Field book of common rocks and minerals _ for identifying the rocks and minerals of the United States and interpreting their origins and meanings Frederic Brewster Loomis 1063 down

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Title: Field book of common rocks and minerals
for identifying the rocks and minerals of the United States and
interpreting their origins and meanings

Author: Frederic Brewster Loomis

Photographer: Walter Everett Corbin

Release date: August 18, 2017 [eBook #55382]
Most recently updated: October 23, 2024

Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK FIELD BOOK
OF COMMON ROCKS AND MINERALS ***

Field Book of
Common Rocks
and Minerals

For identifying the Rocks and Minerals of the United States and
interpreting their Origins and Meanings

By
Frederic Brewster Loomis
Late Professor of Mineralogy and Geology
in Amherst College

With 47 Colored Specimens and over 100 other Illustrations from Photographs by W. E.
Corbin and drawings by the Author

G. P. Putnam’s Sons
New York and London

FIELD BOOK
OF
COMMON ROCKS AND MINERALS

Copyright, 1923, 1948
by
Frederick Brewster Loomis

Twenty-sixth Impression
Revised 1948

All rights reserved. This book, or parts thereof, must not be reproduced in any form without
permission.

Made in the United States of America

Dedicated
TO

MY MOTHER
WHO ENCOURAGED ME WHILE A BOY TO GATHER MINERALS, ROCKS AND FOSSILS.

vii

PREFACE
Everyone, who is alert as he wanders about this world, wants to know what
he is seeing and what it is all about. Here and there with the aid of capable
guides a few have been introduced into the sphere of that wide and
fascinating knowledge of Nature which has been so rapidly accumulated
during this and the latter part of the last century. It is a full treasure house
constantly being enriched, but unfortunately the few who have been
initiated have soon acquired a technical language and habit, so that their
knowledge and new acquisitions are communicated to but few. The public
at large, not having the language nor an interpreter at hand, has come
almost at once to a barrier which few have the time or patience to surmount.

Latterly it has become clear that the largest progress cannot be made if the
knowledge of any branch of Science is confined to a few only. The most
rapid advances have been made where many men are interested and
enthusiastic. In no science should there be a difficult barrier between the
amateur and the professional student. All Nature is equally open for
everyone to study, and there should never be created obstacles as by the use
of terminology not easily acquired by anyone. Of late these barriers have
been in part broken down and competent students have written guides viii
which anyone can follow, and soon begin to know the plants, trees,
birds, insects, etc. So far no one has attempted to make the study of
minerals and rocks so direct and simple that everyone can get a start. Most
books on minerals, and practically all those on rocks are written for school
courses, and to say the least chill any enthusiasm which is naturally aroused
by the finding of interesting looking rocks or minerals.

The purpose of this book is first of all to provide a means of identifying
minerals and rocks by such methods as are practical without elaborate
equipment or previous training: and second to suggest the conditions under
which the various minerals and rocks were formed, so that, at the first
contact, one may get a conception of the events which have anteceded the
mineral or rock which has been found. For this purpose keys have been

worked out for determining the rocks and minerals by such obvious features
as color, hardness, etc. Each mineral or rock is introduced by a summary of
its characters, then the features by which it may be distinguished from any
other similar mineral are given, after which its mode of origin and its
meanings are considered. For those interested in the composition of the
minerals, it is given in chemical symbols with each mineral. Most
classifications of minerals are based on the composition, all the sulphides,
carbonates, etc., being grouped together, but in this book, because the
popular interest and commercial uses are primarily in the metal ix
present, the minerals are grouped in each case about the chief metal,
all the minerals of iron being grouped together, for instance.

A few minerals and rocks which are not strictly common have been
included such as gems and meteorites; the gems because they are of intense
interest to their owners and are often simply perfect examples of a fairly
common mineral; and such forms as meteorites because it is important that,
if one should run across one, it should be recognized, and so not lost to the
world.

The book is freely illustrated, those minerals in which color is important for
identification being illustrated in colors, and those which are black, or in
which the color is not a determining factor, are shown in either
photographic or outline figures.

In the introductory chapter there are explanations of the terms used in
describing minerals, and of the systems in which they are grouped. A
knowledge of the systems may not be a necessity, but it is a great help in
determining minerals, and is very important in understanding why the
individual minerals take the varied forms which are characteristic of them.
These systems will be better understood after a few minerals have been
gathered and examined.

It is hoped the book will help those who have already some knowledge of
rocks and minerals, and especially that it will tempt many to begin an
acquaintance with the rocks and minerals which are all about them, and are
the foundation on which our material progress is built. Rocks and minerals
have some advantages over most objects which are collected in that x

they neither require special preparation before they can be kept, nor do they
deteriorate with time.

The author will appreciate corrections or suggestions as to better
presentation of the material in this book.

F. B. L.

Amherst, Mass.

xi

CONTENTS
PAGE
Preface vii
CHAPTER
I.—An Introduction 3
II.—On the Forms and Properties of Minerals 10
III.—The Minerals 25
IV.—The Rocks 170
V.—Miscellaneous Rocks 248
Bibliography 270
Index 273

xiii

LIST OF PLATES
(AT END OF BOOK)

PAGE
Tourmaline crystals, growing amid feldspar crystals in a cavity in granite,
from Paris, Me.
279
Plate 1.—Basal forms of the isometric system 311
Plate 2.—Basal forms of the tetragonal system. Basal forms of the
orthorhombic system
312
Plate 3.—Basal forms of the monoclinic system. A cross section of the
prism with its edges beveled so that a six-sided prism is formed (pseudo-
hexagonal). Basal form of the triclinic system.
313
Plate 4.—Basal forms of the hexagonal system 314
Plate 5.—Gold in quartz from California (in color) 280
Plate 6.—Native silver in calcite. Argentite, the black masses throughout
the white quartz (in color)
281
Plate 7.—Pyrargyrite as it appears after moderate exposure to the light;
streak at left. Crystal form of pyrargyrite. Prousite as it appears after
moderate exposure to the light; streak at left (in color)
282
Plate 8.—Native copper from Michigan. Chalcopyrite in tetrahedrons and
an occasional octahedron; streak to the left (in color)
283
Plate 9.—Chalcocite crystals with the bluish tarnish. Tetrahedrite crystals;
streak to left (in color)
284

Plate 10.—Tetrahedrons showing characteristic manner in which
tetrahedrite occurs. A cube with the edges beveled and the corners cut in a
form characteristic of cuprite
315
Plate 11.—Cuprite, the red crystals showing characteristic color, others
showing the green tarnish of malachite. Malachite (green) and azurite
(blue), the two minerals shown together as they very commonly occur (in
color)
285
Plate 12.—Limonite. The crystal form in which goethite is found (in color)
286
Plate 13.—Hematite. Clinton iron ore, oolitic. Siderite crystals (in color) 28
7
Plate 14.—Crystal forms of hematite. A typical crystal of magnetite. The
rhombohedron typical of siderite
317
Plate 15.—Pyrite crystals. Marcasite in concretionary form with radiate
structure (in color)
288
Plate 16.—The pyritohedron. The pyritohedron with certain of its edges
beveled by the cube faces, to show the relationship of these two forms
318
Plate 17.—Galena in crystals. Pyromorphite crystals (Green) (in color) 289
Plate 18.—Typical forms for cerrusite. Forms in which anglesite occurs 319
Plate 19.—Sphalerite, some the normal yellow and some crystals with the
reddish tinge. (White is dolomite.) Zincite, streak to the left (in color)
290
Plate 20.—A characteristic form in which sphalerite may occur.
Characteristic form for zincite crystals. Typical form of crystal of willemite
320
Plate 21.—Smithsonite in yellow crystals. Franklinite in octahedral
crystals, streak to left (in color)
291
Plate 22.—Moss agates, showing the dendritic growth of manganitic
minerals, like manganite or pyrolusite. Crystal form of manganite
321
322

Plate 23.—Crystals of green corundum in syenite, from Montana. Typical
crystal forms of corundum
Plate 24.—Arsenopyrite, showing crystals massed so as to be incompletely
developed. Realgar as it usually occurs in powdery incrustations (in color)
292
Plate 25.—Large crystals of stibnite; the light colored face is the one
parallel to which cleavage occurs. Niccolite is a vein in slate (in color)
293
Plate 26.—Cobaltite, silver color, with pink tinge. Smaltite, pink is cobalt
bloom (in color)
294
Plate 27.—Carnotite from Southwest Colorado. Cinnabar (in color) 295
Plate 28.—Cassiterite, twinned crystals. The crystal form in which both
cassiterite and rutile occur when in simple crystals. Multiple twinning
characteristic of rutile
323
Plate 29.—Crystal of spinel. Crystal forms in which dolomite occurs 324
Plate 30.—Two intergrowing or twinned quartz crystals. Diagram of the
typical quartz crystal. A quartz crystal on which the left hand rhombohedron
is represented by small faces, while the right hand rhombohedron has large
faces
316
Plate 31.—Amethyst, not however deep enough colored for gems. Jasper,
with botryoidal surface (in color)
296
Plate 32.—Banded agate from Brazil (in color) 297
Plate 33.—Common opal from Arizona. Siliceous sinter or geyserite from
Yellowstone Park (in color)
298
Plate 34.—Orthoclase, a cleavage piece. Crystal forms of orthoclase.
Diagram of a multiple twin of a plagioclase feldspar
325
Plate 35.—A group of microcline crystals from Pike’s Peak, Colo.
Labradorite, showing multiple twinning (the striation) and the iridescent
play of colors (in color)
299
300

Plate 36.—Crystal form of a pyroxene. Cross sections of a pyroxene crystal
showing the lines of intersection of two cleavage planes. Cross sections of
pyroxenes, showing typical forms taken by crystals. Augite crystals, in
crystalline limestone (in color)
Plate 37.—Diagrams of amphibole crystals. Tremolite in silky fibrous
crystals, asbestos. Hornblende crystals in quartzite
326
Plate 38.—The dodecahedron and the 24-sided figure characteristic of
garnets. The garnet, grossularite. The garnet, alamandite (in color)
301
Plate 39.—Beryl of gem quality. Zircon in syenite (in color) 302
Plate 40.—Cyanite crystals in schist. A crystal of mica, showing basal
cleavage (in color)
303
Plate 41.—Crystal form typical of topaz. A topaz crystal from Brazil.
Crystal form typical of staurolite when simple. A typical twin of staurolite
(in color)
304
Plate 42.—Epidote crystals. Typical forms of epidote crystals. Typical
forms of tourmaline
327
Plate 43.—Serpentine. Chlorite (in color) 305
Plate 44.—The typical form of analcite. A typical natrolite crystal. The
typical crystal form of stilbite. A sheaf-like bundle of fibrous crystals,
typical of stilbite
329
Plate 45.—A group of calcite crystals. Typical forms of calcite 330
Plate 46.—Typical forms of aragonite. Typical form of the anhydrite
crystal
331
Plate 47.—A piece of gypsum looking on the surface of the perfect
cleavage, and showing the two other cleavages as lines, intersecting at 66°.
Twinning is also shown. A simple crystal of gypsum. Twin crystals of
gypsum.
332

Plate 48.—A group of barite crystals. Outline of the typical tabular barite
crystal. The six-sided double pyramid, composed of three interpenetrating
crystals, typical of witherite and strontianite
328
Plate 49.—Apatite crystals in crystalline calcite. The ends of apatite
crystals showing common modes of termination (in color)
306
Plate 50.—A group of fluorite crystals. A group of halite crystals (in color)
307
Plate 51.—Sulphur crystals. Ice crystals, the top one, the end of a
hexagonal prism; the two lower figures multiple twins as in snow flakes
333
Plate 52.—The Devil’s Tower, Wyoming, an example of igneous rock with
columnar structure, and resting on sedimentary rocks
334
Courtesy of the U. S. Geological Survey
Plate 53.—A coarse granite. Graphic granite 335
Plate 54.—Syenite. Gabbro 336
Plate 55.—Basalt-porphyry. The large white crystals are phenocrysts of
plagioclase feldspar. Basalt-obsidian
337
Plate 56.—Amgydoloid 338
Plate 57.—The north face of Scott’s Bluff, Neb., showing sedimentary
sandstones above and clays below. The type of erosion is characteristic of
arid regions
339
Courtesy of the U. S. Geological Survey
Plate 58.—Breccia. Conglomerate 340
Plate 59.—Calcareous shale. Coquina 341
Plate 60.—Foramenifera from chalk; enlarged about 25 diameters. Encrinal
limestone; fragments of the stems, arms and body of crinoids
342
Plate 61.—Amber. Two bottles of petroleum, the left hand one with a
paraffin base, the right hand one with an asphalt base (in color)
308
Plate 62.—Diatomaceous earth magnified 50 times. Two diatoms from the
above enlarged 250 times
343

After Gravelle, by the courtesy of Natural History
Plate 63.—A metamorphic rock, showing the contortion of layers due to
expansion under heat
344
Plate 64.—A conglomerate partly metamorphosed to a gneiss. A typical
gneiss
345
Plate 65.—Mica schist, with garnets. Chlorite schist (in color) 309
Plate 66.—Phyllite. A white marble, with black streaks due to graphite 346
Plate 67.—Serpentine composed of serpentite, hematite, and some calcite
(in color)
310
Plate 68.—Claystones, simple and compound. A lime concretion, which on
splitting disclosed a fern leaf of the age of the coal measures
347
Plate 69.—A septeria from Seneca Lake, N. Y. Pisolite from Nevada 348
Plate 70.—A geode filled with quartz crystals 349
Plate 71.—A quartz pebble from the bed of a New England brook. A
pebble of schist and granite from the foot of Mt. Toby, Mass.
350
Plate 72.—An iron-nickel meteorite, of 23 lbs., which fell in Claiborne
Co., Tenn. An etched slice of an iron meteorite which fell in Reed City,
Osceola, Co., Mich.
351
Plate 73.—A stone meteor, about natural size, which fell in 1875 in Iowa
Co., Iowa
352

xix

FIELD BOOK OF
COMMON ROCKS AND
MINERALS
3

CHAPTER I
AN INTRODUCTION

Why

Why should one be interested in rocks and minerals? Because the whole
world is made of rocks and minerals. They are the foundations on which we
build. From them we draw all our metals, and the extent to which we utilize
our minerals is a measure of the advance of our civilization. Fragments of
rock are the soil from which, by way of the plants, we draw our food, and
ultimately our life. The rocks make wild or gentle scenery, one at least of
the sources of pleasure. Knowledge of rocks and minerals is then
knowledge of fundamentals, of ultimate sources. Between finding the raw
materials and their present uses there are usually many steps (so many that
we forget that the beginning and end are united), as for instance in your
watch. It is made of gold, brass, steel, agate, glass, and perhaps has
luminous radium paint on the hands. It is a long way from finding and
mining gold, chalcopyrite, hematite, carnotite, etc., through the raw
materials, gold, copper, iron, etc., to the finished watch, but the minerals are
the foundations of the watch; and it took centuries to find them and learn
one by one how to use them, from the gold 10,000 years ago down to 4
the radium within the last fifty years. Then too there is joy in going out
into Nature’s wild and raw places, joy in being on the foundations of the
earth, joy in the scenery, in the beauty of the minerals themselves.

But why collect the rocks and minerals? First because this is the way to
know them. Both mineral and rocks require careful examination in order to
see all those fine points by which they are distinguished. It is often
necessary to compare one with another to get in mind the differences of
form, color, streak, though with increasing familiarity these characteristics
are recognized at first sight. It is the repeated examination which makes a
rock tell the story of the country from which it came. Our first attempts to

read the story give us only the most general facts. Nature’s book, written in
the rocks, has to be read closely, often between the lines. Until we are used
to the characters in which the words are written, we read slowly. When they
look at Nature’s book, always open, most people do not read; for they do
not know their letters. Every mineral is a letter, every rock a word, and we
learn to read as we learn the minerals and rocks, and every time we go over
them we get more facts coming out. The place where a rock or mineral
occurs is of course the relation between them, and is involved in reading the
story. No one today is a perfect reader. We are all learning to see more in
the rocks day by day. So it is important to have the rocks and minerals
where they can be handled and repeatedly examined, where we can 5
turn to them in our leisure moments. Don’t stop when you have learned
the name of a mineral or rock. You need more. See what it means. Secondly,
minerals have beauties of form, color, and structure, and they do not fade.
They will be as perfect in ten years as when found. We are all naturally
crows, and love to gather the objects which interest us. It is not a bad habit,
and only needs directing. Cultivate it. Have a hobby, and minerals and rocks
are a good one; for they are like treasures in Heaven which “neither moth
nor rust doth corrupt.” Not only will they give you pleasure, but they will
be a constructive education, training the eye to see, and the mind to think
straight. No one ever regretted the time and effort spent in collecting either
minerals or rocks.

Collecting

In order to make a collection valuable two or three rules must be observed.
In the case of rocks, collect large enough samples so that they will be
characteristic, and clear in their make-up. The standard size for rocks is 3 ×
4 inches on top and one to two inches thick according to the nature of the
rock. Tiny fragments do not give the character of the rock as well, and they
are all the time getting into confusion. Every specimen should be labeled,
with at least its name and the exact locality from which it came.
Composition, structural features, associations, and classification may be
added, the more the better; for each item adds to the information and
interest of the specimen. One may make his own labels or have printed 6

blanks, and may put as much care and art into the labels as desired, the
more the better. One thing is very important and that is to have a number on
the label with a corresponding one on the specimen, so that in case they
should get separated, they may be readily brought together, even by one
who is not familiar with the individual specimens. Lastly, give your
collection as good a place as possible, either in drawers, boxes or in a case.
The specimens are worth being kept in order and where they can be readily
seen and compared. Nature is systematic, and there is a reason for the order
in which rocks and minerals are taken up. It is desirable either that this
order, or some one of the orders of Nature appear in the collection. In this
book the metals are the basis of classification, all those minerals primarily
related to one of the metals being grouped together.

In collecting minerals, the size of the specimens can not be so regularly
followed, but it should be followed when collecting non-crystalline
minerals, and when possible. Crystals however are chosen from a variety of
points of view, as perfection of form, color, examples of cleavage, twinning,
etc.; so that in many cases smaller or larger examples must appear in the
collection. It is always desirable that as many variations of a rock or
mineral as possible should appear in the collection, and in many cases
examples of the matrix from which the crystals came. When crystals are
tiny, it is well to place them in vials, that they may not be lost.

7

Where

Where shall we start in making a collection? Near home. Get the local
minerals and rocks first, and then range as widely as possible. The best
places are bare and exposed rocks, especially where fresh and un-weathered
surfaces are available. Quarries and where there has been blasting along
roads offer fine opportunities. Fissures and cavities in the rocks are
especially likely to have fine crystals, and in all localities continued search
will reveal a surprising number of different minerals. The greatest variety
occur in metamorphic rocks, or where igneous rocks come in contact with
other rocks, but even the sedimentary rocks have a goodly range of
minerals. All through the glaciated regions of the northern United States lie

scattered boulders brought from afar, which will yield a surprising number
of minerals and variety of rocks.

Equipment

One may start with a very simple equipment, a geologist’s or stone mason’s
hammer which can be obtained at any hardware store, being sufficient for
field work. Rocks should be broken, so as to show fresh surfaces and to get
below the disintegrating effects of weathering. At home one should have a
streak plate (a piece of unglazed porcelain), a set of hardness minerals (see
page 20), and a small bottle each of hydrochloric and nitric acid. A pocket
lens is useful in order to see more clearly the form of small minerals. These
things can be purchased of any Naturalist’s Supply Co., like Ward’s Natural
Science Establishment, P.O. 24, Beachwood Sta., Rochester, N. Y., or 8
the Kny-Scheerer Corp., 483 First Ave., New York City. Success
depends upon a quick eye, and persistent hunting. When traveling,
opportunities are offered at frequent intervals to see and get new specimens.

Study Your Collection

Be sure and see the meaning in each rock and mineral. The history of the
country is revealed in its rocks and minerals. Note whether the rocks are
horizontal or folded, whether they change character from place to place, or
vertically. In going over a piece of country you may locate an ancient
mountain system now leveled, by noting a series of metamorphic rocks,
with a central core of granite, the roots of former mountains. Don’t be
afraid to draw conclusions from what you see. Later, when the opportunity
offers, look up the region in the geological folio, bulletin, or map of that
section, and check up your findings. These geological folios and bulletins,
of which there is one for nearly every region, are a great help to collectors
in suggesting where to look for various rocks and minerals. Write to the
Director of the U. S. Geological Survey, Washington, D. C., for a catalogue
of the publications of the United States Survey, or find out from him what
are the maps or folios for the region in which you are interested. These U.

S. publications cost but little. When opportunity presents itself, visit other
collections. In them you will see some of the minerals or rocks which have
puzzled you, and there is nothing quite so satisfactory as seeing the 9
rocks or minerals themselves. No description can always be so
convincing. Then too you will get suggestions as to localities that you can
visit.

Literature

As your collection grows, if you find you have special interest in one or
another branch of the field, you can get books giving more details in that
line; and at the back of this book will be found a list of such books.

10

CHAPTER II
ON THE FORMS AND PROPERTIES OF MINERALS

Rocks

All we know of the earth by direct observation is confined to less than four
miles depth; though by projecting downward the layers of rock that come to
the surface, we may fairly assume a knowledge of the structure down to six
or eight miles depth. This outer portion is often referred to as the “crust of
the earth,” but the idea that the deeper portions are molten is no longer held.
This outer portion is made of rocks, and a rock may be defined as, a mass of
material, loose or solid, which makes up an integral part of the earth, as
granite, limestone, or sand. The rocks (except glassy igneous ones) are
aggregates of one or more minerals; either in their original form like the
quartz, feldspar and mica of granite, or in a secondary grouping, resulting
from the units having been dislodged from their primary position and
regrouped a second time, as in sandstone or clay.

Minerals

Since the rocks are aggregates of minerals, it is best to take up the minerals
first. A mineral may be defined as a natural inorganic substance of definite
chemical composition. It is usually solid, generally has crystalline 11
structure, and may or may not be bounded by crystal faces. A crystal
is a mineral, bounded by symmetrically grouped faces, which have definite
relationships to a set of imaginary lines called axes. There are between
1100 and 1200 minerals, of which 30 are so frequently present, and so
dominant in making up the rocks, that they are termed rock-forming
minerals. About 150 more occur frequently enough so that they can be
termed common minerals, and one may expect to find a fairly large

proportion of them. Some of these are abundant in one part of the country
and rare in others, but this book is written to cover the United States, and so
all those which have a fair abundance are included, though some will only
be found in the west and others mostly in the east. Then there are some
more minerals which are really rare, but which are cherished because of
their beauty of color, and are used as gems. These are mentioned, and many
of the gems are simply clear and beautiful examples of minerals, which in
dark or cloudy forms are much more common. If one finds any of these rare
minerals which are not mentioned in this book, he must turn to one of the
larger mineralogies mentioned in the literature list to determine them.

Crystal Structure

A crystal is a mass of molecules, all of the same composition. A molecule in
its turn is made up of atoms, and each atom is a unit mass of an element.
Thus the calcite molecule is made up of one unit or atom of calcium, one of
carbon, and three of oxygen (CaCO₃). These atoms are held together 12
by an attraction, and make a molecule, and for the study of minerals
the molecule is the unit. The mineral, calcite, is a mass of molecules all like
the one above, and each molecule so small as to be invisible even with the
aid of the most powerful microscope. When calcite is in crystal form, the
molecules, like ranks of soldiers, are arranged each in its place, each at a
definite distance from the other. While each molecule may vibrate or wiggle
within certain limits it does not leave its place. (The comparison with
soldiers is a good one for the molecules of one layer, but it must be
remembered that in a crystal there are also like spacings and ranks up and
down as well.) As long as the molecules remain in fixed ranks, up and
down, forward and back, and sideways, the crystal is perfect. Calcite may
be heated until it melts and becomes liquid. Then the molecules leave their
definite arrangement and move about in all sorts of directions, like the
soldiers after ranks have broken. So long as the molecules are thus free to
move about but keep together, the substance is a liquid. There are cases
when the molecules in this disorder take fixed positions without falling into
ranks. Such minerals are non-crystalline and usually appear glassy. If still
greater heat is applied to the mineral in liquid form, a point is reached (the

vapor point), above which the molecules go flying away from each (like
soldiers in a panic), each seeking to get as far from the other as possible, so
only a container will prevent their dissipation. When in this condition a
mineral is gaseous. When cooled, the reverse order obtains. The 13
molecules of gas gather into a miscellaneous mob or liquid: and if this
is further cooled (but not too suddenly), they fall into ranks and make a
crystal. This may be illustrated with water. When above 212° F. it is steam
(molecules wildly dissipated); when between 212° and 32° it is water
(molecules close to each other, but milling like a herd of cattle); and when
below 32° it is ice, the molecules ranged in perfect order, rank on rank.

Crystal Systems

With all the possible forms that crystals can and do take, there are six
systems of arrangement. First there is the case where ranks, files, and
vertical rows are all equal, and now to be scientific, instead of talking about
ranks, files, etc., we use the term axes to express these ideas; the files or
arrangements from front to back, being called the a axis, the ranks, or side
to side arrangement the b axis, and the vertical arrangement the c axis. (See
Plate 1.) These axes are imaginary lines, but they represent real forces.

Isometric system

When the axes are all equal and at right angles to each other, a crystal is
said to be in the isometric system. The cube is the basal form and each side
is known as a face. The ends of the axes come to the middle of the cube
faces. The essential feature of this system is that whatever happens to one
axis must happen to all, which is another way of saying that all the 14
axes are equal. If we think of the cube as having the corners cut off,
we would have a new face on each of the eight corners, in addition to the
six cube faces. Then if each of these new faces were enlarged until they met
and obliterated the cube faces, an eight-sided figure, the octahedron, would
result. In this the axes would ran to the corners. Another modification of the
cube would be to bevel each of its twelve edges, making twelve new faces

in addition to the six cube faces. If we think of these new faces being
developed until they meet and obliterate the cube faces, there will result a
twelve-sided figure, the dodecahedron. And the 24 edges of the
dodecahedron could be beveled to make a 24-sided figure, and so on. Of
course in Nature the corners are not cut, nor the edges beveled, but as a
result of the interaction of the forces expressed by the axes and the
distribution of the molecules, the molecules arrange themselves in a cube,
octahedron, dodecahedron or combination of these basal forms.

Crystal formation

Crystals are formed in liquids as they cool or evaporate and can no longer
hold the minerals in solution. Crystals start about a center or nucleus, and
molecule by molecule, the orderly arrangement is increased and the crystal
grows, there being no size which is characteristic. If free in the liquid the
crystal grows perfectly on all sides, but if crystals are growing side by side,
there comes a time when they interfere with each other. Then the free faces
continue to grow and the orderly internal arrangement is maintained, 15
though externally there is interference.

Tetragonal system

In the second or tetragonal system one axis (the c axis) is different from
the other two, but all three are still at right angles with each other. This is
saying scientifically that the lines of force are greater or less in one
direction than in the other two, but they act at right angles to each other.
The a and the b axes are equal and anything that happens to one of these
two must happen to the other, but need not happen to the c axis. Thinking of
the molecules that arrange themselves under this system of forces, it is clear
that the simplest form will be a square prism, i.e., front to back, and from
side to side the numbers of molecules will be equal, but up and down there
will be a greater or lesser number. If the eight corners of this prism were
cut, and these corner faces increased in size until they met, the resulting
octahedron would be longer (or shorter) from top to bottom than from side

to side or front to back, but the measurement from front to back would be
equal to the one from side to side. In this system we may have the vertical
edges of the prism beveled, and not have to bevel the horizontal ones, or we
may bevel the horizontal edges and not the vertical ones. There is no
dodecahedron in this system or in any other system than the isometric. The
forms in this tetrahedral system are really a combination of the four sides of
the square prism with such modifications as equally affect them all, with
two ends which may be flat, or pyramidal, or modified pyramidal 16
faces.

Orthorhombic system

The third system has all three axes unequal, but all three are still at right
angles with each other. This is saying that the lines of force in the crystals
are all at right angles to each other but of unequal value. The faces in this
case are all in pairs. What happens at one end of an axis must happen at the
opposite end, but does not need to happen at the ends of any of the other
axes. We are dealing with pairs of faces (one at either end of an axis), and if
three such pairs are combined in the simplest manner, the resulting figure
will be a rectangular prism. If we cut the eight corners of this prism and
enlarge the faces until they meet, the result is an octahedron, in which the
distance from top to bottom, from side to side, or from front to back is not
the same in any two cases. (See Plate 2.) In this system if a face is made by
beveling one edge of the prism there must be a corresponding face on the
edge diagonally opposite, but there does not have to be one on any of the
other edges. However if a corner is cut, that face affects all the axes and so
all the corners must be cut. A great many crystals occur in this system, and
some of them which are prismatic in shape may give trouble, for it is not
uncommon for the vertical edges of the prism to be so beveled, that two of
the original prism faces are obliterated, and the two remaining faces added
to the four new faces make a six-sided prism, which at first glance seems to
belong to the hexagonal system. (See Plate 3, fig. 3.) Close 17
examination however will show that, instead of all the prism faces
being alike, as would be necessary for the hexagonal system, they are really

in pairs, and one pair at least will be distinguished in some way, such as
being striated, pitted, or duller.

Monoclinic system

The fourth system has all the axes unequal, the a axis and the b axis at right
angles to each other, but the c axis is inclined to the a axis, meeting it at
some other than a right angle. The monoclinic system is like the
orthorhombic system except that it leans, or is askew, in one direction. The
result is that the faces at the ends of the b axis are rhombohedral, while the
others are rectangular. As in the foregoing system, the faces are in pairs at
opposite ends of the axes; and as in the orthorhombic system, a face may
occur on one edge and only have to be repeated on the edge diagonally
opposite. The simplest form in this system will be made by combining the
three pairs of faces at the opposite ends of the axes, which gives a prism,
which is rectangular in cross section, but leans backward (or forward) if
placed on end. As in all the systems, if a corner is cut, all must be cut; and if
these corner faces are extended to meet each other, an octahedron results, in
which, as in the prism, no two axes are equal. If this octahedron is properly
orientated (i.e. with the a and b axes horizontal), it will lean forward or
backward. Many minerals belong to this system; and, as in the 18
orthorhombic system, it is not uncommon to have the vertical edges
so beveled that two of the prism faces are obliterated, and the remaining
two prism faces with the four new faces make a six-sided prism, which
seems hexagonal. (See plate 3, figure 3.) However, such a pseudo-
hexagonal prism may be recognized by at least one pair of the faces having
distinguishing marks (striæ, pits, or dullness), instead of all being just alike.

Triclinic system

The fifth or triclinic system has all the axes unequal, and no two of them
intersect at right angles. As in the two preceding systems the faces occur in
pairs at the opposite ends of the axes. This is the most difficult system in

which to orientate a crystal, but fortunately only a few crystals occur in this
system, such as the feldspars.

Hexagonal system

Lastly there is a group of crystals which have four axes, one vertical, and
three in the horizontal plane which intersect each other at angles of 60°, all
these three being equal to each other, but different from the vertical axis.
The simplest form in this system is the six-sided prism. If one corner of this
prism is cut all must be, and if these corner faces are extended to meet each
other, a double-six-sided pyramid results. In this system if one of the
vertical edges of the prism is beveled, all must be, but the horizontal edges
need not be; or the horizontal edges may be beveled and the vertical ones
not. The ends as they are related to the c axis may be developed 19
independently of the prism, and so the prism may be simply truncated
by a flat end, or have pyramids on either end.

Hemihedral forms

In this system it is quite common to have forms which result from the
development of each alternate face of either the prism or the double
pyramid. In the case of the prism, if every alternate face is developed (and
the others omitted) a three-sided prism results, as in tourmaline. In the case
of the double pyramid if the three alternate faces above are united with the
three alternate faces below, a six-sided figure is formed, which is known as
the rhombohedron, as all the faces are rhombohedral in out-line and all
equal. These forms in which only half the faces are developed are known as
hemihedral forms. The same sort of thing may happen in the isometric
system in the case of the octahedron, and also in the case of the octahedron
of other systems. When half the faces of the octahedron are developed, two
above unite with two below and make a four-sided figure, known as a
tetrahedron. (See plate 10.) While tetrahedrons may occur in any of the first
five systems they are not common outside the isometric system.

Twinning

Another modification of the simple forms which will be met occasionally is
twinning. By this is meant two crystals growing together as though placed
side by side on some one of the faces, and then revolved until the two axes
which would normally be parallel are at some definite angle with each
other, 60°, or 180° which is commoner. The surface of contact 20
between the two crystals is called the composition face, and as no
more material can be added on that face the crystals continue to grow
developing the other faces, and we find faces in contact with each other
which should be at the opposite end or other side of the crystals. This
contact of faces which should not come in contact, and the presence of
reentrant angles are indications of twinning. In some minerals the twinning
may be repeated time and again, and if the twinning is on one of the end
faces a branching structure results, as in frost and snow crystals, or the
multiple twinning may be of crystals growing side by side when the final
form will approximate a series of thin sheets placed side by side as in some
feldspars. The peculiar forms characteristic of individual minerals are taken
up under the respective minerals.

Other important properties of minerals are hardness, cleavage, specific
gravity, streak, luster, and color.

Hardness

Hardness may be defined as the mineral’s resistance to abrasion or
scratching. It is measured by comparing a mineral with Moh’s scale, a set of
ten minerals arranged in the order of increasing hardness, as follows:

1 talc
2 gypsum
3 calcite
4 fluorite
5 apatite
6 feldspar
7 quartz

8 topaz
9 corundum
10 diamond

A set for measuring hardness may be purchased from any dealer in 21
mineral supplies. For rough determination, as in the field, the
following objects have the hardness indicated; the finger nail 2¼, a penny 3,
a knife blade about 5.5, and glass not over 6. In testing, a mineral is harder
than the one it will scratch, and softer than the one by which it is scratched.
For instance, if a mineral will scratch calcite and is scratched by fluorite, it
is between 3 and 4 in hardness, say 3.5. When two samples mutually scratch
each other they are of equal hardness. Care must be used in determining
hardness, especially with the harder minerals; for often, when testing a
mineral, the softer one will leave a streak of powder on the harder one,
which is not a scratch. One should always rub the mark to make sure it is
really a groove made by scratching.

Cleavage

Cleavage is the tendency, characteristic of most minerals, and due to the
arrangement of their molecules, to cleave or break along definite planes.
The cleavage of any mineral is not irregular or indefinite, but characteristic
for each mineral, and always parallel to possible or actual faces on the
crystal, and always so described. For instance galena has three cleavages,
all equally good, and parallel to the cube faces; so it is said to have cubic
cleavage. In the same way fluorite has octahedral cleavage, and calcite
rhombic cleavage. In some minerals cleavage is well developed in one
plane, and less developed in other planes, or it may be lacking altogether.
The varying degrees of perfection by which a mineral cleaves are 22
expressed as, perfect or imperfect, distinct or indistinct, good or poor,
etc.

Specific gravity

The specific gravity of a mineral is its weight compared with the weight of
an equal volume of water, and is therefore the expression of how many
times as heavy as water the mineral is. For instance the specific gravity of
pyrite is 5.1, which is saying it is 5.1 times as heavy as water. In a pure
mineral the specific gravity is constant, and an important factor in making
final determinations. As ordinarily obtained, a piece of pure mineral is
weighed in air, which value may be called x. It is then immersed in water
and again weighed, and this value is called y. The difference between the
weight in air and that in water is the weight of an equal volume of water.
Then we have the following formula:

x
specific gravity = .
x-y

Various balances have been devised for making these measurements, but
any balance which will weigh small objects accurately, may be adapted to
specific gravity work, by hanging a small pan under the regular weighing
pan. When using this balance, care is taken to see that the lower pan is
always submerged in water, even while the mineral is being weighed in air,
so that when weighed in water in the lower pan, the weight of this lower
pan has already been considered.

23

Streak

By streak is meant the color of the mineral when powdered. For some
minerals, especially metallic ores, it is of great importance, for it remains
constant, though the color of the surface of the mineral changes materially.
It is most readily determined by rubbing a corner of the mineral on a piece
of unglazed porcelain. Small plates, known as “streak plates” are made for
this purpose.

Luster

The luster of a mineral is the appearance of its surface by reflected light,
and it is an important aid in determining many minerals. Two types of luster
are recognized; metallic, the luster of metals, most sulphides and some
oxides, all of which are opaque on their thin edges; and non-metallic, the
luster of minerals which are more or less transparent on their thin edges,
and most of which are light colored. The common non-metallic lusters are;
vitreous, the luster of glass; resinous, the appearance of resin; greasy, oily
appearance; pearly, the appearance of mother-of-pearl; silky, like silk due to
the fibrous structure; adamantine, brilliant like a diamond; and dull, as is
chalk.

Color

When used with caution color is of the utmost importance in determining
minerals, especially in making rapid determinations. In metallic minerals it
is constant and dependable; but in the non-metallic minerals it may vary,
due to the presence of small amounts of impurities which act as pigments.
Color depends on chemical composition, and when not influenced by 24
impurities is termed natural; but when the color is due to some
inclosed impurity it is termed exotic. In this latter case caution must be used
in making determinations. Many minerals are primarily colorless, but take
on exotic colors as a result of the presence of small quantities of impurities;
for instance, pure corundum is colorless, but with a trace of iron oxide
present becomes red, and is called the ruby, or with a trace of cobalt
becomes blue and is called sapphire.

25

CHAPTER III
THE MINERALS

KEY TO THE MINERALS, BASED ON HARDNESS, COLOR, ETC.

OPAQUE COLORS
Color Hardness Streak Remarks Mineral
Red
scarlet 2.5 scarlet surface tarnishes prousite
black
2.5 vermilion surface scarlet to dark cinnabar
red
ochre 7 white non-crystalline jasper
6 ochre red color red to almost hematite
black
rose 4 white effervesces in warm rhodochrosite
acid
dark 4 orange zincite
2.5 purplish red surface tarnishes pyrargyrite
black
brownish 3.5 brownish cuprite
red
Orange 3.5 white to pyromorphite
yellowish
1-1½ orange realgar
Blue 5.5-6 white in igneous rocks sodalite
azure 4 azure azurite
sky 7 & 4.5 white blade-like crystals cyanite
turquoise 6 blue non-crystalline turquois
2-4 white chrysocolla

Green
malachite 3.5 lighter malachite
green
olive 6.5-7 white in igneous rocks olivine
3.5 white to pyromorphite
yellow
2 white mica-like cleavage chlorite
1 white greasy feel, color talc
light to dark olive
green
yellowish 6.5 white epidote
2.5-4 white color yellow green to serpentine
olive
Yellow
golden 2.5 shining non-crystalline gold
brassy 6 greenish- usually crystalline pyrite
black
6 greenish- color pale brassy marcasite
gray yellow, usually non-
crystalline
5.5 greenish- colors nitric acid millerite
black green
4 greenish- color golden similar chalcopyrite
black to gold
3.5 dark brown purplish tarnish on tetrahedrite
surface
bronze 5.5 pale color with coppery niccolite
brownish- cast
black
4 dark gray- with speedy black pyrrhotite
black tarnish
3 gray-black brownish with bluish bornite
tarnish
2.5 shining coppery red color copper

sulphur 3.5 white to compact masses pyromorphite
yellowish
2 yellow sulphur
1-3 earthy masses carnotite
Brown
violet 1½ shining tarnishes black cerargyrite
yellowish 7.5 white 4-sided prisms zircon
6.5 gray cassiterite
5.5 ochre compact to earthy limonite
yellow masses
5 brownish- goethite
yellow
4.5 black wolframite
3.5 yellowish- sphalerite
brown
3.5 white siderite
grayish 7.5 white often twinned staurolite
6.5 pale brown rutile
3.5 white to earthy masses pyromorphite
yellowish
reddish 7 white dodecahedrons & garnet
trapezohedrons
Black 6.5 gray cassiterite
6 reddish- franklinite
brown
6 black magnetic magnetite
5.5 dark brown chromite
5.5 black yellow precipitate in wolframite
sulphuric acid
5-6 black non-magnetic ilmenite
5-6 brownish- compact masses psilomelane
black

5 brownish- surface often goethite
yellow brownish
3.5 dark brown tetrahedrons tetrahedrite
2.5 silvery fresh surfaces silver silver
color
2.5 scarlet fresh surfaces bright prousite
red
2.5 purplish red fresh surfaces red pyrargyrite
2 black earthy masses pyrolusite
1 steel gray greasy feel graphite
Metallic Gray 2.5 black tarnishes black, chalcocite
bluish, or green
2.5 lead gray sectile argentite
2.5 lead gray cubic cleavage galena
2 lead gray long prismatic stibnite
crystals
1.5 bluish gray in scales molybdenite
steel 5.5 gray black rose color in nitric smaltite
acid
4.5 steel gray very heavy platinum
4 reddish often in striated manganite
black prisms
1 gray with greasy feel graphite
silvery 5.5 black arsenopyrite
2.5 silvery tarnishes black on silver
exposure
reddish 5.5 gray black rose color in nitric cobaltite
acid
pearly 1-1½ shining exposed surfaces cerargyrite
violet brown
White, with 4 white porcelainous masses, magnesite
impurities effervesces in acid

grayish or 2 white earthy masses, greasy kaolinite
yellowish feel
1-3 white earthy masses bauxite
1 white greasy feel, fibrous or talc
scaly
27

TRANSPARENT OR TRANSLUCENT COLORS
Color Hardness Remarks Mineral
Colorless or with faint tinges of color due to impurities
10 in octahedrons diamond
9 in hexagonal prisms corundum
8 in hexagonal prisms topaz
7 in three-sided prisms tourmaline
7 in hexagonal prisms quartz
7 non-crystalline chalcedony
7 or 4.5 cubes with beveled edges boracite
6 non-crystalline, pearly luster opal
5.5 rhombohedrons willemite
5.5 trapezohedrons analcite
5.5 tufts of needle-like crystals natrolite
5.5 sheaf-like bundles of crystals stilbite
5 hexagonal prisms with basal cleavage apatite
5 effervesces in acid smithsonite
5 becomes jelly-like in acid calamine
4.5 monoclinic prisms colemanite
4 in cubes fluorite
3.5 effervesces in acid, but one cleavage aragonite
3.5 effervesces in acid, heavy cerrusite
3 effervesces in acid, rhomboidal cleavage calcite
3 no effervescence, but soluble in nitric anglesite
acid
2.5 in cubes tastes of salt halite

2 soluble in water, sweetish taste borax
2 1 perfect cleavage, and two imperfect gypsum
cleaves at 66 with each other
White or with faint tinges of color due to impurities, such as pink, bluish,
etc.
7 hexagonal prisms quartz
7 non-crystalline chalcedony
7 or 4.5 cubes with beveled edges boracite
6 non-crystalline, pearly luster opal
6 cleavage in 3 directions, good in 2 and feldspar
imperfect in the other
5.5 short eight-sided prisms pyroxene
5.5 long six-sided prisms amphibole
5.5 trapezohedrons analcite
5.5 tufts of needle-like crystals natrolite
5.5 sheaf-like bundles of crystals stilbite
5.5 rhombohedrons willemite
5 effervesces in acid smithsonite
5 becomes jelly-like in acid calamine
4.5 & 7 cubes with beveled edges boracite
4.5 monoclinic prisms colemanite
4 effervesces in acid, porcelainous magnesite
3.5-4 effervesces in acid, heavy, red color in strontianite
flame
3.5 effervesces in acid, heavy, green color in witherite
flame
3.5 effervesces in warm acid, rhomboidal dolomite
cleavage
3.5 effervesces in acid, cleavage in one aragonite
direction only
3.5 effervesces in acid, heavy, does not color cerrusite
flame

3-3.5 no effervescence, cleavage in three anhydrite
directions at right angles
3 effervesces in acid, rhomboidal cleavage calcite
3 tabular crystals, heavy, green color in barite
flame
2-3 cleaves in thin elastic sheets mica
2.5 cleaves in cubes cryolite
2.5 cubes, soluble in water, salty taste halite
2 1 perfect cleavage, and 2 less perfect gypsum
ones
2 cleaves in thin non-elastic sheets chlorite
2 soluble in water, tastes sweet borax
1 greasy feel talc
Green 9 hexagonal prisms oriental
emerald
8 octahedrons spinel
7.5 hexagonal prisms beryl
7 three-sided prisms tourmaline
7 dodecahedrons or trapezohedrons garnet
7 non-crystalline prase or
plasma
6.5-7 non-crystalline, olive color olivine
6.5 yellow green color, rather opaque epidote
6 non-crystalline, pearly luster opal
5.5 short eight-sided prisms pyroxene
5.5 long six-sided prisms amphibole
5 hexagonal prisms apatite
4 cubes fluorite
3.5 effervesces in acid cerrusite
2.5-4 somewhat greasy feel, massive or fibrous serpentine
2 in mica-like scales, non-elastic chlorite
1 greasy feel, fibrous or scaly talc

Red 9 hexagonal prisms ruby
8 octahedrons spinel
7 three-sided prisms tourmaline
7 dodecahedrons or trapezohedrons garnet
7 hexagonal rose quartz
7 non-crystalline jasper or
carnelian
6 pearly luster fire opal
4 cubes, rose tints fluorite
2-3 pink mica-like scales lepidolite
Blue 9 hexagonal prisms sapphire
7 & 4.5 blade-like crystals cyanite
6 non-crystalline masses turquois
5.5-6 in igneous rocks sodalite
4 azure color azurite
3.5 effervesces in acid, heavy cerrusite
2-4 earthy masses, turquoise color chrysocolla
Violet 7 hexagonal prisms amethyst
4 cubes fluorite
Yellow 9 hexagonal prisms oriental topaz
8 octahedrons spinel
8 hexagonal prisms topaz
4 cubes fluorite
Brown 9 hexagonal prisms corundum
8 octahedrons spinel
7.5 four-sided prisms zircon
7 hexagonal prisms smoky quartz
7 three-sided prisms tourmaline
7 non-crystalline flint
6 non-crystalline opal
5.5 short eight-sided prisms pyroxene
5.5 long six-sided prisms amphibole

2-3 cleaves into thin sheets mica
Black 9 hexagonal prisms corundum
8 octahedrons spinel
7 three-sided prisms tourmaline
5.5 short eight-sided prisms pyroxene
5.5 long six-sided prisms amphibole
2-3 cleaves in thin sheets mica
29

The Gold Group

Gold was undoubtedly the first metal to be used by primitive man; for,
occurring as it did in the stream beds, its bright color quickly attracted the
eye, and it was so soft, that it was easily worked into various shapes, which,
because they did not tarnish, became permanent ornaments. The metal is
associated with the very earliest civilizations, being found in such ancient
tombs as those at Kertsch in Crimea and in northern Africa and Asia Minor.
It was used in the cloisonné work of Egypt 3000 years B.C. In America the
Indians, especially to the south, were using it long before the continent was
discovered.

