduction in the high symmetry of such profiles is noted wherever a
breaching by lava outflow has occurred (Fig. 154).

With the composite cone, complexity and corresponding lack of
symmetry is introduced, especially in the partially ruined caldera, and by
the more or less accidental distribution of parasitic cones, as well as by
migrations of the central cone. Peculiarly similar acuminated profiles result
from spatter-cone formation, from the formation of a superchimney spine,
and by the uncovering of the chimney through denudational processes—the
volcanic neck.

Fig. 154.—Character profiles connected with volcanoes.

Another important feature resulting from denudation is the Mesa or
table mountain with its protecting basalt cap above softer rocks. Its profile
most resembles that of table mountains due to differential erosion of
alternately strong and weak horizontally bedded rocks, such as compose the
upper portion of the section in the Grand Cañon of the Colorado. Here,
however, in place of a single unusually strong top layer there are found
several strong layers in alternation with weaker ones so as to produce
additional steps in the profile.

Reading References to Chapters IX and X

General works:—
Paulett Scrope. The Geology of the Extinct Volcanoes of Central France.
John Murray, London, 1858, pp. 258. (An epoch-making work of early
date which, like the following reference, may be studied to advantage
to-day.)

Sir Charles Lyell. Principles of Geology, vol. 1, Chapters xxiii-xxv.
Melchior Neumayr. Erdgeschichte, vol. 1, Allgemeine Geologie, revised
edition by v. Uhlig, 1897, pp. 133-277 (a storehouse of valuable
information clearly presented).
J. D. Dana. Characteristics of Volcanoes, with Contributions of Facts and
Principles from the Hawaiian Islands. Dodd, Mead, and Company,
New York, 1890, pp. 397.
Tempest Anderson. Volcanic Studies in Many Lands, being reproductions of
photographs by the author with explanatory notes. John Murray,
London, 1903, pp. 200, pls. 105.
T. G. Bonney. Volcanoes, their Structure and Significance. John Murray,
London, 1899, pp. 331.
I. C. Russell. Volcanoes of North America. Macmillan, New York, 1897,
pp. 346.
Elisée Réclus. Les volcans de la terre, Belgian Society of Astronomy,
Meteorology, and Physics of the Globe, 1906-1910 (a valuable
descriptive geographical and bibliographical work of reference).
G. Mercalli. I vulcani attivi della terre. Hoepli, Milan, 1907, pp. 421. (A
most valuable work, beautifully illustrated, but in the Italian language.)

Arrangement of volcanic vents:—
Th. Thoroddsen. Die Bruchlinien und ihre Beziehungen zu den Vulkanen,
Pet. Mitt., vol. 51, 1905, pp. 1-5, pl. 5.
R. D. M. Verbeek. Various volumes and atlases of maps covering the Dutch
East Indies and fully cited in the following reference (p. 21).
William H. Hobbs. The Evolution and the Outlook of Seismic Geology,
Proc. Am. Phil. Soc., vol. 48, 1909, pp. 17-27.

Birth of volcanoes:—
F. Omori. The Usu-san Eruption and Earthquake and Elevation Phenomena,
Bull. Earthq. Inv. Com., Japan, vol. 5, No. 1, 1911, pp. 1-37, pls. 1-13.

Fissure eruptions:—
Th. Thoroddsen. Island, IV, Vulkane, Pet. Mitt., Ergänzungsh. 153, 1906,
pp. 108-111.
A. Geikie. Text-book of Geology, 4th ed., pp. 342-346.

Lava domes of Hawaii:—
J. D. Dana. Characteristics of Volcanoes (as above).
C. H. Hitchcock. Hawaii and Its Volcanoes. Honolulu, 1909, pp. 314.

Eruption of Matavanu volcano in 1906:—
Karl Sapper. Der Matavanu-Ausbruch auf Savaii, 1905-1906, Zeit. d.
Gesell. f. Erdk. z. Berlin, vol. 19, 1906, pp. 686-709, 4 pls.
H. J. Jensen. The Geology of Samoa, and the Eruptions in Savaii, Proc.
Linn. Soc., New South Wales, vol. 31, 1906, pp. 641-672, pls. 54-64.
Tempest Anderson. The Volcano of Matavanu in Savaii, Quart. Jour. Geol.
Soc., London, vol. 66, 1910, pp. 621-639, pls. 45-52.

Eruption of Volcano in 1888:—
H. J. Johnston-Lavis. The South Italian Volcanoes. Naples, 1891, pp. 342,
pls. 16.

Eruption of Taal volcano in 1911:—
W. E. Pratt. The Eruption of Taal Volcano, January 30, 1911, Phil. Jour.
Sci., vol. 6, No. 2, Sec. A, 1911, pp. 63-86, pls. 1-14.
F. H. Noble. Taal Volcano, album of views of 1911 eruption, Manila, 1911,
pp. 1-48.

The volcano of Etna:—
G. vom Rath. Der Aetna. Bonn, 1872, pp. 1-33. (A beautiful piece of
descriptive writing from both the geological and scenic standpoints.)

Sartorius von Waltershausen. Der Aetna. Leipzig, 1880, 2 quarto vols.,
pp. 371 and 548.

The eruption of Vesuvius in 1906:—
H. J. Johnston-Lavis. Geological Map of Monte Somma and Vesuvius, with
a short and concise account, etc. Geo. Philip & Son, London, 1891.
H. J. Johnston-Lavis. The Eruption of Vesuvius in April, 1906, Trans. Roy.
Dublin Soc., vol. 9, 1909, Pt. VIII, pp. 139-200, pls. 3-23 (the most
authoritative work upon the subject).
T. A. Jaggar, Jr. The Volcano Vesuvius in 1906, Tech. Quart., vol. 19,
1906, pp. 105-115.
W. Prinz. L’éruption du Vesuv d’avril, 1906, Ciel et Terre, 27e Année, 1906,
pp. 1-49.
Frank A. Perret. Notes on the Electrical Phenomena of the Vesuvian
Eruption, April, 1906, Sci. Bull., Brooklyn Inst. Arts and Sci., vol. 1,
No. 11, pp. 307-312; Vesuvius, Characteristics and Phenomena of the
Present Repose Period, Am. Jour. Sci., vol. 28, 1909, pp. 413-430.
William H. Hobbs. The Grand Eruption of Vesuvius in 1906, Jour. Geol.,
vol. 14, 1906, pp. 636-655.

The spine of Pelée:—
E. O. Hovey. The New Cone of Mont Pelée and the Gorge of the Rivière
Blanche, Martinique, Am. Jour. Sci., vol. 16, 1903, pp. 269-281, pls.
11-14.
A. Heilprin. The Tower of Pelée. Philadelphia, 1904, pp. 62, pls. 22.
A. Lacroix. La montagne Pelée et ses éruptions, Acad. des Sciences, Paris,
1904, Chapter iii.
Karl Sapper. In den Vulkangebieten Mittelamerikas und Westindiens,
Stuttgart, 1905, pp. 172-178.
A. C. Lane. Absorbed Gases of Vulcanism, Science, N.S., vol. 18, 1903, p.
760.

G. K. Gilbert. The Mechanism of the Mont Pelée Spine, ibid., vol. 19,
1904, pp. 927-928.
I. C. Russell. Pelée Obelisk once More, ibid., vol. 21, 1905, pp. 924-931.

The dissection of volcanoes:—
J. W. Judd. Volcanoes, Chapter v.
S. Sekya and Y. Kikuchi. The Eruption of Bandai-San, Trans. Seis. Soc.,
Japan, vol. 13, Pt. 2, 1890, pp. 140-222, pls. 1-9.
R. D. M. Verbeek. Krakatau. Batavia, 1885, pp. 557, pls. 25.
Royal Society. The Eruption of Krakatoa and Subsequent Phenomena.
London, 1888, pp. 494.
G. K. Gilbert. Report on the Geology of the Henry Mountains, U.S.
Geogr. and Geol. Surv., Rocky Mt. Region, Washington, 1877, pp. 22-60.
Sir A. Geikie. Ancient Volcanoes of Great Britain, vol. 2 especially.
D. W. Johnson. Volcanic Necks of the Mount Taylor Region, New Mexico,
Bull. Geol. Soc. Am., vol. 18, 1907, pp. 303-324, pls. 25-30.

CHAPTER XI

THE ATTACK OF THE WEATHER

The two contrasted processes of weathering.—It has already been
pointed out that change and not stability is the order of nature. Within the
earth’s outer shell and upon it rock alteration goes on continually, and from
some portions of its surface the changed material is as constantly migrating
to neighboring or even far distant regions. Before such transportation can
begin the hard rock must first be broken down and reduced to fragments
which the transporting agencies are competent to move.
To accomplish this breaking down, or degeneration, of the rock masses,
either a wide range in temperature or chemical reaction is essential. In the
atmosphere are found such active chemical agents as oxygen and carbon
dioxide, the so-called carbonic acid gas; and these agents in the presence of
water react chemically with the minerals of the rocks and form other
minerals such as the hydrates and carbonates, which are lighter in weight
and more soluble. This chemical attack upon the outer shell of the
lithosphere is described as decomposition.
On the other hand the rock may succumb to changes which are purely
mechanical and are due either to the stresses set up by differences between
surface and interior temperatures, or to the prying action of the frost in the
crevices. Such purely mechanical degeneration of the rocks is in contrast
with decomposition and is described as disintegration. The two processes of
decomposition and disintegration may, however, go on together; and the
changes of volume that are caused by decomposition may result directly in
considerable disintegration, as we are to see.

