Field Book of Common Rocks and Minerals - For identifying the Rocks and Minerals of the United States - and interpreting their Origins and Meanings

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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: Corbin, Walter Everett, Loomis, Frederic Brewster
Language: English
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*** Start of this LibraryBlog Digital Book "Field Book of Common Rocks and Minerals - For identifying the Rocks and Minerals of the United States - and interpreting their Origins and Meanings" ***

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

Twenty-sixth Impression
Revised 1948

Made in the United States of America

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

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
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 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 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.

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

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_) 287
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
Plate 23.—Crystals of green corundum in syenite, from Montana.
Typical crystal forms of corundum 322
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
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_) 300
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

FIELD BOOK OF
COMMON ROCKS AND MINERALS

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 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
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 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.

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 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 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.

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 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 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 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 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, 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 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 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 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 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
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 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 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:

specific gravity = (x)/(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.

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 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.

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 cinnabar
dark red
ochre 7 white non-crystalline jasper
6 ochre red color red to hematite
almost black
rose 4 white effervesces in rhodochrosite
warm acid
dark 4 orange zincite
2.5 purplish red surface tarnishes pyrargyrite
black
brownish 3.5 brownish red cuprite
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 green malachite
olive 6.5-7 white in igneous rocks olivine
3.5 white to yellow pyromorphite
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 serpentine
to olive
Yellow
golden 2.5 shining non-crystalline gold
brassy 6 greenish-black usually crystalline pyrite
6 greenish-gray color pale brassy marcasite
yellow, usually
non-crystalline
5.5 greenish-black colors nitric acid millerite
green
4 greenish-black color golden chalcopyrite
similar to gold
3.5 dark brown purplish tarnish tetrahedrite
on surface
bronze 5.5 pale color with coppery niccolite
brownish-black cast
4 dark gray-black with speedy black pyrrhotite
tarnish
3 gray-black brownish with bornite
bluish 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 yellow compact to earthy limonite
masses
5 brownish-yellow goethite
4.5 black wolframite
3.5 yellowish-brown sphalerite
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-brown franklinite
6 black magnetic magnetite
5.5 dark brown chromite
5.5 black yellow precipitate wolframite
in sulphuric acid
5-6 black non-magnetic ilmenite
5-6 brownish-black compact masses psilomelane
5 brownish-yellow surface often goethite
brownish
3.5 dark brown tetrahedrons tetrahedrite
2.5 silvery fresh surfaces silver
silver color
2.5 scarlet fresh surfaces prousite
bright red
2.5 purplish red fresh surfaces red pyrargyrite
2 black earthy masses pyrolusite
1 steel gray greasy feel graphite
Metallic 2.5 black tarnishes black, chalcocite
Gray 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 smaltite
nitric acid
4.5 steel gray very heavy platinum
4 reddish black often in striated manganite
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 cobaltite
nitric acid
pearly 1-1½ shining exposed surfaces cerargyrite
violet brown
White, with 4 white porcelainous magnesite
impurities masses,
effervesces in acid
grayish 2 white earthy masses, kaolinite
or greasy feel
yellowish
1-3 white earthy masses bauxite
1 white greasy feel, talc
fibrous or scaly

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 calcite
cleavage
3 no effervescence, but soluble in anglesite
nitric 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 feldspar
and 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 strontianite
in flame
3.5 effervesces in acid, heavy, green witherite
color in 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 cerrusite
color flame
3-3.5 no effervescence, cleavage in three anhydrite
directions at right angles
3 effervesces in acid, rhomboidal calcite
cleavage
3 tabular crystals, heavy, green color barite
in 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 serpentine
fibrous
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

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 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 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,
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 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
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 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 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 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 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, 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, 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 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,
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 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
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 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 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 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 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 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 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 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
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.

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 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, 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 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 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 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 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 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
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
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 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, 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 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
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 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
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
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
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, 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.

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.

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
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 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 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
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.

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 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, 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.

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 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 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,
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.

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
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 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 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
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 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 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. 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 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 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, 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
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 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
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.

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
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 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 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
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 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
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
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 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 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 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 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 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, 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 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 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_.

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.,
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 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 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
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.

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 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 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

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
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 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
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
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,
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 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
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;
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
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
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.

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 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 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 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.

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 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 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 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 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.

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.

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 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-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 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
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 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 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 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 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 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 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 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 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. 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 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, 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 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 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.

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 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 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, 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 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 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 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 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.

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, 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.

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 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. 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.

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
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.