Of all the metals gold is the most malleable, and its ductility is remarkable,
for a piece of a grain’s weight (less than the size of a pin head) can be
drawn out into a wire 500 feet long; and it can be beaten into a thin 30
leaf as thin as ¹/₂₅₀₀₀₀ of an inch in thickness, and thus a bit, weighing
only a grain, can thus be spread over 56 square inches.

It forms very few compounds, but has a considerable tendency to make
alloys (i.e., mixtures with other metals without the resulting compound
losing its metallic character). In Nature gold is never entirely pure, but is an
alloy, usually with silver, there being from a fraction of 1% up to 30% of
the silver with the gold, the more silver in the alloy, the paler the color of
the gold. Australian gold is the purest, having but about .3% of silver in it,
while Californian gold has around 10% and Hungarian gold runs as high as
30% of silver. Another alloy fairly abundant in Nature is that with tellurium,
such as calaverite (AuTe₂) which is a pale brassy yellow, similar to pyrite,

but with the hardness of but 2.5. Another combination includes gold, silver
and tellurium, sylvanite, (AuAgTe₄) a silvery white mineral with a hardness
of but 2. Such combinations are known as tellurides and the calaverite is
mined as a source of gold at Cripple Creek, Colo., while the sylvanite is one
of the important ores of gold in South Africa. Occasionally gold is also
found alloyed with platinum, copper, iron, etc. Jewelers make several
alloys, “red gold” being 3 parts gold and 1 of copper, “green gold” being
the same proportions of gold and silver, and “blue gold” being the
combination of gold and iron. Our gold coins are alloys, nine parts gold and
one of copper, to give them greater durability. Most of the gold 31
recovered from nature is found native, i.e., the pure metal, or with
some alloy.

Gold
Au
Pl. 5

Usually non-crystalline, but occasionally showing cube or octahedral faces
of the isometric system; hardness 2.5; specific gravity 19.3; color golden
yellow; luster metallic; opaque.

Gold is mostly found as the metal and is readily recognized by its color,
considerable weight, hardness, malleability, and the fact that it does not
tarnish. It usually occurs in quartz veins in fine to thick threads, scales or
grains, and occasionally in larger masses termed “nuggets.” It is insoluble
in most liquids so that when weathered from its original sites, it was often
washed down into stream beds, to be found later in the sands or gravels, or
even in the sea beaches. When thus found it is termed “placer gold,” and its
recovery is placer mining. Most of the original discoveries of gold have
been in these placer deposits; and from them it has been traced back to the
ledges from which it originally weathered. In the placer deposits the size of
the particles varies from fine “dust” up to large nuggets, the largest found in
California weighing 161 pounds; but the largest one found in the world was
the “Welcome Nugget,” found in Australia, and weighing 248 pounds.
When gold was discovered in California in 1848, this became the chief

source for the world, but later this distinction went to Australia, and now
belongs to South Africa, which today yields over half the annual supply.

The ultimate source of gold is from the lighter colored igneous rocks, 32
like granites, syenites, and diorites, throughout which it is diffused in
quantities too small to be either visible or worth while to extract. It becomes
available only when it has been dissolved out by percolating waters and
segregated in fissures or veins, either in or leading from these igneous
rocks. Generally this transfer of gold has taken place when the rocks were
at high temperatures, and by the aid of water (and perhaps other solvents)
which was also at high temperatures. The presence of gold in sandstones,
limestones, etc., is secondary, as is also its presence in sea water, in which
there is reported to be nearly a grain (about five cents worth) in every ton of
water. Beside the direct recovery of gold from gold mining, a great deal is
obtained from its association with iron, copper, silver, lead and zinc
sulphides, in which it is included in particles too fine to be visible, but in
quantities large enough to be separated from the other metals after they are
smelted.

In the United States gold is found in the Cordilleran region from California
to Alaska, in Colorado, Nevada, Arizona, Utah, the Black Hills of South
Dakota, and in small quantities in the metamorphosed slates of North and
South Carolina, Georgia, and in Nova Scotia.

The Silver Group

Though much commoner than gold, silver did not attract the eye of man as
early, probably because it tarnishes when exposed to air or any other agent
having sulphur compounds in it, and a black film of silver sulphide 33
covers the surface. Its first use was for ornaments, and some of these
found in the ruins of ancient Troy indicate its use as early as 2500 B.C. A
thousand years later it was being used to make basins, vases and other
vessels.

Silver is next to gold in malleability and ductility, so that a grain of silver
can be drawn out into a wire 400 feet long, or beaten into leaves ¹/₁₀₀₀₀₀ of

an inch in thickness. As a conductor of electricity it is unsurpassed, being
rated at 100% while copper rates 93%. Silver is also like gold in the
freedom with which it alloys with other metals, such as gold, copper, iron,
platinum, etc. All our silver coins, tableware, etc., have some copper
alloyed with the silver to give it greater hardness and durability.

Unlike gold, silver freely enters into compounds with the non-metals, which
is the reason that it is not found primarily in its native state, but usually as a
sulphide. Its ultimate source is in the igneous rocks, few granites or lavas,
on analysis, failing to show at least traces of silver. Before it is available as
an ore, or mineral, it has been dissolved from the original magma, and
segregated in fissures or veins, along with such minerals, as quartz, fluorite,
calcite, etc. This seems to have taken place while the igneous rocks were
still hot, and by the agency of vapors and liquids which were also hot. The
presence of silver in sedimentary and metamorphic rocks, or even in sea
water, is secondary.

The primary deposition of silver is usually in the form of sulphides, the
commoner of which are, argentite or silver sulphide, pyrargyrite or 34
silver and antimony sulphide, and prousite, or silver and arsenic
sulphide. Its occurrence as native silver, or the chloride, cerargyrite, is
secondary and due to the reactions which have taken place when sulphide
deposits have been subjected to weathering agents.

The United States produces about 25% of the world’s supply, Mexico some
35%. It is especially found along the Cordilleran ranges of both North and
South America.

Silver
Ag
Pl. 6

Usually non-crystalline, but occasionally showing cube or octahedron faces
of the isometric system; hardness 2.5; specific gravity 10.5; color silvery
white; luster metallic; opaque.

When found in its native state silver is usually in wirey, flakey, or mossy
masses; but sometimes masses of considerable size occur, the most famous
being an 800 pound nugget found in Peru, and another of 500 pounds
weight found at Konsberg, Norway, and now preserved in Copenhagen.
When exposed to the air the surface soon tarnishes and takes on a black
color which must be scraped off to see the real color.

Like gold, silver is usually found associated with other metals, like iron,
copper, lead and zinc; and much of the silver recovered is obtained in
connection with the mining, especially of copper and lead. Some lead ores
have so much silver in them that they are better worth mining for the silver;
galena, for instance, under such circumstances being termed argentiferous
galena. Native silver is a secondary mineral, having been formed by 35
the reduction of some one of its sulphides by water, carrying various
elements which had a greater affinity for the sulphur.

Silver is found along with copper in the Lake Superior region, and in Idaho,
Nevada, and California.

Argentite
AgS
Pl. 6
silver glance

Usually in irregular masses, but sometimes in cubes; hardness 2.5; specific
gravity 7.3; color and streak lead gray; luster metallic; opaque on thin
edges.

Argentite, the simple sulphide of silver, is the chief source from which
silver is obtained. It looks like galena, and has the same hardness, streak
and specific gravity, but can be distinguished by the galena having a very
perfect cubic cleavage while the argentite has no cleavage. Argentite is
easily cut with a knife (sectile). It is usually found in irregular masses, but
sometimes in cubes which make very choice cabinet specimens; and is

associated with such other minerals as galena, sphalerite, chalcopyrite,
pyrite, fluorite, quartz, and calcite.

It occurs in fissures and veins all through the Cordilleran regions, especially
in California, Colorado, Nevada (Comstock Lode), Arizona (Silver King
Mine) and about the shores of Lake Superior.

Pyrargyrite
Ag₃SbS₃
Pl. 7
ruby silver or dark red silver

Usually occurs in irregular masses; hardness 2.5; specific gravity 5.8; color
dark red to black; streak purplish red; luster metallic to adamantine;
translucent on thin edges.

Pyrargyrite, the sulphide of silver and antimony, is distinguished by its 36
dark red color and the purplish streak. It may look like prousite, but is
easily distinguished from the latter which has a scarlet streak. It also at
times looks like hematite and cinnabar, but the hematite has a hardness of 6,
and the latter has the bright red color throughout, while pyrargyrite turns
black when exposed to the light, so that the characteristic red color will be
seen only on fresh surfaces. The characteristic red color can only be kept on
the mineral if it is constantly protected from the light.

Sometimes pyrargyrite occurs in crystals and these belong to the hexagonal
system, and are prisms with low faces on the ends, as on plate 7, and the
mineral is peculiar in that the faces on the opposite ends are unlike.

Pyrargyrite is found mostly in fissures and veins of quartz, fluorite, calcite,
etc., and associated with pyrite, chalcopyrite, galena, etc. It is fairly
common in Colorado in Gunnison and Ouray counties, in Nevada, New
Mexico, Arizona, etc.

Prousite
Ag₃ AsS₃
Pl. 7
light red
silver

Usually occurs in irregular masses; hardness 2.5; specific gravity 5.6; color
scarlet to vermilion; streak the same; luster adamantine; transparent on thin
edges.

In general this mineral is very like pyrargyrite, but has the scarlet color and
streak which are entirely characteristic. It is likely to have the surface
tarnished black, which happens on exposure to light, so that it is 37
essential to be sure that fresh surfaces are being examined.
Occasionally it is found in crystals, of the same type as the preceding
mineral. It is generally found associated with pyrargyrite.

Cerargyrite
AgCl
horn silver

Usually found in irregular masses or incrustations; hardness 1 to 1½;
specific gravity 5.5; color pearly gray, grayish green to colorless, but
turning violet brown on exposure to light; luster resinous; transparent on
thin edges.

This mineral is usually found in thin seams or waxy incrustations, but it
may occur in crystals in which case they are cubes. It is very soft and easily
cut with a knife, which with its tendency to turn violet-brown on exposure
to light, makes it easy to identify. Cerargyrite is a secondary mineral,
resulting from the action of chlorine-bearing water on some one of the
sulphides of silver. It is found in the upper portions of mines, especially
those in arid regions.

The Copper Group

After gold the next metal to be utilized was copper. About 4000 B.C. our
early forefathers found that by heating certain rocks, they obtained a metal
which could be pounded, ground and carved into useful shapes. Curiously
enough the rocks which had the copper also had some tin in them, so that
this first-found copper was not pure, but had from five to ten per cent of tin
in it, making the resulting metal harder, and what we call bronze. It 38
was some thousands of years later before they distinguished the
copper as a pure metal, but it worked and made good tools. The newly
found metal was not as ornamental as gold; but, because it could be made
into tools, it had a tremendous influence on man’s development. As the
bronze tools began to take the place of the stone implements, the “Age of
Bronze” was ushered in. In America the Indians in the Lake Superior region
found native copper weathered out of the rocks and later mined it, and they
too pounded it into knives, axes, needles, and ornaments, but probably
never learned to melt it and mold their tools. At any rate they were not as
far advanced in using this metal when Columbus landed as were the
southern Europeans 6500 years earlier. Since the use of iron became
general, copper has not held such a dominant place, but it still is “the red
metal” which holds the second most important place.

It is malleable and ductile, though not equal to gold or silver in these
respects. It is a good conductor of electricity and a very large amount of
copper is used in electrical manufacture, roofing, wire, etc. It alloys with
other metals; ten parts copper and one of tin being bronze, ten of copper and
one of zinc is brass, and copper with aluminum is aluminum bronze.

Like silver and gold, copper is widely diffused through the igneous rocks,
but before it is available, it must be leached out by solvents and
concentrated in veins, fissures, or definite parts of the lavas or granites. The
primary ores are those which, while the igneous rock was still hot, 39
were carried by hot vapors and liquids into the fissures and there
deposited, mostly as sulphides. There is a long list of these, but in this
country, the following are the commoner ones; chalcocite the sulphide of
copper, chalcopyrite the sulphide of copper and iron, bornite another
combination of copper, iron and sulphur, and tetrahedrite copper and

antimony sulphide. When these primary ores are near enough to the surface
to come in contact with waters carrying oxygen, carbon dioxide or silica in
solution, they may give up their sulphur and take some one of these new
elements and we have such forms as cuprite, the oxide of copper, malachite
and azurite, carbonates of copper, or chrysocolla, the silicate of copper.
Native copper is also a secondary deposit laid down in its present state by a
combination of circumstances which deprived it of its original sulphur. In
general copper mining can not be profitably carried on for ores with
anything less than a half of one percent in them; and the use of such low
grade ores has only been possible for a few years, as the result of inventing
most delicate processes in the smelting.

The United States produces about a quarter of the world’s supply of copper,
with Chile ranking second with about 17%.

Copper
Cu
Pl. 8

Usually in irregular masses; hardness 2.5; specific gravity 8.9; color copper
red; luster metallic; opaque. Native copper, easily determined by its color
and hardness, is generally found in irregular grains, sheets, or masses, 40
on which may sometimes be detected traces of a cube or an octahedral
face, showing that it belongs to the isometric system. The most famous
locality is the Upper Peninsula of Michigan which may be taken as typical.
Here, long before it was known historically, the Indians found and dug out
copper to make knives, awls, and ornaments.

In this region, beds of lava alternate with sandstones and conglomerates.
The copper was originally in the lavas, but has been dissolved out, and now
fills cracks and gas cavities in the lavas, and also the spaces between the
pebbles of the conglomerate. This locality has been very famous both
because of the quantity mined, and also because of the strikingly large
masses sometimes found. Today but little of the ore runs above 2 percent
copper, and it is mined if it has as little as ½ of one percent.

While nowhere near as abundant, native copper occurs in the same way in
cavities and cracks in the trap rocks of New Jersey, and along the south
shore of the Bay of Fundy. It is also known from Oregon, the White River
region of Alaska, and in Arctic Canada.

Chalcopyrite
CuFeS₂
Pl. 8
copper pyrites or yellow copper ore

Occurs in crystals of irregular masses; hardness 4; specific gravity 4.2;
color bronze yellow; streak greenish black; luster metallic; opaque on thin
edges.

Chalcopyrite resembles pyrite, but its color is a more golden yellow, and its
surface tarnishes with iridescent colors. Then too the hardness of 41
chalcopyrite is but 4 as compared with 6 for pyrite. When in crystals
this mineral belongs to the tetrahedral system as the c axis is but .985 in
length as compared with I for the two other axes. This difference is so little
that, to the eye, the octahedron appears to belong to the isometric system.
Chalcopyrite occurs in octahedrons and tetrahedrons (as on plate 8), the
latter being the form where but half of the octahedral faces are developed.
However by far the most frequent mode of occurrence is in irregular
masses.

This is the most important primary ore of copper, and is widely distributed,
being found either in lavas, or in veins, or in fissures connected with
igneous rocks. Apparently the deposits were made, either at the time of
eruptive disturbances or shortly afterward, from vapors or hot solutions
carrying the copper sulphides (and other sulphides) from the molten
igneous rocks. Chalcopyrite is usually associated with pyrite, galena,
sphalerite and chalcocite, as well as quartz, fluorite and calcite. It is found
in all the New England States, in New York, New Jersey, Pennsylvania,
Maryland, Virginia, North Carolina, Tennessee, Missouri, and all the Rocky
Mountain and Pacific Coast States.

Bornite
Cu₃FeS₃
purple copper ore

Occurs in granular or compact masses; hardness 3; specific gravity, 5; color
bronze-brown with a bluish tarnish; streak gray-black; luster metallic;
opaque on thin edges.

Bornite is also known as erubescite, blushing ore, variegated copper, 42
peacock copper, etc., all of which names refer to the highly iridescent
tarnish which fresh faces soon take on when exposed to the air. Though
usually in masses, it is sometimes found in rough cubes of the isometric
system. In this country it is not abundant enough to be used as an ore, but is
likely to be found with other ores like chalcopyrite or chalcocite. In the east
it has been found at Bristol, Conn., and near Wilkesbarre, Penn., while in
the west it may be expected to occur wherever other sulphide minerals of
copper are found.

Chalcocite
Cu₂S
Pl. 9
copper glance

Occurs in fine grained compact masses; hardness 2.5; specific gravity 5.7;
color dark leaden gray; streak black; luster metallic; opaque on thin edges.

Chalcocite is one of the important ores of copper, especially in Arizona and
the Butte District of Montana. It resembles argentite in color and general
appearance, but is readily distinguished by being brittle and having a
tendency to tarnish to bluish or greenish colors on fresh surfaces.
Occasionally it occurs in crystals which are in the orthorhombic system; but
the edges of the prism are so beveled that there are six sides and the prism
resembles a hexagonal prism (see page 16).

In the Butte, Mont., district, the most important copper region in the United
States, fully 50% of the ore is chalcocite, which is a derivative of the
originally deposited chalcopyrite, the latter having lost its iron. In the 43
veins of this district chalcopyrite, bournite, tetrahedrite, and several
other copper minerals not described in this book, occur all together, and
with them also gold, silver and arsenic minerals. The gold amounts to about
2¼ cents per pound of copper, and the silver is in somewhat less quantity.
These veins were first opened to get the silver ores, which were the more
important ones down to a depth of 200 to 400 feet. Below these depths the
copper became much more important. It was the weathering which had
removed a large part of the copper minerals in the upper levels of the veins,
but had left a large part of the silver. Chalcocite is also important in most of
the Utah and Arizona mines.

In the east it has been found at Bristol, Simsbury and Cheshire, Conn., and
in the west it is found in all the Cordilleran States.

Tetrahedrite
Cu₃SbS₃
Pl. 9 & 10
gray copper ore

Occurs in irregular masses and in tetrahedrons of the isometric system;
hardness 3.5; specific gravity 4.7; streak dark brown; luster metallic;
opaque on thin edges.

In its crystalline form the tetrahedrite occurs in tetrahedrons, which
generally have faces formed by beveling the edges and by cutting the
corners, as in the two figures of plate 10. Chalcopyrite may also occur in
tetrahedrons, but its golden yellow color is entirely different from the gray-
black of the tetrahedrite. When in masses the hardness and the streak which
is dark brown, are very characteristic.

In England and Bolivia tetrahedrite is an important ore of copper, but 44
in this country it is simply a copper mineral which is widely

distributed, and associated with most of the mining enterprises, but is in no
case the important ore. It has been found sparingly through the New
England States, at the Kellogg Mines in Arkansas, and abundantly in
Colorado, Montana, Utah, Arizona, Nevada and New Mexico.

Cuprite
Cu₂O
Pl. 9 & 10
red copper ore

Occurs in isometric cubes, octahedrons, and dodecahedrons, or in masses;
hardness 3.5; specific gravity 6; color dark brownish-red; streak brownish-
red; luster metallic; translucent on thin edges.

When in crystals cuprite is easily determined, but when in masses its fresh
surfaces may suggest prousite, but the streak and hardness are quite
different in the two cases. Sometimes its color suggests hematite, but the
latter has the hardness of 6. When found it is often coated with a thin film
of green, which is malachite.

Except when found as native copper, the ore which contains the greatest
percentage of copper is cuprite with 88.8% of copper. It is likely to occur in
any of the deposits of copper ore, where they are in arid climates and above
the level of the underground water, and is very frequently associated with
malachite and azurite. In the Bisbee, Arizona, district cuprite is one of the
important ores.

Besides the normal occurrence described above, cuprite may be found in
two other varieties; one where the crystals have grown side by side 45
and so only the ends have been free for continuous additions of the
mineral, which has resulted in a fibrous mass known as “plush copper ore”
or chalcotrichite; the other an earthy mixture of limonite and cuprite, which
is brick red in color, and termed “tile ore.”

Cuprite is found sparingly in New England, more abundantly at such places
as Summerville and Flemington, N. J., Cornwall, Penn., in the Lake
Superior region, and fairly abundantly in the Cordilleran States.

Malachite
CuCO₃·Cu(OH)₂
Pl. 11

Usually occurs in nodular or incrusting masses; hardness 3.5; specific
gravity 4; color green; streak a lighter green; luster adamantine, silky or
dull; translucent on thin edges.

The vivid green of malachite is usually enough to determine it at once, but
one may be sure by trying a drop of acid on it, in which case it effervesces
as is characteristic of so many carbonates, but this is the only carbonate
which is vivid green. Generally the malachite is in irregular masses, but
crystals are occasionally found. These are extremely small and needle-like,
and belong to the monoclinic system. In the Ural Mountains there is a
locality where these crystals grow in fibrous masses, usually radiating from
the center. Malachite in such nodules has a silky luster. These rare nodules
have furnished the rulers of Russia with a unique and much prized material
for making royal gifts. In European museums and palaces one finds 46
many objects carved from this form of malachite, and marked as gifts
of the czars of Russia.

In the United States malachite is widely distributed, appearing as green
streaks and stains where copper minerals have been exposed to the air. It is
the green tarnish which appears on bronze and copper when exposed to the
weather. It is found in large quantities in New Jersey, Pennsylvania,
Wisconsin, Nevada, Arizona, Utah, New Mexico, etc. The Bisbee mine in
Arizona is the place that has furnished museums with so many of the
handsome specimens of malachite associated with azurite. These are the
most striking specimens for the vividness of their colors that appear in any
collections.

Malachite has been known since about 4000 B.C., the Egyptians having
mines where they obtained it between the Suez and Mt. Sinai. In those early
days it was particularly a child’s charm, protecting the wearer from evil
spirits. It is still used as a stone of lesser value in making some sorts of
jewelry.

Azurite
2CuCO₃·Cu(OH)₂
Pl. 11

Occurs as short prismatic or tabular crystals of the monoclinic system;
hardness 4; specific gravity 3.8; color azure blue; streak lighter blue; luster
vitreous; translucent on thin edges.

Azurite is another very striking mineral fully characterized by its color and
streak. Like malachite it effervesces in acid. It is very near to malachite in
composition, and by increasing its water content, can and freely does
change to the green mineral; so that few specimens of azurite are 47
without traces of malachite. It is found in the same places as
malachite, but is not as abundant in the east.

Azurite with the accompanying malachite is cut and polished to make semi-
precious stones for some forms of jewelry.

Chrysocolla
CuSiO₃·2H₂O

Never occurs in crystals, but in seams and incrustations; hardness 2-4;
specific gravity 2.1; color bluish-green; streak white; luster vitreous;
translucent on thin edges.

This rather rare mineral often appears in opal- or enamel-like incrustations,
and its color is variable ranging from the typical bluish-green to sky-blue or
even turquoise blue. This is a mineral resulting from the action of silica

bearing waters, coming in contact with most any of the copper minerals,
and is found accompanying cuprite, malachite, azurite, etc. It is never in
large enough quantities to be used as an ore, but its striking color attracts
attention and it can be found fairly frequently, especially in the west.

The Iron Group

Pure iron is a chemical curiosity which looks very much like silver. As
obtained from its ores, or as it occurs in Nature, iron always has some
impurities with it, such as carbon, silicon, sulphur and phosphorus, and
these are highest in the crudest iron such as “pig-iron.” Its malleability and
ductility are only a little less than for gold and silver, and so it has a wide
range of qualities for use by man. It is only rarely found native in minute
grains in some of the dark lavas. There is however one remarkable 48
exception to this statement, in that on Disco Island, Greenland, there
is a basaltic rock, from which are weathered great boulders of native iron up
to 20 tons in weight. This iron is very like that occurring in meteorites, and
probably came from great depths in the earth’s interior. The specific gravity
of iron is 7.8. It makes up around 5% of the crust of the earth, and probably
occurs in much larger percentages in the interior of the earth.

Iron was discovered by man later than gold or silver or copper, about 1000
B.C.; but once found it was so much more abundant than any of these that it
soon dominated over copper, and from Roman times to the present has been
the basis of progress in civilization, and these times are well called “the iron
age.”

Iron unites freely with the non-metals, and occurs as sulphides, oxides,
carbonates, etc., and is also present as a secondary metal in that great group
of minerals known as the silicates (see page 97). It alloys with a wide range
of other metals, every combination altering the properties of the iron, and
thus making it useful in a still greater range of manufacture. The
introduction of ¼ to 2½% of carbon into iron makes steel, which is harder
(in proportion to the amount of carbon) and stronger than the pure iron.

Iron compounds are among the most numerous and important of the colors
in Nature’s paint box, limonite furnishing the browns which color the soil
and so many of the rocks, hematite giving the red color to other abundant
rocks, and magnetite often coloring igneous rocks black, while the 49
chlorophyll which gives the green color to plants is an iron
compound, as is also the hemoglobin which gives the red to our blood.

Iron is present in all igneous rocks, and secondarily in the sedimentary and
metamorphic rocks. It is soluble in water, and so is being constantly
transferred from place to place, and changes from one compound to another,
according to the circumstances in which it is placed.

The primary forms are pyrite, magnetite and the silicates. When in
weathered rocks the iron is changed to limonite, siderite or hydrated
silicates. Hematite is an intermediate oxide from which the water contained
in limonite has been driven off by moderate heat or bacterial action.

Limonite
2Fe₂O₃·3H₂O
Pl. 12

Never crystalline, occurs in mammillary, botryoidal and stalactitic forms, or
in fibrous, compact, oolitic, nodular or earthly masses; hardness 5.5;
specific gravity 3.8; color yellow-brown to black; streak yellow-brown;
luster metallic to dull; opaque.

Limonite is a very common mineral, the color, streak and hardness
identifying it readily. Iron rust is its most familiar form. When powdered it
is the ochre yellow used in paints. Being so universally distributed, it is to
be expected it will occur in a variety of ways. First, there is the fibrous type
found lining cavities, in geodes, or hanging in stalactites in caves. This has
a silky luster, an opalescent, glazed or black surface, and is in mammillated
or botryoidal masses. Second, it may occur in compact masses in veins,
where it was deposited by waters; which, circulating through the 50
adjacent rocks, gathered it from the rocks, and, on reaching the open
seams, gave it up again. Third, it may occur in beds on the bottom of ponds,

where it was deposited by waters which gathered it as they flowed over the
surface of the country rocks. Measurements in Sweden show that it may
accumulate in such places as much as six inches in the course of twenty
years. In ponds and swamps, the decaying vegetation forms organic
compounds, which cause the precipitation of the iron from the water, as it is
brought in by the streams. This sort of iron in the bottom of ponds or
swamps is also known as “bog iron.” Another form in which limonite may
occur in ponds, lakes, or even the sea, is in oolitic masses. In this case the
iron forms in tiny balls, with perhaps a grain of sand at the center, and one
coat of iron after another formed around it, like the layers of an onion. If the
resulting balls are tiny this is called oolitic (like fish eggs), but if the balls
are larger it is pisolitic (like peas). Bacteria probably have a good deal to do
with the precipitation of limonite in this manner. Fourth, limonite occurs in
earthy masses, usually mixed with impurities like clay and sand, which are
the residue left behind, where limestones have been dissolved by
weathering. The fifth mode of occurrence is known as gossan, or “the iron
hat,” which is a mass of limonite capping a vein of some sulphide mineral,
like pyrite, chalcopyrite or pyrrhotite, which has been exposed to
weathering; and in these minerals the sulphur has been removed, leaving a
mass of limonite over the vein. This is particularly common in the 51
west. Limonite is quite easily fusible and so was probably the first ore
from which early man extracted iron.

Limonite is iron oxide, with 3 molecules of water of crystallization (or
constitution) associated with every 2 molecules of the oxide. If limonite is
moderately heated the water is driven out and the resulting compound is
hematite, the same oxide, but without the water. In this case and many other
similar cases, as gypsum, opal, etc., we have two or more minerals resulting
from the presence or absence of water in the mineral. The water molecules
have a definite place in the arrangement of molecules which determines the
structure of the mineral. Sometimes the water is driven out at a temperature
around 212 F., in which case it is called, water of crystallization, but in
other cases as gypsum, a considerably higher temperature is required to
drive out the water, and then it is called, water of constitution. In all cases
the removal of the water changes the arrangement of molecules and a new
mineral results, with characteristics of its own.

In this case limonite is only one of a series of minerals which have the
Fe₂O₃ molecule as a basis, and that incorporate more or less water into their
molecular construction as follows:

Turgite 2Fe₂O₃·H₂O
Goethite Fe₂O₃·H₂O
Limonite 2Fe₂O₃·3H₂O
Xanthosiderite Fe₂O₃·2H₂O
Limonite Fe₂O₃·3H₂O

Of these goethite is crystalline, the others non-crystalline. They may 52
occur pure or in all sorts of mixtures, the mixtures usually being
lumped under limonite. The limonite is far the commonest of the series,
goethite is fairly common, but the others are rare as pure minerals.

Limonite is found in all parts of all states and in every country. Though so
common, it is by no means an important source of iron today, only about
one percent of the iron mined in this country coming from this source,
though in Germany, Sweden and Scotland it is relatively much more
important.

Goethite
Fe₂O₃·H₂O
Pl. 12

Occurs in lustrous brown to black orthorhombic prisms, usually terminated
by low pyramids; hardness 5; specific gravity 4; color brown to black;
streak brownish-yellow; luster imperfect adamantine; opaque.

Goethite, named for the poet Goethe, who was interested in mineralogy, is
much less abundant than limonite or hematite, but occurs with them, when
they are in veins. Its usual form is an orthorhombic prism with the edges
beveled, and a low pyramid on either end. The crystals usually grow in
clusters, making a fibrous mass, often radiated, in which case it is known as
“needle iron stone”; or the prisms may be so short as to be almost scales;
when, because of the yellowish-red color, it is called “ruby mica”. It is
found in many states, including Connecticut, Michigan, Colorado, etc.

53

Hematite
Fe₂O₃
Pl. 13 & 14
specular iron

Occurs in compact, mammillary, botryoidal, or stalactitic masses of dark
red to black color, or in earthy masses of bright to dark red; hardness 6;
specific gravity 5.2; color ochre red to black; streak cherry red to dark red;
luster metallic, vitreous, or dull; opaque on thin edges.

Hematite is readily distinguished from other red minerals by its hardness
and streak. It may occur in crystals, which belong to the hexagonal system,
and are usually hemihedral forms of the double pyramid, or
rhombohedrons. These rhombohedrons usually have the edges beveled, as
in Pl. 13, A; or are tabular in form as a result of the beveling of two of the
opposite edges to such an extent that a form like Pl. 13 B results. However
the usual occurrence is in non-crystalline masses, which represent
transformations from limonite by the loss of water of crystallization on the
part of the limonite. In such cases we have fibrous, oolitic or compact
masses, according to the form in which the limonite occurred. The
transformation from limonite into hematite involves some heat to drive out
the water of crystallization, but nothing like what is involved in
metamorphism.

Hematite is the source of 90% of the iron mined in this country. Part of it
comes from the famous Clinton iron ore, a layer a foot or more in thickness;
starting in New York State, and extending all down the Appalachian
Mountains to Alabama, where it is ten or more feet thick and the basis of
the Birmingham iron industries. Then there are tremendous deposits 54
of earthy to compact hematite, probably derived from limonite,
around the west end of Lake Superior. This latter region yields today around
75% of the iron for this country.

Loose earthy masses of hematite are often known as “ochre red,” and were
used by the Indians for war paint. Today the same sort of material is
obtained by powdering hematite and using it for red paint. The red color in
great stretches of rock is due to the presence of small amounts of hematite,
acting as cementing material. The red of the ruby, garnet, spinel, and the
pink of feldspars and calcite are due to traces of hematite.

This mineral is very common and found in every state.

Magnetite
Fe₃O₄
Pl. 14
Magnetic iron ore

Occurs in masses or in isometric octahedrons or dodecahedrons; hardness 6;
specific gravity 5.8; color black; streak black; luster metallic; opaque on
thin edges.

Magnetite is another important ore of iron, and is peculiar in being strongly
magnetic; its name being derived, according to Pliny, from that of the
shepherd Magnes, who found his iron pointed staff attracted by the mineral
when he was wandering on Mount Ida. This magnetic property has been
repeatedly used to locate beds of magnetite, and is very helpful in
separating magnetite from the “black sands,” of which it so often forms a
part. These sands however generally have magnetite with so much titanium
in it that they are unfit for smelting.

Magnetite is found in association with igneous or metamorphic rocks, 55
and often represents limonite or hematite which has been altered as
the result of high temperatures. Some of it, in the igneous rocks especially,
was undoubtedly in the molten magma and has crystallized out from the
magma while it was still hot. It is the form of iron always indicative of
former high temperatures. It is an ore mineral for about 3% of the iron in
this country, but in Scandinavia and some other countries, it plays a leading
role as the source of iron.

It is found in the Adirondack Mountains, in New Jersey, Pennsylvania,
Arkansas, North Carolina, New Mexico, and California.

Siderite
FeCO₃
Pl. 13 & 14
Spathic iron

Occurs in fibrous botryoidal masses or rhombohedral crystals, sometimes
with curved faces; hardness 3.5; specific gravity 3.8; color gray-brown;
streak white; luster vitreous; translucent on thin edges.

Like hematite this mineral belongs to the hexagonal system, and crystallizes
in hemihedral form, making the rhombohedron. Its faces are often curved,
which is rare in minerals, only a few forms like this and dolomite having
other than plane faces. When siderite crystals grow in clusters, the crowding
often results in growth on one face only, making a mass of fibrous
character, and in such cases the surface of the mass is botryoidal in contour.
The mineral is likely to oxidize, losing its gray-brown color, and becoming
limonite. In the United States it is scarcely ever used as an ore for 56
iron, but in Germany and England a great deal of iron is smelted from
this mineral.

It occurs in Massachusetts, Connecticut, New York, throughout the
Appalachian Mountains, and also in Ohio.

Pyrite
FeS₂
Pl. 15 & 16
iron pyrites

Occurs as cubes, octahedrons and pyritohedrons, or in compact masses,
scales or grains; hardness 6; specific gravity 5.1; color brassy yellow; streak
greenish-black; luster metallic; opaque on thin edges.

This is one of the commonest of all minerals. It is found in all kinds of
rocks, with all kinds of associations, in all parts of the world. Its crystals are
isometric, and cubes and octahedrons are abundant. The pyritohedron is
also a common form, and characteristic of this mineral. It is a hemihedral
form derived from a 24-sided form, i.e. the cube with four faces on each
side. On this 24-sided form each alternate face has developed and the others
have disappeared, resulting in a 12-sided form, known as the pyritohedron,
which differs from the dodecahedron in that each of its faces is five-sided

instead of rhomboidal. When in crystals pyrite can not be easily confused
with any other mineral; but when in masses it is often mistaken for gold,
chalcopyrite, pyrrhotite or marcasite. From the first two, the color should be
sufficient to distinguish it, for they are golden yellow. Pyrrhotite is bronze
yellow, and marcasite is paler yellow. Then too in hardness pyrite is 57
much harder than any of these minerals except marcasite. This last is
the one which is most likely to cause real difficulty. Its lighter color, and the
fact that it usually comes in fibrous masses are the best distinctions.

In spite of being so abundant pyrite is scarcely ever used as an ore for iron,
because the sulphur makes the metal “short,” or brittle, and the sulphur is
not easily gotten entirely out of the iron; but pyrite is used largely in the
manufacture of sulphuric acid, so important to many of our industries.

Other sulphides are commonly mixed with pyrite, such as chalcopyrite,
arsenopyrite, argentite, etc.; but the most important impurity is gold, which
is often scattered through the pyrite in invisible particles, and sometimes in
quantities enough to make it worth while to smelt it for the gold.

Pyrite is particularly the form in which the sulphur compounds of iron
appear in rocks which have been highly heated, and is to be expected in
metamorphic rocks and also igneous rocks, especially in fissures and veins
leading from the igneous rocks. It may occur in sedimentary rocks, but in
these last it is usually marcasite.

Marcasite
FeS₂
Pl. 15
white pyrite

Occurs in orthorhombic crystals, usually grouped to make fibrous or
radiating masses, or non-crystalline in masses; hardness 6; specific gravity
4.8; color pale brassy-yellow; streak greenish-gray; luster metallic; opaque
on thin edges.

Marcasite has the same chemical composition, as pyrite, and looks 58
like it, but is lighter colored and usually occurs in fibrous masses. It is
the commoner form in limestones and shales, while pyrite is more likely to
occur in igneous and metamorphic rocks. It seems probable that marcasite
is due to a more hasty precipitation from cold solutions, while pyrite is
deposited more slowly from hot solutions.

Isolated crystals of marcasite are rare; but, if formed, they belong to the
orthorhombic system. Usually some form of twinning is present, and
because of the multiple character of the twinning, marcasite crystals usually
show a ragged outline, with reentrant angles. It is most abundant in radiated
masses, which appear fibrous on the broken surfaces. It decomposes easily,
taking oxygen from the air and forming, even in museum cases, a white
efflorescence or “flower,” which is iron sulphate or melanterite. In moist air
it takes water and decomposes to sulphuric acid which may change the
surrounding limestone to gypsum. Marcasite is found wherever limestones
and shales are the country rock.

Pyrrhotite
Fe₁₁S₁₂
Magnetic pyrites

Occurs in masses; hardness 4; specific gravity 4.6; color bronze; streak
grayish-black; luster metallic; opaque on thin edges.

Tabular crystals are known, but are very rare. They belong to the hexagonal
system. This form is easily distinguished from the other yellow minerals by
being magnetic. It is by no means as abundant as the two preceding
sulphides of iron, but does occur fairly frequently in veins in igneous 59
rocks, and less frequently in limestones, large quantities of sulphuric
acid being made from a deposit in limestone at Ducktown, Tenn. It will be
found in most states. When associated with nickel it is an important source
for the latter mineral, as at Sudbury, Canada. Pyrrhotite is very like a
substance found in meteorites, known as troilite.

The Lead Group

After learning how to get iron from the rocks by rude smelting methods, the
early peoples tried heating various rocks, and some time around 500 B.C.
stumbled upon lead, which is rather easily separated from its ores. This
metal was used through Roman times to make pipes, gutters, etc.

Lead is a soft metal, fairly malleable, but with little ductility, and still less
tensile strength. Though one of the commoner metals, it does not occur as
pure metal in Nature. It is diffused in minute quantities through the igneous
rocks, and also is found in the sedimentary rocks and in the sea water. Its
minerals are few, galena, the sulphide of lead, being the commonest, and at
the same time the form in which lead is primarily deposited. Galena may
also represent a secondary deposition. The other minerals, cerrusite,
anglesite, and pyromorphite are results of modification of the galena when
it lies near enough to the surface to be acted on by weathering agents, like
water and air. Lead minerals are usually associated with zinc minerals, there
being but few places where the minerals of the one group occur 60
without the other. Most lead when first smelted from its ore, contains
a greater or less amount of silver in it, sometimes enough so that the lead
ore is better worth working for the silver than for the lead.

Lead is used in making pipes, gutters, bullets, etc., and in its oxide forms in
the manufacture of paints and glass. Eighty-three parts of lead with 17 parts
of antimony make type metal. Lead and tin alloy to make solder. Lead and
tin with small amounts of copper, zinc and antimony make pewter. The
United States produce about 20% of the world’s supply of this metal.

Galena
PbS
Pl. 17
lead glance

Occurs in cubes or cleavable masses; hardness 2.5; specific gravity 7.5;
color lead-gray; streak lead-gray; luster metallic; opaque.

While there is quite a group of lead-gray minerals, galena is easily
identified by its cleavage, which is perfect in three directions parallel to the
cube faces. Even a moderate blow of the hammer will shatter a mass of
galena into small cubic pieces. The crystals often have the corners cut by
octahedral faces, and occasionally the edges are beveled by dodecahedral
faces. It is not uncommon to find crystals of large size, several inches
across. If galena has 1 to 2% of bismuth as an impurity, curiously enough,
the cleavage changes to octahedral, but this is a rare occurrence.

Galena may occur as a primary mineral in veins associated with igneous
intrusions, or in irregular masses in metamorphic rocks; but it is more 61
often found in irregular masses in limestones, where the limestone has
been dissolved, and the cavities thus formed, filled with secondary deposits
of galena. It also occurs at the contact between igneous rocks and the
adjacent rock, whatever this may be. Sometimes it is found in residual
clays.

Among the most important lead deposits are the Cœur d’Alene district in
Idaho, where galena with a high percentage of silver is mined; the
Leadville, Colo., district where lead, silver and gold occur together in veins;
the Joplin, Mo., district, where lead and zinc ores occur together in irregular
masses in limestones; and the Wisconsin district of similar character.

When found galena is usually associated with sphalerite, argentite
chalcopyrite, pyrite and calcite. It will be found in every state.