The rôle of the percolating water.—In order to effect chemical change
or reaction, it is essential that the substances which are to react must be
brought into such intimate contact with each other as it is seldom possible
to attain except by solution. The chemical reactions which go on between
the gaseous atmosphere and the solid lithosphere are accomplished through
solution of the gases in water. This water, derived from rain or snow,
percolates into the ground or descends along the crevices in the rocks,
carrying with it a certain measure of dissolved air. This air differs from that
of the surrounding atmospheric envelope by containing relatively large
amounts of oxygen and of the other active element carbon dioxide. It
follows from the important rôle thus performed by the percolating water
that the process of decomposition will be relatively important in humid
regions where the atmospheric precipitation is sufficient for the purpose.
Within hot and dry regions there is a
larger measure of rock disintegration, and
distinct chemical changes unlike those of
humid regions take place in the higher
temperatures and with the more concentrated
saline solutions. The discussion of such
changes will be deferred until desert
conditions are treated in another chapter.
Mechanical results of decomposition—
spheroidal weathering.—From an earlier
chapter it has been learned that the rocks of
the earth’s outermost shell are generally
intersected by a system of vertical fissures
which at each locality tend to divide the rock
into parallel and upright rectangular prisms.
It is these joints which offer relatively easy
paths for the descent of the water into the
rocks. In rocks of sedimentary origin there
are found, in addition to the vertical joints,
planes of bedding originally horizontal, and
Fig. 155.—Successive diagrams to in the intrusive and volcanic rocks a
show the effect of decomposition somewhat similar parting, likewise parallel
and resulting disintegration upon
joint blocks so as to produce
to the surface of the ground. The combined
effect of the joints and the additional parting

spheroidal bowlders byplanes is thus to separate the rock mass into
weathering.
more or less perfect squared blocks (Fig.
155, upper figure) which stand in vertical columns.
The water which percolates downward upon the joints, finds its way
laterally along the parting planes, and so subjects the entire surface of each
block to simultaneous attack by its reagents. Though all parts of the surface
of each block are alike subject to attack, it is the angles and the edges which
are most vigorously acted upon. In the narrow crevices the solutions move
but sluggishly, and as they are soon impoverished of their reagents in the
attack upon the rock, fresh solution can reach the middle of the faces from
relatively few directions. The edges are at the same time being reached
from many more directions, and the corners from a still larger number.
The minerals newly formed by these chemical processes of hydration
and carbonization are notably lighter, and hence more bulky than the
minerals from whose constituents they have been largely formed. Strains
are thus set up which tend to separate the bulkier new material from the
core of unaltered rock below. As the process continues, distinct channels for
the moving waters are developed favorable to action at the edges and
corners of the blocks. Eventually, the squared block is by this process
transformed into a spheroidal core of still unaltered rock wrapped in layers
of decomposed material, like the outer wrappings of an onion. These in turn
are usually imbedded in more thoroughly disintegrated material from which
the shell structure has disappeared (Fig. 156).
Exfoliation or scaling.—A fact of
much importance to geologists, but one
far too often overlooked, is that rocks
are but poor conductors for heat. It
results from this that in the bright sun
of a summer’s day a thin skin, as it
were, upon the rock surface may be
heated to a relatively high temperature,
Fig. 156.—Spheroidal weathering of an
although the layer immediately below igneous rock.
it is practically unaffected. The
consequent expansion of the surface layer causes stresses that tend to scale
it off from the layer below, which, uncovered in its turn, develops new
strains of the same sort. This process of exfoliation acquires exceptional

importance in desert regions where the rock surfaces are daily elevated to
excessively high temperatures (see Chapter XV).
Dome structure in granite
masses.—In large granite masses, such
as are to be found in the ranges of the
Sierra Nevada of California, a peculiar
dome structure is sometimes found
developed upon a large scale, and has
had an important influence upon the
breaking down of the rock and upon
Fig. 157.—Dome structure in granite mass,
the shaping of the mountain (Fig. 157).
Yosemite valley, California (after a Such a structure, made up as it is of
photograph by Sinclair). prodigious layers, can have little in
common with the veneers of weathered
minerals which are the result of exfoliation, and it is quite likely that the
dome structure is in some way connected with the relief of these massive
rocks from their load—the rock which once rested upon them, but has been
carried away by erosion since the uplift of the range.
The prying work of frost.—In all countries where winter temperatures
range below the freezing point of water, a most potent agent of rock
disintegration is the frost which pries at every crevice and cranny of the
surface rock. Important in the temperate zones, in the polar regions it
becomes almost the sole effective agent of rock weathering. There, as
elsewhere, its efficiency as a disintegrating agent is directly dependent upon
the nature of the crevices within the rock, so that the omnipresent joints are
able to exercise a degree of control over the sculpturing of the surface
features which is hardly to be looked for elsewhere (see plate 10 A).
Talus.—Wherever the earth’s surface rises in steep cliffs, the rock
fragments derived from frost action, or by other processes of disintegration,
as they become detached either fall or slide rapidly downward until arrested
upon a flatter slope. Upon the earlier accumulations of this kind, the later
ones are deposited, until their surface slopes up to the cliff face as steeply as
the material will lie—the angle of repose. Such débris accumulations at the
base of a cliff (Fig. 158) are known as talus, and the slope is described as a
talus slope, or in Scotland as a “scree.”

S
oil
flow
in
Fig. 159.—Striped ground from soil the
flow of chipped rock fragments upon conti
a slope, Snow Hill Island, West
Antarctica (after Otto Nordenskiöld). nued
prese
nce of thaw water.—So soon as the
rocks are broken down by the
weathering processes, they are easily
moved, usually to lower levels. In part Fig. 158.—Talus slope beneath a cliff.
this transportation may be
accomplished by gravity slowly acting upon the disintegrated rock and
causing it to creep down the slope. Yet even in such cases water is usually
present in quantity sufficient to fill the spaces between the grains, and so act
as a lubricant to facilitate the migration.
Upon a large scale rocks which were either originally incoherent or
have been made so by weathering, after they have become saturated with
water, may start into sudden motion as great landslides or avalanches,
which in the space of a few moments materially change the face of the
country, and by burying the bottom lands leave disaster and misery in their
wake.
Within the subpolar regions, where a
large part of the surface is for much of the
year covered with snow, the underlying
rocks are for long periods saturated with
thaw water, and in alternation are
repeatedly frozen and thawed. Essentially
similar conditions are met with in the high,
snow-capped mountains of temperate or
torrid regions. For the subpolar regions Fig. 160.—Pavement of horizontal
surface due to soil flow,
particularly it is now generally recognized Spitzbergen (after Otto
that somewhat special processes of soil Nordenskiöld).
flow, described under the name
solifluction, are characteristic. The exact nature of these processes is as yet
imperfectly understood, but there can be little doubt concerning the large

rôle which they have played in the transportation of surface materials. Such
soil flow is clearly manifested under different aspects, and it is likely that
by this comprehensive term distinct processes have been brought together.
Possibly the most striking aspect of
the soil flow in subpolar regions is
furnished by the remarkable “stone
rivers” and “rock glaciers”; though the
more generally characteristic are peculiar
stripings or other markings which appear
upon the surface of the ground and thus
betray the movements of the underlying
Fig. 161.—Tree roots entering fissured
rock and prying its sections apart.
materials. Upon slopes it is not
uncommon for the surface to be
composed of angular rock fragments riven by the frost and crossed by broad
parallel furrows as though a gigantic plow had gone over it (Fig. 159). The
direction of the furrows is always up and down the slope, and the striping is
marked in proportion as the slope is steep. Where the bottom is reached, the
furrows are replaced by a sort of mosaic pavement of hexagonal repeating
figures, each of which may be an area of the surface six feet or more across
(Fig. 160, and Fig. 390, p. 368). The depressions which separate the
“blocks” of the pavement are often filled with clay, while the inclosed
surfaces are made up of coarsely chipped stone.
The splitting wedges of roots and trees.—In the mechanical
breakdown of the rocks within humid regions a not unimportant part is
sometimes taken by the trees, which insinuate the tenuous extremities of
their rootlets into the smallest cracks and by continued growth slowly
wedge even the firmer rocks apart (Fig. 161). In a similar manner the small
tree trunk growing within a crevice of the rock may in time split its parts
asunder (Fig. 162).
The rock mantle and its shield in the mat of vegetation.—Through
the action of weathering, the rocks, as we have seen, lose their integrity
within a surface layer, which, though it may be as much as a hundred feet or
more in thickness, must still be accounted a mere film above the underlying
bed rock. The mechanical agents of the breakdown operate only within a
few feet of the surface, and the agents of rock decomposition, derived as
they are from the atmosphere, become inert before they have descended to
any considerable depth. The surface layer of incoherent rock is usually

referred to as the rock mantle (Fig. 163).
Where the rock mantle is relatively deep,
as it is in the states south of the Ohio in the
eastern United States, there is found, deep
below the outer layer of soil, a partially
decomposed and disintegrated rock, of
which the unaltered minerals lie unchanged
in position but separated by the new
minerals which have resulted from the
breakdown of their more susceptible
associates. While thus in a certain sense
possessing the original structure, this Fig. 162.—A large glacial bowlder
altered material is essentially incoherent split by a growing tree near East
Lansing, Michigan (after a
and easily succumbs to attack by the pick photograph by Bertha
and spade, so that it is only at considerably Thompson).
greater depths that the unaltered rock is
encountered.
Because of the tendency of mantle rock
to creep down upon slopes it is generally
found thicker upon the crests and at the
bases of hills and thinnest upon their slopes
(Fig. 164).
In the transformation of the upper
Fig. 163.—Rock mantle consisting
portion of the mantle rock into soil,
of broken rock, above which is additional chemical processes to those of
soil and a vegetable mat. Coast of weathering are carried through by the
California (after a photograph by agency of earthworms, bacteria, and other
Fairbanks).
organisms, and by the action of humus and
other acids derived from the decomposition of vegetation. The bacteria
particularly play a part in the formation of carbonates, as they do also in
changing the nitrogen of the air into nitrates which become available as
plant food. Within the humid tropical regions ants and other insects enter as
a large factor in rock decomposition, as they do also in producing not
unimportant surface irregularities.
How important is the cover of vegetation in retaining the rock mantle
and the upper soil layer in their respective positions, as required for
agricultural purposes, may be best illustrated by the disastrous

consequences of allowing it to be
destroyed. Wherever, by the
destruction of forests, by the excessive
Fig. 164.—Diagram to show the varying
grazing of animals, or by other causes,
thickness of mantle rock upon the the mat of turf has been destroyed, the
different portions of a hill surface (after surface is opened in gullies by the first
Chamberlin and Salisbury). hard rain, and the fertile layer of soil is
carried from the slopes and distributed with the coarser mantle upon the
bottom lands. Thus the face of the country is completely transformed from
fertile hills into the most desolate of deserts where no spear of grass is to be
seen and no animal food to be obtained (plate 5 A). The soil once washed
away is not again renewed, for the continuation of the gullying process now
effectively prevents its accumulation.

