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Title: Geology - The Science of the Earth's Crust
Author: Miller, William J.
Language: English
As this book started as an ASCII text book there are no pictures available.
Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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Transcriber's Note


Italic text is denoted by _underscores_ and bold text by =equal signs=.
Whole and partial numbers are displayed as 2-1/2.



[Illustration: Photo, Ridge Studio; Courtesy Ogden Chamber of
Commerce]

_A great ledge in Ogden Canyon near Ogden, Utah. The rock, still
retaining its stratification, was deposited layer upon layer
horizontally mostly as sand upon the floor of a sea which covered the
region fully 25,000,000 years ago. That the sea was of very early
Paleozoic (i.e., Cambrian) age has been proved by fossils in associated
strata. Long after their deep burial and consolidation within the
earth, the strata were subjected to tremendous mountain-making
pressure, notably altered to a rock called "Quartzite," raised high
above sea level, and tilted almost vertically. Then through long ages
(millions of years) overlying rocks of great thickness have been cut
away (eroded) by weathering and stream action, laying bare the ledge as
we see it to-day._


                        Popular Science Library

                            EDITOR-IN-CHIEF
                          GARRETT P. SERVISS

                                AUTHORS

     WILLIAM J. MILLER      HIPPOLYTE GRUENER      A. RUSSELL BOND
       D. W. HERING      LOOMIS HAVEMEYER      ERNEST G. MARTIN
            ARTHUR SELWYN-BROWN     ROBERT CHENAULT GIVLER
              ERNEST INGERSOLL      WILFRED MASON BARTON
                WILLIAM B. SCOTT     ERNEST J. STREUBEL
                     NORMAN TAYLOR      DAVID TODD
                        CHARLES FITZHUGH TALMAN
                              ROBIN BEACH

                      ARRANGED IN SIXTEEN VOLUMES
                 WITH A HISTORY OF SCIENCE, GLOSSARIES
                          AND A GENERAL INDEX

                             _ILLUSTRATED_

                            [Illustration]

                             VOLUME THREE

                      P. F. COLLIER & SON COMPANY
                               NEW YORK


                            Copyright 1922
                    By P. F. Collier & Son Company

                       MANUFACTURED IN U. S. A.



                                GEOLOGY

                   The Science of the Earth's Crust


                                  BY


                           WILLIAM J. MILLER
                  Professor of Geology, Smith College


                            [Illustration]


                      P. F. COLLIER & SON COMPANY
                               NEW YORK



PREFACE


In the preparation of this book the author has attempted to present, in
popular form, the salient points of a general survey of the whole great
science of geology, the science which deals with the history of the
earth and its inhabitants as revealed in the rocks.

The use of technical and unusual terms has been reduced to a minimum
compatible with a reasonable understanding of the subject by the
layman. Each of the relatively few scientific terms is explained where
first used in the text, and a glossary of common geological terms has
been appended.

The matter of illustrations has received very careful attention, and
only pictures, maps, and diagrams are used which actually illustrate
important features of the text. A special point has been made to
introduce only cuts of simple construction comparatively free from
technicalities. Nearly every illustration is accompanied by a really
explanatory title.

A number of the pictures are from the author's collection of
photographs, and many of the line-cuts have either been made or
considerably modified by the author. Among the numerous sources of
illustrations, special mention should be made of the United States
Geological Survey, the New York State Museum, the American Museum
of Natural History, the University of Chicago Press, and various
individuals, full credit being given wherever due.

                                                  William J. Miller.

                                                  Northampton, Mass.



CONTENTS


  CHAPTER                                                    PAGE

      I. Introduction                                           9

     II. Weathering and Erosion                                22

    III. Stream Work                                           30

     IV. The Sea and Its Work                                  51

      V. Glaciers and Their Work                               59

     VI. The Action of Wind                                    71

    VII. Instability of the Earth's Crust                      76

   VIII. Volcanoes and Igneous Rocks                           99

     IX. Waters Within the Earth                              113

      X. How Mountains Come and Go                            130

     XI. A Study of Lakes                                     142

    XII. How the Earth May Have Originated                    158

   XIII. Very Ancient Earth History                           164

    XIV. Ancient Earth History                                179

     XV. Medieval Earth History                               208

    XVI. Modern Earth History                                 221

   XVII. Evolution of Plants                                  249

  XVIII. Geological History of Animals (Excluding
           Vertebrates)                                       259

    XIX. Geological History of Vertebrate Animals
           (Including Man)                                    281

     XX. Mineralogy                                           307

    XXI. Economic Geology                                     342

  Glossary of Common Geological Terms                         377



LIST OF ILLUSTRATIONS


Ledge of Rock with Nearly Vertical Strata      _Frontispiece_

  PLATE                                                    FACING PAGE

   1 Granite Weathering to Soil Leaving Residual
       Cores of Joint Blocks (_Upper picture_)                 32

     Looking-Glass Rock, Utah. Stratified Sandstone
       Sculptured by Wind Erosion (_Lower picture_)            32

   2 Grand Canyon of the Yellowstone River. A
       Channel Worn by Erosion                                 33

   3 Gorge of the Niagara River Below the Falls.
       A Sample of Recent Erosion                              64

   4 Winding Stream in the St. Lawrence Valley
       with Flood Plain (_Upper picture_)                      65

     Davidson Glacier, Alaska, Showing Action on
       the Valley's Walls and Floor (_Lower picture_)          65

   5 Swift Current Valley in Glacier National
       Park, Formed by Stream and Glacier Action               96

   6 Yosemite Valley from Western Entrance. The
       Result of Glacial Action                                97

   7 An Upbend Fold or Anticline in Maryland                  128

   8 Ledge of Igneous Rock Showing Joints
       (_Upper picture_)                                      129

     Fault Fracture in Limestone Formations
       (_Lower pictures_)                                     129

   9 Molten Lava Flowing Over a Cliff in Hawaii
       (_Upper picture_)                                      160

     Dikes of Granite Cutting Old Rock
        (_Lower picture_)                                     160

  10 Lassen Peak, California, in Eruption
        (_Left picture_)                                      161

     Devil's Tower, Wyoming, Once a Mass of Molten
       Rock Forced Through Strata (_Right picture_)           161

  11 Mammoth Hot Springs Terrace in Yellowstone
       National Park (_Upper picture_)                        224

     Crater Lake, Oregon, Formed by the Subsidence
       of a Volcano (_Lower picture_)                         224

  12 Archeozoic Rock, Oldest Known Rock Formation
       on Earth (_Upper picture_)                             225

     Twisted Mass of Stratified Limestone, Surrounded
       by Granite (_Lower picture_)                           225

  13 Paleozoic Rock, Covered with Oldest Known
       Animal Remains                                         256

  14 "Stone-Lily" Stems in Ordovician Strata (_Upper
       left picture_)                                         257

     Various Brachiopods in Ordovician Strata (_Upper
       right picture_)                                        257

     Stratified Limestone of Middle Ordovician Era
       Containing Fossils (_Lower picture_)                   257

  15 A Landscape of the Coal Age (_Upper picture_)            288

     Fossil Fern on a Piece of Shale (_Lower picture_)        288

  16 Restoration of Huge Animals of the Mesozoic Era          289

  17 Skeleton of Two-Legged Carnivorous Dinosaur
       (_Large picture_)                                      320

     Earliest Known Bird Form (_Insert_)                      320

  18 Skeleton of Large Flying Reptile (_Upper picture_)       321

     Skeleton of a Swimming Reptile of the Mesozoic
       Era (_Lower picture_)                                  321

  19 Appalachians Along New River, Peneplain Upraised Again   352

  20 Glacial Bowlder Left by Ice Sheet (_Upper picture_)      353

     Esker, Deposited by a Stream in the Great
       Glacier (_Lower picture_)                              353



CHAPTER I

INTRODUCTION


Earth features are not fixed. The person of ordinary intelligence,
surrounded as he is by a great variety of physical features, is, unless
he has devoted some study to the subject, very likely to regard those
features as practically unchangeable, and to think that they are now
essentially as they were in the beginning of the earth's history. Some
of the most fundamental ideas taught in this book are that the physical
features of the earth, as we behold them to-day, represent but a single
phase of a very long-continued history; that significant changes
are now going on all around us; and that we are able to interpret
present-day earth features only by an understanding of earth changes in
the past.

_Geology_, meaning literally "earth science," deals with the history of
the earth and its inhabitants as revealed in the rocks. The science is
very broad in its scope. It treats of the processes by which the earth
has been, and is now being, changed; the structure of the earth; the
stages through which it has passed; and the evolution of the organisms
which have lived upon it.

_Geography_ deals with the distribution of the earth's physical
features, in their relation to one another, to the life of sea and
land, and human life and culture. It is the present and outward
expression of geological effects.

As a result of the work of many able students of geology during the
past century and a quarter, it is now well established that our planet
has a definitely recorded history of many millions of years, and that
during the lapse of those eons, revolutionary changes in earth features
have occurred, and also that there has been a vast succession of
living things which, from very early times, have gradually passed from
simple into more and more complex forms. The physical changes and the
organisms of past ages have left abundant evidence of their character,
and the study of the rock formations has shown that within them we have
a fairly complete record of the earth's history. Although very much yet
remains to be learned about this old earth, it is a remarkable fact
that man, through the exercise of his highest faculty, has come to know
so much concerning it.

The following words, by the late Professor Barrell, admirably summarize
the significance of geological history. "The great lesson taught by the
study of the outer crust is that the earth mother, like her children,
has attained her present form through ceaseless change, which marks
the pulse of life and which shall cease only when her internal forces
slumber and the cloudy air and surf-bound ocean no more are moving
garments. The flowing landscapes of geologic time may be likened to a
kinetoscopic panorama. The scenes transform from age to age, as from
act to act; seas and plains and mountains of different types follow and
replace each other through time, as the traveler sees them succeed each
other in space. At times the drama hastens, and unusual rapidity of
geologic action has, in fact, marked those epochs since man has been
a spectator upon the earth. Science demonstrates that mountains are
transitory forms, but the eye of man through all his lifetime sees no
change, and his reason is appalled at the conception of a duration so
vast that the milleniums of written history have not accomplished the
shifting of even one of the fleeting views which blend into the moving
picture."[A]

[A] Central Connecticut in the Geologic Past, pp. 1-2.

Or in the words of Tennyson:

  There rolls the deep where grew the tree.
  O, earth, what changes hast thou seen!
  There where the long street roars, hath been
  The stillness of the central sea.
  The hills are shadows, and they flow
  From form to form, and nothing stands;
  They melt like mist, the solid lands,
  Like clouds they shape themselves and go.

The following statement of some of the more definite important
conclusions regarding earth changes may serve to make still clearer the
general scope of the science of geology. The evidences upon which these
conclusions are based are discussed in various parts of this book. For
untold millions of years the rocks at and near the earth's surface
have been crumbling; streams have been incessantly sawing into the
lands; the sea has been eating into continental masses; the winds have
been sculpturing desert lands; and, more intermittently and locally,
glaciers have plowed through mountain valleys, and even great sheets of
ice have spread over considerable portions of continents. Throughout
geologic time, the crust of the earth has shown marked instability.
Slow upward and downward movements of the lands relative to sea level
have been very common, in many cases amounting to even thousands of
feet. Various parts of the earth have been notably affected by sudden
movements (resulting in earthquakes) along fractures in the outer
crust. During millions of years molten materials have, at various
times, been forced into the earth's crust, and in many cases to its
surface. Mountain ranges have been brought forth and cut down. The site
of the Appalachian Mountains was, millions of years ago, the bottom of
a shallow sea. Lakes have come and gone. The Great Lakes have come into
existence very recently (geologically), that is to say, since the great
Ice Age. A study of stratified rocks of marine origin shows that all,
or nearly all, of the earth's surface has at some time, or times, been
covered by sea water. Over certain districts the sea has transgressed
and retrogressed repeatedly. Organisms have inhabited the earth for
many millions of years. In earlier known geologic time, the plants and
animals were comparatively simple and low in the scale of organization,
and through the succeeding ages higher and more complex types were
gradually evolved until the highly organized forms of the present time,
including the human race, were produced.

The rocks of the earth constitute the special field of study for the
geologist because they contain the records of events through which the
earth and its inhabitants have passed during the millions of years of
time until their present conditions have been reached. All the rocks of
the earth's crust may be divided into three great classes: _igneous_,
_sedimentary_, and _metamorphic_.

_Igneous_ rocks comprise all those which have ever been in a molten
condition, and of these we have the _volcanic_ rocks (for example,
lavas), which have cooled at or near the surface; _plutonic_ rocks (for
example, granites), which have cooled in great masses at considerable
depths below the surface; and the _dike_ rocks which, when molten, have
been forced into fissures in the earth's crust and there cooled.

_Sedimentary_ rocks comprise all those which have been deposited under
water, except some wind-blown deposits, and they are nearly always
arranged in layers (stratified). Such rocks are called strata. They may
be of mechanical origin such as clay or mud which hardens to _shale_;
sand, which consolidates into _sandstone_; and gravel, which when
cemented becomes _conglomerate_. They may be of organic origin such as
limestone, most of which is formed by the accumulation of calcareous
shells; _flint_ and _chert_, which are accumulations of siliceous
shells; or _coal_, which is formed by the accumulation of partly
decayed organic matter. Or, finally, they may be formed by chemical
precipitation, as beds of _salt_, _gypsum_, _bog iron ore_, etc.

_Metamorphic_ rocks include both sedimentary and igneous rocks which
have been notably changed from their original condition. Traces or
remains of plants and animals preserved in the rocks are known as
fossils. The term originally meant anything dug out of the earth,
whether organic or inorganic, but for many years it has been strictly
applied to organic remains. Many thousands of species of fossils are
known from rocks of all ages except the oldest, and more are constantly
being brought to light, but these represent only a small part of
the life of past ages because relatively few organic remains were
deposited under conditions favorable for preservation in fossil form.
The fossils in the rocks are, however, a fair average of the groups of
organisms to which they belong. It is really remarkable that such a
vast number of fossils are imbedded in the rocks, and from a study of
these many fundamental conclusions regarding the history of life on our
planet may be drawn.

As early as the fifth century B. C., Xenophanes is said to have
observed fossil shells and plants in the rocks of Paros, and to
have attributed their presence to incursions of the sea over the
land. Herodotus, about a century later, came to a similar conclusion
regarding fossil shells in the mountains of Egypt. None of the
ancients, however, seemed to have the slightest conception of the
significance of fossils as time markers in the history of the earth.
(See discussion below.)

In the Middle Ages, distinguished writers held curious views regarding
fossils. Thus Avicenna (980-1037) believed that fossils represented
unsuccessful attempts on the part of nature to change inorganic
materials into organisms within the earth by a peculiar creative force
(_vis plastica_). About two centuries later, Albertus Magnus held
a somewhat similar view. Leonardo da Vinci (1452-1519), the famous
artist, architect, and engineer, while engaged in canal building in
northern Italy, saw fossils imbedded in the rocks, and concluded
that these were the remains of organisms which actually lived in
sea water which spread over the region. During the seventeenth and
eighteenth centuries, many correctly held that fossils were really
of organic origin, but it was commonly taught that all fossils
represented remains of organisms of an earlier creation which were
buried in the rocks during the great Deluge (Noah's Flood). William
Smith (1769-1839), of England, was, however, the first to recognize
the fundamental significance of fossils for determining the relative
ages of sedimentary rocks. This discovery laid the foundation for the
determination of earth chronology which is of great importance in the
study of the history of the earth. (See discussions below.)

Organic remains, dating as far back as tens of millions of years, have
been preserved in the rocks of the earth in various ways. A very common
kind of fossilization is the preservation of only the hard parts of
organisms. Thus the soft parts have disappeared by decomposition, while
the hard parts, such as bones, shells, etc., remain. In many cases
practically complete skeletons of large and small animals which lived
millions of years ago have been found intact in the rocks. Fossils
which show none of the original material, but only the shape or form,
are also very abundant. When sediment hardens around an imbedded
organism, and the organism then decomposes or dissolves away, a cavity
or fossil mold only is left. Casts of organisms or parts of them are
formed by filling shells or molds with sediment or with mineral matter
carried in solution by underground water. Only rarely have casts of
wholly soft animals been found in ancient rocks. In other cases both
original form and structure are preserved, but none of the original
material. This is known as petrifaction which takes place when a plant
or hard part of an animal has been replaced, particle by particle,
by mineral matter from solution in underground water. Not uncommonly
organic matter, such as wood, or inorganic matter, such as carbonate
of lime shells, has been so perfectly replaced that the original
structures are preserved almost as in life. The popular idea that
petrified wood is wood which has been changed into stone is, of course,
incorrect. It is doubtful if flesh has ever been truly petrified. In
many cases mainly the carbon only of organisms has been preserved.
This is also true of plants where, under conditions of slow chemical
change or decomposition, the hydrogen and oxygen mostly disappear,
leaving much of the carbon with original structures often remarkably
preserved. Fine examples are fossil plants in the great coal-bearing
strata. Much more rarely entire organisms have been preserved either
by freezing or by natural embalmment. Most remarkable are the species
of mammoths and rhinoceroses, extinct for thousands of years, bodies
of which, with flesh, hide, and hair still intact, have been held in
cold storage in the frozen soils of Siberia, or other cases. Insects
have been perfectly preserved in amber, as, for example, in the Baltic
region. This amber is a hardened resin in which the insects were
caught while it was still soft and exuding from the trees. Finally, we
should mention the preservation of tracks and trails of land and water
animals. Thousands of tracks of long-extinct great reptiles occur in
the sandstones and shales of the Connecticut Valley of Massachusetts.
The footprints were made in soft sandy mud which hardened and then
became covered with more sediment.

Few fossils occur in other than the sedimentary rocks. Most numerous,
by far, are fossils in rocks of marine origin, because on relatively
shallow sea bottoms, where sediments of the geologic ages have
largely accumulated, the conditions for fossilization have been
most favorable. Among the many conditions which have produced great
diversity in numbers and distribution of marine organisms during
geologic time are temperature, depth of water, clearness of water,
nature of sea bottom, degree of salinity, and food supply. River
and lake deposits also not uncommonly contain remains of organisms
which inhabited the waters, but also others which were carried in.
"Surrounding trees drop their leaves, flowers, and fruit upon the mud
flats, insects fall into the quiet waters, while quadrupeds are mired
in mud or quicksand and soon buried out of sight. Flooded streams bring
in quantities of vegetable debris, together with carcasses of land
animals drowned by the sudden rise of the flood" (W. B. Scott).

In the study of the many changes which have taken place in the history
of the earth, a fundamental consideration is the determination of
the relative ages of the rocks, especially the strata. How can the
geologist assign a rock formation of any part of the earth to a
particular age in the history of the earth? How can it be proved that
certain rock formations in various parts of the earth originated
practically at the same time? There are two important criteria.
First, in any region where the strata have not been disturbed from
their normal order, the older strata underlie the younger because the
underlying sediments must have been deposited first. Now, the total
thickness of the stratified series of the earth has been estimated to
be no less than 200,000 feet and only a small part of this is actually
present in any given locality or region. It is, therefore, evident
that the order of superposition of strata is in itself not sufficient
for the determination of the relative ages of all the strata in even
a considerable portion of a single continent, not to mention its utter
inadequacy in building up the geological column of the whole earth.
When, however, the second criterion, namely, the fossil content of the
strata, is used in direct connection with the order of superposition,
we have the real basis for determining the relative ages of strata for
all parts of the earth. The discovery of this method was very largely
due to the painstaking field work in England by William Smith about the
beginning of the nineteenth century.

It is a well-established fact that organisms have inhabited the earth
for many millions of years and that, through the geologic ages,
they have continuously changed, with gradual development of higher
and higher types. Tens of thousands of species have come and gone.
Accepting this fact, it is then clear that strata which were formed
at notably different times must contain notably different fossils,
while strata which accumulated at practically the same time contain
similar fossils, allowing, of course, for reasonable differences in
geographical distribution of organisms as at the present time. Each
epoch of earth history or series of strata has its characteristic
assemblage of organisms. In short, "a geological chronology is
constructed by carefully determining, first of all, the order of
superposition of the stratified rocks, and next by learning the fossils
characteristic of each group of strata.... The order of succession
among the fossils is determined from the order of superposition of
the strata in which they occur. When that succession has been thus
established, it may be employed as a general standard" (W. B. Scott).
It should, however, be borne in mind that precise contemporaneity of
strata in widely separated districts can rarely, if ever, be determined
because of the very great length of geologic time and the general
slowness of the evolution of organisms. Rocks carrying remarkably
similar fossils may really be several thousand years different in age;
but this is, indeed, a very small limit of error when one considers the
vast antiquity of the earth. Much very accurate and satisfactory work
has been done, especially in Europe and North America, in correlating
strata and assigning them to their places in the geological time table
(see below), but a vast amount of work yet remains to be done before
the task is complete.

Certain types or species of organisms are much more useful than others
in the determination of earth chronology. Best of all for world-wide
correlations are species which were widely distributed and which
persisted for relatively short times. Thus any species which lived in
the surface waters of the ocean and was easily distributed over wide
areas, while, at the same time, it existed as such only a short time,
is the best type of chronologic indicator.

The known history of the earth has been more or less definitely divided
into great eras and lesser periods and epochs, constituting what may
be called the geologic time scale. In the accompanying table the era
and period names, except those representing earlier time, are mostly
world-wide in their usage. Epoch names, being more or less locally
applied, are omitted from the table. Very conservative estimates of
the length of time represented by the eras and the most characteristic
general features of the life of the main divisions are also given.

PRINCIPAL DIVISIONS OF GEOLOGIC TIME

(Modified after U. S. Geological Survey.)


  =========================================================================
             |              |                                    |Millions
     Era.    |    Period.   |     Characteristic life.           |of years
             |              |                                    |estimated
  -----------+--------------+------------------------------------+---------
             |              |"Age of man." Animals and plants    |
             |Quaternary.   |of modern types.                    |
             +--------------+------------------------------------+
  Cenozoic   |              |"Age of mammals." Rise of highest   |3 to 5.
             |Tertiary.     |animals except man. Rise and        |
             |              |development of highest orders of    |
             |              |plants.                             |
  -----------+--------------+------------------------------------+---------
             |              |"Age of reptiles." Rise and         |
             |Cretaceous.   |culmination of huge land reptiles   |
             |              |(dinosaurs), of shellfish with      |
             |              |complexly partitioned coiled        |
             +--------------+shells (ammonites), and of great    |
             |              |flying reptiles. First appearance   |
  Mesozoic   |Jurassic.     |(in Jurassic) of birds and mammals; |5 to 10.
             |              |of cycads, an order of palm-like    |
             +--------------|plants (in Triassic); and of        |
             |              |angiospermous plants, among         |
             |Triassic.     |which are palms and hardwood        |
             |              |trees (in Cretaceous).              |
  -----------+--------------+------------------------------------+---------
             |              |"Age of amphibians." Dominance of   |
             |Permian.      |club mosses (lycopods) and plants   |
             +--------------+of horsetail and fern types.        |
             |              |Primitive flowering plants and      |
             |Pennsylvanian.|earliest cone-bearing trees.        |
             |              |Beginnings of back-boned land       |
             +--------------+animals (land vertebrates). Insects.|
             |Mississippian.|Animals with nautiluslike coiled    |
             |              |shells (ammonites) and sharks       |
             |              |abundant.                           |
             +--------------+------------------------------------+
             |              |"Age of fishes." Shellfish          |
             |Devonian.     |(mollusks) also abundant. Rise of   |
             |              |amphibians and land plants.         |
             +--------------+------------------------------------|
             |           A {|Shell-forming sea animals dominant, |
             |           g {|especially those related to         |
             |           e {|the nautilus (cephalopods). Rise    |
  Paleozoic  |             {|and culmination of the marine       |17 to 25.
             |Silurian.  o {|animals sometimes known as sea      |
             |           f {|lilies (crinoids) and of giant      |
             |             {|scorpionlike crustaceans            |
             |           I {|(eurypterids). Rise of fishes and   |
             |           n {|of reef-building corals.            |
             +-----------n-{+------------------------------------+
             |           v {|Shell-forming sea animals,          |
             |           e {|especially cephalopods and          |
             |Ordovician.r {|mollusk-like brachiopods, abundant. |
             |           t {|Culmination of the buglike marine   |
             |           e {|crustaceans known as trilobites.    |
             +-----------b-{+------------------------------------+
             |           r {|Trilobites and brachiopods most     |
             |Cambrian.  a {|characteristic animals. Seaweeds    |
             |           t {|(algæ) abundant. No trace of        |
             |           e {|land animals found.                 |
  -----------+-----------s-{+------------------------------------+---------
             |              |First life that has left distinct   |
  Proterozoic|Algonkian.    |record. Crustaceans, brachiopods,   |
             |              |and seaweeds.                       |
  -----------+--------------+------------------------------------+
             |              |Organic matter in form of graphite  |25 to 50+
  Archeozoic |Archean.      |(black lead), but no determinable   |
             |              |fossils found.                      |
  -----------+--------------+------------------------------------+---------

The length of time represented by human history is very short compared
to the vast time of known geological history. The one is measured
by thousands of years, while the other must be measured by tens of
millions of years. Just as we may roughly divide human history into
certain ages according to some notable person, nation, principle, or
force as, for example, the "Age of Pericles," the "Roman Period,"
the "Age of the French Revolution," or the "Age of Electricity," so
geologic history may be subdivided according to great predominant
physical or organic phenomena, such as "the Appalachian Mountain
Revolution" (toward the end of the Paleozoic era), the "Age of Fishes"
(Devonian period), or the "Age of Reptiles" (Mesozoic era).

In the study of earth history, as in the study of human history, it
is important to distinguish between events and records of events.
Historical events are continuous, but they are by no means all
recorded. Records of events are often interrupted and seemingly sharply
separated from each other.



CHAPTER II

WEATHERING AND EROSION


All rocks at and near the surface of the earth crumble or decay.
The term "weathering" includes all the processes whereby rocks are
broken up, decomposed, or dissolved. A mass of very hard and seemingly
indestructible granite, taken from a quarry, will, in a very short
time, geologically considered, crumble (Plate 1). During the short
span of the ordinary human life weathering effects are generally of
very little consequence, but during the long ages of geologic time the
various processes of weathering have been slowly and ceaselessly at
work upon the outer crust of the earth, and such tremendous quantities
of rock material have been broken up that the lands of the earth have
everywhere been profoundly affected.

Most of us have noticed buildings and monuments in which the stones
show marked effects of weathering. A good case in point is Westminster
Abbey, London, in which many of the stones are badly weathering, some
of the more ornamental parts having crumbled beyond recognition since
the building was erected in the thirteenth century. In many countries,
tombstones and monuments only one or two centuries old are so badly
weathered that the inscriptions are scarcely if at all legible.

What are some of the processes of nature whereby rocks are weathered?
In cold countries, and often in mountains of generally mild climate
regions, the alternate freezing and thawing of water is a potent agency
in breaking up rocks where the soils are thin or absent. On freezing,
water expands about one-tenth of its volume and exerts the enormous
pressure of over 2,000 pounds per square inch. Nearly all relatively
hard rock formations are separated into more or less distinct blocks by
natural cracks called "joints" (Plate 8). Very commonly the rocks also
contain minute crevices, fissures, and pores. Repeated freezing and
thawing of water which finds its way into such openings finally causes
even the most resistant rocks to break up into smaller and smaller
fragments. A very striking example of difference in climatic effect
upon a given rock mass is the obelisk in Central Park, New York. For
many centuries this famous monument stood practically without change in
the dry, frostless climate of Egypt, but very soon after its removal to
the moist, frosty climate of New York, it began to crumble so rapidly
that it was necessary to cover it with a coating of glaze to protect it
from the atmosphere.

Temperature change, especially in dry regions, is also an important
agency for mechanical breaking up of rocks. On high mountains and on
deserts, a daily range of temperature of from 70 degrees to 80 degrees
is frequent. Due to heat absorption, rocks in desert regions, during
the day, not uncommonly reach temperatures of fully 120 degrees, while
during the night, due to heat radiation, their temperature falls
greatly. During the heating of the outer portion of the rock, the
various minerals each expand differently, thus setting up a series
of stresses and strains tending to cause the minerals to pull apart.
The outer portions of the rocks which are subjected to unstable and
relatively rapid temperature changes, often crack or peel off in slabs
or flakes, this process being called exfoliation. Stone Mountain in
Georgia, and some of the mountains of the southern Sierra Nevada range
in California, are excellent examples of mountains which are being
rounded off by exfoliation. The principle is the same as that which
causes the "spalling" of stones in buildings during fires.

Masses of débris consisting of more or less angular rock fragments of
all sizes commonly occur at the bases of cliffs and mountains. They
represent materials which have weathered off the ledges mainly by frost
action and temperature changes.

Where electrical storms are frequent, lightning often shatters
portions of rock ledges. Many such cases have come under the writer's
observation in the Adirondack Mountains of New York. The total effect
of lightning as a weathering agency is, however, relatively small.

Another minor weathering effect is the mechanical action of plants. The
principle is well illustrated by the breaking or tilting of sidewalks
by the wedging action of the growing roots of trees. In many places the
roots of plants growing in cracks in rocks, exert powerful pressure
causing the rocks or blocks of rocks to wedge apart.

Let us now briefly consider some of the chemical processes of
weathering. The solvent effect of perfectly pure water upon rocks is
very slight and slow. But such water is not found in nature because
certain atmospheric gases, especially oxygen and carbonic acid gas, are
always present in it, and they notably increase the solvent power of
the water. Such water has the power to slowly but completely dissolve
the common rock called limestone which consists of carbonate of lime.
This material is then carried away by the streams. Rocks, like certain
sandstones which contain carbonate of lime cementing material, are
caused to crumble due to removal of the cement in solution. Carbonic
acid gas in water also has the power to chemically alter various
minerals in many common rocks and thus the rocks fall apart and the
carbonates which result from the action usually are carried away in
solution. One of the most important changes of this kind takes place
when the very common mineral feldspar is attacked by water containing
carbonic acid gas and the mineral alters to a soluble carbonate, kaolin
(or clay) and silica.

The oxygen, both of the air and that which is contained in water, is
a very important chemical agent of decomposition of many rocks. Water
at the surface and the upper part of the crust of the earth as well
as moisture in the air are also important chemical agents which bring
about rock decay. We are all familiar with the rusting of iron which
is due to the chemical union of the iron with oxygen, thus forming an
iron oxide which in turn commonly unites with water from air or earth.
Now, many rocks contain iron, not as such, but held in combination with
other substances in the form of various minerals, and this iron of the
rocks, where subjected to the oxygen and moisture of air or water,
slowly unites with the oxygen and water to form a hydrated iron oxide
which is essentially iron-rust. The minerals containing considerable
iron are, therefore, decomposed and the rocks crumble. There are
various iron oxides, usually more or less hydrated, ranging in color
from red through brown to yellow, and these constitute probably
the most common and striking colors of the rocks of the earth. The
gorgeously colored Grand Canyon of the Yellowstone River is a very fine
example of large scale coloring due to development of much hydrated
oxide of iron during the weathering of lava rock, the process having
been aided by the action of heated underground waters.