Cerrusite
PbCO₃
Pl. 18
White lead ore

Occurs in fibrous or compact masses, or in orthorhombic crystals, usually
on galena; hardness 3.5; specific gravity 6.5; colorless; streak white; luster
adamantine; transparent on thin edges.

While the crystals of this mineral simulate hexagonal, they are actually
orthorhombic, the simple form being an octahedron with two of its edges
beveled, making double six-sided pyramids (see Pl. 18 A.) Usually prism
faces are present. Twinning is common, both the simple contact sort, as
shown on Plate 18 B, and also the sort in which three crystals have grown
through each other, so as to make a six-rayed crystal. The 62
considerable weight, and the fact that it effervesces in acid serve to
identify cerrusite. When pure it is colorless, but impurities cause it to appear
white, gray or grayish-black, and sometimes it has a tinge of blue or green.

It is likely to occur wherever galena is found, as a secondary mineral
derived from the galena. In this country it is not used as an ore, for, as in the
Leadville district, veins which have cerrusite near the surface change at
moderate depths, and galena takes the place of the cerrusite. It is found all
down the Appalachian Mountains, and in all the Cordilleran States.
Especially fine specimens have come from the Cœur d’Alene district in
Idaho.

Anglesite
PbSO₄
Pl. 18

Occurs in grains and masses, or in tabular and prismatic orthorhombic
crystals; hardness 3; specific gravity 6.3; colorless; luster adamantine;
transparent on thin edges.

Two modes of occurrence are characteristic, one in cavities in galena, the
other in concentric layers around a nucleus of galena. In the former case
fine crystals are developed, in the latter the mineral is in masses. The
crystals look like those of barite, but are soluble in nitric acid while the
barite is insoluble. Sometimes the crystals are prismatic with pyramidal
faces instead of the tabular form.

It is found in the lead mines associated with galena, and in this country is
not used as an ore for lead, but in Mexico and Australia it is abundant 63
enough to be mined as an ore. Exposed to water which has carbon

dioxide in it, and most surface waters have some, it readily changes to
cerrusite. It is found in Missouri, Wisconsin, Kansas, Colorado, and
Mexico.

Pyromorphite
Pb₅Cl(PO₄)₃
Pl. 17
Green lead ore

Occurs in small barrel-shaped hexagonal crystals, and in fibrous or earthly
masses; hardness 3.5; specific gravity 7; color green to brown; luster
resinous; translucent on thin edges.

Pyromorphite is found in the upper levels of lead mines, and is formed by
the decomposition of galena. Its green color (sometimes shading off toward
brown), considerable weight and resinous luster, serve to distinguish this
mineral. The crystal form is that of a simple hexagonal prism, with the ends
truncated. It is found in Phœnixville, Penn., Missouri, Wisconsin, Colorado,
New Mexico, etc.

The Zinc Group

Zinc and copper made the brass of early Roman times; but even then, zinc
was not known as a separate metal, the brass being made by smelting rocks
in which both zinc and copper occurred, the zinc never being isolated until
much later. Some time in the later Roman times it seems to have been
obtained separately, but then and all down through the Middle Ages zinc
and bismuth were confused. Our earliest record of zinc being smelted, as
we know it today, was about 1730 in England. In those earlier days, 64
the product, zinc, or bismuth, or both together, were known as
“spelter,” and this name has clung to zinc in mining and commercial circles;
so that today, if one looks for quotations in the newspaper, he often finds
zinc under the head of spelter.

Zinc, like lead, is diffused in small quantities through all the igneous rocks.
In places it is segregated in fissures or veins leading from the igneous rocks,
along the contact between igneous rocks and either sedimentary or
metamorphic rocks, in limestones where solution cavities have been formed
and later filled with zinc minerals, and as a residue where limestones have
been weathered away. In all these places it is closely associated with lead.

The sulphide, sphalerite, is the primary mineral, and the other minerals, like
zincite, smithsonite, calamine, willemite, franklinite, etc., are secondary,
resulting from modifications of the original sphalerite. In connection with
zinc minerals the region of Franklin Furnace, N. J., is especially interesting,
for at that place are found two large metamorphosed deposits containing a
wide range of zinc minerals, several of which are not found anywhere else.

Zinc is soft and malleable, but is only slightly ductile, and has little tensile
strength. It alloys with several metals, and in this form is most useful today;
three parts of copper to one of zinc making brass; four or more parts of
copper and one of zinc, making “gold foil”; copper and zinc (a little more
zinc than copper) making “white metal”; three parts of copper to one of zinc
and one of nickel making German silver; etc. Zinc is also used in large 65
quantities in galvanizing iron, sheets of iron being dipped into melted
zinc and thus thinly coated. It is also used in batteries and a wide range of
chemical industries.

Sphalerite
ZnS
Pl. 19 & 20
zinc blende, black jack

Occurs in grains, in fibrous or layered masses, or in isometric crystals;
hardness 3.5; specific gravity 4; color yellow-brown to almost black; streak
light yellow to brownish; luster resinous to adamantine; translucent on thin
edges.

When in crystals sphalerite occurs most commonly either in dodecahedrons
or in tetrahedrons (hemihedral forms of the isometric octahedron). The
cleavage is fairly good and parallel to the faces of the dodecahedron. The
difficulty usually is to get large enough crystalline masses to see this
cleavage clearly, but by examining the angles between the faces of cleavage
pieces they will be found to be the same as those on a dodecahedron. When
the mineral is pure, it has the color of resin, but sometimes it is reddish to
red-brown, and then it is called “ruby zinc,” more often it is dark brown due
to the presence of iron as an impurity. This is what the miners call “black-
jack.” The presence of iron also tends to make the streak darker. The
hardness, streak and cleavage will usually determine this mineral readily.

Sphalerite is the primary ore of zinc and is usually found in fissures and
veins leading from masses of igneous rocks, or along the surface of 66
contact where igneous rocks like granite or lavas come against such
metamorphic rocks as gneisses, schists, or crystalline limestones. In the
region of Joplin, Mo., however, the sphalerite is of secondary character,
having been gathered by waters circulating through the limestones, and
deposited in them in irregular pockets. This Joplin district has produced
more zinc than any other in the world. The United States annually produces
about 25% of the world’s supply of this metal.

Sphalerite is always associated with galena, and such other minerals as
argentite, pyrite, chalcopyrite, fluorite, quartz, calcite and barite, are very
apt to be present. It will be found in almost every state, especially in
fissures and veins, and less frequently in cavities in limestones.

Zincite
ZnO
Pl. 19 & 20
red zinc ore

Usually occurs massive, but may be found in crystals; hardness 4; specific
gravity 5.6; color deep red; streak orange; luster subadamantine; translucent
on thin edges.

When in crystals zincite forms in hexagonal prisms with hexagonal
pyramids on the ends. This is rather rare, most of the zincite being found in
massive form. The cleavage is parallel to the prism faces and perfect. The
deep red color and orange streak are wholly characteristic.

This mineral is so common at Franklin Furnace, N. J., as to be an important
ore, but it is very seldom found elsewhere. This district, as mentioned 67
before, is a peculiar one for zinc minerals. The zinc beds are in a
metamorphosed limestone, and into this are intruded numerous dikes of
granite. Probably the zinc was originally present in the bed of limestone as
smithsonite, calamine and other secondary minerals of zinc. When intruded
by the hot granite the smithsonite (carbonate) may well have been altered to
the oxide, zincite; while the calamine (hydrous silicate) became the simple
silicate, willemite.

Willemite
ZnSiO₄
Pl. 20

Occurs in masses or in crystals; hardness 5.5; specific gravity 4.1; color
pale yellow when pure; luster resinous; translucent on thin edges.

Willemite is another of the minerals which are distinctively characteristic of
Franklin Furnace, and found elsewhere very rarely. It is so common there as
to be one of the principal ores, and mostly occurs in irregular masses, but is
also found in crystals. These are hexagonal prisms, with a three-sided
(rhombohedral) pyramid on the ends. The color when pure is whitish or
greenish-yellow, but with small amounts of impurities it may be flesh-red,
grayish-white or yellowish-brown. When in crystals it is easily determined;
but when massive it looks like calamine, and can only be distinguished by
placing a bit of the mineral in a closed tube and heating it, in which case
calamine will give off water vapor, while willemite will not.

This mineral is one of those resulting from metamorphic alteration and is
derived from calamine, when the latter loses its water of 68
crystallization. It is common at Franklin Furnace, N. J., and also

found occasionally elsewhere, as at Salida, Colo., and in Socorro Co., New
Mexico.

Calamine
Zn₂(OH)₂·SiO₃

Occurs as crystalline linings in cavities, or as botryoidal or stalactitic
masses; hardness 5; specific gravity 3.4; colorless to white; luster vitreous.

Calamine resembles both smithsonite and willemite when in non-crystalline
masses. From the smithsonite it is easily separated by the fact that in nitric
acid the smithsonite effervesces and the calamine does not. From willemite
it is harder to distinguish, but a piece may be placed in a closed tube and
heated. If it is calamine water vapor will be given off, if willemite nothing
happens. When calamine occurs in crystals these are orthorhombic and
mostly tabular, and the crystals are peculiar in that the two ends are
terminated differently.

Both this and smithsonite are secondary minerals and usually occur together
when zinc is found in limestones. It is abundant at Franklin Furnace and
Sterling Hill, N. J., and also found at Phœnixville, Penn., in Wythe Co., Va.,
and Granby, Mo.

Smithsonite
ZnCO₃
Pl. 21
Dry bone

Usually occurs as incrustations, grains, earthy or compact masses, and as
crystals; hardness 5; specific gravity 4.4; color white, yellow, greenish or
bluish; streak white; luster vitreous; transparent on thin edges.

When pure this mineral is colorless, but, as it occurs, it is usually 69
white, or tinged with some shade of yellow, green, or blue, but in all

cases its streak is white. The crystals are rhombohedrons often with edges
beveled or corners cut by other faces. It resembles calamine and willemite,
but is readily separated from either of these by the acid test, for smithsonite
effervesces when acid is placed on it.

Next to sphalerite, smithsonite is the commonest of the zinc minerals. It is a
secondary mineral, resulting from the action of lime-charged water acting
on sphalerite, and so is likely to be found wherever zinc minerals occur in a
limestone region. In the Wisconsin-Illinois-Iowa district it serves as a minor
ore of zinc, and is termed here “dry bone.” It is also found in the Missouri
and Arkansas districts, and in Europe is an important ore for zinc.

Franklinite
(ZnMn)Fe₂O₄
Pl. 21

Occurs in compact grains or masses, and in isometric octahedrons; hardness
6; specific gravity 5; color black; streak reddish-brown; luster metallic;
opaque on thin edges.

This is a mineral peculiar to the Franklin Furnace region, from which it gets
its name. It looks like magnetite, but its reddish-brown streak and lack of
magnetism distinguish it. When it occurs in octahedrons, the edges are
rounded, while those of magnetite are sharp. It is a complex and variable
oxide of zinc, iron and manganese, which has resulted from the
metamorphism of the beds in which it occurred probably being 70
originally something quite different.

The Manganese Group

Though manganese was known in the mineral pyrolusite in early times, it
was then thought to be magnetite or magnetic iron ore. It was not until 1774
that it was isolated and recognized as a distinct element.

Manganese is one of the lesser elements in the crust of the earth, making
less than .07 of one percent, but as an alloy with other metals, especially
iron, it has attained a considerable importance to man. It is used chiefly
with iron, 20% of manganese making the alloy, spiegeleisen, a combination
which occurs in Nature in Germany, and from 20% to 80% making
ferromanganese. These alloys are in great demand because they make an
especially tough steel essential in the manufacture of munitions. The
sources for manganese are the oxide ores, manganite, pyrolusite and
psilomelane, which have been formed as secondary minerals, as a result of
the weathering of silicates which carry manganese. They occur widely
enough, but throughout the United States the deposits are small, and this is
one of the elements in which this country is not self-sufficient. The largest
producer of manganese is Russia; however she consumes almost all of her
output at home, and our supply comes from the next largest producers,
India, the Union of South Africa, and the Gold Coast. A shift in trade may
be expected when Brazil’s recently discovered ore body in Matto Grosso is
brought into full production. Besides being used as an alloy, 71
manganese is employed in making paints and dyes, for clearing glass,
and for some types of electric batteries.

Pyrolusite
MnO₂

Occurs in earthy or fibrous masses; hardness 1-2; specific gravity 4.8; color
black; streak black; luster dull; opaque.

Pyrolusite occurs in soft masses and incrustations, usually leaving a sooty
mark on the fingers. Sometimes it seems to be in crystals, but these are
pseudomorphs which have the form of manganite, from which the
pyrolusite has formed as a result of the water having been driven from the
manganite. Frequently pyromorphite and manganite will be found together,
and in some cases the outer part of a mass or crystal will be pyrolusite,
while the center is still manganite. Psilomelane is another oxide of
manganese with water and may appear very like pyrolusite, but both
manganite and psilomelane have much greater hardness than does

pyrolusite. If there is difficulty in deciding about pyrolusite, it may be
placed in a closed tube and heated. It will not be affected by the heat, while,
under the same circumstances, both manganite and psilomelane will give
off water vapor.

Pyrolusite usually occurs in black streaks or pockets in residual clays which
have formed as a result of the decomposition of limestones. It may also
occur in dendritic forms in seams and crevices (see manganite). It is found
in Vermont, Massachusetts, Virginia, Arkansas, Colorado, California, etc.

72

Psilomelane
MnO₂·H₂O

Occurs in compact botryoidal or stalactitic masses; hardness 5-6; specific
gravity 4.2; color black; streak brownish-black; luster metallic; opaque on
thin edges.

Psilomelane is very like pyrolusite, and often occurs with it. It is
distinguished by its greater hardness, and the fact, that when heated in a
closed tube, it gives off water vapor. From manganite it is more easily
distinguished, for it never occurs in crystals, while the manganite is usually
crystalline. This and pyrolusite are the principal ores of manganese.

Wad is an impure form of psilomelane, having some iron oxide mixed with
the manganese oxide, usually limonite; or the impurity may take the form of
a copper, cobalt, lithium or barium oxide.

Psilomelane is found at Brandon, Vt., in Arkansas, Colorado, California,
etc.

Manganite
Mn₂O₃·H₂O
Pl. 22

Occurs in prismatic crystals, or in columnar or fibrous masses; hardness 4;
specific gravity 4.4; color steel gray; streak reddish-black; luster
submetallic; opaque on thin edges.

This is the form taken by manganese oxide when it crystallizes in the
presence of moisture, and pyrolusite frequently changes to manganite when
exposed to moisture. The crystals are orthorhombic prisms, with striated
sides and the ends truncated. These prisms usually occur in bundles and
give the mineral a fibrous appearance. Manganite is not hard to identify, the
striations on the crystals and the streak being very characteristic. 73

In seams and tiny crevices this mineral, and often pyrolusite, grows in a
branching manner, resembling tree-like or “mossy” masses. This is termed
dendritic, and the growths of manganese minerals are called dendrites. One
of the most curious of these is when the “mossy” growth is inclosed in
chalcedony, making the so-called moss agate. These moss agates are
abundant through the Rocky Mountains and are frequently cut for semi-
precious stones. The finest ones however come from India and China.

Manganite is found in the Lake Superior region, Colorado, etc.

Rhodochrosite
MnCO₃

Occurs in compact cleavable masses; hardness 4; specific gravity 3.5; color
rose to dark red; streak white; luster vitreous; translucent on thin edges.

This usually occurs in pink to red masses which cleave readily parallel to
the faces of the rhombohedron. When it is found in crystals, which are rare,
these too are rhombohedrons. It is usually found in veins as a gangue
mineral with copper, silver or zinc ores. Its beautiful color and the fact that
it effervesces in acid serve to distinguish this mineral. It is found at
Branchville, Conn., at Franklin Furnace, N. J., and in veins with silver in
Colorado, Nevada, and Montana.

The Aluminum Group

Though aluminum is one of the most abundant of all the metals, making
some 8% of the crust of the earth, its union with other elements is so 74
firm, that only recently have methods been found for getting the metal
free. It was first isolated in 1846, but up to 1890 the extraction of aluminum
was so expensive, that it could not be widely used. About that time
electrical processes were applied to its extraction, and since then the price
has steadily dropped, until now it is under $.20 per pound. It is very
malleable, and ductile, and has high tensile strength. Exposed to the air,
water or ordinary gases, it does not tarnish; and it is very light, an equal
bulk weighing about a third as much as iron. The combination of lightness
and strength, and the fact that it is a good conductor of electricity, have
made it available for a wide range of uses, such as electrical apparatus,
delicate instruments, boats, aeroplanes, and domestic utensils.

It is an essential component of all the important rocks, except sandstone and
limestone, and combines to a greater or less degree in a host of minerals.
Though present in clays, shales, argillites, feldspars, and micas, it is only
from bauxite that it has been successfully extracted. Aside from the small
number of simple compounds of aluminum grouped here, it also takes a part
in the make-up of a large series of minerals termed silicates, treated a little
further on in this book.

It alloys with other metals, especially copper. The union of copper and a
small amount of aluminum makes aluminum-bronze, which looks like gold
and is used for watch chains, pencil-cases, etc., and also for the antifriction
bearings of heavy machinery. A small amount added to steel prevents 75
air holes and cracks in casting.

Corundum
Al₂O₃
Pl. 23

Occurs in cleavable masses or in hexagonal crystals; hardness 9; specific
gravity 4; colorless, red, yellow, blue, or gray; luster vitreous to
adamantine; translucent to transparent on thin edges.

Corundum is readily recognized by its hardness, second only to that of the
diamond. The crystals may be simple six-sided prisms, hexagonal pyramids
or combinations of the two. The cleavage is usually described as parting, for
it is by no means perfect, but when it is recognizable it is parallel to the
faces of a rhombohedron, and cleavage pieces may appear almost cubic.

When in clear and perfect crystals this mineral is one of the most highly
prized of all the gems. Clear and colorless it is known as the “Oriental
white sapphire”; when tinged with blue it is the sapphire; when colored
yellow, the “Oriental topaz”; when green, the “Oriental emerald”; when
purple, the “Oriental amethyst” and when red, the ruby. Sapphires range
from colorless to deep blue, the value depending on the shade of the blue,
and increasing as the color deepens. The Oriental topaz can easily be
confused with the true topaz, which is a much commoner and less valuable
gem, but can be distinguished by the hardness, topaz having a hardness of
but 8. The name emerald is applied to several green gems, mostly to beryl,
which is not so hard and is the true emerald. The Oriental emeralds have a
value about the same as diamonds. Rubies of clear and deep color are 76
the rarest of all gems, ranging in value about three times as high as
diamonds of equal size. The most sought-for shade is the so-called “pigeon-
blood red,” and the value of a stone of this sort is almost dependent on the
whim of the buyer. The best of the rubies come from granites or
metamorphosed limestones in Burma; the best sapphires from Ceylon,
though both of these, and some of the other corundums of gem quality, have
been found in North Carolina and Montana.

Around these stones, which have been used so long among the Hindus,
Persians, Jews, Egyptians, and Christians, a wealth of lore has been woven.
The sapphire was Saturn’s stone, and a talisman to attract Divine favor.
Where tradition makes the stone on which the ten commandments were
written the sapphire, it is probable that, what was really meant, is lapis
lazuli, as is also the case when sapphires are mentioned as building stones
for the celestial gates. The ruby in ancient lore is termed “lord of stones,”

“gem of gems” etc., and so protected its wearer that he was safe from injury
in peace or war.

When corundum is colored brown by impurities of iron, it is termed
corundum, when black by greater quantities of iron, it is emery. These
varieties are far the commonest form in which corundum occurs, and when
ground to finer or coarser powder make the commercial emery. Emery is
likely to be found in sands, making so-called “black sands,” where it has
accumulated as a result of the weathering to bits corundum-bearing 77
rocks. In some one of its forms, corundum is found in Massachusetts,
Connecticut, New York, New Jersey, and all down the Appalachian
Mountains, also in Colorado, Montana, California, etc.

Bauxite
Al₂O₃·2H₂O

Occurs in grains, or oolitic or clay-like masses; hardness 1-3; specific
gravity 2.5; color white to yellowish-white or reddish-brown.

Bauxite never comes in crystals, but is usually in earthy masses, which have
resulted from the decomposition of granitic or volcanic rocks, in
circumstances where hot alkaline waters were present. This explanation
seems to apply especially to the deposits in France, which were first the
chief source of the bauxite, and may be applicable to those in Georgia and
Alabama. Some of the other deposits, however, do not seem to have had
any hot water available, and the deposit appears more like simple
decomposition of the underlying rocks by alkaline waters.

In many cases bauxite resembles limonite in being a mixture of two or more
aluminum oxides with water of crystallization, such as Al₂O₃·H₂O,
Al₂O₃·2H₂O and Al₂O₃·3H₂O. This is particularly true of the bauxite which
resulted from the decomposition of rocks by surface water.

Bauxite is the ore from which aluminum is obtained. The deposits are not
large, but the United States has its share of them. It is found in Alabama,
Arkansas, Georgia, Missouri, Tennessee, and California.

78

Cryolite
Na₃AlF₆
Ice stone

Occurs in pseudo-cubic crystals or massive; hardness 2.5; specific gravity
3; color white; luster vitreous; transparent on thin edges.

Cryolite is a relatively soft mineral, colorless to white as snow; for which
reason, and partly also because it comes mostly from Greenland it is called
“ice stone.” It is really monoclinic but the inclination of the c axis is so
slight, that, unless examined carefully, the crystals appear to be cubic. Until
about 1900 great quantities of this mineral were shipped from West
Greenland, and from them the metal aluminum was extracted. When
bauxite was discovered, it was found to be considerably cheaper to make
the aluminum from that mineral, and now cryolite is no longer sought.
Aside from its occurrence in Greenland some cryolite is found in Colorado,
near Pike’s Peak.

The Arsenic Group

The metal, arsenic, is a dark steel gray in color, when the surface is fresh,
but it soon tarnishes. It is very brittle and easily powdered under the
hammer, and its only use as a metal, is for an alloy with lead in making
shot. Its compounds find a wider use. The white powder called “arsenic” is
arsenous acid, and is used mostly in making poisons, which fortunately are
easily detected in animal tissues. Copper arsenate, (Scheele’s green) is a
pigment used in making green paint, and formerly in the green colors of
wall paper. A combination of arsenous acid, copper oxide and acetic acid is
the well known Paris Green, so much used for an insecticide. Beside 79
these uses, arsenic serves a large number of other purposes, as in
making glass and enamel, embalming fluids, and various medicines.

Curiously arsenic plays a double part, acting part of the time as a metal, as
in the two following minerals, and part of the time as a non-metal, as in

cobaltite, niccolite, etc.

Arsenopyrite
FeAsS
Pl. 24

Occurs in well formed crystals, grains, or masses; hardness 5.5; specific
gravity 6; color silver-white; streak black; luster metallic; opaque on thin
edges.

When in crystals, they are usually short prisms of the orthorhombic system,
either end being terminated with a low roof. Though usually described as
silver-white in color, there is always a brassy cast to the color. Its
appearance is much like cobaltite and smaltite, but it can be easily
distinguished from both these by putting a piece in nitric acid. The
arsenopyrite will not materially change the color of the fluid, but the other
two turn it rose-red, and all give off the smell of sulphur. It looks sometimes
like marcasite, but that is yellower, and has the fibrous structure, not found
in arsenopyrite.

It is found in veins or in metamorphic rocks, associated with argentite,
galena, sphalerite, chalcopyrite and pyrite. It is distinctly a mineral formed
by deposition from hot vapors or hot water rising from either lavas, or in the
course of metamorphism.

It is found in New Hampshire, Vermont, Massachusetts, Connecticut, 80
New York, New Jersey, California, etc.

Realgar
AsS
Pl. 24

Occurs in incrustations or scattered grains; hardness 1.5 to 2; specific
gravity 3.5; color orange; streak orange; luster resinous; opaque on thin

edges.

Crystals are very rare, but when found are short monoclinic prisms. The
color is aurora-red, changing to orange as soon as it is exposed to the air.
This and the streak are entirely characteristic. It is a mineral associated with
hot vapors or hot waters, and is found about volcanoes, as deposits from the
hot water of the geysers in Norris Basin, Yellowstone Park, and in veins,
associated with barite, stibnite, quartz, etc., as in Massachusetts, Utah,
California, etc.

Orpiment
As₂S₃

Occurs as incrustations or powdery masses; hardness 1 to 2; specific gravity
3.5; color lemon yellow; streak yellow; luster resinous.

This mineral is very like realgar in its physical properties, and likely to
occur with it. It gives the lemon yellow color to the basins about hot
springs, as in the Yellowstone Park, and about volcanoes. It also comes in
veins with realgar.

Molybdenum

Molybdenum is a rare metal, silvery-white in color, brittle and very difficult
to fuse. It is used mostly as an alloy of steel, to make certain grades of tool
steel. The world’s greatest supply is obtained from Climax, Colorado,
where the principal ore mineral is molybdenite.

81

Molybdenite
MoS₂

Occurs in scales or scaly masses, occasionally in tabular hexagonal crystals;
hardness 1.5; specific gravity 4.7; color lead-gray; streak bluish-gray; luster
metallic; opaque.

This mineral is the chief source for the metal molybdenum. Its extreme
softness and greasy feel will distinguish it at once from any other mineral
except graphite, which has much the same qualities, but its scaly character
and the more bluish tinge in streak and color will distinguish these two.

It occurs in granites, gneisses, and metamorphic rocks in Colorado, New
Mexico, Maine, Connecticut, New Hampshire, New York, Pennsylvania,
etc.

Antimony

Antimony is another hard, brittle metal, of bluish-white color. Exposed to
the air at ordinary temperatures it does not tarnish; and this combined with
its hardness make it useful for such alloys as Britannia metal, type metal,
and pewter. Only one of its minerals, stibnite, is common enough for
mention.

Stibnite
Sb₂S₃
Pl. 25
gray antimony

Occurs in prismatic or needle-like crystals; hardness 2; specific gravity 4.5;
color lead-gray; streak lead-gray; luster metallic; opaque.

The crystals of stibnite are orthorhombic and usually elongated, the sides
striated and the ends with low pyramids on them. Sometimes the long 82
crystals are curved or even twisted. There is a well-developed
cleavage parallel to face b in the figure. While the color is similar to that of

galena, the form and cleavage are so different that stibnite is easily
determined.

The ancients used stibnite to color their eyebrows, now it is the source for
the metal antimony. Hungary and Japan are famous for the fine large
crystals they produce; but moderate sized crystals may be found in this
country. It occurs in veins along with pyrite, galena, cinnabar, and realgar,
with quartz, calcite or barite as gangue minerals.

Stibnite has been found in Arkansas, California, Nevada, and Utah.

The Nickel Group

Nickel as a metal is silvery-white in color, rather hard, and does not tarnish
when exposed to the air. When pure it is malleable and fairly ductile. It is
highly useful for plating other metals to protect their surfaces. Alloyed with
steel, it makes a product of extreme hardness. Copper, zinc, and nickel
make the well known German silver.

Nickel has a fairly large range of minerals, but they do not occur with any
abundance in the United States, so that we have to import most all of our
nickel. In the earlier days New Caledonia produced most of the world’s
supply, but recently since the finding of large nickel deposits near Sudbury,
Canada, this locality has not only outstripped New Caledonia, but 83
now produces four-fifths of the world’s supply. In this country but two
nickel minerals will be found at all common.

Niccolite
NiAs
Pl. 25
copper nickel

Occurs in masses; hardness 5.5; specific gravity 7.4; color pale coppery-
yellow; streak pale brownish-black; luster metallic; opaque on thin edges.

Niccolite is very seldom in crystals, but if they do occur they are hexagonal.
The mineral looks a little like smaltite, but in case there is any question of
the determination, dissolve a piece in nitric acid, and if niccolite, it will
color the solution green.

Niccolite is usually associated with copper and silver ores, and in this
country has been found at Chatham, Conn., and Silver Cliff, Colo. It may be
associated with pentlandite, a sulphide of iron and nickel, which is similar
in appearance, but not so hard, and occurs in small grains throughout dark
lavas. The particles of pentlandite are however so small, that they are
seldom noticeable, but at Sudbury, Canada, this is the chief ore of nickel.

Millerite
NiS
capillary pyrites

Occurs in needle-like or fibrous crystals; hardness 3.5; specific gravity 5.5;
color brass-yellow; streak greenish black; luster metallic; opaque on thin
edges.

The fibrous crystals of millerite belong to the orthorhombic system. The
color and streak suggest pyrite, but the crystals are long and slender, 84
while pyrite is in cubes, octahedrons, etc. If there is any doubt of the
identity of this form, place a piece in nitric acid, and if it is millerite, it will
color the acid green.

It may occur in veins associated with cobalt and silver minerals, or as a
secondary mineral as at Gap Mine, Penn., or in cavities in sedimentary
rocks. In the last case it usually is in needle-like crystals growing through
calcite crystals, as at St. Louis, Mo., Keokuk, Iowa, and Antwerp, N. Y.

The Cobalt Group

As a metal, cobalt is hard, brittle, and of a grayish color, tinged with red. It
was not recognized as a separate element until 1735, and even today is one
of the minor metals. Cobalt, chromium and a little tungsten make the alloy
stellite, which has come into large use in making high-speed tools. The
oxide of cobalt (CoO) is “smalt,” used to give the blue color to porcelain,
pottery, glass, tiles, etc. Invisible ink is made by diluting cobalt chloride in a
large quantity of water. This solution is a faint pink color and practically
invisible on paper, but if heated it loses water and turns blue in color, and is
perfectly visible.

Cobalt is another of the metals, of which the United States does not have an
adequate supply. Sweden, Norway and India were the chief sources of
supply until cobalt was found near the town of Cobalt in Ontario, Canada,
and now this district furnishes 90% of the world’s supply.

85

Cobaltite
CoAsS
Pl. 26
cobalt glance

Usually crystalline in cubes, pyritohedrons or octahedrons; hardness 5.5;
specific gravity 6.1; color reddish silver-white; streak grayish-black; luster
metallic; opaque on thin edges.

In color cobaltite may appear very like arsenopyrite, especially if the
reddish tinge is not strong, in which case the mineral can be definitely
determined by putting a piece in nitric acid. If it is cobaltite the solution will
be colored rose-red, if arsenopyrite there will be no change of color. The
forms of the crystals are the same as those of pyrite, but the color will easily
distinguish cobaltite from pyrite. This pink color is characteristically
present either in or about cobalt minerals, being sometimes called “cobalt
bloom.” It is a cobalt-arsenic-oxide with water of crystallization
(Co₃As₂O₈·8H₂O), which results from the exposure of cobalt and arsenic
minerals to air and moisture. It is the pink color on the figures of both
cobaltite and smaltite. In Sweden, Norway and India, this is the chief ore

for cobalt, but in the United States it is rather rare, but is found in Oregon,
and at Cobalt, Canada.

Smaltite
(CoNi)As₂
Pl. 26
gray cobalt ore

Usually occurs in masses; hardness 5.5; specific gravity 6.2; color tin-white
to steel-gray; streak grayish-black; luster metallic; opaque on thin edges.

While very like cobaltite, smaltite is almost never found in crystals, but
when crystals are found, they are cubes. The color is tin-white but 86
there is usually a pink tinge visible due to the presence of small
amounts of “cobalt bloom.” If in any doubt about the determination of this
mineral, put a piece in nitric acid. If it colors the acid rose-pink, and is non-
crystalline it is pretty surely smaltite; if the acid is not affected it is
arsenopyrite.

Smaltite is found in Kentucky, Missouri, Colorado, Idaho, California, and at
Cobalt in Canada.

Chromium

This metal gets its name in recognition of the many colors (chroma
“color”), in which its compounds appear. Chromic oxide is a vivid green,
used to color porcelains, pottery, tiles, etc., and also as a substitute for the
arsenical greens formerly used in wall-paper. The chromate of lead is the
pigment, well known to artists as “chrome yellow,” and the bichromate of
potassium is bright red. The metal is obtained in at least two different
forms; one hard, brittle and so resistant to heat as to be infusible at
temperatures which would volatilize platinum; the other as a powder which
burns brightly if heated in air. While used in paints, dyes, etc., its greatest
importance is for the making of ferro-chrome steel, which is used where

resistance to sudden shock is required, as in armor plate, automobile
springs, ball bearings, etc. With tungsten and cobalt it makes the alloy,
stellite, as noted above.

Chromium was used in relatively small quantities before the first 87
world war, and we imported our supplies from Turkey, India, New
Caledonia, and Rhodesia. During the last war we started a large-scale
development of low-grade ores in Montana, and can now supply all of our
needs from this source.

Chromite
FeCr₂O₄
chromic iron

Occurs in grains, masses, or isometric octahedrons; hardness 5.5; specific
gravity 4.4; color black; streak dark-brown; luster submetallic; opaque on
thin edges.

In form, color and streak chromite resembles magnetite and franklinite.
From the magnetite it is distinguished by being non-magnetic; from the
franklinite, by being insoluble in hydrochloric acid, while the franklinite is
soluble. Chromite furnishes practically all the chromium used in the arts
and manufactures. It is a mineral associated with high temperatures, and
therefore found in dark lavas, serpentine, and olivine. It occurs in
Pennsylvania, Maryland, New Jersey, Montana, Oregon, Wyoming, and
California.

Tungsten

This element is obtained either as a heavy dark-gray metal, which is very
hard and difficult to fuse, or as a dark-gray powder. It is used as an 88
alloy with iron, one part of tungsten to nine of steel, to make the
ferrotungsten, which has extraordinary hardness, and is used mostly for
high-speed tools. Tungsten is also one of the three metals (cobalt, chromium

and tungsten) which are alloyed together to make stellite. Some of the
tungsten supply is also used to make the films in incandescent lamps, and in
some of the chemical industries. It has but one important ore, wolframite,
and this is found in the United States in but small quantities; so that we
ordinarily have to import the greater part of what we use. During the last
war, under the stimulus of high prices and the urge of necessity, we did find
and produce substantial quantities of tungsten. China is the world’s largest
producer of tungsten ore with Burma second, and the United States a poor
third.

Wolframite
(FeMn)WO₄

Occurs in monoclinic crystals or in crystalline masses; hardness 5.5;
specific gravity 7.4; color dark-brown to black; streak nearly black; luster
submetallic; opaque on thin edges.

If in crystals the form will serve to distinguish this mineral from cassiterite
and ilmenite, the two which it most resembles; but if it is massive the only
sure way to decide is to put a piece in strong sulphuric acid; if it dissolves
and throws down a yellow precipitate (tungstic acid) it is wolframite.

Like the two other minerals mentioned above it occurs in veins in 89
igneous rocks, being associated with high temperatures. As it is
almost insoluble in water, like cassiterite and ilmenite, it is likely to occur
with them in the sands which are the result of the disintegration of the rocks
which carried the minerals; and so a large part of the supply today comes
from placer deposits.

It is found in Connecticut, North Carolina, Missouri, Colorado, and
California.

Radium, Uranium and Vanadium

These three metals are all rare and occur together. Radium, discovered in
1898, is a heavy metal which has proved very useful because of its radio-
activity, that is, its power of giving off or radiating tiny particles of matter
known as X-rays, part of which are charged with positive electricity, and
part of them with negative electricity. The ability of these rays to pass
through other substances has made possible photographing the denser
substances within those less dense, as the bones within the flesh, or metal
within leather or wood, etc. The rays have proved of great value
medicinally, and are also used to make objects luminous in the dark. These
X-rays are also used in the study of the ultimate structure of matter, as it can
be thus obtained in such small units.

Uranium is another element which is radio-active and can be used for many
of the same purposes as radium.

Vanadium, the third of these associated metals, and the commonest of 90
the group, is not radio-active. It is a silvery-white metal, mostly used
as an alloy with steel to give it great hardness.

Carnotite
K₂O·2U₂O₃·V₂O₅·3H₂O
Pl. 27

Occurs in earthy masses; color yellow.

This mineral is included here, not because it is common, but because it is of
such great interest. It is the chief source of supply in the United States of
radium, uranium and vanadium. It is a lemon-yellow earth or powder,
which looks a little like orpiment. It is however found in a sandstone,
instead of where hot waters have deposed minerals. From a ton of this ore
about 10 pounds of uranium oxide, 55 pounds of vanadium and ¹/₁₀₀₀th of a
gram of radium are obtained. Carnotite is found in south-west Colorado and
south-east Utah, and on Carrizo Mountain on the line between Arizona and
New Mexico.

Mercury

Mercury, or quicksilver, is the only metal which is liquid at ordinary
temperatures. It is silvery-white in color, with a striking metallic luster, and
at the low temperature of 662° F., boils and changes to a colorless vapor.
Mercury alloys with certain metals, these alloys being known as 91
amalgams. In this way it is especially useful for the recovery of gold
and silver, the mercury being added to crushed ore, the gold or silver
uniting with the mercury in a liquid amalgam, which is then drawn off and
heated to a temperature above 662° F., at which temperature the mercury
volatilizes and is recovered, while the gold or silver remains behind.
Mercury also forms a solid amalgam with tin which is used to coat glass,
the high metallic luster making the most effective looking glass. It is also
used in medicines (calomel, corrosive sublimate, etc.), for scientific
instruments (thermometers, barometers, etc.), in cosmetics, in paints for
ship bottoms, etc.

Though there are some 25 minerals of mercury, only one is common or
important as a source of the metal, cinnabar. The United States is self-
sufficient as far as mercury is concerned, producing just about as much as it
uses. The leading producers are Spain, Austria, Italy, and the United States.
Commercially mercury is quoted as quicksilver, and in flasks of 75 pounds
each.

Cinnabar
HgS
Pl. 27

Occurs in massive or earthy form, or in minute crystals in cavities; hardness
2.5; specific gravity 8; color scarlet to dark red; streak vermilion; luster
adamantine; translucent on thin edges.

The bright-red color and the streak are usually enough to identify this
mineral at once, but some of the darker varieties resemble hematite or
zincite in appearance, but both these have much greater hardness. 92

When in crystals they are tiny hexagonal prisms with pyramids on the end.
Cinnabar is usually found in or near metamorphic or igneous rocks, either
in veins leading from the igneous rocks, or in metamorphic rocks, or it may
occur disseminated through metamorphic rocks. It is associated with quartz
or calcite, and may occur with other sulphides like pyrite, galena, argentite,
etc. It is most abundant in California, but is also found in Oregon,
Washington, Idaho, Arizona, Nevada, Utah, Texas, and Montana.

Tin

Tin has been known since early Roman times, and the mines at Cornwall,
England, were worked from that time all through down to the present, but
now they are becoming of minor importance as they approach exhaustion.
The metal is silvery-white, does not easily tarnish, is malleable, but has
little ductility and little tensile strength. Tin is mostly used in making tin
plate, a thin sheet of steel covered with tin, the tin being only 1 to 2% of the
total weight. This tin plate is mostly made into tin cans, and used as
containers for food. Some tin is used in making solder, tin-foil, tubes for
paste, vaseline, etc., and around 1000 tons per year for weighting silk. This
“weighting” makes the silk heavier by about 25% and gives it a “rustle,”
which, while much in evidence, is really indicative that the silk is not pure.
The United States produces very little tin, most of the world’s supply
coming from the Malay Peninsula, Dutch East Indies, China, and 93
Bolivia, with small amounts from several other countries.

Cassiterite
SnO₂
Pl. 28
tin stone

Occurs in tetragonal crystals, massive, or in grains and pebbles; hardness
6.5; specific gravity 7; color black or dark-brown; streak gray; luster
adamantine; translucent on thin edges.

The crystals are short prisms with pyramidal ends. Twinning is common.
Cassiterite also occurs in fibrous masses, and when it is weathered from its
original location, is so insoluble and hard, that it remains as grains and
pebbles, making placer-deposits, from which today three quarters of the
supply is obtained. If pure, the crystals would be colorless, but impurities of
iron and titanium give it the dark-brown to black color. Cassiterite may
appear very like rutile, the crystalline forms being identical, but the reddish
tinge of color in the rutile will separate the two.

Cassiterite is one of those minerals which result from deposition at very
high temperatures, probably from vapors, and is found in the veins in
igneous rocks, such as light-colored granites, gneisses, syenites, etc. While
not mined in this country it is found in small quantities in Maine,
Massachusetts, New Hampshire, Virginia, Alabama, Wyoming, Montana,
and California.

Titanium

Titanium, as a metal, is a heavy, gray, iron-like powder, which is chiefly
useful as an alloy with iron, giving it toughness, and preventing 94
bubbles and cracks in casting. It is not as rare as some other metals
which have found a wider use.

Rutile
TiO₂
Pl. 28

Occurs in tetragonal crystals, and in grains; hardness 6.5; specific gravity
4.2; color red to reddish-brown; streak yellowish-brown; luster metallic to
adamantine; translucent on thin edges.

Rutile usually occurs in crystals, which are either short and stout, or in
needle-like crystals. Twinning is common. In form and general appearance
it resembles cassiterite, but the reddish color, and the yellowish-brown

streak will distinguish the rutile. It is found in similar rocks, granites,
gneisses, syenites, and mica-schists, the two minerals cassiterite and rutile
often occurring together. This is also true of the grains, which have been
weathered out and are found in sands and gravels of placer deposits. It is
found in small quantities in all the New England States, New York, and all
down the Appalachian Mountains, especially at Graves Mountain, Ga., and
in Arkansas and Alaska.

Ilmenite
FeTiO₃

Occurs in granular masses, as black sand, or as tabular hexagonal crystals;
hardness 5-6; specific gravity 4.7; color black; streak brownish-red to black;
luster metallic; opaque on thin edges.

When ilmenite occurs in crystals they are tabular and resemble hematite in
its darker varieties, but the streak readily distinguishes the two. In masses it
looks like magnetite, but the lack of magnetism serves to distinguish these
two minerals. It is very likely to be associated with cassiterite, rutile, 95
or magnetite in grains which have weathered out of the original rock,
and have resisted solution and wear. Sands with a large amount of the above
mentioned minerals are termed “black sands,” some of which are important
for one or another of these minerals.

Ilmenite is a mineral formed at high temperatures, and probably often
deposited from hot vapors. It is found in granites, syenites, and gneisses.
Among the better known localities are Orange, N. Y., Litchfield, Conn.,
Florida, California, etc.

Platinum

This metal is steel-gray in color, very malleable and ductile, almost
infusible and resists the action of acids. It is one of the “noble” metals,
much rarer than gold, and so has become popular for jewelry. It is also used

in the manufacture of sulphuric-acid, in nitrogen-fixation plants, for
chemical utensils, in the electrical industries, and in dentistry. Platinum in
its occurrence is associated with the certain other equally rare elements, like
iridium, palladium and osmium. Its use has increased rapidly of late, but the
supply has not kept up with the demand, so that, whereas in 1906 platinum
and gold were about equally valuable, now the platinum brings about five
times as much as the gold.

Platinum
Pt

Occurs in grains or nuggets; hardness 4.5; specific gravity 19 (21 if pure);
color steel-gray; luster metallic; opaque.

This rare metal is mostly found in placer-deposits, often with gold. It 96
comes originally from dark igneous rocks, like peridotite, pyroxenite,
etc., and platinum is found to be associated with the nickel ores of Sudbury,
Canada. While formerly 90% of the world’s supply of platinum came from
placer mines in the Ural Mountains, today more than half is produced in
Canada and about a fifth in Russia. In the United States it is found in
California, Oregon, Nevada, and Alaska.

The Magnesium Group

Magnesium is a silvery-white metal, easily tarnished by exposure to moist
air. Because of its light weight, less than twice the weight of water, and
strength, it is being substituted for aluminum, especially in airplanes, where
the question of weight is crucial. It is also used in automobile and ship
production and other machine industries, and in the manufacture of flares
and incendiary bombs. Magnesium is obtained chiefly from magnesite,
dolomite, and in the United States as a result of a recently developed
process, from sea water. Magnesium has a considerable number of 97
minerals, of which three are taken up here and several more under the

head of silicates, where both magnesium and silicon are combined in a
mineral.