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 hornblende and/or augite Mica and/or hornblende with pyroxene augite and/or hornblende
and/or mica
+quartz -quartz +quartz -quartz +olivine -olivine +olivine -olivine

Dense rhyolite trachite dacite (felsite) andesite (felsite) basalt augitite or
hornblendite
Porphyritic rhyolite-porphyry trachite-porphyry dacite-porphyry andesite-porphyry basalt-porphyry augitite-porphyry
Granitoid granite syenite quartz-diorite diorite olivine-gabbro gabbro peridotite pyroxenite
Fragmental rhyolite, tuff trachite, tuff Dacite, tuff or andesite tuff or Basalt tuffs and breccias
or breccia or breccia breccia breccia

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, 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 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 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 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.

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 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.

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 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.

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.

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 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 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 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 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.

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 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 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
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 talus
resulting from weathering
2. The same fragmentary material breccia
cemented
3. Unsorted material resulting from soil
rock weathering
4. Coarse fragments rounded by the gravel
action of water and wind
5. The same material cemented conglomerate
6. Finer material deposited by water sand
or wind
7. The same material cemented sandstone
8. The finest material, mostly clay
kaolin, deposited by water
9. The finest material, deposited by loess
wind
10. The same material cemented shale
11. Fine particles of lime, pure or marl
impure
12. The same material cemented limestone
13. Unassorted material left by the till
glacial ice
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, diatomaceous earth, etc.
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.

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
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. 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 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.

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.

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 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 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 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 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, 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 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 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.

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 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 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, 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 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 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 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.

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,
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 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 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 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 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 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 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 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 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 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 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
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.

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
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, 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
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, 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 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 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.

_Loose sediment_ _Consolidated sediment_ _Metamorphic
equivalent_

gravel conglomerate gneiss
sand (quartz) sandstone quartzite
mud (sand and clay) shale schist
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 schist
trachite, rhyolite, etc.

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 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 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.

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
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 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 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, 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
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 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.

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.

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 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 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 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 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.

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 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 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 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 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 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 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 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
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 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
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 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 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 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.

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

Table of Geologic Time

_Eras_
_Periods and their _Important Physical _Important
Duration in Millions Events_ Organic Events_
of Years_

Cenozoic
Quaternary
Recent Youthful land forms Dominance of man.
having high relief
formed.
Pleistocene Epoch 2 M.Y. Period of Heidelberg,
glaciation; four Neanderthal, and
great ice advances. Crô-Magnon man;
extinction of
large mammals.
Tertiary
Pliocene Epoch 10 M.Y. Continuing Intermigration of
world-wide land North and South
elevation. American mammals.
Transformation of
ape to man.
Miocene Epoch 18 M.Y. Cordilleras, Alps, Culmination of
Himalayas formed. modern types of
Widespread mammals. Apes
vulcanism-basalt appear in Old
flows in World.
northwestern United
States.
Oligocene Epoch 10 M.Y. Land dominant; seas Carnivores and
marginal. ungulates develop
into importance.
Eocene Epoch 20 M.Y. Extensive Dawn of the
sedimentation; seas dominance of
marginal. mammals. Reptiles
subordinate.
Cretaceous 65 M.Y. Widespread Climax and
epicontinental culmination of
seas. Laramide reptiles,
revolution at close especially
of period—Rocky dinosaurs; first
Mountains formed. flowering plants
and grasses.
Mesozoic
Jurassic 38 M.Y. Continent emergent; Rise of birds and
shallow seas on flying reptiles,
western North first modern
America. trees.
Triassic 35 M.Y. Continent emergent; Rise of
seas marginal. dinosaurs,
cycads, and
ammonites.
Paleozoic
Permian 35 M.Y. World-wide Extinction of
continental uplift most Paleozoic
and mountain fauna and flora.
building. First modern
Widespread insects.
glaciation.
Pennsylvanian 48 M.Y. Continent Great
alternately rising coal-forming
and sinking. forests, of ferns
and seed-ferns.
Mississippian 35 M.Y. Low lands and Culmination of
widespread crinoids,
submergence. numerous sharks.
Devonian 40 M.Y. Widespread First known land
submergence, local animals, first
vulcanism. forests.
Silurian 28 M.Y. Widespread First lung fishes
submergence, local and scorpions,
deserts. abundant 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
mountain building. invertebrates
probably present,
but remains are
lacking.
Archeozoic 800 ± M.Y. World-wide Blue-green algae
intrusive igneous present,
activity; some primitive
sediments. one-celled plants
and animals
probably present.

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.
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.