Most of the soils of the earth are the direct result of weathering.
Important exceptions are soils which have been transported by the
action of water, ice, or wind. Although the process of weathering is
very slow and relatively superficial, it is, nevertheless, true that
in many places, the products of weathering form faster than they can
be carried away. Such weathered materials accumulate in their place
of origin to form soils. The upper few hundred feet of the earth's
crust is everywhere more or less fractured and porous and the rocks
are there affected in varying degrees by most of the ordinary agents
of weathering. In such cases, outside the areas which were recently
covered by ice during the great Ice Age, it is common to find the loose
soil grading downward into rotten rock, and this in turn into the fresh
practically unaltered bedrock. Soils of this kind are generally not
more than ten or twenty feet deep, though under exceptional conditions,
as in parts of Brazil, they attain depths of several hundred feet.

In order to make still clearer some of the above principles of
weathering and also to give the reader some understanding of the most
common types of residual soils, we shall consider what happens to a
few rather definite types of ordinary rocks when they are subjected to
weathering. A very simple case is that of a sandstone, the mineral
grains (mostly quartz) of which are held together by carbonate of
lime. The lime simply dissolves and is carried away, while many of
the mineral grains may remain to form a soil of nearly pure sand.
Where oxide of iron forms the cementing material, the rock yields less
readily to weathering, and the sandy soil will be yellowish brown or
red according to the climate. Another simple case is that of limestone
which when perfectly pure yields no soil because it is all soluble.
Pure limestone is, however, rare, and the various mineral impurities
in it, being to a considerable degree insoluble, tend to remain to
form a residual soil which may vary from sandy to clayey, and usually
brown or red due to the setting free of oxides of iron. According to
one estimate a thickness of about 100 feet of a certain fairly impure
limestone formation in Virginia must weather to yield a layer of soil
one foot thick. Soils of this kind, which are usually rich, are common
in many limestone valleys of the Appalachian Mountains. In the case of
shale rock, which is hardened mud, the cementing materials are removed,
some chemical changes in the minerals may take place, and the rock
crumbles to a claylike soil. What happens to a very hard, resistant
igneous rock like granite when attacked by the weather? Such a rock
always consists mainly of the two very common minerals feldspar and
quartz, usually with smaller amounts of other minerals such as mica,
hornblende, augite, or magnetite. The feldspar, which when fresh is
harder than steel, slowly yields when attacked by water containing
carbonic acid gas and crumbles or decays to a mixture of kaolin (clay),
carbonate of potash, and silica (quartz). Clay is an important
constituent of most good soils, while the carbonate of potash is
essential as a food for most plants. Due to yielding of the grains
or crystals of feldspar, the granite falls apart (see Plate 1). The
grains of quartz remain chemically unchanged, though they may be more
or less broken by changes of temperature, and the other minerals, which
are mostly iron-bearing, yield more or less to weathering, resulting
in a variety of products, among which are oxides of iron. A typical
granite, therefore, gives rise to a good heavy soil which is yellow,
brown or red according to climate. Such granite soils are common in
many parts of the Piedmont Plateau from Maryland to Georgia. Most of
the dark-colored igneous rocks, like ordinary basaltic lava, contain
much feldspar, various iron-bearing minerals, and little or no quartz.
Such rocks yield to the weather like granite but, because of lack of
quartz, the soils are more clayey. Rich soils of this kind occur in the
great lava fields of the northwestern United States and in the Hawaiian
Islands.

The importance of the breaking down of feldspar under the influence of
the weather, as above described, not only from the standpoint of soil
development, but also as regards the wearing down of the lands of the
earth, is difficult to overemphasize because that mineral is by far the
most abundant constituent of the earth's crust.

The term "erosion" is one of the most important in geologic science.
It comprises all the processes whereby the lands of the earth are
worn down. It involves the breaking up of earth material, and its
transportation through the agency of water, ice, or wind. Weathering,
including the various subprocesses as above described, is a very
important process of erosion. By this process much rock material is
got into condition for transportation. Another process of erosion,
called "corrasion," consists in the rubbing or bumping of rocks
fragments of all sizes carried by water, ice, or wind against the
general country rock, thus causing the latter to be gradually worn
away. A fine illustration of exceedingly rapid corrasion of very hard
rock was that of the Sill tunnel in Austria, which was paved with
granite blocks several feet thick. Water carrying large quantities of
rock fragments over the pavement at high velocity caused the granite
blocks to be worn through in only one year. Ordinarily in nature,
however, the rate of wear is much slower than this. Pressure exerted
upon the country rock by any agency of transportation may cause
relatively loose joint blocks, into which most rock formations are
separated, to be pushed away. This process, called "plucking," is
especially effective in the case of flowing ice.



CHAPTER III

STREAM WORK


Most streams are incessantly at work cutting or eroding their way into
the earth's crust and carrying off the products of weathering. By this
means the general level of lands is gradually being reduced to nearer
and nearer sea level. Base level of erosion is reached when any stream
has eroded to its greatest possible depth, and a whole region is said
to be base-leveled when, by the action of streams, it has been reduced
to a practically flat condition. A region of this kind is known as a
"peneplain."

To one who has not seriously considered the matter, the power of even
moderately swift water to transport rock débris seems incredible.
A well-established law of transportation by running water is that
the transporting power of a current varies as the sixth power of its
velocity. For example, a current which is just able to move a rock
fragment of a given size will, when its velocity is merely doubled, be
able to move along a piece of similar rock sixty-four times as large!
That this must be the case may be readily proved as follows: A current
of given velocity is just able to move a block of rock, say, of one
cubic inch in the form of a cube. A cubic block sixty-four times as
large has a face of sixteen square inches. By doubling the velocity of
the current, therefore, twice as much water must strike each of the
sixteen square inches of the face of the larger block with twice the
force, thus exerting sixty-four times the power against the face of the
larger block, or enough to move it along. This surprising law accounts
for the fact that in certain floods, like the one which rushed over
Johnstown, Pennsylvania, in 1889, locomotives, massive iron bridges,
and great bowlders were swept along with great velocity. It is obvious,
then, that ordinarily swift rivers in time of flood accomplish far more
work of erosion (especially transportation) than during many days or
even some months of low water.

Few people have the slightest idea as to the enormous amount of earth
material which the rivers are carrying into the sea each year. The
burden carried by the Mississippi River has been carefully studied for
many years. Each year this river discharges about 400,000,000 tons of
material in suspension; 120,000,000 tons in solution; and 40,000,000
tons rolled along the bottom. This all represents earth material
eroded from the drainage basin of the river. It is sufficient to cover
a square mile 325 feet deep, or if placed in ordinary freight cars
it would require a train reaching around the earth several times to
contain it. Since the drainage basin of the Mississippi covers about
1,250,000 square miles, it is, therefore, evident that this drainage
area is being worn down at the average rate of about one foot in
3,840 years, and this is perhaps, a fair average for the rivers of
the earth. The Ganges River, being unusually favorably situated for
rapid erosion, wears down its drainage basin about one foot in 1,750
years. It has been estimated that nearly 800,000,000 tons of material
are annually carried into the sea by the rivers of the United States.
According to this the country, as a whole, is being cut down at the
rate of about one foot in 9,000 years. In arriving at this figure it
should, of course, be borne in mind that the average level of hundreds
of thousands of square miles of the western United States, particularly
the so-called Great Basin, is practically not being reduced at all
because none of the streams there reach the sea.

Deposition of sediment is an important natural consequence of erosion.
The destination of most streams is the sea, and where tides are
relatively slight the sediments discharged mostly accumulate relatively
near the mouths of the rivers in the form of flat, fan-shaped delta
deposits. Some rivers, like the Ganges, which carry such unusual
quantities of sediment, are able to construct deltas in spite of
considerable tides. Deltas also form in lakes. In most cases, however,
rivers enter the sea where there are considerable tides and their
loads are more widely spread over the marginal sea bottom. But in many
cases some of the sediment does not reach the mouth of the stream. It
is, instead, deposited along its course either where the velocity is
sufficiently checked, as is the case over many flood-plain areas of
rivers, or where a heavily loaded, relatively swift stream has its
general velocity notably diminished. An excellent example of the latter
type of stream is the Platte River, which is swift and loaded with
sediment in its descent from the Rocky Mountains, but, on reaching
the relatively more nearly level Nebraska country, it has its current
sufficiently checked to force it to deposit sediment and build up its
channel along many miles of its course, and this in spite of the fact
that it still maintains a considerable current. In a mountainous arid
region a more or less intermittent stream at times of flood becomes
heavily loaded with rock débris and rushes down the mountain side.
On reaching the valley floor the velocity is greatly checked and most
of the load is deposited at the base of the mountain, successive
accumulations of such materials, called alluvial cones or fans, having
not uncommonly built up to depths of hundreds, or even several thousand
feet.

[Illustration: Plate 1.--(_a_) Granite Weathering to Soil near
Northampton, Mass. Under the action of weathering all of the once hard,
fresh, mass of granite has crumbled to soil except the fairly fresh
rounded masses which are residual cores of "joint blocks." (_Photo by
the author._)]

[Illustration: Plate 1.--(_b_) Looking-Glass Rock, Utah. The rock is
stratified sandstone sculptured mainly by wind erosion, that is, by
the wind driving particles of sand against it. (_Photo by Cross, U. S.
Geological Survey._)]

[Illustration: Plate 2.--Grand Canyon of the Yellowstone River in
Yellowstone National Park. The great waterfall 308 feet high is shown.
The large swift river has here sunk its channel (by erosion) to a
maximum depth of 1,200 feet during very recent geological time, and the
process is still going on. The wonderful coloring is due to iron oxides
set free during weathering of the lava rock. (_Photo by Hillers, U. S.
Geological Survey._)]

Any newly formed land surface, like a recently drained lake bed or
part of the marginal sea bottom which has been raised into land, has
a drainage system developed upon it. In the early or youthful stage
of such a new land area lying well above sea level, under ordinary
climatic conditions a few streams only form and these tend to follow
the natural or initial slope of the land. These streams carve out
narrow, steep-sided valleys, and all of them are actively engaged
in cutting down their channels, or, in other words, none of them
have reached base level, and flood plains and meandering curves are
therefore lacking. During this youthful stage there are no sharp
drainage divides; gorges and waterfalls are not uncommonly present;
and the relief of the land in general is not rugged. A good example of
youthful topography is the region around Fargo, North Dakota, which is
part of the bed of a great recently drained lake. The Grand Canyon of
the Yellowstone River is an excellent illustration of a youthful valley
cut in a high plateau of geologically recent origin. (Plate 2.)

As time goes on, a region in youth gradually gives way to a region in
maturity, during which stage the maximum number (usually a network)
of streams in broader V-shaped valleys have developed; divisions of
drainage are sharp; the maximum ruggedness of relief has developed;
the larger streams only have cut down so near base level that winding
(meandering) courses and flood plains are well developed along them;
and waterfalls and gorges are rarely present. An almost perfect example
of a region in maturity is that around Charleston, West Virginia.

The old-age stage develops next in the history of the region, during
which only a moderate number of streams remain, most of these being
at or close to base level so that sweeping curves or meanders (Plate
4) and cut-off meanders or "ox bows" and wide flood plains are
characteristic and common. The relief is greatly subdued and the term
"rolling country" might be applied to the moderately hilly region.
Divisions of drainage are, of course, not at all sharp and the valleys
are wide and shallow. Oxbow lakes are common, but gorges and waterfalls
are absent. A region typical of old-age topography is that around
Caldwell, Kansas.

Finally, after the remaining low hills have been cut down, the region
is in the condition of a broad monotonous plain, practically devoid
of relief, over which the sluggish streams pursue very winding, more
or less shifting or indefinite courses. For the attainment of this
final stage (called a "peneplain") in the normal cycle of erosion
a proportionately very long time is necessary, because the rate of
erosion becomes slower and slower as the region is being cut down.
Then, too, some change of level between the land and the sea is very
likely to take place before the peneplain stage is reached. It is
doubtful if any extensive region was ever brought to the condition of
a perfect peneplain. Some masses of more resistant or more favorably
situated rocks are almost sure to maintain at least moderate heights
above the general plain level. Geologically recently upraised, fairly
well developed peneplains are southern New England and the great
region of eastern Canada. The remarkably even sky lines of these
regions mark the peneplain level before the uplift took place, and
occasionally masses, called "monadnocks" from Mount Monadnock in
southern New Hampshire, rise above the general level. The valleys in
such an uplifted peneplain region have been carved out by streams
since the uplift began. We have positive evidence that more or less
well-developed peneplains of considerable extent existed in various
parts of the earth at various times during the many millions of years
of known earth history.

The normal cycle of erosion which, as outlined above, tends toward the
peneplain condition may be interrupted at any stage by other processes.
An excellent case in point is the upper Mississippi Valley, which had
reached the old-age stage, even approximating a peneplain, just before
the great Ice Age. Then, during the withdrawal of the vast sheet of ice
from the region toward the close of the Ice Age, extensive deposits
(moraines, etc.) of glacial débris were left irregularly strewn over
the country, giving rise to many low hills, lake basins, and altered
drainage lines, in some cases with resultant gorge development. Some
distinct features of a youthful topography are, therefore, plastered
over what was otherwise a region well along in old age. The general
district around the Dells of Wisconsin River well illustrates this
principle.

Changes in level between land and sea which take place during the
erosion of a region may also disturb the normal cycle of erosion. For
example, a region in old age may be considerably upraised so that
the streams have their velocities notably increased. Such a region
is said to be "rejuvenated" and the streams, which are revived in
activity, begin to cut youthful valleys in the bottom of the old ones
and, after a time, the general surface of the region is subjected to
vigorous erosion and a new cycle of erosion will be carried out unless
interfered with in some way, as by relative change of level between the
land and the sea. In this connection the history of the topography in
the general vicinity of Harrisburg, Pennsylvania, may be of interest by
way of illustration of the principle just described. The long, narrow,
parallel Appalachian Mountain ridges there rise to about the same
level, causing a remarkably even sky line as viewed from one of the
summits. This even sky line marks approximately the surface of what was
a peneplain late in the Mesozoic era. Early in the succeeding Cenozoic
era, the broad peneplain was notably upraised to nearly the present
altitudes of the ridge tops. The revived Susquehanna River left the
old course which it had on the peneplain surface, and began to carve
out its present valley, while tributaries (subsequent streams) to it
developed along belts of weaker rock and thus they formed the present
parallel valleys separated by belts of more resistant rocks which stand
out as ridges. In this way, the mature stage of topography was reached.
Very recently, geologically, the region has been rejuvenated enough to
cause the larger streams to appreciably sink their channels below the
general valley floors. The reader will find a general discussion of
movements of the earth's crust in a succeeding chapter.

[Illustration: Fig. 1.--The submerged Hudson River channel is clearly
shown by the contour lines on the sea floor. Figures indicate depth of
water in fathoms. Geologically recent sinking of the land has caused
the "drowning" of the river valley. (Coast and Geodetic Survey).]

If, for example, a region along the seaboard has reached the mature
stage of erosion, and the land notably subsides relative to sea level,
the tidewater will enter the lower valleys to form estuaries and the
valleys are said to be "drowned." The large streams, or at least their
lower courses, are thus obliterated and also the general erosion of the
region is distinctly diminished. The recently sunken coast of Maine
well illustrates the idea of "drowned valleys." The drowned valley of
the lower Hudson River is another fine example.

[Illustration: Fig. 2.--Sketch maps showing how the Shenandoah River
captured the upper waters of Beaverdam Creek in Virginia. The abandoned
valley of the creek across Blue Ridge is now called a "wind gap."
(After B. Willis.)]

What is termed stream "piracy" is of special interest in connection
with stream work. By this is meant the stealing of one stream or
part of a stream by another. We shall here explain only one of the
various ways by which stream capture may be effected. One of two
fairly active streams, flowing roughly parallel to each other, is more
favorably situated and has cut its channel deeper. Its tributaries
are, therefore, more favorable to extension of headwaters and, in
time, one of its tributaries eats back far enough to tap a branch of
the less favorably situated stream so that the waters of this branch
are diverted into the more favorably situated stream. The Shenandoah
River of Virginia has been such a pirate. This river developed as a
tributary of the Potomac. By headward extension toward the south, the
Shenandoah finally tapped and diverted the upper waters of the smaller,
less favorably situated Beaverdam Creek. The notch or so-called "wind
gap" through which the upper waters of Beaverdam Creek formerly flowed
across the Blue Ridge is still plainly visible. Such abandoned water
gaps, known as "wind gaps," are common in the central Appalachian
Mountain region.

A remarkable type of river is one which has been able to maintain
its course through a barrier, even a mountain range, which has been
built across it. Thus, the Columbia River, after flowing many miles
across the great lava plateau, has maintained its course right across
the growing Cascade Range by cutting a deep canyon while the mountain
uplift has been in progress. In a similar manner the Ogden River of
Utah has kept its westward course by cutting a deep canyon into the
Wasatch Range which has geologically recently, though slowly, risen
across its path. In no other way can we possibly explain the fact that
such a river, rising on one side of a high mountain range, cuts right
across it.

A feature of minor though considerable popular interest is the
development of "potholes" by stream action. Where eddies occur, in
rather active streams, rock fragments of varying sizes may be whirled
around in such manner as to corrode or grind the bedrock, resulting
in the development of cylinder-shaped "potholes." Such holes vary in
diameter up to fifty feet or more in very exceptional cases. In the
production of large "potholes" many rock fragments are worn away and
new ones supplied to continue the work. Locally, some stream beds are
honeycombed with "potholes."

[Illustration: Fig. 3.--Grand Canyon, Arizona. (From Darton's "Story of
the Grand Canyon.")]

Strikingly narrow and deep valleys, called gorges and canyons, are
rather exceptional features of stream action. Most wonderful of all
features of this kind is the Grand Canyon of the Colorado River in
Arizona. In fact, this canyon takes high rank among the most remarkable
works of nature. The canyon is over 200 miles long, from 4,000 to 6,000
feet deep, and from 8 to 15 miles wide. Contrary to popular opinion,
this mighty canyon is not a result of some violent process, such as
volcanic action, or the sudden sinking of part of the earth's crust.
Nor is it the result of the scouring action of a great glacier. It is
simply a result of the operation of the ordinary processes of erosion
where the conditions have been exceptionally favorable. Some of the
favorable conditions have been, and are, a large volume of very swift
water (Colorado River) continually charged with an abundance of rock
fragments for the work of corrasion, and a great thickness of rock
which the river must cut through before reaching base-level. Aridity of
climate also tends to preserve the canyon form. The whole work has been
accomplished in very late geological time, and the tremendous volume of
rock which has been weathered and eroded to produce the canyon has all
been carried away by the Colorado River and accumulated in the great
delta deposit near where the river empties into the Gulf of California.
Even now the canyon is growing deeper and wider because the very active
Colorado River is still from 2,000 to 3,000 feet above sea level.
Standing on the southern rim near Grand Canyon station at an altitude
of nearly 7,000 feet, and looking down into the canyon, one beholds a
vast maze of side canyons, high, vertical rock walls which follow very
sinuous courses, giving rise to a steplike topography, and countless
rock pinnacles, towers, and mesas often of mountain-like proportions.
The side canyons are the result of erosion by tributaries to the main
river which have gradually developed and worked headward as the main
river has cut down. The mountain-like sculptured forms which rise out
of the canyon are erosion remnants, or, in other words, masses of rock
which were more favorably situated against erosion by either the main
river or any of its tributaries. All of the rocks of the broader, main
portions of the canyon are strata of Paleozoic age, arranged as a vast
pile of almost horizontal layers, including sandstone, limestone,
and shale. Some of these layers, being distinctly more resistant
than others, stand out in the canyon wall in the form of conspicuous
cliffs, in some cases hundreds of feet high. The very striking color
bands (mostly light gray, red, and greenish gray), which may be traced
in and out along the canyon sides, represent the outcropping edges
of variously colored rock layers. Far down in the canyon lies the
steep-sided, V-shaped inner gorge, or canyon which is fully 1,000 feet
deep. The rocks are there not ordinary strata, but rather metamorphic
and igneous rocks, mostly dark gray, not in layers, and about uniformly
resistant to erosion. There is reason to believe that this inner gorge
has developed mainly since a distinct renewed uplift (rejuvenation)
of the Colorado Plateau after the river began its canyon cutting. The
narrow, steep-sided inner gorge may thus be readily accounted for and
the general lack of steplike forms on its sides is due to essential
uniformity of the rock material as regards resistance to erosion.

[Illustration: Fig. 4.--Profile and structure section across the
line A-A in Fig. 3. Length of section 10 miles, vertical scale not
exaggerated. The main relief features, and the relations of the rocks
below the surface are shown. The granite and gneiss are of Archeozoic
age, and the overlying nearly horizontal strata are of Paleozoic age.
(After Darton, U. S. Geological Survey.)]

The wonderful King's River Canyon of the southern Sierras in California
is remarkable for its combined narrowness and depth. It is a steep
V-shaped canyon whose maximum depth is 6,900 feet, carved out in
mostly solid granite by the action of weathering and running water.
Some idea of the vast antiquity of the earth may be gleaned from the
fact that this tremendously deep canyon has been produced by erosion
in one of the most resistant of all known rocks in very late geologic
time! Conditions favorable for cutting this canyon have been volume and
swiftness of water and a liberal supply of grinding tools.

Among the many other great canyons of the western United States brief
mention may be made of the Grand Canyon of the Yellowstone River
in the National Park. The plateau into which the river has cut its
steep-sided, narrow, V-shaped canyon, with a maximum depth of 1,200
feet, has been geologically recently built up by outpourings of
vast sheets of lava. The large volume of very swift water, aided by
decomposition and weakening of the ordinarily very hard rock by the
action of the hot springs, has been able to carve out this deep canyon
practically within the last period of earth history. The deepening
process is still vigorously in progress. The wonderful coloring of the
rock, mostly in tones of yellow and brown, is due to the hydrated iron
oxides developed during the decay of the iron-bearing minerals of the
lava, the chemical action having been greatly aided by the action of
the hot waters. (See Plate 2.)

In regard to its origin, the marvelous Yosemite Valley, or canyon,
falls in a somewhat different category, and it is discussed beyond in
connection with the work of ice. Suffice it to say here that running
water has been a very important factor in its origin.

In New York and New England there are many gorges which have developed
by the action of running water since the Great Ice Age. Famous among
these are Ausable Chasm and Watkins Glen of New York, and the Flume in
the White Mountains of New Hampshire.

[Illustration: Fig. 5.--Sketch map showing the retreat of the crest
of Niagara Falls from 1842 to 1905, based upon actual surveys. The
retreat of the inner part of the Horseshoe Fall was more than 300 feet.
(Modified by the author after Gilbert, U. S. Geological Survey.)]

Before leaving our discussion of the work of running water, we should
briefly consider waterfalls. True waterfalls originate in a number of
ways. Most common of all is what may be termed the "Niagara type"
of waterfall. Niagara Falls merit more than passing mention not only
because of their scenic grandeur, but also because of the unusual
number of geologic principles which their origin and history so clearly
illustrate. Niagara Falls are divided into two main portions, the
Canadian, or so-called "Horseshoe Fall," and the "American Fall,"
separated by a large island. The crest of the American Fall is about
1,000 feet long and nearly straight, while the crest of the Canadian
Fall is notably curved inward upstream, and it is about 3,000 feet
long. The height of the Falls is 167 feet. Downstream from the Falls
there is a very steep-sided gorge about 200 feet deep and seven miles
long. The exposed rocks of the region are nearly horizontal layers of
limestone underlain with shales. Relatively more resistant limestone
forms the crest of the falls, and directly underneath are the much
weaker shales. Herein lies the principle of this type of waterfall
because, due to weathering and the swirling action of the water, the
weaker underlying rocks erode faster, thus causing the overlying rock
to overhang so that from time to time blocks of it already more or less
separated by cracks (joints), fall down and are mostly carried away
in the swift current. Thus the waterfall maintains itself while it
steadily retreats upstream. Careful estimates based upon observations
made between 1827 and 1905 show that the Canadian Fall retreated at
the rate of from three to five feet per year, while the American Fall
retreated during the same time at the rate of only several inches
per year. It has been well established that Niagara Falls came into
existence soon after the ice of the great Ice Age had retreated
from the district. The falls started by plunging over a limestone
escarpment, situated at what is now the mouth of the gorge seven miles
downstream from the present falls. If we consider the rate of recession
of the falls to have been always five feet per year, the length of time
required to cut the gorge would be something over 7,000 years. But
the problem is not so simple, because we know that, at the time of,
or shortly after, the beginning of the falls, the upper Great Lakes
drained farther north and not over the falls; and that this continued
for a considerable, though unknown, length of time. During this
interval the volume of water in Niagara River was notably diminished,
and hence the recession of the falls must have been slower. On the
other hand, judging by the width of the gorge, the length of the crest
of the falls has generally been considerably less than at present,
which in turn means greater concentration of water over the crest
and more rapid wear. Various factors considered, the best estimates
for the age of the falls vary from 7,000 to 50,000 years, an average
being about 25,000 years. Although this figure is not precise, it is,
nevertheless, of considerable geologic interest because it shows that
the age of Niagara Falls (and gorge) is to be reckoned in some tens of
thousands of years, rather than hundreds of thousands or millions of
years. Although waterfalls of the Niagara type are the most common of
all, it is by no means necessary that the particular rocks should be
limestone and shale.

Another common kind of waterfall may be termed the "Yosemite type,"
so named from the high falls in the Yosemite Valley of California. At
the great falls of the Yosemite, the rock is a homogeneous granite and
the undermining process does not operate. Yosemite Creek first plunges
vertically over a granite cliff for 1,430 feet to form the Upper Falls,
which must rank among the very highest of all true water falls. The
water then descends about 800 feet by cascading through a narrow gorge,
after which it makes a final vertical plunge of over 300 feet. A brief
history of the falls is about as follows. A great steep-sided V-shaped
canyon several thousand feet deep had been carved out by the action
of the Merced River which now flows through the valley. Then, during
the Ice Age, a mighty glacier plowed through the canyon, filling it
to overflowing. The granite of this district having been unusually
highly fractured by great vertical joint cracks was relatively easy
prey for ice erosion. Due to its great weight, the erosive power of
the ice was most potent toward the bottom, successive joint blocks
were removed, and the valley was thus widened and the sides steepened
or even commonly made practically vertical. (See Plate 6.) After the
melting of the ice, certain of the streams, like Yosemite Creek and
Bridal Veil Creek, were forced to enter the valley by plunging over
great perpendicular, granite cliffs which are in reality joint faces.
This type of waterfall does not retreat, but it constantly diminishes
in height by cutting into the crest. A number of other high falls of
this kind occur in the Yosemite region, and also in other mountain
valleys which formerly contained glaciers, as in the Canadian Rockies,
the Alps, and Norway, the rocks in these regions being of various kinds.

In the case of the "Yellowstone type" of waterfall a different
principle is involved, namely, a distinctly harder or more resistant
mass of rock which extends vertically across the channel of the stream.
At the Great Falls of the Yellowstone a mass of relatively fresh, hard
lava lies athwart the course of the river, while just below it the lava
has been much weakened by decomposition. The harder rock therefore
acts as a barrier, while, in the course of time, the weak rock on the
downstream side has been worn away until a waterfall 308 feet high has
developed. This waterfall does not retreat very appreciably, but it is
probably increasing in height, due both to the scouring action of the
water at the base of the fall and the unusual clearness of the river
water here, thus causing little wear at the crest. It should be noted,
in this connection, that the channel just on the upstream side of the
barrier cannot be cut down faster than the top of the barrier itself.

The famous Victoria Falls of the Zambezi River in South Africa,
represents a relatively uncommon type of waterfall. Considering height
of the fall, length of crest, and volume of water, this is perhaps
the greatest waterfall in the world. The Zambezi River, a mile wide,
plunges over 400 feet vertically into a chasm only a few hundred feet
wide and at right angles to the main course of the stream. The general
country rock is hard lava, but locally a narrow belt of the rock has
been highly fractured vertically, due to earth movements or faulting
(see explanation beyond) and therefore weakened and more subject
to weathering than the general body of the lava rock. This belt of
weakened rock has been easy prey for erosion by the Zambezi River
and the chasm has there developed. In fact the chasm is still being
increased in depth. Leaving the chasm toward one end, the river flows
through a narrow zig-zag gorge whose position has been determined by
big joint cracks. The mile-wide crest of the falls is interrupted by
a good many ledges and even small islands. The thundering noise of
this great waterfall is most impressive, but a good complete view is
impossible because most of the chasm is constantly filled with dense
spray.

Still another type of waterfall develops by the removal of joint blocks
by the action of running water. Falls of this type are fairly common
though they seldom attain really great heights. Where the rock in the
bed of a stream is traversed by well-developed vertical joint cracks,
slabs of rock cleaved by the joints may fall away due to weathering or
they may be pushed away by pressure of the water. Such a fall retreats
upstream by removal of joint blocks even in comparatively homogeneous
rocks. Taughannock Falls, 215 feet high, in southern central New York,
has developed by this manner in a shaly sandstone. The several falls
(one 50 feet high) in the famous gorge at Trenton Falls, in central New
York, have developed in this way in limestone.



CHAPTER IV

THE SEA AND ITS WORK


It is well known that the waters of the sea cover nearly three-fourths
of the surface of the earth. We think of the United States as being a
large piece of land of over three million square miles--but the sea
is about forty-five times as large, that is, it covers approximately
140,000,000 square miles. It is a remarkable fact that the average
depth of the great oceans of the earth is nearly two and one-half
miles. If the sea were universally present everywhere with the same
depth, it would be almost two miles deep. Yet this vast body of water
is an extremely thin layer when compared with the earth's diameter
of 8,000 miles. The Pacific is the deepest of the oceans with an
average depth of about two and three-fourths miles. The deepest ocean
water ever sounded is 32,114 feet (over six miles), not far from
the Philippine Islands. This is known as the Planet Deep, and was
discovered in 1912. Second deepest is 30,930 feet near the island of
Guam. In the Pacific Ocean there are five places where the water is
over five miles deep and eleven places were it is over four miles deep.
The deepest sounding ever made in the Atlantic Ocean was 27,972 feet,
not far from Porto Rico.