Spinel
MgAlO₄
Pl. 29

Occurs mostly as isometric octahedrons; hardness 8; specific gravity 3.5;
color, red, yellow, green, or black; streak white; luster vitreous; transparent
on thin edges.

This is a rather rare mineral, but, when in clear crystals is considered one of
the gems. It was early confused with corundum, and the red variety called
ruby, as it was found in the same gem-bearing sands in Ceylon, Burma, and
Siam. However the form of the isometric octahedron as compared with the
hexagonal prism of the corundum, together with the lesser hardness are
sufficient to distinguish the two easily. The crystals are usually octahedrons,
but may have the corners cut or the edges beveled. Twins are not
uncommon.

The standard color is a clear deep-red, and such a spinel is known in the
gem trade as a spinel-ruby. If the color is rose-red, it is a Balas ruby; if
orange, it is rubicelle, if of a violet tinge, almandine. When small quantities
of other elements replace the magnesium, the color is greatly changed. For
example a little iron present gives the crystals a dark-green to black color,
and the spinel is known as ceylonite. If there is both iron and chromium
present, the color becomes yellowish or greenish-brown, and this variety is
picotite. When the impurities are iron and copper, the color becomes 98
grass-green, and it is called chlorospinel. A form, in which the
magnesium is completely replaced by iron, is black in color and termed
hercynite, and occurs fairly abundantly in Westchester Co., N. Y. From
Amity, N. Y., to Andover, N. J., there is a belt of granular limestone in
which spinel of all colors is found. St. Lawrence Co., N. Y., is also a rich
locality. Bolton, Mass., Newton, Sterling, and Sparta, N. J., North Carolina,
Alabama, and California all yield spinel.

Magnesite
MgCO₃

Occurs in cleavable or compact porcelain-like masses; hardness 4; specific
gravity 3.1; color white to gray; luster vitreous; translucent on thin edges.

Magnesite is white and brittle, and cleaves perfectly parallel to the faces of
the rhombohedron, but it seldom occurs in crystals. It will effervesce in
warm hydrochloric acid and has some resemblance to calcite, but can be
distinguished by the greater hardness. It is still more like dolomite, both
having the same color and cleavage, both effervescing in warm
hydrochloric acid; but the magnesite has half a point greater hardness and
the porcelainous appearance. Magnesite is used in toilet preparations, paper
making, and mixed with asbestos, as a covering for heating pipes.

Magnesite is found in Massachusetts, Pennsylvania, Texas, and in large
deposits in California and Washington.

99

Dolomite
(MgCa)CO₃
Pl. 19 & 29

Occurs in crystals, or in cleavable or granular masses; hardness 3.5; specific
gravity 2.8; color white to pink or gray; streak white; luster vitreous;
transparent on thin edges.

Dolomite crystallizes in the hexagonal system, in rhombohedrons
(hemihedral form), which are more or less modified by faces on the corners
or edges. The cleavage is parallel to the rhombohedron, and it will
effervesce in warm hydrochloric acid. Sometimes the crystal faces are
curved, and when this is the case, dolomite is easily determined. Usually
however dolomite resembles both calcite and magnesite. From the calcite it
is distinguished by the greater hardness, and from magnesite by lesser
hardness and not being porcelainous in appearance. Some of the commoner

forms are shown on Plate 29, crystals like C being found embedded in
anhydrite and gypsum.

Magnesium is a common element and is likely to be present wherever lime
is being deposited, so dolomite crystals are common, and much of the
limestone is dolomitic.

It may be found in almost any limestone section of the country. Some of the
finest crystals of dolomite however come from Roxbury, Vt., Smithfield, R.
I., Hoboken, N. J., Lockport, Rochester, and Niagara Falls, N. Y., etc.

Silicon, Silica and the Silicates

Silicon is one of the non-metallic elements, and does not occur as such in
Nature. When isolated it is either a dark-brown powder, or steel-gray 100
crystals. However silicon is next to oxygen in its importance in
making the crust of the earth. Forty-seven per cent of the surface rocks are
composed of oxygen, and 28% of silicon, the latter appearing in a host of
minerals. The oxide of silicon is termed silica (SiO₂), its crystal form being
quartz, the commonest of all minerals. In non-crystalline form silica is also
widely distributed, as chalcedony and opal, even appearing in the tissues of
animals and plants, as in the feathers of birds, the shells of certain Protozoa
(Radiolaria), the spicules of sponges; and in plants, as the shells of diatoms,
and in the stalks of grasses, especially cereals and bamboo. Silica in the
form of sand is widely used in making glass, porcelain, china, etc., and in
the various cements.

Then there are a considerable number of acids of silicon, which do not
occur in Nature, but their salts do, and make a host of minerals, which are
known as the silicates, such as mica, feldspar, hornblende, etc. Either as
quartz, or as silicates, silicon is represented in most all the igneous and
metamorphic rocks and in many of the sedimentary rocks.

Quartz
SiO₂
Pl. 30

Occurs as hexagonal crystals, or in grains or masses; hardness 7; specific
gravity 2.65; colorless when pure; luster vitreous; transparent on thin edges.

Quartz is not hard to identify. Its hardness and the crystal-form separate it
from most all other minerals. It is the most common mineral, making 101
12% of the earth’s crust. The usual crystal form is a hexagonal prism
with the sides horizontally striated, and a six-sided pyramid on one or both
ends. This six-sided pyramid is really two rhombohedrons, a right-handed
one and a left-handed one, so that the alternate faces of the pyramid may
show peculiarities, for instance three may be large and three small, as in
Fig. B, Plate 30, or the alternate ones may be duller or etched in some
manner. The crystals are clear and when pure colorless, but there is a
tendency for some slight impurity to color them almost any hue.

The most perfect double-ended crystals form only where growth is possible
in all directions, as in clay. In cavities and caves there is an opportunity for
the crystals to grow in toward the open spaces, and in such places, one finds
fine large crystals; the Alps, Brazil, Japan, and Madagascar being especially
famous localities. The largest quartz crystal on record is one 25 feet in
circumference which came from Madagascar. In this country the caves at
Little Rock, Ark., have furnished some very fine large crystals. Smaller, but
very clear crystals, come from about Herkimer, N. Y. Some of these have
been used as “Rhine-stones” and as cheap imitations of diamonds. Clear
quartz is beautiful enough to be a gem, but it is too common to interest
people as jewelry, however many objects of art have been carved from it.
One of these took the form of crystal balls, which, through the Middle Ages
particularly, developed into a form of mysticism. The gazing into the crystal
ball was supposed to give some people supernatural vision. It seems 102
to be a form of hypnotism, gazing at the bright reflecting surface
tiring the eye, and making possible visions, which are subjective rather than
anything external.

Silica is slightly soluble in water, especially when it is alkaline; so that most
river-, lake-, and sea-waters have some silica in solution, and are carrying it
from one place to another. The waters, which percolate through the rocks,
carry even more, and when they come out into open spaces, they give up
some of the silica, making crystals lining these openings, whether fissures
or cavities. Not infrequently these silica-bearing waters dissolve out some
other crystal, and then deposit in its place silica, thus making a crystal
which has the form of what was dissolved, rather than that of quartz. Such a
form is known as a pseudomorph.

When molten masses of igneous rock were cooling the quartz crystals had
their faces interfered with as they grew, and we have resulting crystalline
quartz, simply filling in the spaces between the other crystals, such as
feldspar and mica, in the granite. Quartz is a large component in many
igneous rocks, also in metamorphic rocks, and certain sedimentary rocks
like sandstone are almost wholly made up of quartz grains. Quartz is also
the gangue mineral in many veins. In this case it seems to have been
deposited from hot water or vapors, as they rose from cooling magmas.
With it are associated all sorts of metallic ores as has been suggested.

Quartz has been largely used to make imitations of other much rarer 103
minerals, sometimes in its crystalline form to imitate the diamond, at
other times ground and made into a “paste,” which is colored to imitate
other gems. This paste is a mixture of about 4 parts of quartz, 5 parts of red
lead and 1 part of potassium carbonate, melted and cooled slowly. It is clear
and has a brilliant luster like the diamond. If some coloring matter is put
into it it can be used for rubies, sapphires, etc. When there is any reason to
think that this is being used, it is easily detected by being so much softer
than any of the true gems, and even than true quartz. Quartz will scratch
glass readily, but this imitation has only the hardness of very soft glass, or
about 5.

Varieties of Quartz

Rock crystal is the term applied to quartz when it is clear and colorless.

Milky quartz is the milky variety, the whiteness being due to imperfections
in the crystallization, such as cracks, bubbles, etc.

Smoky quartz is the cloudy brown-colored variety, which results from the
presence of small quantities of organic matter (hydrocarbons) in the quartz.
If the color is so dark as to be almost black it is termed morion. In the
above cases the color will disappear if the stone is heated. Pebbles of smoky
quartz from Cairngorm, Scotland, have been so widely used as
semiprecious stones that they have come to be known as cairngorms.

Citrine, or false topaz, is a clear yellow variety, the color again due to the
presence of organic matter. It is distinguished from true topaz by the 104
lesser hardness, this having the hardness of 7, while true topaz has a
hardness of 8.

Amethyst is quartz with a violet color, due to the presence of small
quantities of manganese. To be suitable for cutting into gems, the color
must be deep or the small pieces will appear almost colorless. It is widely
used today as a semiprecious stone in jewelry; and in the fifteenth century it
had the traditional virtue of making the wearer sober-minded, whether he
had taken too freely of wine, or was over excited by love-passion.

Rose quartz gets its pale-red color from the presence of a small amount of
titanium. It is widely distributed, but is more abundant in the Black Hills of
South Dakota.

Aventurine is quartz which has inclosed tiny scales of mica or hematite
giving it a spangled appearance.

Prase is a green quartz, the color being due to the inclusion of fibrous
crystals of green actinolite.

Cat’s Eye is a quartz which has inclosed silky fibers of asbestos. When this
is cut parallel to the fibers, the effect is opalescent. The colors are greenish,
yellowish-gray, and brown. This form, however, is not to be confused with
the true or Oriental Cat’s Eye, which is chrysoberyl and has the hardness of
8.

Chalcedony
SiO₂

Non-crystalline, occurring in botryoidal, stalactitic or concretionary masses;
hardness, 7; specific gravity, 2.65; color white when pure; luster waxy;
translucent to transparent on thin edges.

In addition to the crystalline form, silica is freely deposited in an 105
amorphous or cryptocrystalline form which has the same properties
as quartz, except the crystal faces. This is called chalcedony, and it occurs
in seams, cavities and free surfaces. When the surface of a chalcedony
deposit is free it has a waxy luster. It is generally very brittle and breaks in a
peculiar splintery manner. Like quartz it also has a great many varieties,
according to the impurities present. Its wide distribution, hardness, and the
manner in which it can be chipped have made this a most important stone in
the history of the development of civilization. The early men first broke it
into rough tools, such as knives, axes, spear points, etc., and used these as
cutting tools, of one sort or another, because they held their edge better than
most stones. We apply, to the people who used only these chipped stones as
tools, the term “Men of the Old Stone Age,” or the period is termed the
Palæolithic Age. Later men learned how to grind the edge to a smoother
outline, and this much shorter period is termed the Neolithic Age. The use
of flints for the first tools is world-wide, and the American Indian when
discovered was still using chalcedony in its rough-hewn state.

“There the ancient Arrow-maker
Made his arrow heads of sandstone,
Arrow heads of chalcedony,
Arrow heads of flint and jasper,
Smoothed and sharpened at the edges,
Hard and polished, keen and costly.”

Chalcedony is the proper term to use when the color is white to 106
translucent, in which case the surfaces are usually botryoidal and
waxy.

Carnelian is chalcedony which is clear red in color and translucent. This is
one of the first stones used for ornamental purposes and for engraving.
Carnelians with figures engraved on them were used by the Egyptians,
Assyrians and The Children of Israel, at least 2000 B.C.; and the Egyptian
scarabs of the fifth or sixth century B.C., were often carved from this variety
of chalcedony, as well as from jasper and agates.

The brownish varieties are termed sard.

Chrysoprase is an apple-green variety of chalcedony the color being due to
the presence of nickel oxide. This is by no means as common as most of the
varieties of chalcedony, and was long prized as a gem.

Plasma is chalcedony with a leek- to emerald-green color, and the same
stone when it has small red spots of jasper in it is termed blood-stone, or
heliotrope. These red spots are said by tradition to be drops of the blood of
Christ.

Jasper is a deep red chalcedony, the color being due to hematite, which is
so abundant as to make it opaque. A brown variety colored by limonite is
also called jasper, and even green jaspers are found. In all cases the opaque
character is common.

Flint is an impure brown chalcedony, usually forming concretions. The
color is due to organic matter. Flint is mostly found in limestone or chalk,
and the concretions are the result of the small particles of silica scattered
through the rock being dissolved, and then reprecipitated about some 107
organic center. Generally the silica was obtained by the dissolution of
small fossils, like the shells of diatoms or sponge spicules.

Hornstone and Chert are simply impure varieties of flint, brown in color,
and with a splintery fracture.

Agate, Plate 32, is a banded or cloudy chalcedony which has formed in a
cavity, the layers of different color representing deposition from water,
carrying first silica with one impurity, then later, silica with another
impurity. Gradually the cavity has been thus filled with silica; and when the
mass is freed by the weathering away of the surrounding rock, these banded

masses are found. Sometimes the manner of deposition has changed, and
while the outer part of the cavity was filled with chalcedony, the central part
will contain quartz crystals. On account of the beauty of the colors, and the
unusual way in which they may be developed, agates are widely used for
semiprecious jewelry and objects of art, and this has been true since ancient
times, the name itself coming from the River Achates in Sicily. The center
for cutting and polishing agates is at Oberstein, Germany, where this work
has been carried on since the middle of the fifteenth century. In spite of the
many fine natural colors in agates, they are sometimes artificially colored,
in many cases by methods which are kept as “trade secrets.” The color
seldom penetrates far; so that even slight chipping reveals whether an
inferior agate has been taken and colored up, or whether the stone is 108
natural. Moss agates are chalcedony which has inclosed dendritic
masses of some one of the manganese compounds as shown under
manganite, p. 73.

Onyx is a variety of agate where the bands are alternately black and white;
while sardonyx is agate with red or brown bands alternating with the white.
Such agates as these are especially desirable for cameo work, where the
figure is carved in the chalcedony of one color, and the other color makes
the background.

Silicified or agatized wood is a form of chalcedony, where silica has
replaced wood, molecule by molecule; so that in good specimens, all the
structure of the wood is still retained, and when thin sections are made it
can be studied under the microscope almost as well as modern wood. This
takes place under water, usually, if not always, in fresh water. Such
fossilized wood is widely distributed in the western United States, the most
famous cases being the Fossil Forest of Arizona, now a National
Reservation, and the fossil trees in the Yellowstone National Park.

Opal
SiO₂·H₂O
Pl. 33

Non-crystalline, massive, stalactitic or nodular; hardness, 6; specific gravity
2; all colors; luster vitreous, resinous, or pearly; transparent on thin edges.

Opal differs from chalcedony in having water, usually about 10%,
incorporated in its structure. This is water of crystallization, and not firmly
held; so that, if opal is heated in a closed tube to above 100 C., it is given
off as a vapor. Opal is distinguished from chalcedony by its lesser 109
hardness, and the resinous to pearly luster. It forms in cavities, in
layers often of extreme thinness.

Opal is originally the product of the dissolution of silicate minerals in hot
acid waters, the resulting gelatinous silica, when it is deposited and
hardened, becoming the opal. There are many varieties, some of them
highly prized as gems in spite of the moderate hardness and opacity of the
mineral. Gem-quality opal gets its opalescent character from the successive
deposition of thin films of opal, the light penetrating and being reflected
from different films. This breaks up the white light and causes the play of
colors which is the charm of this gem.

Precious opal, in which the play of colors is finest, comes mostly from
Hungary, Mexico, and Queensland. The opal was a favorite stone from
before Roman times, and in its early history was a charm against the “evil
eye.” During the nineteenth century for some reason it came to be
considered an unlucky stone.

Fire opal is a hyacinth-red to honey-yellow variety, which has a fire-like
play of color, and is found in Mexico and Honduras.

Common opal does not have the play of color, but comes in a variety of
colors; is waxy or greasy in luster; and occurs mostly as fillings of seams or
cavities, especially those in igneous rocks, like the steam holes in lavas, etc.
It is found in Cornwall, Penn., in Colorado, California, etc.

Opal-agate is a variety in which there are color bands, and it is widely
distributed.

Opalized wood is formed in exactly the same manner as agatized 110
wood, much of the fossil wood called silicified being really opalized.

Siliceous sinter is the porous mass of opal which is so frequently deposited
about hot springs and geysers. It is readily recognized by its porous
character.

The shells of the diatoms, which are microscopic plants, are made of opal;
and while they are so small, there is certainly no other plant so abundant or
omnipresent, living as it does in every pool, lake, or sea by the millions.
These shells are very indestructible so that they accumulate at the bottom of
ponds, bogs, and sea-bottoms, making at times extensive deposits. This
material in quantities is termed diatomaceous earth, or tripolite (from
Tripoli where it was first used commercially). It is used as a polishing
powder for metals, marble, glasses, etc.

The Feldspars

The term feldspar is a family name for a large variety of very common
minerals, which altogether make up nearly 60% of the crust of the earth,
being the predominant part of granites, gneisses, and lavas. In composition
they are silicates of aluminum, together with potassium, sodium and
calcium, and their mixtures. They may be tabulated as follows:

1. KAlSi₃O₈, orthoclase, the silicate of aluminum and potassium.
2. NaAlSi₃O₈, albite, the silicate of aluminum and sodium.
3. CaAlSi₂O₈, anorthite, the silicate of aluminum and calcium.
4. Mixtures of 1 and 2 are alkalic feldspar.
5. Mixtures of 2 and 3 are plagioclase feldspar.

Orthoclase is monoclinic, but the rest of the feldspars are triclinic. If 111
crystals are available they may be short and stout, or tabular and thin,
but as the feldspars are mostly components of the igneous rocks, where
perfect crystals have not had a chance to grow, they are mostly determined
by their hardness and cleavage. The hardness of all the feldspars is 6 or very
close to it.

They all have three planes of cleavage, two of which are good and intersect
either at 90° as in orthoclase, or at about 86° as in the plagioclase series;

while the third cleavage plane is imperfect. In figure 1, Plate 34, a and b are
the two perfect cleavages, while c is the imperfect one. Breaking into such
cleavage masses as the one illustrated is characteristic of feldspar. The
specific gravity ranges from 2.55 to 2.75. The luster is vitreous, and the
color white, ranging to various shades of gray and pink, and, sometimes in
recent lavas, colorless.

Twinning is very common and helps to distinguish orthoclase from the
plagioclase feldspars. In orthoclase the twins are simple, that is, only two
crystals growing together, and are united on one of the faces, as if one of
them had been revolved 180° with the other; or, while related to each other
as in the preceding case, they may seem to grow through each other. On
plate 34 are three orthoclase crystals showing this simple type of twinning.
The first (A) is a simple crystal; the second (B) shows the simplest type of
twinning where the left-hand crystal has revolved 180° on the p face, 112
and the end is composed, half of the upper end of one crystal, and
half of the lower end of the adjacent crystal. The presence of reëntrant
angles calls attention to the twinning. The third figure (C) is a case of
intergrowing crystals.

In the plagioclase feldspars twinning is multiple, a large number of crystals,
each thin, sometimes as thin as paper, growing side by side, the first one in
normal position, the next at 180° with it, the third revolved 180° to the
second and thus parallel to the first, and so on. The result is first of all a
striated appearance, and second that, as plagioclase crystals have their
prism faces intersecting at 86°, there is a series of low roofs and valleys,
which are best seen by holding the piece of feldspar so the light reflects
from a cleavage face, when it will appear striated; then by tilting it about 8
degrees a second set of reflections, also appearing striated, will appear. The
light was first reflected from one side of the roofs, and in the second case
from the other side. Figure D, Pl. 34, is a diagram showing the relation of
the individual crystals in a multiple twinned piece of plagioclase, in which
the crystals are represented as rather large. Plate 35, under labradorite,
shows a photograph of a cleavage piece, on which is readily seen the
striation which is characteristic of the plagioclase feldspars.

Mixtures of albite and anorthite occur in bewildering numbers, one or the
other predominating, and each mixture being uniform throughout the crystal
and in the whole mass; so each combination is a mineral, each with its
special properties; but the different plagioclase feldspars are so 113
similar in appearance, that by the naked eye it is impossible to
separate the closely related ones. This can be done under the microscope by
studying the angles at which light is cut off, and also by chemical analyses.
For our purposes six types will suffice to illustrate the group, and their
composition may be indicated as follows.

Albite is albite with up to 15% of anorthite mixed with it.

Oligoclase is albite with from 15-25% of anorthite mixed with it.

Andesite is albite with from 25-50% of anorthite mixed with it.

Labradorite is anorthite with from 25-50% of albite mixed with it.

Bytownite is anorthite with from 15-25% of albite mixed with it.

Anorthite is anorthite with up to 15% of albite mixed with it.

The best method for distinguishing these feldspars of the plagioclase group
is to measure the angle between the two perfect cleavage faces, and even
this requires careful measurement. The angles between these faces are as
follows:

Orthoclase 90°
Microcline 89° 30′
Oligoclase 86° 32′
Andesite 86° 14′
Labradorite 86° 14′
Bytownite 86° 14′
Anorthite 86° 50′

Orthoclase
KAlSi₃O₈

Occurs in granites, syenites, gneisses and light-colored lavas; hardness, 6;
specific gravity, 2.57; color white to gray or pink; cleavage in two
directions perfect and at 90°, in the third direction imperfect; luster 114
vitreous; translucent on thin edges.

Orthoclase is monoclinic, and when formed in cavities develops as crystals,
but it is usually a constituent of igneous rocks, in which case the crystals
have not had the opportunity to develop the crystal faces, and the orthoclase
is in grains or irregular masses; and the best way of determining the mineral
is the cleavage, the two perfect cleavage planes intersecting at right angles.
Twinning is frequent but of the simple type, only two crystals being united,
similar to either B or C on plate 34.

It is found in granites, gneisses or lavas, wherever they occur, being
especially characteristic of the granites of the Rocky Mountains.

Microcline
KAlSi₃O₈
Pl. 35

Occurs in granites and gneisses as crystals or irregular masses; hardness, 6;
specific gravity, 2.56; color white to gray, pink, or greenish; luster vitreous;
translucent on thin edges.

Microcline has the same composition as orthoclase, but is in the triclinic
system, the c axis being inclined a half degree away from a right angle with
the b axis. This is best seen in the cleavage pieces, the two perfect cleavage
planes meeting at 89° 30′, and this is the only test for determining this
mineral by the unaided eye. Pike’s Peak is the best known locality for
microcline, and there it occurs in fine large crystals of greenish color, which
are known as Amazon stone.

115

Albite
NaAlSi₃O₈

Occurs in small crystals, or more often in lamellar masses in granites or in
seams in metamorphic rocks; hardness, 6; specific gravity, 2.62; color white
to gray; luster vitreous.

Albite may occur in simple crystals, in which case the two perfect cleavage
planes meet at an angle of 86° 24′. However, it is much more frequently
found twinned in the multiple manner, the individual crystals often being as
thin as paper. This gives rise to a fine striation on the end of a crystal, or on
the surface made by the imperfect cleavage plane. Where the crystals are
extremely thin, the surface may have a pearly luster. Albite types of granite
often inclose secondary minerals, that are prized as gems, such as topaz,
tourmaline, and beryl.

It is found at Paris, Me., Chesterfield, Mass., Acworth, N. H., Essex Co., N.
Y., Unionville, Penn., and in Virginia, and throughout the Rocky Mountains.

Oligoclase
(NaCa)AlSi₃O₈

Generally found in cleavable masses in granites and lavas, rarely in
crystals; hardness, 6; specific gravity, 2.65; color white, greenish or pink;
luster vitreous; translucent on thin edges.

Oligoclase is a plagioclase feldspar and is distinguished by its two perfect
cleavage planes meeting at an angle of 86° 32′, but otherwise it is very like
albite. Crystals are not common, and it occurs mostly in masses, making
one of the components of granite or lava.

It is found in St. Lawrence Co., N. Y., Danbury and Haddam, Conn., 116
Chester, Mass., Unionville, Penn., Bakersville, N. C., etc.

Labradorite
(NaCa)AlSi₃O₈
Pl. 35

Usually found in cleavable masses in granites and lavas; hardness, 6;
specific gravity, 2.71; color gray or white, often with a play of colors; luster
vitreous; translucent on thin edges.

Labradorite is distinguished by having the two perfect cleavage planes meet
at 86° 14′. The iridescent play of color is also very characteristic and is
generally present. It is due to the inclusion of minute impurities. This
feldspar is usually associated with granites or lavas in which the dark
minerals predominate. It gets its name from being the feldspar of the
granites of Labrador, and is also found in the granites of the central part of
the Adirondack Mountains and the Wichita Mountains of Arkansas.

The Pyroxene Group

The minerals of this group are generally associated with feldspars, and
make the dark-colored component of granites, gneisses and lavas. This is
especially true of those which have some iron in the crystal. Pyroxenes are
salts of metasilicic acid (H₂SiO₃), in which the hydrogen (H) has been
replaced by calcium, magnesium, iron, etc. The commoner minerals are
orthorhombic or monoclinic, and all agree in their crystal habit, being short
stout prisms, with the vertical edges so beveled that a cross section is eight-
sided. The cleavage is good in two directions, parallel to the beveling 117
faces (m in figure b, Plate 36), and they intersect at an angle of 87°.
This is very characteristic, and if one has a crystal broken across, it is easy
to see and measure this angle of intersection. These pyroxenes have the
same chemical composition as the corresponding series of amphiboles, but
the two are distinguished by several features. Pyroxenes are short and stout
crystals, while amphiboles are long and either blade- or needle-like;
pyroxenes are eight-sided in cross section, while amphiboles are six-sided;
in pyroxenes the cleavage planes intersect at 87°, while in amphiboles they
intersect at 55°. The minerals of this group are most frequently one of the

components of a lava or granite, and are less frequently associated with
metamorphic rocks. Three are common; enstatite, hypersthene, and augite.

Enstatite
MgSiO₃

Usually occurs in lamellar or fibrous-lamellar masses in dark lavas;
hardness, 5.5; specific gravity, 3.3; color gray, bronze or brown; luster
vitreous, translucent on thin edges.

Enstatite rarely occurs in crystals, but when it does they are orthorhombic.
Usually it is in irregular masses with the cleavage angles, typical of
pyroxene. The color is light, that is gray or brownish, and the streak white
or nearly so. In most respects it is similar to hypersthene, which has the
same composition, except that a large part of the magnesium is replaced by
iron, and there are all sorts of gradations between the two minerals. When
some iron takes the place of magnesium, the color darkens to, or 118
towards bronze, until when about a third of the magnesium is so
replaced, and the color is fully bronze, this variety is called bronzite.
Bronzite is present in some of the dark lavas like gabbro and peridotite.
Enstatite is found in the Adirondack Mountains, at Brewster and Edwards,
N. Y., etc.

Hypersthene
(MgFe)SiO₃

Occurs in cleavable masses in dark lavas; hardness, 5.5; specific gravity,
3.4; color dark-brown or greenish-brown; luster vitreous; translucent on
thin edges.

Hypersthene is a pyroxene in which magnesium and iron are present in
about equal quantities. It is similar to enstatite, except that the color is
darker, and the streak gray or brownish-gray in color. These two minerals
grade into each other, so that there are cases where it is simply a matter of

preference as to which name should be given to the mineral. This form is
associated with dark lavas, of the gabbro or peridotite type, in such places
as the Adirondack Mountains, Mount Shasta in California, Buffalo Peaks,
Colo., etc.

Augite
CaMg(SiO₃)₂, MgAlSiO₆ + Fe₂O₃
Pl. 36

Usually occurs in short stout monoclinic crystals; hardness, 5.5; specific
gravity, 3.3; color dark-green to black; luster vitreous; translucent on thin
edges.

Augite is a complex pyroxene having some iron and aluminum always
present in it, but the amount not a fixed quantity. It is by far the 119
commonest of the pyroxenes and has a wide distribution, both in the
sorts of lavas in which it appears, and in the world. It is commonly the dark
component of such lavas, as gabbros and peridotites, and also is common in
metamorphic rocks, especially impure crystalline limestones. It is found at
Raymond and Mumford, Me., Thetford, Vt., Canaan, Conn., in Westchester,
Orange, Lewis and St. Lawrence Counties of N. Y., in Chester Co., Penn., at
Ducktown, Tenn., Templeton, Canada, etc.

The Amphibole Group

The amphiboles are a group of minerals made up of the same chemical
elements as the pyroxenes, but with the molecular arrangement different,
which appears in the forms of the crystals. The commoner ones are all
monoclinic but contrast with the pyroxenes as follows. Amphiboles are long
and slender crystals, while pyroxenes are short and stout; amphiboles are
six-sided, while pyroxenes are eight-sided; amphiboles have the two perfect
cleavages intersecting at 55° and 125°, while those of pyroxene intersect at
87° and 93°. With the above in mind it is easy to place the minerals in their
proper group, but inside the group it is not always so easy to distinguish one

from another. This group is associated rather with metamorphic rocks than
with igneous rocks, with which the pyroxenes are mostly associated. The
three commoner minerals of the group are tremolite, actinolite, and
hornblende.

120

Tremolite
(CaMg)₃(SiO₃)₄
Pl. 37

Occurs in long prismatic crystals or in columnar or fibrous masses;
hardness 5.5; specific gravity, 3; color white to gray; luster vitreous;
transparent on thin edges.

The long prismatic crystals of tremolite occur especially where dolomitic
limestones have been altered by metamorphism. Sometimes these crystals
grow side by side, making fibrous masses, where the long slender crystals
can be picked apart with the fingers, and yet are flexible, and tough enough
so that they can be felted together. This is termed asbestos, which, because
it is infusible and a poor conductor of heat, is much used to make insulators,
fire-proof shingles, and all sorts of fireproof materials. The varieties in
which the crystals are finer and silky in appearance, like the one illustrated
on Plate 38 are termed amianthus. There are other minerals, such as
actinolite and serpentine, which occur in the same manner, and are also
called asbestos, the serpentine variety being just now the most important
commercially.

Tremolite is found at Lee, Mass., Canaan, Conn., Byram, N. J., in Georgia,
etc.

Actinolite
(CaMgFe)₃(SiO₃)₄

Occurs in radiating crystals, or in fibrous masses; hardness, 5.5; specific
gravity 3; color pale- to dark-green; luster vitreous; translucent on thin

edges.

Except for its green color, this mineral is very like tremolite. The difference
between the two is due to the small amount of iron in the actinolite. It is
usually found in schists, and the radiating character of the crystal 121
groups is enough to determine the mineral, if it is already clear that it
is one of the amphiboles. Occasionally it occurs with the crystals parallel to
each other, making one of the forms of asbestos.

Actinolite is found at Warwick, Edenville, and Amity in Orange Co., N. Y.,
at Franklin and Newton, N. J., Mineral Hill and Unionville, Penn., Bare
Hills, Md., Willis Mt., Va., etc.

Hornblende
(CaMgFe)₃(SiO₃)₄CaMgAl₂(SiO₄)₃
Pl. 37

Occurs in well-defined crystals, in grains and in masses; hardness, 5.5;
specific gravity 3.2; color black, dark-green, or dark-brown; luster vitreous;
translucent on thin edges.

In composition hornblende corresponds to augite, but occurs in long
slender, six-sided crystals with cleavage planes intersecting at 55°, so that it
is a typical amphibole. It occurs in a very wide range of rocks, such as
granite, syenite, diabase, and gabbro; and in such metamorphic rocks as
schists and gneisses; and sometimes igneous rocks are made up almost
entirely of hornblende, when they are known as amphibolites or
hornblendite. It is found all through the New England States, down along
the Piedmont Plateau, through the Blue Ridge Mountains, and in many of
the western mountainous areas.

The Garnet Group

The garnets are a series of double silicates, which occur with surprisingly
uniform characters. They are all isometric, and occur either as 122
dodecahedrons, or as the 24-sided figure (the trapezohedron), which
is formed by the beveling of the edges of the dodecahedron, and developing
these new faces to the exclusion of the dodecahedron faces. Combinations
of the dodecahedron and trapezohedron (36 faces) may occur. All the
garnets have a hardness of 7 to 7.5, and the specific gravity runs from 3.2 to
4.3, according to the composition. In size they run from as small as a grain
of sand up to as large as a boy’s marble, and occasionally even to four
inches in diameter. The color varies with the composition, from colorless to
yellow, red, violet, or green. There is no cleavage, and the luster is always
vitreous.

Garnets are usually accessory minerals, found in metamorphic rocks,
though they are sometimes also present in granites and lavas. They are
always segregations which have taken place in the presence of high
temperatures. When clear and perfect several of the garnets are used as
gems. On the other hand some of the common garnets occur in such
quantities that they are crushed and used as abrasives, for such work as
dental polishes, or for leather and wood polishing.

The following is the composition of some of the commoner garnets.

Ca₃Al₂(SiO₄)₃ = grossularite
Mg₃Al₂(SiO₄)₃ = pyrope
Fe₃Al₂(SiO₄)₃ = almandite
Mn₃Al₂(SiO₄)₃ = spessartite
Ca₃Fe₂(SiO₄)₃ = andradite
Ca₃Cr₂(SiO₄)₃ = uvarovite
123
Grossularite is chiefly found in crystalline limestones, which have
resulted either from contact with lavas, or from general metamorphism of
impure limestones. These garnets are colorless to white, or more often
shades of yellow, orange, pink, green or brown, according to traces of
impurity which they may contain. The cinnamon-colored variety from
Ceylon is termed cinnamon stone, and is a fairly popular gem.

Pyrope is a deep-red color and when perfect is highly prized as a gem. It is
found in dark-colored igneous rocks, like lavas, or serpentines. Some of the
finest come from South Africa, where they are found in company with the
diamond.

Almandite is dark-red to brown in color, the brownish-cast distinguishing it
from pyrope. It is one of the garnets known as “common garnet.” In some
cases it is clear and deep colored enough to be used as a gem, but mostly it
is muddy in appearance. The name almandite comes from Alabanda, a city
of the ancient district of Caria, Asia Minor, whence garnets were traded to
ancient Rome. The finest garnets “Sirian garnets” came from the city of
“Sirian” in Lower Burma, and were supposed to have been found near
there, but careful investigation shows that no garnets occurred near there,
and this town was therefore, even at that early time, a distributing point for
garnets, found probably further to the east. The “Sirian” garnet had a violet
cast and now the term is used to indicate a type of garnet, rather than a
locality.

Spessartite is dark-hyacinth-red, or red with a violet-tinge, and is one of
the less-common garnets. It is usually found in granites. The finest 124
garnets of the type come from Amelia Court House, Va., which has
yielded some ranging from one up to a hundred carats.

Andradite is another garnet which is termed “common garnet.” It is red in
color, but with a yellowish-cast which distinguishes it from almandite, but
these two are not easy to separate. It is found mostly in metamorphosed
limestones. One variety is black in color and called malanite. It is found in
lavas. The common yellowish-red garnets are found through New England
and the Piedmont Plateau.

Uvarovite is a rare garnet of emerald-green color, found in association with
chromium ores.

The number of localities for garnets is so great that a list would suggest
most of the regions where metamorphic rocks occur, as all over New
England, throughout the Piedmont Plateau, the Rocky Mountains, etc.
Certain fine clear garnets, found in Montana, northeastern Arizona, and
northwestern New Mexico are sold under the trade name of “Montana,

Arizona or New Mexico rubies.” These are of fine quality and are mostly
collected by the Indians from the ant hills and scorpion’s nests of those
regions.

Garnets are among the earliest stones mentioned in ancient languages, as
would be expected from the way these hard and beautiful crystals weather
out of the much softer metamorphic rocks, like schists. In the past they,
with most any other translucent red stone, were included under the name
carbuncle. This, however, is not the name of any mineral, but refers 125
rather to a mode of cutting, en cabochon or with a convex surface.

Glucinum

Glucinum is a rare metal, silvery-white in color, malleable, and melting at a
fairly low temperature. It is found in the mineral beryl, from which has
come the alternative name beryllium. The name comes from the sweet taste
of its salts. Except for beryl its minerals are rare, and the metal has found
but few uses for man.

Beryl
Gl₃Al₂(SiO₃)₆
Pl. 39

Occurs in hexagonal crystals in granites, gneisses and mica schists;
hardness, 7.5; specific gravity, 2.7; color usually some tint of green; luster
vitreous; transparent on thin edges.

When this mineral occurs in coarse hexagonal prisms, with or without faces
on the ends, it is known as beryl; when the crystals are clear and perfect and
of a dark-green color, they are of gem value and are termed emerald; when
of a light-green color, they are aquamarine; and when bright-yellow in
color, they are the golden beryl. There is little difficulty in determining
beryl, for only apatite occurs in such crystals, and is green, and this latter
mineral has a hardness of only 5. There is an imperfect basal cleavage.

Ordinary beryl is fairly common in granites of the pegmatite sort, and less
common in gneisses and mica-schists. This type often furnishes crystals of
large size, up to two and three feet in diameter.

Beryl which is free from cracks and inclosures, so it can be used as a 126
gem, is so rare, that the emerald has a value above that of the
diamond, and second only to the ruby. It is one of the gems with a long
history, having been quarried on the west coast of the Red Sea at least 1650
B.C. by the Egyptians. To early people it had a power to quicken the prophet
instinct and made the wearer see more clearly. The Spanish conquistadores
found fine emeralds among the treasures of both Mexico and Peru. In the
United States, Stony Point, N. C., was a notable locality for these gems, but
now seems to have been exhausted. The name emerald has been applied to
many other green stones, usually with some geographical modification, as
“Oriental emerald” which is green corundum, “Brazilian emerald” which is
tourmaline, etc.

Giant beryls have been found at Acworth and Grafton, N. H., and at
Royalston, Mass. Localities for ordinary beryl are Albany, Norway, Bethel,
Hebron, Paris, and Topsham, Me., Barre, Goshen and Chesterfield, Mass.,
New Milford and Branchville, Conn., Chester and Mineral Hill, Penn.,
Stony Point, N. C., and many other localities in the Appalachians; also
Mount Antero, Colo., and in the Black Hills of South Dakota.

Sodalite
Na₄Al₃Cl(SiO₄)₃

Occurs in irregular masses, sometimes in dodecahedrons; hardness, 5.5-6;
specific gravity, 2.3; color deep-blue to colorless; streak white; luster
vitreous; translucent on thin edges.

This striking mineral, with its deep-blue to azure color, is not easily 127
confused with any other. It is characteristic of soda-rich igneous
rocks such as syenite and some lavas. In this country it is found at
Litchfield, Me., and Salem, Mass.

Zircon
ZrSiO₄
Pl. 39

Usually occurs in tetrahedral crystals in igneous rocks; hardness, 7.5;
specific gravity, 4.7; color brown; luster vitreous; translucent on thin edges.

Zircon, the mineral of the rare earth element zirconium, nearly always
occurs in light-colored igneous rocks, like syenite. It may occur in schists or
gneisses, but in these rocks the crystals are of microscopic size. Because of
their great hardness and insolubility, zircon crystals resist weathering and
are often found, along with gold, cassiterite, or magnetite, in sands which
have resulted from the disintegration of syenite rocks.

Zircon refracts and disperses light to a degree second only to the diamond,
so that clear crystals are sought as gems. They are often called “Matura
diamonds” because of their abundance at Matura, Ceylon. When the
crystals are colorless or smoky they are termed jargons or jargoons; when
of a red-orange hue, they are hyacinth or jacinth. Most of the zircon of
gem-quality comes from Ceylon, where it is picked up as rolled-pebbles
from the beds of brooks.

The most remarkable American locality for zircon is near Green River, in
Henderson Co., N. C., where it is found abundantly in a decomposed
pegmatite dike, from which many tons have been obtained. It is also found
at Moriah, Warwick, Amity and Diana, N. Y., at Franklin Furnace, 128
and Trenton, N. J., in the gold-bearing sands of California, etc.

Cyanite
Al₂SiO₅
Pl. 40

Occurs in long blade-like crystals in gneisses and schists; hardness, 7 at
right angles to the length, and 4.5 parallel to the length; specific gravity,
3.6; color blue; luster vitreous; translucent on thin edges.

There are only a few blue minerals, and the way in which cyanite occurs in
long thin blade-like crystals is entirely characteristic. If more is still wanted
to determine this mineral, its unique character in having the great hardness
7 when scratched parallel to the length, and only 4.5 when scratched
crossways, will settle any doubts.

The mineral sillimanite has the same composition as cyanite, but is fibrous
in habit and has the hardness 6.5. If cyanite is heated to 1350° C. it changes
its character and becomes sillimanite.

Cyanite is found as an accessory mineral in metamorphic rocks, such as
gneiss and schist, at Chesterfield, Mass., Litchfield and Oxford, Conn., in
Chester Co., Penn., in North Carolina, etc.

The Mica Group

The micas are very common minerals, easily recognized by their very
perfect basal cleavage, as a result of which thin sheets, often less than a
thousandth of an inch in thickness, readily split off. These are tough and
elastic, which distinguishes mica from the chlorite group in which 129
there is similar basal cleavage, but the sheets are not elastic.

Micas are complex silicates of aluminum, with potassium, iron, lithium,
magnesium and hydrogen. They are one of the principle components of
many granites, gneisses, and schists. This mineral is always crystalline,
being in the monoclinic system, but occurring in six-sided prisms. The
cleavage is so dominant a character that the crystal form is usually
overlooked, as it is seldom requisite in determining this mineral. The size of
the sheets of mica depend on the size of the crystals, the larger sheets
expressing great slowness in cooling from the original magmas. Sometimes
the crystals may be two or even three feet in diameter. The hardness is not
great, ranging between 2 and 3. The specific gravity lies between 2.7 and
3.2. The color varies according to the composition, from silvery-white,
through gray, pink, and green to black. The luster is vitreous to pearly,
sometimes gleaming in the darker-colored varieties. The commoner types of
mica are as follows:

Muscovite, H₂KAl₃(SiO₄)₃ or potash mica.
Lepidolite, LiK(Al₂OH·F)Al(SiO₃)₃ or lithia mica.
Biotite, (HK)₂(MgFe)₂Al₂(SiO₄)₃ or iron mica.
Phlogopite, H₂KMg₃Al(SiO₈)₃ or magnesia mica.

Muscovite is colorless, silvery-white, gray or sometimes pale-green or
brown. It gets its name from Moscow where it was early used for window
panes, and it is still used for stove and furnace doors, as well as in 130
electric work, for a lubricant, etc.

The best crystals occur in granites, in the coarse varieties of which large
crystals may be obtained. It is found also as small scales in gneisses and
schists, and when weathered from its original rocks it may be present in
sandstones and shales. Muscovite is always in its origin an elementary
component of deep-seated igneous rocks, like granite; but is never a
component of extruded lavas. Sericite is muscovite which has been
secondarily produced by the alteration of other minerals into muscovite, as
when feldspar, cyanite, topaz, etc., have been modified by the presence of
heat and hot vapors, when near lavas that have come in contact with other
rocks. Muscovite is very resistant to alteration by weathering, but when it
does change, the greater part of it becomes kaolin. It is found at Acworth
and Grafton, N. H., in plates, sometimes a yard across at Paris, Me.,
Chesterfield and Goshen, Mass., Portland and Middletown, Conn., at
Warwick, Edenville, etc., N. Y., and all down the Appalachian Mts., also in
the Rocky Mts., the Cascade Range, etc.