INDEX

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
Antimony, 81
Antimony, gray, 81
Apatite, 160
Aquamarine, 125
Aragonite, 147
Argentite, 35
Argillite, 242
Arkose, 206
Arsenic group, 78
Arsenopyrite, 79
Asbestos, 120, 140
Augite, 118
Aventurine, 104
Azurite, 46

B
Barite, 154
Barium group, 154
Basalt, 188
Batholith, 174
Bauxite, 77
Beryl, 125
Beryl, golden, 125
Beryllium, 125
Bibliography, 270
Biotite, 129, 130
Bitumen, 228
Black jack, 65
Bloodstone, 106
Bog lime, 213
Bombs, 191
Boracite, 164
Borax, 165
Bornite, 41
Brass, 64
Breccia, 191, 198
Brittania metal, 81
Bronze, 38
Bronze Age, 38
Bronzite, 118
Bytownite, 113

C
Calamine, 68
Calaverite, 30
Calcite, 144
Calcium, 143
Carbon, 156
Carbonite, 222
Carbuncle, 124
Carnelian, 106
Carnotite, 90
Cassiterite, 93
Cat’s eye, 104
Celestite, 153
Cerargyrite, 37
Cerrusite, 61
Ceylonite, 97
Chalcedony, 104, 106
Chalcocite, 42
Chalcopyrite, 40
Chalcotrichite, 45
Chalk, 213
Chert, 107
Chlorite, 140
Chlorospinel, 98
Chromite, 87
Chromium, 86
Chrysocola, 47
Chrysolite, 134, 140
Chrysoprase, 106
Cinnabar, 91
Cinnamon stone, 123
Citrine, 103
Clay, 207
Clay, ball, 208
Clay, brick, 209
Clay, china, 208
Clay, fire, 208
Clay, paving brick, 209
Clay, sewer-pipe, 209
Clay, slip, 209
Clay, stoneware, 209
Clay stones, 250
Cleavage, 21
Cleavage, slaty, 234
Coal, 217
Coal, bituminous, 212, 220
Coal, cannel, 221
Coal, hard, 222
Coal, soft, 220
Cobalt, 84
Cobalt bloom, 85
Cobalt glance, 85
Cobalt gray ore, 85
Cobaltite, 83
Coke, 220
Colemanite, 165
Collecting, 5, 7
Color, 23
Concretions, 248
Concretions, flint, 253
Concretions, lime, 251
Concretions, other, 255
Concretions, sandstone, 253
Conglomerate, 202
Copper, 37, 39
Copper, blushing, 42
Copper, glance, 42
Copper, grey, 43
Copper, peacock, 42
Copper, plush, 45
Copper, purple, 41
Copper, red, 44
Copper, variegated, 42
Copper, yellow, 40
Coquina, 213
Coral, 146
Coral rock, 214
Corundum, 75
Crude oil, 227
Cryolite, 78
Crystal balls, 101
Crystal formation, 14
Crystal rock, 103
Crystal structure, 11
Crystal systems, 13-18
Cuprite, 44
Cyanite, 128

D
Dacite, 187
Dense, 176
Diamond, 157
Diamonds, Matura, 127
Diamonds, slave’s, 133
Diatoms, 231
Dikes, 174
Diorite, 182
Dog-tooth spar, 145
Dolomite, 99
Dry bone, 68

E
Earth, diatomaceous, 23
Elements, listed, 267
Emerald, 125
Emerald, Oriental, 75
Emery, 76
Enstatite, 117
Epidote, 134
Equipment, 7
Erubescite, 42
Extrusive, 173

F
Feldspar, 110
Feldspar, alkalic, 111
Felsite, 186
Felsitic, 176
Ferromanganese, 70
Flagstone, 207
Flint, 106
Fluorine, 162
Fluorite, 162
Fossils, 266
Fragmental, 176
Franklinite, 69
Freestone, 207

G
Gabbro, 183
Galena, 60
Garnet group, 121
Garnet, Sirian, 123
Geodes, 255
German silver, 82
Gilsonite, 229
Glassy, 176
Glucinum, 125
Gneiss, 237
Goethite, 51, 52
Gold, 31
Gold foil, 64
Gold group, 29
Gossan, 50
Granite, 178
Granite, graphic, 179
Granitoid, 176
Graphite, 156, 219
Gravel, 201
Graywacke, 206
Grit, 206
Grossularite, 122, 123
Guano, 230
Gumbo, 210
Gypsum, 150