Many substances are known to be in solution in sea water, but in spite
of this the composition is remarkably uniform. The most abundant
substance by far in solution is common salt. In every 100 pounds of
sea water, there are 3.5 pounds of mineral matter of various kinds
dissolved. Nearly 78 per cent of the dissolved matter is common salt.
The principal other constituents in solution are chloride and sulphate
of magnesia, and the sulphates of lime and potash. All other dissolved
mineral substances together make up less than one per cent of the
total. It has been estimated that if all the dissolved mineral matter
should be brought together, it would form a layer 175 feet thick over
the whole sea bottom. The salts of the sea have been mostly supplied by
the rivers, which in turn have derived them from the disintegration and
chemical decay of the rocks.

If we make a general comparison with the surface of the land, the
floor of the ocean is a vast monotonous plain. None of the sea bottom
compares with the ruggedness of mountains, and even the more level
portions of land surface show many sharp minor irregularities such as
stream trenches. But the sea bottom is characterized by its smoothness
of surface. There are under the sea, however, mountain-like ridges,
plateaus, submarine volcanoes and valleys known as "deeps." But these
rarely show ruggedness of relief like similar features on land.

One of the most remarkable relief features of the ocean bottom is the
so-called "continental shelf." This is a relatively narrow platform
covered by shallow water bordering nearly all the lands of the earth.
Seaward, the depth of water is greatest, and it is seldom over 600 or
800 feet. The continental shelves of the world cover about 10,000,000
square miles or about one-fourteenth of the area of the sea.

Viewed in a broad way, there are two great classes of marine deposits;
first, those laid down comparatively near the borders of the land,
that is, on the continental shelf and continental slope, and second,
the abysmal deposits laid down on the bottom of the deep ocean. Those
found along and near the continental borders are largely land-derived
materials, that is to say, they are mostly sediments carried from the
land into the sea by rivers, and to a lesser extent rock material
broken up by waves along many shores. Practically all such land-derived
material is deposited within 100 to 300 miles of the shores. The
continental border deposits are extremely variable. Near shore they
are chiefly gravel and sands, while farther out they become gradually
finer, and on the continental slope only very fine muds are deposited.
These deposits usually contain more or less organic materials and
shells or skeletons of organisms. In some cases the shells or skeletons
of organisms predominate or even exist to the exclusion of nearly
all other material, as is true of the coral deposits or reefs which
form only in shallow water. Deposits like those just described as
accumulating on the bottom of the shallow sea, comparatively near the
lands, are of great significance to the geologist because just such
marine deposits now consolidated into sandstone, conglomerate, shale,
and limestone, are so widely exposed over the various continents.
A knowledge of the conditions under which shallow sea deposits are
now forming, is, therefore, of great value in interpreting events of
earth history as they are recorded in similar rocks which have been
accumulating through millions of years of time. One specific instance
will make this matter clearer. Using the method outlined in Chapter I
for the determination of earth chronology, and our knowledge of present
conditions under which shallow sea deposits are formed, it has been
well established that a shallow sea spread over fully four-fifths of
the area of North America during the middle Ordovician period of the
early Paleozoic era. Beyond this main conclusion, a careful study of
these rocks has revealed many important facts regarding the physical
geography, life, and climate of that time. The importance of this whole
matter is still further emphasized by the statement that five-sixths of
the exposed rocks of the earth are strata--mostly of shallow sea origin.

The deposits on the deep sea bottom are very largely either organic or
the shells and skeletons of organisms which have fallen to the bottom
from near the surface as already explained. Most common of these are
the deep sea "oozes" which are made up of the remains and shells of
tiny organisms called "foraminifers." These "oozes" cover about 50
million square miles of the sea bottom down to depths of from two to
three miles.

At depths greater than from two to three miles, a peculiar red clay is
the prevailing deposit. This is most extensive of all, covering an area
of 55 million square miles, or nearly the total area of lands of the
earth. Some remains of organisms are mixed with this clay, but since
most of the shells are of carbonate of lime and very thin, they are
dissolved without reaching the bottom in the deep sea water which is
under great pressure and rich in carbonic acid gas.

The deep sea deposits, both "oozes" and red clay, do, however, contain
some land-derived and other materials. Thus off the west coast of
Africa some dust carried by the prevailing winds from the Sahara
Desert, is known to fall in the deep sea several hundred miles from
shore. Volcanic dust is carried for many miles and deposited in the
deep sea--particularly in the South Pacific Ocean. Bits of porous
volcanic rock called "pumice" sometimes float long distances out over
the deep sea, before becoming water soaked. Icebergs often drift far
out from the polar regions over the deep sea, and on melting the rock
débris which they carry is dropped to the sea bottom. Also, particles
of iron and dust from meteorites ("shooting stars") have been dredged
from the deep sea.

One important geological significance of the deep sea deposits is
the proof which they furnish that, from at least as far back as the
beginning of the Paleozoic era, fully twenty-five million years ago, to
the present time, the two great deep ocean basins--the Atlantic and the
Pacific--have maintained essentially the same positions on the earth.
This is proved by the fact that nowhere, on any continent among the
rocks of all ages, as old at least as the early Paleozoic, do we find
any really typical deep-sea deposits. There is then no evidence that a
deep sea ever spread over any considerable part of any continent, and
this in spite of the fact that marine deposits of shallow water origin
furnish abundant evidence of former sea extensions. The shallow seas
have at various times spread over large portions of the continents.

On many rocky coasts the waves are incessantly pounding and wearing
away the rocks. In such places the sea, like a mighty horizontal saw,
is cutting into the borders of the lands. The finer materials produced
by the grinding up of the rocks are carried seaward by the undertow.
But, if the land remains stationary with reference to the sea, this
landward cutting by the waves reaches a limit. Since even big waves
have very little effect in water 100 or 200 feet deep, a shelf is cut
by the waves and this shelf, not many miles wide, is covered by shallow
water. The finer ground-up rock materials carried out by the undertow
are dumped just beyond the edge of the shelf which is thus built out
seaward as a terrace. In traveling over this shelf and terrace, the
waves, due to friction, lose their power. With gradually sinking land,
a much wider shelf may be cut, because the power of the waves is then
allowed to continue.

It might be of interest to cite a few cases of relatively rapid coast
destruction by the waves which have come under human observation. A
remarkable example is the island of Heligoland on which is (or was)
located the powerful German fort which guards the entrance to the Kiel
Canal. In the year 800 A. D. this island had 120 miles of shore line;
in 1300 it had 45 miles of shore; in 1649 only 8 miles; and in 1900
but 3 miles of shore line remained. In southeastern England "whole
farms and villages have been washed away in the last few centuries,
the sea cliffs retreating from 7 to 15 feet a year." A church located
a mile from the sea shore near the mouth of the Thames river, in the
sixteenth century, now stands on a cliff overlooking the sea. An island
in Chesapeake Bay covered over 400 acres in 1848, and the waves have
since reduced it to about fifty acres. Study showed that the relatively
soft unconsolidated strata of the Nashaquitsa Cliffs on the island of
Martha's Vineyard, were cut back at the rate of 5-1/2 feet per year,
between 1846 and 1886.

If part of the relatively smooth sea bottom should be raised into land,
the resulting shore line would of course, be regular and free from
indentations or sharp embayments. Examples of such coast which are
very young are at Cape Nome, Alaska; the northern coast of Spain; and
the west coast of northern South America. Soon, however, such a shore
line is attacked, and, either where the waves are greatest or the rocks
are weakest, indentations will result and the whole coast is gradually
eaten back until the power of the waves is largely spent in traveling
across the shallow water shelf. Sand bars are then built across the
mouths of the bays or indentations which later the rivers gradually
fill up with sediment. The result is a relatively straight or regular
old shore line. The coast of Texas has about reached this stage.

If a portion of the relatively rugged land surface should become
submerged under the sea, a very irregular, deeply indented shore line
would result, due to the entrance of tidewater into the valleys. The
deeply indented coast of Maine is a fine example of a very irregular
youthful shore line produced by geologically recent sinking of a
rugged, hilly region so that tidewater backs for miles into the
lower reaches of the river valleys. The promontories and islands are
undergoing rapid wear, and the development of bars across the inlets
has scarcely begun. Other excellent examples are the coasts of Norway
and southern Alaska. Such a coast is then attacked by the ocean waves
and the promontories are cut back until the broad shallow water shelf
is formed, after which sand bars are built across the remaining
embayments and the shore line becomes relatively regular.

It is, then, a remarkable fact that, whether shore lines originate
by emergence of sea bottom, or by sinking of land, there is a very
strong tendency on the part of nature to develop regular shore lines.
It should be stated that the principles of wave work and shore-form
development just outlined apply almost equally well to lakes,
especially large ones.

Before leaving this subject of shore-line development, mention should
be made of the fact that bars and beaches are often built part way
or wholly across embayments of the coast with surprising rapidity.
To illustrate, Sandy Hook, New Jersey, is advancing northward, while
Rockaway Beach, New York, is extending westward, the tendency being
to close up the entrance to New York harbor and to make the line of
seashore more nearly regular. Records show that Rockaway Beach actually
advanced westward more than three miles between the years 1835 and
1908.



CHAPTER V

GLACIERS AND THEIR WORK


A glacier may be defined as a mass of flowing ice. The motion may
not be that of flowage in the usually accepted sense of the term. A
discussion of the various theories of glacier motion will not here be
attempted. Glaciers form only in regions of perpetual snow, but they
commonly move down far below the line of perpetual snow of any given
region. In the polar regions they may form near sea level, while in the
tropics they form at altitudes of two to three miles, and there only
rarely. In southern Alaska, the lower limit of perpetual snow is about
5,000 feet above sea level, and many of the glaciers come down to sea
(Plate 4), while in the Alps, the lower limit of perpetual snow is at
about 9,000 feet, and the glaciers descend as much as 5,000 feet below
it.

In regions of perpetual snow there is a tendency for more or less snow
to accumulate faster than it can be removed by evaporation or melting.
As such snow accumulates it gradually undergoes a change, especially in
its lower parts, first into granulated snow (so-called "névé") and then
into solid ice. Snow drifts in the northern United States often undergo
similar transformation, after a few months first to névé, and then to
ice. This transformation seems to be brought about mainly by weight of
overlying snow which compacts the snow crystals; by rain or melting
snow percolating into the snow to freeze and fill spaces between the
snow crystals; and by the actual growth of the crystals themselves.
When ice of sufficient thickness has accumulated (probably at best
several hundred feet), the spreading action or flowage begins and a
glacier has developed. Renewed snowfalls over the gathering ground keep
up the supply of ice.

There are several types of glaciers: valley or alpine glaciers; cliff
or hanging glaciers; piedmont glaciers; ice caps; and continental ice
sheets. A valley or alpine glacier consists essentially of a stream
of ice slowly flowing down a valley and fed from a catchment basin of
snow within a region of perpetual snow. In the Alps, where glaciers of
this sort are very typically shown, they vary in length up to eight or
nine miles. Perhaps the grandest display of great valley glaciers is in
southern Alaska where they attain lengths up to forty or fifty miles
and widths of one or two miles (Plate 4).

Hanging or cliff glaciers are in many ways like valley glaciers, but
they are generally smaller; they develop in snow-filled basins above
the snow line usually on steep mountain sides; and they do not reach
down into well-defined valleys. Most of the glaciers of the Glacier
National Park in Montana and many of those in the Cascade Mountains are
of this type. Mount Rainier in Washington is one of the most remarkable
single large mountain peaks in the world, in regard to development of
glaciers over it. Great tongues of ice, starting mostly at 8,000 to
10,000 feet above sea level, flow down the sides of the mountain for
distances of to four and even six miles. The total area of ice in this
remarkable system of radiating glaciers on this one mountain is over
forty square miles. These Mount Rainier glaciers are in general best
classified as intermediate in type between valley and hanging glaciers.

[Illustration: Fig. 6.--Map of Mount Rainier, Washington, showing its
wonderful system of glaciers which covers more than 40 square miles.
Dotted portions represent moraines. (U. S. Geological Survey.)]

In some high latitude areas, as in Iceland and Spitzbergen, snow and
ice may accumulate on relatively level plains or plateaus and slowly
spread or flow radially from their centers. These are called ice caps.
Ordinary ice caps usually do not cover more than some hundreds of
square miles.

Continental glaciers or ice sheets are, in principle, much like ice
caps, but they are larger. Greenland is buried under an ice sheet of
moderate size (about 500,000 square miles), the motion being outward in
all directions toward the sea. Tongues of ice, like valley glaciers,
are commonly sent off from the main body of ice across the land
border of Greenland into the sea. The size of the great ice sheet of
Antarctica is not definitely known, but it covers probably at least
several million square miles. Two continental ice sheets of special
interest to the geologist are those which existed during the great
Ice Age of the Quaternary period. One of these then covered nearly
4,000,000 square miles of North America, while the other covered about
600,000 square miles of northern Europe. The main facts regarding
the Ice Age are given in a succeeding chapter. The facts brought out
in the present discussion of existing glaciers will greatly aid in
understanding the Ice Age.

How fast do glaciers flow? Based upon many observations, we may say
that an average rate of flow for the glaciers of the world is not more
than a few feet per day. A very exceptional case is a large glacier,
branching off as a tongue from the ice sheet of Greenland, which is
said to move sixty to seventy-five feet per day. Some of the great
Alaskan glaciers have been found to flow from four to forty feet per
day. Most glaciers of the Alps move only one to two feet per day. A
glacier advances only when the rate of motion is greater than the rate
of melting of its lower end and vice versa in the case of retreat. Thus
it is true, though seemingly paradoxical, to assert that a glacier has
a constant forward motion even when it is retreating by melting.

By watching the changing position of marked objects placed in the ice,
it has been proved that, in a valley glacier, the top moves faster
than the bottom; the middle moves faster than the sides; the rate of
motion increases with thickness of ice, slope of floor over which it
moves, and temperature.

Ice, like molasses candy, tends to crack when subjected to a relatively
sudden force, and where the ice rides over a salient on the bed of
the glacier, transverse cracks or fissures often develop. Due to more
rapid motion of the central part of a valley glacier, stresses and
strains are set up and crevasses are formed, usually pointing obliquely
upstream. Where the ice tends to spread laterally in a broad portion
of a valley, longitudinal cracks may develop. Crevasses vary in size
up to several feet in width and hundreds of feet in depth. Owing to
the forward motion of the ice, old fissures tend to close up and new
ones form, and, aided by uneven melting, the surface of a glacier is
generally very rough.

Like running water, ice may have considerable erosive power when it is
properly supplied with tools. The total erosive effect which has been,
and is now being, accomplished by ice compared with that of running
water is, however, slight. One of the main processes by which ice
erosion is accomplished is "corrasion" due to the rubbing or grinding
action of hard rock fragments frozen into the bottom and sides of the
glacier. Thick ice, shod with hard rock fragments and flowing through a
deep, narrow valley of soft rock, is especially powerful as an erosive
agent because the abrasive tools are supplied; the work to be done is
easy; and the deep ice causes great pressure on the bottom and lower
sides of the valley. Rock surfaces which have been thus subjected to
ice erosion are characteristically smoothed and more or less scratched,
striated, or ground due to the corrosive effects of small and large
rock fragments. This affords one of the best means of proving the
former presence of a glacier over a region or in a valley. A typical
V-shaped stream cut (eroded) valley is changed into one with a U-shaped
profile or cross section by glacier erosion (Plate 5).

Another important process of ice erosion is "plucking," which consists
in pushing among already more or less loosened joint blocks by the
pressure of the moving ice. The pressure thus exerted, especially by
a deep valley glacier, may be enormous. This process was an important
factor in the development of the famous Yosemite Valley, a very brief
account of whose history it will now be instructive to give.

[Illustration: Plate 3.--The Gorge of Niagara River Below the Great
Falls. The strata (containing fossils) were accumulated on the bottom
of the Silurian sea which overspread the region at least 18,000,000
years ago. Since the Ice Age or within 20,000 to 40,000 years, the
river has carved out the gorge. (_Courtesy of the Haines Photo Company,
Conneaut, Ohio._)]

[Illustration: Plate 4.--(_a_) A Winding Stream in the St. Lawrence
Valley of New York. Due to its low velocity the stream cuts its channel
down very little, but it swings or "meanders" slowly from one side of
its valley to the other, developing a wide flood plain. The stream once
flowed against the valley wall shown at the middle left. (_Photo by the
author._)]

[Illustration: Plate 4.--(_b_) Davidson Glacier, Alaska. This glacier
is at work slowly grinding down the valley floor and cutting back its
walls, thus changing the original stream-cut, V-shaped profile, like
that of Plate 5. (_Photo by Wright, U. S. Geological Survey._)]

The Yosemite Valley, about 7 miles long, less than one mile wide,
and from 2,000 to 4,000 feet deep, lies on the western slope of the
Sierra Nevada Mountains of California. Great cliffs of granite,
mostly from 1,000 to over 3,000 feet high, bound the valley on either
side. The floor of the valley is wide and remarkably flat (Plate 6).
Just prior to the Ice Age, by the processes of erosion already set
forth, the Merced River had carved out a great steep-sided V-shaped
canyon commonly from 1,000 to 3,000 feet deep. During the Ice Age,
two glaciers joined to form an extra deep powerful glacier, which
flowed through a deep part of the Merced Canyon and modified it into
the Yosemite Valley, essentially as we see it to-day. Because the ice
was shod with many fragments of hard rock (granite), and the pressure
at the bottom and lower sides of the glacier (several thousand feet
thick) was so great, the V-shaped stream-cut canyon was changed to a
U-shaped canyon with very steep to even vertical walls. A factor of
great importance which notably aided the erosive power of the glacier
in this case was the existence of an unusual number of large vertical
joint cracks in the granite in this local region. The plucking action
of the ice was thus very greatly facilitated and great slabs of rock,
separated by the vertical joints, especially toward the lower sides
and bottom of the valley, were pushed away one after another by the
ice. When the ice disappeared, great precipitous joint faces from 1,000
to 3,000 feet high were left along the valley sides. At its lower
end the glacier left a dam of glacial débris (moraine) across the
valley, thus causing a lake to form over the valley floor. The wide
flat bottom of the valley was caused by filling up of the lake with
sediment. The uniqueness of the Yosemite Valley is, then, due to a
remarkable combination of several main factors; one, the presence of
a large swift river well supplied with tools which carved out a deep
V-shaped canyon; two, a mighty glacier which plowed its way through
this canyon and converted it by erosion into a U-shaped canyon;
three, the weakening of the rock by many joint cracks, thus greatly
facilitating the ice erosion; and four, a postglacial lake covering
the valley floor which became filled with sediment. As a result of the
ice work, several streams, tributary to the main stream (Merced River)
which flows through the bottom of the valley, were forced to plunge
over great vertical rock walls (joint faces), thus producing high and
beautiful true waterfalls, including the very high Upper Yosemite Fall
where Yosemite Creek makes a straight drop of 1,430 feet. A tributary
valley like that of Yosemite Creek, which ends abruptly well above
the main valley, is known as a "hanging" valley. The valley of Bridal
Veil Creek is another good example. (See Plate 6.) Valleys which were
once occupied by active glaciers are generally characterized by their
U-shaped cross sections and their hanging (tributary) valleys, but
the great height and steepness of the valley walls in Yosemite are
exceptional.

A type of glacial erosion which is of special interest is the
sculpturing of so-called "cirques" or "amphitheaters" in mountains
within the region of perpetual snow. Where the main mass of snow and
ice in the catchment basin or gathering ground of a valley glacier
pulls away from the snow and névé on the upper slopes, the rock wall
is more or less exposed in the deep crevasse. During warm days water
fills the joint cracks in the rocks down in this crevasse (so-called
"Bergschrund"), and during cold nights the water freezes and forces
the blocks of rock apart. This is greatest toward the bottom of the
crevasse and so, by this excavating or quarrying process, vertical or
very steep walls are developed around a great bowlike basin or cirque.
Such cirques, now free from glacial ice, with precipitous walls 500 to
2,000 feet high and one-fourth of a mile to one-half of a mile across,
are common in the Sierra Nevada and Cascade Ranges and in the Rocky
Mountains.

What becomes of the materials eroded by the ice? An answer to this
question involves at least a brief discussion of the deposition of
glacial débris, this constituting an important feature of the work
of ice. The débris transported by a glacier is carried either on
its surface or within it, or pushed along under it. It is generally
heterogeneous material ranging from the finest clay through sand and
gravel, to bowlders of many tons' weight. Various types of glacial
deposits are abundantly illustrated by débris left strewn over much
of the northeastern United States and some reference to these will be
made.

Most valley glaciers carry considerable débris on their surfaces, this
representing material which falls or is carried down from the valley
walls upon the margins of the ice, thus forming marginal moraines.
When two glaciers flow together, one marginal moraine from each will
coalesce to form a medial moraine. The material carried along at the
bottom of a glacier is called the ground moraine. Where it contains
much very fine grained material with pebbles or bowlders scattered
through its mass, it is called "till" or "bowlder clay." The pebbles or
bowlders of the ground moraine are commonly facetted and striated as a
result of having been rubbed against the bedrock on which the glacier
moved. Ground moraine material is the most extensively developed of
all glacial deposits. It is so widely scattered over the glaciated
northeastern portion of the United States that most of the soils
consist of it, having been left strewn over the country during the
melting of the vast ice sheet.

When a glacier remains practically stationary for some time, more or
less material which it carries is piled up at its lower end to form
a terminal moraine. Repeated pauses during general glacier retreat
permit the accumulations of so-called recessional moraines. A wonderful
display of recessional moraines occurs from the Great Lakes south,
where they are festooned one within another and remain almost exactly
as they were formed during pauses in retreat of great lobes of ice
during the closing stages of the Ice Age. A great terminal moraine
marks the southernmost limit of the ice sheet during the Ice Age,
a very fine illustration being the ridge of low irregular hills
extending the whole length of Long Island. Some of the material in that
morainic ridge was transported by the ice from northern New England.

Considerable rock débris is transported within the ice, and such
"englacial" material in part results from rock débris which falls
on the surface in the catchment basin and becomes buried under new
snowfalls which change to ice, and in part from material which falls
into the crevasses in the glacier farther down the valley. Marked
objects thrown into the catchment basin have, after many years, emerged
at or near the end of the glacier; thus the rate of motion can be very
accurately told. A very remarkable case of transportation through the
body of a glacier is the following: In 1820, three men were buried
under an avalanche in the catchment basin of the Bossons Glacier in
the Alps. Forty-one years later several parts of the bodies, including
the three heads together with some pieces of clothing, emerged at the
foot of the glacier after traveling most of its length at the rate of
eight inches per day. The heads were so perfectly preserved after their
remarkable journey in cold storage that they were clearly recognized by
former friends!

Where a valley floor slopes downward away from the end of a glacier,
waters emerging from the ice, heavily loaded with rock débris, cause
more or less deposition of the débris on the valley floor often for
miles beyond the ice front. Such a deposit is called a "valley train."
When the ice front pauses for a considerable time upon a rather flat
surface, the débris-laden waters emerging from the ice develop an
"outwash plain" by deposition of sediment rather uniformly over the
flat surface. A very fine example is the plain which constitutes most
of the southern half of Long Island just beyond the southern limit of
the great terminal moraine ridge.

A type of glacial deposit of particular interest is the "drumlin"
which is, in reality, only a special form of ground moraine material
(commonly till), and, therefore, essentially unstratified. Typical
drumlins are low, rounded mounds of till with roughly elliptical bases
and steeper fronts facing the direction from which the ice flowed.
Their long axes are always parallel to the direction of ice movement.
In height they commonly range from 50 to 200 feet. Their mode of origin
is not yet definitely known, but they form near the margins of broad
lobes of ice either by erosion of earlier glacial deposits, or by
accumulation beneath the ice under peculiarly favorable conditions,
as perhaps in the longitudinal crevasses. One of the finest and most
extensive exhibitions of drumlins in the world is in western New York
between Syracuse and Rochester. Thousands of drumlins there rise above
the general level of the Ontario plain, the New York Central Railroad
passing through the very midst of them. Drumlins are also abundant in
eastern Wisconsin.

Another type of glacial deposit in the form of low hills is the "kame"
which, unlike the drumlin, always consists of more or less stratified
material. Kames are seldom over 200 feet high, and they are of various
shapes. In many cases they form irregular groups of hills, and in
other cases fairly well defined kame ridges. Kames form as deposits
from débris-laden streams emerging from the margins of glaciers, the
water sometimes rising as great fountains because of the pressure. Such
deposits are now actually in process of formation along the edge of
the great Malaspina Glacier of Alaska. Kames are commonly associated
with terminal and recessional moraines. "Eskers" are similar except
that they are long winding low ridges of stratified material deposited
by débris-laden streams, probably in longitudinal fissures in the ice
near its margin. (See Plate 20.)

Glacial bowlders, or "erratics" are blocks of rock or bowlders left
strewn over the country during the melting of the ice. They vary in
size from small pebbles to those of many tons of weight, and most of
them were derived from ledges of relatively hard, resistant rocks. (See
Plate 20.) Erratics have very commonly been carried a few miles from
their parent ledges, while more rarely they have traveled even hundreds
of miles. They are extremely abundant in New York and New England,
many occurring even high up on the mountains. In some cases erratics
of ten or more tons' weight have been left in such remarkably balanced
positions on bedrock that a child can cause one of them to swing back
and forth slightly. Such a bowlder is literally a "rocking stone."
In the Adirondack Mountains the writer recently observed a rounded
erratic of very hard rock fourteen feet in diameter resting in a very
remarkably balanced position on top of another large round glacial
bowlder.



CHAPTER VI

THE ACTION OF WIND


Only during the last quarter of a century have geologists come to
properly appreciate the really important geological work of the wind.
One reason for this is the fact that people live mostly in humid
regions where the soils are largely effectually protected against
wind action by the vegetation. But even in such regions, wind action
is by no means negligible. One has but to observe the great clouds of
dust raised by strong wind from freshly cultivated fields during a
little dry weather in the late spring. Much of this dust is carried
considerable distances, often miles, and in some cases young crops
are injured by removal of soil from around the roots, while in other
cases young plants are buried by deposition of the wind-blown material
over them. In humid regions, the action of the wind is perhaps most
strikingly exhibited along and near shores of sea and lakes, where
loose dry sands are picked up and transported in great quantities,
often depositing them as sand dunes, which may form groups of hills
covering considerable areas. Very conspicuous examples are the sand
dunes of Dune Park in northern Indiana, and the dunes along the coast
of New Jersey.

But the action of wind is most strikingly effective in desert and
semiarid regions. The importance of the work of wind is made more
impressive when we realize that about one-fifth of the land of the
earth is desert.

In deserts some of the ordinary agents of weathering and erosion
are either absent or notably reduced in effectiveness. Thus, stream
action is, in general, reduced to a minimum; weathering effects due
to moisture in the air are notably reduced, and either frost action,
or wedge work of ice, is relatively unimportant due to lack of water.
Change of temperature between night and day is, however, unusually
important as a process whereby rocks are broken up due to relatively
rapid expansion and contraction in deserts because such temperature
changes are exceptionally great, and rocks and soils are almost
everywhere directly exposed, being free from vegetation.

The finer grained materials, especially sand grains, in deserts
are picked up by the wind and driven, often with great velocity,
against barren rock ledges and large and small rock fragments. By
this process (corrasion) the rocks are worn and often polished by the
materials blown against them. The principle is that of the artificial
sand-blast, used in etching glass, or cleaning and polishing building
and decorative stones. Under favorable conditions wind-driven sand
accomplishes noticeable erosion in a surprisingly short time. Thus, in
a hard wind storm, a plate glass window in a lighthouse on Cape Cod was
worn to opaqueness, while in a few weeks or months the directly exposed
window glass may there be worn through.

The great erosive effect of wind-driven sand is relatively close to
the ground because the larger and heavier fragments are not lifted to
very considerable heights. For this reason ordinary telegraph poles
are difficult to maintain in desert regions because, unless they are
specially protected, they are soon cut down by sand swept against
their bases. In the desert regions of our Southwestern States cliffs
rising above the general level of the country are often undercut by
wind erosion, sometimes with the development of large caverns. (See
Plate 1.) Even the high portions of great ledges are there more or less
fantastically sculptured by wind erosion, the softer portions being
more deeply cut into than the harder. The famous sphinx of Egypt has
been notably roughened by action of this kind.

The enormous power of high winds to transport rock material in desert
regions is strikingly illustrated by the great sand storms of the
Sahara Desert, where sand and dust, forming clouds with cubic miles of
volume, sweep for many miles across the country. Some one has estimated
that every cubic mile of air in such a storm contains more than
100,000 tons of rock material. It is said that an army of 50,000 men
under Cambyses was buried under the sands of a storm in the desert of
northern Africa.

Dust from some of these storms is known to be driven hundreds of miles
out over the Atlantic Ocean, there to settle in the sea. In mountainous
desert regions, like the Great Basin of our Western States, the general
tendency is for the rock materials wind-eroded from the mountains to
be carried into the intermontane basins or valleys. Some basins of
this sort are believed to contain depths of 1,000 to 2,000 feet of
wind-blown material.

A special kind of wind-blown material called "loess," is a sort
of fine-grained yellow, or brown loam which, though relatively
unconsolidated, has a remarkable property of standing out as high
steep cliffs or bluffs along the banks of streams. Many thousands of
square miles of northern China are covered with loess. Among many other
regions, thousands of square miles of parts of the States of Iowa,
Nebraska, and Kansas are covered with loess, which, in this case, is
believed to be fine material gathered by winds from the region just
after the retreat of one of the ice sheets of the great Ice Age, when
there was very little vegetation to hold down the loose soils of
glacial origin.

Much as snowdrifts are formed, so, in many places, the wind-driven
sands are built up into sand hills or so-called "dunes." Dunes are very
common in many places, as for example, along our middle Atlantic coast;
in Dune Park of northern Indiana; and in the great arid and semiarid
regions of the Western States. Where there is a distinctly prevailing
direction of wind, the sand is blown to the leeward side from the
windward side, and the dunes are caused to migrate in the direction of
the wind. The burial and destruction of forests, and the uncovering of
the dead trees is not uncommonly caused by migration of sand dunes, all
stages of this phenomenon being well exhibited in Dune Park, Indiana.
The rate of dune migration is very variable, but study in a number of
places has shown a rate of from a few feet to more than 100 feet per
year. Arable lands, buildings, and even towns have been encroached upon
and buried under drifting sand. An interesting example is a church in
the village of Kunzen, on the Baltic seashore which, in a period of
sixty years, became completely buried under a dune and then completely
uncovered by migration of the dune. Much destruction has been wrought
by shifting sands on the Bay of Biscay, where farms and even villages
have been overwhelmed. The ruins of the ancient cities of Babylon and
Nineveh are buried mostly under wind-blown sand and dust. There is
good reason to believe that the climate of central and western Asia is
now notably drier than it was a few thousand years ago, and this may
help to explain the burial of many old cities and villages there under
wind-blown deposits.