Plate 5.

A. Once wooded region in China now reduced to desert through deforestation
(after Willis).

B. “Bad Lands” in the Colorado Desert (after Mendenhall).

Reading References to Chapter XI

Decomposition and disintegration:—
George P. Merrill. The Principles of Rock Weathering, Jour. Geol., vol. 4,
1896, pp. 704-724, 850-871. Rocks, Rock Weathering, and Soils.
Macmillan, New York, 1897, Pt. iii, pp. 172-411.
Alexis A. Julien. On the Geological Action of the Humus Acids, Proc. Am.
Assoc. Adv. Sci., vol. 28, 1879, pp. 311-410.

Corrosion of rocks:—
C. W. Hayes. Solution of Silica under Atmospheric Conditions, Bull. Geol.
Soc. Am., vol. 8, 1897, pp. 213-220, pls. 17-19.
M. L. Fuller. Etching of Quartz in the Interior of Conglomerates, Jour.
Geol., vol. 10, 1902, pp. 815-821.
C. H. Smyth, Jr. Replacement of Quartz by Pyrites and Corrosion of Quartz
Pebbles, Am. Jour. Sci. (4), vol. 19, 1905, pp. 282-285.

Dome structure of granite masses:—

G. K. Gilbert. Domes and Dome Structure of the High Sierra, Bull. Geol.
Soc. Am., vol. 15, 1904, pp. 29-36, pls. 1-4.
Ralph Arnold. Dome Structure in Conglomerate, ibid., vol. 18, 1907, pp.
615-616.

Soil flow:—
J. Gunnar Andersson. Solifluction, a Component of Subaërial Denudation,
Jour. Geol., vol. 14, 1906, pp. 91-112.
Otto Nordenskiöld. Die Polarwelt und ihre Nachbarländer, Leipzig, 1909,
pp. 60-65.
Ernest Howe. Landslides in the San Juan Mountains, Colorado, etc., Prof.
Pap., 67 U. S. Geol. Surv., 1909, pp. 1-58, pls. 1-20.
G. E. Mitchell. Landslides and Rock Avalanches, Nat. Geogr. Mag., vol.
21, 1910, pp. 277-287.
William H. Hobbs. Soil Stripes in Cold Humid Regions and a Kindred
Phenomenon, 12th Rept. Mich. Acad. Sci., 1910, pp. 51-53, pls. 1-2.

Relation of deforestation to erosion:—
N. S. Shaler. Origin and Nature of Soils, 12th Ann. Rept. U. S. Geol. Surv.,
1891, Pt. 1, pp. 268-287.
W. J. McGee. The Lafayette Formation, ibid., pp. 430-448.
F. H. King. Soils. Macmillan, New York, 1908, pp. 50-54.
Bailey Willis. Water Circulation and Its Control, Rept. Nat. Conserv.
Com., 1909, vol. 2, pp. 687-710.
W. J. McGee. Soil erosion, Bull. 71, U. S. Bureau of Soils, 1911, pp. 60,
pls. 33.

CHAPTER XII

THE LIFE HISTORIES OF RIVERS

The intricate pattern of river etchings.—The attack of the weather
upon the solid lithosphere destroys the integrity of its surface layer, and
through reducing it to rock débris makes it the natural prey of any agent
competent to carry it along the surface. We have seen how, for short
distances, gravity unaided may pile up the débris in accumulations of talus,
and how, when assisted by thaw water which has soaked into the material, it
may accomplish a slow migration by a peculiar type of soil flow. Yet far
more potent transporting agencies are at work, and of these the one of first
importance is running water. Only in the hearts of great deserts or in the
equally remote white deserts of the polar regions is the sound of its
murmurings never heard. Every other part of the earth’s surface has at some
time its running water coursing in valleys which it has itself etched into the
surface. It is this etching out of the continents in an intricate pattern of
anastomosing valleys which constitutes the chief difference between the
land surface and the relatively even floor of the oceans.
The motive power of rivers.—Every river is born in throes of Mother
Earth by which the land is uplifted and left at a higher level than it was
before. It is the difference of elevation thus brought about between
separated portions of the land areas that makes it possible for the water
which falls upon the higher portions to descend by gravity to the lower. This
natural “head” due to differences of elevation is the motive power of the
local streams, and for each increase in elevation there is an immediate
response in renewed vigor of the streams. The elevated area off which the
rivers flow is here termed an upland.

The velocity of a stream will be dependent not only upon the difference
in altitude between its source and its mouth, but upon the distance which
separates them, since this will determine the grade. The level of the mouth
being the lowest which the stream can reach is termed the base level, and
the current is fixed by the slope or declivity. The capacity to lift and
transport rock débris is augmented at a quite surprising rate with every
increase in current velocity, the law being that the weight of the heaviest
transportable fragment varies with the sixth power of the velocity of the
current. Thus if one stream flows twice as rapidly as another, it can
transport fragments which are sixty-four times as heavy.
Old land and new land.—The uplifts of the continents may proceed
without changes in the position of the shore lines, in which case areas,
already carved by streams but no longer actively modified by them, are
worked upon by tools freshly sharpened and driven by greater power. The
land thus subjected to active stream cutting is described as old land, and has
already had engraved upon it the characteristic pattern of river etchings,
albeit the design has been in part effaced.
If, upon the other hand, the shore line migrates seaward with the uplift,
a portion of the relatively even sea floor, or new land, is elevated and laid
under the action of the running water. As we are to see, stream cutting is to
some extent modified when a river pattern is inherited from the uplift. The
uplift, whether of old land only or of both old land and new land, marks the
starting point of a new river history, usually described as an erosion cycle.

Fig. 165.—Two successive forms of gullies from the earliest stage of a
river’s life (after Salisbury and Atwood).

The earlier aspects of rivers.—Though geologists have sometimes
regarded the uplift of the continents as a sort of upwarping in a continuous

curved surface, the discussions of river histories and the pictorial
illustrations of them have alike clearly assumed that the uplift has been
essentially in blocks and that the elevated area meets the lower lying
country or the sea in a more or less definite escarpment. The first rivers to
develop after the uplift may be described as gullies shaped by the sudden
downrush of storm waters and spaced more or less regularly along the
margin of the escarpment (Fig. 165). These gullies are relatively short,
straight, and steep; they have precipitous walls and few, if any, tributaries.

Fig. 166.—Partially dissected upland (after Salisbury and Atwood).

With time the gully heads advance into the upland as they take on
tributaries; and so at length they in part invest it and dissect it into
numerous irregularly bounded and flat-topped tables which are separated by
cañons (Fig. 166). At the same time the grade of the channel is becoming
flatter, and its precipitous walls are being replaced by curving slopes, as
will be more fully described in the sequel. It is because of this progressive
reduction of grades with increasing age that the early stages of a river’s life
are much the most turbulent of its history. The water then rushes down the
steep grades in rapids, and is often at times opened out in some basin to
form a lake where differences of uplift have been characteristic of
neighboring sections. For several reasons such basins in the course of a

stream are relatively short lived (Chapter XXX), and they disappear with
the earlier stages of the river history.
The meshes of the river network.—From the continued throwing out
of new tributaries by the streams, the meshes in the river network draw
more closely together as the stages of its history advance. The closeness of
texture which is at last developed upon the upland is in part determined by
the quantity of rainfall, so that in New Jersey with heavy annual
precipitation the meshes in the network are much smaller than they are, for
example, upon the semiarid or arid plains of the western United States. Its
design will, however, in either case more or less clearly express the plan of
rock architecture which is hidden beneath the surface (Chapter XVII).
The upper and lower reaches of a river contrasted.—From the fact
that the river progressively invades new portions of the upland and lays the
acquired sections under more and more thorough investment, it has near its
headwaters for a long time a frontier district which may be regarded as
youthful even though the sections near its mouth have reached a somewhat
advanced stage. The newly acquired sections of river valley may thus
possess the steep grade and precipitous walls which are characteristic of
early gullies and cañons and are in contrast with the more rounded and flat-
bottomed sections below. Lateral streams, from the fact that they are newer
than the main or trunk stream to which they are tributary, likewise descend
upon somewhat steeper grades (Fig. 167).

Fig. 167.—Characteristic longitudinal sections of the upper portion of a
river valley and its tributaries (after scaled sections by Nussbaum).

The balance between degradation and aggradation.—We have seen
that the power to transport rock fragments is augmented at a most surprising
rate with every increase in the current velocity. While the lighter particles of
rock may be carried as high up as the surface of the water, the heavier ones
are moved forward upon the bottom with a combined rolling and hopping
motion aided by local eddies. Those particles which come in contact with
the bottom or sides of the channel abrade its surface so as ever to deepen
and widen the valley. This cutting accomplished by partially suspended

débris in rapidly moving currents of water is known as corrasion and the
stream is said to be incising its valley.
As the current is checked upon the lower and flatter grades, some of its
load of sediment, and especially the coarser portion, will be deposited and
so partially fill in the channel. A nice balance is thus established between
degradation and the contrasted process known as aggradation. The older
the river valley the flatter become the grades at any section of its course,
and thus the point which separates the lower zone of aggradation from the
upper one of degradation moves steadily upstream with the lapse of time.
The accordance of tributary valleys.—It is a consequence of the great
sensitiveness of stream corrasion to current velocity that no side stream may
enter the trunk valley at a level above that of the main stream—the tributary
streams enter the trunk stream accordantly. Each has carved its own valley,
and any abrupt increase in gradient of the side streams near where they
enter the main stream would have increased the local corrasion at an
accelerated rate and so have cut down the channel to the level of the trunk
stream.
The grading of the flood plain.—All rivers are subject to seasonal
variations in the volume of their waters. Where there are wet and dry
seasons these differences are greatest, and for a large part of the year the
valleys in such regions may be empty of water, and are in fact often utilized
for thoroughfares. In the temperate climates of middle latitudes rivers are
generally flooded in the spring when the winter snows are melted, though
they may dwindle to comparatively small streams during the late summer.
In the upper reaches of the river the current velocities are such that the
usual river channel may carry all the water of flood time; but lower down
and in the zone of aggradation, where the current has been checked, the
level of the water rises in flood above the banks of its usual channel and
spreads over the surrounding lowlands. As a deposit of sediment is spread
upon the surface, the succession of the annual deposits from this source
raises the general level as a broad floor described as the flood plain of the
river.
The cycles of stream meanders.—The annual flooding with water and
simultaneous deposition of silt is not, however, the only grading process
which is in operation upon the flood plain. It is characteristic of swift
currents that their course is maintained in relatively straight lines because of