Lepidolite is pink or lilac in color and occurs in scaly masses, mostly in
granites. It does not come in large crystals. Lepidolite is found at Paris and
Hebron, Me., Middletown, Conn., Pala, Calif., etc.

Biotite is dark-brown or black mica. Like muscovite it is very common,
making one of the chief components of granites, gneisses and schists; 131
and, unlike muscovite, it may occur in extrusive lavas, like trachyte,
andesite, and basalt. It resists weathering much less than muscovite, so that,
when the rocks of which it is a component disintegrate, biotite is usually
altered to kaolin and other compounds. It is likely to occur in good-sized
crystals, especially at Topsam, Me., Moriah, N. Y., Easton, Penn., etc.

Phlogopite is pale-brown, often coppery in color, and is most likely to
occur in serpentines, or crystalline limestones or dolomites, often in fine
crystals, of good size. While one of the less abundant micas, this is found at
Gouverneur, Edwards, and Warwick, N. Y., Newton, N. J., and Burgess,
Canada.

Topaz
Al₂F₂SiO₄
Pl. 41

Occurs in crystals mostly; hardness, 8; specific gravity, 3.5; colorless to
pale-yellow; luster vitreous; transparent on thin edges.

Topaz may be colorless, but is more often some shade of yellow, and at
times brown or even blue. Its hardness is characteristic, there being but few
minerals as hard, and it is used to represent the hardness 8 in the Moh’s
scale. The crystals are orthorhombic prisms, with the edges of the prism
beveled and often striated. The ends of crystals usually terminate with a
basal plane, parallel to which there is good cleavage. Between this basal
plane and the prism faces there are usually several sets of small faces as
indicated on Plate 41.

This mineral, as is also true of most minerals containing fluorine, is one of
those which have crystallized out from hot vapors, escaping from 132
igneous magmas. It is associated with such minerals, as tourmaline,
beryl, fluorite, and cassiterite, and occurs mostly in cavities or seams, in or
near granites.

Ordinary topaz, which means crystals that are imperfect by reason of tiny
cracks and impurities is not very rare, but crystals which are perfect and
clear in color are considered gems. Most of the gem-topaz is some shade of
yellow, but may be brown or blue, never, however, pink, as is often seen in
jewelry. The “pinking” is artificial, and done by packing yellow or brown
topaz in magnesia, asbestos, or lime, and then heating it slowly to red heat,
after which it is cooled slowly. If underheated the color is salmon, if
overheated all color disappears. Topaz has been a gem for centuries, the

earliest records coming from Egypt. The name comes from topazios,
meaning to seek, because the earliest known locality, from which it was
gathered, was a little island of that name in the Red Sea, and this island was
often surrounded by fog and hard for those early mariners to find. Here by
mandate of the Egyptian kings the inhabitants had to collect topazes, and
deliver them to the gem-cutters of Egypt for polishing.

Several yellow stones are called topaz, as the “Oriental topaz” which is
corundum and more valuable than topaz itself; and several varieties of
yellow quartz, which go under such names as “Saxon,” “Scotch,”
“Spanish,” and “smoky” topaz. When topaz occurs colorless as in Siberia,
the Ural Mountains, and in the state of Minas Geraes, Brazil, in all of 133
which places it is found as pebbles in brooks, it goes under the name
of “slave’s diamonds.” Brazil is today the chief source of gem-quality
topaz.

Ordinary topaz is found in this country at Trumbull, Conn., Crowder’s Mt.,
N. C., Thomas Mts., Utah, in Colorado, Missouri, and California, etc.

Staurolite
FeAl₅OH(SiO₆)₂
Pl. 41

Occurs in orthorhombic crystals; hardness, 7.5; specific gravity, 3.7; color
brown; luster resinous; translucent on thin edges.

This mineral occurs about equally abundantly in simple crystals similar to
the outline on Plate 41, and in twins which have grown through each other
either at 90° or at 60°. The color is either brown or reddish-brown. In all
cases it is an accessory mineral, occurring in metamorphic rocks, usually
schists, though less frequently in slates and gneisses.

From the seventeenth century on, it has been used as a baptismal stone, and
worn as a charm, legends stating that it fell from the heavens. Fine crystals
have been found in Patrick County, Va., and there is in this region the
legend, that when the fairies heard of the crucifixion of Christ, they wept

and their tears falling crystallized in the form of crosses, such as the one
shown on Plate 41.

Staurolite is found in the schists of New England as at Windham, Me., or
Chesterfield, Mass., and all down the east side of the Appalachian
Mountains to Georgia.

134

Olivine
(MgFe)₂SiO₄
Peridot or Chrysolite

Occurs in grains and irregular masses in dark lavas; hardness 6.5 to 7; specific gravity 3.3;
color bottle- to olive-green; luster vitreous; translucent on thin edges.

Olivine rarely occurs in crystals, but when it does they belong to the orthorhombic system.
The dark-green grains or masses are recognized by the color, considerable hardness and
indistinct cleavage. Serpentine may have a similar color, but its hardness is only 4. In
hydrochloric acid olivine decomposes to a gelatinous mass.

Olivine is typically one of the constituents of the dark lavas, like basalt, gabbro, or
peridotite. It is also a common mineral in meteorites. Olivine, in the presence of water,
alters to other minerals, especially serpentine, with great facility.

It occurs fairly widely wherever the dark lavas are present, as in the White Mountains of N.
H., in Loudoun Co., Va., in Lancaster Co., Penn., and in many localities in the Rocky
Mountains and Cascade Range.

Epidote
Ca₂(AlOH)(AlFe₂)(SiO₄)₃
Pl. 42

Occurs in grains or columnar masses; hardness, 6.5; specific gravity 3.4; color green,
usually a pistachio or yellow-green; luster vitreous; translucent on thin edges.

Rarely epidote occurs in crystals, which belong to the monoclinic system, and may be either
short like the diagrams on plate 42 or long and needle-like. The color and hardness will
suffice to determine this mineral, as almost no other has the peculiar yellowish-green 135
color which is characteristic of this form.

Epidote occurs primarily in metamorphic rocks at or near the contact with igneous rocks; or
it may be a secondary mineral resulting from the weathering of granites, especially along
seams. It sometimes occurs with hornblende in highly folded schists, as in New York City. It
is often a mineral which has resulted from the alteration of other minerals, as pyroxene,
amphibole, biotite, or even feldspars.

It is found at Chester and Athol, Mass., Haddam, Conn., Amity, Munroe and Warwick, N.Y.,
East Branch, Penn., in the Lake Superior region, in the Rocky Mountains, etc.

Tourmaline
(FeCrNaKLi)₄Mg₁₂B₆Al₁₆H₈Si₁₂O₆₃
Pl. 42 & frontispiece

Occurs in three-sided prismatic crystals; hardness, 7; specific gravity, 3.1; colorless, red,
green, brown, or black; luster vitreous; transparent on thin edges.

Tourmaline is readily distinguished from other minerals, as it always occurs in long to short
prisms, which are three-sided in cross section. There is also a tendency for the sides to be
curved as seen on the end view of D, Pl. 42. Frequently the vertical edges of the prism are
beveled with one, two or three faces, grouped about each of the three original edges, and
there are often striations on the prism faces. The ends are terminated by a low
rhombohedron and again there may be a host of modifying faces on the edges and corners of
the end. The common varieties are brown or black in color, but occasionally there 136
may occur green, red, yellow or almost any color. When the crystals are perfect, that
is free from impurities and without tiny cracks, tourmaline becomes a gem of popularity and
value.

Tourmaline is very complex in composition and may vary considerably, the sodium,
potassium, lithium, magnesium, and iron being either more or less abundant or even lacking.
The color is to some extent dependent on the proportions of these elements present, the dark
varieties having more iron, and the light colored tourmalines lacking it. This mineral is one
of those which form from superheated vapors, escaping from molten magmas. It will
therefore occur in veins, often associated with copper minerals, in crystalline limestones, or
in cavities in granites, where it is associated with such minerals, as beryl, apatite, fluorite,
topaz, etc.

If heated tourmaline crystals develop electricity, with the effect of making one end a
positive and the other a negative pole, and then will attract bits of straw, ashes, etc. It was
first introduced into Europe about 1703 from India, and its vogue as a gem has greatly
increased since it was found on Mount Mica near Paris, Me. This Paris, Me., locality was
discovered by two boys, amateur mineralogists, Elijah L. Hamlin and Ezekiel Holmes, who
in 1820 were returning home from a trip hunting for minerals, when, at the root of a tree,
they discovered some gleaming green substance. It proved to be gem-quality tourmaline. A
snow storm that night buried their “claim,” but next spring it was visited and several 137
fine crystals found. Later this locality was systematically worked, and over $50,000
worth of tourmaline taken from the pegmatite seam in the granite, which lay under the
crystals found on the surface. The figure in the frontispiece is one of the crystals from there.

Well known localities are Paris and Hebron, Me., Goshen and Chesterfield, Mass., Acworth
and Grafton, N. H., Haddam and Munroe, Conn., Edenville and Port Henry, N. Y., Jefferson
Co., Colo., San Diego Co., Calif., etc.

Kaolinite
H₄Al₂Si₂O₉
Kaolin

Usually found in whitish clay-like masses; hardness, 2; specific gravity, 2.6; color white to
grayish or yellowish; luster dull.

Kaolinite does not generally occur in crystals, though crystals of microscopic size and
monoclinic forms have been found. It is a secondary mineral resulting from the
decomposition by weathering of feldspars, the calcium, potassium or sodium having been
replaced by water. When found in place it is generally white or nearly white, and is
characterized by its greasy feel.

As granites or other feldspar-bearing rocks are weathered away, the kaolin is washed out by
water, and with other fine material is carried down into lakes or the sea, where it settles to
the bottom and is known as clay. Clay is kaolin with more or less impurities.

Pure kaolin is used for the manufacture of china and white porcelain ware; but when it is
impure, especially when it has iron in it, baking causes the product to turn red or brown, so
that it is only suitable for making tile, bricks, etc. 138

It is found almost anywhere that feldspar rocks are, or have been, exposed to weathering.

Talc
H₂Mg₃(SiO₃)₄

Occurs in scales, or in fibrous, scaly or compact masses; hardness, 1; specific gravity, 2.7;
color white, gray or pale-green; luster pearly; translucent on thin edges.

This mineral is as soft as any, only graphite and molybdenite being of the same hardness,
but both these latter two have a black streak, while the streak of talc is white. The greasy
feel is also characteristic. Talc is very seldom found in crystals, but if they are found, they
will appear like flakes and have a hexagonal cross section, though in reality they belong to
the monoclinic system.

Talc is a secondary mineral which usually results from the exposure of magnesium silicates,
such as pyroxenes or amphiboles, to moisture. In this case, in-as-much as the original rocks
were metamorphic in origin, the talc therefrom will occur in old metamorphic regions. Some
talc is also formed by the action of silica-bearing waters on dolomite. This is likely to be the
case near the contact between dolomite and igneous rocks. Talc is closely related to
serpentine and likely to be found in the same regions.

Talc has come to have a considerable use. Some of it is compact and then called soapstone,
and this was used by the ancient Chinese to make images and ornaments; and our North

American Indians used it to make large pots, to serve as containers for liquids. Some 139
of these pots have been carved out with great skill, so as to be fairly light in
proportion to what they would hold. Pipes and images were also carved from soapstone.
Today we still cut soapstone into slabs to make mantels, laundry tubs and sinks. The scaly
and fibrous varieties are ground, and used in making paper, paint, roofing, rubber, soap,
crayons, toilet powders, etc. The United States produce and use over half the world’s
production, our industries requiring over 100,000 tons of talc a year. Of this 38% goes into
paper, 23% into paint, 18% into roofing, and so on down to toilet powder which uses 2½%,
or 2,500 tons a year.

Talc is found in metamorphosed regions, that is in New England, all down the east side of
the Appalachian Mts., in the Rocky Mts., and the Cascade Ranges, with a large number of
local occurrences. New York State is the leading producer.

Serpentine
H₄Mg₃Si₂O₉
Pl. 43

Occurs in compact, granular or fibrous masses; hardness, 3; specific gravity, 2.6; color
green; luster greasy; translucent on thin edges. Serpentine is never in crystals. Its color and
hardness serve to distinguish it. Like talc it is a secondary mineral resulting from the
alteration, in the presence of moisture, of pyroxenes, amphiboles, and especially, olivine. As
these are often in metamorphic rocks, the serpentine is likely to be associated with
metamorphic rocks. Some serpentine is also the result of the action of silica-bearing water
on dolomite, and this is likely to occur in areas of sedimentary rocks. The fibrous 140
variety of serpentine, chrysolite, usually occurs in seams or veins, and when the fibers
are long, it is used as asbestos. This form of asbestos is the one most used commercially
today, as there are remarkably large deposits of it in the Province of Quebec, which provide
the major part of the world supply. In the United States it is also found in California and
Arizona but only in moderate quantities.

Massive serpentine is used in considerable quantities as an ornamental stone, the green color
varied with streaks and blotches of white, yellow and red, due to various impurities, making
it very effective. It is, however, only suitable for interior work as the weather quickly spoils
the polished surface. This is further discussed under serpentine rock, page 245.

Serpentine is found at Newfane, Vt., Newburyport, Mass., Brewster, Antwerp, etc., N. Y.,
Hoboken, N. J., in Pennsylvania, Maryland, etc.

Chlorite
H₈(MgFe)₅Al₂(SiO₆)₃

Pl. 43

Occurs in monoclinic crystals of six-sided outline, or in scaly flakes or masses; hardness, 2;
specific gravity 2.8; color green; luster pearly on cleavage faces; translucent on thin edges.

Chlorite is a family name, covering a series of closely related minerals, so similar in
appearance that they are best considered under this common name. In many respects they
resemble mica, in the shape of the crystals and the remarkable basal cleavage. At first glance
it is easy to confuse the two, but chlorite scales are not elastic, and when bent, stay 141
bent, instead of snapping back like mica. In fact they look like more or less rotted
micas. This is more than appearance, for chlorites form as a result of the alteration of micas
in the presence of moisture. They are then secondary, and will be found where mica-rocks
have been weathered, as in granites and schists.

They may be expected anywhere that micas have been long exposed, as in New England,
the Rocky Mountains, or the Sierra Nevada or Cascade Ranges. Special localities are
Brewster, N. Y., Unionville and Texas, Penn., etc.

The Zeolites

The zeolites are a group of white minerals, with a pearly luster, light weight, and easy
solubility in acids; which, because their contained water is lightly held, readily boil before
the blowpipe. They are all secondary minerals, which result from the decomposition of
feldspars, when exposed to weathering. They are almost universally found in seams and
cavities of disintegrating lavas. From a group of a dozen or so, three are common enough to
be considered here. They may be found by watching such places, as where trap rock is being
quarried for road material, or being blasted for any reason.

Analcite
Na₃Al₂Si₄O₁₃ + 2H₂O
Pl. 44

Occurs as trapezohedrons in seams and cavities in lavas; hardness, 5.5; specific gravity, 2.2;
colorless, white or pink; luster vitreous; transparent on thin edges.

Analcite usually occurs in the 24-sided form, known as a trapezohedron, as illustrated 142
in figure A, Pl. 44; but it may also occur in cubes with the three faces of the
trapezohedron on each corner. Small crystals are often colorless, but the larger ones are
either white or pink, and are opaque. While the form is the same as that of garnets, the color,
lesser hardness, and the occurrence in lavas will serve to distinguish this mineral. If placed
in hydrochloric acid analcite dissolves to a gelatinous mass.

It is always found in seams and cavities in lavas, as at Bergen Hill and Weehawken, N. J.,
Westfield, Mass., in the Lake Superior region, etc.

Natrolite
Na₂Al₂Si₃O₁₀ + 2H₂O
Plate 44

Occurs as bristling crystals in seams and cavities in lavas; hardness, 5.5; specific gravity,
2.2; colorless; luster vitreous; transparent on thin edges.

Natrolite occurs as beautiful bristling tufts of needle-like crystals, each crystal an
orthorhombic prism with a very low pyramid on the end. This mineral is so easily fusible
that it can be melted in a candle flame, giving to the flame the characteristic yellow color
due to sodium. In hydrochloric acid it dissolves to a gelatinous mass. It is always a
secondary mineral in cavities and seams in disintegrating lavas, and the tuft-like manner of
growth is so characteristic, that once seen, it will always be recognized.

Natrolite is found at Weehawken and Bergen Hill, N. J., at Westfield, Mass., in the Lake
Superior region, etc.

143

Stilbite
H₄(CaNa₂)Al₂(SiO₃)₆ + 4H₂O
Pl. 44

Usually occurs in sheaf-like bundles of fibrous crystals; hardness, 5.5; specific gravity 2.2;
colorless to white, yellow or brown; luster vitreous; transparent on thin edges.

Stilbite crystals are really monoclinic, but on account of almost universal twinning, appear
as if orthorhombic. Like the two foregoing minerals, stilbite is found in the seams and
cavities of disintegrating lavas. It is readily recognized by its habit of forming in sheaf-like
bundles of fibrous crystals. It may also, but more rarely, occur in radiating masses. In
hydrochloric acid it is completely dissolved. It is found in lavas, at Weehawken and Bergen
Hill, N. J., in the Lake Superior region, etc.

Calcium

Calcium is one of the most abundant of metals, but never occurs as such in nature, nor is it
used as a metal by man. In its metallic form it is yellowish-white, and intermediate between
lead and gold in hardness. Exposed to air it soon tarnishes by oxidation, and in water rapidly
decomposes the water, forming the oxide. However, it has a great affinity for other

elements, and makes a large number of minerals, the most important of which are calcite,
aragonite, gypsum and fluorite, while it is an essential component of some garnets,
anorthite, epidote, amphibole and pyroxene. It is very widely distributed as limestone, and is
found in solution in most all natural waters, and in the shells and bones of many animals and
some plants.

144

Calcite
CaCO₃
Pl. 45

Occurs in well defined crystals in incrustations, and in stalactitic, oolitic, and granular
masses; hardness, 3; specific gravity 2.7; colorless to white, or when impure, yellow, brown,
green, red or blue; luster vitreous to dull; transparent on thin edges.

Next to quartz, calcite is the most abundant of all minerals, and occurs in an almost endless
variety of forms, over 300 having been described. It belongs to the hemihedral section of the
hexagonal system, the form of the crystals being all sorts of variations of the rhombohedron,
and combinations of left and right handed rhombohedrons. The cleavage is entirely uniform,
in three directions, parallel to the faces of the rhombohedron, and at an angle of 74° 55′ with
each other. Crystals may occur in the form characteristic of the cleavage, but not often. The
commonest forms are a more or less elongated scalenohedron, made by combining right and
left handed rhombohedrons, so that the resulting pyramid is six-sided, as in figure C, Plate
45. Such a scalenohedron may be combined with other forms in a great variety of ways. The
six-sided prism with the ends terminated by one or more sets of rhombohedral faces is also
fairly common. Twinning occurs occasionally.

The quickest way to determine calcite is by the hardness (3), combined with the fact that it
effervesces, when hydrochloric acid is dropped upon it.

An interesting feature of this mineral is its marked property of deflecting light rays, so that a
line or object placed behind a piece of clear calcite appears double. It was with pieces 145
of calcite from Iceland that this was first seen; so that large transparent crystals of
calcite are still called Iceland spar; and such calcite is used to make the Nichol’s prisms for
microscopes, which are so useful in the study of minerals. This power of refracting light is
present in all minerals, but not to such a marked degree as in calcite. The elongated
scalenohedrons of calcite are often called “dog-toothed spar” from a fancied resemblance
between them and the dog’s tooth.

Calcite is present in solution in the water of the sea and most streams, from which it is
withdrawn by many animals and some plants, to make their shells, and bones. The
foraminifera, some sponges, the echinoderms, corals and molluscs all draw large quantities
from the water in which they live, and build more or less permanent structures from it.
These shells when they fall to the bottom, or after being broken to bits, accumulate on the

bottom and make limestone, which is widely distributed over the country. This same
limestone, when metamorphosed and crystalline, is marble.

Calcite then is readily soluble in water, and streams flowing along crevices and fissures in
limestone dissolve out great cavities or caves, like the Mammoth Cave of Kentucky. Other
water, percolating through the limestone, comes to these cavities saturated with lime in
solution and drips from the roofs and walls; then as part of the water evaporates, it deposits
part of its lime in icicle-like masses, hanging from the roof. Such masses of non- 146
crystalline calcite are called stalactites. Below on the floor of the cave, conical
masses are built up in the same manner where the dripping water falls on the floor. These
are stalagmites. In these limestone caves and in smaller cavities many of the most beautiful
crystals grow. Somewhat similarly, when hot water from deep springs comes to the surface,
it cools and can not carry as much lime, and so around the spring is laid down layer after
layer of non-crystalline calcite making a mass known as travertine. Sometimes this is
colored by iron or other impurities and a banded effect results. Such travertine as the
“Suisun marble” from California, “California onyx,” “Mexican onyx,” and “satin spar” all
belong to this class.

The coral animals, especially in tropical waters precipitate an enormous amount of lime,
until whole reefs are built of lime in this non-crystalline form. In places it is hundreds of
feet thick and hundreds of miles in extent. Some of this coral has become popular for
personal adornment. This is particularly a small, fine-grained variety, Corallum rubrum,
which lives almost exclusively in the Mediterranean Sea. This coral is red in color, varying
all the way from a deep red to white. It grows in small masses, three pounds being a good
sized mass, in water 60 to 100 feet deep, requires some ten years to develop a full-sized
mass. The making of this into beads and ornaments is an Italian industry. The demand is
growing, while at the same time the supply is diminishing, and search is being widely made
for more such coral, but up to the present time with little success. This precious coral 147
is much worn as a protection against the “evil eye” and is widely imitated, apparently
with as much protection to the wearer. When coral beads are offered cheap, they are
probably something else, red gypsum being much used. This and all imitations can be
readily detected by trying a drop of acid in the bead. Coral will effervesce, but gypsum and
other substitutes will not.

The bulk of the shells of most molluscs is made of lime, but the mother-of-pearl layer inside
is usually aragonite. The chalk of the cliffs on either side of the English channel is lime, and
composed of the shells of single celled animals. See p. 213. When lime is deposited in loose
porous masses, as around grass, etc., and below hot springs, this mass is termed calcareous
tufa.

Calcite will be found almost everywhere, some of the localities for the finest crystals being
Antwerp and Lockport, N. Y., Middletown, Conn., the caves of Kentucky, Warsaw, Ill.,
Joplin, Mo., Hazel Green, Wis., etc.

Aragonite
CaCO₃
Pl. 46

Occurs in crystals, in columnar or fibrous masses, or incrustations; hardness, 3.5; specific
gravity, 2.9; colorless, white or amber; luster vitreous; transparent on thin edges.

Aragonite has the same chemical composition as calcite, but it crystallizes in the
orthorhombic system, either in simple forms like A on Plate 46, or twinned, so as to make
forms which seem hexagonal. When in simple crystals its form easily distinguishes it from
calcite and dolomite, but when twinned it appears much like either of these two 148
minerals. From calcite it can then be distinguished by its greater hardness and the fact
that it has cleavage in one direction only, and that imperfect. The cleavage is the only easy
method of distinguishing it from dolomite. However, aragonite is most always easily
distinguished by its habits, for it generally forms long slender crystals, which appear more
like fibers than crystals. Neither calcite nor dolomite is at all fibrous.

Aragonite is much less abundant than calcite, and has resulted, either from deposition from
hot waters, or from waters having sulphates in solution as well as lime. Much of the
travertine, and many stalagmites and stalactites are composed of aragonites, forming as
outlined under calcite. The mother-of-pearl layer in the shells of bivalves is generally
aragonite. The pearly luster of this layer is due to its being formed by the successive
deposition of one thin layer upon another; so that light falling on the mother-of-pearl,
penetrates, part of it to one layer and part to another, and is then reflected. Certain molluscs
have this layer composed of especially thin layers, one, the Unios or freshwater clams, the
other, the “pearl oysters” or Aviculidæ, these latter, however, being only distantly related to
the edible oysters. In the cases, where molluscs of either of these two families are of large
size, large pieces of mother-of-pearl can be recovered, and are used for buttons, handles, and
various ornamental objects. A further peculiarity of these same molluscs is the formation of
pearls in the sheet of flesh, lining the shells. The pearls are round or rounded 149
concretions of aragonite. At the center there is a grain of sand, or more often a tiny
dead parasite. Either was an irritant to the mollusc, and to be rid of it, a layer of aragonite
was secreted around it. Then as the mollusc continued to grow and secrete layers for its
shell, it also added each time another layer around the sand-grain or parasite, until in time a
pearl of noticeable, and then of considerable size resulted. These have all the pearly luster of
the mother-of-pearl in a sphere which tends to make the luster even more marked.

Pearls were in use as ornaments in China some twenty-three centuries before Christ, and in
India over 500 B.C. They were very highly prized by the Romans and since their times the
rulers of India have shown a remarkable fondness for them. Today the finest come from the
Gulf of Persia and the Red Sea, while still others are found about Australia and in the
Caribbean Sea. In the United States not a few are collected every year from the fresh water
clams, some of them beautifully tinted with pink or yellow.

Aragonite is found widely, as at Haddam, Conn., Edenville, N. Y., Hoboken, N. J., New
Garden, Penn., Warsaw, Ill., etc.

Anhydrite
CaSO₄
Pl. 46

Occurs in cleavable or granular masses, rarely in crystals; hardness, 3-3.5; specific gravity,
2.9; color white, gray, bluish or reddish; luster pearly on cleavage faces; transparent on thin
edges.

When anhydrite occurs in crystals, they are orthorhombic, like the diagram on Plate 150
46. Usually, however, it is found in beds or layers, which were deposited by the
evaporation of sea water, and so it is associated with salt. Anhydrite has three cleavage
planes which are at right angles to one another, which produce rectangular or cube-like
forms. Mostly anhydrite is associated with gypsum, from which it differs by its greater
hardness, pseudo-cubic cleavage, and the fact that anhydrite is not readily soluble in acid,
while gypsum is. Chemically it differs from gypsum in not having water of crystallization,
which gypsum does have. The anhydrite is likely to occur as veins and irregular masses in
beds of gypsum. Calcium sulphate is precipitated from sea water when 37% of the water has
been evaporated, and it may be deposited either as anhydrite or as gypsum, the factors,
which decide as to which of these two minerals it will be, being as yet unknown. After
deposition, if exposed to moisture, the anhydrite may change to gypsum, irregular masses
often remaining unchanged.

It is found in salt mines in Elsworth Co., Kan., in limestone cavities at Lockport, N. Y., in
veins in Shasta Co., Calif., etc.

Gypsum
CaSO₄ + 2H₂O
Pl. 47

Occurs in crystals, in cleavable masses, or in fibrous masses; hardness, 2; specific gravity,
2.3; colorless, white, amber, gray, or pink; luster vitreous, silky or pearly; transparent on
thin edges.

Gypsum crystals are monoclinic as seen on Plate 47, the perfect ones usually 151
occurring in clay, as at Oxford, O., or in cavities; while crystals of less perfect
outline, but with fine cleavages, are found in Utah, Kansas, and Colorado. The cleavage is
very perfect in one direction, making it possible to strip off thin sheets almost like mica, and
less perfect in two other directions, which appear on the smooth surface of the first cleavage

as lines intersecting at 66° 14′. Twinning is also common in such a way, that the two united
crystals make forms similar to arrowheads. These cleavages and the twinning show nicely in
the photograph of gypsum on Plate 47.

Gypsum is distinguished from anhydrite by its lesser hardness, its cleavage and by being
soluble in acids.

Most gypsum occurs in beds or granular masses which were deposited from evaporating
sea-water, coming down when 37% of the water was lost. Such beds are often very
extensive and are quarried as a source of gypsum to make plaster of Paris, stucco, neat
plaster, Keene’s cement, plaster and wall board, partition tiles, etc. The use of the gypsum
for plaster of Paris and all these other uses is based on its affinity for water of
crystallization. The gypsum is first heated to about 400° C., which drives off the water of
crystallization, and causes it to crumble to a powder, which is the plaster of Paris. When
water is added, it is taken up and the powder “sets,” or recrystallizes back to gypsum. This
simple reaction has made it very useful, for making moulds, casts, hard finish on walls, as
stucco, etc.

When the granular type of gypsum is fine grained, it is known as alabaster, which is 152
used for carving vases, statuettes, ornaments, etc. The fibrous variety is called satin
spar, and is sometimes used for cheap jewelry and ornaments, but it is very soft and quickly
wears out. At Niagara Falls there is a considerable trade in objects carved from this satin
spar, tourists buying them on the assumption that the mineral is native and comes from
under the falls. Most of it, however, comes from Wales, the small amount of gypsum of that
region being mostly granular.

Gypsum is found all across the United States, as in New York, Michigan, Virginia, Ohio,
Alabama, South Dakota, Wyoming, Colorado, Utah, California, etc.

The Strontium Group

Strontium is a pale-yellow metal, ductile and malleable, but oxidizing quickly when
exposed to the air. It does not occur in its native state in Nature, but always as some
compound, usually either the carbonate or sulphate. It resembles barium, but differs in
giving to the flame a brilliant red color, on which account the compounds of strontium are
used mostly in making red fire in fireworks.

Strontianite
SrCO₃

Occurs in needle-like crystals, or in columnar or fibrous masses; hardness, 3.5-4; specific
gravity, 3.6; color white, pale-green or pale shades of yellow; luster vitreous; transparent on

thin edges.

Strontianite is orthorhombic, but appears as if hexagonal, since its general habit is to have
three twin crystals grow together in such a way as to make a six-sided double 153
pyramid. In this it is very like witherite, both these minerals appearing externally
much alike. They can be readily distinguished, however, by holding a piece in the flame. If
it gives a red color to the flame it is strontianite, if green, it is witherite. It effervesces
readily in hydrochloric acid.

Strontianite is found in veins and cavities in limestone, where it has been deposited after
being leached from the limestone by percolating waters. Though known at several localities
it is not now being mined in this country, what we use being imported mostly from
Germany.

It is found at Schoharie, Chaumont Bay and Theresa, N. Y., in Mifflin Co., Penn., etc.

Celestite
SrSO₄

Occurs in crystals, cleavable masses and fibrous; hardness, 3; specific gravity, 3.9; colorless,
white, pale-blue, or reddish; luster vitreous; transparent on thin edges.

Celestite, the sulphate of strontium, is very like barite in external appearance and habit. It is
orthorhombic and occurs in tabular crystals. Its cleavage is perfect on the basal plane, and
imperfect in one other direction. The ready way of distinguishing celestite from barite is to
hold a piece in the flame. If it is celestite it will color the flame red, if barite, green.

Celestite is mostly found in veins or cavities in limestone, where it has been deposited by
percolating waters, after having been leached from the limestone. Some years ago an
important deposit of celestite was found on Strontian Island in Lake Erie, but that was 154
soon worked out and now no veins are being worked in this country. It is also found
at Chaumont Bay, Schoharie and Lockport, N. Y., in Kansas, Texas, West Virginia,
Tennessee, etc.

The Barium Group

Barium is another metal which does not occur in its native state in Nature. It has only been
isolated as a yellow powder, which, exposed to air or water, soon changes to one of the
oxides. Both barium and its compounds are peculiar in causing a green color, whenever
exposed to the flame. Two of its compounds are fairly abundant, the sulphate, barite, and the
carbonate, witherite. The former is the more abundant and has come to be fairly widely
used, something over 100,000 tons being annually consumed in the United States, to make

the body in flat finish paints for interior work and light colors, for a filler in rubber goods,
linoleum, oil cloth, glazed paper, and for a wide range of chemical compounds.

Barite
BaSO₄
Pl. 48
heavy spar

Occurs in crystals or in lamellar, nodular or granular masses; hardness 3; specific gravity,
4.5; colorless, white or almost any color; luster vitreous; transparent on thin edges.

Barite occurs in orthorhombic crystals, which are tabular in form, and usually have the
edges beveled, as in figure A, Plate 48. There is cleavage in three directions, a rather perfect
basal cleavage, and two less perfect cleavages, which are at right angles to the basal 155
cleavage plane, and intersect each other at 78°.

The tabular form, the cleavage, the heavy weight, and the fact that a piece of barite put into
the flame colors it green, all serve to distinguish this mineral.

Barite is a secondary mineral of aqueous origin, which has been deposited in veins and
cavities in igneous, metamorphic, or sometimes sedimentary rocks. It is most likely to occur
in veins in igneous or metamorphic rocks, the barium having been dissolved from certain
feldspars and micas by percolating water, and then redeposited in the fissures, as the water
came into them. If in sedimentary rocks, the barite veins are usually in limestones. Barite is
quite likely to be a gangue mineral for the ores of lead.

It is found at Hatfield and Leverett, Mass., Cheshire, Conn., Pillar Point, N. Y., Cartersville,
Ga., in Virginia, North Carolina, South Carolina, Missouri, Kentucky, Tennessee, Alabama,
Illinois, Wisconsin, Nevada, California, Alaska, etc.

Witherite
BaCO₃
Pl. 48

Occurs in crystals, or in granular or columnar masses; hardness, 3.5; specific gravity, 4.3;
color white to gray; luster vitreous; translucent on thin edges.

Witherite is not an abundant mineral. Its crystals are really orthorhombic, but they are
usually twinned, three crystals growing through each other in such a manner that the
resulting crystal appears like a six-sided double pyramid, similar to the one figured on Plate
48. The commonest mode of occurrence is in compact masses. Witherite effervesces 156
when cold acid is dropped upon it, which, with its heavy weight, and the green color

it gives to the flame, serves to distinguish the mineral. It is used for medicines, in chemical
industries, and a considerable amount is made into rat poisons. The chief locality for
witherite is in northern England, but in this country it is found along with barite, especially
at Lexington, Ky., and in Michigan.

Carbon

Carbon is an element widely distributed in nature, occasionally appearing in its elementary
form, as graphite or the diamond, but much more important in its compounds. Small
quantities are present in the air as carbon dioxide, CO₂, immense quantities occurring in the
carbonate minerals, which have been considered under their respective metallic salts, as
calcite, malachite, siderite, cerrusite, smithsonite, witherite, etc., and still other large
quantities being represented in organic compounds, which occur as rocks under the heads of
petroleum, coal, etc. The occurrence of limestones, graphite, coal or petroleum is always
indicative of the activity of living organisms, and in some cases is the only indication of life
in the earlier rocks.

Graphite
C
Plumbago

Occurs in hexagonal scales or flakes, in layered masses, or earthy lumps; hardness, 1;
specific gravity, 2.1; color black or steel-gray; streak gray; luster metallic; opaque on thin
edges.

Like the diamond graphite is pure carbon, but in this case it is in non-crystalline form. 157
It occurs in both igneous and metamorphic rocks. In the former case it is either in
flakes in the rock, or in veins, and has been derived directly from the molten magmas,
having either precipitated in the hardening granite or lava, or having been carried into the
fissures and there precipitated to make the veins of graphite. In either case the graphite
probably represents organic deposits which have been melted into the igneous magma at the
time of its formation. Graphite may also occur in metamorphic rocks, beds of coal or other
organic deposits being altered by the heat. Such beds are often of considerable extent and
economic importance.

The extreme softness, greasy feel, and the dark-gray streak readily distinguish graphite.

It is widely used in making crucibles and furnace linings for foundries, lead pencils, paint,
lubricating powders, etc.

Graphite is found at Brandon, Vt., Sturbridge, Mass., Ashford, Conn., in Essex, Warren and
Washington Cos., N. Y., Clay, Chilton and Coosa Cos., Ala., Raton, N. M., Dillon, Mont.,

etc.

Diamond
C

Occurs in octahedral crystals; hardness, 10; specific gravity, 3.5; colorless to yellow, brown,
blue, etc., luster adamantine; transparent on thin edges.

Like graphite the diamond is pure carbon, but in this case in crystal form. It is the hardest of
all minerals, and as brilliant as any; so that in spite of being by no means the rarest, it 158
may easily be considered the most popular of all gems. Tiny diamonds have been
made artificially under great heat and pressure; so that this mineral is thought of as forming
in Nature in dark igneous lavas at great depths. The diamond has good cleavage parallel to
the octahedron faces, and, in spite of some traditions to the contrary, is brittle.

There are not many diamond localities, the most famous being the Kimberley district of
South Africa, which produces many times as many diamonds as all the others put together,
though all the time some are being found in Borneo and Brazil. A very few have been found
in the United States, only one locality however yielding them in the original matrix. That is
at Murfreesboro, Ark., where they are mined in a disintegrating peridotite (a dark lava,
mostly peridot), which has been extruded through the sedimentary rocks of that region. This
matrix is similar to the “blue earth,” the matrix of the diamonds of South Africa, which
occurs in “pipes,” representing the necks of ancient volcanoes. The American diamonds are
of small size, averaging considerably less than a third of a carat in weight, which does not
allow great value to the individual diamonds.

From time to time, especially large diamonds have been found in different parts of the
world, the largest being the Cullinan diamond, found at the Premier Diamond Mine of South
Africa. It weighed 3025 carats or about a pound and a quarter, and was valued at over
$3,000,000. It was presented to King Edward VII, who had it cut into 11 brilliants, 159
four of which are larger than any other diamond yet found. Other famous diamonds,
like the Kohinoor, 106 carats, found in India in 1304; the Regent, 136 carats, also found in
India; the Orloff, 193 carats, set in the eye of an Indian idol; the South Star, 125 carats, the
largest ever found in Brazil; the blue Hope, etc., have in many cases romantic and
interesting stories woven about them.

Though for ages diamonds have been highly prized gems, it is only in comparatively recent
times that cutting and polishing have been resorted to, for the purpose of enhancing their
brilliancy. This is done by grinding reflecting faces on the original stone, by the aid of discs
of iron or tin in which diamond dust has been embedded. Diamond chips and cloudy or
imperfect diamonds are used for making tools for cutting glass, rock drills, etc.

Phosphorus

The element phosphorus at ordinary temperatures is an almost colorless, faintly yellow,
solid substance of glistening appearance and waxy consistency. In Nature it does not occur
pure, but always as one of its compounds. It is of great importance to man for it is one of the
essentials for plant growth and also for the higher animals, being required for the bones and
to some extent for nervous tissue. Originally it is found in all the igneous rocks. Some of the
phosphorus is removed by solution and carried to other regions and to the sea. From 160
this distribution it comes into the sedimentary rocks, and, when they are altered by
heat, into the metamorphic rocks. Thus it has a wide, though by no means even, distribution.
The soils formed by disintegration of these rocks probably all have some phosphorus in
them; but where there is vigorous plant growth, it soon tends to become exhausted, and must
be renewed. For this reason the use of phosphates has become of prime importance in
Agriculture. The possession of beds of rock carrying phosphorus has come to be of
international importance. The United States is particularly fortunate in this respect, and
produces over 25% of the world’s supply of phosphates. Most all the phosphorus is
recovered either from phosphate minerals, the most important of which is apatite, or from
the non-crystalline and impure mixtures of phosphate minerals and other substances,
discussed under phosphate rock.

Apatite
Ca₅F(PO₄)₃
Pl. 49

Occurs in crystals, concretionary nodules, or in bedded masses; hardness, 5; specific gravity,
3.2; color reddish-brown or green, rarely white or colorless; luster vitreous; translucent on
thin edges.

Apatite occurs in hexagonal prisms, usually with the ends truncated by a basal plane, and
with one or more sets of pyramidal faces between the prism and the basal plane. Crystals
range in size from tiny to over a foot in diameter. There is but one cleavage and that is basal.
The crystal form, cleavage, and hardness will easily determine this mineral. Apatite is 161
usually associated with igneous or highly metamorphic rocks, such as granites,
gneisses, and crystalline limestones. While the phosphoric acid of apatite is highly desirable
for use in fertilizers, the crystals do not occur in sufficient abundance to make them
commercially available, and non-crystalline phosphate rocks are resorted to for this purpose.

Crystals of apatite are found at Norwich and Bolton, Mass., Rossie and Edenville, N. Y.,
Suckasunny and Hurdstown, N. J., Leiperville, Penn., Wilmington, Del., etc. Templeton,
Canada, is perhaps the best known locality for fine apatite.

Turquois
H₅[Al(OH)₂]Cu(OH)(PO₄)₄

Occurs in seams and incrustations; hardness, 6; specific gravity, 2.7; color bluish-green;
streak blue; luster waxy; translucent to opaque on thin edges.

In this country this complex phosphate of aluminum and copper is found in streaks and
patches in volcanic rocks, but in Persia comes from metamorphic rocks. To the Persians it
was a magical stone, protecting the wearer from injuries, and among the Pueblo Indians it
was regarded as of religious value in warding off evil. The best turquois comes from Persia,
but it has been found at several points in the United States, as in Los Cerrillos and Burro
Mts., N. M., in Mohave Co., Ariz., San Bernardino Co., Cal., in Nevada and Colorado.

162

Fluorine

At ordinary temperatures the element fluorine is a colorless gas, which was not obtained
pure until 1888, because it could not be contained in vessels of glass, gold, platinum, etc. At
that time it was made and kept in a vessel composed of an alloy of platinum and iridium. Its
most important compound is hydrofluoric acid, a fuming liquid, which is mostly used to
etch or dissolve glass. It occurs in several minerals, like tourmaline, turquois, etc., but the
only one used to obtain the hydrofluoric acid is fluorite.

Fluorite
CaF₂
Pl. 50
Fluor spar

Occurs in crystals and cleavable masses; hardness, 4; specific gravity, 3.2; colorless or some
shade of violet, green, yellow, or rose; luster vitreous; transparent on thin edges.

Fluorite usually occurs in beautiful cubic crystals, often with the edges and corners beveled
by smaller faces, and occasionally in twins, which seem to have grown through each other.
There is perfect cleavage parallel to each of the octahedral faces, which often, as in the
illustration on Plate 50, show as cracks cutting off the corners.

Since fluorite loses weight and color on heating, it is concluded that the colors are due to the
presence of hydrocarbon compounds. The red and the green fluorite when heated to above
212° F. become phosphorescent, as may be seen if they are thus heated and exposed to the
light, then taken into the dark.

Fluorite is quite commonly the gangue mineral associated with metallic ores, and is 163
also likely to occur with topaz, apatite, etc. It is generally in such places that it seems
to have been deposited from hot vapors, rising from igneous magmas.

It is the only mineral at all common from which fluorine can be obtained, and is used for
making hydrofluoric acid, and other chemical compounds of this element. It is, however, of
much greater importance as a flux in reducing iron, silver, lead and copper ores. In the
industries it finds a place, being used to make apochromatic lenses, cheap jewelry, and for
the electrodes in flaming arc lamps.

Fluorite is widely distributed, some of the better known localities being Trumbull and
Plymouth, Conn., Rossie and Muscalonge Lake, N. Y., Gallatin Co., Ill., Thunder Bay, Lake
Superior, Missouri, etc.

Halite
NaCl
Pl. 50
Salt

Occurs in crystals, and in cleavable and granular masses; hardness, 2.5; specific gravity, 2.1;
colorless to white; luster vitreous; transparent on thin edges.

Halite is common salt, occurring in cubic crystals, with perfect cubic cleavage. Its form,
hardness, taste, and solubility in water make it easy to determine.

Halite is the most abundant salt in sea water, making about 2.5% out of the total of 3.5% of
solids in solution. It is also a prominent, when not the leading, salt in solution in the waters
of inland lakes, like Great Salt Lake, or the Dead Sea, there being 20% of halite in the
former and 8% in the latter, though the total of solid in solution in the water of the 164
Dead Sea is greater than that in Great Salt Lake.