CHAPTER VII

INSTABILITY OF THE EARTH'S CRUST


The crust of the earth is unstable. To the modern student of
geology the old notion of a "terra firma" is outworn. The idea of
an unshakable, immovable earth could never have emanated from the
inhabitants of an earthquake country. In general we may recognize
two types of crustal movements--slow and sudden. To most people the
sudden movements accompanied by earthquakes are more significant and
impressive because they are more localized and evident, and often
accompanied by destruction of property, or quick, though minor, changes
in the landscape. But movements which take place slowly and quietly are
often of far greater significance in the interpretation of the profound
physical changes which have affected the earth during its millions of
years of known history.

[Illustration: Fig. 7.--Structure section across the Hudson River
Valley near West Point, New York. The shafts and tunnel, 1,200 feet
below sea level, in solid rock, show the position of the New York City
aqueduct from the Catskills. The Preglacial valley has been submerged
and filled with Postglacial sediment to a depth of nearly 800 feet.
(Redrawn by the author after Berkey, from New York State Museum
Bulletin.)]

A few well-known examples will serve to prove that upward, downward,
and differential movements of the earth's crust have actually taken
place not only in the remote ages of geologic time, but also that
such movements have geologically recently taken place, and that
similar movements are still going on. It is very important that the
reader thoroughly appreciate the fact that crustal disturbances,
often profound ones, do take place, because this is one of the most
fundamental tenets of geologic science. Let us consider the case of the
Hudson-Champlain-St. Lawrence Valley region. That the whole region was
once notably higher (at least 1,000 feet) than at present is proved by
the drowned character of the Hudson Valley, in which tidewater extends
northward for 150 miles to near Troy. Where the New York City Aqueduct
passes under the Hudson River near Newburgh, the bedrock bottom of the
old river channel is now about 800 feet below sea level as determined
by drilling. This old channel is there filled up nearly to sea level
with glacial and postglacial rock débris, which shows that the old
channel must have been cut before the oncoming of the ice of the great
Ice Age. Before the Ice Age, then, the lower Hudson Valley must have
been considerably more than 800 feet higher than at present, because it
then contained a river with sufficient current to be an active agent
of erosion, carving out the canyonlike valley in the vicinity of West
Point. This conclusion is strongly reenforced by the fact that the old
valley of the Hudson River has been definitely traced as a distinct
trench across the shallow sea bottom for about 100 miles eastward from
the entrance to New York harbor. Toward the eastern end of this trench
the depth of water is now considerably over 1,000 feet, and thus it is
obvious that, preceding the Ice Age, the earth's crust in the vicinity
of New York City must have been much higher than at present, so that
the Hudson River was able to erode its now completely drowned channel.
Somewhat similar evidence has also established the fact that the lower
St. Lawrence Valley region was much higher before the Ice Age. It is
evident, therefore, that the general Hudson-St. Lawrence Valley region
is now notably lower with reference to sea level than it was before the
Ice Age. That this was caused by actual sinking of the earth's crust
rather than by a rise of sea level is proved by the fact that similar
changes of level between land and sea did not take place at the same
time even along the Atlantic and Gulf coast of our Southern States.

We shall now proceed to the next step in the geologically recent
history of earth-crust movements in the Hudson-Champlain-St. Lawrence
Valley region by asserting that, since the Ice Age, the land was
actually notably lower than at present. In fact, the land was enough
lower to allow tidewater to extend up the St. Lawrence Valley into
the Ontario basin, and all through the Champlain-Hudson Valley. Many
beaches, bars, and delta deposits formed in these arms of the sea are
still plainly preserved, in some cases with shells and bones of marine
animals in them, now hundreds of feet above sea level. These marine
deposits are highest above sea level in the northern portion of the
Champlain Valley, where they lie at an altitude of 700 feet or more and
their altitude steadily diminishes southward to about 300 to 400 feet
in the general vicinity of Albany, and to near sea level in the general
vicinity of New York City. Obviously, then, the land stood lower during
part or all of the interval of not more than a few tens of thousands
of years since the Ice Age than at present. This leads us to the third
important conclusion regarding earth movements in this region, namely,
that still later the land has undergone a differential uplift, the rate
having steadily increased toward the north where the total uplift is
many hundreds of feet. We have discussed this region somewhat in detail
because the principles of slow up and down movements of the earth's
crust are there so plainly recorded.

Among many other regions where earth movements similar to those above
described have taken place, brief mention may be made of Norway. The
great fjords of Norway were, just before the Ice Age, stream-cut
valleys which were then more or less modified by glacial erosion,
and after the Ice Age the rivers in them were drowned due to land
subsidence. The kind of evidence is like that above given for the lower
Hudson River. Since the subsidence there has been partial reelevation,
as proved by the fact that along the sides of the larger fjords marine
terraces and beaches may be traced with gradually increasing altitude
for many miles (150 or more) back into the country where they are
hundreds of feet above tidewater.

Scandinavia is of still further special interest because very
appreciable earth movements have there come under human observation.
Marks carefully placed along the shores of Sweden by the government
have proved that during the last 150 years the southern end of the
country has actually subsided several feet, while from Stockholm north
the land has risen in increasing amount, reaching a maximum of seven or
eight feet. In southern Sweden, at Malmo, a certain street now at times
becomes covered by wind-driven high water, and during excavations made
some years ago an older street eight feet below the present one was
found.

A theory which appears to be in perfect harmony with the facts to
account for the subsidence and partial reelevation of central eastern
North America and Scandinavia since the beginning of the Ice Age is
that the great weight of ice during the Ice Age pressed the land down,
and that since the removal of the ice there has been an appreciable
tendency for the land to spring back.

Certain crustal movements which have occurred about the Bay of Naples
are of very special interest because actual human history dates can be
placed upon them. Most remarkable are the records in connection with
the temple of Jupiter Serapis which was built near the shore before
the Christian era. The land sank about five feet and a new pavement
had to be constructed; then, by the middle of the third century A. D.,
the temple rose to well above sea level. By about the ninth century
the land had subsided fully thirty feet, so that marble columns of
the temple were bored full of holes as high as twenty-one feet above
their bases by marine-shelled animals, species of which still live in
the bay. Then a slow uplift of twenty-three feet began, bringing the
bases of the columns two feet above sea level by 1749. Since that time
a slight sinking has taken place and this seems to be still going on.
Three of the marble columns with the borings still stand in upright
position.

While the movements just described were taking place, the island of
Capri, twenty miles across the Bay of Naples, has slowly sunk to an
amount estimated at thirty or forty feet as proved by evidence from the
famous Blue Grotto. About the beginning of the Christian era a large
ancient wave-cut cave, part of which is now called the Blue Grotto, had
its floor above sea level, and it was used by certain Romans as a cool
place to retire to from the heat. In order to obtain better light an
opening was cut through its upper portion. The land has sunk so much
that at the present time even part of the artificial opening (through
which tourists pass) is now under water.

By way of illustrating remarkable contrasts in direction of crustal
movements on very considerable scales in a given region, we shall
briefly mention some facts regarding part of the coast of southern
California and the neighboring islands of Santa Catalina and San
Clemente, respectively twenty-five and fifty miles offshore. Those
movements were not, however, checked up by human history records.
The mainland at San Pedro has clearly risen 1,240 feet, as proved by
the presence of unusually perfect coast terraces (so-called "raised
beaches"), while San Clemente has risen 1,500 feet as proved by the
raised beaches into which deep, youthful V-shaped stream-cut valleys
have been sunk, and a shore line characteristic of recent notable
uplift. It is a remarkable fact that at the same time the intervening
island (Santa Catalina) has notably sunk, as proved by the nature
of its shore line, and the distinctly more mature character of its
topography.

We are, however, by no means dependent upon lands along sea shores
for evidences of slow rising and sinking of land. Thus, by careful
measurements it has been shown that the general region of the Great
Lakes is now differentially rising toward the northeast at the rate
of about five inches per 100 miles per century. At Chicago the rise
of water is estimated at about nine inches per century, which means
increase of flowage through the Chicago Canal. At this rate the upper
lakes would, in some thousands of years, drain through this canal to
the Mississippi. A well-preserved shore line of the large ancestor
of Lake Ontario shows a steady increase in altitude at the rate of
several feet per mile toward the northeast from near Niagara to the St.
Lawrence Valley, thus proving a tilting of the land since the shore
line was formed.

Shore lines of the great ancestor of Great Salt Lake also show warping
of the earth's crust, some parts of a definite shore line being several
hundred feet higher than others.

Very significant evidence pointing to profound crustal movements
consist in the finding of fossil remains of marine animals in the
strata high above sea level, very commonly from one to three miles, in
many parts of the world, especially in the high mountains. In Wyoming,
nearly horizontal strata of the Mesozoic Age carrying marine fossils
lie two miles or more above sea level. The fact that given formations,
carrying marine fossils representing certain definite portions of
geologic time, are found at various altitudes up to several miles in
many parts of the world, shows that the land in those places has
really risen relative to sea level.

It should not be presumed from the above discussion that the sea level
itself has never changed. Thus, the vast areas of thick ice sheets in
both North America and Europe during the Great Ice Age represented
sufficient water withdrawn from the sea to very appreciably lower its
level. All land-derived materials, carried into the sea mainly by
rivers, displace sea water, with consequent rise of its level. If all
existing lands were worn down and carried into the sea, its level would
be raised some hundreds of feet. Subsidence of any part of the ocean
bottom would cause a lowering of sea level. There is a strong reason
to believe that some such shiftings of sea level have occurred during
the vast lapse of geologic time. During certain periods erosion of the
land predominated, and during other periods building up of the land
predominated, as pointed out in the chapters on geologic history. It is
not thought that shifting of sea level has ever amounted to more than a
few hundred feet, at least not during the millions of years of the more
clearly recorded earth history.

We have thus far considered slow upward and downward movements of the
earth's crust without notable structural changes in the rocks. Another
type of crustal disturbance causes more or less profound changes in
the structures of the rocks themselves. Just how the earth originated
is a matter of uncertainty, but we can be sure that for many millions
of years it has been a shrinking body. The outer, or crustal, portion
of the earth, in adjusting itself to the contracting interior, has had
many pressures, stresses, and strains set up within it. As results of
such forces the rocks at and near the earth's surface have in various
places, and at various times, been broken (faulted) and subjected to
sudden movements (see discussion beyond), while those well within the
crustal portion, that is to say a few miles or more down, have, in
many cases, been bent (folded), or even crumpled. For these reasons
the surface and near-surface crustal portions are called the "zone of
fracture," while the more deeply buried portions comprise the "zone
of flowage." In the zone of flowage the rocks, where subjected to
great lateral pressure, act like plastic materials and therefore bend
rather than break, because of the great weight of overlying materials.
Laboratory experiments have confirmed the findings of geologists in
this regard. Small masses of rocks properly inclosed in nickel-steel
cylinders have been subjected to slow differential pressures equivalent
to those which obtain twenty to forty miles within the earth. Under
such conditions rocks have been made to change shape very notably
without fracturing. Both geological observations and experiments have
led us to conclude that not even small fractures or crevices can remain
open at a depth greater than ten or twelve miles even in the hardest
rocks.

From time to time, during the long history of the earth, forces of
lateral pressure have been slowly exerted along more or less localized
zones or belts within the earth's crust, and the rocks have been
deformed chiefly by bending or folding, especially in those regions
where mountains of the folded type have developed. Movements of this
type are considered beyond in the chapter on mountains. Rock folds vary
in size from microscopic to miles across, and they exhibit many shapes.
Plate 7 will give the reader a good idea of actual rock folds of
common sizes and shapes in various places. Folded structures are most
clearly discernible in sedimentary rocks, because of their stratified
(layered) arrangement. Since folds in hard rocks rarely, if ever,
develop except at a depth of some miles within the earth, they show at
the surface only where great thicknesses of overlying materials have
been stripped off by erosion.

[Illustration: Fig. 8.--An outcrop of stratified crystalline limestone
(or marble) exhibiting two small sharp folds--a syncline on the
left and an anticline on the right--near Lenox, Mass, These folds
developed during the great mountain-making disturbance at the end of
the Ordovician period fully 20,000,000 years ago. (After Dale, U. S.
Geological Survey.)]

From the standpoint of our consideration of slow earth-crust movements,
it is important to bear in mind that lateral pressure in the zone of
flowage has not only notably deformed rocks, but that, as a result
of the buckling forces, given rock masses have, in many cases, been
notably shifted downward or upward--mainly upward--from their original
positions. Folded strata carrying shells of sea animals are commonly
found thousands of feet above sea level in many of the great mountain
ranges of the world. During the process of folding on a large scale the
crust of the earth is very appreciably shortened at right angles to
the direction of applied pressure, due to squeezing or bending of the
strata. In the case of the Appalachian mountains of Pennsylvania it has
been estimated that such shortening amounts to about twenty-six miles
or, in other words, that the strata originally spread out horizontally
across an area whose width was about 100 miles have been squeezed or
folded into an area whose width is twenty-six miles less.

[Illustration: Fig. 9.--Structure section showing the profile of the
mountains and relations of rocks below the surface near Livingston,
Montana. The strata were crowded together until they bent into great
sharply defined folds at the time of the Rocky Mountain Revolution
several million years ago. Then the rocks broke along the fault
fracture and the mass on the right was shoved over upon the mass on the
left. (After U. S. Geological Survey.)]

We shall now turn to a consideration of sudden earth movements and some
of their effects, including earthquakes. Mention has already been made
of the fact that, when pressures and strains are set up in the outer
portion ("zone of fracture") of the earth's crust, the rocks yield
mainly by breaking or fracturing because the rocks not being under a
great load of overlying material are there brittle. The earth's crust
has been fractured on small and large scales in many places during
the long space of geologic time. Where one block of earth's crust
has slipped or moved past another along a fracture we have what is
called a "fault." Such displacements of rock masses vary in amount
from less than an inch to some miles, and they constitute one of the
most important features of the architecture of the outer portion of
the earth. There are two types of faults fundamentally different as
to cause. In one type (so-called "normal fault") the rocks suddenly
yield to a force of tension; a fracture develops and the earth block on
one side of the fracture or fault drops with reference to that on the
other. In the other type (so-called "thrust faults") the rocks yield
suddenly to a force of compression or lateral thrust, and one block
of earth is pushed or thrust partly over another along the surface of
fracture or fault. (See Plate 8.)

Faults range in length up to hundreds of miles, those from one to
twenty miles in length being very common. Where an earth block has been
displaced thousands of feet along a fault surface, it is not to be
understood that the whole displacement resulted from a single movement,
but rather from a series of sudden movements separated by greater or
less intervals of time. Each sudden movement along a fault surface
produces a vibration of the earth near by. Many such sudden movements
are known to have caused violent earthquakes. Displacements of twenty
to fifty feet, as a result of single movements, are definitely known
to have taken place in various regions during the last fifty years;
and rarely, if ever, has any sudden displacement of as much as several
hundred feet occurred. Cliffs and steep slopes very commonly result
from faulting, but, because of the long lapse of time required for the
repeated movements in the case of great faults, the cliffs or steep
slopes begin to wear back and become more or less subdued long before
the last of the movements take place. In regions where movements along
great faults have long since ceased, the original steep slopes may be
completely obliterated by erosion.

[Illustration: Fig. 10.--Vertical sections through strata illustrating
common kinds of faults: a, "normal faults" where one mass simply sinks
below another; b, a "thrust fault" where one mass is shoved over
another. (After U. S. Geological Survey.)]

How does the geologist determine the actual amount of displacement,
especially in the case of a large fault in stratified rocks? First, the
various formations of the region, where unaffected by faulting, are
carefully studied, especially in regard to the character and thickness
of each, and their relative geologic ages or normal order as they
were deposited one layer above the other. Then, in the simple case of
a normal-fault surface at right angles to horizontal strata, it is
only necessary to find out what two formations or parts of formations
come together along the fault fracture, and the actual amount of
displacement is readily determined. Where strata and normal fault
surfaces lie at various angles, and also in thrust faults, those angles
must be determined in addition to the data above named. In many mining
regions, where valuable deposits are affected by faulting, accurate
knowledge of the direction and amount of displacements of faults is of
great economic importance.

A few examples of normal faults from well-known districts will now be
briefly described. The whole eastern front of the central and southern
Sierra Nevada Range of California is a great, steep fault slope, from
a few thousand to ten thousand or more feet high and hundreds of miles
long, of such recent geologic age that it has been only moderately
affected by erosion. In fact, it is well known that the southern
two-thirds of the range is a great tilted fault block, the total
displacement having resulted from repeated sudden movements since about
the middle of the present geologic era. A great fault also extends
along the eastern base of the great Wasatch Range of Utah and the steep
slope thousands of feet high is a fault scarp only slightly modified
by erosion. Renewed movements along this profound fault have very
recently taken place as proved by the presence of fresh fault scarps
in loose deposits which have accumulated across the mouths of some of
the canyons, as, for example, near Ogden. In fact, practically all of
the north-south ranges of the Great Basin from Utah to California are
essentially a series of tilted fault blocks. Another great fault, less
conspicuous from the topographic standpoint, is hundreds of miles long
in the Coast Range Mountains of California. At the time of the San
Francisco earthquake of 1906 there was a renewed sudden movement along
this great fracture. The eastern one-half of the Adirondack Mountains
of New York is literally a mosaic of hundreds of fault blocks. Many
of these faults are from two to thirty miles long and they commonly
show displacements of from a few hundred to 2,000 or more feet. A
glance at the geological map (in colors) of the vicinity of the great
copper mines at Bisbee, Arizona, shows most of that region to contain
a network of normal faults which separate it into a mosaic of fault
blocks. In each of the examples of faults just given a block of earth
has sunk relative to the other, or in other words, each is a "normal
fault."

We shall now turn to some large scale cases of faults in which great
masses of earth have been pushed one over another--so-called "thrust
faults." In the southern Appalachian Range, and especially well
exhibited in the vicinity of Rome, Georgia, one portion of the mountain
mass has literally been shoved over another, at a low angle over a
fault surface many miles long, for fully seven miles westward. Both the
tremendous weight of rock material actually translated and the number
of sudden movements required in the operation stagger the imagination.
It is safe to say that during the long time of this great operation
violent earthquakes were not uncommon. In the Rocky Mountains of the
northern United States and southern Canada there is the greatest known
thrust fault on the continent. It is hundreds of miles long, and the
actual displacement is commonly at least several miles. In the Glacier
National Park of Montana it has been established that the front range
portion of the Rockies has actually been pushed at least seven miles,
and possibly as much as twenty miles, eastward over a fault surface,
and out upon the Great Plains. In some cases rocks of the Prepaleozoic
Age have there been pushed upon rocks of the late Mesozoic Age, thus
locally upsetting the geologic column.

[Illustration: Fig. 11.--East-west profile and vertical structure
sections fifty-two miles long in the Mohawk Valley region of New York,
showing numerous tilted fault blocks which notably influence the
topography. Vertical scale exaggerated. The rocks are Prepaleozoic and
early Paleozoic in age. (Modified by the author after Darton, New York
State Museum.)]

The Wasatch Range of Utah, in addition to the great normal fault along
its western base, contains a remarkable system of thrust faults. In
the region now occupied by the Wasatch Mountains a number of parallel
(thrust) faults were developed close together and the broken pieces of
the earth's crust between them were pushed up, the rocks on one side
of each crack riding up over those on the other side until a great
mountain range was formed where once lay a plain. In the Ogden Canyon
one great earth block of Prepaleozoic (Algonkian) Age has been shoved
thousands of feet over late Paleozoic (Carboniferous) rock, which
latter has in turn been thrust over early Paleozoic (Cambrian) rock.
This thrust faulting was accomplished before the development of the
geologically recent normal fault along the western base of the range.

[Illustration: Fig. 12.--Vertical (structure) section through a part
of the earth's crust several miles long in Ogden Canyon, Utah, showing
the system of great thrust faults. Prepaleozoic (Algonkian) rocks have
been pushed far over upon late Paleozoic (Carboniferous) strata, which
latter have in turn been shoved over early Paleozoic (Cambrian) strata,
etc. (After U. S. Geological Survey.)]

Any sudden movement of part of the crust of the earth, due to a natural
cause, produces a trembling or shaking called an earthquake. Though
earthquakes are generally classed among the most terrifying of all
natural phenomena, those which have occurred during human historic
times have had scarcely any geological or topographical effects of
real consequence on the face of the earth. Locally, the effects may
be notable and the destruction of life and property may be great. The
earth may be locally cracked and rent, small fault scarps may develop,
landslides and avalanches may result from the shaking of the earth,
buildings may be demolished, and sea waves may be rolled upon the
land. On the other hand, many earthquakes, called "tremors," are too
slight to be noticed by people, though they are recorded by specially
constructed instruments called "seismographs." We have already stated
that actual sudden displacements causing earthquakes have amounted to
twenty or even fifty feet right along fault fractures, but during the
vibrations or quakings, which are often so destructively sent out into
the neighboring country, the earth's surface rarely actually moves
more than a small fraction of an inch. Because of the suddenness of
the movement objects on the surface may be moved inches or even feet.
Violent shocks may last one or two minutes and cause the whole earth
to tremble, though at distant points only seismographs record the
movement. It is probably true that some part of the earth is shaking
all the time.

Studies during the last fifty years have made it certain that the main
cause of earthquakes is the sudden slipping of earth blocks past each
other along fault fractures, the sudden slipping furnishing the impulse
which sends out the vibrations into the surrounding more or less
elastic crust of the earth. The low rumbling to roaring sound, which
sometimes immediately precedes an earthquake, is probably due to the
grinding of the rocks together below the surface.

Earthquakes generally accompany volcanic outbursts of the violent
or explosive type, and in such cases subterranean explosions cause
both the eruptions and the quakings of the earth. It is well known
that the principal volcanic districts or belts of the earth are also
the belts of most frequent earthquakes, but this does not mean that
volcanic action causes most of the earthquakes. Active volcanoes and
earthquakes are so commonly associated in the same belts because those
belts no doubt represent portions of the crust which are now most
actively yielding to the forces directly resulting from the shrinkage
of the earth. Within the volcanic belts many earthquakes take place
unaccompanied by any volcanic action, and many others take place
far from volcanoes. Some earthquakes have been caused by the impact
of great landslides or avalanches, or by the sudden caving in of
underground openings.

Brief descriptions of a few typical carefully studied earthquakes
during recent years will serve to make the main features of earthquakes
still clearer to the reader.

The violent Japanese earthquake of 1891 was caused by the sinking of
a block of earth forty miles long from two to thirty feet below that
on the other side of a fault fracture. There was also considerable
horizontal shifting, and cracks developed in the adjacent region. A
distinct fault scarp, fifteen to twenty feet high, developed, and in
some cases extended right across cultivated fields.

[Illustration: Fig. 13.--Map of the United States, showing the large
areas over which three of the greatest of our earthquakes were actually
felt by people. These earthquakes were recorded in many parts of the
world by delicate instruments: New Madrid, 1811; Charleston, 1886; San
Francisco, 1906.]

[Illustration: Fig. 14.--Sketch map showing the trace of the great
fault fracture along which a renewed sudden movement of as much as
twenty feet took place to cause the San Francisco earthquake of 1906.
(After U. S. Geological Survey.)]

The San Francisco earthquake of 1906 was produced by renewed movement
along the great fault which extends lengthwise through the Coast Range
Mountains for several hundred miles. It is literally correct to say
that, for 250 miles along this great earth fracture, one part of the
Coast Range instantaneously slipped from two to twenty-two feet past
the other. More or less of the movement extended at least several
thousand feet down into the earth. In this case both sides slipped and
the movement was very largely horizontal rather than vertical. The
land on the east side of the fault moved south and that on the west
side moved north, the amount diminishing away from the fault on each
side so that some miles out the actual crustal movement was only a few
inches. When one thinks of the tremendous volumes of earth material
involved in this shifting of the earth's crust, is it any wonder
that such destructive earthquake waves were produced? Many buildings
were wrecked, several hundred people were killed, the disastrous San
Francisco fire resulted, water mains were broken, and fences and roads
crossed by the fault were dislocated as much as fifteen to twenty feet.

[Illustration: Plate 5.--Swift Current Valley in Glacier National Park,
Montana. This was once a deep V-shaped canyon carved out (eroded)
by stream action. Then a great valley glacier slowly plowed its way
through it during the Ice Age and, by ice erosion, the present nearly
straight U-shaped canyon has resulted. (_Photo by Campbell, U. S.
Geological Survey._)]

[Illustration: Plate 6.--View in the Yosemite Valley from Near the
Western Entrance. The great rock called "El Capitan," on the left rises
3,500 feet above the river, and Bridal Veil Falls on the right is 620
feet high. All the rock is granite, the nearly vertical walls of which
have resulted from the action of a great glacier which plowed its way
through the valley during the Ice Age; the valley walls have been cut
back by the removal of large vertical joint blocks. The flat bottom of
the valley has resulted from the filling with sediment of a postglacial
lake in the valley. (_Photo by F. N. Kneeland, Northampton, Mass._)]

During the great earthquake on the coast of Alaska in 1899 notable
changes took place along the shore for some miles, one portion having
suddenly risen as much as forty-seven feet, while another portion sank
below sea level.

[Illustration: Fig. 15.--Map showing the principal earthquake regions
of the world.]

In 1886 the earthquake centering near Charleston, S. C., was preceded
by rumbling and roaring noises and the slight quaking increased to
violent shaking which lasted more than a minute. Eight minutes later
a rather violent earthquake shock took place, followed during the
next ten or twelve hours by less severe shocks. Most buildings in the
city were wrecked or more or less badly damaged, and some people
were killed. The shocks were so violent that the quaking was actually
felt by people over an area of more than 2,000,000 square miles, the
disturbance having spread at the rate of about 150 miles per minute.
Near Charleston openings and fissures were formed through which sand
and muddy water were ejected, but the cause of the disturbance was most
likely slipping of the old very hard rocks below the loose deposits of
the Coastal Plain.

From 1811 to 1813 a series of violent earthquakes developed in the
general vicinity of New Madrid, Missouri. In an area of over 2,000
square miles, now called the "sunk country," many portions suddenly
sank giving rise to small fault scarps or cliffs, and various lake
basins were formed. Development of a fissure caused a local change in
the course of the Mississippi River.

In 1897, Assam, India, was shaken by an earthquake of unusual
magnitude, which lasted 2-1/2 minutes. An area of 150,000 square miles
was disastrously shaken, and the shocks were distinctly felt over
an area of 750,000 square miles. A number of notable fault scarps
developed, the movement on one having been thirty-five feet.



CHAPTER VIII

VOLCANOES AND IGNEOUS ROCKS


Not only because of the great power and terrifying grandeur of violent
eruptions, but also because of their destruction of life and property,
volcanoes stand out in the popular mind as among the most real and
important of all geological phenomena. But great volcanic outbursts,
like violent earthquakes, are in truth only outward, sensible,
relatively minor manifestations of the tremendous earth-changing forces
below the surface. They are far less important as geological agencies
than the mighty interior forces which cause parts of continents to
be slowly upraised and the rocks folded, or even than the incessant
action of streams whereby the lands are cut down. Even as an igneous
agency, volcanoes are notably less important than the development and
shifting of molten materials within the earth's crust. Volcanic action
is, however, not only conspicuous, but it is also of real significance
as a means of changing the earth, such action having taken place since
very early known geologic time. After bringing out the main facts and
principles of volcanoes, aided by descriptions of specific eruptions,
we shall turn to a consideration of igneous activity within the earth's
crust.

Mount Vesuvius in Italy is perhaps the most famous active volcano in
the world. Its eruptions have been more or less carefully studied for
a longer time than any other. The eruption in the year 79 A. D. was
really a tremendous explosion causing a large part of the old crater to
be blown away, and sending immense volumes of rock fragments, mostly
finely divided (so-called "ashes") into the air which completely buried
the small city of Pompeii. Water from the great clouds of condensing
steam, mixed with "ashes," formed muddy floods which overwhelmed
Herculaneum. Little or no lava was erupted. Since that time the crater
has been more or less active and the present cone, 4,000 feet high, has
been built up. During the last fifty years the greatest eruptions took
place in 1872 and 1906, when, streams of molten rock flowed down the
sides of the mountain.

[Illustration: Fig. 16.--Map showing the distribution of active and
recently active volcanoes of the world.]

One of the greatest volcanic explosions ever recorded was that of the
island of Krakatoa, between Sumatra and Java, in 1883. The greater part
of the island was blown away and there was water 1,000 feet deep,
where just before the island stood hundreds of feet high. About a cubic
mile of rock material was sent into the air mostly in the form of fine
dust--some of it for seventeen miles--and completely hid the sun,
causing total darkness during the eruption. Dust fell over an area of
several hundred thousand square miles. Several days after the explosion
ships more than 1,000 miles away were dust covered. Such enormous
quantities of a light porous lava called "pumice" fell and floated
upon the sea that navigation was badly obstructed many miles from the
volcano. Extremely fine dust gradually spread through the whole earth's
atmosphere, causing the extraordinary red sunsets for several months.
The sound of the explosion was heard for hundreds of miles. Great
sea waves 50 to 100 feet high were stirred up and they swept inland
for several miles over the low-lying coast lands of neighboring Java
and Sumatra, overwhelming hundreds of villages and drowning tens of
thousands of people.

[Illustration: Fig. 17.--The great hole left after the top of Mt.
Katmai in southern Alaska was blown off in 1912 by one of the most
tremendous volcanic explosions in the annals of human history.
The water in the lake is hot. (After Griggs, National Geographic
Magazine.)]

One of the greatest explosions on record was that of Katmai volcano,
several thousand feet high, on the coast of Alaska, in June, 1912.
Not only was the top of the mountain completely blown off, but also a
great crater pit, three miles wide across the top and several thousand
feet deep, was developed in the stump of the former mountain. Volcanic
dust fell to a depth of several feet within twenty-five to fifty miles
of the mountain. Dust accumulated to a depth of nearly a foot in the
village of Kodiak, 100 miles east of the mountain, where total darkness
prevailed for more than two days. A lake of very hot water now occupies
the bottom of the great new crater. The noise of the explosion was
heard for at least 750 miles.

[Illustration: Fig. 18.--Diagrammatic vertical or structure section
through a portion of the earth illustrating the common modes of
occurrence of igneous rocks. P, deep-seated (plutonic) igneous rock; S,
strata; D, dikes; M, mass of igneous rock forced between strata bending
them upward; F, feeding channel of volcano; V, volcano; L, lava sheets.
(By the author.)]

One of the most frightful volcanic catastrophes of recent years was the
eruption of Mont Pelée, island of Martinique, West Indies, in 1902.
In this case, also, no lava was poured out, but violent explosions
sent great clouds of very highly heated gases and vapors, mingled with
incandescent dust, thousands of feet into the air. One of these great
clouds rushed down the mountain at hurricane speed and destroyed the
city of St. Pierre with its 30,000 inhabitants. After the main eruption
a spine or core of hard rock began to rise out of the crater and it
slowly grew to a height of 1,000 feet in several months, after which it
began to crumble away. This spine probably represented nearly frozen
lava which solidified as it was gradually forced out of the mountain.