the inertia of the rapidly moving water. In proportion as their currents
become sluggish, rivers are turned aside by the smallest of obstructions; and
once diverted from their straight course, a law of nature becomes operative
which increases the curvature of the stream at an accelerated rate up to a
critical point, when by a change, sudden and catastrophic, a new and direct
course is taken, to be in its turn carried through a similar cycle of changes.
This so-called meandering of a stream is accompanied by a transfer of
sediment from one bend or meander of the river to those below and from
one bank to the other. Inasmuch as the later meanders cross the earlier ones
and in time occupy all portions of the plain to the same average extent, a
process of rough grading is accomplished to which the annual overflow
deposit is supplementary.
The course of the current in
consecutive meanders and the cross
sections of the channel which result
directly from the meandering process
will be made clear from examination
of Fig. 168. So soon as diverted from Fig. 168.—Map and sections of a stream
meander. The course of the main current
its direct course, the current, by its is indicated by the dashed line.
inertia of motion, is thrown against
the outer or convex side so as to scour or corrade that bank. Upon the
concave or inner side of the curve there is in consequence an area of slack
water, and here the silt scoured from higher meanders is deposited. The
scouring of the current upon the outer bank and the filling upon the inner
thus gives to the cross section of the stream a generally unsymmetrical
character (Fig. 168 ab). Between meanders near the point of inflection of
the curve, and there only, the current is centered in the middle of the
channel and the cross section is symmetrical (Fig. 168 cd).
The scour upon the convex side of a meander causes the river to swing
ever farther in that direction, and through invasion of the silted flood plain
to migrate across it. Trees which lie in its path are undermined and fall
outward in the stream with tops directed with the current (Fig. 169).
Whenever the flood plain is forested, the fallen trees may be so numerous
as to lie in ranks along the shore, and at the time of the next flood they are
carried downstream to jam in narrow places along the channel and give the
erroneous impression that the flood has itself uprooted a section of forest
(see p. 418).

The cut-off of
the meander.—As
the meander
swings toward its
extreme position it
becomes more and
more closely
looped. Adjacent Fig. 170.—Diagrams to
loops thus show the successive

Fig. 169.—Tree in part undermined
approach nearer positions of stream
meanders and the
upon the outer bank of a and nearer to each relatively stationary
meander. other, but in the point near the sharpest
successive curvature.
positions a nearly stationary point is established
near where the river makes its sharpest turn (Fig. 170, G, and Fig. 454, p.
417). At length the neck of land which separates meanders is so narrow that
in the next freshet a temporary jamming of logs within the channel may
direct the waters across the neck, and once started in the new direction a
channel is scoured out in the soft silt. Thus by a breaking through of the
bank of the stream, a so-called “crevasse”, the river suddenly straightens its
course, though up to this time it has steadily become more and more sharply
serpentine. After the cut-off has occurred, the old channel may for a time
continue to be used by the stream in common with the new one, but the
advantage in velocity of current being with the cut-off, the old channel
contains slacker water and so begins to fill with silt both at the beginning
and the end of the loop. Eventually closed up at both ends, this loop or “ox-
bow” is entirely separated from the new channel, and once abandoned of
the stream is transformed into an ox-bow lake (Fig. 171 and p. 415).
Meander scars.—Swinging as it
occasionally does in its meanderings
quite across the flood plain and against
the bank of the earlier degrading river
in this section, the meander at times
scours the high bank which bounds the
flood plain, and undermining it in the
same manner, it excavates a recess of
amphitheatral form which is known as

a meander scar (Fig. 172). At length Fig. 171.—An ox-bow lake in the flood
plain of a river.
the entire bank is scarred in this
manner so as to present to the stream a series of concave scallops separated
by sharp intermediate salients of cuspate form.
River terraces.—Whenever the
river’s history is interrupted by a small
uplift, or the base level is for any
reason lowered, the stream at once
Fig. 172.—Schematic representation of a begins to sink its channel into the flood
series of river terraces. a, b, c, e,
successive terraces in order of age. d, d, plain. Once more flowing upon a low
d, d, terrace slopes formed of meander grade, it again meanders, and so
scars. produces new walls at a lower level,
but formed, like the first, of
intersecting meander scars. Thus there is produced a new flood plain with
cliff and terrace above, which is known as a river terrace. A succession of
uplifts or of depressions of the base level yields terraces in series, as they
appear schematically represented in Fig. 172. Such terraces are to be found
well developed upon most of our larger rivers to the northward of the Ohio
and Missouri. The highest terrace is obviously the remnant of the earliest
flood plain, as the lowest represents the latest.
The delta of the river.—As it approaches its mouth the river moves
more and more sluggishly over the flat grades, and swings in broader
meanders as it flows. Yet it still carries a quantity of silt which is only laid
down after its current has been stopped on meeting the body of standing
water into which it discharges. If this be the ocean, the salinity of the sea
water greatly aids in a quick precipitation of the finest material. This
clarifying effect upon the water of the dissolved salt may be strikingly
illustrated by taking two similar jars, the one filled with fresh and the other
with salt water, and stirring the same quantity of fine clay into each. The
clay in the salt water is deposited and the water cleared long before the
murkiness of the other has disappeared.
By the laying down of the residue of its burden of sediment where it
meets the sea, the river builds up vast plains of silt and clay which are
known as deltas and which often form large local extensions of the
continents into the sea. Whereas in its upper reaches the river with its
tributary streams appears in the plan like a tree and its branches, in the delta
region the stream, by dividing into diverging channels called distributaries

(Fig. 458, p. 420), completes the resemblance to the tree by adding the
roots. From the divergence of the distributaries upon the delta plain the
Greek capital letter Δ is suggested and has supplied the name for these
deposits. Of great fertility, the delta plains of rivers have become the
densely populated regions of the globe, among which it is necessary to
mention only the delta of the Nile in Egypt, those of the Ganges and
Brahmaputra in India, and those of the Hoang and Yangtse rivers in China.
The levee.—When the snows thaw upon the mountains at the
headwaters of large rivers, freshets result and the delta regions are flooded.
At such times heavily charged with sediment, a thin deposit of fertile soil is
left upon the surface of the delta plain, and in Egypt particularly this is
depended upon for the annual enrichment of the cultivated fields. Though at
this time the waters spread broadly over the plain, the current still continues
to flow largely within the normal channel, so that the slack water upon
either side becomes the locus for the main deposit of the sediment. There is
thus built up on either side of the channel a ridge of silt which is known as a
levee, and this bank is steadily increased in height from year to year (Fig.
452).
To prevent the danger of floods upon the inhabited plains, artificial
levees are usually raised upon the natural ones, and in a country like
Holland, such levees (dikes) involve a large expenditure of money and no
small degree of engineering skill and experience to construct. So important
to the life of the nation is the proper management of its dikes, that in the
past history of China each weak administration has been marked by the
development of graft in this important department and by floods which have
destroyed the lives of hundreds of thousands of people.
Wherever there has been a markedly
rapid sinking upon a delta region, and
depressions are common in delta territory,
no doubt as a result of the loading down of
the crust, the river may present the
paradoxical condition of flowing at a
higher level than the surrounding country.
Between the levees of neighboring
distributaries there are peculiar saucer-
shaped depressions of the country which FigMississippi
. 173.—“Bird-foot” delta of the
River.
easily become filled with water. At the

extremity of the delta the levee may be the only land which shows above
the ocean surface, and so present the peculiar “bird-foot” outline which is
characteristic of the extremity of the Mississippi delta, though other
processes than the mere sinking of the deposits may contribute to this result
(Fig. 173).
The sections of delta deposits.—If now we leave the plan of the delta
to consider the section of its deposits, we find them so characteristic as to
be easily recognized. Considered broadly, the delta advances seaward after
the manner of a railroad embankment which is being carried across a lake.
Though the greater portion of the deposit is unloaded upon a steep slope at
the front, a smaller amount of material is dropped along the way, and a
layer of extremely fine material settles in advance as the water clears of its
finely suspended particles (Fig. 174). Simultaneous deposits within a delta
thus comprise a nearly horizontal layer of coarser materials, the so-called
top-set bed; the bulk of the deposit in a forward sloping layer, the so-called
fore-set bed; and a thin film of clay which is extended far in advance, the
bottom-set bed (Fig. 174, 2). If at any point a vertical section is made
through the deposits, beds deposited in different periods are encountered;
the oldest at the bottom in a horizontal position, the next younger above
them and with forward dip, and the youngest and coarsest upon the top in
nearly horizontal position (Fig. 174, 3).
It has been estimated that the surface
of the United States is now being pared
down by erosion at the average rate of an
inch in 760 years. The derived material
is being deposited in the flood plain and
delta regions of its principal rivers. Some
513 million tons of suspended matter is
in the United States carried to tidewater
each year, and about half as much more
goes out to sea as dissolved matter. If
this material were removed from the
Fig. 174.—Diagrams to show the nature Panama Canal cutting, an 85-foot sea-
of delta deposits as exhibited in section.
level canal would be excavated in about
73 days. The Mississippi River alone carries annually to the sea 340 million
tons of suspended matter, or two thirds of the entire amount removed from
the area of the United States as a whole. It is thus little wonder that great

deltas have extended their boundaries so rapidly and that the crust is so
generally sinking beneath the load.