The great salt deposits are mostly the result of the evaporation of the water of arms or
isolated portions of former oceans; the salt, gypsum, etc., left by the drying sea, having been
buried beneath later sediments. Other bodies of salt represent the disappearance of ancient
lakes. There are also the curious “salt domes” of Louisiana and Texas, which are immense,
roughly circular, subterranean masses of salt extending to as yet unknown depths which are
thought to have been formed by masses of salt from some deep source bed pushing their
way upward through the overlying formations by plastic flowage. As the upthrust took place
the sediments were arched into domes. Some of these domes are today important sources of
rock salt.

There are extensive beds of salt under parts of New York, Michigan, Ohio, Oklahoma,
Kansas, etc., which are mostly worked by drilling wells into the salt layer, then introducing
hot water to dissolve the salt. The brine thus formed is pumped to the surface, and the salt

recovered by evaporation in pans. During the process, skeleton crystals of salt with concave
faces may form, but in Nature the crystals are uniformly solid cubes.

Boracite
Mg₇Cl₂B₁₆O₃₀

Occurs in small crystals or granular masses; hardness of crystals, 7; of the masses, 4.5;
specific gravity 3; colorless to white; luster vitreous; transparent to translucent on thin
edges.

Small crystals, associated with salt and gypsum, occur in the beds and incrustations, 165
which result from the drying up of alkaline lakes, especially in Nevada and southern
California. The crystals are orthorhombic, but appear like perfect cubes, with the edges
beveled and part of the corners cut. They are not easily dissolved in water, but quickly go
into solution in hydrochloric acid.

Colemanite
Ca₂B₆O₁₁ + 5H₂O

Occurs in crystals or compact masses; hardness, 4.5; specific gravity, 2.4; colorless to white;
luster vitreous; translucent on thin edges.

The crystals when they occur, are monoclinic; but usually colemanite is a bedded deposit,
which has resulted from the drying up of a saline lake. It was first found in Death Valley,
Cal., in 1882, then near Daggett, Cal., and since then in several similar locations in Nevada
and Oregon. The deposits are of all grades of purity, the colemanite being mixed with
varying quantities of mud. Today this mineral is the chief source of borax, which is used in
medicines, cosmetics, colored glazes, enamel, and as a preservative.

Borax
NaB₄O₇ + 10H₂O

Occurs in crystals or in powdery incrustations; hardness, 2; specific gravity, 1.7; colorless to
white; luster vitreous; translucent on thin edges.

The crystals are tiny and monoclinic, this mineral being usually obtained by the evaporation
of the saline waters of such lakes as Clear and Borax Lakes in southern California, or from
the muds of salt marshes, like Searles Borax Marsh in California. Originally most of 166
our borax came from a large saline lake in Tibet, but now most of it is obtained from
colemanite. Borax is soluble in water, giving it a sweetish taste.

Sulphur
S
Pl. 51

Occurs in crystals, incrustations or compact masses; hardness, 2; specific gravity, 2; color
yellow; streak yellow; luster resinous; translucent on thin edges.

Aside from the numerous compounds, such as the sulphides of the metals like pyrite, galena,
sphalerite, etc., and the sulphates, like gypsum, barite, anglesite, etc., sulphur occurs in its
elemental form in Nature. In this case it may be in crystals, which are orthorhombic and
usually occur as octahedrons, with the upper and lower ends truncated, either by a basal
plane, or by a lower octahedron, or by both. Incrustations and compact masses are, however,
much the commoner mode of occurrence. The incrustations are found mostly about volcanic
regions, where the sulphur has risen from the molten lavas as a sublimate, and on cooling
has been deposited in crevices or on the adjacent surfaces. Irregular masses of sulphur are
often found where sulphide minerals, like pyrite or galena have been decomposed in such a
way as to leave the sulphur behind. The extensive beds of sulphur are usually associated
with gypsum, and are thought to be the result of water, containing bituminous matter, so
acting on gypsum as to remove the calcium and oxygen as lime, and leave the sulphur.
Finally many waters carry sulphates in solution, from which the sulphur may be 167
precipitated by certain sulphur bacteria, making thus incrustations on the bottom of
ponds or lakes.

Sulphur is used for making matches, gunpowder, fireworks, insecticides, in medicine,
vulcanizing rubber, etc. It is widely distributed, however, most of the present world’s
production is from deposits associated with the “salt domes” of Texas and Louisiana. A
“caprock” of gypsum and anhydrite overlies many of these which often contains elemental
sulphur. Wells are drilled into this, and the sulphur is melted by the introduction of hot
steam. This melted sulphur is then pumped to the surface and run into molds.

Some of the best known localities are Sulphurdale, Utah, Cody and Thermopolis, Wyo.,
Santa Barbara Co., Cal., Humboldt Co., Nev., and about the hot springs of the Yellowstone
Park.

Ice
H₂O
Pl. 51
water

Occurs solid as ice, snow and frost, or liquid as water; hardness, 2; specific gravity, .92;
colorless to white; luster adamantine; transparent on thin edges.

Though we seldom think of ice, and its liquid form, water, as a mineral, still it is one, and
perhaps the most important of all minerals, as well as the most common. Ice melts at 32° F.
and vaporizes at 212° F., being then termed steam. Because it is so common and liquid at
ordinary temperatures it acts as a solvent for a host of other minerals, and is therefore the
agent by which they are transported from place to place and redeposited in veins and beds.

Not only does water act as a transportation agent for minerals in solution, but is also 168
the agent of erosion and weathering. Water vaporizes slowly when exposed to the air
at all temperatures above freezing, and so it is slowly rising from the surface of the sea or
lakes or moist ground into the air, where it would accumulate until the air was saturated, if
the air would only keep still and at a uniform temperature. The air will hold a given amount
of water vapor, which is, for example, 17 grams per cubic meter when the temperature is
68° F., but at 59° F. it will hold only 12½ grams, or at 50° F. only 9 grams. Thus the air is
more or less completely saturated at higher temperatures, and when the temperature is
lowered the air can not hold all it has taken up, and it is precipitated in dew, rain or snow,
most often as rain. When the rain falls it mechanically carries away, and more or less slowly
transports to other places particles of rock, being thus the agent of erosion; and when it is
slowed down, as on entering the quiet water of a lake or the sea, it drops the mechanically
carried sediment and makes sedimentary deposits.

Another very important and unique feature of water is that on freezing it expands about ¹/
₁₁th of its former bulk, so that, as a result, ice floats, and also wherever water in crevices is
frozen, the crevices are enlarged. In locations where this freezing and melting take place
repeatedly throughout a year, there the breaking up of rocks is rapid.

This is hardly the place to take up a complete discussion of water, but its action as a solvent,
mechanically, and in freezing, melting, and vaporizing is the basis of a large part of 169
the study of geology.

When water crystallizes, as in forming ice, it is in the hexagonal system. It tends to twinning
and a snow-flake is made up of a large number of twinned crystals, each diverging from the
other at 60°. When ice is formed in the air or on the surface of water it forms these complex
and beautiful multiple twins, of which but a couple are suggested here. Beneath the surface
the hexagonal crystals grow downward into the water, parallel to each other, making a
fibrous structure, which is very apparent when ice is “rotten,” which is the time at which the
surfaces of the prisms are separating, because the molecules leave the crystal in the reverse
order to which they united with it. Frost in marshy or spongy ground will often show this
fibrous growth beautifully.

170

CHAPTER IV
THE ROCKS

Broadly speaking a rock is an essential part of the crust of the earth, and includes loose
material, like sand, mud, or volcanic ashes, as well as compact and solid masses, like
sandstone and granite. Rocks are aggregates of minerals, either several minerals grouped
together, as are mica, quartz and feldspar to make granite, or large quantities of a single
mineral, like quartz grains to make sandstone.

The rocks are most conveniently classified according to their mode of origin, into three
main groups, igneous, sedimentary, and metamorphic. The igneous rocks are those which
have solidified from a molten magma, like lavas, granites, etc. The sedimentary rocks are
those which represent accumulations of fragments or grains, derived from various sources,
usually the weathering of other rocks, and deposited by such agents as water, wind and
organisms. Metamorphic rocks are those which were originally either igneous or
sedimentary, but have been altered by the actions of heat, pressure and water, so that the
primary character has been changed, often to such an extent as to be obscured.

Rocks once formed in any of the above ways are being constantly altered in character by the
various processes of nature. Those exposed on the surface are weathered to pieces, 171
and the fragments are transported by wind or water to accumulate elsewhere as
sedimentary rocks. Those buried deep beneath the surface are affected by the high
temperature and pressure of the depths of the earth and thus metamorphosed. For instance a
granite exposed on the surface is slowly weathered, some parts being carried away in
solution by the rain water, others less soluble remaining as grains of quartz, mica or kaolin.
These are transported by water and sorted, the finer kaolin being carried to still and deep
water, the quartz and mica accumulating in some lowland as sand. This sand will in time be
cemented to a sandstone, later slowly buried beneath the surface. If buried deep it will feel
the effect of the interior temperature, which increases as one goes down at the rate of one
degree F. for every 50 feet. If this should be in a region where folding and mountain-making
takes place, the material under the folds would be melted (because of the relief from
pressure which would permit the high temperature to act freely) and become igneous rock,
either coming to the surface as lava, or remaining below the surface and making a granite or
similar rock; while the sedimentary material not melted but near enough to the molten
material to be affected, would be metamorphosed, in this case to a quartzite. Much of the
interest and profit in studying rocks, will come from the understanding which they will give
as to the history of that particular part of the earth’s crust where they are found.

172

Igneous Rocks

Igneous rocks are those which have formed from material that has been melted, which
involves temperatures around 1300° C.; or, if there is water in the original material,
temperatures as low as 800° C. will suffice. Considering the increase of temperature to be a
degree for every 50 feet downward, this involves the rocks having been at depths of 5 to 10
miles below the surface. While at such depths the temperature must be high enough to melt
rocks, the great pressure of the overlying rocks seems to keep them solid; for we know that
the center of the earth is solid, as is shown by a variety of observations, such as the rate at
which earthquake waves are transmitted through the earth, the lack of tidal effects, etc.
However, there is every reason to believe that if the pressure is removed from the rocks
which are five to ten miles below the surface, there is heat enough at those depths to melt
them. When the crust of the earth is folded, as when mountain ranges are formed, the areas
under the arches or upward folds are relieved of pressure. Then those rocks, which are under
the arches and are relieved, become molten. The molten magma may well up and fill the
space beneath the arch where it would cool again very slowly; or, if there is fissuring during
the folding, some of the molten material may be forced out through the fissures and pour out
over the surface as lava. Another area in which pressures may be locally relieved is in the
region of faulting, where the crust of the earth is broken into blocks, between which 173
there are readjustments, some being tipped one way, some another, some uplifted.
Here again there would be areas of relieved pressure and molten magmas would form, some
of them solidifying in place, others rising to the surface.

The molten material is termed the magma, and when it reaches the surface, great quantities
of water vapor and other gases escape: or these gases may even escape from magmas which
do not reach the surface, rising through fissures. As these hot vapors pass through the
fissures, they are cooled, and may deposit part or all of their dissolved compounds in the
fissure, making veins. Lava is the magma minus the vapors. Magmas vary greatly from
place to place, indicating that they are formed locally and do not come from any general
interior reservoir, as has sometimes been suggested.

When the molten magmas escape to the surface, they are termed extrusive, and as they
spread out in a layer this is termed a sheet. This rise and overflow may be quiet, and from
time to time one outpouring may follow another making sheet after sheet. Or after one
outpouring, the pressure below may cease for a time and allow the lava to solidify and make
a cap or cover over the opening. Before more lava can rise, this cover must be removed.
This usually happens in an explosive manner, the lava below, with the increasing pressure
exerted by its expanding gases, finally exerting enough pressure, so that the cover is broken,
or shattered and thrown in thousands of fragments into the air, as happened at Mt. 174
Pelée on the Island of Martinique in 1902. The fragments thrown into the air are often
termed volcanic ashes, though this is not a good word for them, for they have not been
burned.

In case the molten magmas under the relieved areas do not reach the surface they are termed
intrusive. Such magmas may remain in the space under a mountain fold, or be forced in
fissures part way to the surface. When the magma is forced into more or less vertical cracks
and there solidifies, and these are exposed by erosion, they are termed dikes. Sometimes the

magmas have risen part way to the surface and then pushed their way between two
horizontal layers of rock and there hardened, in which case they are termed sills, when
uncovered. The Palisades along the Hudson River are the exposed edge of a sill. Again the
molten magmas may well up and spread between two horizontal layers, but come faster than
they can spread horizontally, and then the magma takes the form of a half sphere, and the
overlying layers of rock are domed up over it. Such a mass is termed a laccolith. In all these
cases the mass of igneous rock is only discovered when the overlying rocks have been
eroded off. The great mass of molten magma under the arches of mountain ranges simply
cools slowly into a granitic type of rock. These masses are exposed when the thousands of
feet of overlying rock are eroded off. When these masses are exposed, if of but a few miles
in extent, they are called stocks, but, if of many miles in length and breadth, they are
batholiths, and are very characteristic of the heart of mountain ranges. 175

In all the above cases the exterior of the molten mass cools first, and forms a shell around
the rest. The shell determines the size of the mass. As the cooling continues into the interior,
it also solidifies, and as all rocks shrink on cooling, cracks develop, separating the mass into
smaller pieces. There is usually no regularity about these cracks and the mass is divided into
blocks from six inches to three feet in diameter. However, in some cases, especially in sills
and laccoliths where the cooling is slower, the shrinkage may be marked by a regular system
of cracks which bound the rock into more or less regular hexagonal columns. The Palisades
and the Devil’s Tower in Wyoming (See Plate 52) show this structure. The Devil’s Tower is
the remnant of a laccolith, all except the central core of which has been eroded away. All of
the above terms have nothing to do with composition, but refer entirely to the manner of
occurrence.

While the igneous rocks are classified according to their composition, the rate at which they
cooled has much to do with their texture, and certain names apply to the texture. For
instance when the molten lava cools very rapidly, there is no time for the formation of
crystals, and the resulting rock is glassy or non-crystalline. If the cooling is slow as in large
bodies, crystals have time to form and grow to considerable size as in granites. Between
these all grades may occur; and one classification of igneous rocks expresses their rate of
cooling, in such terms as the following.

Glassy—lavas which have cooled so quickly that they are without distinct 176
crystallization, such as obsidian, pitchstone, etc.

Dense or felsitic—lavas which have cooled less rapidly, so that crystals have formed, but in
which the crystals are too small to be identified by the unaided eye, such as felsite or basalt.

Porphyritic—magmas from which, in solidifying, one mineral has crystallized out first and
the crystals have grown to considerable size, while the rest have remained small.

Granitoid—magmas which have solidified slowly, so that all the minerals have crystallized
completely, and the component crystals are large enough to be recognized readily, as in
granite.

Fragmental—a term applied to the fragments which have resulted from explosive eruptions
of igneous rocks. These fragments may be loose or consolidated. Volcanic ashes are typical.

Porous—a term applied to the lava near the upper surface, which is filled with gas cavities,
such as pumice.

Amygdoloidal—is the term applied to porous lavas, when the cavities have been filled by
other minerals, such as calcite or some of the zeolites.

In determining a rock, first decide whether it is igneous, sedimentary or metamorphic. The
igneous character is recognized by its being either glassy, or composed of masses of crystals
irregularly arranged, there being neither layering nor bedding.

177
CLASSIFICATION OF IGNEOUS ROCKS

Texture Excess of light colored minerals Excess of dark colored minerals
Glassy obsidian, perlite, pumice, pitchstone scorias, trachylyte, basalt-obsidian
Feldspar
orthoclase Feldspar Plagioclase No feldspar
Mica and/or augite and/or
hornblende Mica and/or hornblende and/or
and/or augite hornblende with pyroxene mica
-
+quartz -quartz +quartz -quartz +olivine olivine +olivine -olivine
Dense rhyolite trachite dacite andesite basalt augitite or
(felsite) (felsite) hornblendite
Porphyritic rhyolite- trachite- dacite- andesite- basalt- augitite-
porphyry porphyry porphyry porphyry porphyry porphyry
Granitoid granite syenite quartz- diorite olivine- gabbro peridotite pyroxenite
diorite gabbro
Fragmental rhyolite, trachite, Dacite, andesite Basalt tuffs and breccias
tuff or tuff or tuff or tuff or
breccia breccia breccia breccia
178
When it is located as igneous, turn to the key on page 177 and decide as to which
type of texture is present. If glassy, the color, luster and type of construction will place it. If
the rock is crystalline, first decide whether feldspar is present, and if present, what type:
then determine the dark mineral, and lastly whether quartz or olivine is present. In dense
rocks the presence of quartz may be determined by trying the hardness, for none of the other
constituents of igneous rocks have so great hardness. For example, if it is found that a rock

is composed of orthoclase hornblende and quartz, and the texture is granitoid, it is granite:
or if the rock is plagioclase feldspar and pyroxene of any sort, it is gabbro, etc.

Granite
Pl. 53

The combination of orthoclase feldspar (or microcline), quartz, and either mica, hornblende
or augite is termed granite, if the texture is coarse enough so the individual minerals can be
recognized with the unaided eye. The rock is light-colored because the feldspar and quartz
dominate. Accessory minerals may be present such as apatite, zircon, beryl or magnetite.
Varieties of granite are distinguished according to the dark mineral present. When this is
muscovite, it is a muscovite-granite; when it is biotite, a biotite-granite; if it is hornblende, a
hornblende-granite; etc. The size of crystals in granite varies widely. When they are as
small as ¹/₁₂ of an inch in diameter, it is termed fine grained; from ¹/₁₂ to ¼ of an inch, it is
medium-grained; when larger, it is coarse-grained. In some cases the crystals may be over a
foot in diameter which is known as giant granite.

Originally granite was a great mass of molten magma, which has cooled very slowly, 179
having been intruded or thrust up in great stocks or batholiths beneath overlying
rocks, which acted as a blanket to prevent rapid cooling. These overlying rocks, in their
turn, have been acted upon by the heat and metamorphosed. Granite is particularly likely to
have been formed under mountain folds; so that, after the mountains have been more or less
completely eroded away, the great masses of granite have come to the surface to mark the
axes of the ranges; and even after the mountains have been wholly worn away, the granite
remains to mark the sites on which they stood.

In the granite mass itself, there are often veins and dikes, which probably resulted from the
shrinkage of the cooling granite, and they are filled with a different and usually coarser
granite known as pegmatite. This pegmatite formed from the residual magmatic material, so
that as some of the elements had already crystallized out, the granite in these dikes is of
different composition. The extreme coarseness of these pegmatites seems to be due to the
character of the mineralizing agents left in the dikes. In some of these pegmatites the
feldspar and quartz are so intergrown, that when broken along the cleavage surface of the
feldspar, the quartz appears like cuneiform characters, and this variety has been given the
name graphic granite (See Plate 53).

When granite is exposed to weathering, the feldspar is the first mineral to be decomposed,
altering eventually into carbonates, quartz and kaolin. The dark minerals are only 180
slightly less susceptible and they break down into carbonates, iron oxides and kaolin.
The original quartz remains unchanged. Of these products the carbonates, some of the iron
oxide and a little of the quartz are carried away in solution. The kaolin and some of the iron
oxide is in fine particles and they are carried by the water until it comes to the lakes or the
sea. The quartz is left in coarser grains, which are more slowly transported, and deposited in
coarser or finer sand and gravel beds.

Granites are widely used for building stone, because they can be worked readily in all
directions, and have great strength and beauty. The color depends largely on the color of the
feldspar, which may be white or pink, in which case the granite will be gray to pink.

Granites occur throughout New England, the Piedmont Plateau, the Lake Superior Region,
the Black Hills, Rocky Mountains, Sierra Nevada, etc.

Syenite
Pl. 54

The combination of orthoclase and either mica, hornblende, or augite is syenite, the texture
being coarse enough so that the individual minerals can be distinguished by the unaided eye.
It differs from granite in the absence of quartz. Syenite is a light-colored rock with the
feldspar predominating. Minerals like apatite, zircon, or magnetite may occur in it, as
accessory minerals. The foregoing would be an ideal syenite, but usually there is some
plagioclase feldspar also present. If this occurs in such quantities as to nearly equal 181
the orthoclase feldspar, the rock is termed a monzonite; if it predominates, the rock
becomes a diorite. The presence of quartz would make this rock into a granite. Such a
compound rock has its type form, and when the proportions of the component minerals are
changed, it grades into other types.

Like the granite, syenite is an intrusive rock, which occurs in stocks and batholiths along the
axes of present or past mountain ranges. The original magma welled up under the mountain
folds, where it cooled slowly, metamorphosing the adjacent rocks. Like granite it has only
been exposed after a long period of erosion has removed the overlying layers of rock.

Syenites are not as abundant as granites, but they occur in the White Mountains, near Little
Rock, Ark., in Custer Co., Colo., etc.

Quartz-Diorite

The combination of plagioclase feldspar, quartz and either mica or hornblende makes
quartz-diorite, sometimes called tonalite. The above would be the typical quartz-diorite, but
there is usually some orthoclase present, which if it equals the plagioclase feldspar in
amount makes this into a monzonite; or if it dominates, it makes the rock a granite. Quartz-
diorite is darker colored than the two preceding rocks, the dark minerals being about as
abundant as the light-colored ones, such as feldspar and quartz. For this reason the weight is
also somewhat greater.

Like the others this is an intrusive rock, occurring in stocks and batholiths, and 182
indicative of great molten masses thrust up under mountain folds, and only exposed

after the overlying rocks have been weathered away. It is by no means an abundant type of
rock, but occurs at Belchertown, Mass., Peekskill, N. Y., in the Yellowstone Park, etc.

Diorite

Plagioclase feldspar with hornblende or mica, or with both, is known as diorite. It is
distinguished from quartz-diorite by the absence of quartz. There is generally some augite in
it, but if this should be equal to, or exceed the hornblende, the rock is then a gabbro. There
may also be a small amount of orthoclase present, without taking this rock out of the diorite
class, but if the orthoclase feldspar becomes dominant, then the rock is a syenite. Thus there
is gradation into other groups in all directions. Apatite, magnetite, zircon, and titanite often
occur in small quantities as accessory minerals. Generally the hornblende is in excess of the
feldspar, so that the rock is a dark-colored one.

Diorites occur in much the same manner as granites, being in stocks, batholiths or dikes, and
are often associated with granites and gabbros. They are great intruded masses, associated
with mountain making, and like the preceding rocks, cooled far below the surface, and have
been exposed only after great thicknesses of overlying rocks have been weathered away.

Peekskill, N. Y., the Sudbury nickel district in Canada, Mt. Davidson above the Comstock
Lode in Nevada, etc., are typical localities for finding diorite. 183

Olivine-Gabbro

The combination of plagioclase feldspar with augite (or any of the pyroxenes) and olivine
makes olivine-gabbro. The feldspar is usually one of those with considerable calcium in it,
like labradorite; and as the dark minerals predominate, the rock is dark-colored. It is an
intrusive rock, usually in dikes or stocks, where it solidified far below the surface, and was
only exposed after the overlying rocks were weathered off. It is by no means an abundant
type of rock, but is found in the Lake Superior Region, and near Birch Lake, Minn.

Gabbro
Pl. 54

Plagioclase feldspar with any one of the pyroxenes, most commonly augite, is gabbro. There
is a wide range in the relative proportions of the two minerals making gabbro. At one
extreme are rocks made entirely, or almost entirely, of plagioclase feldspar, which are
known as anorthosites, and occur in parts of the higher mountains of the Adirondacks like
Mt. Marcy, in several places in eastern Canada, etc. Then there are the typical gabbros
where the feldspar and augite are more or less equally represented. At the other extreme

come those gabbros in which the pyroxene predominates, in the most marked cases the
feldspar being entirely lacking, and the rock being termed a pyroxenite. When the pyroxene
of a gabbro is either enstatite or hyposthene (usually the latter) the gabbro is often 184
called norite. Magnetite, biotite, and hornblende may occur in small quantities as
accessory minerals.

Gabbro is a common intrusive rock, occurring in stocks, batholiths, and dikes, and often
varies considerably in different parts of the mass. Like granite the mass solidified far below
the surface, under some mountain fold, and has only been exposed as the result of
weathering away the layers of overlying rock. Gabbros appear much like diorites, but are
distinguished by the fact that the dark mineral is one of the pyroxenes, instead of an
amphibole or a mica. They are widely distributed, being found in the White Mountains, near
Peekskill, N. Y., Baltimore, Md., about Lake Superior, in Wyoming, the Rocky Mts., etc.

Peridotite

A rock made up of olivine and augite (or any of the pyroxenes) is peridotite. As it contains
no feldspar, and both augite and olivine are dark-green to black in color, these rocks are
always dark green to black in color and of considerable weight. They are usually rather
coarsely crystalline. Peridotite is usually associated with gabbro, making dikes which lead
from the main gabbro mass. Less frequently it occurs independently, making up an intrusive
mass. Hornblende and mica may be present in small quantities, as accessory minerals.

In general these are rather rare rocks, making dikes connected with stocks or batholiths of
gabbro. Peridotite is found near Baltimore, Md., in Custer Co., Colo., in Kentucky, etc.

185

Pyroxenite

This represents the extreme among coarsely crystalline igneous rocks, a whole mass made
up of one mineral, and that some one of the pyroxene group. If the mineral can be exactly
determined, the rock may be still more definitely named. For instance if it is all augite, then
the rock would be called augitite. Like the preceding rocks, pyroxenite is an intrusive rock,
usually found in dikes, which are connected with gabbro, and it represents the segregation
of one mineral out of the gabbro, and its solidification at one point. Hornblende, magnetite
and pyrrhotite may be present as accessory minerals. This is not a common rock, but it
illustrates the fact that all possible combinations do occur, if the circumstances have
warranted it. It is found near Baltimore, Md., Webster, N. C., and in Montana.

Rhyolite

This is a combination of orthoclase feldspar, quartz, and either hornblende, mica or augite in
which the crystals are of such small size that they can not be identified with the naked eye.
In composition it corresponds to granite, but it is much finer in texture. It differs from
trachite by having quartz while the latter has none. This can usually be determined by trying
the hardness as none of the other minerals are as hard as 7. It is much harder to distinguish it
from dacite which differs only in having plagioclase feldspar in place of the orthoclase, and
only the microscope will enable one to make this distinction. Where the distinction 186
cannot be made these light-colored lavas are often called felsite.

Rhyolite is usually an extrusive lava, occurring in sheets, but sometimes it is intrusive,
occurring in sills, dikes, and laccoliths. In all these cases the lava has solidified so rapidly,
that the crystals are tiny, and only the general effect of a crystalline structure is
distinguishable. Rhyolites may occur with porphyritic structure, in which case the presence
of the larger feldspar crystals will help to distinguish whether they are orthoclase or not,
making the determination easier. The color of rhyolites is green, red or gray, always a
decided light shade.

Rhyolites are abundant in the western states, as in the Black Hills, the Yellowstone Park,
Colorado, Nevada, California, etc.

Trachite

The combination of orthoclase feldspar with mica, hornblende or augite is termed trachite, if
the texture is dense. It is usually an extrusive lava of light color (green, red or gray), and
corresponds in composition to syenite. It can be distinguished from rhyolite by having no
quartz, and so nothing to show a hardness above 5.5; but it is difficult to distinguish it from
andesite, which differs only in having plagioclase feldspar in place of orthoclase. It
sometimes occurs with a porphyritic structure, in which case the feldspar crystals are
usually large enough to be distinguished.

Trachites are not abundant in America, but some are found in the Black Hills of South
Dakota, in Custer Co., Colo., and in Montana.

187

Dacite

The union of plagioclase feldspar, quartz, and either hornblende or mica is termed dacite, if
the texture is dense. It is an extrusive lava, occurring mostly in sheets and dikes. It
corresponds in composition to quartz-diorite. As the texture is dense it is difficult to
distinguish dacite from rhyolite, for both have quartz and differ only in the character of the
feldspar, so it is quite common to use the term felsite which does not distinguish between
the two, and only states that the rock is dense, light-colored and extrusive. When, as often

occurs, the texture is porphyritic, and the feldspars are the large crystals, then exact
determination is fairly easy.

Dacites are rather common, occurring on McClelland Peak, Nev., in the Eureka district,
Nev., on Lassen’s Peak, Calif., Sepulchre Mt. in the Yellowstone Park, etc.

Andesite

The union of plagioclase feldspar with mica, hornblende or augite, makes andesite if the
texture is dense. The lack of quartz, and so no mineral which has a hardness of over 5.5,
makes it possible to distinguish andesite from dacite or rhyolite, but it is hard to distinguish
this rock from trachite, which differs only on having orthoclase feldspar in place of
plagioclase. When the texture is porphyritic and the feldspars are the large crystals, then it is
easy to make the distinction. Andesite gets its name from being the characteristic lava of the
Andes Mountains, and is the commonest of all the extruded, light-colored lavas, being the
lava of hundreds of flows throughout the western United States. 188

The union of plagioclase feldspar and biotite is the commonest type. Plagioclase with
hornblende or augite is less common, and, when they do occur, they are usually
distinguished as hornblende-andesite or augite-andesite. Magnetite, apatite and zircon may
be present as accessory minerals.

The lavas of Mt. Hood, Shasta, Rainier and others of the volcanic peaks of the Cascade
Range, those at Eureka and Comstock in Nevada, in the Yellowstone National Park, and the
porphyries of many peaks in Colorado, like the Henry Mts., etc., which are exposed
laccolithic intrusions, are all andesites, as are many more.

Basalt

The combination of plagioclase feldspar with olivine and augite (or any other pyroxene)
makes a heavy, dark-colored, black to dark-brown rock which, if its texture is dense or
porphyritic, is termed basalt. This usually has more or less magnetite in it as an accessory
mineral, indeed the magnetite may be so abundant as to be a component part of the rock.
This magnetite makes trouble for anyone trying to use a compass on or about basalt rocks.
These are extrusive or intrusive rocks and correspond in composition to gabbro.

Basalts are among the commonest of igneous rocks, and are popularly designated “trap,”
much used as a road ballast on account of its toughness, which is largely due to its dense
texture. The coast of New England is seamed with dikes of basalt, and through the 189
Adirondack and White Mountains there are a host of these dikes. The crests of such
mountains, as the Holyoke Range, the Tom Range, the Talcott Mts., East and West Rocks at
New Haven, etc., are all basalt sheets. The Palisades, First Wachung and Second Wachung

Mountains of New Jersey are sills of basalt. The Lake Superior region is crisscrossed with
basalt dikes. That greatest of all lava fields the Columbia Plateau, covering over 200,000
square miles on the Snake and Columbia Rivers in Oregon, Washington and Idaho, is all
basalt. So it goes all down through Nevada, New Mexico and California.

Porphyry
Pl. 55

This is a term which properly refers to texture alone, indicating a lava, which has cooled in
such a manner that one mineral has crystallized out of the magma first and developed to a
larger size, while the mass of the material formed tiny crystals in which the larger ones are
embedded. The large crystals are technically known as phenocrysts. The surrounding mass
of tiny crystals is termed the matrix. This porphyritic structure is especially characteristic of
lavas which have been extruded in large masses, and of intruded lavas in such places as sills
and laccoliths.

The term porphyry today has the above precise meaning. It is a much abused word, and has
had all sorts of meanings. In the past it was first used to refer to lavas in general, then it
came to be applied to lavas which had been erupted before Tertiary times, that is to all 190
ancient lava sheets. This idea soon proved incorrect, lavas being of the same
composition whether ancient or recent. In the West the word is often colloquially used today
to designate almost every kind of igneous rock occurring in sheets or dikes, if in any way
connected with ore deposits.

When the composition of a rock with porphyritic textures can be determined, the name due
to the composition is coupled with that due to texture, making such terms as trachite-
porphyry, basalt-porphyry, etc.

Tuff

Tuff, a term not to be confused with tufa on page 215, is the name used to designate the
finer fragmental ejecta of volcanic eruptions, which are also often referred to as “volcanic
ash,” but the word, ash, conveys the false impression that the rock is a remnant of something
burned, and is therefore not a good term. When first ejected, tuff is loose material, but it is
usually soon cemented to make a more or less firm mass of rock, for which the term, tuff, is
still retained. In some cases, while still loose, it is carried by streams to a distance and
deposited in more or less sorted and layered beds: and the finer tuff is often carried by the
winds and laid down, at a considerable distance from its source, in so called “ash beds.” In
both these cases, sedimentary characteristics have been added to the tuff, and layering which
is characteristic of sedimentary deposits, is present. These transported tuff beds are really
sedimentary, but as there is little change in the material, they are referred to here and 191
not again. These tuff beds are not at all uncommon in the sedimentary deposits of

Tertiary age in the Rocky Mountain region. The coarser material of volcanic eruptions
usually goes under the head of breccia.

Breccia

This term is used to describe the coarse fragmental ejecta of volcanic eruptions. It is also
used, in the section under sedimentary rocks, in a broad sense to include all angular unworn
fragmental material, whether of igneous or sedimentary origin. For this reason, when
dealing with igneous rocks, it is usual to designate the fragments according to their
composition, making such terms as trachite-breccia, rhyolite-breccia, etc.

While still loose (and also even when cemented into beds of rock), it is customary to
designate the smaller fragments, from the size of a grain of wheat up to an inch or two in
diameter, as lapilli; the larger fragments, from two inches up to a foot or so in diameter, as
bombs; and the largest masses, often tons in weight, as volcanic blocks.

Obsidian
Pl. 55

Lavas, which have cooled so quickly that crystals have not had time to form, have a glassy
appearance, and are termed obsidian. If the color is dark, due to the presence of large
amounts of those elements which make dark minerals, this lava is termed basalt-obsidian.
Obsidian is characterized by its glassy texture, a hardness around 6, and by breaking 192
with a conchoidal fracture, so called because the surface is marked by a series of
concentric ridges, something like the lines of growth on a shell. Obsidians vary greatly in
color, but are usually red or green to black, and translucent on thin edges. While glassy, all
the obsidians contain embryonic crystals, which appear like dust particles floating in the
glassy matrix, or there may even be a few larger crystals present, which are often arranged
in flow lines. Most all large masses of obsidian have streaks or layers of stony material in
them where crystallization has set in, in a limited way.

Near the upper surface, obsidians usually have gas cavities scattered through them, and
these may be small and few, or large and numerous. Indeed the cavities may be so numerous
as to dominate and give the rock a frothy appearance. In this case, if the cavities are small
and more or less uniform, the rock is called pumice; if they are larger it is scoria. If, as often
happens when the lava is ancient and has been buried beneath other rocks, the cavities have
been filled with some secondary mineral, then the lava is called an amygdoloid.

Obsidian is found in many localities, especially where there are recent volcanoes, the most
famous places being the obsidian cliffs in the Yellowstone Park, those near Mono Lake in
California, and many other localities in the Rocky Mountains, the Sierra Nevadas, and the
Cascade Mountains.

193

Pitchstone

This is very like obsidian in appearance, but differs in that the glassy material contains from
five to ten per cent of water in its composition, the most obvious effect of which is to make
the luster resinous, instead of vitreous, as is characteristic of obsidian. The colors are
commonly red, green or brown. Pitchstone is associated with recent volcanoes, and some
fine specimens have come from Silver Cliffs, Colo., and various parts of New Mexico and
Nevada.

Perlite
pearlstone

Perlite is a glassy lava, containing two to four per cent of water, which, on cooling, has
cracked into numerous rounded masses, with a concentric structure, reminding one of the
layers of an onion.

Scoria

While lava is cooling, there is a constant escape of gases, mostly steam, and as these rise
through the molten mass they make cavities, near the upper surface, that portion on top
often becoming frothy. If this solidifies quickly so that the gas cavities are preserved it is
scoria. When the gas cavities are small and uniformly distributed, the rock is called pumice,
and often used as a scouring agent. When the cavities are large and irregular the term scoria
is generally used. Molten lavas may form various structures, according to the conditions
under which they cool, dripping through cracks or from the roof of caves, which often form
where the molten lava escapes from a hardened shell, and making stalactites, stalagmites,
etc. The very thin lava of the Hawaiian volcanoes may even be blown by the wind 194
into fine threads, known as “Pele’s hair.”

The presence of the gas cavities is so characteristic of the upper surface of lavas which have
been extruded; that, where one is dealing with older lavas, now buried beneath other rocks,
this fact helps to determine whether the mass is a sheet, rather than a sill; for, in the case of
the sill, the lava was forced between layers of sedimentary rocks, and the burden of the
overlying rocks did not permit the escape of steam and therefore the upper surface of sills
does not have the scoriaceous structure.

Amygdoloid
Pl. 56

When the upper surface of a lava is filled with steam holes, and this lava has been buried
beneath other rocks, the seeping waters slowly bring such minerals as quartz, calcite and
zeolites and fill the cavities. Such a rock is known as an amygdoloid. It is often confused
with porphyry; but, if examined closely, it will be seen that the outlines of the gas cavities
are rounded, while the outlines of a crystal, like a phenocryst, are always angular. This will
be clear if the amygdoloid on Plate 56 is compared with the porphyry on Plate 55.

The Sedimentary Rocks

To this class belong all those rocks which have been laid down by water or wind, or are the
results of organic depositions. They include loose material like sand or day, and also 195
the same materials, when cemented into more or less solid rocks, like sandstone or
shale. So long as the material has not been altered from what it was when laid down, the
rock is termed sedimentary.

In general the material of which these rocks are composed comes from the weathering and
disintegration of other rocks. This does not apply to the organic deposits, for each type of
which there is a peculiar mode of formation. To illustrate the typical formation of
sedimentary rocks, we may look at the fate of a granite when exposed. At once the surface is
attacked by changes of temperature, frost and rain. The various minerals of the granite
expand and contract with every change of temperature, but each component mineral has a
different coefficient of expansion under heat, so that minute cracks are quickly formed
between the minerals. Water gets into these cracks and begins to dissolve the minerals.
Feldspar is the most easily attacked, part of it being dissolved and carried away, a small part
changing to quartz, and by far the largest part changing to kaolin. The dark mineral is also
attacked and partly dissolved, and partly changed to kaolin and iron oxides. The quartz
resists solution almost completely. Of these products the kaolin and iron oxides are carried
far away and deposited in still water. The quartz and perhaps some of the dark mineral are
heavier and carried more slowly, being deposited as sand. This happens to granite
everywhere, but in the regions where there is frost the action is greatly hastened; for 196
water gets into the cracks and expands every time it freezes and thus widens the
cracks rapidly, which greatly facilitates the entrance and movement of water in the rock. In a
similar way any original rock will be disintegrated, and the residue, after the soluble part has
been carried away, becomes sand or clay or mud.

Particles of quartz, kaolin, and lime, separately, or mixed, loose or more or less cemented,
with accompanying impurities, make up the great bulk of the sedimentary rocks. They are
usually arranged in layers, of varying thickness, as they were laid down by water or the
wind. In the same way layered accumulations which are either products of plants or animals,
or parts of the plants or animals, are considered sedimentary, as for instance, coal, chalk,
petroleum, etc.

A Classification of Sedimentary Rocks

Inorganic origin:
1. Coarse fragmentary material resulting from weathering talus
2. The same fragmentary material cemented breccia
3. Unsorted material resulting from rock weathering soil
4. Coarse fragments rounded by the action of water and gravel
wind
5. The same material cemented conglomerate
6. Finer material deposited by water or wind sand
7. The same material cemented sandstone
8. The finest material, mostly kaolin, deposited by water clay
9. The finest material, deposited by wind loess
10. The same material cemented shale
11. Fine particles of lime, pure or impure marl
12. The same material cemented limestone
13. Unassorted material left by the glacial ice till
14. The same material cemented tillite
Organic Origin:
15. Limes made from shells, etc. coquina, chalk, coral rock,
etc.
16. Silica from the shells of plants, etc. diatomaceous earth, etc.
17. Carbon from plants peat, lignite, coal, etc.
18. Hydrocarbons from animals petroleum, asphalt, amber,
etc.
19. Phosphates from animals guano, phosphate rock, etc.
197

Talus

Where weathering is very active, especially on or below steep mountain slopes, a mass of
loose, angular fragments accumulates. This material is termed talus, a term which refers
only to the physical character of the material, and not at all to its composition. If weathering
continues these fragments will be further broken up into one of the finer grained 198
rocks, which the water can carry away and deposit elsewhere. There is little or no
layering in talus. If the talus is not carried away but is cemented where it was formed, the
resulting mass is termed breccia, but this is not very commonly the case.

Breccia
Pl. 58

The term breccia is used to cover all those rocks which are composed of angular fragments,
of any composition, and above sand in size, when they are cemented into a solid mass, by
any sort of cementing agent. Here the term is used in its broad sense, as compared with the
way it was used under igneous rocks.

Breccias may result from the cementing of talus, but more often the breaking up of the
material into angular fragments was due to other causes, such as crushing along a fault
plane, or in the movements involved in mountain making. In such cases the breccia is of
limited extent, but may occur repeatedly in the same neighborhood. Limestone, which has
been crushed and then recemented, often makes a rock which takes a good polish and is
used in several localities as an ornamental stone in place of marble, in fact often goes in
trade circles under the name of “marble.” The breccia figured on Plate 58 is such a
limestone.

Soil

Over most of the earth’s surface there is a covering of rock waste, the product of
weathering, some of which is unassorted, and some of it sorted by water or wind. 199
This is all termed soil. It is an ever-moving cover resulting from the decomposition of
the underlying rocks, to which have been added in places layers of rock waste brought from
afar by the streams. Some soils are rock waste which had been carried clear to the ocean and
deposited on the floor of the sea, and is now above sea level, because the floor of the sea has
been elevated. Inasmuch as the underlying rocks vary in composition, and as there are areas
of transported material, it is clear that the composition of soils must vary from place to
place, both as to composition and texture.

Soils range from the finest, composed mostly of clay, to coarse ones, composed of sand,
gravel or even boulders. Clay, the finest grained soil, is composed of particles only about ¹/
₁₀₀₀th of a millimeter in diameter, of which it would take 720,000 billion particles to make a
gram’s weight. Ordinary soils however have about 2 to 5 million particles to the gram.

The average specific gravity of soil with the usual amount of humus in it is from 2.55 to
2.75. In this case however the specific gravity is of less importance than is the volume
weight. A cubic foot of water weighs 62½ pounds, that of soil from 75 to 80 pounds, the
extremes being 30 lb. for peaty soil and 110 lb. for calcareous sand. The terms “heavy” and
“light,” used in agriculture do not refer to the volume weight, for clay which is actually
relatively light (70-75 lb. per cubic foot) is classed as a “heavy” soil; while sand, of much
greater actual weight, is classed as a “light” soil. These terms as used in agriculture 200
refer to the ease with which the soils are worked, and to their penetrability by plant
roots.

Soil is usually divided into an upper darker-colored layer, termed loam, and into a lower,
lighter-colored layer, termed subsoil. The presence of humus, resulting from the
decomposition of plant and animal remains is the factor which darkens the color and
distinguishes the loam; so that loam is a complex of inorganic rock particles plus more or
less humus, colloid compounds, bacteria, living plants and animals. The subsoil is mainly
rock particles. The distinctions between these two layers break down in arid soils, and often
also in swampy regions.

It is this layer of soil on which the water of every rain and flood works, picking part of it up
and carrying it along, step by step, to the sea. Though the amount moved on any one day is
small, the sum of all the soil transported is enormous, a large river carrying annual
incredible amounts. For instance the Mississippi annually deposits in the Gulf of Mexico
476,900,000 metric tons (2204 lb. to the metric ton), of which about a third is in solution. At
this rate it takes about 7000 to 9000 years to remove a foot from over the whole drainage
basin. This is considerably slower than is the case of some other rivers. While on the one
hand soil is being continuously carried away from the surface, on the other hand it is being
constantly renewed from below, by the weathering action of water, air and temperature.