H
Halite, 163
Hardness, 20
Hardpan, 216
Heavy spar, 154
Heliotrope, 106
Hematite, 53
Hemihedral forms, 19
Hercynite, 98
Hexagonal system, 18
Hornblende, 121
Hornstone, 107
Hyacinth, 127
Hypersthene, 118

I
Ice, 167
Iceland spar, 145
Ice stone, 78
Ilmenite, 94
Intrusive, 174
Iron, 47
Iron, bog, 50
Iron, chromic, 87
Iron, magnetic, 54
Iron pyrites, 56
Iron, spathic, 55
Iron, specular, 53
Isometric system, 13

J
Jacinth, 127
Jargons, 127
Jargoons, 127
Jasper, 106
Jet, 222

K
Kaolin, 137, 208
Kaolinite, 137

L
Labels, 5
Labradorite, 113, 116
Laccolith, 174
Lapilli, 191
Lava, 173
Lead, 59
Lead glance, 60
Lead, green ore, 63
Lead, white ore, 61
Lepidolite, 129, 130
Lignite, 218, 219
Limestone, 212
Limestone, encrinal, 214
Limestone, hydraulic, 214
Limestone, lithographic, 214
Limonite, 49, 51
Loess, 210
Luster, 23

M
Magma, 173
Magnesite, 98
Magnesium group, 96
Magnetite, 54
Malachite, 45
Malanite, 124
Malta, 229
Manganese group, 70
Manganite, 72
Marble, 243
Marble, Suisun, 146
Marcasite, 57
Marl, 211
Mercury, 90
Meteorites, 262
Mica group, 128
Microcline, 113, 114
Millerite, 83
Mineral tables, 25
Minerals, defined, 10
Molybdenite, 81
Molybdenum, 80
Monoclinic system, 17
Monzonite, 181
Morion, 103
Mother-of-pearl, 148
Muscovite, 129

N
Natrolite, 142
Natural gas, 227
Needle iron stone, 52
Niccolite, 83
Nickel, copper, 83
Nickel group, 82

O
Obsidian, 191
Ochre red, 54
Ochre yellow, 49
Oligoclase, 113, 115
Olivine, 134
Olivine-gabbro, 183
Onyx, 108
Onyx, Californian, 146
Onyx marble, 215
Onyx, Mexican, 146
Oolites, 254
Opal, 108
Opal-agate, 109
Opal, common, 109
Opal, fire, 109
Opal, precious, 109
Ophicalcite, 246
Ophiolite, 246
Orpiment, 80
Orthoclase, 110, 113
Orthorhombic system, 16

P
Paste, 103
Pearls, 148
Pearlstone, 193
Peat, 218, 219
Pebbles, 256
Pegmatite, 179
Peridot, 134
Peridotite, 184
Perlite, 193
Petroleum series, 224, 227
Pewter, 60
Phenocrysts, 189
Phlogopite, 129, 131
Phosphate, 160, 230
Phosphorus, 159
Phyllite, 242
Picotite, 97
Pisolite, 255
Pitchstone, 193
Plagioclase, 111
Plasma, 106
Platinum, 95
Plumbago, 156
Porous, 176
Porphyritic, 176
Porphyry, 189
Prase, 104
Prousite, 36
Psilomelane, 72
Pumice, 193
Pyrargyrite, 35
Pyrite, 56
Pyrite, capillary, 83
Pyrite, magnetic, 58
Pyrite, white, 57
Pyritohedron, 56, 318
Pyrolusite, 71
Pyromorphite, 63
Pyrope, 122, 123
Pyroxene group, 116
Pyroxenite, 185
Pyrrhotite, 58

Q
Quartz, 100
Quartz-diorite, 181
Quartz, milky, 103
Quartz, rose, 104
Quartz, smoky, 103
Quartzite, 239
Quicksands, 204
Quicksilver, 90

R
Radium, 89
Realgar, 80
Rhinestones, 101
Rhodochrosite, 73
Rhyolite 185
Rock, phosphate, 230
Rocks, 170
Rocks, defined, 10
Rocks, igneous, 172
Rocks, igneous, classified, 177
Rocks, metamorphic, 232
Rocks, metamorphic, classified, 236
Rocks, sedimentary, 194
Rocks, sedimentary, classified, 196
Rubicelle, 97
Ruby, 75
Ruby, Balas, 97
Ruby mica, 52
Rutile, 94