CHAPTER XIII

EARTH FEATURES SHAPED BY RUNNING WATER

The newly incised upland and its sharp salients.—The successive
stages of incising, sculpturing, and finally of reducing an uplifted land area,
are each of them possessed of distinctive characters which are all to be read
either from the map or in the lines of the landscape. Upon the newly
uplifted plain the incising by the young rivers is to be found chiefly in the
neighborhood of the margins. In this stage the valleys are described as V-
shaped cañons, for the valley wall meets the upland surface in sharp salients
(plate 12 A), and the lines of the landscape are throughout made up from
straight elements. Though the landscapes of this stage present the grandest
scenery that is known and may be cut out in massive proportions, often with
rushing river or placid lake to enhance the effect of crag and gorge, they
lack the softness and grace of outline which belong only to the maturer
erosion stages. The grand cañon of the Colorado presents the features
characteristic of this stage in the grandest and most sublime of all examples,
and the castled Rhine is a gorge of rugged beauty, carved out from the
newly elevated plateau of western Prussia, through which the water swirls
in eddying rapids (Fig. 175).
The stage of adolescence.—As the
upland becomes more largely invaded as a
consequence of the headward advance of
the cañons and their sending out of
tributary side cañons, the sharp angles in
which the cañon walls intersect the plain
become gradually replaced by well- Fig. 175.—Gorge of the River Rhine
near St. Goars, incised within an

rounded shoulders. Thus the lines in the uplifted plain which forms the
hill tops.
landscape of this stage are a combination
of the straight line with a simple curve convex toward the sky (Fig. 176). In
this stage large sections of the original plateau remain, though cut into small
areas by the extensions of the tributary valleys.
The maturely dissected upland.—
Continued ramifications by the rivers
eventually divide the entire upland area into
separated parts, and the rounding of the
Fig. 176.—V-shaped valley with
shoulders of valleys proceeds
well-rounded shoulders simultaneously until of the original upland
characteristic of the stage of no easily recognizable compartments are to
adolescence. Allegheny plateau in be found. Where before were flat hilltops are
West Virginia.
now ridges or watersheds, the well-known
divides. The upland is now said to be completely dissected or to have
arrived at maturity. The streams are still vigorous, for they make the full
descent from the upland level to base level, and yet a critical turning point
of their history has been reached, and from now on they are to show a
steady falling off in efficiency as sculpturing agents.
Viewed from one of the hilltops, the
landscape of this stage bears a marked
resemblance to a sea in which the
numberless divides are the crests of
billows, and these, as distance reduces Fig. 177.—View of a maturely
their importance in the landscape, fade dissected upland from one of its
hilltops, Klamath Mountains,
away into the even line of the horizon (Fig. California (after a photograph by
177). Fairbanks).
The Hogarthian line of beauty.—
Since the youthful stage of the upland, when the lines of its landscape were
straight, its character rugged, and its rivers wild and turbulent, there has
been effected a complete transformation. The only straight line to be seen is
the distant horizon, for the landscape is now molded in softened outlines,
among which there is a repeated recurrence of the line of beauty made
famous by Hogarth in his “Analysis of Beauty.” As well known to all art
students, this is a sinuous line of reversed or double curvature—a curve
which passes insensibly at a point of inflection from convex to concave
(Fig. 178). The curve of beauty is now found in every section of the hills,

and it imparts to the landscape a gracefulness and a measure of restfulness
as well, which are not to be found in the landscapes of earlier stages in the
erosion cycle. In the bottoms of the valleys also the initial windings of the
rivers within their narrow flood plains add silver beauty lines which stand
out prominently from the more somber background of the hills.
Considered from the commercial
viewpoint, the mature upland is one of the
least adaptable as a habitation for highly
civilized man. Direct lines of communication
run up hill and down dale in monotonous
alternation, and almost the only way of Fig. 178.—Hogarth’s line of
beauty.
carrying a railroad through the region,
without an expenditure for trestles which would be prohibitive, is to follow
the tortuous crest of a main divide or the equally winding bed of one of the
larger valleys.
The final product of river
sculpture—the peneplain.—When
maturity has been reached in the
history of a river, its energies are
devoted to a paring down of the valley
slopes and crests so as to reduce the
Fig. 179.—View of the old land of New
England, with Mount Monadnock rising
general level. From this time on hill
in the distance. summits no longer fall into a common
level—that of the original upland—for
some mount notably higher than others, and with increasing age such
differences become accentuated. There is now also a larger aggradation of
the valleys to form the level floors of flood plains, out of which at length
the now slight elevations rise upon such gentle slopes that the process of
land sculpture approaches its end. Gradually the vigor of the stream has
faded away, and can now only be renewed through a fresh uplift of the land,
or, what would amount to the same thing, a depression of the base level.
Upland and river have reached old age together, and the approximation to a
new plain but little elevated above base level is so marked that the name
peneplain is applied to it. Scattered elevations, which because of some
favoring circumstance rise to greater heights above the general level of the
peneplain, are known as monadnocks after the type example of Mount
Monadnock in New Hampshire (Fig. 179).

The river cross sections of
successive stages.—To the successive
stages of a river’s life it has been
common to carry over the names from
the well-marked periods of a human
life. If neglecting for the moment the
general aspect of the upland, we fix
our attention upon the characteristic
Fig. 180.—Comparison of the cross
sections of river valleys for the different
cross sections of the river valley, we
stages of the erosion cycle. find that here also there are clearly
marked characters to distinguish each
stage of the river’s life (Fig. 180). In infancy the steep, narrow, and sharp-
angled cañon is a characteristic; with youth the wider V-form has already
developed; in adolescence the angles of the cañon are transformed into
well-rounded shoulders, and the valley broadens so as in the lower reaches
to lay down a flood plain; in maturity the divides and the double curves of
the line of beauty appear; while in the decline of old age the valleys are
extremely broad and flat and are floored by an extended flood plain.
The entrenchment of meanders with renewed uplift.—Upon the
reduced grades which are characteristic of the declining stage of a river’s
life, the current has little power to modify the surface configuration. On the
old land of this stage a renewed uplift starts the streams again into action.
This infusion of driving power into moving water, regarded as a machine
capable of accomplishing certain work, is like winding up a clock that has
run down. Once more the streams acquire a velocity sufficient to enable
them to cut their valleys into the land surface, and so a new erosional cycle
may be inaugurated upon the old land surface—the peneplain. After such an
uplift has been accomplished and the rivers have sunk their early valleys
within the new upland, we may look out from this now elevated surface and
the eye take in but a single horizontal line, since we view the plain along its
edge.

Fig. 181.—The Beavertail Bend of the Yakima Cañon in central
Washington (after George Otis Smith).

By the uplift the meanders of the earlier rivers may become entrenched
in the new upland, the wide lobes of the individual meanders being now
separated by mountains where before had been plains of silt only. The New
River of the Cumberland plateau and the Yakima River of central
Washington (Fig. 181) furnish excellent American examples of intrenched
meanders, as the Moselle River does in Europe. Upon the course of the
latter river near the town of Zell a tunnel of the railroad a quarter of a mile
in length pierces a mountain in the neck of a meander lobe in which the
river itself travels a distance of more than six miles in order to make the
same advance. The Kaiser Wilhelm tunnel in the same district penetrates a
larger mountain included in a double meander of the river. Although
intrenched, river meanders are still competent to scour and so undermine
the outer bank, and with favoring conditions they may by this process erode
extended “bottoms” out of the plateau. (See Lockport quadrangle, U. S. G.
S.)
The valley of the rejuvenated river.—Whenever a new uplift occurs
before an erosional cycle has been completed, the rivers become intrenched,
not in a peneplain, but in the bottoms of broad valleys. The sweeping curves
which characterize mature landscapes may thus be brought into striking
contrast with the straight lines of youthful cañons which with V-sections
descend from their lowest levels (Fig. 182). The full cross section of such a
valley shows a central V whose sharp shoulders are extended outward and
upward in the softened curves of later erosion stages.
The arrest of stream erosion by the more resistant rocks.—The
capacity of a river to erode and carry away the rock material that lies along

its course is dependent not only upon the
velocity of the current, but also upon the
hardness, the firmness of texture, and the
solubility of the material. Particularly in
arid and semiarid regions, where no
mantle of vegetation is at hand to mask
the surfaces of the firmer rock masses,
Fig. 182.—A rejuvenated river valley differences of this kind are stamped
(after a photograph by Fairbanks). deeply upon the landscape. The rock
terraces in the Grand Cañon of the Colorado together represent the stronger
rock formations of the region, while sloping talus accumulations bury the
weaker beds from sight.
Each area of harder rock which rises
athwart the course of a stream causes a
temporary arrest in the process of valley
erosion and is responsible for a noteworthy
local contraction of the river valley. The
valley is carved less widely as well as less
deeply, and since a river can never corrade Fig. 183.—Plan of a river
below its base, a “temporary base level” is narrows.
for a time established above the area of
harder rock. Owing to the contraction of the valley under these conditions,
the locality is described as a river narrows (Fig. 183). The narrows upon the
Hudson River occur in the Highlands where the river leaves a broad
expanse occupied by softer sediments to traverse an island-like area of hard
crystalline rocks. Within the narrows of a river the steep walls,
characteristic of youth and the turbulent current as well, are often retained
long after other portions of the river have acquired the more restful lines of
river maturity. The picturesque crag and the generally rugged character of
river narrows render them points of special interest upon every navigable
river.
The capture of one river’s territory by another.—The effect of a hard
layer of rock interposed in the course of a stream is thus always to delay the
advance of the erosional process at all levels above the obstruction. When a
stream in incising its valley degrades its channel through a veneer of softer
rocks into harder materials below, it is technically described as having
discovered the harder layer. Where several neighboring streams flow by

similar routes to their common base level,
those which discover a harder rock will
advance their headwaters less rapidly into
the upland and so will be at a disadvantage
in extending their drainage territory. A
stream which is not thus hindered will in
the course of time rob the others of a
portion of their territory, for it is able to
erode its lower reaches nearer to base level
and thus acquire for its upper reaches,
where erosion is chiefly accomplished, an
advantage in declivity. The divide which
separates its headwaters from those of its
less favored neighbor will in consequence
migrate steadily into the neighbor’s
territory. The divide is thus a sort of Fig. 184.—Successive diagrams to
boundary wall separating the drainage illustrate repeated river piracy
basins of neighboring streams, and any and the development of “trellis
drainage”, (after Russell).
migration must extend the territory of the
one at the expense of the other. As more and more territory is brought under
the dominion of the more favored stream, there will come a time when the
divide in its migration will arrive at the channel of the stream that is being
robbed, and so by a sudden act of annexation draw off all the upper waters
into its own basin. By this capture the stream whose territory has been
invaded is said to have been beheaded. By this act of piracy the stronger
stream now develops exceptional activity because of the local steep grades
near the point of capture, and with this newly acquired cutting power the
invader is competent to advance still further and enter the territory of the
stream that lies next beyond. The type of drainage network which results
from repeated captures of this kind is known as “trellis drainage” (Fig.
184), a type well illustrated by the rivers of the southern Appalachians.
In general it may be said that, other conditions being the same, of two
neighboring streams which have a common base level, that one which takes
the longest route will lose territory to the other, since it must have the flatter
average slope. Stream capture may thus come about without the discovery
of hard rock layers which are more unfavorable to one stream than another.