Of special interest to us, though not of great importance is the only
active volcano in the United States. In May, 1914, Mount Lassen (or
Lassen Peak), a long inactive volcano in northern California, suddenly
burst forth explosively and during the next several years hundreds of
eruptions occurred. Little or no lava appeared, but great clouds of
steam and dust often shot into the air from one to three miles above
the top of the mountain, which lies over 10,000 feet above sea level.
(Plate 10.) Great quantities of dust have accumulated for miles around
the mountain. At this writing (October, 1920) the mountain is again
active.

It should not be presumed, however, that all, or nearly all, volcanoes
are of the explosive type. Others of the more quiet type are well
exemplified by the two great Hawaiian volcanoes, Mauna Loa and
Kilauea. Any but relatively very minor explosions rarely, if ever,
occur, the product of such volcanoes being almost wholly lava, which
flows down the mountainsides in molten streams. The Hawaiian Islands
have, in fact, been almost entirely built up by successive eruptions
of lava, the building-up process having begun well below sea level.
Mauna Loa rises to nearly 14,000 feet above the sea, but, due to the
fact that the streams of lava have spread so far, the mountain has an
exceptionally low angle of slope which makes it difficult to realize
that it is so high. Considering its submarine portion, Mauna Loa
really rises nearly 30,000 feet above the sea floor. Although Kilauea
lies nearly 4,000 feet above sea level on the flank of Mauna Loa, and
only twenty miles distant from it, the two volcanoes are singularly
independent in regard to their eruptions. Each mountain has a crater
irregularly oval in shape, nearly three miles long, bounded by almost
vertical walls of hard lava, in some cases arranged in terraces. The
floors of the great crater pits are relatively level, and consist of
black lava in which are lakes of molten and even boiling lava. The
black lava floor is, in each case, only a frozen or hardened crust upon
a great column of molten lava extending down into the mountain. Prior
to an eruption of Mauna Loa the lava column rises hundreds of feet in
the crater, but during recent years the lava seldom, if ever, flows out
over the crater rim. Instead, it breaks through the mountainsides at
various altitudes, the great flow of 1919 having started at an altitude
of about 8,000 feet. This stream of liquid rock, fully one-half of a
mile wide, flowed for weeks down the mountainside and into the ocean,
the waters of which, in contact with the highly heated lava, were
thrown into terrific commotion. In 1885 a stream of lava several miles
wide flowed forty-five miles. In one case, lava traveled the first
fifteen miles in two hours, but this is an unusually great rate of
speed. Lava streams in general seldom move faster than one or two miles
per hour, and as the liquid rock gradually cools and becomes more and
more viscous, the speed diminishes to zero. Almost incredible volumes
of steam emanate from streams of molten lava.

In 1840 an outflow of lava took place from the side of Kilauea Mountain
and ran into the sea. Since that time the floor of the great crater
pit (quoting Professor W. H. Hobbs) "has been essentially a movable
platform of frozen lava of unknown and doubtless variable thickness
which has risen and descended (hundreds of feet) like the floor of an
elevator car between its guiding ways. The floor has, however, never
been complete, for one or more open lakes are always to be seen, that
of Halemaumau, located near the southwestern margin, having been
much the most persistent. Within the open lakes the boiling lava is
apparently white hot at a depth of but a few inches below the surface,
and in the overturnings of the mass these hotter portions are brought
to the surface and appear as white streaks marking the redder surface
portions. From time to time the surface freezes over, the cracks open
and erupt at favored points along the fissures, sending up jets and
fountains of lava, the material of which falls in pasty fragments
that build up driblet cones. Small fluid clots are shot out, carrying
threadlike lines of lava glass behind them, the well-known 'Pelée's
hair.' Sometimes the open lakes build up congealed walls, rising above
the general level of the pit, and from their rim the lava spills over
in cascades to spread out upon the frozen floor."

In some regions, like the Columbian Plateau of the northwestern United
States and the Deccan of India, each covering about 200,000 square
miles, vast quantities of lava have been poured out layer upon layer to
depths of even thousands of feet. Distinct volcanic cones or mountains
in those regions are either absent or too scarce to look to as sources
of so much lava. Such lava floods were probably mostly erupted from
great fissures in the earth's crust, the fluidity to spread many miles.

Some idea of the quantitative geological importance of volcanism may be
conveyed to the reader when we assert that, according to a conservative
estimate, fully one-half of a million cubic miles of molten rocks have
been poured out upon the surface of the earth through volcanic action
in relatively recent geological time! The Cascade Range with its lofty
peaks, including Mount Shasta and Mount Rainier, each rising more than
14,000 feet above the sea, has been built up very largely by volcanic
action during the last era of geologic time. Many other mountain peaks
and various ranges have been similarly developed either wholly or in
part. The great chain of Aleutian Islands extending hundreds of miles
into the sea, is the scene of much volcanic activity where a great
mountain range is now literally being born out of the sea by the
processes of vulcanism.

Before this the reader has more than likely wondered about the source
of the heat, vapors (mainly water), and power involved in volcanic
action. Answers to these questions are closely tied up with the precise
cause (or causes) of volcanic action which remains one of the most
uncertain of the larger problems of geologic science. Before briefly
discussing the causes, a few additional facts should be stated. First,
in regard to the heat, a careful determination of the temperature
of the molten lava of Kilauea in 1911 showed it to be 1,260 degrees
Centigrade, or 2,300 degrees F. This is, however, a relatively low
temperature, because many lavas from other regions show melting points
all the way up to at least 2,000 degrees Centigrade (3,600 degrees
F.). Water in the form of steam is quantitatively one of the greatest
products of volcanoes. A fair idea of the tremendous volumes of water
involved may be gained from the statement that a careful estimate shows
that fully 460,000,000 gallons of water in the form of steam erupted
from a single secondary cone of Mount Etna during a period of 100
days. Among other gases which are given off in greater or less amounts
during volcanic activity are carbonic acid gas, sulphureted hydrogen,
sulphur dioxide, and hydrochloric acid. Some idea of the power back of
volcanoes may be gained not only from the tremendous explosions such
as those above described, but also from the fact that the pressure
necessary to raise the column of lava from sea level to the top of
Mauna Loa (nearly 14,000 feet) is about 1,150 atmospheres, or about
17,000 pounds per square inch. The actual pressure must there be much
greater because the lava is forced up from far below sea level.

A long-held idea that a relatively thin crust covers a molten interior,
and that downward pressure of this crust due to earth contraction
causes molten rocks to be forced out, has been too thoroughly disproved
to now be at all seriously entertained. The fact that near-by volcanoes
commonly erupt entirely independently, as in the case of Mauna Loa
and Kilauea, shows that there can be no universal liquid beneath a
relatively thin crust. Other arguments against liquidity of the earth's
interior are that the earth acts like a body nearly as rigid as steel
against the powerful tide-producing forces, and that earthquake waves
which pass through the earth to a depth of at least 2,000 miles are the
kind which require a solid medium for transmission.

Let us then briefly consider more plausible views regarding the cause
of volcanic action. First of all we may be sure that the earth is
highly heated inside. Measurements in many deep borings show that the
temperature increases at the rate of about 1 degree F. for each 50 to
60 feet downward, to depths greater than a mile. Accordingly, on the
basis of 1 degree rise in 50 feet, at depths of 20 to 35 miles, the
temperature must be great enough (2,120 degrees to 3,590 degrees F.),
to cause all ordinary rocks to melt _if they were at the surface_. At
such depths, however, the downward pressure upon the rocks is so great
that their melting points are notably raised, and there is every reason
to believe that under ordinary conditions the rocks 20 to 35 miles down
are not molten. If we adhere to the older (nebular) hypothesis of earth
origin, the interior heat of the earth is left over from the cooling,
once molten, earth. On the basis of another (planetesimal) hypothesis,
the earth's heat is due to the steady, powerful action of gravity
causing the earth to contract. In any case, the earth is hot inside as
proved by deep well records and igneous phenomena in general, and it is
a contracting or shrinking body as proved by the many large scale zones
of wrinkling or folding of rocks. If, then, highly heated solid rocks
at reasonable distances down in any part of the earth are subjected
to relief of pressure by an earth movement such as upward crumpling
of the crust, or by readjustment of large fault blocks, such heated
solid rocks would become molten. The very earth movement which brings
about relief of pressure and melting may very reasonably be regarded
as the power which forces some of the newly formed molten material
higher up into the earth's crust, and even out upon the surface. This
view harmonizes with the well-known fact, already mentioned, that the
main belts of active volcanoes are also the main belts of active earth
movements, such as earthquakes.

Another source of power behind volcanic action is steam pressure. We
have already mentioned the fact that vast amounts of water in the form
of steam escape from volcanoes or even from streams of molten lava. The
violent volcanic explosions are quite certainly all, or nearly all,
direct results of sudden giving way of volcanoes to steam pressure
which accumulates during greater or less periods of time, and with
little or no possibility of escape, without rupturing the mountain.
Steam alone, or combined with some of the other gases so common as
volcanic products, may also aid in forcing out molten rock. What is
the source of the steam and other gases or vapors? According to one
view they were originally in the earth, while according to another view
the water at least has been absorbed by the molten rocks from surface
waters which worked their way downward. At least two arguments oppose
the second hypothesis: first, that not a few volcanoes are really many
miles from the sea or other bodies of water, while downward percolation
of rain water would fall far short of supplying the tremendous
quantities of water ejected, and second, any water taken up by molten
rock must be absorbed within a very few miles of the surface because,
as we have learned, farther down there are no openings large enough to
permit the downward passage of water, but, as a matter of fact, the
very upper part of the earth's crust is just the place where molten
rocks begin to give up their water, often with terrific violence.

We may now turn to a consideration of the other very important kind of
igneous activity, namely, the rise and transfer of molten materials
within the earth's crust, but not to the surface. The quantity of such
deep-seated (so-called "plutonic") igneous rock material which has
been intruded into the earth's crust within known geologic time, is
far greater than that which has been forced to surface, that is the
so-called "volcanic" material. The plutonic rocks are always thoroughly
crystallized, and they are generally coarser grained than the volcanic
rocks.

Where molten materials have been forced into cracks or fissures in the
crust of the earth and there congealed, we have a very common mode of
occurrence called "dikes" (Plate 9). In many regions often one set of
dikes was formed, after which one or more succeeding injections from
the same or different deep-seated bodies of molten rock took place,
and some of the later dikes were forced to cut across earlier ones.
Dikes of all lengths up to at least thirty miles, and of all widths up
to many hundreds of feet, are known, but they are generally less than
a mile long and not more than a few feet or rods wide. They have been
intruded into all kinds of rock formations--igneous, sedimentary, and
metamorphic. Dikes are common in many parts of the world and they often
excite the interest of lay-men. They are wonderfully displayed along
the southern coast of Maine. Plate 9 shows small dikes where the molten
material was forced from a larger mass into a body of older dark
rock. The Palisades of the Hudson River, just north of New York City,
consists of a layer of igneous rock several hundred feet thick which,
in the molten condition, was forced nearly horizontally between layers
of sandstone millions of years ago, that is in the early Mesozoic era.
The palisade or columnar structure was caused by cracking of the rock
during the cooling and contraction. This is the explanation of most
columnar structures of igneous rocks, exceptionally fine exhibitions
being at the Giant's Causeway in Ireland, and Devil's Tower, Wyoming
(Plate 10).

A type of occurrence not so common, but of special interest, is where
a body of molten rock rising in nearly horizontal strata becomes
cooler and therefore stiffer or more viscous and, losing its power to
penetrate, forces its way between the layers causing the strata to be
arched or domed over it. Sufficient removal of overlying material by
erosion has revealed many fine examples of this type of occurrence.

Another type of interest is the volcanic neck, which is the core or
plug filling the feeding channel of a volcano. In certain regions, like
parts of Arizona and New Mexico, extinct volcanic mountains may be all
cut away by erosion, except the central cores or necks which, both
because they are more resistant and are last to be reached by erosion,
stand out conspicuously as great towers on the landscape (Plate 9).

Most important of all from the quantitative standpoint, however, are
the great bodies of igneous rocks, ranging up to many miles across,
which, in a molten condition, were forced irregularly into the earth's
crust from unknown depths.

The common rock called granite belongs in this category of rocks, which
are the best and most extensively developed of all igneous types.
The roots or cores of great mountain ranges often consist of such
rocks which are exposed to view only after removal of great thickness
of overlying material. Immense areas of granite and other plutonic
rocks of extra deep-seated origin are exposed, because of removal of
overlying material by erosion, in southeastern Canada, the Adirondack
Mountains, New England, the Piedmont Plateau of the Atlantic Coast, and
in the Sierra Nevada Mountains. All the rock forming the lofty walls of
Yosemite Valley is granite, which was forced into the earth's crust in
relatively late Mesozoic time, and which has since been laid bare by
erosion.



CHAPTER IX

WATERS WITHIN THE EARTH


It has been estimated that approximately 1,500 cubic miles of water
fall upon the surface of the United States each year. About one-half
of this goes back into the atmosphere by evaporation; about one-third
of it flows away in surface streams; and the remaining one-sixth
enters the crust of the earth. Considerable water which enters the
earth returns to the surface as springs, by capillarity of soils and
rocks, or by being drawn up into plants and evaporated. Some idea of
the amount of ground water may be gleaned from the statement, based
upon a careful estimate, that all the water in the rocks and soils of
the first 100 feet below the surface of the United States would make a
layer seventeen feet thick. In most humid regions the soils and loose
rock formations are saturated with water at greater or less depths
(usually less than 100 feet) below the surface. The surface of this
saturated layer is called the ground-water level, or more familiarly
the "water table." The water table shifts up and down more or less
according to variation in rainfall.

In addition to the water held in the loose rocks and soils near the
earth's surface, large quantities occur in definite layers (usually
strata) of porous rocks which very commonly extend at various angles,
hundreds or even thousands of feet into the earth. A very fine
illustration of this principle is the case of the Dakota sandstone
formation of Nebraska. Almost anywhere across the State a well drilled
through a bed of clay and into the porous sandstone layer encounters
water. (Figure 19.) Another principle is also well illustrated,
namely, that water in such a porous layer may actually travel hundreds
of miles, water obtained from a well sunk to the Dakota sandstone
having actually traveled under the surface of the State all the way
from the eastern face of the Rocky Mountains, where rain and melting
snow entered the upturned and exposed porous rock layer. Another good
example is Iowa, where certain porous rock layers outcropping in the
northwestern and northeastern corners of that and adjacent States
gradually bend down under the State, reaching the greatest depths (up
to 3,000 feet) far in the interior. From wells 3,000 feet deep near
Boone, Iowa, it is, therefore, a fact that some of the water pumped
out of the earth actually traveled underground all the way from beyond
the corners of the State. This sort of travel of underground water is
common in many parts of the world. It should be clearly understood
that such water does not flow freely as in a pipe along subterranean
passageways, but rather it slowly works its way between the grains of
porous rock. Where such water moves distinctly downward, and the porous
layer has both above and below it an impervious rock layer like shale
or clay, it gradually gets under greater and greater pressure. In some
cases such pressure has actually been found by deep drilling to be
equivalent to that of a column of water several thousand feet high. The
rate of motion of water in porous underground rock layers is very slow,
data from various sources indicating a rate of speed of not more than
one-fifth of a mile a year in coarse porous sandstone, while in many
rocks it cannot be more than ten to fifty feet per year.

[Illustration: Fig. 19.--Vertical (structure) section from the Rocky
Mountains to Omaha, Nebraska, illustrating a widespread underground
porous rock layer (the Dakota sandstone) charged with water under
pressure, the clay formation acting as a cap rock. (After Darton, U. S.
Geological Survey.)]

We still have to consider a third mode of occurrence of waters within
the earth. Many formations, like granite and other types of crystalline
rocks are neither in definite layers, nor are they sufficiently porous
to allow water to really flow through them. Where such rocks extend
far down from or near the surface, how does rain water descend? It
does so along cracks or fractures (both joints and faults) which we
have learned are almost universally abundantly present in all hard
rocks in the upper (or zone of fracture) portion of the earth's crust.
Joint cracks are generally very irregular in direction and spacing,
while fault fractures are usually fairly regular and straight. Many
cracks are not wide enough to allow anything like good passageways for
water, while others are sufficiently open to allow water to travel
along them for hundreds, or even thousands of feet. In canyons of the
West, springs not rarely emerge from the bottoms of great, nearly
vertical ledges of granite and other hard crystalline rocks, the waters
certainly having entered the rocks hundreds, or even some thousands of
feet, higher. In rocks of the kind here considered it is evident, then,
that the movements of subterranean waters must be mostly exceedingly
irregular and usually not in great quantities. In many deep mines of
the world, underground water causes little or no trouble except often
near the surface. Occasionally a shaft or tunnel strikes a prominent
joint or fault fracture filled with water.

What we might really call underground streams may occur only under
exceptional conditions in rocks other than limestone, but in limestone
they are not uncommon because the slow solubility of the rock allows
underground waters to slowly enlarge the passageways to form distinct
channels. Echo River, which flows through Mammoth Cave, is a fine case
in point.

Most water by far which emerges as springs, was at one time surface
water. A simple, but common case is where rain water soaking through
porous soil (e.g., sand) or rock, sinks to the top of an underlying
impervious layer (e.g., clay) along whose surface it flows until it
reaches the side of a valley where a spring results. In fact, wherever
the water table is crossed by the surface of the ground, water must
either seep or flow out. Where underground streams which are common in
limestone regions reach the surface on hill or valley sides, springs
result. Another source of springs is where under proper conditions of
slope a porous rock layer, charged with water well below the surface,
appears at a lower level than its source of water. Still another type
of spring is where a fissure or fracture crosses a water-bearing layer
in which the pressure is great enough to cause the water to rise to the
surface along the relatively open fissure or fracture.

In various localities we hear of springs in seemingly paradoxical
situations on tops of hills and even mountains. Such a mystery is not
difficult to clear up. In the first place, such springs are rarely at
the summit of the hill or mountain. A case well known to many persons
is the small, but never-failing spring a little below the summit of
Mount Whiteface, a peak in the Adirondacks, rising 3,000 feet above the
general level of the immediately surrounding country. In this case, a
mass of highly fractured rock, subjected to much rainfall and lying
above the level of the spring, is sufficiently large easily to contain
and give forth enough water to account for several such springs. In
rare cases, however, springs or flowing wells are located on summits,
and in such places it is only necessary to bear in mind some of the
principles above set forth, but mainly the facts that water may travel
under pressure long distances underground, and that the point of
emergence may be on a hill which is actually lower than the source of
the water far away.

The economic significance of underground waters is forcibly brought
to our attention when we realize that 75 per cent of the people of
the United States depend upon wells for their water supply. Many city
supplies, most farm supplies, and much irrigation water come from
wells. The 3,000,000 people of Iowa, for example, are dependent upon
underground waters from wells varying in depth from a few feet to
several thousand feet.

[Illustration: Fig. 20.--Ideal section illustrating the chief requisite
conditions of artesian wells. A, a porous stratum; B and C, impervious
beds below and above A, acting as confining strata; F, the height of
the water level in the porous bed A, or, in other words, the height of
the reservoir or fountainhead; D and E, flowing wells springing from
the porous water-filled bed A. (After U. S. Geological Survey.)]

Most wells are simply dug to depths a little below the water table. In
humid climate regions the depths seldom exceed fifty feet. The water
encountered in such wells is rarely under pressure. In some regions of
deep soils or loose formations, wells are actually bored with an auger
to depths of as great as 200 feet. Deep wells in relatively hard rocks
are always drilled to depths of even thousands of feet. In such cases
the purpose is to strike either a porous rock layer charged with water,
or a crack or fissure filled with water, the water almost always being
under pressure (sometimes very great), under such conditions. These are
called artesian wells whether the water under pressure actually flows
out at the surface or not.

We may now inquire as to the necessary conditions for artesian wells.
This may best be done by the aid of diagrams. Figure 20 illustrates a
very common case where a porous layer, lying between impervious layers,
passing under a valley, comes to the surface of the hills on each side
where the water enters the porous layer. On sinking a well to the
water-charged layer, the water rushes through the hole to a greater or
less distance above the surface. In Figure 21 the porous and impervious
layers are simply tilted, and the water under pressure rises through
the free opening to the surface. Wells of this kind are also common
in the Atlantic Coastal Plain of the United States. In another case,
less comprehensible to the layman, the porous water-bearing stratum
curves downward under a hill or mountain, water entering it where it
is exposed on each side. Under such conditions a flowing artesian well
cannot be drilled at or near the summit, but since the water is under
pressure it will rise in the hole to a level approaching that of the
lowest part of the outcrop of the porous layer on either side of the
hill or mountain. This is essentially the condition of things toward
the interior of Iowa, where water from the deeper wells rises 2,000
feet or more in the holes, but does not reach the surface.

[Illustration: Fig. 21.--Section illustrating the thinning out of a
porous water-bearing bed. A, inclosed between impervious beds B and C,
thus furnishing the necessary conditions for an artesian fountain at D.
(After U. S. Geological Survey.)]

The drilling of deep wells, where records, including samples of rock
materials brought up, have been kept, has been a great aid to the
geologist in determining, or rendering more precise, the knowledge of
not only the kinds of rocks underground, but also the thicknesses and
structural relations of the formations.

In yet another way deep wells are of special significance, that is in
regard to the light which they throw upon the subterranean temperature
of the earth. Very recently the deepest well in the world was drilled
near Fairmont, West Virginia, to a depth of 7,579 feet, in quest of
oil or gas. At a depth of 7,500 feet, the temperature was found to be
168 degrees F. Allowing for a near-surface temperature 50 degrees,
this means an average rate of increase downward of 1 degree in 62
feet. The second deepest well is near Clarksburg, West Virginia, sunk
to a depth of 7,386 feet, with a temperature of 172 degrees at the
7,000-foot level, or at the rate Of 1 degree in 57 feet, allowing for
a near-surface temperature of 50 degrees. It is a remarkable fact,
that little or no water was encountered all the way down. A well 7,348
feet deep in southeastern Germany gave a temperature of 186 degrees
at the bottom, or a rate of increase of 1 degree in 54 feet. These
three records are about the average for the deep holes of the world.
Next to the deepest mining shafts in the world are in the copper
mining region of northern Michigan, where over 5,000 feet (counted
vertically, not down the slope) down the temperature is nearly 90
degrees the year round. The rate of increase is here less than in most
wells of such depth, because of the cooling air currents. Many years
ago a rather remarkable experiment in well drilling was tried by the
city of Budapest, Hungary, the attempt being to get a supply of water
at the brewing temperature of 176 degrees in order to encourage the
manufacture of beer. After getting a good supply of water at a depth
of 3,120 feet and a temperature of 158 degrees, work was stopped. In
building the two great tunnels (St. Gotthard and Simplon) through
portions of the Alps, such high temperatures were encountered that work
was continued only under great difficulties. In the famous Comstock
gold and silver mine of Nevada, over forty years ago, temperatures as
high as 157 degrees were encountered in the shafts at a depth of only
2,000 feet or a little over, the exceptional temperature for such depth
no doubt being due to occurrence of the ores in geologically recent
igneous rocks which have not yet cooled to the normal temperature for
the depth of 2,000 feet.

From the sanitary standpoint, wells are of very great significance,
especially in view of the fact that such a large proportion of people
depend upon well water. It is generally understood that typhoid
fever is more common in the country than in cities, in spite of what
might reasonably be expected. What are some of the causes leading
up to such a situation? The idea that water purifies itself after
flowing a relatively short distance is, in many cases, far from being
true, especially when we are dealing with underground water. Actual
observations prove that germ-laden water may travel surprisingly far
underground. Germ-laden water from barns, cesspools, or outhouses
spreads notably on sinking to the water table and it is easy to see
how so many wells become contaminated. On general principles, a
geologist is especially wary of water from a well in a barn yard.
The well for human use at least should be located out of reasonable
range of such contamination. Under the condition of the diagram a
well or spring some distance down the side of the hill may actually
be unfit for use, though a serious situation is much less likely to
develop there. Nor should one assume that by locating the well on
the uphill side of the house, and the outhouses or cesspool on the
downhill side, safety is assured. From what we have learned in regard
to earth movements, and the tilting of strata from their original
positions, we know how the movement of water in the saturated zone
near the surface may be downhill roughly following the hill slope,
while in a tilted porous layer of rock farther below the surface the
movement of water may be in just the opposite direction. A well drilled
into the solid rock for safety on the uphill side of a house might
derive its water from this very same porous layer, whose water has
been contaminated from a cesspool or other source down the side of
the hill. Such a case is by no means a theory or a rarity. There is
also real danger of contamination in cases where the water flows more
like streams underground through cracks or fissures in hard or dense
rocks, or through channels developed by solution in limestone. It may
happen that water becoming contaminated from barn sites, cesspools, or
outhouses finds its way along such a channel to the side or bottom of
a well. The author well remembers the case of a farmer whose house,
barn, and well were close together on a little limestone terrace and
who continued to use the well water although he complained of its
disagreeable taste, especially after a rain when he could "taste the
barn in it."

[Illustration: Fig. 22.--Diagram illustrating a danger of contamination
of wells by impure underground water. S, soil; B, bedrock; P, porous
stratum. Impure water from cesspool moves through porous layer to
bottom of well. (By the author.)]

Finally, in this connection, it may be said that wells should be
located in the light of the principles above explained, best of all
upon the advice of some one with geological training, and that, to
insure safety to health from the well water, sanitary analyses (at
small cost) should be made once or twice a year. A bad well should be
abandoned and a new one sunk.

A large amount of money has been wasted upon, and much mystery and
superstition has surrounded so-called "water witches," or those who
claim some special or supernatural power of locating supplies of
underground water. Most common of all devices used is the so-called
"divining rod," which is a forked stick of willow, witch-hazel, or
other wood, according to the seemingly special requirement of the
operator. Certain mechanical and electrical devices are also employed.
With one fork of the divining rod grasped in each hand and the main
part of the stick upright, the operator walks about until, due to
some "mysterious" influence, a place is found where underground
water pulls the upright portion of the stick downward in spite of
the grasp of the holder. Some operators even claim to know just how
deep a well must be sunk. Without any attempt to question the honesty
of all operators, geologists are in full accord with the following
quotation from a paper published by M. L. Fuller, for the United States
Geological Survey: "The uselessness of the divining rod is indicated
by the facts that it may be worked at will by the operator, that he
fails to detect strong water currents in tunnels and other channels
that afford no surface indications of water, and that his locations
in limestone regions where water flows in well-defined channels are
no more successful than those dependent upon mere guesses. In fact,
its operators are successful only in regions in which ground water
occurs in definite sheets in porous material, or in more or less
clayey deposits, such as pebbly clay or till. In such regions (which
are extremely common) few failures can occur, for wells can get water
almost anywhere. Ground water occurs under certain definite conditions,
and just as surface streams may be expected wherever there is a valley,
so ground water may be found where certain rocks and conditions exist.
No appliance, either mechanical or electrical, has yet been devised
that will detect water in places where plain common sense will not show
its presence just as well. The only advantage of employing a 'water
witch,' as the operator of the divining rod is sometimes called, is
that crudely skilled services are thus occasionally obtained, since
the men so employed, if endowed with any natural shrewdness, become,
through their experience in locating wells, better observers of the
occurrences and movements of ground water than the average person."

It should not be assumed, however, from the above statement that the
location or foretelling of underground water is mostly hopeless from
a scientific point of view. In most regions the kinds of rocks which
would be pierced by wells can be more or less accurately foretold by
careful studies of the rocks exposed at the surface. But foretelling
the underground water is often much more uncertain. Where the geologic
structure or arrangement of rocks in a region is fairly regular, as
in the case of most sedimentary rocks, and a few scattering deep
wells have been drilled, with records preserved, the geologist, by
combining such data with his surface studies, can do much toward
putting the facts regarding the underground waters of the region on a
scientific basis. There are many such regions, an excellent case in
point being Iowa, regarding which State the United States Geological
Survey has published a report containing data by the use of which
it is possible to foretell almost exactly what formations would be
pierced by drilling from 1,000 to 3,000 feet or more, the thickness
of each, which ones are water-bearing and, in many cases, even the
character of the mineralization of the water for almost any part of
the State. Such knowledge, through the years, is worth untold millions
of dollars to the State. Where the rocks are igneous and rather
uniformly dense, usually little or nothing can be accurately foretold
about the underground water supplies, because in such rocks the water
follows exceedingly variable and irregular cracks and fissures. In
metamorphic rocks the difficulties are usually about as great. In
limestone regions, with humid climate, much water travels in channels
underground, but these are so exceedingly irregular that there is no
way of locating them by surface studies. In humid climates it seldom
happens, however, that a well does not reach at least a fair supply of
water within a few thousand feet even in rock formations in which the
water travels along irregular cracks and channels.

Certain other important features of the geological work of underground
water should be brought to the attention of the reader. One of these
is its power to dissolve mineral substances of many kinds more or less
rapidly. As already pointed out, limestone is especially susceptible to
solution in water, both surface and underground.

The carbonate of lime taken into solution from limestone is the
principal substance which causes so-called "hard water." Most of the
solution takes place in the upper part of the zone of fracture of the
earth's crust and the dissolved substances are carried along generally
to the lower levels where they tend to deposit (and crystallize),
filling fissures, cracks, and even tiny spaces between mineral grains.
Cracks and fissures thus filled by mineral matter from solution
are called "veins." In many mining regions valuable ores and other
substances have been deposited from underground water solutions and
concentrated in veins. In many places underground waters with certain
substances in solution travel through various rocks or encounter
solutions of other substances and, as a result of chemical action, many
new mineral combinations result. Such actions through the millions
of years of geologic time have effected great changes in many rock
formations. In the case of petrification, like that of petrified wood,
the buried organism slowly decomposes cell by cell, and particle by
particle it is replaced by mineral matter from underground water
solutions. In this manner the remarkable so-called petrified forests
(not really forests) of Arizona and the Yellowstone Park were formed,
the petrifying material there having been the very common substance
called silica which is the same in composition as the familiar mineral
quartz. Mineral matter carried in solution in surface streams is
derived from ground waters which reach the surface. An idea of the
tremendous quantity of mineral matter thus removed may be gained from
the statement that by careful determination the Mississippi River
carries 120,000,000 tons in solution into the Gulf of Mexico each year.

[Illustration: Fig. 23.--Structure section and part of landscape in a
limestone region showing how caves and natural bridges are formed by
the dissolving action of underground water. AA, limestone; BB, sink
holes; DD, caves and galleries; and an arch (natural bridge) which is
the remnant of a large cave. (After Shaler, U. S. Geological Survey.)]