S
Salt, 163
Sand, 202
Sandstone, 205
Sapphire, 75
Sapphire, Oriental white, 75
Sardonyx, 108
Satin spar, 146
Schist, 240
Schistosity, 234
Scoria, 192, 193
Septeria, 252
Sericite, 130
Serpentine, 139, 245
Shale, 210
Shale, oil-bearing, 225
Sheet, 173
Siderite, 55
Silica, 99
Silicates, 99
Silicon, 99
Sill, 174
Sillimanite, 128
Silver, 34
Silver, dark red, 35
Silver, German, 65
Silver glance, 35
Silver group, 32
Silver, horn, 37
Silver, light red, 36
Silver, ruby, 35
Sinter, 110
Slate, 241
Smalt, 84
Smaltite, 85
Smithsonite, 68
Soapstone, 244
Sodalite, 126
Soil, 198
Solder, 60
Specific gravity, 22
Speigeleisen, 70
Spelter, 64
Spessartite, 122, 123
Sphalerite, 65
Spinel, 97
Spinel-ruby, 97
Stalactites, 146
Stalagmites, 146
Staurolite, 133
Steatite, 244
Stellite, 84, 88
Stibnite, 81
Stilbite, 143
Stock, 174
Streak, 23
Strontianite, 152
Strontium group, 152
Sulphur, 166
Syenite, 180
Sylvanite, 30

T
Talc, 138
Talus, 197
Tetragonal system, 15
Tetrahedrite, 43
Tile ore, 45
Till, 215
Tillite, 217
Time chart, 268
Tin, 92
Tin stone, 93
Titanium, 93
Tonalite, 181
Topaz, 131
Topaz, false, 103
Topaz, Oriental, 75
Topaz, Saxon, 132
Topaz, Scotch, 132
Topaz, smoky, 132
Topaz, Spanish, 132
Tourmaline, 135
Trachite, 186
Trap, 188
Travertine, 146, 215
Tremolite, 120
Triclinic system, 18
Tripolite, 110
Tufa, calcareous, 147
Tuff, 190
Tungsten, 87
Turgite, 51
Turquois, 161
Twinning, 19
Type metal, 60

U
Uintaite, 229
Uranium, 89
Uvarovite, 122, 123

V
Vanadium, 89
Verde antique, 247
Volcanic ash, 190
Volcanic blocks, 191

W
Water, 167
White metal, 64
Willemite, 67
Witherite, 153
Wolframite, 88
Wood, agatized, 108
Wood, opalized, 109
Wood, silicified, 108

X
Xanthosiderite, 51

Z
Zeolites, 141
Zinc, 63
Zinc blende, 65
Zinc red ore, 66
Zinc, ruby, 65
Zincite, 66
Zircon, 127

Plate Frontispiece

[Illustration: Tourmaline crystals, growing amid feldspar crystals
in a cavity in granite, from Paris, Me.]

Plate 5

[Illustration: Gold in quartz, from California]

Plate 6

[Illustration: Native silver in calcite]

[Illustration: Argentite, the black masses throughout the white
quartz]

Plate 7

[Illustration: Pyrargyrite as it appears after moderate exposure to
the light.]

[Illustration: Crystal form of Pyrargyrite]

[Illustration: Prousite as it appears after moderate exposure to the
light]

Plate 8

[Illustration: Native copper from Michigan]

[Illustration: Chalcopyrite in tetrahedrons and an occasional
octahedron.]

Plate 9

[Illustration: Chalcocite crystals with the bluish tarnish]

[Illustration: Tetrahedrite crystals]

Plate 11

[Illustration: Cuprite, the red crystals showing characteristic
color, other showing the green tarnish of malachite]

[Illustration: Malachite (green) and azurite (blue), the two
minerals shown together as they very commonly occur]

Plate 12

[Illustration: Limonite]

[Illustration: The crystal form in which goethite is found, _p_ is
the prism faces, _b_ and _c_ are faces formed by beveling the edges
of the prism, _o_ is the pyramidal face characteristic of the ends]

Plate 13

[Illustration: Hematite, Clinton iron ore, oolitic]

[Illustration: Siderite crystals]

Plate 15

[Illustration: Pyrite crystals]

[Illustration: Marcasite in concretionary form with radiate
structure]

Plate 17

[Illustration: Galena in crystals]

[Illustration: Pyromorphite crystals (green)]

Plate 19

[Illustration: Sphalerite, some the normal yellow and some crystals
with the reddish tinge. (White is dolomite)]

[Illustration: Zincite]

Plate 21

[Illustration: Smithsonite in yellow crystals]

[Illustration: Franklinite in octahedral crystals]

Plate 24

[Illustration: Arsenopyrite, showing crystals massed so as to be
incompletely developed]