Water and wind gaps.—In the
Allegheny plateau rivers cross, the
range of harder rocks in deep mountain
narrows which upon the horizon
appear as gateways through the barrier
of the mountain wall. Such gateways
are sometimes referred to as “water
Fig. 185.—Sketch maps to show the earlier gaps”, of which the Delaware Water
and the present drainage condition
about the Blue Ridge near Harper’s Gap is perhaps the best known
Ferry. example, though the Potomac crosses
the Blue Ridge at the historic Harper’s
Ferry through a similar portal. The valley of the tributary Shenandoah has
been the scene of an interesting episode in the struggle of rival streams
which is typical of others in the same upland region. The records which
may be made out from the landscapes show clearly that in an earlier but
recent period, when the general surface stood at a higher level which has
been called the Kittatinny Plain, the younger Potomac of that time and a
younger but larger ancestor of Beaverdam Creek each crossed the Blue
Ridge of the time through similar water gaps (Fig. 185, map, and Fig. 186).
The Potomac of that time was, however, the more deeply intrenched, and
possessing an advantage in slope it was able to advance the divide at the
head of its tributary, the Shenandoah, into the territory of Beaverdam Creek.
Thus the beheading of the Beaverdam by the Shenandoah was
accomplished (Fig. 185, second map) and its upper waters annexed to the
Potomac system. With the subsequent lowering of the general level of the
country which yielded the present Shenandoah Plain, the former water gap
of Beaverdam Creek was abandoned of its stream at a high level in the
range. Known as Snickers Gap, it may serve as a type of the “wind gaps” of
similar origin which are not altogether uncommon in the Appalachian
Mountain system (Fig. 186).
Character profiles.—For humid
regions the landscapes possess
characters which, speaking broadly,
depend upon the stage of the erosion
cycle. For the earliest stages the straight
Fig. 186.—Section to illustrate
line enters as almost the only element in the history of Snickers Gap.
the design; as the cycle advances to

adolescence the rounded forms begin to replace the angles of the immature
stages, and with full maturity the lines of beauty alone are characteristic. As
this critical stage is passed irregularity of feature and ever more flattened
curves are found to correspond to the decline of the river’s vital energies.
There are thus marks of senility in the work of rivers (Fig. 187).

Fig. 187.—Character profiles of landscapes shaped by stream erosion in
humid climates.

Reading References for Chapters XII and XIII

General:—
Sir John Playfair. Illustrations of the Huttonian Theory of the Earth.
Edinburgh, 1802, pp. 350-371.
J. W. Powell. Exploration of the Colorado River of the West and its
Tributaries. Washington, 1875, pp. 149-214.
G. K. Gilbert. Report on the Geology of the Henry Mountains. Washington,
1877, pp. 99-150. (A classic upon the work of rivers.)
C. E. Dutton. Tertiary History of the Grand Cañon District (with atlas),
Mon. 2, U. S. Geol. Surv., 1882, pp. 264.
W. M. Davis. The Rivers and Valleys of Pennsylvania, Nat. Geogr. Mag.
vol. 1, 1889, pp. 203-219; The Triassic Formation of Connecticut, 18th
Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 144-153.
Sir A. Geikie. The Scenery of Scotland. London, 1901, pp. 1-12.

I. C. Russell. Rivers of North America. Putnam. New York, 1898, pp. 327.
M. R. Campbell. Drainage Modifications and their Interpretation, Jour.
Geol., vol. 4, 1896, pp. 567-581, 657-678.
Henry Gannett. Physiographic Types, U. S. Geol. Surv., Topographic
Atlas, Folios 1-2, 1896, 1900.
W. M. Davis. The Geographical Cycle, Geogr. Jour., vol. 14, 1899, pp. 481-
504.

The flood plain:—
Henry Gannett. The Flood of April, 1897, in the Lower Mississippi, Scot.
Geogr. Mag., vol. 13, 1897, pp. 419-421.
W. M. Davis. The Development of River Meanders, Geol. Mag., Decade iv,
vol. 10, 1903, pp. 145-148.
W. S. Tower. The Development of Cut-off Meanders, Bull. Am. Geogr.
Soc., vol. 36, 1904, pp. 589-599.

River terraces:—
W. M. Davis. The Terraces of the Westfield River, Massachusetts, Am. Jour.
Sci., vol. 14, 1902, pp. 77-94, pl. 4; River Terraces in New England,
Bull. Mus. Comp. Zoöl., vol. 38, 1902, pp. 281-346.

River deltas:—
G. K. Gilbert. The Topographic Features of Lake Shores, 5th Ann. Rept. U.
S. Geol. Surv., 1885, pp. 104-108; Lake Bonneville, Mon. I, U. S.
Geol. Surv., 1890, pp. 153-167.
Charts of Mississippi River Commission.
G. R. Credner. Die Deltas, ihre Morphologie, geographische Verbreitung
und Entstehungsbedingungen, Pet. Mitt. Ergh. 56, 1878, pp. 1-74, pls.
1-3.

The peneplain:—

W. M. Davis. Plains of Marine and Subaërial Denudation, Bull. Geol. Soc.
Am., vol. 7, 1896, pp. 377-398; The Peneplain, Am. Geol., vol. 23,
1899, pp. 207-239.

Intrenchment of meanders:—
W. M. Davis. The Seine, the Meuse, and the Moselle, Nat. Geogr. Mag.,
vol. 7, 1896, pp. 189-202.

Stream capture:—
N. H. Darton. Examples of Stream Robbing in the Catskill Mountains,
Bull. Geol. Soc. Am., vol. 7, 1896, pp. 505-507, pl. 23.
Collier Cobb. A Recapture from a River Pirate, Science, vol. 22, 1893, p.
195.
William H. Hobbs. The Still Rivers of Western Connecticut, Bull. Geol.
Soc. Am., vol. 13, 1902, pp. 17-22, pl. 1.
Isaiah Bowman. A Typical Case of Stream Capture in Michigan, Jour. Geol.,
vol. 12, 1904, pp. 326-334.

CHAPTER XIV

THE TRAVELS OF THE UNDERGROUND WATER

The descent within the unsaturated zone.—Of the moisture
precipitated from the atmosphere, that portion which neither evaporates into
the air nor runs off upon the surface, sinks into the ground and is described
as the ground water. Here it descends by gravity through the pores and open
spaces, and at a quite moderate depth arrives at a zone which is completely
saturated with water. The depth of the upper surface of this saturated zone
varies with the humidity of the climate, with the altitude of the earth’s
surface, and with many other similarly varying factors. Within humid
regions its depth may vary from a few feet to a few hundred feet, while in
desert areas the surface may lie as low as a thousand feet or more.
The surface of the zone of the lithosphere that is saturated with water is
called the water table, and though less accentuated it conforms in general to
the relief of the country (Fig. 188). Its depth at any point is found from the
levels of all perennial streams and from the levels at which water stands in
wells.

Fig. 188.—Diagram to show the seasonal range in the position of the water
table and the cause of intermittent streams.

During the season of small precipitation the water table is lowered, and
if at such times it falls below the bed of a valley, the surface stream within
the valley dries up, to be revived when, after heavier precipitation, the water
table has in turn been raised. Such streams are said to be intermittent, and
are especially characteristic of semiarid regions (Fig. 188).
Wherever in descending from the surface an impervious layer, such as
clay, is encountered, the further downward progress of the water is arrested.
Now conducted in a lateral direction it issues at the surface as a spring at
the line of emergence of the upper surface of the impervious layer (Fig.
189).

Fig. 189.—Diagram to show how an impervious layer conducts the
descending water in a lateral direction to issue in surface springs.

The trunk channels of descending water.—While within the
unconsolidated rock materials near the surface of the earth, it is clear that
water can circulate in proportion as the materials are porous and so
relatively pervious. As the pore spaces become minute and capillary, the
difficulty of permeation through the materials becomes very great. Thus in
the noncoherent rocks it is the coarse gravel and the layers of sand which
serve as the underground channels, while the fine clays have the effect of an
impervious wall upon the circulating waters. In coarse sand as much as a
third of the volume of the material is pore space for the absorption and
transmission of water. Even under these favorable conditions the movement
of the water is exceedingly slow and usually less than a fifth of a mile a
year.
Within the hard rocks it is the
sandstones which have the largest
pore spaces, but in nearly all
consolidated rocks there are
additional spaces along certain of the
bedding planes, the joint openings
(Fig. 190), and the crushed zones of
displacement, so that these parting

planes become the trunk channels, so Fig. 190.—Sketch map of the Oucane de
Chabrières near Chorges in the High
to speak, of the circulating water. It is Alps, to illustrate the corrosion of
along such crevices that in the course limestone along two series of vertical
of time the mineral matter carried in joints (after Martel).
solution by the water is deposited to
produce the ore veins and the associated crystallized minerals.
The caverns of limestones.—Where limestone formations have a
nearly flat upper surface, a large part of the surface water enters the rock by
way of the joint spaces, which it soon widens by solution into broad
crevices with well-rounded shoulders. At joint intersections solution of the
limestone is so favored that the water may here descend in a sort of vertical
shaft until it meets a bedding plane extending laterally and offering more
favorable conditions for corrosion. Its journey now begins in a lateral
direction, and solution of the rock continuing, a tunnel may be etched out
and extended until another joint is encountered which is favorable to its
further descent into the formation. By this process on alternating shafts and
galleries the water descends to near the surface of the water table by a series
of steps, and is eventually discharged into the river system of the district
(Fig. 191). Within the larger caverns the water at the lowest level usually
flows as a subterranean river to emerge later into the light from beneath a
rock arch.
From the plan of a system of connecting
caverns it may often be observed that the
galleries of the several levels are alike
directed along two rectangular directions
Fig. 191.—Diagram to show the which indicate the master joint directions
relation of caverns in limestone to
the river system of the district within the limestone formation. This is
and to the “swallow holes” upon especially clear from the map of the galleries
the surface. in the explored portions of the Mammoth
Cave (Fig. 192).
Swallow holes and limestone sinks.—Above the caverns of limestone
formations there are selected points where the water has descended in the
largest volume, and here funnel-shaped depressions have been dissolved out
from the surface of the rock. In different districts such depressions have
become known as “sinks”, “swallow holes”, entonnoirs, and Orgeln.
Wherever the depressions have a characteristic circular outline, there can be
little doubt that they are the product of solution by the descending water,

and have relatively small connections only with the subterranean caverns.
They have thus naturally collected upon their bottoms the insoluble clay
which was contained in the impure limestone as well as a certain amount of
slope wash from the surface. Inasmuch as the clays are impervious to water,
the bottoms of these swallow holes are better supplied with moisture than
the surrounding rock surfaces, and by nourishing a more vigorous plant
growth are strongly impressed upon the landscape (Fig. 193).