201

Gravel

Gravel is a mass of loose fragments of rock, which have been rounded by water and
deposited with little or no sorting, so that larger and smaller pebbles and sand all occur
together. It is the deposit laid down by comparatively fast water in inland lakes or along the
storm-beaten shores of the sea. Where a swift stream enters quiet water, as where it empties
into a lake, there it quickly drops its coarse material as gravel, usually thus building a delta.
Gravel also occurs in stream beds, where for any reason the rate of flow is checked. During
the recent glacial period, the ice sheet brought down great masses of unsorted material,
which was deposited as till, or in moraines. Much of this was then picked up by the running
water and moved longer or shorter distances, so that, all over the glaciated country of the
northern and eastern United States, there are unusually large numbers of gravel deposits.
Gravels are all water laid, and usually show more or less clearly the bedded or stratified
structure.

The size of the component pebbles of gravel ranges from great boulders to fine sand, and the
finer gravels grade into the coarser sands, the line between gravel and sand being drawn at
about the size of a pea, the coarser being gravel, the finer sand.

Gravel is widely used as ballast for railroads and in making highways, because of its
tendence to pack well, while the hard pebbles resist wear. It is also widely used in concrete
work, bonding in well with the cement, and making it go from three to five times as far.

202

Conglomerate
Pl. 58

Conglomerates are composed of rounded pebbles and sand of varying sizes, cemented
together into a solid rock. The pebbles may run up to boulders in size, but they have all been
more or less rounded by water, and transported some distance. The pebbles may all be of the
same composition, or may represent a variety of rocks. When the pebbles are all, or most all,
of one sort, the resulting conglomerate is termed a quartz-conglomerate, a limestone-
conglomerate, a gneiss-conglomerate, etc. So too the cementing material varies in kind,
silica, calcite and iron oxide being the commonest. The color will depend on both the
component pebbles and the cement, sometimes one dominating, sometimes the other. There
are some of the quartz- and limestone-conglomerates which can be cut and polished to make
very handsome stone.

Conglomerates represent consolidated gravels, and always indicate an aqueous origin, quite
often the delta of an ancient stream, or the invasion of the sea over the land; so they have
become of importance to geologists in interpreting past events.

Sand

Sand is a mass of small rock particles, from the size of a pea down to ¹/₅₀₀ of an inch in
diameter. The material may be any sort of rock, or a mixture of two or more kinds. Sand
may be the result of the disintegration of older rocks at the point where it is now found, in
which case the grains have the shapes they had in the original rock; but more often 203
the sand grains have been transported greater or lesser distances, and in the process
have been more or less rounded.

Those sands, which lie where they were formed are called residual, and such sand is usually
composed of a mixture of angular grains, some harder and others softer, such as quartz,
feldspar, mica and hornblende, all mixed together. Where the sand has been transported,
only the more resistant minerals have remained, such as quartz, magnetite, cassiderite, etc.;
with which there are at times rarer minerals, such as gold, platinum, garnets or topaz. Such
sands are known as gold-bearing, topaz-bearing, etc.

The sands from different localities differ greatly. The streams gather the rock particles, and
sort them according to the size, which the water flowing at any given rate can carry. When
the water is slowed down, it drops all the particles above the size which the new rate of
speed can handle. The grains of sand from the bed of a stream are usually more or less
angular. The further they are carried, the more they are knocked together and rounded; so
that after being carried to the sea, and then thrown up on the beaches, they have been well
rounded, especially the larger grains. As the air is less viscid than the water, sand which is
transported by the wind, is even more rounded; so that desert sands show the most complete
rounding, indeed are even polished; and this is true even of the smaller grains. It is the wind-

blown, or desert sands, which flow so evenly in an hourglass. Between the angular residual
sands and the polished desert sands, there are of course all grades. Glacial sands are 204
angular or “sharp” almost to the degree characteristic of residual sands; and lake-
shore sands are between river sands and sea sands in the degree of rounding.

Sands made of particles of lime, calcareous sands, are less resistant to wear than are those
of quartz. In regions where the water is “soft” (free from lime), they do not last long, as they
are dissolved; but in a limestone region where the water is “hard” (saturated with lime), the
grains are not so quickly dissolved and may accumulate into beds of great thickness, as in
Florida. Along some shores of the ocean, there occur “green sands,” which are ordinary
quartz sands mixed with the dark green mineral glauconite, which is a potassium iron
silicate, forming on the ocean bottom as a result of the action of decaying animal matter on
iron-bearing clays and potassium-bearing silicates, like feldspar. This is particularly
characteristic of some of the sands along the coast of New Jersey.

In places, especially in the beds of rivers, there occur “quicksands.” This is a deposit of fine
sand, mixed with a considerable amount of clay, and saturated with water; so that it will not
support the weight of a man or an animal. Much that goes under the name of quicksand is a
fluid mud, covered with a thin layer of sand.

Sand is used for a wide variety of commercial purposes, and under these conditions gets
various trade names; for instance “glass sand” is a pure, colorless to white, quartz sand,
which is used as one of the components in making glass. It must be free from 205
impurities, as these color the glass, and much of the sand used for this purpose is
quartz, crushed to a fine sand-like condition. “Moulding sand” is a rather fine-grained quartz
sand, with a small but very definite admixture of clay, and this is used to make the moulds
for castings in foundries. “Polishing sand” is one composed of angular fragments of quartz,
usually from stream beds or glacial deposits, or even crushed quartz, and is used for cutting
and polishing marble, for sandpaper, and for polishing wood and softer stones. There are
many other special uses, like building, ballast, filters, furnaces, etc., in which quartz sand is
used, being screened if necessary to get the right sizes.

Sandstone

When sand of any sort is cemented so as to make a solid rock, it is termed sandstone, which
varies widely according to the size, color and composition of the grains, and also with the
sort and amount of the cement. When the size of the grains is larger than that of a pea,
sandstone grades into conglomerate; when smaller than ¹/₅₀₀th of an inch, especially if
mixed with clay, it grades into shale. There are all grades of firmness, due to the amount and
kind of cement, ranging from those which have little or no cement, but are compact as a
result of the pressure of the overlying rocks, to those in which the cement has filled all the
pore spaces. In general there is a considerable amount of space between the grains of sand;
so that a sandstone will absorb large amounts of water, up to 25% of its bulk. In moist 206
climates where it freezes, this makes many sandstones unsuitable for use as building

stones, as they are likely to spale, or chip off, as is seen in the “brown stone” so much used
in New York City.

Sandstones are usually bedded rocks and are relatively easy to quarry, and most of them are
not so firmly cemented, but that they can be readily worked or cut into shape by the stone
cutter; and so, certain sandstones are very popular for building stone or for trimming on
buildings, where they are not too much exposed to the weather.

Sandstone gets a variety of names according to the cement.

Siliceous sandstone is cemented with silica and usually very hard.

Calcareous sandstone is cemented with lime and usually rather soft.

Ferruginous sandstone is cemented with one of the iron oxides.

Argillaceous sandstone is held together with clay impurities, and is usually both soft and of
undesirable color.

According to their composition there is also a number of varieties.

Arkose is a sandstone composed of quartz and feldspar grains, usually derived from the
disintegration of granite and not transported far.

Graywacke is a sandstone composed of quartz, feldspar, and some other mineral, like
hornblende-augite, etc., also derived from the disintegration of granites and not transported
far.

Grit is a term applied to a coarse sandstone, composed of angular quartz fragments, 207
and used to a considerable extent for millstones.

Flagstone is a thin bedded sandstone, often with mica, which splits easily and uniformly
along the bedding planes; so that it can be quarried in large slabs. It was widely used for
sidewalks before the advent of concrete.

Freestone is a thick-bedded sandstone, not over hard, so called, because it can be worked
freely and equally well in all directions.

Clay

Clay is a term used to describe a mass of fine particles, the most prominent property of
which is plasticity when wet. Clays range from masses of pure kaolin to masses of kaolin
and related minerals mixed with as much as 60% of impurities, which may be sand, lime,
iron oxides, etc. The particles of a fine clay range around ¹/₁₀₀₀ of a millimeter in diameter,
while the impurities may be, and usually are, of larger size, up to the size of sand grains.

All clays are of secondary origin, the result of weathering, especially of feldspars, though
clays may also result from the weathering of serpentines, gabbros, etc. In some cases after
the weathering of feldspar or limestones, the clay may remain just where it was formed, as a
residual deposit; but, being so fine-grained, it is usually transported by rain water or by the
wind and deposited somewhere else as a sedimentary bed. The quiet waters of a lake are
favorable places for such deposits, and many clay beds represent former lake bottoms.
Impure clays are often laid down on the flood plains of sluggish streams. In fresh 208
water the settling of the clay is a very slow process, requiring days, or when very
fine, weeks, before the water wholly clears. In salt water, however, the clay sort of
coagulates, the particles gathering together in tiny balls, which settle rapidly, so that the
water is soon clear.

According to their mode of origin clays are classified as residual, sedimentary, marine,
swamp, lake, flood-plain, eolian, etc. But when their uses are considered a very different
classification is made, based mostly on their composition, and we speak of China clays or
kaolins, fire or refractory clays, paving-brick clays, sewer-pipe, stone-ware, brick, gumbo
and slip clays.

The kaolin or china clays are residual clays, usually resulting from the decomposition of
pegmatite dikes. They must be white when burned, free from iron oxides, and fairly plastic.
A good deal of china clay is made by crushing feldspar.

Ball clays are sedimentary clays which remain white when burned, are usually very plastic,
and free from iron oxides. They are mostly used in the making of various sorts of china.

Fire clays may or may not have iron oxides in them, but they must be free or nearly free
from fluxing materials, such as lime, magnesia and the alkalies (sodium and potassium
compounds). They may be more or less plastic, the essential quality being their ability to
withstand high temperatures without fusing. Silica (as sand) tends to diminish the refractory
quality; so that a clay otherwise suitable, if it has sand in it, becomes at best a second 209
grade fire clay. In coal mining sections it is customary to term those beds of clay
either above or below the coal, “fire clay”; but this is an unfortunate designation, for though
some of them are true fire clays, the most of them are not.

Stone-ware clays are those with considerable sand and up to five per cent of fluxing
materials. They must be plastic enough to be readily worked, and then burn to a dense body
at comparatively low temperatures.

Sewer-pipe clays must be plastic, and carry a considerable amount of fluxing material, as
the surface of the pipe is expected to vitrify in the burning.

Brick clays are low grade clays and vary greatly in composition. The main requisites are
that they mould easily and bake hard at relatively low temperatures with as little warping
and cracking as possible. As most clays shrink both in the air drying and in the baking, sand
is added when the clay is being mixed. The color is mostly due to the presence of iron
impurities. If there are iron oxides and little or no lime, the brick bakes to a red color, but if

there is an excess of lime over the iron oxides, it bakes to a cream or buff color, which on
vitrifying turns green.

Paving-brick clays range from surface clays, to semirefractory clays, shale being often
used. The essential component is enough fluxing material, so that the bricks shall begin to
vitrify, or fuse, at not too high temperatures.

Slip clays are those with a high percentage of fluxing material; so that, when baked at
moderate temperatures, the surface fuses into a glassy brown or green glaze. 210

Adobe is an impure calcareous clay, widely used in the western United States for making
sun-dried bricks.

Gumbo is a term applied to fine-grained plastic clays which shrink too much in the burning
to be useful in manufactures. They can be burned to make an excellent ballast for railroads
and highways. They are especially abundant in the Middle Western States.

Loess

This is the name given to a fine grained homogeneous clay-like material, which is a mixture
of clay, fine angular fragments of sand, flakes of mica and more or less calcareous matter. It
is usually without stratification, and cleaves vertically, so that, when eroded, it forms steep
cliffs. Loess covers great areas in the Mississippi Valley, in the Rhine Valley, and in North
Central China. By some it is thought to be an accumulation of dust in those regions where
the prevailing winds were of diminished velocity and where the grass or other vegetation
has served to catch and hold the material; by others it is thought of as a river and lake
deposit; and by still others it is thought to be due to the combination of the two modes, wind
and flood. The writer inclines to the first view expressed.

Shale
Pl. 59

When pure or impure clays, or loess, are consolidated, they are all grouped under the name
shale. It usually possesses a layered or stratified structure, which makes it possible to 211
split it into thin layers. Of all the sedimentary rocks shale is the commonest, and it
may occur in all the places where clay could occur, but the most widely distributed shale is
that which made the sea bottom of former times and is more or less calcareous, like the
piece on Plate 59, in which bits of shells are still visible. Shale has the same wide variation
in composition as has clay, the various types being designated according to the impurity
which is present, as:

argillaceous shale, made mostly of clay,

arenaceous shale, shale with more or less sand as an impurity,

calcareous shale, or one with more or less lime as an impurity,

ferruginous shale, or one with iron compounds as impurities,

bituminous shale, or one colored black by the presence of organic matter, remains of either
plants or animals.

While of no value as building material, shale may be ground or crushed, and used as a
substitute for any corresponding clay, and thus many manufacturers use shale in making
fire-clay products, bricks, tile, etc.

Marl

Where limestones or shells of any sort have been pulverized, and mixed with more or less
impurities, especially clay, the resulting unconsolidated mass is known as marl. It is usually
associated with marine formations, and is the finer débris which has settled on the ocean
bottom well out from shore, that is out beyond the sandy and mud deposits. Finding it 212
therefore usually indicates a sea bottom recently elevated. It is very characteristic of
the southern coastal states, from Maryland all along to Texas.

Limestone

Any mass of marl, or aggregate of calcareous shells, corals, etc., which has become
consolidated is known as limestone. It may, and usually does, have a wide range of
impurities, chief of which are clay, sand, iron oxides, and bituminous matter, like plant or
animal remains. Pure limestone is white, but due to impurities it ranges through grays,
greens, browns, to black, and even red, but this last is rarer. It is easily identified by the
presence of calcium carbonate, which effervesces in hydrochloric acid. It most often
represents deposits in fairly deep water on ocean bottoms of the past, but there is also a wide
range of limestones which were formed in fresh water.

Limestone is often burned at temperatures just above 900° C, at which point carbon dioxide
goes off as a gas, and leaves calcium oxide, or lime. When this is mixed with water it makes
calcium hydroxide, or slaked lime, which is mixed with sand to give it body, and is used as
mortar. When exposed to the air, the slaked lime gives up water, and takes back from the air
carbon dioxide, and again becomes calcium carbonate with its original hardness. Limestone
is also used as one of the elements in all cements. It is also considerably used as a building
stone, which, however, suffers in moist climates from the solution of its lime by rains, 213
but has stood up very well in dry climates.

The varieties of limestone are mostly distinguished according to their mode of origin, some
of them being as follows.

Bog Lime is a white calcareous powdery deposit on the bottom of ponds in limestone
regions, a deposit precipitated from solution by the action of the plants inhabiting the ponds.

Coquina (Plate 59) is the rock formed by the rather loose consolidation of shells and shell
fragments. It is particularly characteristic of tropical regions, and is very abundant near St.
Augustine, Fla., in which region it was, and still is, cut into blocks and used for building
stone. In that mild climate it has stood very well.

Chalk (Plate 60) is a soft fine-grained limestone, formed in the ocean by the accumulation
of myriads of the tiny shells of Foramenifera, which are single celled animals, living either a
floating life near the surface of the sea, or a creeping life on the bottom. Chalk is composed
mostly of the shells of floating Foramenifera, which when the animals died, settled to the
bottom and there accumulated, mostly at depths of 600 feet or more. When the mass of
unconsolidated shells is dredged up from depths of 50 to 2000 fathoms, it is known as
Foramenifera ooze. Chalk beds are then indications of an uplifted sea bottom. When
consolidated, if pure or nearly so, it makes a white chalk, and the beds may be of
considerable thickness, as is the case of the famous cliffs near Dover on either side of the
English Channel. One of Huxley’s most famous lectures is the one on chalk, found in 214
his Essays and Lay Sermons.

Coral Rock is made by the cementation of fragments of corals. The binding material, as in
most stones, is lime; and this sort of rock is associated with coral reefs of either the past or
the present. One of the best illustrations of this being the “Dolomite Mountains” in Tyrol.
Coral rock, like coquina, has been cut into blocks and used as building stone, as in
Bermuda.

Encrinal Limestone (Plate 60) is a rock made by the cementation of fragments of the
skeleton of crinoids. These animals belong to the group, echinoderms, and are now extinct
except for a few so called “sea-lilies.” They were animals with a central mouth surrounded
by long, jointed, flexible arms in multiples of five, and below this a small body inclosed in
calcareous plates, all at the top of a long jointed stem. They lived in the sea and in the earlier
geological times must have been very abundant; for their remains are so common in places
as to make whole layers of limestone.

Hydraulic Limestone is a fine-grained, compact, yellowish limestone with from 13 to 17%
of sand, and some clay; which, when it is burned at a temperature a little higher than that
used in burning lime, makes a product, that, while not as strong as Portland cement, still like
it sets under water.

Lithographic Limestone is a very fine-grained, compact limestone with clay impurities, the
finest of the grain making it usable for making the stone plates used in lithographic printing.
On slabs of this limestone figures are drawn in reverse with a special crayon. Then 215
the slab is treated with acid, those parts which are not protected by the drawing being

etched away, while the points protected by the drawing remain in low relief. From this slab
figures can then be printed.

Travertine is a general name, applied to calcareous deposits from fresh water lakes or
streams, and has been precipitated either as a result of cooling or evaporation. Some
travertines are porous, while others are dense; some are white, while others are colored,
often beautifully, by impurities in the water.

Porous deposits of travertine, when made on grass or other like substances, are known as
tufa or calc sinter. Such masses are common around Caledonia, N. Y., Mammoth Hot
Springs in the Yellowstone Park, etc.

Onyx marble is a dense travertine, usually formed as a result of the deposition of lime from
the water of springs. It is often banded, due to the presence of impurities in the water at one
time, and their absence at other times.

Till

Till is an unconsolidated mass of boulders, pebbles, sand and fine clay, the unsorted material
left behind by glaciers when they melted. The boulders and pebbles, while they show some
wear, are not rounded like those that have been transported by streams, but have a more or
less angular shape; and some of them are polished or striated on one side, where, while
frozen in the ice, they were rubbed along the bottom.

One of the most recent geological events in America was the 216
extension of the ice sheet, now covering Greenland, down over north
and northeastern North America, until it extended as far south as northern
New Jersey, the Ohio River and the Missouri River, and as far west as the
Rocky Mountains, but not over the Great Basin, the Cascade Ranges or
Alaska. This great mass of ice, thousands of feet thick, moved from two
centers, one either side of Hudson Bay, scraping up the loose soil, and
grinding off the exposed surfaces of the underlying rock. All this material it
carried southward, until the melting along its lower margin equaled the rate
at which it advanced. When the melting was faster than the advance the
glacial sheet retreated. At the southern limit of the advance this débris was
dropped, either making long ridges (moraines) or while the ice was
retreating, thicker or thinner sheets. This deposited débris is till.

The soil, and especially the subsoil, in all the regions formerly covered by
the ice sheet, is made up very largely of this till; which, where it is
undisturbed is often called “hardpan.” When till is mixed with humus it
becomes loam. This mixture of material, varying all the way from the fine
powdered products of the ice grinding to the great boulder it picked up and
carried south, is characteristic of this or any other glaciated country. When
this section of country was settled, the boulders and stone were a hindrance
to cultivation, and were picked up and piled into stone walls, which are one
of the first features to strike the eye.

217

Tillite

When till is consolidated into solid rock, it is known as tillite. In several
cases it has been found buried far beneath the more recent sedimentary
rocks; testifying that there were other glacial periods beside the last one
which furnished the till.

The Coal Series

Disregarding minor constituents, the plants are largely made up of cellulose,
which is a combination of carbon, hydrogen, and oxygen, (C₆H₁₀O₅). If this
is heated in the air, where there is plenty of oxygen, it disintegrates, or
burns, making carbon dioxide and water; but if the heating is done where
the oxygen is excluded, as in a kiln, the hydrogen and oxygen will be driven
off and the carbon will remain behind as charcoal. In Nature similar
reactions go on, but more slowly. Vegetable matter, exposed to the air,
disintegrates into carbon dioxide and water, and there is no solid residue.
However, if the vegetable matter is under water, which excludes the air
more or less completely including the oxygen in it, then disintegration still
takes place, but the products formed are water, (H₂O) marsh gas (CH₄), and
some carbon dioxide (CO₂), but a considerable part of the carbon remains
behind and accumulates.

Thus in bogs, swamps and ponds, where dead vegetation, especially that
growing in the water, piles up, the oxidation is incomplete; so that there
gradually accumulates on the bottom a layer of brown to black mud, 218
known as peat. More plant remains are constantly being added, and
the layer may increase to several feet in thickness. The decomposition is
incomplete and some oxygen and hydrogen remain, but the carbon is in a
constantly increasing ratio and in proportion far above that in cellulose. In
the cold northern climates sphagnum moss is the most efficient peat
producing plant, but in temperate and tropical climates the moss is replaced
by the leaves, twigs, trunks, etc., of trees, bushes, and vines.

If these peat beds are buried beneath a layer or layers of sediment,
especially clay, the peat is sealed up and oxidation stops almost entirely.
With the pressure of the superincumbent beds, the peat becomes more and
more compact, and changes to a dark-brown or black color. It is then known
as lignite. If this lignite is buried still deeper, with consequently more
pressure and more time, it changes into the still denser black bituminous
coal. This is as far as it will go unless some new agent is added to the forces
already working.

The next step in the series of changes forming coal is associated with
mountain making. In case the layers of rock containing beds of coal are
folded, and that presupposes at least a moderate increase in heat, the

bituminous coal is altered to anthracite, which is still denser, and so hard
that it breaks with a conchoidal fracture. Alteration may be carried a step
still farther, in case the rocks between which lie beds of coal are effected by
such high temperatures as accompany metamorphism. Then all the 219
associated hydrogen, oxygen and moisture are driven off, and only
the carbon remains, which is then known as graphite. All steps between the
stages especially designated occur. The following represent steps only in the
series of changes.

Peat

Peat is a mass of unconsolidated vegetable matter, which has accumulated
under water, and in which the original plant remains are still, at least in part,
discernible. It contains a large amount of water, so that before it can be used
as a fuel, it is cut out in blocks, which are piled up and left for a time to dry
before using. It burns with a long flame and considerable smoke. This
country is so well supplied with other fuels, that so far peat has been but
little used.

Lignite
brown coal

Lignite is more compact than peat, and is found buried to some depth under
layers of clay or sandstone. It is dark brown to black in color, and still
retains pretty clear traces of the plants from which it was derived. It also
usually contains a considerable amount of moisture, and when this is dried
out, it tends to crumble badly, so that it is undesirable to handle it much, or
to ship it far, before using. It has a fair fuel value and is fairly widely used;
but it is very desirable that some method be found, by which lignite could
be treated to obtain its by-products, and at the same time make it more
compact, so it would not crumble with the handling incident to using 220
it in furnaces. There are extensive lignite deposits in this country in
North and South Dakota, Montana, Wyoming, Colorado, New Mexico,
Texas, Louisiana, and Mississippi.

Bituminous Coal
soft coal

This type of coal is compact, black in color, and breaks readily, but does not
crumble as badly as lignite. It contains considerable water, and still has
some hydrogen and oxygen compounds in it. Bituminous coal is the product
of plant remains which have been preserved for long periods, (millions of
years), sealed from the air by the overlying beds of rock. The pressure has
made it compact, and nearly all traces of the original plants have
disappeared.

Bituminous coal is our most abundant fuel, occurring the world over in
seams from less than an inch in thickness to some over fifteen feet thick.
The United States is peculiarly fortunate in the abundant and easily
accessible deposits of this type of coal, in Pennsylvania, West Virginia,
Ohio, Kentucky, Tennessee, Indiana, Illinois, Michigan, Iowa, Missouri,
Kansas, Nebraska, Texas, Utah, and Colorado.

The volatile constituents, hydrogen and oxygen compounds, of bituminous
coal may be driven off by heating the coal in closed ovens, and the residual
mass is known as coke, almost pure carbon. This is distillation, and the
ovens in which this is done, without trying to save the volatile products, are
called bee-hive ovens, while the more modern ovens which save the 221
by-products are called by-products ovens. A ton of bituminous coal
treated in the typical by-products oven, will yield on the average 1410 lb. of
coke, 7.1 gallons of tar, 18.9 pounds of ammonia sulphate, etc., 2.4 gallons
of light oils, 10440 cubic feet of illuminating gas, about half of this last
being used to furnish the heat for the distillation. The coal-tar dye industry
is built on the tar thus produced. Toluol, benzol, etc., come from the light
oils; and half the gas produced is available for household illumination, etc.
Coke is demanded, as it is a superior fuel for melting iron ores, iron and
steel, and is made regardless of whether the by-products are used. The coke
thus produced is hard, clean, and vesicular; but for some reason as yet
unknown, by no means all bituminous coal will produce a coke which has
this porous structure. These latter are known as “non-coking,” and are of
little use to the steel industry.

Cannel Coal

This is a compact variety of non-coking bituminous coal, with a dull luster
and a conchoidal fracture. It contains the largest proportion of volatile
hydrocarbon compounds of any variety of coal; so that when the supply of
petroleum and natural gas gives out, this will be one of the important
sources of obtaining substitutes. Cannel coals occur in Ohio, Indiana, and
eastern Kentucky. This cannel coal owes its peculiar fatty nature to the
material from which it is derived, it being supposed to have resulted from
the accumulation of the spores of lycopod trees, and their conversion 222
to jelly-like masses by bacteria in the fresh-water marshes of those
ancient days.

Anthracite
hard coal

Anthracite coal is hard, black, has a luster, and breaks with a conchoidal
fracture. It contains but a low percentage of volatile matter, and so burns
with a short flame, and less smoke, than is the case with the other coals. It is
always associated with folded rocks, and appears to have been formed as a
result of the combined pressure and the higher temperatures, which
accompanied mountain making. Still the temperature was not high enough
to metamorphose the adjacent rocks. Most of our anthracite comes from
northeastern Pennsylvania.

Carbonite

Carbonite is natural coke. It occurs in coal seams which have been cut by
dikes or intrusions of igneous rocks, the coal having been thus coked by
natural processes. It is not vesicular like artificial coke, for which reason it
is not useful as a fuel. Some carbonite is found in the Cerillos coal field of
New Mexico, in Colorado, and Virginia.

Jet

Jet is a dense variety of lignite, a fossil wood of black color, which takes a
high polish and cuts easily into various ornamental shapes. It has been used
for ornaments since early ancient times, beads of jet being found in the
early bronze period in England, the supply probably coming from the
Yorkshire coast, whence the principal supply comes even to the 223
present day. In Switzerland and Belgium it was used still earlier, even
as far back as the Palæolithic age. Jet seems then to have had a talismanic
value, and to have been worn to protect the owner. About 700 A.D. crosses
and rosaries began to be made of jet, the custom starting at Whitby Abbey,
the material being obtained nearby, so that it came to be known as “Whitby
jet,” and in the eighteenth century became very popular. In recent times it
has been used mostly as jewelry suitable for mourning.

Amber
Pl. 61

Amber is a gum which oozed from coniferous trees and was petrified. It is
associated with lignite beds of middle Tertiary age. It is usually pale-yellow
in color, but at times has a reddish or brownish tinge, and is more or less
transparent. It occurs in rounded irregular lumps, up to ten pounds in
weight, though most pieces are smaller; and is mostly picked up along
certain coasts where it is washed ashore by the waves. Since the earliest
records amber has been cast up on the shores of the Baltic, and it was used
by peoples as early as in the stone age for ornaments and amulets. It has
been found among the remains of the cave dwellers of Switzerland, in
Assyrian and Egyptian ruins of prehistoric age, and in Mycenæ in the
prehistoric graves of the Greeks, the first recorded reference to it being in
Homer, and the Greek name for amber being elektron from which our word
electricity comes. All these finds were of Baltic amber which was doubtless
gathered and traded by those early men. Even down to the present 224
many men make their living, riding along the shore at low tide and
hunting for the amber washed ashore by the waves. As early as 1860 the
German geologists concluded that the source of the amber must be lignite

beds outcropping beneath the sea level, and started mining for the amber
with fair success, so that today two types of Baltic amber are distinguished,
“sea stone” which is washed ashore, and “mine stone” taken from the
mines. Beside the Baltic locality, it is found along the shores of the Adriatic,
Sicily, France, China, and occasionally of North America.

Some pieces of amber are found with insects inclosed and preserved almost
as perfectly as if collected yesterday. They were apparently entangled in the
gum while still viscid and completely embedded, before fossilization.

The Petroleum Series

Certain sedimentary rocks contain larger or smaller quantities of natural
gas, petroleum, mineral tar and asphalt. These are compounds of carbon and
hydrogen, or hydrocarbons, and range from gases to solids, each being a
mixture of two or more hydrocarbon compounds. The crude petroleum may
have either a paraffin base or an asphalt base: in the former case, when the
gas, gasoline, kerosene, etc., have been removed by distillation, the solid
residue will be paraffin, as in most of the Pennsylvania crude oils; while in
the latter case, the solid residue will be an asphalt, as in most of the 225
California and Texas crude oils. In the case of the paraffin series all
the compounds belong to the paraffin group, while the asphalt is due to the
presence, in addition to the paraffin group, of some of the benzine series of
hydrocarbons.

Petroleum is found in sands and shales, which were originally deposited on
ancient sea bottoms, the shales generally being the real source of the
petroleum. The oil was once the fatty portion of animal bodies (perhaps to
some extent of plant bodies), and was separated during decomposition as a
result of bacterial activity. Oil thus produced is in tiny droplets, which have
a great affinity for clay. After being freed by the bacteria, the oil droplets in
muddy water attach themselves to particles of clay, and as the clay settles
the oil is carried down with it, the two eventually making a bituminous
shale. In clear water, or in water which is in motion, the oil droplets rise to
the surface and eventually distill into the air.

The oil, or petroleum, may stay diffused through the shales, in which case
we have oil-bearing shales, with sometimes as much as 20% of oil. Were
there but ¹/₁₀₀₀ of a per cent of oil in a layer of shale 1500 feet thick, this
would amount to 750,000 barrels per square mile which is equal to a rich
production from wells. When the oil in shale amounts to three per cent or
more, it is commercially usable. There are large stretches of petroleum-
bearing rocks in New York, Pennsylvania, Ohio, Indiana, and all the way
out to the Pacific coast, some of them with oil so abundant, that a 226
blow of the hammer will cause them to smell of petroleum.

In case these oil-bearing shales have been heavily overburdened and
compressed, the petroleum may have been more or less completely pressed
out of them. Then the droplets uniting have formed a liquid, which has
moved out from the shale, and gone wherever it could find open spaces.
Sandstones have frequently offered their pore space, and as it filled, have
been thus saturated with petroleum. If the sandstones were open to the air,
or if fissures extended from them to the surface, the oil has escaped to the
surface and evaporated into the air. But in those cases where the sandstone
(or other permeable rocks) was covered by an impervious layer, like a dense
shale or clay, the oil was confined below the covering layer of rock. Crude
oil is lighter than water; so that when natural gas, petroleum and water were
all present in the rocks, the gas lies on top, the petroleum next, and the
water underneath. With this in mind it is easy to see, that in slightly folded
or undulating layers of rock, the gas and petroleum would be caught under
upraised folds and domes. This is the basis of prospecting for oil.

If petroleum-bearing layers are depressed far enough beneath the surface to
be affected by the high temperatures of the earth’s interior, or have been
near volcanic activity, of course the petroleum has been distilled by natural
processes, and at most only the residues, like paraffin or asphalt, have
remained. For this reason it is impossible to find petroleum in 227
igneous or metamorphic rocks.

Natural gas

Natural gas is the lightest portion of crude oil, and consists mostly of marsh
gas (“fire damp,” CH₄) together with other light hydrocarbons, like ethane
(C₂H₆), ethylene (C₂H₄), and some carbon dioxide and monoxide. It is
colorless, odorless, and burns with a luminous flame. Mixed with air it is
explosive. It is found in sedimentary rocks, mostly sandstones, either with
or without petroleum. Usually it is under considerable pressure, and escapes
with great force wherever a hole permits. In time the gas all escapes
through the hole or well, and then the well “runs out.” If petroleum is
present under the natural gas, the hole may become an “oil well,” from
which petroleum may be pumped, until it in turn is exhausted. The end of
an oil supply is usually indicated by the appearance of water in the well.
Natural gas is mostly associated with oil districts, as in Pennsylvania, Ohio,
Illinois, Texas, California, etc.

Petroleum Crude Oil
Pl. 61

Petroleum is a mixture of paraffin compounds all the way from the gases,
through gasoline, kerosene, lubricating oils, and vasoline to paraffin. In
some of the crude oils there is also an admixture of compounds from the
benzine series, in which case, when all the volatile compounds have been
distilled off, an asphalt remains. The different components of petroleum
may be separated out by heating the crude oil in closed tanks, and 228
drawing off the various substances at the proper temperatures.

Petroleum occurs in sedimentary rocks of marine origin, usually rocks
which also contain the shells of some of the animals, the soft parts of which
made the oil. To have been preserved the millions of years since the
petroleum was first formed, the oil-bearing layers must have been covered
by some impervious layer of rock, beneath the domes and anticlines of
which the oil has lain ever since. When such a dome or anticlinal fold is
perforated by a well, the released oil flows to the surface with a greater or
less rush, according to the pressure. Wells may keep flowing for 20 years,
sometimes more, sometimes much less. Those which flow with the greatest
pressure usually are relatively short lived, at times lasting only a year or

two. When this easily obtained oil is exhausted, there is an even greater
supply to be obtained by the distillation of the bituminous shales. Petroleum
never occurs in igneous or metamorphic rocks, but is found in either
sandstones or shales, in places favorable for accumulation, all across that
great stretch of ancient sea bottoms, extending from the Appalachian
Mountains to the Rocky Mountains, and in the Great Basin between the
Rocky Mountains and the Sierra Nevada Range, and also to the west of the
Sierras.

Bitumen

Where petroleum has escaped through pores in the rocks, or by way of
fissures, and has come to the surface of the earth, the lighter 229
components, thus exposed to the air, have vaporized and escaped,
leaving behind a more or less solid residue, which is known as bitumen. If
the escape was through a fissure, the bitumen may have accumulated in the
fissure until it was filled, making vein bitumen. Or the escape may have
been so rapid that the petroleum formed a pool or lake from the surface of
which evaporation took place. In time such a pool will give off the gases
and volatile compounds, only a residue remaining to make a pitch lake, like
the one at Rancho Le Brea near Los Angeles, or an asphalt lake like the one
on the island of Trinidad. On account of their varying hardness and
composition, some of these bitumens have received special names; as:

Albertite, a black bitumen with a brilliant luster on broken surfaces, a
hardness between 1 and 2, and a specific gravity a shade over 1.

Grahamite, a black bitumen, which is brittle, but has a dull luster, a
hardness of 2, and a specific gravity of 1.15.

Gilsonite or Uintaite, a black bitumen with a brilliant luster and a
conchoidal fracture, a hardness of 2 to 2½, and a specific gravity of 1.06.

Malta is a semi-liquid viscid natural bitumen, which has a considerable
distribution in California.

The above varieties of bitumen look a good deal like coal, but are easily
distinguished by their lightness (weight about half that of coal), and the fact
that with only moderate heat they melt, and become a thick liquid like tar.

230

Guano

Guano is the accumulation of the excrement of birds (or of other animals
like bats) on areas so dry that, though soluble, it is not leached and washed
away. It may also contain some of the bones and mummified carcasses of
the birds which died on the spot. The greatest of these deposits are on
several small islands, just off the west coast of Peru, and now “farmed” by
the Peruvian government. In this country there are no true guano beds,
except a few accumulations of bat guano in certain caves of Kentucky and
Texas, but these are not large enough to become of commercial importance.

Phosphate Rock

Phosphate rock is one composed chiefly of calcium phosphate along with
various impurities, such as clay and lime. It occurs in beds, irregular
masses, or as concretionary nodules in limestone or sand.

The bedded varieties are in the older sedimentary rocks, in which the
phosphate runs from a small percentage up to as high as 85%. Ultimately
the phosphate came from either animal excrement, or from bacterial
decomposition of animal carcasses and bones. In all the beds it seems to be
true that in the first instance the phosphate was laid down as a disseminated
deposit in marine beds, usually limestones. Later by the action of water
leaching through the rocks, the phosphate was dissolved, and then
redeposited elsewhere in a more concentrated form. This may be either in
the underlying sandstones, but is more often in limestones, replacing the
original lime.

In these secondary deposits, if the phosphate has been laid down in 231
cavities, the resulting phosphate will be in nodular masses. In the

case of the Florida and Carolina deposits, these nodules have been freed
from their matrix and washed along the river beds, remaining as pebbles in
the river sands. The bed deposits are mostly in Kentucky and Idaho. The
commercial use for such phosphate rocks is of course the making of
fertilizers.

Diatomaceous Earth
Pl. 62

Diatoms are tiny single-celled plants living in uncounted millions in the
fresh and salt water. Each diatom builds around itself two shells which fit
into each other like the cover and box of a pill-box, and each shell is
marvelously ornamented. The shells are composed of silica of the opal type.
In size the diatoms range from ¹/₅₀₀₀ of an inch in diameter up to the size of
a pin head, and they live in such numbers that ordinary surface waters have
hundreds of them to the quart, and where they are flourishing up to 250,000
in a quart. When the plants die, or in order to reproduce abandon the shells,
these shells fall to the bottom of the pond or the sea, and there accumulate,
often making a layer from a few inches thick up to hundreds of feet in
extreme cases. If unconsolidated, this mass of tiny shells is known as
diatomaceous earth; but if they are consolidated it is called tripolite, so
named because the first of them used commercially came from Tripoli.

As the shells are tiny and uniform in size and have a hardness of 6, 232
the diatomaceous earth is used to make a great variety of polishes,
scouring soaps, tooth paste, as a filler in certain kinds of paper, in making
waterglass, as an absorbent for nitroglycerine, and as packing in insulating
compounds, where asbestos would otherwise be used.

Deposits of freshwater diatoms are found all over the United States, usually
in thin layers of limited extent, especially in Massachusetts, New York,
Michigan, etc. The marine deposits of diatoms are on a much larger scale,
there being beds of diatoms in Anne Arundel, Calvert and Charles Counties,
Md., up to 25 or 30 feet in thickness. In Santa Barbara County, Cal., there is
one bed 2400 feet thick and another 4700 feet thick, beside many other
smaller ones. The enormous former wealth of life indicated by these great

deposits may be suggested, when it is remembered that it takes about
120,000,000 to make an ounce in weight. They reproduce on an average
about once in five days, so that from a single diatom the offspring possible
under favorable conditions would amount to over 16,000,000 in four
months or over 60 tons in a year. Of such an order is the potential increase
of animals or plants, no matter how small, if the rate of reproduction is
high.

Metamorphic Rocks

Either a sedimentary or an igneous rock, which has been altered by the
combined activities of heat, pressure and chemical action, becomes a
metamorphic rock. The process is essentially one, during which the 233
layers of rock come under the influence of such temperatures as are
associated with the formation of granite or lavas. Such material as is
actually melted becomes igneous rock, but adjacent to the masses actually
melted are other rocks which do not melt but, according to the temperature,
are more or less changed, and these are the metamorphic rocks. At a
distance from the molten masses the changes are minor, but close to the
molten magmas extensive changes take place. Though not actually melted
the rock near the heat center may be softened, usually is, in which case
pebbles and grains or even crystals become soft and plastic, and, as a result
of the great pressure, are flattened, giving the rock, when it cools again, a
striated appearance. At these high temperatures the water in the rock and
also some other substances vaporize, and the hot steam and vapor are active
agents in making a great many chemical changes. In some cases material
like clay is changed into micas, or chlorite, etc.; in other cases the elements
of a mineral will be segregated and large crystals will appear scattered
through the metamorphic rock, such as garnets, staurolites, etc.

If one studies a layer of rock both near and far from the molten mass, all
grades of change will appear. For example, at a distance a conglomerate
maybe unaltered; somewhat nearer the molten mass, the heat and steam
may have softened (but not melted) the pebbles and then the pressure has
flattened them as though they were dough; and nearest the molten mass, the
outlines of the pebbles are lost, only a layered effect remaining, and many

of the materials have changed into new minerals, like mica, garnets, 234
etc., but still the layered effect is preserved.

One of the effects of heat and pressure is to flatten the component particles
of the rock, so that it tends to split in a direction at right angles to the
direction of the pressure, just as particles of flour are softened and flattened
under the pressure of the roller; and then when the crust is baked it splits or
cleaves at right angles to the direction in which the pressure was exerted by
the roller. This tendency to split is not to be confused with either the
layering, characteristic of sedimentary rocks, nor the cleavage characteristic
of minerals. It has nothing to do with the way the particles were originally
deposited, nor with their cleavage; but is due to the pressure, and resembles
the pie crust splitting, being irregular and flaky. This is designated
schistosity if irregular and slaty cleavage if regular. Schistosity refers to the
flaky manner of splitting into thin scales as in mica schists. Slaty cleavage
is more regular, this being due to the fact that the material of which slate is
made is small particles of clay of uniform size.

The metamorphic rocks are generally more or less folded, as they are
always associated with mountain making. These major folds are of large
size, from a hundred feet across to several miles from one side to the other.
Such folds may also occur in sedimentary rocks or even in igneous rocks
and simply express the great lines of yielding, or movement of the crust of
the earth. In addition to this there is minor folding or contorting 235
which is characteristic of metamorphic rocks only. When the rocks
were heated by their nearness to the molten igneous magmas, they must
expand, but being overburdened by thick layers of other rocks, there is no
opportunity for yielding vertically, so the layers crumple, making minor
folds from a fraction of an inch to a few feet across. Such crumpling, which
is so very conspicuous especially where there are bands of quartzite in the
rock, is entirely characteristic of metamorphic rocks. It is seen on hosts of
the rocks about New York City, all over New England, and in any other
metamorphic region. Plate 63 is a photograph of such a crumpled rock
which has been smoothed by the glacial ice.

The metamorphic rocks are the most difficult of all the rocks to determine
and understand, because the amount of change through which they have

gone is greatest, but for this same reason they offer the most interest, for the
agents which caused the changes are of the most dramatic type of any that
occur in Nature. From one place to another a single layer of metamorphic
rock changes according to the greater or less heat to which it was subjected,
making a series of related rocks of the same composition but with varied
amount of alteration. For this reason in naming metamorphic rocks, a type
is named, and from that there will be gradations in one or more directions,
both according to composition, and according to amount of heat involved. If
it is possible to follow a given layer of metamorphic rock from one place to
another this is of great interest; for by this means, many variations in 236
the type will be found, both those resulting from a different amount
of heat, and those due to the local changes in the composition of the original
rock.

One further consideration has to be kept in mind. When a rock is
metamorphosed the high temperatures either drive off all water, or the water
may be used up in the making of some of the complex minerals. When such
a metamorphic rock later comes near the surface and is exposed to the
presence of ground water, and that leaching down from the surface into the
rocks, several of the minerals formed at high temperatures will take up this
water and make new minerals such as serpentine, chlorite, etc. They are
always associated with metamorphic rocks, and have been metamorphic
rocks, but since then have become hydrated, forming minerals not at all
characteristic of high temperature.

The following shows the relation of the sedimentary and igneous rocks to
their metamorphic equivalents.