[Illustration: Realgar as it usually occurs in powdery
incrustations]

Plate 25

[Illustration: Large crystal of stibnite, the light colored face is
the one parallel to which cleavage occurs]

[Illustration: Niccolite as a vein in slate]

Plate 26

[Illustration: Cobaltite, silver color, with pink tinge]

[Illustration: Smaltite, pink is cobalt bloom]

Plate 27

[Illustration: Carnotite from southwest Colorado]

[Illustration: Cinnabar]

Plate 31

[Illustration: Amethyst, not however deep enough colored for gems]

[Illustration: Jasper, with botryoidal surface]

Plate 32

[Illustration: Banded Agate from Brazil]

Plate 33

[Illustration: Common Opal from Arizona]

[Illustration: Siliceous sinter or Geyserite from The Yellowstone
Park]

Plate 35

[Illustration: A group of Microcline crystals from Pike’s Peak,
Colo.]

[Illustration: Labradorite, showing multiple twinning (the
striation), and the iridescent play of colors]

Plate 36

[Illustration: Crystal form of a pyroxene; _a_ and _b_ prism faces,
_m_ the beveled edge between two prism faces]

[Illustration: Cross section of a pyroxene crystal showing the lines
of intersection of the two cleavage planes]

[Illustration: Cross sections of pyroxenes, showing typical forms
taken by crystals]

[Illustration: Augite crystals, in crystalline limestone]

Plate 38

[Illustration: The dodecahedron and the 24-sided figure
characteristic of garnets]

[Illustration: The garnet, grossularite]

[Illustration: The garnet alamandite]

Plate 39

[Illustration: Beryl of gem quality]

[Illustration: Zircon in syenite]

Plate 40

[Illustration: Cyanite crystals in schist]

[Illustration: A crystal of mica, showing basal cleavage]

Plate 41

[Illustration: Crystal form typical of topaz]

[Illustration: A topaz crystal from Brazil]

[Illustration: Crystal form typical of staurolite when simple]

[Illustration: A typical twin of staurolite]

Plate 43

[Illustration: Serpentine]

[Illustration: Chlorite]

Plate 49

[Illustration: Apatite crystals in crystalline calcite]

[Illustration: The ends of apatite crystals showing common modes of
termination]

Plate 50

[Illustration: A group of fluorite crystals]

[Illustration: A group of halite crystals]

Plate 61

[Illustration: Amber]

[Illustration: Two bottles of petroleum, the left hand one with a
paraffin base, the right hand one with an asphalt base]

Plate 65

[Illustration: Mica schist, with garnets]

[Illustration: Chlorite schist]

Plate 67

[Illustration: Serpentine, composed of serpentine, hematite, and
some calcite]

Plate 1

Basal forms of the isometric system

[Illustration: Cube]

[Illustration: Octahedron]

[Illustration: Dodecahedron]

Plate 2

Basal forms of the tetragonal system

[Illustration: A square prism]

[Illustration: Octahedron]

Basal forms of the orthorhombic system

[Illustration: A Rectangular prism]

[Illustration: Octahedron]

Plate 3

Basal forms of the monoclinic system

[Illustration: The rectangular prism askew]

[Illustration: The octahedron]

[Illustration: A cross section of the prism with its edges beveled
so that the _b_ faces are obliterated by the _m_ faces, and a
six-sided prism is formed (pseudo-hexagonal)]

[Illustration: Basal form of the triclinic system]

Plate 4

Basal forms of the hexagonal system

[Illustration: The six-sided prism]

[Illustration: The double pyramid]

[Illustration: The rhombohedron]

Plate 10

[Illustration: Tetrahedrons showing characteristic manner in which
tetrahedrite occurs]

[Illustration: A cube with the edges beveled and the corners cut in
a form characteristic of cuprite]

Plate 30

[Illustration: Two intergrowing or twinned quartz crystals]

[Illustration: Diagram of the typical quartz crystal, _p_ prism
faces, _l_ left hand rhombohedron, _r_ right hand rhombohedron]

[Illustration: A quartz crystal on which the left hand rhombohedron
is represented by small faces while the right hand rhombohedron has
large faces]

Plate 14

[Illustration: Crystal forms of hematite, _A_ the rhombohedron with
the edges beveled; _B_ the tabular form, resulting from the
excessive development of the two _o_ faces opposite each other]

[Illustration: A typical crystal of magnetite]

[Illustration: The rhombohedron typical of siderite]

Plate 16

[Illustration: The pyritohedron]