Fig. 192.—Plan of a portion of Mammoth
Cave, Kentucky (after H. C. Hovey).

Certain of the depressions above
caverns are, however, less regular in
outline, and their bottoms are
occupied by a mass of limestone
rubble. In some instances, at least,
these depressions appear to be the
result of local incaving of the cavern
roofs. An incaving of this nature may Fig. 193.—Trees and shrubs growing
close up an earlier gallery in the luxuriantly upon the bottoms of sinks
cavern and divert the cave waters to a

new course. The destruction of the within a limestone country (after a
photograph by H. T. A. de L. Hus).
roofs of caverns through this process
of incaving may continue until only relatively small remnants are left. From
long subterranean tunnels the caves are thus transformed into subaërial rock
bridges that have become known as “natural bridges.” The best-known
American example is the Natural Bridge near Lexington, Virginia. Much
grander natural bridges have been formed in sandstone by a totally different
process, and must not be confused with these limestone remnants of
caverns.
The sinter deposits.—Just as water can dissolve the calcareous rocks
with the formation of caverns, it can under other conditions deposit the
material which has thus been taken into solution. Its power to hold
carbonate of lime in solution is dependent upon the presence of carbonic
acid gas within the water. Water charged with gas and dissolved lime
carbonate is said to be “hard”, and if the gas be driven off by boiling or
otherwise, the dissolved lime is thrown out of solution and deposited in a
form well known to all housekeepers.
Hard water flowing in a surface stream, if dashed into spray at a
cascade, may deposit its lime carbonate in an ever thickening veneer
wherever the spray is dashed about the falls. This material, when cut in
section, has waving parallel layers and is known as travertine or calcareous
sinter. Some of the most remarkable deposits of this nature may be seen at
the cascade of Tivoli near Rome, and most of the Roman buildings have
been constructed from travertine that has been quarried in the vicinity.
The growth of stalactites.—Water, after percolating slowly through the
crevices of limestone, where it becomes charged with the carbonic acid gas
and with dissolved carbonate of lime, may trickle from the roof of a cavern.
Emerging from the narrow crevice, it may give off some of its contained
gas and is usually subject to evaporation, with the result that the lime
carbonate is left adhering to the rock surface from which evaporation took
place. If the water collects upon the cavern roof so slowly that it can
entirely evaporate before a drop can form, the entire content of carbonate
will be left adhering to the roof. Evaporation is most rapid near the margins
and over the center of each drop as it develops, and the deposit which is left
thus takes the form of tiny white rings at those points upon the crevice
where there is the easiest passage for the trickling water. To the outer
surface of these rings water will first adhere and then evaporate, as it will

also slowly ooze through the passage in the ring, but here without
evaporation until it reaches the lower surface. A pendant structure will,
therefore, develop, growing outward in all directions by the deposition of
concentric layers which are thickest near the roof, and downward into the
form of a rock “icicle” through evaporation of the water which collects near
the tip. These pendant sinter formations are known as stalactites and are
thus formed of concentric layers arranged like a series of nested
cornucopias with a perforation of nearly uniform caliber along the axis of
the structure (Fig. 194).
Formation of stalagmites.—
Wherever the water percolates
through the roof of the cavern so
rapidly that it cannot entirely
evaporate upon the roof, a portion
falls to the floor, and, spattering as it
strikes, builds up a relatively thick
cone of sinter known as a stalagmite,
and this is accurately centered
beneath a stalactite upon the roof. In Figof. 194.—Diagrams to show the manner
formation of stalactites, stalagmites,
proportion as the cavern is high, the and sinter columns beneath parallel
dropping water is widely dispersed as crevices upon the roofs of caverns (in
it strikes the floor, with the formation part after von Knebel).
of a correspondingly thick and blunt
stalagmite. As this rises by growth toward the roof, it often develops upon
its summit a distinct crater-like depression (Fig. 194, lower figure). When
the process is long continued, stalactites and stalagmites may grow together
to form columns which may be ranged with their neighbors like the pipes of
an organ, and like them they give out clear tones when struck lightly with a
mallet. At other times the columns are joined to their neighbors to form
hangings and draperies of the most fantastic and beautiful design (Fig. 195).

Fig. 195.—Sinter formations in the Luray caverns, Virginia.

In remote antiquity limestone caverns afforded a refuge to many species
of predatory birds and animals as well as to our earliest ancestors. The
bones of all these denizens of the caves lie entombed within the clays and
the sinter formations upon the cavern floors, and they tell the story of a
fierce and long-continued warfare for the possession of these natural
strongholds. The evidence is clear that these cave men with their primitive
weapons were able at times to drive away the cave bears, lions, and hyenas,
and to set up in the cavern their simple hearths, only in their turn to be
conquered by the ferocity of their enemies. Some of the European caves
have yielded many wagonloads of the skeletons of these fierce predatory
animals, together with the simple weapons of the primitive man.
The Karst and its features.—Most so-called limestones have a large
admixture of argillaceous materials (clays) and of siliceous or sandy
particles. Such impurities make up the bulk of the clays and muds which are
left behind when the soluble portions of the limestone have been dissolved.
Swallow holes we have found to be characteristic features within such
districts. When limestones are more nearly pure, as in the Karst region east
of the Adriatic Sea, similar features are developed, but upon a grander scale,
and certain additional forms are encountered. In place of the sink or
swallow hole, there appears the “karst funnel” or doline, a deep, bowl-
shaped depression having a flat bottom. Such funnels may be 30 to 3000

feet across and from 6 to 300 feet in
depth (Fig. 196). Though in one or two
instances known to be the result of the
break down of cavern roofs (Fig. 197),
yet like the swallow holes of other
regions these larger funnels appear
generally to be the work of solution by
the descending waters. Where they
have been opened in artificial cuttings
along railroads or in mines, the original
rock is found intact at the bottom, with
small crevices only going down to Fig. 196.—Map of the dolines of the
lower levels. Over the bottoms of the Karst region near Divača.
dolines there is spread a layer of fertile
red clay, the terra rossa, like that which is obtained as a residue when a
fragment of the limestone has been dissolved in laboratory experiments.
A desert from the destruction of
forests.—Between the dolines is found a
veritable desert with jutting limestone angles
and little if any vegetation. The water which
falls upon the surface either runs off quickly
or goes down to the subterranean caverns by
Fig. 197.—Cross section of the
doline formed by inbreak of a
which so much of the country is
cavern roof. The Stara Apnenka undermined. Hence it is that the gardens
doline in Carinthia (after which furnish the sustenance for the
Martel). scattered population are all included within
the narrow limits of the doline bottoms. Although to-day so largely a barren
waste, we know that the Karst upon the Adriatic was in remote antiquity a
heavily forested region and that it supplied the myriads of wooden piles
upon which the city of Venice is supported. The vessels which brought to
this port upon the Adriatic its ancient prosperity were built from wood
brought from this tract of modern desert. In the days of Venetian grandeur
the fertile terra rossa formed a veneer upon the rock surface of the Karst
and so retained the surface waters for the support of the luxuriant forest
cover. After deforestation this veneer of rich soil was washed by the rains
into the dolines or into the few stream courses of the region, thus leaving a
barren tract which it will be all but impossible to reclaim (plate 6 A).

Upon the steeper slopes over the
purer limestones, the rain water runs
away, guided by the joints within the
rock. There is thus etched out a more or
less complete network of narrow
channels (Fig. 190, p. 181), between
which the remnants rise in sharp blades
to produce a structure often simulated
Fig. 198.—Sharp Karren of the upon the fissured surface of a glacier that
Ifenplatte Allgäu (after Eckert).
has been melted in the sun’s rays (Fig.
401). These almost impassable areas of karst country are described as
Schratten or Karrenfelder (Fig. 198).
The ponore and the polje.—To-day large areas of the Karst are devoid
of surface streams, nearly all the surface water finding its way down the
crevices of the limestone into caverns, and there flowing in subterranean
courses. The foot traveler in the Karst country is sometimes suddenly
arrested to find a precipice yawning at his feet, and looking down a well-
like opening to the depth of a hundred feet or more, he may see at the
bottom a large river which emerges from beneath the one wall to disappear
beneath the other. These well-like shafts are in the Austrian Karst known as
Ponores, while to the southward in Greece they are called Katavothren.

Plate 6.

A. Barren Karst landscape near the famous Adelsberg grottoes.
(Photograph by I. D. Scott.)

B. Surface of a limestone ledge where joints have been widened through
solution. Syracuse, N.Y. (Photograph by I. D. Scott.)