One interesting effect of the dissolving power of underground water
in limestone regions is the development of caves or caverns. Most
remarkable of all is Mammoth Cave, Kentucky, with its hundreds of
miles of passageways and galleries. This marvelous work of nature is
all a result of the action of underground water which has dissolved
and carried away vast quantities of limestone. Echo River, which flows
through the cavern, is still carrying on the work aided by various
underground tributaries. The stalactites and stalagmites, which are
so strikingly displayed in many caves, as at Luray, Virginia, in
which water with carbonate of lime drips or oozes from the roof and,
due mainly to evaporation, deposits the lime. Many wonderful and
fantastic effects are thus produced. Where part of the roof of a cave
is dissolved out, or falls in, a "sink hole" results. Where all but a
portion of the roof of a cave or underground channel has fallen in,
a natural bridge, like the famous one in Virginia, results, though
natural bridges are also formed by other means.

In concluding this chapter we shall briefly discuss hot underground
waters, hot springs, and geysers. There are two well-known ways by
which underground waters may become heated. One is by the movement of
water downward into the normally heated portion of the earth, the rate
of increase downward being, as above stated, 1 degree F. for about
50 to 60 feet. Water descending two miles would, therefore, attain a
temperature of about 200 degrees F. In some regions such a temperature
may be reached at depths considerably less. Such water (under pressure)
taking a short course to the surface (forming springs) at a lower
level would retain much of its heat taken up far below the surface.
In regions where there are great down-folds of the strata (i.e.,
synclines), as in the central to southern Appalachians, conditions
appear to be favorable for such warm or hot springs, as, for example,
at Hot Springs, Virginia. A second cause of the heating of underground
water is by the descent of surface waters into contact with masses of
still hot igneous rock of relatively recent geologic age. In some such
cases the water does not go more than some hundreds of feet down and
when, under proper conditions, it returns to the surface hot and even
boiling springs may result.

[Illustration: Plate 7.--An Upbend Fold (_anticline_) in the
Appalachian Mountain Strata Near Hancock, Maryland. The strata were
deposited in horizontal layers upon the sea bottom, covering the region
many millions of years ago in middle Paleozoic time. At the time of
the Appalachian Mountain revolution, near the end of Paleozoic time,
this and many other folds developed well below the surface. Removal
of overlying material by erosion has laid bare the fold as we see it
to-day. (_Photo by Russell, U. S. Geological Survey._)]

[Illustration: Plate 8.--(_a_) A Ledge of Igneous Rock (Granite) in
Northern New York. This illustrates so-called "joints" or natural
cracks, commonly separating most hard rock masses into more or less
prismatic blocks. (_Photo by the author._)]

[Illustration: Plate 8.--(_b_) A Fault Fracture in a Ledge at East
Canada Creek in the Mohawk Valley, New York. The Ordovician limestone
formation in thin layers on the right has sunk hundreds of feet along
vertical fault to the left of middle, bringing it sharply against the
older (Cambrian) massive formation on the left. The hole is artificial.
(_Photo by Darton, U. S. Geological Survey._)]

Geysers are periodically eruptive hot springs found only in a few of
the volcanic regions of the world. They are most wonderfully displayed
in the Yellowstone National Park, where they send columns of hot
water to all heights up to 250 feet at various intervals of time.
Almost incredible amounts of hot water are sent into the air every
day in the geyser basins of Yellowstone Park. The single geyser "Old
Faithful," which erupts at intervals of about seventy minutes, sends
a column of water several feet in diameter to heights of from 125 to
150 feet. During each eruption about 1,500,000 gallons of water are
sent forth, or every day enough to supply the need of a fairly large
city. A very brief explanation of the cause of geyser eruptions may
be stated as follows: The very irregular, narrow, geyser tube extends
nearly vertically downward into yet uncooled lava. The tube is more or
less rapidly filled by underground water. The bottom, or near-bottom,
portion of the water gradually becomes heated by the lava until finally
the boiling point is reached for that depth. But, because of the
pressure of the overlying water column, the boiling point at that depth
is considerably greater than for the surface. A little steam develops
far down and this causes the whole column of water above it to lift
slightly, thus relieving the pressure on the superheated water far
down. This relief of pressure allows much of the superheated water far
down to flash into steam, which violently forces the column of water
out of the geyser tube.



CHAPTER X

HOW MOUNTAINS COME AND GO


Mountains constitute the grandest relief features of the earth, and
some of the most profound lessons of earth changes may be learned by
studying them. To the layman who views great mountains in all their
grandeur and massiveness, the expression "everlasting hills" seems
appropriate. But the geologist knows that even the loftiest mountains
are only temporary features on the face of the earth. Like organisms,
they come and go. For example, where the great Rocky Mountains now
stand was only a few million years ago (in late Mesozoic time) the
bottom of an interior sea. Where the Appalachians now stand there were
no mountains late in the Paleozoic era (not less than ten or twelve
million years ago), but instead sea water covered the district. Then
the Appalachians were formed, lifting their heads much higher than at
present, after which they were cut down almost to sea level, and then
once more upraised. The Coast Range Mountains of our Pacific Coast have
come into existence since the middle of the present (Cenozoic) geologic
era. Every mountain, like every organism, has a life history, in some
cases simple, and other cases complex. All pass through stages of
birth, youth, maturity, old age, and death. Some rear their heads and
disappear after a short (geological) existence. Others continue their
growth and persist much longer, while still others undergo periods of
profound rejuvenation.

Among the various processes by which mountain ranges have been
formed, the folding and accompanying general uplift of strata are
the most important. In fact, in most of the great mountain ranges of
the world the folded structure is conspicuously developed, so much
so that they may well be called "folded mountains." Very commonly,
however, mountains of this type have also been subjected to more or
less fracturing of the rocks (faulting), and not uncommonly they have
also been subjected to igneous activity, including both intrusion and
extrusion of molten material. It is among the folded mountains of
greater or less degree of complexity that the "greatest exhibitions of
geologic phenomena are seen and the lessons which geology as a sciences
teaches may be learned. If one desires to know the history of a region,
one turns naturally to its mountain ranges, for here may be found the
upturned and dissected strata, a study of whose kinds, thickness,
and fossils throws light upon past events, while their foldings and
dislocations show the nature and results of those great dynamic
agencies which, from time to time, have operated upon the outer portion
of the earth, and given to it the broad distinctive features which
characterize it to-day." (L. V. Pirsson.) Among the great mountains we
may also see wonderful exhibitions of the results of weathering and
erosion, especially the work of rivers and glaciers.

We can, perhaps, best convey to the reader some of the main facts
and principles regarding folded mountains by considering certain
observations which may be readily made in a short trip across a folded
range of not too complex kind--for example, across the Appalachian
range along the line of the Baltimore and Ohio Railroad, west of
Washington, or the Pennsylvania Railroad, west of Philadelphia. It
would be most evident that the mountains consist of strata, that is
sedimentary rocks, such as sandstone, shale and limestone, which were
deposited under water. A few measurements would reveal the fact that
thousands of feet in thickness of strata are represented. Careful
measurements by geologists have, in fact, shown that the strata were
originally piled up layer upon layer to a thickness of 25,000 to 30,000
feet. The fact that they are strata of such great thickness proves that
sediments must there have accumulated under water for some millions
of years at least. Closer examination of a few good exposures (i.e.,
outcrops) would further reveal the presence of fossil shells and
impressions of marine organisms, thus definitely leading us to conclude
that the strata were accumulated under sea water, which, of course,
means that the present site of the mountain range was once sea floor.

Examination of the rock materials also establishes the fact that the
strata are such as were deposited in relatively shallow sea water--that
is to say, none are at all of the sort which are now forming under
really deep ocean water. Most of the strata represent original
sands (and even gravels) and muds which could have accumulated only
relatively near shore, that is within about 100 miles, which harmonizes
with a statement made in a preceding chapter to the effect that very
little land-derived sediment is at present depositing more than 100
miles out from shore. The coarse materials (sands and gravels) could
not, of course, be carried many miles out, while many of the strata are
covered with ripple marks, thus positively proving their shallow-water
origin. We conclude, therefore, that the Appalachian strata are of
marine, shallow-water origin. But we have already stated that these
strata are at least 25,000 feet thick. How, then, do we reconcile
these two seemingly paradoxical statements? All that is necessary is
to realize that the floor of the shallow sea, in which the sediments
eroded from adjacent land were being deposited, slowly, though more
or less irregularly, subsided or sank during the long ages (millions
of years) of their accumulation. It would carry us too far afield to
really attempt an explanation of this remarkable type of geologic
phenomenon, and it must suffice to suggest that, starting with the
earth's crust in equilibrium, the very weight of accumulating strata
would tend to destroy that equilibrium and so cause subsidence.

In our trip across the mountains it would be strikingly evident that
the strata are no longer in their original horizontal position, as
they were piled up layer upon layer, but that they have been notably
disturbed and thrown into folds (Plate 7), large and small, some masses
of the strata having been bent upward (anticlines) and others downward
(synclines). Such folded structures could have been developed only by a
great force of lateral compression in the earth's crust within the zone
of flowage. Under compression the strata were mashed together, notably
bent into curves (folds), and more or less upraised. It would also be
readily observed that the main axes of the folds extend essentially
parallel to the main trend of the mountain range, thus proving that
the force of compression was exerted at right angles to the trend of
the range.

[Illustration: Fig. 24.--Diagrammatic sections illustrating the
development of a typical folded mountain range. Upper figure: A, the
old land eroded to furnish sediments deposited under the adjacent
sea at C. Middle figure: strata (C) folded as they would appear if
unaffected by erosion, and a down-warp (B) between A and C. Lower
figure: condition after profound erosion, and filling of B with
sediment. (Drawn by the author.)]

Using a biological analogy, a brief history of a typical folded
mountain range may be stated as follows: First, there is the prenatal
or embryonic stage when the materials of the range are gathering, that
is when the sediments are piling up layer upon layer relatively near
shore on a sinking sea bottom. Next comes the birth of the range when,
due to the great lateral compressive force, the strata are thrown into
folds and forced to appear above sea level, the range thus literally
being born out of the sea. During the next, or youthful stage,
the range grows (with increasing altitudes) because of continued
application of the compressive force. Even during the youthful growing
stage weathering and erosion attack the range and tend to reduce it.
Then comes the stage of maturity, when the upbuilding (compressive)
force and the tearing down (erosive) force about counterbalance each
other. At this time the range has reached its maximum height and
ruggedness of relief, with ridges and valleys higher and deeper than
at any other time. The old-age stage sets in when the upbuilding power
wanes or actually ceases, and erosion dominates or reigns supreme.
Slowly but surely, unless there be a renewal by an upbuilding power,
the range is cut down until little or nothing of it remains well
above sea level, or, in other words, until a peneplain is developed.
This last stage may truly be called the death of the range. Usually,
however, some local portions of the disappearing range, which are more
resistant or more favorably situated against erosion, are left standing
to at least moderate heights above the general level of the plain of
erosion.

The above normal order of events may be disturbed at any stage,
especially after maturity, by renewed uplift when the streams are
revived in activity and increased ruggedness results. Even after the
whole range as a relief feature has been planed away, the site of the
range may be uplifted and a new cycle of erosion started.

By the use of two well-known examples we shall not only illustrate the
above principles of mountain history, but also show that no less than a
few million years must be allowed for the growth and decay of a great
folded range. During the last (Permian) period of the Paleozoic era
the Appalachian strata began to buckle and the yielding to pressure
continued till well into the succeeding (Triassic) period. The climax
was reached about the close of the Permian. Then, throughout the
Mesozoic era, erosion reduced the central Appalachians to a great plain
(peneplain) near sea level, after which, about the beginning of the
present (Cenozoic era), the site of the former range was distinctly
upraised (without folding of the rocks), causing the revived streams to
begin their work of carving out the present ridges and valleys, this
work still being in progress.

In the case of the Sierra Nevadas, the strata were folded into a lofty
mountain range relatively late in the Mesozoic era and, by the middle
of the Cenozoic era, the old-age stage of erosion was well advanced
when the range was not more than a few thousand feet high. Then (in
the middle of the Cenozoic era) uplift, accompanied by faulting on a
large scale, but not by folding, took place, and the range was notably
rejuvenated to about its present height. All the remarkably deep
canyons of the Sierras have been carved out since the rejuvenation.

How is the geological birthday of a mountain range determined? In the
preceding paragraph we stated that the Appalachians were folded and
born out of the sea about the close of the Paleozoic era. This is
readily proved by calling attention to two facts. First, the youngest
strata involved in the folding are of Permian, or late Paleozoic Age
in the geologic column, as proved by their fossil content, etc.,
and obviously the folding must have taken place after they had been
deposited. Clearly, then, the folding could not have taken place before
very late Paleozoic time. Second, the oldest strata resting upon the
folded rocks are of early (not the very earliest) Mesozoic Age, and
these strata are somewhat tilted but not folded. Obviously, then, the
folding must have occurred before the nonfolded strata were deposited,
which means that the folding must have been essentially completed
in not later than early Mesozoic time. Or, in the case of the Rocky
Mountains, we know that strata were there folded late in the Mesozoic
era or very early in the Cenozoic era, because folded rocks as late
in age as late Mesozoic (Cretaceous) have resting upon them, in some
places, nonfolded strata of early Cenozoic (Tertiary) Age. The figure
clearly shows how the Ordovician strata must have been folded before
the next (Silurian and Devonian) strata were deposited upon them in
southeastern New York.

[Illustration: Fig. 25.--Diagram illustrating the topography and
folded structure of the Appalachian Mountains west of Harrisburg,
Pennsylvania. The valleys have been etched out of belts of weak rocks,
while outcropping resistant rocks stand out to form ridges. Note the
course of the Susquehanna River across the mountain ridges, this being
a "superimposed river" (see text, p. 233). (Drawn by A. K. Lobeck.)]

[Illustration: Fig. 26.--Only slightly tilted strata of Silurian and
Devonian ages resting upon folded strata of Cambrian and Ordovician
ages in an east-west section across the Catskill Mountains and Hudson
Valley of New York. The folding took place at the time of the Taconic
Mountain Revolution toward the end of the Ordovician period. (Drawn by
the author.)]

As already suggested, however, folding is not the only method by
which mountains are formed. Many ranges are either entirely due to
the tilting of earth blocks by faulting or fracturing of the earth,
or their present altitude, at least, is a direct result of faulting.
Such may be called block mountains. They are wonderfully represented
by the various north-south ranges rising some thousands of feet above
the general level of the Great Basin region of Utah and Nevada.
These ranges are, in short, somewhat eroded edges of approximately
parallel-tilted fault blocks lying between the Sierra Nevada Range and
the Wasatch Range. In southeastern Oregon a series of nearly parallel
block mountains, up to forty miles in length and over 1,000 feet in
height, show very steep eastern fronts only slightly modified by
erosion.

Another mode of origin of mountains is by the rise of molten material
to the surface, especially where a chain of volcanoes is located. Thus
the Cascade Mountains from northern California through Oregon and
Washington, including Mounts Lassen, Shasta, Pitt, Baker, St. Helens,
and Rainier, are very largely the result of volcanic action. The long
chain of Aleutian Islands of Alaska, referred to in our study of
volcanoes, is an excellent example of a great mountain range now being
built up out of the sea by volcanic action. More locally molten rocks
under pressure may not reach the surface but instead simply bulge or
dome the strata over them, as in the case of the group known as the
Henry Mountains of Utah, and also in other parts of the West.

In still other cases mountains of considerable area and altitude have
resulted from erosion of uplifted regions where the uplift has been
practically unaccompanied by either folding, faulting, or igneous
activity. Any low-lying area, regardless of the character of its rocks,
structure, or previous history, may be notably upraised and simply
subjected to erosion. An excellent illustration is afforded by the
Catskill Mountains of New York, where numerous deep valleys and narrow
ridges have been carved out of upraised nearly horizontal strata. The
so-called "Bad Lands" region of parts of South Dakota and Wyoming is
also essentially of this type, where deep, narrow valleys and sharp
ridges have been etched out of high, relatively soft, nearly horizontal
strata, resulting in an almost impassable maze of mountains. In the
high, recently upraised Colorado Plateau of parts of Arizona, New
Mexico, Colorado, and Utah, nearly horizontal strata are being etched
out, the result being numerous buttes, mesas (flat-topped hills and
mountains) and deep canyons, including the Grand Canyon with its maze
of peaks and pinnacles, many of them rising like mountains out of the
canyon depths.

Mountains of the pure types just described are not the prevailing
ones of the earth. Most mountains and their structures, as we see
them to-day, are products of two or more of the processes of folding,
faulting, igneous action, and erosion. A few well-known examples will
suffice to make this matter clearer. Thus, the Appalachian Mountains
originally developed by severe folding of thick strata. After
considerable erosion, numerous small and large thrust faults developed,
some of the dislocations amounting to miles. Then the whole range was
cut down nearly to sea level by erosion, after which the district was
upraised (without folding) mostly from 2,000 to 4,000 feet, and the
present long, narrow mountain ridges and valleys have been carved out
by stream erosion. Thus folding, faulting, and erosion all enter into
the height and structure of the Appalachians.

A lofty mountain range still more complex in its history is the Sierra
Nevada of California. First, thick strata were highly folded, upraised,
and intruded by great masses of molten granite. Erosion then proceeded
to cut the range down to hills, after which a great fracture (fault)
developed along the eastern side and the Sierra Nevada earth block was
notably tilted with steep eastern front and long western slope. Erosion
has considerably modified the eastern fault face, and the deep canyons
like Yosemite, King's River and American River, have been carved out
of the western slope of the great tilted fault block. Geologically
recently the central to northern portion of the range has been affected
by volcanic action, streams of lava in some cases having flowed down
the valleys.



CHAPTER XI

A STUDY OF LAKES


Lakes are ephemeral features on the face of the earth. Compared to the
tens of millions of years of known earth history, lakes, even large
ones, are very short lived. They may, in truth, be regarded as merely
results of the temporary obstructions to drainage. Lake basins are
known to originate in many ways, and there are various means by which
they are destroyed. Not attempting an exhaustive, scientific treatment
of the subject, our present purpose may be well served by describing
and explaining some of the better known and more remarkable lakes of
the world.

Even a cursory examination of a large map of the world reveals the fact
that the regions of most numerous lakes are those which were recently
occupied by glaciers--either the vast ice sheets of the Glacial epoch
or mountain (or valley) glaciers. This is because more lakes of the
present time have come into existence as direct or indirect results
of glaciation than by any other cause. A considerable number of these
lakes occupy rock basins which have been eroded or excavated by the
direct action of flowing ice. Small lakes of this sort are commonly
found in the upper parts of valleys formerly occupied by mountain or
so-called Alpine glaciers, because there the excavating power of such
glaciers was especially effective. More rarely rock basins have been
scoured out by glaciers farther down their valleys. Many lakes occupy
rock basins excavated by ice in the high Sierra Nevada and Cascade
Ranges, in the Rocky Mountains from Colorado into Canada, in the Alps,
and in the mountains of Norway. Few, if any of them are, however,
large or famous. Other lakes, some of very considerable size, occupy
rock basins scoured out by the passage of the great ice sheets of
the Glacial epoch in North America and Europe, though they are less
common than formerly supposed. Some of the many lake basins of Ontario,
Canada, are quite certainly of this origin, as might well be expected,
because the power of the great ice sheet was there in general notably
greater than south of the Great Lakes where the tendency was to unload
or deposit the eroded materials as shown by the great accumulations of
glacial débris (moraines).

Where the ice walls of certain existing glaciers form dams across
valleys, waters are ponded, a small lake of this kind occurring
alongside the Great Aletsch Glacier of the Alps, where its wall is
slowly moving past a tributary valley. Lakes of this kind also occur
in Greenland and in Alaska, but none are of considerable size. During
the Great Ice Age, however, literally thousands of large and small
lakes were formed, both during the advance and the retreat of the
ice, wherever the glacier wall blocked valleys which sloped downward
toward the ice. New York State furnishes many fine examples of large
and small lakes of this sort. Thus, when the great glacier was melting
in northern New York, waters hundreds of feet deep and many miles long
were ponded between two ice lobes--one retreating eastward and the
other westward from the Mohawk Valley. An ice dam lake was also formed
a little later, when an ice wall blocked the northern part of the
Black River Valley just west of the Adirondack Mountains and caused a
lake covering about 200 square miles. One of the largest of all known
ice dam lakes has been called Lake Agassiz, which attained a maximum
length of over 700 miles and a width of 250 miles in the Red River of
the North region of eastern North Dakota, western and northwestern
Minnesota, and northward into Canada, most of its area having been
in Canada. It began as a small lake with southward drainage into the
Mississippi when the great northward retreating ice sheet formed a dam
across the valley of the Red River of the North. The retreating ice
continued to block the northward drainage until the vast lake, covering
a greater territory than all of the present Great Lakes combined,
was developed. Beaches, bars, deltas and the outflow channel of this
remarkable lake are wonderfully well preserved. Lake Winnipeg is a mere
remnant of great Lake Agassiz.

Many ponds and small lakes occupy basins formed by irregular
accumulations of glacial (morainic) materials. Still others lie in
depressions which formed by the melting of masses of ice which became
wholly or partly buried by ice deposits, or by sediments washed into
bodies of water which were held up by ice dams. Depressions of the
latter kind are commonly found as pits or so-called "kettle holes"
below the general level of sand flats or sand plains of glacial lake
origin.

Most common of all lake basins of glacial origin are those formed by
accumulation of glacial débris or morainic materials acting as natural
dams across valleys. This is, in fact, the most common of all ways by
which existing lake basins, some of them very large, have been formed.
Most of the thousands of ponds and lakes of Minnesota, Wisconsin, and
northern New York belong in this category.

In the Adirondack Mountains, for example, most of the lakes, like the
well-known Lake Placid, Saranac Lakes, Long Lake, and Schroon Lake,
have their waters ponded by single dams of glacial débris across
valleys. In some cases a series of such dams blockades a valley
and forms a chain of lakes like the well-known Fulton Chain in the
Adirondacks. Less commonly the lake may have its waters ponded by two
natural dams of glacial débris, one across a valley at each end of a
lake. A very fine, large scale example of the last-named type is the
famous Lake George in the southeastern Adirondacks. It is over 30 miles
long and from 1 to 2-1/2 miles wide. It lies in the bottom of a deep,
narrow mountain valley, mountain sides rising very steeply from a few
hundred feet to 2,000 or more feet above its shores. There are many
islands, especially in the so-called "Narrows," thus greatly enhancing
the scenic effect. The valley itself has been produced by a combination
of faulting and erosion. There was a preglacial stream divide at the
present location of the "Narrows." This divide was somewhat reduced by
ice erosion when the deep, narrow body of ice plowed its way through
the valley during the Ice Age. During the retreat of the ice heavy
morainic accumulations were left as dams across the valley at each end
of the lake.

Another remarkable body of water, similar to Lake George in its origin,
is Chautauqua Lake of western New York, famous for its Chautauqua
assemblies. It lies 1,338 feet above sea level, with its northern end
near the edge of the steep front of the plateau overlooking Lake Erie.
Chautauqua Lake really consists of parts of two valleys, one sloping
north and the other sloping south, each dammed by glacial deposits.

The famous Alpine lakes--Garda, Como, and Maggiore--have resulted from
deposition of glacial morainic materials under conditions different
from those above described. In these cases great mountain or valley
glaciers once flowed down the valleys and spread out part way upon the
Italian plain. Great accumulations of glacial débris took place around
the borders of the glacier lobes, and, after retreat of the ice, the
glacial deposits acted as dams ponding the waters far back into the
mountain valleys.

The origin and history of the Great Lakes constitutes one of the most
interesting and remarkable chapters in the recent geological history of
North America. Most of the salient points have been well worked out and
they may be very briefly summarized, as follows: Before the Ice Age the
Great Lakes did not exist, because the region, prior to that time, had
been land subjected to erosion for millions of years--a time altogether
too long for any lake to survive. Their sites were occupied by broad,
low, stream-cut valleys which were quite certainly locally somewhat
deepened by ice erosion during the Ice Age. Ice erosion is, however,
altogether insufficient to account for the great closed basins.
The two most important factors entering into the formation of the
basins of the Great Lakes were doubtless the great glacial (morainic)
accumulations acting as dams along the south side, and the tilting
of the land downward on the north side of the region. In support of
this explanation it has been established that the great dumping ground
of ice-transported materials from the north was in general along the
southern side of the Great Lakes and southward. It has also been well
established that, late in the Ice Age, the land on the southern side
of the Great Lakes region was lower than at present, as proved by the
tilted character of beaches of the well-known extinct glacial lakes
which were the ancestors of the present lakes. Such a down-warp of the
land must have helped to form the closed basins by tending to stop the
southward and southwestward drainage of the region.

[Illustration: Fig. 27.--Sketch map showing a very early stage in the
history of the Great Lakes when two relatively small lakes in front of
the ice wall separately drained into the Mississippi River. (Drawn by
the author from map by Taylor & Leverett.)]

[Illustration: Fig. 28.--Lake Whittlesey stage of the Great Lakes
history when the ice had retreated far enough to allow the eastern and
western ice margin waters to join with a single outlet past Chicago.
(Drawn by the author from a map by Taylor & Leverett.)]

[Illustration: Fig. 29.--The Algonquin-Iroquois stage of the Great
Lakes when their whole area was ice-free, and all their waters drained
through the Mohawk-Hudson Valleys of New York into the Atlantic Ocean.
(After Taylor, published by New York State Museum.)]

We shall now very briefly trace out the principal stages in the
history of the Great Lakes during the final retreat of the vast ice
sheet. This may best be done by the aid of maps which need only brief
explanation. When the ice sheet had retreated far enough northward to
uncover the very southern end of the Lake Michigan basin and a little
beyond, a small glacial lake (Lake Chicago) developed against the ice
wall. Its outlet was through the Illinois River and thence into the
Mississippi. At the same time a larger glacial lake, held up by the ice
wall, developed over the western part of the Erie basin and beyond.
Its outlet was through the Wabash River. With further retreat of the
ice a large lake (Whittlesey) covering considerably more than the area
of Lake Erie developed, with outlet westward across Michigan into the
enlarged Lake Chicago which continued to drain into the Illinois River.
During a still later stage of ice withdrawal the remarkable set of
three glacial lakes existed--Lakes Duluth, Chicago, and Lundy. Each of
these large lakes had its own outlet. Lake Duluth covered about half
of the Lake Superior basin and drained through the St. Croix River
into the Mississippi. Lake Chicago expanded to cover nearly all of the
Michigan basin and continued to drain through the Illinois River. Lake
Lundy covered not only more than the area of the Erie basin, but also
considerable territory north of Detroit, and drained eastward alongside
the ice lobe of the Ontario basin through the Mohawk and Hudson
valleys of New York, and into the Atlantic Ocean. Just after the ice
completely withdrew from the area now occupied by the Great Lakes, but
still blocked the St. Lawrence Valley, the vast body of water called
Lake Algonquin more than covered the sites of the present Superior,
Michigan, and Huron. At this time the land was distinctly lower toward
the northeast than at present, causing the outlets to the west to
be abandoned. The great Lake Algonquin poured its waters eastward
through the Trent River channel of Ontario, Canada, into glacial Lake
Iroquois, which was the great ancestor of Lake Ontario. Lake Iroquois,
in turn, had its outlet eastward through the Mohawk and Hudson Valleys
of New York. For part of the time at least, Lake Erie maintained a
separate existence discharging into Lake Iroquois near Buffalo. During
the Algonquin-Iroquois stage the combined area of all the lakes was
notably greater than the present area of the Great Lakes. The volume
of water discharged by the lakes through the Mohawk Valley of New York
was doubtless greater than that which now goes over Niagara Falls.
Gradually, as the St. Lawrence ice lobe waned, the outlet waters of the
lakes began to move alongside the ice through the St. Lawrence Valley.
Finally the ice withdrew far enough to free the St. Lawrence Valley and
the waters of the Great Lakes region dropped to a still lower level,
bringing about the Nipissing Great Lakes stage not greatly different
from the present. East and northeast of the Lakes the land was low
enough to allow tidewater (the so-called Champlain Sea) to extend
through the Hudson, Champlain, and St. Lawrence Valleys, and possibly
into the Ontario basin, as proved by the occurrence of marine beaches
and fossils. The waters in the Erie and Ontario basins covered about
the present areas, while the Nipissing Lakes, which covered a little
more than the present areas of the three upper Great Lakes, had their
outlet through the Ottawa River channel into tidewater (Champlain
Sea). Postglacial warping of the land has brought the whole region to
the present condition.

[Illustration: Fig. 30.--Map showing next to the present stage of the
Great Lakes history when the land was lower on the north and the upper
(Nipissing) lakes drained through the Ottawa River Valley into an arm
of the sea (Champlain Sea) which reached through the Champlain and
Hudson Valleys. (After Taylor, published by New York State Museum.)]

Many lakes, including some remarkable ones, occupy basins which are
directly due to movements of the earth's crust--either faulting or
warping. An example of a lake occupying part of a fault basin is the
famous Dead Sea of Palestine. This lake lies in the lowest part of the
Jordan Valley, which has geologically recently come into existence
by the sinking of a long, narrow block of earth for several thousand
feet between two great earth fractures (faults). The Dead Sea covers
about 500 square miles and its surface lies about 1,300 feet below
sea level, which makes it the lowest lake in the world. Almost equally
remarkable is the fact that its depth is about 1,300 feet, so that the
lowest part of the lake basin is 2,600 feet below sea level. The lake
contains approximately 24 per cent salt, mostly common table salt,
causing it to be a thick brine in which there is neither plant nor
animal life--hence the name "Dead Sea." At one time, probably just
after the Ice Age, the lake was much larger and deeper, when it filled
a considerable part of the Jordan Valley and had an outlet to the
south. During the high-level stage the water was fresh, but gradually,
as the climate became drier, evaporation was greater than intake,
the outlet was abandoned, and the mineral matter (mostly chloride of
magnesia and common table salt) carried by the streams in solution
into the shrinking lake steadily accumulated until the high degree of
salinity of the present time has been reached.