[Illustration: The pyritohedron with certain of its edges beveled by
the cube faces, to show the relationship of these two forms]

Plate 18

Typical forms for cerrusite

[Illustration: The pyramid, _n_ the prism face, _m_ the beveled
prism, _p_ the octahedral face, and _o_ the edge of the octahedral
faces beveled]

[Illustration: The simple type of twinning]

[Illustration: A multiple twin where three crystals grow through
each other]

[Illustration: Forms in which anglesite occurs: _l_ the pyramid
face, _p_ the prism face, _o_ the vertical edge of the prism
beveled, _m_ the horizontal edge of the prism beveled, _n_ a further
beveling of the horizontal edge of the prism. _D_ the tabular, _E_
the prismatic form]

Plate 20

[Illustration: A characteristic form in which sphalerite may occur;
being the combination of, _d_ the dodecahedron, _o_ the octahedron,
and _t_, a 24-sided figure]

[Illustration: Characteristic form for zincite crystals, _n_ the
hexagonal prism, and _p_ pyramidal faces on it]

[Illustration: Typical form of crystal of willemite: _p_ the prism,
_r_ rhombohedron faces on end, ½ _r_ a second lower rhombohedron]

Plate 22

[Illustration: Moss agates, showing the dendritic growth of
manganitic minerals, like manganite or pyrolusite]

[Illustration: Moss agates]

[Illustration: Crystal form of manganite]

Plate 23

[Illustration: Crystals of green corundum in syenite, from Montana]

[Illustration: Typical crystal forms of corundum: _A_ the elongated
prism with the alternate corners cut by rhombohedral faces, _B_ the
tabular prism, _C_ the double pyramid]

Plate 28

[Illustration: Cassiterite, twinned crystals]

[Illustration: The crystal form in which both cassiterite and rutile
occur when in simple crystals, _p_ prism faces, _m_ beveling of the
prism, _o_ octahedral face, _n_ beveling of the edge between
octahedral faces]

[Illustration: Multiple twinning characteristic of rutile]

Plate 29

[Illustration: Crystal of Spinel]

Crystal forms in which dolomite occurs

[Illustration: _A_ the cleavage form, rhombohedron with the faces
curved]

[Illustration: _B_ the rhombohedron with the corners cut, as it
often occurs]

[Illustration: _C_ the form found in gypsum or anhydrite]

Plate 34

[Illustration: Orthoclase, a cleavage piece, _a_ and _b_ the perfect
cleavage planes, and _c_ the imperfect cleavage plane]

Crystal forms of orthoclase

[Illustration: _A_ the simple crystal]

[Illustration: _B_ the twinned form]

[Illustration: _C_ the twinned form in which the crystals are
intergrowing]

[Illustration: Diagram of a multiple twin of a plagioclase feldspar]

Plate 37

Diagrams of amphibole crystals

[Illustration: _A_ a typical crystal]

[Illustration: _B_ cross section showing the intersection of
cleavage planes]

[Illustration: _C_ and _D_ cross sections to show variations in
outline]

[Illustration: Tremolite in silky fibrous crystals. Asbestos]

[Illustration: Hornblende crystals in quartzite]

Plate 42

[Illustration: Epidote crystals]

[Illustration: Typical forms of epidote crystals; _p_ prism faces,
_m_, _n_, _x_, and _y_ beveled edges of the prism, _o_ octahedral
faces]

Typical forms of tourmaline

[Illustration: _A_ side view; _B_ and _C_ ends to show terminations;
_p_ prism faces, _m_ beveling of prism edges, _r_ a low rhombohedron
on the end, _s_ the opposite rhombohedron, _b_ basal face, and the
other faces represent bevelings]

Plate 48

[Illustration: A group of barite crystals]

[Illustration: Outline of the typical tabular barite crystal]

[Illustration: The six-sided double pyramid, composed of three
interpenetrating crystals, typical of witherite and strontianite]

Plate 44

[Illustration: The typical form of analcite]

[Illustration: A typical natrolite crystal]

[Illustration: The typical crystal form of stilbite]

[Illustration: A sheaf-like bundle of fibrous crystals, typical of
stilbite]

Plate 45

[Illustration: A group of calcite crystals]

Typical forms of calcite

[Illustration: _A_ the rhombohedron formed by cleavage]

[Illustration: _B_ a rhombohedral crystal truncated by the basal
plane]

[Illustration: _C_ the scalenohedron]

[Illustration: _D_ the scalenohedron truncated by the rhombohedron]

[Illustration: _E_ the scalenohedron on a prism]