Metamorphic
Loose sediment Consolidated sediment equivalent
gravel conglomerate gneiss
sand (quartz) sandstone quartzite
mud (sand and shale schist
clay)
clay shale slate or phyllite
marl limestone marble

peat bituminous coal anthracite to graphite
coarse igneous rocks such as gneiss
granite, syenite, etc.
fine igneous rocks such as trachite, schist
rhyolite, etc.
237
In working out the past history of any given region, much of it is
done on the basis of this series of equivalents. The finding of limestone, for
instance, indicates that the given area was at one time under the sea to a
considerable depth, that is from 100 to 1000 feet, but not ocean-bottom
depths which run in tens of thousands of feet. Marble indicates the same
thing, and so one can go on through all these types of rock.

Gneiss
Pl. 64

Gneiss is an old word used by the Saxon miners, and is often very loosely
used. Here it is used in its structural sense, and a gneiss may be defined as:
a banded metamorphic rock, derived either from a sedimentary or an
igneous rock, and is composed of feldspar, quartz, and mica or hornblende,
and is coarse enough, so that the constituent minerals can be determined by
the eye. It corresponds to a granite, or some sedimentary rock like gravel or
conglomerate.

Due to the action of pressure, all the gneisses are banded, and the original
constituent particles or crystals are distorted. The lines of banding may be
long or short, straight, curved or contorted. When the banding is not
conspicuous, the gneiss tends toward a granite. When the banding is thin
and the structure appears flaky, the gneiss tends toward a schist. The color
varies according to the constituent minerals, from nearly white, through red,
gray, brown, or green to nearly black. Plate 64 shows one gneiss which is in
a less advanced stage, the pebbles being simply flattened and the matrix
partly altered to micaceous minerals, and a second gneiss which is so 238
far advanced that the original constituents are all altered to other
minerals and only the banded structure remains. This latter type would have

required but little more heat to have completed the melting and changed this
to a granite.

Gneisses are very compact and have little or no pore space in them. They
are hard and strong and resist weathering well, so that they are widely used
as building stone: but they are not as good as granite for this purpose, as
they split more readily in one direction and can not therefore be worked so
uniformly as can granite.

There are many varieties of gneiss, based either on their origin,
composition, or their structure, as follows:

Granite-gneiss is one derived by metamorphism from granite.
Syenite-gneiss is one derived by metamorphism from syenite.
Diorite-gneiss is one derived by metamorphism from diorite.
Gabbro-gneiss is one derived by metamorphism from gabbro.
Biotite-gneiss is one composed of quartz, feldspar and biotite.
Muscovite-gneiss is one composed of quartz, feldspar and muscovite.
Hornblende-gneiss is one composed of quartz, feldspar and hornblende.
Banded-gneiss is one in which the banded structure shows clearly.
Foliated-gneiss is one in which there is thin irregular layering.
Augen-gneiss is one which has concretionary lumps scattered through it.

Gneisses have a wide distribution over all New England, most of 239
Canada, the Piedmont Plateau, the Lake Superior region, the Rocky
Mountains, the Sierra Nevada and the Cascade Ranges.

Quartzite

Quartzite is metamorphosed sand or sandstone, and frequently grades into
one or the other. It is a hard compact crystalline rock, which breaks with a
splintery or conchoidal fracture. It is distinguished from sandstone by the
almost complete lack of pore spaces, its greater hardness and by its
crystalline structure. In practice it may be distinguished by the fact that a
sandstone in breaking separates between the grains of sand, while a
quartzite breaks through the grains.

Some quartzites are almost pure quartz, but others contain impurities of
clay, lime or iron, which were in the original sandstone. These alter in the
metamorphism to such accessory minerals as feldspar, mica, cyanite,
magnetite, hematite, calcite, graphite, etc. The color of quartzite when pure
is white, but may be altered to red, yellow, or green by the presence of these
accessory minerals.

On account of the difficulty of working the quartzites, they are not much
used in building, though they are very durable. When crushed they often
make excellent road ballast, or filling for concrete work. The pure varieties
are sometimes ground and used in the manufacture of glass. 240

According to the accessory mineral, the following varieties may be
distinguished; chloritic-quartzite, micaceous-quartzite, feldspathic-
quartzite, etc.

Quartzites are common in the New England, the Piedmont Plateau, and
Lake Superior metamorphic regions, and also in many western localities.

Schist
Pl. 65

Schist is a loosely used term, but is used here in its structural sense. It
includes those metamorphic rocks which are foliated or composed of thin
scaly layers, all more or less alike. The principle minerals are recognizable
with the naked eye. In general schists lack feldspar, but there are some
special cases in which it may be present. Quartz is an abundant component
of schists; and with it there will be one or more minerals of the following
groups: mica, chlorite, talc, amphibole or pyroxene. Frequently there are
also accessory minerals present, like garnet, staurolite, tourmaline, pyrite,
magnetite, etc.

All schists have the schistose structure, and split in one direction with a
more or less smooth, though often irregular, surface. At right angles to this
surface they break with greater or less difficulty and with a frayed edge. As
they get coarser, the schists may grade into gneisses, losing their scaly

structure: while on the other side, as the constituent minerals become finer
and so small as to be difficult of recognition, schists may grade into slates.

The varieties of schist are based on the mineral associated with the 241
quartz; as mica-schist, chlorite-schist, hornblende-schist, talc-schist,
etc.

The color also is due to the constituent minerals other than quartz and
ranges widely, mica-schists being white to brown or nearly black, chlorite-
schists some shade of green, hornblende-schists from dark green to black,
talc-schists white, pale-green, yellowish or gray, etc.

Schists are found all over the same regions as gneisses and quartzites, i.e.,
New England (especially good exposures of schist being seen about New
York City), the Lake Superior region, Rocky Mountains, etc. Beside these
regions where it occurs native, there are boulders of schist all over the
glaciated areas of eastern and northern United States.

Slate

Slate is a metamorphic rock which will split into thin or thick sheets, and is
composed of grains so fine as to be indistinguishable to the unaided eye.
The cleavage is the result of pressure during metamorphism, and has
nothing to do with the bedding or stratification of the sedimentary rock
from which it was derived. The original bedding planes may appear as
streaks, often more or less plicated, and running at any angle with the
cleavage. If these bedding streaks are abundant or very marked, they may
make a slate unsuitable for commercial uses. The slaty cleavage may be
very perfect and smooth so that the rock splits into fine sheets, in which
case it is often used for roofing slate; but by far the greater part of the 242
slates have a cleavage which is not smooth or perfect enough so that
they can be so used. Slates are the metamorphic equivalents of shales and
muds, and represent the effect of great pressure but with less heat than is
associated with schists or phyllite, and consequently with less alteration of
the original mineral grains.

The color ranges from gray through red, green and purple to black. The
grays and black are due to the presence of more or less carbonaceous
material in the original rock, the carbon compounds having changed to
graphite. The reds and purple are due to the presence of iron oxides, and the
green to the presence of chlorite.

While the particles of slate are so small as to be indistinguishable to the
unaided eye, the use of thin sections under the microscope shows that slate
is composed mostly of quartz and mica, with a wide range of accessory
minerals, like chlorite, feldspar, magnetite, hematite, pyrite, calcite,
graphite, etc.

According to their chief constituents slates may be distinguished as
argillaceous-slate or argillite, bituminous-slate, calcareous-slate, siliceous-
slate, etc.

Slate will be found here and there in the metamorphic areas of New
England, the Piedmont Plateau, the Lake Superior region, and in many
places in the west.

Phyllite
Pl. 66

Phyllite is a thinly cleavable, finely micaceous rock of uniform
composition, which is intermediate between slate and mica schist. In this
case the flakes of mica are large enough to be distinguishable to the 243
eye, but most of the rest of the material can only be identified with
the aid of a microscope. It is mostly quartz and sericite. Phyllite represents a
degree of metamorphism greater than for slate, but less than for schist; and
it may grade into either of these other rocks. Garnets, pyrite, etc., may be
present as accessory minerals. The color ranges from nearly white to black,
and it is likely to occur in the same places as do slates.

Marble
Pl. 66

This is a broad term, and includes all those rocks composed essentially of
calcium carbonate (limestones) or its mixture with magnesium carbonate
(dolomite), which are crystalline, or of granular structure, as a result of
metamorphism. It takes less heat to metamorphose a limestone, and for this
reason the marbles have a more crystalline structure than most metamorphic
rocks; and they do not have the tendency to split or cleave which is so
characteristic of most metamorphic rocks. It is only when there is a large
amount of mica present that the typical schistosity appears. Commercially
the term marble is used to include true marble and also those limestones
which will take a high polish; but in this book, and geologically speaking,
no rock is a marble unless it has crystalline structure.

Marbles range widely in color according to their impurities. Pure marble is
white. Carbonaceous material in the antecedent limestone is changed to
graphite in the metamorphic process, and makes the marble black, 244
but appears usually in streaks or spots, rather than in any uniform
color. An all black “marble” is usually a limestone. The presence of iron
colors the marble red or pink. Chlorite makes it green, etc.

Various accessory minerals are common in marbles, such as mica,
pyroxene, amphibole, grossularite among the garnets, magnetite, spinel,
pyrite, etc., through a long list.

Because it cuts readily in all directions and takes a high polish, marble is
widely used as a building stone. In the moist climate of the United States it
suffers in being soluble in rain water when used on the outside of a
building: but for interior decoration it furnishes some of the finest effects.

The largest marble quarries are developed in Vermont, Massachusetts, New
York, Pennsylvania, Georgia, Alabama, Colorado, California, and
Washington.

Steatite
Soapstone

Steatite is a rock composed essentially of talc, which is associated with
more or less impurities, such as mica, tremolite, enstatite, quartz, magnetite,
etc. It is found in and with metamorphic rocks, and is a rock which has been
modified by hydration from a metamorphic predecessor. It was probably
first a tremolite or enstatite schist, in which, after the metamorphic rock
came into the zone where ground water exists, the tremolite or enstatite was
altered to talc, the impurities remaining much as they were in the first place.

It is bluish-gray to green in color, often soft enough to cut with a 245
knife, and has a greasy feel. It is very resistant to heat and acids; for
which reasons it has proved very useful commercially in making
hearthstones, laundry tubs, and fire backs; and, when powdered, in making
certain lubricants. The Indians, in the days before Columbus, took
advantage of the ease with which it is cut, to make from it large pots for
holding liquids, which are today among the greatest treasures in collections
of Indian relics. They also carved pipe-bowls and various ornaments and
amulets from soapstone.

It is found in Vermont, Massachusetts, New York, New Jersey,
Pennsylvania, Maryland, Virginia, North Carolina, Georgia and California.

Serpentine
Pl. 67

Pure serpentine is the hydrated silicate of magnesium, as described among
the minerals on page 138. Serpentine rock is serpentine with more or less
impurities, such as pyroxene, amphibole, olivine, magnetite, chromite,
calcite, magnesite, etc. It often also contains mica and such garnets as
pyrope, as accessory minerals. Serpentine, like steatite, always occurs in
and with metamorphic rocks, and was originally a metamorphic rock, but
has since been changed by the hydration of its silicates, when it came into
the zone in which ground water is present. In the first instance it was some

sort of shale, clay and dolomite, which was metamorphosed to an
amphibole or pyroxene schist. When this was exposed to the action of
ground water, the amphibole or pyroxene minerals were changed to 246
serpentine, resulting in a rock composed mostly of serpentine, but
retaining the impurities which were in the metamorphic rock, and perhaps
adding to them such amphiboles and pyroxenes as were not altered during
the hydration process. The above is the commonest type of serpentine rock.
It can and sometimes has been formed in a similar way from an igneous
predecessor, by the hydration of its silicate minerals. In this latter case the
serpentine would not be a modified metamorphic rock, but a modified
igneous one. It is a case where such a rock as a diorite or a gabbro is
exposed to ground water and the pyroxene present altered to serpentine. A
serpentine formed in this way would be a very impure one.

Serpentine rock is used as an ornamental stone for interior decoration,
because it takes a high polish and has pleasing colors, various shades of
green. It is however decidedly soft and will stand very little exposure to
weather, and it is also filled with seams which make it difficult to get out
large slabs.

Serpentine rock occurs fairly commonly in the metamorphic belt of New
England and the Piedmont Plateau, and in some of the western states,
especially California, Oregon, and Washington.

Ophiolite
Ophicalcite

This name is given to marbles which are streaked and spotted with
serpentine. They are a mixture of green serpentine and a white or nearly
white calcite, magnesite or dolomite in variable proportions. 247

Ophicalcite occurs in and with metamorphic rocks, and represents an
impure limestone which has been metamorphised, the lime becoming
marble, and the impurities becoming such silicates as pyroxene, amphibole,
or olivine. This metamorphic rock has then come into the zone of ground-
water and the silicate minerals have been changed by hydration to

serpentine. Ophicalcite is then a metamorphic rock, in which secondary
chemical changes have since taken place. It may have a wide range of
accessory minerals present, such as magnetite, chromite, pyrope among the
garnets, olivine, etc. Verde antique is a trade name for one of the ophiolites.

While not abundant, ophicalcite is in good demand as an ornamental stone
for interior work; for it takes a high polish, and is beautiful; but, on the
other hand, it will not stand exposure to the weather for the calcite is
soluble, and there are numerous seams and cracks in it making it difficult to
obtain large slabs.

It occurs in Quebec, Canada, in the Green Mountains of Vermont, and in the
Adirondack Mountains.

248

CHAPTER V
MISCELLANEOUS ROCKS

There are a few rocks which do not fit into any of the three groups
described, such as concretions, geodes, meteorites, etc., and they are
gathered together here. There is also one type of rock, which really belongs
among the minerals, but is likely not to be so recognized at first glance, and
that is the material filling veins. These last are sometimes designated “vein
rocks,” but are really massive deposits of one, two or more minerals, and
should be referred to the minerals when found.

Concretions

In the sedimentary rocks there frequently occur inclusions of a nature
different from the surrounding rock. In shape they are usually rounded,
nodular, spherical, discoidal, ovate, flattened, elongated or ring-shaped, or
combinations of the foregoing, making often curious and fantastic forms. In
size they range from a fraction of an inch in diameter to several feet
through. When broken, they may show a nucleus, around which more or
less concentric layers have formed, or neither nucleus nor concentric 249
structure may be visible. The layered structure of the surrounding
rock in some cases continues right through the nodular mass. These
structures are called concretions, and their formation in all cases is at least
due to similar reactions.

In general the concretions differ from the surrounding rock in composition,
but are usually composed of some one of its impurities, of lime in the clays
or silica in limestones, of iron oxide in sandstone, etc. They seem to have
originated as a result of the solution of the minor mineral, and then its
redeposition around some center or nucleus. In many cases the nucleus is
organic, such as a leaf, a shell, a bone, etc., so that when the concretion is

split, in its center will be found the perfect imprint of the leaf, or the shell of
a mollusk, or a bone of a higher animal, sometimes a whole skeleton. Again
the nucleus may be inorganic like a grain of sand; and in still other cases no
nucleus can be found, though there was probably one in the beginning.
What has happened is somewhat like the case of accessory minerals in
igneous and metamorphic rocks. A layer of sediment was laid down,
including in it, here and there, something foreign to the run of the rock.
Later when the water leaches through this rock, impregnated with lime for
instance, it comes to the point where a leaf is decomposing. The products of
the leaf decomposition are different from what is already present in
solution, and may precipitate some of the lime in that neighborhood. As
long as leaf decomposition continues the precipitation in that region 250
will continue and increase the size of the concretion. This sort of
action accounts for many of the concretions, especially those about organic
remains. In some other cases where there is no nucleus, as the flint in chalk,
what has taken place is that the small amounts of silica in the lime have
been dissolved, and then around some center has constantly been added
more and more non-crystalline silica until a mass of flint has accumulated.
There may be a considerable variety of ways to account for different
concretions, but in all cases solutions of one mineral have come in contact
with solutions of a different kind, and precipitation about a center has
resulted.

Clay stones
Pl. 68

Of all the concretions these are perhaps the commonest, being found in the
clays of all types and in many regions. They are made of lime and
precipitated around some nucleus of foreign matter. The shapes vary widely,
usually discs, flattened ovals or even rings, in most all cases however
flattened. This is indicative of the water moving though the clay more freely
in some layers than others. Often clay stones occur so abundantly that two
or more have grown together making fantastic shapes, sometimes
resembling animals, and all sorts of fancied but unrelated objects. As the
clay stones have grown the clay has not been pushed aside, but has been

incorporated within the concretion; so that when a concretion is dissolved in
acid, it yields not only the lime, which is its reason for being, but also 251
a large amount of clay.

Claystones are found in clays most anywhere, usually occurring in certain
layers and being absent from others.

Lime concretions

These are found mostly in shales which carry a high percentage of clay as
impurities, and are characteristic of the older geological formations,
especially ancient sea bottoms. They are likely to have as a nucleus some
shell, fish bone, or a leaf, which when the concretion is split, reveals a
wonderfully preserved portion of an animal or a plant, which was buried
millions of years ago. The lime concretion is closely related to the
claystone, and is really a claystone which has been buried so long that the
surrounding matrix has changed to a shale instead of remaining clay.

One of the most famous localities for these lime concretions is Mazon
Creek, Illinois, where thousands of these concretions have been picked up
and split to study the organic remains included. The commonest objects
found are fern leaves, like the one on Plate 68. But about once in a thousand
times they inclose a spider or insect, and once in ten thousand times the
skeleton of an amphibian, which is of especial interest, as here have been
thus found the remains of the very earliest of the land animals. These
remains were inclosed in these concretions during the coal age, probably
50,000,000 years ago, and once inclosed all the hard parts have been 252
as well preserved after that long interval, as they were immediately
after being inclosed in the concretion. Lime concretions range from less
than an inch in diameter to several feet through. They are not confined to
shales, but sometimes occur in sandstones, in this case also usually having
as a nucleus either a shell, or the bone, or bones, of some animal.

They are likely to be found anywhere in the limestone belt, from the
Appalachian Mountains to the Rocky Mountains, or in the Great Basin, or
on the Pacific Coast. Often they have been mistaken for turtles and other

objects. A good many of the cases where the head or body of animals
“petrified with all the flesh” are reported, it is one of these concretions
which has a shape sufficiently like the part described, for the imagination to
construct the rest.

Septeria
Pl. 69

Septeria are lime concretions, which, after they had formed, have shrunk
and developed a series of cracks running through them in all sorts of
directions, and since then the cracks have been filled with various minerals,
such as calcite, dolomite, and siderite. These make a series of veins which
intersect the concretion, in a sort of network. Septeria are mostly of
considerable size, ranging from six inches in diameter to several feet
through. They are characteristic of the shales of ancient sea bottoms,
especially those of Devonian age in New York, and Pennsylvania, and those
of Cretaceous age in Wyoming, Montana and the Dakotas.

253

Flint concretions

The silica in limestones is often segregated into nodular masses of varying
sizes, to make concretions of flint. Such masses have grown in the
limestone, and, while growing, have either pushed away, or dissolved the
adjacent limestone, so that the flint nodule is pure silica. They are especially
characteristic of the chalk beds, and of ancient limestones which formed on
the floor of the sea, like the Helderberg Limestone of New York,
Pennsylvania, Ohio, etc. When thin sections are cut through these flints, and
examined under the microscope, many remnants of the shells of plants and
animals are still recognizable. A nucleus is seldom found, but in some cases
there is a fossil in the nodule about which the concretion doubtless formed.
The spicules of sponges, shells of diatoms, and of radiolarians seem to have
contributed most of the material from which flint concretions are formed. In
addition to the silica there are frequently inclosed in these nodules the

horny jaws of various sea worms, and a host of spiny balls the relationships
of which are still unknown.

Sandstone concretions

There are two types of sandstone concretions, first those which are
cemented with lime, and second those cemented with iron oxide. The
concretions bound by lime are especially characteristic of sandstones which
were laid down as river deposits, either in the channels or on the flood
plains, and also the sandy deposits resulting from wind deposition. In these
cases the concretions will mostly be found to have formed around 254
some organic nucleus, most frequently about a bone, or group of
bones, of some ancient animal. In this country they are mostly found in the
arid and semiarid sections of the West, where the present day wind erosion
exposes the harder parts of bluffs, etc.

The second type of sandstone concretion is the one in which the cement is
most often limonite, less often hematite. These concretions are less dense
than the lime ones, and in some cases the limonite is only precipitated at a
distance from the nucleus, which has resulted in the formation of a hollow
shell, filled with loose sand. This is especially characteristic of certain
concretions, found in a gravel or coarse sand in the region of Middletown,
Del.

Oolites

In large bodies of water like the sea and some larger lakes we find
concretions which have formed, or are still forming, about tiny grains of
sand, which are still being moved about by the waves and currents. In such
cases not only are great masses of concretions formed but they have very
clearly marked the concentric layering, which shows that they have
increased in size, sometimes more rapidly and sometimes more slowly.
Where great masses of such concretions have formed the resulting rock
appears like a great mass of small eggs, whence the term oolite. The cement

may be any one of several substances, but lime, silica, and hematite are
perhaps the most common. Here and there are found larger or smaller
masses of this oolite. In some cases it would appear that the material 255
was precipitated by the action of bacteria. Such for instance is
probably the origin of the Clinton iron ore, a bed of oolitic hematite,
extending from New York State all down the Appalachian Mountains to
Alabama.

Pisolite
Pl. 69

When the concretions, formed in exactly the same manner as in the case of
oolite, are of a size bigger than a pea, then the rock is known as pisolite.

Other Concretions

Though less abundant concretion may form from still other substances.
Hematite has been mentioned, and when concretions are made of this
material, either they have been deposited by bacteria, or were formed as
limonite and the water of crystallization of this latter mineral driven off.

Manganese concretions are found on the floor of the ocean at maximum
depths, and brought to the surface by dredging.

Geodes

Geodes are nodules, which, when broken open, are found to be hollow and
the cavity lined with one or more minerals. They represent a special case of
minerals in a cave. There was in the first place a cavity in the surrounding
rock, usually of sand or clay. As the water leached through the surrounding
rock, it became saturated with one or more minerals and then coming 256
into the cavity, deposited the minerals, either as crystals, or as a non-

crystalline mass, lining the cavity. Thus the inside is often a beautiful
cluster of bristling crystals, or it may be simply layer on layer of
chalcedony of any color. Before this process had gone so far as to
completely fill the cavity, erosion had dislodged the mass, and it has been
found. One usually recognizes that it is a geode by the fact that it is far too
light to be a solid rock, and then it may be carefully broken. They are
characteristic of certain formations; so that having accidentally broken the
first one, others can be carefully opened to display the beauty of the interior.
The geode illustrated on Plate 70 is lined with quartz crystals, but near by
were found many others, some of which had chalcedony and some jasper as
a lining. Such crystallined nodules are usually called geodes so long as they
occur in a softer matrix so that they are easily dislodged, and until they
reach a size of three or four feet in diameter.

Pebbles

When picked up either from brook beds, sea beaches, or the open plain,
there are few forms of rock which tell a story of the past more completely
than do pebbles; and any one, who enjoys reading a story written in form,
structure and composition, will find in pebbles one of the most satisfying
and at the same time testing exercises. The story may be complex or simple
according to what has happened to the parent rock, and to that is 257
added what happened since the pebble left the ledge where it was a
part of a great mass. One must not forget to take into consideration where
the pebble was found and the character of its associates. This sort of
exercise is recommended to all interested in rocks. It will yield something
upon first trying, and more on prolonged study; and the fullness with which
it is done will test one’s knowledge of the meaning of rocks as nothing else
will do. As a sample of this sort of exercise let us take the two pebbles
illustrated on Plate 71.

The upper one is a common quartz pebble picked up in a New England
brook bed. Such pebbles are common all over the country formerly covered
by the glacial ice sheet. It is crystalline quartz, but the individual crystals
are not distinguishable, and such quartz is typical as the filling of veins. It
therefore goes back to a time when the rocks were fissured, probably in

connection with the folding accompanying mountain making far to the
north in Canada. Into the fissures thus formed seeped the water which had
been leaching through the adjacent rocks, and it was saturated with silica
which it had dissolved from those rocks. In the open fissure the quartz was
deposited as crystals, which grew finally filling the fissure and crowding
each other so that all the faces were obliterated. The quartz vein was
complete, but it must have been far below the surface of the ground. Time
must have passed, thousands of years of it, until, in the weathering away of
the mountain system, the many feet of overlying rock were removed 258
and this vein was brought to the surface. As the quartz is harder than
the adjacent rocks, the vein soon projected as a ledge. The effect of changes
of temperature in alternately expanding and contracting the rocks developed
cracks, into which water worked its way, and then the breaking was
hastened by the expansion which takes place when water freezes, and in
exposed regions is so effective, because the freezing and thawing are so
often repeated. Finally an angular fragment of quartz was dislodged and lay
on the surface, resistant to the solvent power of the rain. In this case this
happened just before the advance of the great ice sheet. When that came to
the place where the fragment lay, it was picked up along with all other loose
material and partly shoved in front of, but probably mostly carried frozen in
the ice, and journeyed one, two, three hundred, perhaps a thousand miles.
This took many years for the ice moved only a few feet a day. Finally
however it came to the point where the ice melted as fast as it advanced,
and our quartz fragment was dropped at the front of the ice sheet along with
other great masses of till. Here there was abundant water, partly from the
melting of the ice, and partly from the storms which must develop where
there are such contrasts in temperature, as there would be over the ice, on
one hand, and over the bare land in front of the ice on the other hand. A
torrent picked up our fragment and started it on a second journey, banging
against other stones as it rolled along down the stream bed, every time it
struck another stone bruising the corners which soon became 259
rounded. Thus from time to time during high water the quartz
fragment, becoming rounder every time it moved, journeyed down stream,
until it came to the point where the stream emptied into a lake. Here the
current was checked and the stone dropped to the bottom along with other
larger stones to make the delta at the mouth of the stream. There it lay as
long as the lake existed, and would be lying now, but that in New England a

tilting movement of the land tipped the north end of the lake up and the
water all ran out. Then the stream began to flow over its own delta and in
time of freshet tore a channel down through the old delta carrying the
pebble still further down, until it came to the level stretch which represented
the old lake’s bottom and there it dropped the pebble in its bed. And there it
was found and picked up to become the pebble which told the above story
of its life, and to repeat it as often as anyone will look at it with a seeing
eye.

The second pebble is quite a different one. It was picked up in a gravel bank
along a railroad cut, just at the foot of Mt. Toby in Massachussetts, and the
writer has used it many times to test his students, to see if they could read
the story which it tells.

It consists of two sorts of rock, the one, angular fragments of a hornblende
schist, the other, a fine-grained granite filling all the spaces between the
fragments of schist, even in cracks less than a quarter of an inch wide. The
schist is the older rock and in its first appearance represents a deposit 260
of mud (clay and sand) on the floor of the ocean, well out from the
shore, and somewhere off to the east of Mt. Toby, perhaps ten miles,
perhaps more, from the place where it was found. This was back in early
Palæozoic times, millions of years ago.

This deposit was buried by further layers of sediment on the sea bottom and
cemented into a shale. Then during a mountain making period the region
was folded, and the sediments were altered by the combined pressure and
heat, our layer of rock becoming a hornblende schist. After that happened
considerable time must have passed, but just how much is not indicated by
the pebble, before another period of disturbance took place, during which
this deep seated schist was faulted, and shattered to fragments along the line
of breaking. This accounts for the angular fragments. Then into the fissure
thus formed was pressed a molten magma, which while liquid enough to
flow and be squeezed into every opening could not have been very hot; for
not even the corners of the schist fragments are melted or altered, so as to
appear any different from the mass of the schist. The molten magma cooled
rather slowly, making a fine-grained granite. This must all have taken place

far below the surface, or the magma would have cooled into a felsite or
dense lava.

Again a long time must have elapsed, while the rock overlying our piece
was eroded away, so it could come to the surface. Just about the time it did
come to the surface, the Connecticut Valley was formed by a great 261
block, 95 miles long by fifteen to twenty miles wide, dropping down
six or eight thousand feet (probably not all at once but by one or two
hundred feet at a time) between two north and south faults. This took place
in the Triassic Period. Of course the streams then began to wash sand and
stones of all sizes into the hole. Our pebble was one of these. While still an
angular fragment, lying perhaps ten miles east of the Connecticut Valley, a
stream started it moving, and as it rolled along the brook bed, it was
battered and rounded to its present shape, and finally tumbled over a
waterfall to the bottom of the great hole, which had been formed as
described above. Here with other stones it formed part of a coarse gravel,
coarsest near the sides of the hole, and finer toward the middle; for the
material was further distributed in the bottom of the valley. Our stone
stayed pretty near the side and was soon buried beneath hundreds of feet of
similar material. The leaching water dissolved enough iron rust so that this
acted on the lower layers as a cement and bound the whole mass into a
conglomerate.

Here for some millions of years our pebble rested, while above it was piled
sand and gravel and a couple of sheets of lava, until the hole was filled, and
our pebble was near the bottom of the mass. Later movements of the land
raised the whole region, fully six thousand feet, and erosion went on for
other millions of years. The conglomerate and sandstone wore away faster
than the metamorphosed rocks on either side of the filled valley, so 262
that a new valley, the present Connecticut Valley, came into
existence.

When our pebble finally came near to the surface on the side of Mt. Toby (a
mound of conglomerate which somehow was protected and wore down a
little less rapidly than the conglomerate on either side of it), it was just
about the time of the glacial period. The great ice sheet went over the
mountain removing all the loose material and some more of the solid

conglomerate. This brought our pebble to the surface, but too late to be
moved by the ice. However as soon as the ice left the Mt. Toby region, the
rains fell, and in the further weathering of the conglomerate, the cement
holding our pebble in place was dissolved and it was freed. At once a tiny
brook started it rolling down the side of the mountain, a brook so small that
when the pebble reached the foot of the slope it did not have power to carry
it further. Here there gathered a fan-shaped mound of such pebbles, known
as an alluvial fan. It rested here not over a couple of thousand years, when
the Central Vermont R. R. cut a groove through the fan, using the material
for ballast, and here the pebble was found and brought home.

Meteorites

Meteorites can hardly be called common, but there is always a chance of
finding one, and their interest is so great, that none should escape because
unrecognized.

Meteorites are visitors to the earth from space, and they bring to us 263
knowledge of the composition of planets and solar systems, other
than our own. It is of interest to note, that while they have brought to us
some combinations of elements which do not occur in the earth, still they
have not brought any element with which we were not already familiar.
They are popularly known as “falling” or “shooting stars,” though of course
they are not stars, but only small masses of matter which are entirely
invisible until they come inside our atmosphere.

In space there are many small (compared with the size of the earth) chunks
of matter, each pursuing its solitary way around the sun, or wandering
through space along paths entirely unrelated to the sun. From time to time
one of these passes near enough to the earth, so as to be influenced by its
attraction, and then comes rushing toward it at tremendous speed, 20 to 30
miles per second. As soon as it comes into the atmosphere, even the very
attenuated atmosphere, a couple of hundred miles above the surface,
friction heats the surface of the meteor until it glows, and by that light we
see the so-called shooting star, often with a trail of luminous matter
streaming out behind. Of course in using this term “shooting star,” we

understand the meteor is no star, for they are bodies as big as our sun,
shining at distances billions of miles away.

As the meteor rushes through the atmosphere it may all burn up, no large
fragment reaching the earth’s surface. The luminous matter streaming 264
out behind is material which has melted and dripped off the main
mass. As this oxidizes and cools, that part which did not become gaseous
will finally fall to the earth as fine dust. When however a meteor actually
falls to the earth, its surface is still hot, though probably there has not been
time enough for much heat to be transmitted to the interior. At any rate they
do not show any alteration due to this cause. On landing and sometimes
before they land meteors break into two or more pieces. When found the
surface always shows the effects of the heat generated by the friction of
passing through the air, the surface being smoothed, and covered with
stream lines and melted out pits and hollows, and the outer surface
consisting of a thin crust, making an appearance, which once seen, can
hardly be mistaken.

There are two types of meteorites, those made wholly or largely of iron with
some nickel, and appearing like great chunks of iron, and those which are
stony and resemble a granite boulder. In collections the first sort, i.e. iron
meteorites, are most abundantly represented, because most easily
recognized when found. They consist of masses of iron and nickel with
small amounts of other elements, ranging in size from the Cape York
meteorite, which fell in northern Greenland in 1894 and was later brought
by Peary to the American Museum, and weighs some 36 tons, down to
small grains as small as a grain of wheat. The largest one which has fallen
in the United States was the Willamette meteorite weighing some 15 tons,
and falling 19 miles south of Portland, Oregon. These and all iron 265
meteorites have the iron in crystalline form which is readily seen if
the meteorite is cut, and the surface thus made polished, then etched with
acid, which is put on and quickly washed off. Every meteorite has its
particular pattern, as illustrated on Plate 72, and by these patterns can be
identified. Meteorites have a high value and are eagerly sought by certain
large institutions and collectors. Since the crystalline structure is so
characteristic of each fall, when a new meteorite is found, it is usually cut in
two, and one part retained by the finder or some institution; while the other

part is cut into small pieces, an inch or two on a side and a quarter of an
inch thick, but each large enough to show the characteristic pattern. These
are distributed largely by sale to other collectors. Thus a great meteorite
collection consists of a few large meteorites and a great many small
portions of other meteorites.

The second type of meteorite is the stony meteorite. Where meteorites have
been located as they fell and recovered, the majority of them were of this
type, so that probably more than half of the meteorites which fall are of the
stony type. However when the stony meteorite is exposed to weathering it
takes only a very short time before the surface is eroded off and then such a
meteorite looks like any other boulder and probably most of them fail to be
recognized, and so have been lost. Because they have so much greater
variety, they are in many ways of greater interest than the iron type.

It is desirable that every one have his eye out for meteorites, and 266
when found it is desirable that the fact should be reported to some
one of the great institutions which collect them, such as the National
Museum in Washington, or the American Museum in New York. Each one
should be on record even if it is desired to keep it in a private collection.

Fossils

In the sedimentary rocks one is apt to find remains of some of the animals
and plants that lived at the time the rock was forming. While the soft parts
of animals decompose rapidly, shells and bones are likely to be buried in
the sediments, and if the conditions have been favorable, these remains may
be preserved more or less perfectly. All through the millions of years that
sedimentary rocks have been forming in the sea, in lakes, on river flood
plains and in wind swept deserts, there was an abundance of life, as much
as there is today; and our knowledge of that life is derived from these
buried fossil remains, so that fossils have a great historic interest.

However as there have lived and died several times as many different kinds
of animals as live today, the study of fossils becomes a separate subject,
which cannot be treated in this book. Should any collector of rocks and

minerals come upon fossils, he is opening a new field, and it will be
necessary to turn to other sources for their identification. General books on
this subject are scarce, but one or two are given in the literature list.

267

A List of the Elements, the Abbreviations Used for Them,
and Their Atomic Weight, Which Is Approximately the
Number of Times Heavier They Are Than Hydrogen.

Name Oxygen = 16
Aluminium, Al 27
Antimony, Sb 122
Argon, Ar 40
Arsenic, As 75
Barium, Ba 137
Beryllium, Be 9
Bismuth, Bi 209
Boron, B 11
Bromine, Br 80
Cadmium, Cd 112
Cæsium, Cs 132
Calcium, Ca 40
Carbon, C 12
Cerium, Ce 140
Chlorine, Cl 35
Chromium, Cr 52
Cobalt, Co 59
Columbium, Cb 93
Copper, Cu 64
Dysprosium, Dy 162
Erbium, Er 167
Europium, Eu 152

Fluorine, F 19
Gadolinium, Gd 157
Gallium, Ga 70
Germanium, Ge 63
Glucinum, Gl 9
Gold, Au 197
Hafnium, Hf 179
Helium, He 4
Holmium, Ho 165
Hydrogen, H 1
Indium, In 115
Iodine, I 127
Iridium, Ir 193
Iron, Fe 56
Krypton, Kr 84
Lanthanum, La 139
Lead, Pb 207
Lithium, Li 7
Lutecium, Lu 175
Magnesium, Mg 24
Manganese, Mn 55
Mercury, Hg 201
Molybdenum, Mo 96
Neodymium, Nd 144
Neon, Ne 20
Nickel, Ni 59
Nitrogen, N 14
Osmium, Os 190
Oxygen, O 16
Palladium, Pd 107
Phosphorus, P 31
Platinum, Pt 195

Potassium, K 39
Præseodymium, Pr 141
Protoactinium, Pa 231
Radium, Ra 226
Radon, Rn 222
Rhenium, Re 186
Rhodium, Rh 103
Rubidium, Rb 85
Ruthenium, Ru 102
Samarium, Sm 150
Scandium, Sc 45
Selenium, Se 79
Silicon, Si 28
Silver, Ag 108
Sodium, Na 23
Strontium, Sr 88
Sulphur, S 32
Tantalum, Ta 181
Tellurium, Te 128
Terbium, Tb 159
Thallium, Tl 204
Thorium, Th 232
Thulium, Tu 169
Tin, Sn 119
Titanium, Ti 48
Tungsten, W 184
Uranium, U 238
Vanadium, V 51
Xenon, Xe 131
Ytterbium, Yt 173
Yttrium, Y 89
Zinc, Zn 65

Zirconium, Zr 91
268

Table of Geologic Time

Eras
Periods and
their Duration
in Millions of Important Physical Important Organic
Years Events Events
Cenozoic
Quaternary
Recent Youthful land forms Dominance of man.
having high relief
formed.
Pleistocene 2 M.Y. Period of glaciation; Heidelberg,
Epoch four great ice Neanderthal, and
advances. Crô-Magnon man;
extinction of large
mammals.
Tertiary
Pliocene 10 M.Y. Continuing world- Intermigration of
Epoch wide land elevation. North and South
American mammals.
Transformation of
ape to man.
Miocene 18 M.Y. Cordilleras, Alps, Culmination of
Epoch Himalayas formed. modern types of
Widespread mammals. Apes
vulcanism-basalt appear in Old World.
flows in
northwestern United
States.
Oligocene 10 M.Y. Land dominant; seas Carnivores and
Epoch marginal. ungulates develop

into importance.
Eocene 20 M.Y. Extensive Dawn of the
Epoch sedimentation; seas dominance of
marginal. mammals. Reptiles
subordinate.
Cretaceous 65 M.Y. Widespread Climax and
epicontinental seas. culmination of
Laramide revolution reptiles, especially
at close of period— dinosaurs; first
Rocky Mountains flowering plants and
formed. grasses.
Mesozoic
Jurassic 38 M.Y. Continent emergent; Rise of birds and
shallow seas on flying reptiles, first
western North modern trees.
America.
Triassic 35 M.Y. Continent emergent; Rise of dinosaurs,
seas marginal. cycads, and
ammonites.
Paleozoic
Permian 35 M.Y. World-wide Extinction of most
continental uplift and Paleozoic fauna and
mountain building. flora. First modern
Widespread insects.
glaciation.
Pennsylvanian 48 M.Y. Continent alternately Great coal-forming
rising and sinking. forests, of ferns and
seed-ferns.
Mississippian 35 M.Y. Low lands and Culmination of
widespread crinoids, numerous
submergence. sharks.
Devonian 40 M.Y. Widespread First known land
submergence, local animals, first forests.
vulcanism.

Silurian 28 M.Y. Widespread First lung fishes and
submergence, local scorpions, abundant
deserts. corals.
Ordovician 65 M.Y. 60% of North Climax of
America below sea. invertebrate
dominance, first
vertebrate.
Cambrian 105 M.Y. Widespread First abundant
submergence. invertebrate fauna,
trilobites dominant.
Proterozoic 700 ± M.Y. Long periods of Bacteria and
granite intrusion, seaweeds present.
sedimentation, and Most invertebrates
mountain building. probably present, but
remains are lacking.
Archeozoic 800 ± M.Y. World-wide intrusive Blue-green algae
igneous activity; present, primitive
some sediments. one-celled plants and
animals probably
present.
270

BIBLIOGRAPHY

MINERALOGY

Getting Acquainted with Mineralogy. By G. L. English, 1936, McGraw-Hill
Book Co. A beginning textbook of mineralogy.

Introduction to the Study of Minerals and Rocks. 3rd Edition, by A. F.
Rogers, 1937, McGraw-Hill Book Co. Describes the commoner
minerals systematically.

Dana’s Textbook of Mineralogy. 4th Edition, revised by W. E. Ford, 1932,
John Wiley and Sons. Detailed descriptions of minerals, their physical
properties, and their occurrence.

Manual of Mineralogy. 15th Edition, by E. S. Dana, revised by C. S.
Hurlburt, 1941, John Wiley and Sons. A textbook of mineralogy.

MINERAL ECONOMICS, GEOPOLITICS

World Minerals and World Peace. By C. K. Leith, J. W. Furness, and
Cleona Lewis, 1943, The Brookings Institution. Physical, economic,
and political trends in the mineral industry.

Minerals in World Affairs. By T. S. Lovering, 1943, Prentice-Hall.

Minerals Yearbook. U. S. Bureau of Mines. An annual volume presenting
statistical data on the production of the mineral resources of the United
States. Reports on individual minerals or rocks may be had separately.

ECONOMIC GEOLOGY

Mineral Deposits. 4th Edition, by W. Lindgren, 1933, McGraw-Hill Book
Co. The manner of occurrence and origin of mineral deposits.

Elements of Engineering Geology. 2nd Edition, by H. Ries and T. L.
Watson, 1947, John Wiley and Sons.

This Fascinating Oil Business. By M. W. Ball, 1940, Bobbs-Merrill Co. A
simple and elementary description of the petroleum industry.

Geology of Coal. By O. Stutzer and A. C. Noe, 1940, University of Chicago
Press.

GENERAL GEOLOGY

Down to Earth. By C. Croneis and W. C. Krumbein, 1936, University of
Chicago Press. An introduction to geology, profusely illustrated.

Textbook of Geology Part I—Physical Geology. 4th Edition, by C. R. 271
Longwell, A. Knopf, and R. F. Flint, 1939, John Wiley and
Sons. A standard text on geology.

Field Geology. 4th Edition, by F. H. Lahee, 1941, McGraw-Hill Book Co.
Recognition and interpretation of geologic structures and topographic
forms as they are observed, and methods of geologic work.

PRECIOUS STONES

A Book of Precious Stones. By J. Wodiska, 1910, G. P. Putnam’s Sons.
Written for jewelers, but of general interest.

The Curious Lore of Precious Stones. By G. F. Kunz, 1913, Lippincott.
Legends and stories of the gem minerals.

The Magic of Jewels and Charms. By G. F. Kunz, 1915, Lippincott.

Popular Gemology. By R. M. Pearl, 1948, John Wiley and Sons. Scientific
and industrial uses of gems, current information about their locality
and production.

FOSSILS

An Introduction to the Study of Fossils. By H. W. Shimer, 1933, Macmillan
Co. An introductory textbook about fossil plants and animals.

Invertebrate Paleontology. By W. H. Twenhofel and R. P. Shrock, 1935,
McGraw-Hill Book Co.

Textbook of Geology Part II—Historical Geology. 4th Edition, by C.
Schuchert and C. O. Dunbar, 1941, John Wiley and Sons. The story of
the development of life through the ages.

273

INDEX

A B C D E F G H I J K L M N O P Q R S T U V
W X Y Z

A
Actinolite, 120
Adobe, 210
Agate, 107
Agate, moss, 73, 108
Alabaster, 152
Albertite, 229
Albite, 110, 113, 115
Almandine, 97
Almandite, 122, 123
Aluminum bronze, 74
Aluminum group, 73
Amazon stone, 114
Amber, 223
Amethyst, 104
Amethyst, Oriental, 75
Amianthus, 120
Amphibole group, 119
Amygdoloid, 194
Amygdoloidal, 176
Analcite, 141
Andesite, 113, 187
Andradite, 122, 124
Anglesite, 62
Anhydrite, 149
Anorthite, 110, 113
Anorthosite, 183
Anthracite, 218, 222