Elsewhere the karst river may emerge from its subterranean course in a
broader depressed area bounded by vertical cliffs, from which it later
disappears beneath the limestone wall. Such depressions of the karst are
known as poljen, and appear in most cases to be above the downthrown
blocks in the intricate fault mosaic of the region. Some of these steeply
walled inclosures have an area of several hundred square miles, and
especially at the time of the spring snow melting they are flooded with
water and so transformed into seasonal lakes (Fig. 199 and p. 422). It
appears that at such times the cave galleries of the region with their local
narrows are not able to carry off all the water which is conducted to them;
and in consequence there is a temporary impounding of the flood waters in
those portions of the river’s course which are open to the sky and more
extended. The rush of water at such times may bring the red clay into the
subterranean channels in sufficient quantity to clog the passages. The
Zirknitz Lake usually has high water two or three times a year, and
exceptionally the flooding has continued for a number of years. It has thus

in some districts been necessary to afford relief to the population through
the construction of expensive drainage tunnels.
The conditions which are typified in
the Karst area to the east of the Adriatic
Sea are encountered also in many other
lands; as, for example, in the Vorarlberg
and Swiss Alps, in Lebanon, and in Sicily.
The return of the water to the
surface.—Water which has descended
from the surface and been there held
between impervious layers, may be under Fig. 199.—The Zirknitz seasonal
the pressure of its own weight or “head”; lake within a polje of the Karst
and will later find its way upward, it may (after Berghaus).
be to the surface or higher, where a
perforation is discovered in its otherwise impervious cover. Such local
perforations are produced naturally by lines of fracture or faulting (widened
at their intersections), and artificially through the sinking of deep wells. The
water, which at ordinary times reaches the surface upon fissures, is usually
concentrated locally at the intersections of the fracture network, where it
issues in lines of fissure springs (Fig. 200); but at the time of earthquakes
the water may rise above the surface in lines of fountains (p. 83), or
occasionally as sheets of water which may mount some tens of feet into the
air.
In contrast to the flow of surface springs,
which varies with the season through wide
ranges both in its volume and in temperature
of the water, the volume of fissure springs is
but slightly affected by the seasonal
precipitation, and the water temperature is
maintained relatively constant. Rock is but a
poor heat conductor, and the seasonal
Fig. 200.—Fissure springs arranged temperature changes descend a few feet only
upon lines of rock fracture at into the ground. Thus water which rises from
intersections, Pomperaug valley, depths of a few hundred feet only is apt to be
Connecticut.
icy cold, while from greater depths the effect
of the earth’s internal heat is apparent in a uniform but relatively higher
temperature of the water. Such “warm” or thermal springs are apt to contain

considerable mineral matter in solution, both because the water is far
traveled and because its higher temperature has considerably increased its
solvent properties.
It has long been recognized that lines of junction of different rock
formations at the base of mountain ranges are localities favorable for the
occurrence of thermal springs. These junction lines are usually within zones
where by movement upon fractures the widest openings in the rock have
formed, and the catchment area of the neighboring mountain highland has
supplied head for the ground water. A map of the hot springs within the
Great Basin of the western United States would present in the main a map
of its principal faults.
Artesian wells.—From the natural fissure spring an artesian well differs
in the artificial character of the perforation of the impervious cover to the
water layer. The water of artesian wells may flow out at the surface under
pressure, or it may require pumping to raise it from some lower level. Ideal
conditions are furnished where the geological structure of the district is that
of a broad basin or syncline. The water which falls in a neighboring upland
is here impounded between two parallel, saucer-like walls and will flow
under its head if the upper wall be perforated at some low level (Fig. 201,
3).

Fig. 201.—Schematic diagrams to illustrate the different types of artesian
wells, (1) A non-flowing well; (2) flowing wells without basin structure
caused by clogging of the pervious formation; (3) flowing wells in an
artesian basin. The dotted lines are the water levels within the pervious
layers (after Chamberlin).

A monoclinal structure may furnish artesian conditions when the
generally pervious layer has become clogged at a low level so as to hold

back the water (Fig. 248, 2). Pumping wells may be used successfully even
when such clogging does not exist, for the slow-moving underground water
flows readily in the direction of all free outlets (Fig. 201, 1).
Hot springs and geysers.—Thermal springs whose temperature
approaches the boiling point of water are known as hot springs. A geyser is
a hot spring which intermittently ejects a column of water and steam. Both
hot springs and geysers are to be found only in volcanic regions, and appear
to be connected with uncooled masses of siliceous lava. In two of the three
known geyser regions, Iceland and New Zealand, the volcanoes of the
neighborhood are still active, and the lavas of the Yellowstone National
Park date from the quite recent geological period which immediately
preceded the so-called “Ice Age.”
Wherever found, geysers are in the low levels along lines of drainage
where the underground water would most naturally reappear at the surface.
Their water has penetrated to considerable depths below the surface, but has
been chiefly heated by ascending steam or other vapors. The water journey
has been chiefly made along fissures, as is shown by the cool springs which
often issue near them. Though some hot springs and geysers may disappear
from a district, others are found to be forming, and there is no good reason
to think that geysers are rapidly dying out, as was at one time supposed.
The action of a geyser was first satisfactorily explained by the great
German chemist Bunsen after he had made studies of the Icelandic geysers,
and the mechanics of the eruption was later strikingly illustrated in the
laboratory by an artificial geyser constructed by the Irish physicist Tyndall.
In many respects this action is like that of the Strombolian eruption within a
cinder cone, since it is connected with the viscosity of the fluid and the
resistance which this opposes to the liberation of the developing vapor. In
the case of the geyser, a column of heated water stands within a vertical
tube and is heated near the bottom of the column.
Though the water may at its surface have the normal boiling
temperature and be there in quiet ebullition, the boiling point for all lower
levels is raised by the weight of the column of superincumbent liquid, and
so for a time the formation of steam within the mass is prevented. In Fig.
202 is shown a cross section of the Icelandic Geysir from which our name
for such phenomena has been derived, and to this section have been added
the actual observed temperatures of the water at the different levels as well

as the temperatures at which boiling
can take place at these levels. From
this it will be seen that at a depth of 45
feet the water is but 2° Centigrade
below its boiling point. A slight
increase of temperature at this level,
due to the constantly ascending steam,
will not only carry this layer above the
boiling point, but the expansion of the
steam within the mass will elevate the
Fig. 202.—Cross section of Geysir, Iceland, upper layers of the water into zones
with simultaneously observed
temperatures recorded at the left, and
where the boiling points are lower, and
the boiling temperatures for the same thus bring about a sudden and violent
levels at the right (after Campbell). ebullition of all these upper portions.
Thus is explained the almost universal
observation that just before geysers erupt the hot water rises in the bowls
and generally overflows them.
The water ejected from the geyser is considerably cooled in the air; and
after its return to the tube must be again heated by the ascending vapors
before another eruption can occur. The measure of the cooling, the time
necessary to fill the tube, and the supply of rising steam, all play a part in
fixing the period which separates consecutive eruptions. If the top of the
tube be narrowed from its average caliber, as is commonly observed to be
true of the geysers within the Yellowstone National Park, the escape of the
steam is further hindered, and frequent geyser eruption promoted.
An artificial geyser for demonstration of the phenomenon in the lecture
room is represented in Fig. 203. The cut has been prepared from a
photograph of an apparatus designed by Professor B. W. Snow of the
University of Wisconsin. In this design the tube is contracted so as to have a
top diameter one fourth only of what it is at the bottom, where heat is
directly applied by multiple Bunsen lamps. The water once sufficiently
heated, this artificial geyser erupts at regular intervals of time which are
dependent upon the dimensions of the apparatus and the quantity of heat
applied.
In case of natural geysers a considerable quantity of heat escapes
between eruptions in steam which issues quietly from the bowl of the
geyser. If this heat be retained by plugging the mouth of the tube with a

barrowful of turf, as is sometimes
done with the geyser Strokr in
Iceland, eruption is promoted and so
takes place earlier. Another method
of securing the same result is to
increase the viscosity of the water
through the addition of soap, as was
accidentally discovered by a
Chinaman who was utilizing the
geyser water in the Yellowstone Park
for laundry operations. After this
discovery it became a common
custom to “soap” the Yellowstone
geysers in order to make them play;
but this method was prohibited under
heavy penalty after the disastrous
eruption of the Excelsior Geyser.
The deposition of siliceous
sinter by plant growth.—Geysers
are known only from areas of
siliceous volcanic lava, and this may
perhaps have its cause in the easier Fig. 203.—Apparatus for simulating
solution of the geyser tube from such geyser action in the lecture room (by
materials. The silica dissolved in the courtesy of Professor B. W. Snow).
heated waters is again deposited at
the surface to form siliceous sinter or geyserite. This material forms terraces
surrounding the geysers or is built up into mounds which are often quite
symmetrical, such as those of the Bee Hive and Lone Star geysers of the
Yellowstone Park (Fig. 204).
The greater part of this separation of silica from the heated geyser
waters is due to the action of plants or algæ that are able to grow in the
boiling waters and which produce the beautiful colors in the linings to the
hot springs. The wonderful variety of the tints displayed is accounted for by
the fact that the algæ take on different colors at different temperatures. The
silica is deposited from the water in the gelatinous hydrated form, which,
however, dries in the sun to a white sand. The growth within the pools goes
on in a manner similar to that of a coral reef, the algæ dying below and

there becoming encased in the rock
lining while still continuing to grow
upon the surface. Whereas sinter of
this nature, when deposited by
evaporation alone, can produce a
maximum thickness of layer of a
twentieth of an inch each year, the
growth from alga deposition within
limited areas may be as much as eight
inches during the same period.

Fig. 204.—Cone of siliceous sinter built up Reading References for Chapter
about the mouth of the Lone Star Geyser XIV
in the Yellowstone National Park.

General:—
F. H. King. Principles and Conditions of the Movements of Ground Water,
19th Ann. Rept. U. S. Geol. Surv., 1899, Pt. ii, pp. 59-294, pls. 6-16.
C. S. Slichter. The Motions of the Underground Waters, Water Supply
Paper No. 67, U. S. Geol. Surv., 1902, pp. 1-106, pls. 1-8; Field
Measurements of the Rate of Movement of Underground Waters, ibid.,
No. 140, 1905, pp. 1-122, pls. 1-15.
M. L. Fuller. Occurrence of Underground Water, ibid.. No. 114, 1905, pp.
18-40, pls. 4; Bibliographic review and index of papers relating to
underground waters published by the United States Geological Survey,
1879-1904, ibid., No. 120, 1905, pp. 1-128.

Caverns:—
E. A. Martel. Les abimes, les eaux souterraines, les cavernes, les sources,
la spélæologie. Delagrave, Paris, pp. 578. (Lavishly illustrated.)
H. C. Hovey. Celebrated American Caverns. Cincinnati, 1896, pp. 228; The
Mammoth Cave of Kentucky. Louisville, 1897, pp. 111.
J. W. Beede. Cycle of Subterranean Drainage in the Bloomington
Quadrangle, Proc. Ind. Acad. Sci., 1910, pp. 1-31.