Great Salt Lake, Utah, is a remarkable lake whose history has been
carefully studied. It occupies the lowest position of an extensive
basin which, in turn, forms but part of the whole great district of
Utah which has geologically recently sunk thousands of feet on the
west side of the great fault already described as occurring along the
western base of the Wasatch Mountains. At present the lake covers
about 2,000 square miles, but its area fluctuates considerably. It is
scarcely believable that this big lake has an average depth of only
fifteen feet and a maximum depth of only fifty feet. It lies 4,200 feet
above sea level, and it carries about 18 per cent salts in solution.
Most abundant by far is common table salt, of which there are no less
than 5,000,000,000 tons in solution. The waters also contain about
900,000,000 tons of other salts. Should the lake completely disappear
by evaporation, these salts would be deposited. Allowing for cars 40
feet long and of 40 tons capacity, a train more than 1,000,000 miles
long would be required to carry the salts. What has been the source of
these salts? Great Salt Lake is not, as supposed by some, a remnant
of an ocean once covering the region. Briefly, the explanation is as
follows: At one time, when the climate was moister, the basin now only
in part occupied by the lake was filled to overflowing with an outlet
north into the Snake and Columbia rivers. That great body of water
(called "Lake Bonneville") covered nearly 20,000 square miles and its
depth was about 1,000 feet deeper than now, the present depth being
very small. Because it had an outlet that lake was, of course, fresh.
Beaches and shore lines 1,000 feet above the present lake, and at
various lower levels, are still wonderfully well preserved. When, due
to climatic change, evaporation exceeded intake by streams, the outlet
was cut off. But slowly, as the lake shrank, streams (especially the
Jordan River) carried a little salt in solution, the percentage of salt
increasing until the present stage has been reached. In a real sense,
much of the salt was once in the sea, because it has been dissolved out
of strata which accumulated under sea water long before the basin of
Great Salt Lake came into existence.

Another famous lake, which also occupies part of a basin due to
faulting, is Lake Tahoe in the Sierra Nevada Mountains, near Truckee,
California. This lake, whose length is 21 miles, and width 12 miles,
lies 6,225 feet above sea level. On almost all sides steep mountains
rise several thousand feet above its waters. Its great depth of
1,635 feet makes it, so far as known, the second deepest lake in
North America, Crater Lake, Oregon, only outranking it. The water
is exceedingly clear. An experiment some years ago showed that a
white disk eight inches in diameter could actually be seen through
a thickness of 216 feet of its water. "The statement sometimes made
that 'Tahoe is an old volcanic crater' is not true. The region about
the lake shows evidences of volcanic activity of various kinds, and
the lake waters themselves have probably been dammed at times by
outpourings of lava. A lava flow appears to have temporarily filled
the outlet channel below Tahoe City. The lake, however, lies in a
structural depression--a dropped (fault) block in the earth's crust."
(U. S. Geological Survey.)

The basin of the largest lake in the world--the Caspian Sea--has
resulted from warping of the earth's crust. It has an area of 170,000
square miles, a maximum depth of 3,200 feet, and its surface is about
90 feet below sea level. The composition of its water and some of its
animal life indicate that it was once an arm of the sea. It has been
detached or cut off by an upwarp of the land between it and the Black
Sea region. If this great lake is a cut-off arm of the sea, with no
outlet, how do we explain the fact that its salinity is much less than
that of the ocean? Toward the north, where it is shallow and fed by so
much river water, it is, in fact, almost fresh water. Even the southern
one-half carries not over 1 per cent of salt. The explanation is that
a steady current passes through a narrow passageway into a gulf or bay
on its eastern side where evaporation is much greater than over the
general surface of the Caspian. The salt is, therefore, gradually
accumulating at the estimated rate of 350,000 tons per day in this
gulf, while the sea itself is becoming fresher.

The basin of Lake Champlain, about 100 miles long, was occupied by
tidewater geologically very recently (that is, since the Ice Age),
but it has been cut off by uplift of the land on the north, since
which time the waters of the lake have been completely rinsed out and
freshened.

Many lake basins directly result from volcanic action. In many parts of
the world lakes, usually of small size, occupy craters of volcanoes as,
for example, in the Eifel region of Germany, the Auvergne district of
France, and near Rome and Naples in Italy. Such a lake of exceptional
interest fills part of the great crater, several thousand feet deep,
which resulted from the explosion of Mt. Katmai, Alaska, in 1912. The
water of this lake, more than a mile wide and of unknown depth, is hot.

One of the most unique and beautiful lakes of the world is Crater Lake
in the Cascade Mountains of southern Oregon. It partly fills a great,
nearly circular hole, six miles in diameter, with a maximum depth of
about 4,000 feet, in the top of a mountain (Plate 11). The lake is
over five miles in diameter and nearly 2,000 feet deep, making it the
deepest in North America. Its surface is about 6,200 feet above sea
level. Precipitous rock walls rising 500 to 2,000 feet completely
encircle the lake, the main body of whose water is of a marvelous
deep, sapphire-blue color, while the shallow portions around some of
the shore are of emerald-green. Crater Lake has very little intake
except direct rainfall and snowfall, and its water is fresh. The great
hole was not produced by an explosion like that of Katmai, but rather
by the sinking of the top of a once much greater mountain. That the
mountain was once about the size and shape of Mt. Shasta is proved
by the fact that deep glaciated valleys lead up the slopes and end
abruptly at the very rim of the present mountain. Obviously these
valleys were scoured out in recent geologic time by glaciers whose
sources were several thousand feet up on a former cone-shaped mountain.
That the mountain top sank rather than exploded is proved by the
absence of volcanic débris over the sides and base of the mountain.

Still another way by which lakes are formed by volcanic action is
by streams of lava blocking valleys. The famous Sea of Galilee in
Palestine was thus formed by a stream of lava, which geologically
recently flowed down from the east into the Jordan Valley and across
it, where it cooled to form a dam ponding the waters of the Jordan
River. Because the river flows through the lake, its water is fresh.
One of the most remarkable facts about this lake is that its surface
lies nearly 700 feet below sea level. A number of lava-dam lakes are
known in the Sierra Nevada and Cascade Mountains.

A very interesting case of a lake basin, formed by cutting off an arm
of the sea without any movement of the earth's crust, is the Salton
Sink of southern California. This basin, many miles long and wide, lies
below sea level, its lowest point being 287 feet below tide. The Gulf
of California formerly reached much farther north and into California
where it covered the site of the Salton Sink. Gradually the Colorado
River, always loaded with sediment, built a broad delta deposit right
across the gulf, the northern end of which thus became cut off,
leaving a big salt lake. But the river flowed into the gulf, while in
the dry climate the evaporation was great enough to gradually dry away
the salt lake. This was the condition of things until 1904, when much
of the river at a time of flood got out of control and, following the
general course of a great irrigating canal, it flowed for several years
into the lowest part of the Salton Sink, partly filling it to form a
lake 45 miles long, 17 miles wide, and 83 feet deep. Since 1907 the
lake has been notably decreasing in size, and it may entirely disappear.

Other ways by which lakes, mostly relatively small ones, may develop
are by landslides blocking valley drainages; by streams cutting across
winding curves leaving so-called "oxbow lakes" which are common, for
example, along the lower Mississippi River; by wave and wind action
along shores of lakes or sea; by filling so-called "sink holes" which
result from dissolving or falling in of roofs of caves; and by beavers
through whose industry dams are built across valleys or streams.

Some of the most common ways by which lakes may be destroyed are the
following: by being filled with sediment carried in by streams, or by
vegetation, or by both; by cutting down outlets; by evaporation due
to a change in climate; by removal of the ice dam in certain types of
glacial lakes; and by movements or warping of the earth's crust.



CHAPTER XII

HOW THE EARTH MAY HAVE ORIGINATED


The problem of the origin of the earth is essentially astronomical
rather than geological, because geological history is considered to
have begun when common earth processes, such as erosion, deposition,
and transportation of sediments, etc., were brought into play. It
is quite certain, however, that the earth in its pregeologic state
gradually merged into its geological condition. For this reason the
geologist is interested in the more important doctrines or hypotheses
which have been put forth to account for the origin of the earth.
In fact, one of the few hypotheses which must be taken seriously is
largely the work of a geologist. The most acceptable hypothesis not
only best satisfies the facts regarding the earth's astronomical
relationships, but also best harmonizes with our knowledge of the
oldest known rocks and their history.

Since the problem of the origin of the earth is an essential part of
the problem of the origin of the solar system, the following well-known
facts should be clearly in the mind of the reader. Eight planets,
including the earth, revolve in nearly circular paths around the
central sun, whose diameter is 866,000 miles. The radius of the solar
system is at least 2,800,000,000 miles, this being the distance of
the outermost known planet (Neptune) from the Sun. Neptune requires
164 years for a trip around the sun, while the earth, which averages
about 93,000,000 miles from the sun, makes its circuit once a year. The
planets all revolve around the sun in the same direction, and in nearly
the same plane. The sun and all eight planets rotate on their axes
in the same direction, the earth's rotation being accomplished every
twenty-four hours. Most of the planets have one or more smaller bodies
called satellites revolving about them, such as Earth, with its one
satellite (the moon), and Saturn, with its eight satellites, etc. It is
well known that this solar system is only a very small part of the vast
universe, as shown by the facts that no star is nearer the earth than
several trillion miles, and that some stars are so far away that light
traveling at the rate of 186,000 miles per second requires a thousand
years to reach the earth!

Toward the end of the eighteenth century the famous nebular or ring
hypothesis was set forth by the astronomer named Laplace. This assumes
an original very hot incandescent mass of gas spheroidal in shape and
greater in diameter than the present solar system. This mass rotated
in the direction of rotation of our sun and its planets. Loss of heat
by radiation caused the mass to shrink, and this in turn not only made
it rotate faster, but also caused the centrifugal force (i.e., the
force whose direction was from the center) in its equatorial portion
to gradually become stronger. Finally a time came when the force of
gravity (i.e., the force whose direction was toward the center) and the
centrifugal force became equal and a ring was left (not thrown) off,
while the rest of the mass of gas continued to shrink. After a time the
material of the ring collected to form the outermost planet. The other
planets were similarly formed from other rings which were left off as
contraction of the great mass of gas went on. The sun represents the
remainder of the great mass of rotating gas.

What is the bearing of this nebular hypothesis upon the early
geological history of the earth? According to the hypothesis the earth
must once have been much more highly heated and larger than now. It
condensed to a liquid and then it cooled enough to permit the formation
of a solid crust over a liquid interior. It then had a hot dense
atmosphere containing all the water of the earth in the form of vapor,
and this atmosphere steadily became thinner due to absorption by the
earth. When the pressure and temperature conditions became favorable,
much of the water vapor condensed to form the ocean and the atmosphere
gradually changed to its present condition. According to this view the
oldest rocks of the earth must have been igneous because they resulted
from the solidification of the outer part of the molten globe.

Within recent years certain serious objections to the nebular
hypothesis have been raised, and Chamberlin and Moulton have formulated
the planetesimal or spiral hypothesis as an attempt at a more rational
explanation of the origin of the solar system. Some of the objections
to the older doctrine are that among the many thousands of known
nebulæ in the universe very few only are of the Laplacian or ring
type, while spiral forms are abundant. Spectroscopic study shows
that the nebulæ are not gaseous, but made up of either liquid or
solid particles, and that the leaving off of rings would necessitate
the assumption of an intermittent process which could scarcely have
operated under the conditions of the hypothesis.

[Illustration: Plate 9.--(_a_) Molten Lava Flowing Over a Cliff Into
Water in the Hawaiian Islands. (_After Diller, U. S. Geological
Survey._)]

[Illustration: Plate 9.--(_b_) Dikes of Granite (Light Gray) Cutting
an Old Dark Rock. While the granite on the right was being forced in
molten condition upward into the earth's crust, tongues of it (dikes)
were sent off into the adjacent rock. (_Photo by Howe, U. S. Geological
Survey._)]

[Illustration: Plate 10.--(_a_) Lassen Peak, Northern California, in
Eruption August 22, 1914. The great cloud of steam and volcanic ash
rose several miles. This is the only active volcano in the United
States proper, and it is now included in Lassen Volcano National Park.
(_By permission of R. E. Stinson, Red Bluff, Cal._)]

[Illustration: Plate 10.--(_b_) Devil's Tower, Wyoming. This great mass
of rock was forced in molten condition through strata which, because
of their weakness, have been eroded away all around the hard igneous
rock. This is probably the core or neck of a former volcano. (_Photo by
Darton, U. S. Geological Survey._)]

[Illustration: Fig. 31.--Diagram showing the origin and character of a
spiral nebula according to the planetesimal hypothesis of the origin of
the solar system. (Modified after Moulton.)]

Anything like a full understanding of the planetesimal hypothesis
would be difficult to obtain, and, in the brief space at our disposal,
we shall attempt to make clear only a few of the salient points.
According to this hypothesis the solar system was, during a previous
stage of its evolution, a great, flat, spiral nebula, made up of finely
divided solid or possibly liquid particles called planetesimals,
among which were scattered some larger "knots" or masses. Each tiny
particle and larger mass or knot is considered to have traveled in
its own particular orbit or path about a central very large mass--the
future sun. It is even suggested that the spiral nebula originated by
disruption of one star by a swift-moving passing star. Each disrupted
particle and large mass at first started straight for the large passing
star, but because of change of position of the latter the particles and
larger masses were gradually pulled around so that their paths curved
into spirals. Because of crossing of paths, the larger masses or knots
gradually increased in size by accretion of the small particles or
planetesimals. Meteors (so-called "shooting stars") which now strike
the earth are thought to be disrupted materials still being gathered
in, though very slowly at present. After the passing star got well out
of range, the spiral paths of the disrupted masses gradually changed to
nearly circular, due to a wrapping-up process around the central body
(sun) which then controlled the movements of the both larger masses
(future planets) and small masses (planetesimals).

Let us now inquire briefly into the bearing of this planetesimal
hypothesis upon the early geological history of the earth. According
to this doctrine the earth was never in the form of a highly heated
gas, nor was it ever necessarily hotter than now. Instead of beginning
as a much larger body which has gradually diminished in size, the
earth steadily grew, up to a certain stage, by ingathering of
planetesimals. Increase in size caused the force of gravity to increase
and this caused not only steady contraction of the earth's matter,
but also a development of greater internal heat. The earth has been
getting smaller ever since the force of compression has predominated
over the building-up process, because of the diminishing supply of
planetesimals. Due to steadily increasing internal pressure and heat
the various gases, including water vapor, have been driven out of the
earth to form an atmosphere which has gradually become larger and
denser. After sufficient accumulation of water vapor, condensation
and rainfall took place; the waters of the earth began to gather to
form the oceans; and the ordinary geologic processes of erosion and
deposition of strata were initiated. According to this view stratified
rocks could have been formed very early in the history of the earth,
and in this connection it is interesting to note that the oldest known
rocks are actually of sedimentary origin.



CHAPTER XIII

VERY ANCIENT EARTH HISTORY

(_Archeozoic and Proterozoic Eras_)


We shall now consider the older rocks of the Earth, including those
of Archeozoic, Proterozoic, and Paleozoic ages. What are the salient
points in the very early history of the earth (not including the
evolution of organisms) shown by these very ancient rocks? Beginning
with the oldest known rocks, it will be our purpose to trace out the
principal recorded events of earth history in the regular order of
their occurrence. As in human history, so in earth history the recorded
events of very early times are fewest and most difficult of all to
understand. In spite of this difficulty it is best to begin with the
oldest known rocks or, as Le Conte has said, "to follow the natural
order of events. This has the great advantage of bringing out the
philosophy of the history--the law of evolution." Because of limitation
of space we shall give special attention to the physical history of
North America, but the general principles brought out apply almost
equally well to the other continents.

The Archeozoic rocks contain the earliest known records of geological
history, or, in other words, the oldest recorded ordinary geological
processes such as weathering and erosion, deposition of strata,
igneous activity, etc. Although we are here dealing with the most
obscure records of any great rock system, partly because the rocks
have been so profoundly altered (metamorphosed), and partly because of
the absence of anything like definitely determinable fossils, it is,
nevertheless, true that certain very important conclusions have been
reached regarding this very ancient geological era.

Among the very oldest of all known rocks of North America are the
Grenville strata, so named from a town in the St. Lawrence Valley.
In fact, no rocks elsewhere in the world have been proved to be more
ancient. The Grenville series consists of a great mass of sediments
(strata)--original muds, sands, and limes--which were deposited layer
upon layer under water (Plate 12). The widespread extent and character
of the series in southeastern Canada and the Adirondacks, and more than
likely far beyond these limits, make it certain that the Grenville
strata were accumulated on the bottom of a relatively shallow sea very
much as sediments are now piling up on shallow sea bottoms. Thus,
the most ancient definitely known condition of the region where the
Grenville strata are exposed was an expanse of the sea covering the
whole area. Wherever, in other parts of the world, the Archeozoic rocks
have been studied, stratified rocks also seem to be the very oldest
which are recognizable, but up to the present time no such rocks have
been proved to be any older than, or even as old as, the Grenville
series.

It may occur to the reader to ask, how long ago did the Grenville
ocean exist? There are grave difficulties in the way of answering
this question in terms of years since we have nothing like an exact
standard for such a measurement or comparison. Although we must concede
that not even approximate figures can be given, it can, nevertheless,
be demonstrated by several independent lines of reasoning that the
time must be measured by at least tens of millions of years, a very
conservative estimate of the minimum time which must be allowed
being about 50 million years. In any case, the time is so utterly
inconceivable to us that the important thing to bear in mind is that
the great well-known events of earth history, which have transpired
since the existence of the Grenville ocean, require a lapse of many
millions of years, as shown by revolutionary changes in geographic and
geologic conditions such as the long periods of erosion, the enormous
accumulations of sediment, the repeated spreading out and disappearance
of sea water over many portions of the earth, and the building up and
tearing down of great mountain ranges at various times. The ideas here
expressed will be much better appreciated by the reader after following
through the salient points in the history of North America as set forth
in the succeeding pages.

Again, the reader may ask, by what line of reasoning do we conclude
that these stratified rocks are so exceedingly ancient? All rocks of
Archeozoic Age, including strata as well as certain younger igneous
rocks (see below), invariably occupy a basal position in relation to
all other rock systems. They constitute a complex lot of crystalline
metamorphic rocks, combining certain characteristics which lie below
the base of the determined sedimentary succession. Where rocks with
the characteristics of the Archeozoic are separated from the oldest
Paleozoic (Cambrian) strata by the great sedimentary or metamorphic
system known as the Proterozoic (see below), we may be sure that we
are dealing with Archeozoic rocks. If the series of rocks in question
belongs in the Archeozoic system, all that remains is to determine its
age position in that system. This can usually readily be done because
wherever they have been studied the Archeozoic rocks may be subdivided
into two groups of rocks, a sedimentary and an igneous. Where the
igneous rocks, mainly granites and related types, occur associated with
the sedimentary rocks (e.g., Grenville), they very clearly were forced
or intruded, while molten, into the sedimentary rocks, thus proving
these latter to be the older.

Since the Archeozoic strata of the Adirondack Mountains, southeastern
Canada, and also all, or nearly all, other known districts are mostly
badly disturbed, tilted, and more or less bent or folded, and since
neither top nor bottom of the piles of strata has ever been recognized
as such, it is impossible to give anything like an exact figure for
the thickness of the series. Continuous successions of strata have,
however, been observed in enough places to show that they were commonly
deposited layer upon layer to a thickness of at least some tens of
thousands of feet. A thickness of over 100,000 feet has been reported
from southeastern Canada. The clear implication is that the Archeozoic
sea which received sediments must have existed for a vast length of
time which must be measured by at least some millions of years, because
in the light of all our knowledge regarding the rate of accumulation
of sediments a very long time was necessary for the piling up of such
thick masses of strata. It does not, however, necessarily follow that
the Grenville ocean was many thousands of feet deep where deposition
took place. In fact, the very character of the original sediments
(muds, sands, and limes) clearly indicates that the Archeozoic sea in
which they accumulated was, for most part at least, of shallow water
because such sediments have rarely, if ever, been carried out into
an ocean of deep water. The great ocean abysses of to-day are not
receiving any appreciable amount of land-derived sediment. Thus we are
forced to conclude that in Archeozoic time, as well as many times in
later ages, the shallow sea bottom gradually sank while the sediments
accumulated. Even more conclusive proof of such subsidence has been
obtained from the study of so-called "folded" mountain ranges of
Paleozoic and later time, an excellent example being the Appalachian
Range.

Having established the sedimentary origin and great antiquity of
the Grenville series, we are led to the interesting and important
conclusion that these oldest known rocks are not the most ancient
which ever existed, because the Grenville strata must have been
deposited layer upon layer, upon a floor of still older rocks. If such
still older rocks are anywhere exposed to view, they have never been
recognized as such. Again, the fact that the most ancient known rocks
were deposited under water carries with it the corollary that there
must have been lands at no great distances from the areas of deposition
because, then as now, such sediments as muds and sands could have been
derived only from the wear or erosion of lands, and have been deposited
in layers under water adjacent to those lands. But we are utterly in
the dark regarding any knowledge of the location or character of such
very ancient lands.

The most ancient known strata, as we see them to-day, do not look like
ordinary sediments such as shales, sandstones, and limestones. They
have been profoundly changed from their original condition, that is
to say, they have undergone metamorphism. The Archeozoic strata now
exposed to view were formerly buried at least some miles below the
earth's surface, the overlying younger rocks having since been removed
by erosion through the millions of years of time. Far below the earth's
surface, under conditions of relatively high temperature, pressure, and
moisture, the materials of the strata were completely crystallized into
various minerals. The surfaces of separation of the very ancient layers
of sediment are still usually more or less clearly present (Plate 12).
Original limestone has been changed into crystalline limestone or
marble; sandstone has been changed into quartzite, and shale, sandy
shale, and shaly sandstone have been changed into various schists and
gneisses.

In western Ontario there are also stratified rocks (called the Keewatin
series) which seem to be of about the same age as the Grenville strata
farther east. A point of special interest in connection with the
Keewatin strata is the presence of layers of lava in portions of the
series, thus proving that molten rock materials were poured out on
the earth's surface during the most ancient known era of the earth's
history.

After the accumulation of the very ancient Archeozoic sediments igneous
activity took place on grand scales when great masses of molten rock
were forced (intruded) into the sediments from below. Masses of molten
materials are known to have been thus intruded at several different
times, but of these the most common by far cooled to form a great
series of granite and closely related rocks. The general effect was
to break the old strata up into patches or masses of varying sizes as
clearly shown by the present distribution and modes of occurrence of
these igneous rocks. In most cases the strata were pushed aside by,
or tilted or domed over, the upwelling molten floods--in many cases
the molten materials were, under great pressure, intimately forced or
injected into the strata; numerous large and small masses of strata
were caught up or enveloped (as inclusions) in the molten floods; in
some cases there was local digestion or assimilation of the strata
by the molten materials, while in still other places large bodies of
strata seem to have been left practically intact and undisturbed. Such
igneous rocks, which are very widespread, are all of the plutonic or
deep-seated types; that is, they were never forced up to the earth's
surface like lavas, but they solidified at considerable depths (at
least some thousands of feet) below the surface. We see them exposed
to-day only because a tremendous amount of overlying rock materials
has been removed by erosion. These igneous rocks are generally easily
distinguished from the old sediments of Grenville age because of their
more general homogeneity in large masses, and their lack of sharply
defined bands or layers of varying composition. The fact that the
minerals have always crystallized to form medium to coarse-grained
rocks shows that these rocks solidified under deep-seated conditions,
since it is well known that surface flows (lavas) are much finer
grained commonly with more or less of the rock not crystallized
at all. Slow cooling under great pressure favors more complete
crystallization with growth of larger crystals.

As we have just learned, the very character and structure of the
Archeozoic rocks now exposed to view show conclusively that they were
formerly deeply buried, and the inference is perfectly plain that the
overlying rock materials were removed by erosion. Profound erosion of
any land mass means that the land must have stood well above sea level,
and thus we come to the important conclusion that the great mass of
Archeozoic rocks (both strata and igneous rocks) were upraised well
above sea level. Just when the uplift occurred cannot be positively
stated, but in every region where the matter has been studied it took
place before the strata of the next geological era began to deposit
as shown by the fact that such later strata rest upon the profoundly
eroded surface of the Archeozoic rocks. Such an erosion surface, called
an "unconformity," marks a gap in the geological record of the district
where it occurs. There is much to support the view that the uplift
was concomitant with the great igneous intrusions, especially the
granite. It is reasonable to believe that the same great force which
caused the welling up of such tremendous bodies of liquid rock into
the earth's crust might easily have caused a decided uplift of a whole
large region, but even so the process must have been geologically slow.
In regard to the height of those ancient lands, the character of the
topography, and the drainage lines we are as yet utterly in the dark.
The fact that many thousands of feet in thickness of materials were
removed by erosion to expose the once deeply buried rocks, does not
necessarily imply that the lands at any time had great height, because
it is possible that while elevation slowly progressed, much material
was steadily removed by erosion. In the light of our knowledge of the
origin and growth of mountain ranges of later time there is little
doubt that at least some of the Archeozoic lands were raised to such
mountain heights.

Thus far in our study of the Archeozoic rocks attention has been
mainly directed to southeastern Canada and the Adirondack mountains,
where careful studies have been made. In all parts of the world
where the most ancient known (Archeozoic) rocks have been studied in
detail the same general principles apply. Particular attention has
been given to the Archeozoic rocks south of Lake Superior, and in the
Piedmont Plateau of the eastern United States. In the accompanying
map Archeozoic rocks are widely exposed to view within the areas
shown in black. It has been estimated that Archeozoic rocks appear
at the surface over about one-fifth of the land area of the earth.
Where they are not at the surface it is believed that they everywhere
exist under cover of later rocks. In other words, Archeozoic rocks are
considered to be almost universally present either at or under the
earth's surface. This is true of the rocks of no other age. Special
mention should be made of the fine exhibitions of Archeozoic rocks in
Scandinavia and the Highlands of Scotland.

[Illustration: Fig. 32.--Map showing the surface distribution of
Archeozoic and Proterozoic rocks in North America. (Redrawn by the
author after U. S. Geological Survey.)]

All known evidence leads us to the remarkable conclusion that
the climate of much, or possibly all, of Archeozoic time was not
fundamentally different from that of to-day. There must have been
weathering of rocks, rainfall, and streams much as at present as
proved by the character and composition of the stratified rocks which
formed in that remote era. The presence of graphite ("black lead") in
crystalline flakes scattered through many of the strata shows that the
climate must have been favorable to some form of life, because graphite
thus occurring quite certainly represents the remains of organisms,
this matter being more fully discussed in a succeeding chapter. In
passing it may be stated that climatic zones were then probably
scarcely if at all marked off, as they quite certainly were not even
during Paleozoic time. One of the great contributions of geology to
human knowledge is that during the tens of millions of years from
Archeozoic times to the present the earth's climate has undergone no
fundamental change or evolution. In the earlier ages there was greater
uniformity of climate over the earth, and, during known geologic
time there have been rather localized relatively minor fluctuations
giving rise to glaciers, deserts, etc., but there has been no real
evolution of climate at all comparable to the marvelous evolution of
organisms--both animals and plants.

We shall now turn our attention briefly to a consideration of the
second great subdivision of geologic time--the Proterozoic era. Rocks
of Proterozoic Age comprise all of those which were formed after the
Archeozoic rocks and before the deposition of the earliest Paleozoic
(Cambrian) strata, these latter being rather definitely recognizable
because they contain fossils characteristic of the time. Cambrian
strata are, in fact, the oldest rocks which contain anything like
an abundance of fossils, so that the separation of rocks of either
Archeozoic or Proterozoic Age from the earliest Paleozoic is seldom
difficult. But how may we separate the Proterozoic rocks from the
Archeozoic? Fossils afford us no aid whatever, because no determinable
fossils have been found in rocks as old even as the earlier
Proterozoic. The two great groups of very ancient rocks do, however,
show a number of differences which must be considered together.
Thus, igneous rocks distinctly predominate in the Archeozoic, while
stratified rocks predominate in the Proterozoic. All Archeozoic strata
are thoroughly metamorphosed (changed from their original condition),
while large masses of the Proterozoic strata are only moderately
metamorphosed, or even unaltered, and therefore look much like ordinary
strata of later ages. Archeozoic rocks have almost invariably been
notably deformed by more or less folding, tilting, etc., while the
Proterozoic rocks show relatively much less deformation. Another
important criterion is the fact that the Proterozoic rocks, wherever
they have been studied in relation to the Archeozoic rocks, always
rest upon a profoundly eroded surface of the latter, that is, an
unconformity separates the two great sets of rocks. This erosion
surface is of still further interest because it is the very oldest one
known, none having been recognized within the Archeozoic group itself.
Even where the Proterozoic strata have been considerably metamorphosed
and deformed, this old erosion surface may be recognized, and if the
rocks below that surface possess the characteristics of the Archeozoic
rocks as described above, the two great very ancient rock groups may be
distinguished. One of the triumphs of geology during the last 25 to 30
years has been the recognition of the great rock group (Proterozoic)
between the Archeozoic and Paleozoic, thus bringing to light the
records of an era which lasted many millions of years.

The length of time represented by the Proterozoic era is by many
believed to have been fully as long as all succeeding eras--Paleozoic,
Mesozoic, and Cenozoic--combined. Twenty million years would be a
very conservative estimate for the duration of the era. What is the
nature of the evidence as recorded in the rocks which lead us to
conclude that the Proterozoic era lasted such a vast length of time?
The great thickness of Proterozoic strata (over 30,000 feet in the
Lake Superior region), in the light of what we have already learned
regarding the present rate of wear (erosion) of lands and deposition
of the eroded materials under ordinary conditions, clearly implies
millions of years of time for their accumulation. But the Proterozoic
strata as we now see them are in most places not a continuous pile,
that is they were not accumulated layer upon layer without notable
interruption. Thus, the thick Proterozoic group of the Lake Superior
region has been divided into four distinct, mainly sedimentary series
separated from each other by erosion surfaces (unconformities). Each
erosion surface represents a long time when the area was elevated and
underwent profound wear before the next series of strata accumulated on
the worn surface. That such times of erosion were geologically long is
proved not only by the profound alteration (metamorphism) of one set of
strata before another accumulated, but also by the fact that granite,
which, as we have learned, is never exposed except where much overlying
material has been eroded, actually formed parts of surfaces of earlier
Proterozoic rocks upon which later ones were deposited. In the Lake
Superior region there are not only three great erosion surfaces
(unconformities) within the Proterozoic group, but also one at the base
separating it as a whole from the Archeozoic group, and another at the
top separating it from the Paleozoic group. It is, therefore, fair to
conclude that the amount of time (millions of years) represented by
these great erosion intervals was fully as great as the time needed for
deposition of the existing Proterozoic strata.

In the Lake Superior region the older Proterozoic strata are nearly all
more or less folded and altered (metamorphosed), and they have been
intruded by considerable bodies of molten rock, mostly granite. The
later Proterozoic strata have been much less deformed and in many cases
they are practically unaltered. In this region a very remarkable event
took place in late Proterozoic time. This was volcanic activity on a
grand scale. We may gain some idea of the stupendous and long-continued
volcanic outpourings from the fact that, based upon actual measurements
of thickness, lava sheets, averaging about 100 feet thick, poured out
one upon another until a pile about six miles high had accumulated.