Plate 46

Typical forms of aragonite

[Illustration: _A_ the simple crystal]

[Illustration: _B_ a needle-like form, twinned]

[Illustration: _C_ cross section to show how the form may appear
six-sided]

[Illustration: Typical form of the anhydrite crystal]

Plate 47

[Illustration: 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]

[Illustration: A simple crystal of gypsum]

[Illustration: Twin crystals of gypsum]

Plate 51

[Illustration: Sulphur crystals]

[Illustration: Ice crystals, the top one, the end of a hexagonal
prism; the two lower figures multiple twins as in snow flakes]

Plate 52

[Illustration: The Devil’s Tower, Wyoming, an example of igneous
rock with columnar structure, and resting on sedimentary rocks.
Courtesy of the U. S. Geological Survey]

Plate 53

[Illustration: A coarse granite]

[Illustration: Graphic granite]

Plate 54

[Illustration: Syenite]

[Illustration: Gabbro]

Plate 55

[Illustration: Basalt-porphyry. The large white crystals are
phenocrysts of plagioclase feldspar]

[Illustration: Basalt-obsidian]

Plate 56

[Illustration: Amgydoloid]

Plate 57

[Illustration: The north face of Scott’s Bluff, Neb., showing
sedimentary sandstones above and clays below. The type of erosion is
characteristic of arid regions. Courtesy of the U. S. Geological
Survey]

Plate 58

[Illustration: Breccia]

[Illustration: Conglomerate]

Plate 59

[Illustration: Calcareous shale]

[Illustration: Coquina]

Plate 60

[Illustration: Foramenifera from Chalk; enlarged about 25 diameters]

[Illustration: Encrinal Limestone; fragments of the stems, arms and
body of Crinoids]

Plate 62

[Illustration: _A_ diatomaceous earth magnified 50 times]

[Illustration: _B_ and _C_ two diatoms from the above enlarged 250
times. After Gravelle, by the courtesy of Natural History]

Plate 63

[Illustration: A metamorphic rock, showing the contortion of layers
due to expansion under heat]

Plate 64

[Illustration: A conglomerate partly metamorphosed to a gneiss. Note
the flattened pebbles and the alternation of the intermediate
material to mica scales, etc.]

[Illustration: A typical gneiss]

Plate 66

[Illustration: Phyllite]

[Illustration: A white marble, with black streaks due to graphite]

Plate 68

[Illustration: Claystones, simple and compound]

[Illustration: A line concretion, which on splitting disclosed a
fern leaf of the age of the coal measures]

Plate 69

[Illustration: A septeria from Seneca Lake, N. Y.]

[Illustration: Pisolite]

Plate 70

[Illustration: A geode filled with quartz crystals]

Plate 71

[Illustration: A quartz pebble from the bed of a New England brook]

[Illustration: A pebble of schist and granite from the foot of Mt.
Toby, Mass.]

Plate 72

[Illustration: An iron-nickel meteorite, of 23 lbs. which fell in
Claiborne Co., Tenn.]

[Illustration: An etched slice of an iron meteorite which fell in
Reed City, Osceola Co., Mich.]

Plate 73

[Illustration: A stony meteorite, about natural size, which fell in
1875, in Iowa Co., Iowa]

PUTNAM’S
NATURE FIELD BOOKS
Companion books to this one

Mathews American Wild Flowers
American Trees and Shrubs
Wild Birds and Their Music
Durand Wild Flowers in Homes and Gardens
My Wild Flower Garden
Common Ferns
Lutz Insects
Loomis Rocks and Minerals
Eliot Birds of the Pacific Coast
Armstrong Western Wild Flowers
Alexander Birds of the Ocean
Anthony North American Mammals
Thomas Common Mushrooms
Sturgis Birds of the Panama Canal Zone
Miner Seashore Life
Breder Marine Fishes of the Atlantic Coast
Morgan Ponds and Streams
Longyear Rocky Mountain Trees and Shrubs
Olcott Field Book of the Skies
Putnam
Beebe The Shore Fishes of Bermuda
Tee-Van
Schrenkeisen Fresh-Water Fishes of North America North of
Mexico

Transcriber’s Notes

—Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by
_underscores_.

—Silently corrected a few typos.

—Reconstructed an image caption (Pisolite) on Plate 69.

—Generated a cover image based on elements in the book.

*** End of this LibraryBlog Digital Book "Field Book of Common Rocks and Minerals - For identifying the Rocks and Minerals of the United States - and interpreting their Origins and Meanings" ***

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