In parts of the Grand Canyon of the Colorado tilted Proterozoic
strata may be seen resting upon the profoundly eroded surface of the
Archeozoic rocks of the inner gorge. The Proterozoic strata, 12,000
feet thick, consist of practically unaltered sandstones, shales, and
limestones, associated with some layers of basaltic lava. An erosion
surface (unconformity) separates the whole group into two distinct
series, and the group is separated from the overlying nearly horizontal
Paleozoic (Cambrian) strata in the walls of the Canyon by another
erosion surface.

More recently the Proterozoic strata so finely displayed in the
Rocky Mountains of Montana and southern Canada have been studied.
These strata, at least two or three miles thick, are mostly
unaltered sandstones, shales, and limestones, associated with some
metamorphic and igneous rocks. As usual, these strata rest upon
the eroded Archeozoic. They were more or less upturned and folded
before deposition of the succeeding Paleozoic strata. Satisfactory
subdivisions have not yet been worked out.

In North America most of the areas shown on the accompanying map
contain more or less Proterozoic rocks. Rocks of this age are known to
some extent in all continents where their general relationships seem
to be much like those of North America. They have perhaps been most
carefully studied in Scandinavia and the Highlands of Scotland, where
the strata portions are about two miles thick.

The climate of Proterozoic time must, for most part, have been about
like that of to-day except, of course, for its much greater uniformity
over the earth. About a dozen years ago very typical glacial deposits
were discovered within the early Proterozoic rocks of western Ontario,
Canada. A climatic condition favorable for the development of glaciers
so early in the history of the earth is, to say the least, directly
opposed to an idea (based upon the nebular hypothesis) long held that
the climate of early geologic time must have been much warmer than that
of the present.



CHAPTER XIV

ANCIENT EARTH HISTORY

(_Paleozoic Era_)


Beginning with the earliest Paleozoic, the legible records of events
of earth history are far more abundant and less defaced than those
of earlier times. Stratified rocks of the ordinary kinds greatly
predominate over the igneous and metamorphic rocks, and the strata
are in general far less disturbed than those of the Archeozoic and
Proterozoic groups. From the earliest Paleozoic we have also the first
abundant records (fossils) of the life of the earth, so that the
ordinary methods of subdividing and determining the relative ages of
the Paleozoic and later strata, as well as correlating the subdivisions
(formations) in widely separated regions, can be used. From here on in
our discussion of earth history we shall be able to trace the salient
features of the changing outlines of the face of the earth, the coming
and going of the seas over the lands, and the evolution of animals and
plants with a considerable degree of definiteness and satisfaction.

First, we shall trace out, in the regular order of their occurrence,
the main physical history events of Paleozoic time, leaving a
consideration of the evolution of life for other chapters. Because
of limitation of space, our attention will be almost wholly centered
upon the continent of North America, but the reader should bear in
mind that the general principles and facts set forth apply with about
equal force to most other continents. In Europe the wonderful records
of Paleozoic history are found in strata, whose estimated maximum
thickness is about 100,000 feet! It must not be thought, however, that
all these strata are piled up in a single locality, but the figure does
actually represent the sum total of the greatest thickness of the many
subdivisions (formations) of the Paleozoic rocks in different portions
of the continent. In North America the maximum thickness of all
Paleozoic rocks seems to be no less than 50,000 feet. More than 25,000
feet of strata may actually be observed piled layer upon layer in the
highly folded and deeply eroded central Appalachian Mountains. The
great thickness of the strata, combined with the facts that the fossils
show that many marvelous, mostly progressive, changes took place among
living things, that seas came and went repeatedly over many parts of
the continent, and that great changes took place in the configuration
of the land, force us to conclude that Paleozoic time must have lasted
for many millions of years.

Just before the opening of the Paleozoic era practically all of North
America appears to have been dry land, which had undergone so much
erosion that it was low and far less rugged in relief than at present.
This we know, because the rather widespread early Paleozoic (Cambrian)
strata almost everywhere rest upon deeply eroded rocks of either
Archeozoic or Proterozoic age. Considering both the time involved and
the wide area affected, we have no record of anything like such a
profound erosion interval since the beginning of the Paleozoic era. It
seems that the constructive or upbuilding forces within the earth were
then remarkably quiescent, while the destructive forces (erosion) were
almost unhampered in their work of cutting down the land.

Have we any definite idea of the relations of land and water in North
America during the first or Cambrian period of the Paleozoic era? In
the affirmative answer to this question, certain principles will be
brought out which the reader should keep in mind as we trace out the
succeeding great physical changes in the history of North America.
It should, however, be remembered that, in the brief space at our
disposal, only the most general, or the most significant localized,
physical changes in the long and intricate known history of the
continent since the opening of the Paleozoic era can be brought out.

In early Cambrian time a narrow arm of the sea (like a strait) extended
from the Gulf of St. Lawrence southward across eastern New York and
over the site of the present Appalachian Mountains connecting with the
Gulf of Mexico on the south. On the west, a much larger and broader arm
of the sea (like a mediterranean) extended from Alaska southward over
the site of the Rocky Mountains of Canada and across the sites of the
Columbia Plateau to Great Basin of the western United States. All the
rest of the continent was land, apparently almost or wholly devoid of
high mountains.

By what process of reasoning do we conclude that arms of the early
Cambrian sea reached across eastern and western North America? First,
wherever marine strata of definitely determined early Cambrian age now
occur, the early Cambrian sea must have existed because those strata
were obviously deposited in that sea. Second, to those areas we must
add others from which it can be demonstrated that early Cambrian marine
strata have been removed by erosion. Enough field work along these
lines has been done in North America to render it practically certain
that the relations of land and water during early Cambrian time were
essentially as above outlined.

[Illustration: Fig. 33.--Map showing the relations of land and water
in North America during early Cambrian time, at least 25,000,000 years
ago. Lined areas represent land. (Principal data from a map by Willis
published in the Journal of Geology.)]

[Illustration: Fig. 34.--Map showing the relations of land and water
in North America during Middle Ordovician time. Lined areas represent
land. (Principal data from a map published by Willis in the Journal of
Geology.)]

As Cambrian time went on, the marine waters gradually spread from
south to north across most of the Mississippi Valley area, causing the
eastern and western arms of the sea to be connected, thus forming an
interior continental sea. Otherwise the relations of land and water
were much as in early Cambrian time. We know that the sea transgressed
northward across the Mississippi Valley district because, on the
south, the whole Cambrian system of strata (lower, middle, and upper)
is present, while, farther north, only middle and upper Cambrian are
present, and, farthest north, only upper Cambrian strata occur. This
progressive northward overlap of younger and younger (later) Cambrian
strata upon the old rock floor proves, that the Cambrian sea steadily
spread farther and farther northward over the Mississippi Valley area.
That this spreading sea was shallow is amply demonstrated by the
deposits it left, such as shales, conglomerates (i.e., consolidated
gravels) and sandstones, often ripple-marked. The Cambrian strata of
North America vary in thickness from less than 1,000 feet to about
12,000 feet.

In the Mississippi Valley the Cambrian strata are unaltered and almost
undisturbed from their original horizontal position. In the Appalachian
Mountains of the east, and the Rocky Mountains of the west, the strata
are commonly notably folded and faulted. In some places, as in western
New England, the strata have been notably altered (metamorphosed).

The best estimates for the duration of the Cambrian period range from
2,000,000 to 3,000,000 years. It is a remarkable fact that, during this
great lapse of time, North America was unaffected by any great physical
disturbances such as mountain making, emergence of large tracts of
land, or igneous activity. The one great physical event of the Cambrian
was the gradual submergence of a considerable portion of the continent.

That the climate of the earliest Cambrian was at least locally
favorable for the existence of glaciers, is proved by the occurrences
of true glacial deposits in rocks of that age in China, Norway, and
Australia. It is a remarkable fact that the glacial materials of China
occur along the Yangtse River, thus demonstrating that conditions for
glaciers then existed at a latitude as far south as New Orleans. These
evidences of glaciation directly refute the old idea, based upon the
nebular hypothesis, that the climate of the Paleozoic was distinctly
warmer than now. The glacial evidence, added to our knowledge of the
character and world-wide distribution of many identical species of
animals, leads us to conclude that early Paleozoic climate was not
essentially different from that of very recent geologic time, but that
the climate was then much more uniform than at present.

During the second or Ordovician period of the Paleozoic era, the
progressive submergence of Cambrian time continued until a climax
was reached toward the middle of the period when fully four-fifths
of the continent was submerged under shallow sea water. Since middle
Ordovician marine strata are more widespread than the rocks of any
succeeding age, we can be reasonably sure that so much of the continent
was never again covered by the sea. In fact, so far as the records
have been interpreted, this came nearest to being a universal flood in
the whole known history of the continent. By the very character of the
rocks deposited (seldom over a few thousand feet thick), we can be sure
that the middle Ordovician continental sea was everywhere far shallower
than the great ocean abysses of to-day. Because the lands were so low
and restricted, relatively little land-derived sediment washed into the
sea. But the shallow sea water was inhabited by millions of animals,
the shells of many of which slowly accumulated to build up the thick
bodies of limestone strata (Plate 14) which constitute the main bulk of
rock of early and middle Ordovician age. The famous Trenton limestone,
named from a locality in central New York, with its great abundance of
fossils, was formed mostly by the accumulation of shells of animals
during middle Ordovician time.

Later in the Ordovician there was a considerable shift in level between
land and water causing a withdrawal of much of the widespread sea. As
a result of the generally more elevated lands, erosion proceeded more
vigorously, and sands and muds were more abundantly deposited in the
restricted sea, these sediments having consolidated to form the shales
and sandstones which predominate among the upper Ordovician rocks.

A principle above briefly explained in the discussion of the Cambrian
may be reemphasized here. It is as follows: In making a map to show the
relations of land and water, say during middle Ordovician time, the
geologist is by no means dependent only upon actual surface exposures
of middle Ordovician strata. Such exposures fall far short of giving an
adequate conception of the former or even present real extent of such
strata. In many places originally present Ordovician strata have been
removed by erosion. An excellent case in point is the Adirondack region
of northern New York. On the west side of the Adirondacks a great pile
of marine Ordovician strata 1,500 feet thick end abruptly on the gently
sloping flank of the mountains, thus clearly proving that the strata
formerly extended at least twenty to thirty miles eastward. Again, in
the southern Adirondacks a small area of very typical marine middle
Ordovician strata lies fully fifteen miles from the general area of
such rocks to the south. This small body of rock is very clearly only
an erosion remnant of a general sheet of middle Ordovician rock which
once covered the whole intervening district. In many other regions
the middle Ordovician strata are definitely known to be concealed
under cover of later rocks, as in the Mississippi Valley, where the
actual surface exposures constitute only a fraction of the middle
Ordovician strata which underlie nearly all the valley, as proved by
deep well drillings, study of the scattering outcrops, etc. In still
other places, middle Ordovician strata, associated with other rocks,
are highly folded, as in the Appalachians, where such strata outcrop
in only narrow belts following the trend of the folds. In short, then,
wherever it can be proved that middle Ordovician marine strata are
visible at the surface, or are concealed under other rocks, or were
once present, we can be sure that the middle Ordovician sea existed.
Exactly this principle applies to any subdivision of geologic time.

[Illustration: Fig. 35.--Structure section showing rocks representing
three geologic eras separated by millions of years of time. Length
of section 12 miles, vertical scale much exaggerated. At the bottom
are Archeozoic (Precambric) rocks and resting upon them on the left
are early Paleozoic strata 1,500 feet thick. A glacial lake deposit
of late Cenozoic age lies on the Archeozoic rock toward the right. It
is evident that the Paleozoic strata formerly extended much farther
eastward. (By the author as published in a New York State Museum
Bulletin.)]

The Ordovician period closed with a great mountain-making disturbance
in eastern North America, and at the same time all, or nearly all, of
the continent was land. Throughout most of the Cambrian and Ordovician
periods, the strata accumulated to a thickness of thousands of feet in
the marine waters which spread over the eastern border of New York,
the sites of the Green Mountains of Vermont, the Berkshire Hills of
Massachusetts, and southward at least as far as Virginia, over the area
of the Piedmont Plateau. At, or toward the close of the Ordovician
period, a great compressive force in the earth's crust was brought to
bear upon the mass of strata and they were tilted, highly folded, and
raised above sea level into a great mountain range known to geologists
as the Taconic Range. It is quite the rule throughout this region of
Taconic disturbance to find the strata either on edge or making high
angles with the plane of the horizon. Many of the folds were actually
overturned, and in some cases notable thrust faults developed, that is,
the upper strata broke across and great masses were shoved over each
other. These facts all go to show that the mountain-making compressive
force applied to the region was of rather an extreme type. Since the
origin of the Taconic Range a tremendous amount of erosion has taken
place, so that literally only the roots of the range are now exposed in
the Green Mountains, Berkshire Hills, Highlands-of-the-Hudson, and the
northern Piedmont Plateau.

How do we know that the Taconic disturbance took place toward the close
of the Ordovician period? By way of answer to this question two facts
need to be considered. First, relatively late (or young) Ordovician
strata are involved with the folds, thus proving that the folds
formed after those late Ordovician sediments were deposited. Second,
undisturbed strata formed during the middle of the next (Silurian)
period, rest upon the eroded edges of the folds, which proves that the
folds must have developed well before middle Silurian time because the
only time they were subjected to erosion must have been during early
Silurian time.

[Illustration: Fig. 36.--Structure section showing profile
and underground relations of the rocks across part of the
Highlands-of-the-Hudson region in southeastern New York. Length of
section, sixteen miles. The rocks are mostly of Prepaleozoic Age, but
with belts of highly infolded early Paleozoic strata toward the middle
right. (After Berkey, New York Museum Report.)]

Mention should also be made of the profound metamorphism (alteration)
of the Cambrian and Ordovician strata along the main axis of the range,
where the intense compression, aided by heat and moisture, caused the
deeply buried portions of the strata to become plastic, and hence
they became more or less foliated (cleavable) and crystallized into
various metamorphic rock types, the limestone having changed to marble,
the shale to slate or schist, and the sandstone to quartzite. Thus
we explain the rocks of the extensive marble quarries of Vermont and
western Massachusetts, the slate quarries of central eastern New York,
and the Berkshire schist of the Berkshire Hills of Massachusetts.

One of the grandest and most significant of all the profound geological
processes is the birth and history of a great folded mountain range.
Since the Taconic Range affords us such an excellent example of a
large-scale, well-understood folded range of great antiquity we may do
well to consider it in the light of certain other broad relationships.
The great compressive force which folded and upraised the Taconic
Mountains did not accomplish its work rapidly in the ordinary human
history sense of the word. The force was slowly and irresistibly
applied, and the strata well below the surface were gradually bulged
or folded, or fractured where near the surface, the length of time
required for the operation having been, at the very least calculation,
some hundreds of thousands of years, and more than likely a million
years or more. Such a length of time is, however, so short compared
with all known earth history, that we are accustomed to refer to the
formation of such a mountain range as simply an event of geological
history.

Even before such a range attains its maximum height a very considerable
amount of erosion has already taken place. When the first fold appears
above sea level, erosion begins its work and continues with increasing
vigor as the mountain masses get higher and higher. Thus we have
warfare between two great natural processes--the building up and the
tearing down. After a time the building-up process wanes and then
ceases, while the tearing-down process (erosion) continues either
until the whole range has been completely worn down or until some
rejuvenating force causes a renewed uplift. Here is an example of one
of the remarkable procedures of nature. After millions of years of work
causing the deposition of thousands of feet of strata, piled layer upon
layer on the sea floor, a force of lateral pressure is brought to bear
and a mountain range is literally born out of the sea. No sooner is the
range well formed than the destructive processes (erosion) unceasingly
set to work to destroy this marvelous work. But the sediments derived
from the wear of the range are carried into the nearest ocean again to
accumulate and, perchance, after long ages, to be raised into another
range; and so the process may be often repeated. From this we learn
that the mountain ranges of the earth are by no means all of the same
age. The original Adirondacks were formed long before the Taconics,
which originated millions of years before the Appalachians, these
latter having been folded up long before the Sierras. The Rockies,
followed by the Coast Ranges, are each younger than the Sierras as
regards their original folding and uplift. Among foreign countries
special mention should be made of the British Isles, where Ordovician
strata thousands of feet thick were, late in the period, notably folded
and upraised, the crustal disturbance having been accompanied by great
intrusions of molten rocks and vast outpourings of lavas, so that
this region ranks among the greatest of the ancient volcanic areas of
Europe.

[Illustration: Fig. 37.--Map showing the general relations of land and
water in North America during middle and late middle Devonian time
fully 15,000,000 years ago. (After Willis, courtesy of the Journal of
Geology.)]

We shall now turn our attention to a very brief consideration of the
salient points in the physical history of North America during the
next great period (Silurian) of the Paleozoic era. As a result of
the physical disturbance late in the Ordovician the great interior
sea was largely or wholly expelled from the continent, and this was
essentially the condition of the continent at the beginning of the
Silurian. But this condition was of short duration, for early in the
Silurian the sea again began to spread, gradually increasing in extent
to a climax in about the middle of the period. At this time the famous
and extensive Niagara limestone, so named from the rock at the crest
of Niagara Falls, was deposited. Except for the newly formed Taconic
Range, standing out as a bold topographic feature along the middle
Atlantic Coast, and a somewhat wider extent of land, the condition of
the continent during middle Silurian time was very similar to that of
middle Ordovician time.

[Illustration: Fig. 38.--Map showing the general relations of land and
water in North America during middle Mississippian time. (After Willis,
courtesy of the Journal of Geology.)]

Soon after mid-Silurian time the seas became greatly restricted almost
to disappearance as such. In the eastern United States and southeastern
Canada strata of that particular age are found only in parts of
Ontario, New York, Ohio, Michigan, and from Pennsylvania southward
to West Virginia, where they are characterized by red shales and
sandstones, and salt and gypsum deposits. Such materials containing
few fossils very clearly indicate deposition in either extensive
lagoons or more or less cut-off arms of the sea under arid climate
conditions rather than in ordinary marine water.

Still later in the Silurian the interior seas were partially restored,
as shown by the fact that true marine strata corresponding to that
age not only cover the salt and gypsum deposits, but are notably more
extensive than they. About the close of the Silurian period almost all
of the continent was dry land.

Unlike the Ordovician period, the Silurian closed without any
mountain-making disturbance or great uplift of land. The Silurian
period, like the preceding Ordovician and Cambrian, seems to have been
free from any more than slight igneous activity as, for example, in
Maine and New Brunswick. The total thickness of Silurian strata in
North America is seldom more than a few thousand feet.

The salient features of the physical history of the next, or Devonian
period, are much like those of the preceding Silurian. At the beginning
of the Devonian almost all of the continent was dry land, but soon a
long, narrow arm of the sea extended across the eastern side of the
continent from the Gulf of St. Lawrence southward through western
New England, southeastern New York and throughout the Appalachian
district, thus reminding us of the long, narrow sound which occupied
almost exactly the same territory during the early part of the Cambrian
period. In the west the only water was a small embayment reaching
across southern California into Nevada. By middle Devonian time these
water areas had considerably expanded. During relatively late Devonian
time the sea was so expanded as to cover much of the Mississippi
Valley area, the Appalachian Mountains and St. Lawrence Valley areas,
and most of the site of the Rocky Mountains, except for an island
of considerable size reaching from New Mexico through Wyoming. The
main lands were most of northeastern North America, a large land area
extending from Florida to Nova Scotia, and a large area on the western
side of the continent from California to Alaska.

A remarkable formation of late Devonian Age should be briefly
described. In southeastern New York and the northern Appalachian
region there was a tremendous accumulation of sediments which have
consolidated into sandstone, together with some shale and conglomerate.
This so-called "Catskill" formation is from 1,500 to 8,000 feet thick
and is well shown as the main body of rock in the Catskill Mountains.
It is largely a shallow-water deposit of essentially nonmarine origin,
as proved by coarseness of material, ripple marks, and nonmarine
fossils. All evidence points to the origin of this remarkable formation
as a great delta deposit built out into the shallow interior sea.
Notable thinning toward the west, with increasing fineness of grain
of material, shows that the sediment came from the east, no doubt
carried by a large river from the small continental land mass (called
"Appalachia") on the eastern side of North America.

The maximum thickness of the North American Devonian seems to be about
15,000 feet in the northern Appalachian region, but elsewhere it
generally ranges from 1,000 to 4,000 feet thick. In North America the
subdivisions of the Devonian strata of New York are taken as a standard
for comparison, both because of the wonderful completeness and almost
undisturbed character of the rocks there, and because they have been
so carefully studied. The Devonian system is there fully 4,000 feet
thick, with scarcely a minor subdivision missing, and it covers a wide
area (one-third of the State) with many excellent outcrops. There was
practically uninterrupted deposition of Devonian strata in southern
New York. It is doubtful if there is greater refinement of knowledge
regarding the Devonian or any other Paleozoic system of strata anywhere
else in North America.

During middle to late Devonian time the region from southern New
England to Nova Scotia and the St. Lawrence Valley was notably
disturbed by earth movements, the lands having been considerably
elevated and the rocks more or less folded. The great delta deposit of
late Devonian time, already described as being thousands of feet thick
in New York and Pennsylvania, was formed by one or more streams which
carved much sediment from the newly upraised lands. Accompanying the
uplift and folding of the rocks considerable masses of molten granite
were forced into the earth's crust and some molten rock was forced to
the surface, producing volcanoes. Much of the granite may now be seen
at the surface in various portions of the region, while deeply eroded
volcanoes occur near the city of Montreal.

Except for the disturbance of the region from New England to the St.
Lawrence, the Devonian period seems to have closed rather quietly, with
fairly widespread sea water over the land as already outlined. This is
proved by the fact that the early strata of the next period mostly rest
in regular order upon the undisturbed late Devonian strata.

For many years the term "Carboniferous" period was used to
designate a single period of geologic time which, in America at
least, is now divided into two periods--the Mississippian and
Pennsylvanian--corresponding, respectively, to the earliest and latest
Carboniferous. In regard to the relations of land and water during the
Mississippian period, the general statement may be made that the sea,
already fairly extensive in the late Devonian, continued to spread
until during the second half of the Mississippian, when most of the
United States west of the eastern border of the Appalachians (except
the Pacific Northwest), and also the Rocky Mountain region through
Canada, were submerged.

A significant physical change marked the close of the Mississippian.
This was the withdrawal of sea water from nearly all of the continent,
the emergence of the land having been generally sufficient to allow
considerable erosion. The fact that the Mississippian and the next,
or Pennsylvanian, strata are separated by the most extensive distinct
erosion surface in the whole Paleozoic group of rocks is the chief
reason for considering those two sets of strata to have formed during
separate periods of geologic time.

In eastern North America the Mississippian strata vary in thickness
from a few hundred feet to a maximum of about 5,000 feet in eastern
Pennsylvania. In the West, where the thickness is commonly several
thousand feet, limestone greatly predominates. There appears to have
been vigorous volcanic activity during the period from northern
California to Alaska.

Certain profound crustal disturbances marked the close of the period in
western Europe, resulting in upturning and folding of rocks during the
process of mountain forming from Ireland to Germany, and from Bohemia
to southern France. Abundant intrusions and extrusions of molten rocks
accompanied the disturbances.

We turn next to a consideration of the Pennsylvanian period, which is
of very special interest, because within the rocks of that age in North
America, Europe, and China occur the greatest known coal deposits. The
period opened with almost all of North America dry land undergoing
more or less erosion. Early in Pennsylvanian time marine water began
to overspread the western side of the continent, especially most of
the western two-thirds of the area of the United States, where strata
thousands of feet thick piled up. The sea was most widespread before
the middle of the period, when the relations of land and water were
about as shown by Figure 39.

Over the site of the Appalachians and most of the eastern half of the
Mississippi Valley area the land either stood near sea level and was
often swampy or marshy, or at other times it was a little below sea
level, allowing tidewater to overspread the area. Such conditions
alternated repeatedly, usually more or less locally, over different
parts of the districts in which the great coal mines of the east are
located. Under such conditions strata from 1,000 to 8,000 feet thick
accumulated. Remarkable physical geography of this kind resulted in the
growth and accumulation of vast quantities of vegetable matter which
has changed into the world's greatest coal beds. Similar conditions
prevailed over parts of Nova Scotia, New Brunswick, and Rhode Island,
where strata fully 13,000 feet thick accumulated.

"Perhaps the most perfect resemblance to coal-forming condition is
that now found on such coastal plains as that of southern Florida
and the Dismal Swamps of Virginia and North Carolina. Both of these
areas are very level, though with slight depressions in which there
is either standing water or swamp condition. In both regions there is
such general interference with free drainage that there are extensive
areas of swamp, and in both there are beds of vegetable accumulations.
In each of these areas there is a general absence of sediment and
therefore a marked variety of vegetable deposit. If either of these
areas were submerged beneath the sea, the vegetable remains would be
buried and a further step made toward the formation of a coal bed.
Reelevation, making a coastal plain, would permit the accumulation of
another coal bed above the first, and this process might be continued
again and again." (H. Ries.) But it is not necessary to assume repeated
oscillations of a swamp area up and down as the only way of accounting
for a succession of coal beds one above another in a given region,
because a general, but intermittent, subsidence, with possibly some
upward movements, would occasionally cause the prolific plant life
of a swamp to be killed, after which sediment would deposit over the
site. Shoaling of water by accumulation of sediment would permit the
development of more swamp plant life.

[Illustration: Fig. 39.--Map showing the general relations of land and
water, including the great coal-plant swamp areas (vertical lines), in
North America during the Pennsylvanian period at least 10,000,000 years
ago. Lined areas represent land. (After Willis, courtesy of the Journal
of Geology.)]

In most coal-mining districts there are at least several coal beds, one
above another. In Illinois there are nine; in Pennsylvania at least
twenty; in Alabama, thirty-five; and in Nova Scotia seventy-six, but
not all are important commercially. Each coal bed in such a region
represents a swamp which existed in Pennsylvanian time at least ten
or twelve million years ago, and in which there grew a luxuriant
vegetation. Many individual swamps of that time were of wide extent.
The famous Pittsburgh bituminous coal bed represents probably the
largest one of all. It extends from western Pennsylvania into parts
of Ohio and West Virginia over an area of fully 15,000 square miles.
More than 6,000 square miles of it are being worked and the coal bed
averages seven feet in thickness over an area of 2,000 square miles.
Among the various anthracite coal beds of the same age in eastern
Pennsylvania the Mammoth bed is exceptionally thick, reaching a maximum
of fifty feet or more.

In order that the reader may not gain the impression that coal beds
make up a very considerable bulk of the strata in coal-mining regions,
we should state that, on the average, coal actually constitutes less
than 2 per cent of the containing strata.

Some idea of the tremendous length of the geologic ages may be gained
by a consideration of the time which must reasonably be allowed for
the accumulation of so many coal beds and their containing strata. It
has been estimated that a luxuriant growth of vegetation would produce
100 tons of dried organic matter per hundred years. Compressed to
the specific gravity of coal (1.4) this would form a layer less than
two-thirds of an inch deep on an acre. During the chemical alteration
of vegetable matter to coal about four-fifths of the organic matter
disappears in the form of gases. On this basis, then, it would take
about 10,000 years to accumulate the vegetable matter represented in
a coal bed one foot thick. When we realize that the total thickness
of the coal beds of the Pennsylvanian system of strata in the great
mining regions is commonly from 100 to 250 feet, we conclude that the
time they represent is from 1,000,000 to 2,500,000 years. It seems
most reasonable that the time necessary for the deposition of the
containing strata must have been at least as long. It is, therefore, a
fair conclusion that the Pennsylvanian period lasted from 2,000,000 to
5,000,000 years.

That the climate of the great Coal Age was warm (not tropical), very
moist, and uniform, is borne out by such facts as the following,
according to D. White: The succulent nature of the plants with their
spongy leaves indicates prolific growth in moist, mild climate; lack of
yearly rings of growth points to lack of distinct seasons; as in the
case of many existing plants the aerial roots signify a warm, moist
climate; plants of to-day nearest like the coal plants thrive best
in warm, moist regions; vegetable matter at present accumulates best
in temperate rather than tropical climates, because there decay is
not so rapid; and the remarkable uniformity of climate over the earth
is clearly indicated by finding fossil plants of almost or exactly
identical types in rocks of Pennsylvanian Age from the Polar regions to
the Tropics. The more remarkable plants of the great Coal Age time are
described in the chapter on the evolution of plants (Plate 15).

During the last (Permian) period of the Paleozoic era the marine waters
of the west, and the alternating shallow tidewater, swamps, and near
sea level lands of the east gradually gave way to dry lands, so that
by the close of the period marine water covered only a small part of
the Southwest from Oklahoma across central Texas to southern California
and northwestern Mexico, where strata as much as several thousand feet
thick formed. In the middle western part of the area of the United
States, especially from northern Texas to Nebraska and Wyoming, the
climate was arid and red strata (so-called "Red Beds"), salt, and
gypsum were extensively deposited on land and in great salt lakes or
more or less cut-off arms of the sea. Strata commonly from 2,000 to
7,000 feet thick were there deposited. Similar conditions prevailed
in parts of Nova Scotia, New Brunswick, and Newfoundland, where strata
8,000 feet thick accumulated. Over the site of most of the Appalachians
the coal swamp conditions, with local sea incursions, continued from
the preceding period, as shown by the character of the strata (1,000
feet thick) containing some coal.

Vigorous volcanic activity which, as already mentioned, began in the
Mississippian period from northern California to Alaska continued not
only through the Pennsylvanian and Permian but also into the early
Mesozoic era, as shown by the great quantities of volcanic materials
associated with rocks of those ages.

The Permian presents a puzzling combination of climatic conditions
which causes it to stand out in marked contrast against the generally
mild and uniform climates of nearly all of preceding Paleozoic time.
Most remarkable of all are the records of a great Ice Age during early
Permian time. One surprising fact is the widespread distribution of the
glacial deposits in both the north and south temperate zones, and even
well within the torrid zone. They are perhaps most extensive and best
known in Australia, South Africa, India, and Brazil. Glacial deposits
almost certainly of the same age on smaller scales occur in eastern
Massachusetts, southern England, eastern Russia and the Caucasus
region. Although the areas occupied by the Permian glaciers, which in
many cases must have been extensive ice sheets, cannot be accurately
delimited, it is, nevertheless, quite certain that the ice was notably
more extensively developed than it was during the great "Ice Age"
of late (Quaternary) geologic time. Another surprising fact is that
certain of the glaciers must have come down to, or nearly to, sea
level, as shown by the direct association of marine strata with glacial
deposits. Thus, in southern Australia at least eight beds of glacial
materials (some of them 100 to 200 feet thick) occur within true marine
strata 2,000 feet thick. A third remarkable fact is that the Permian
Ice Age, like the Quaternary Ice Age, had interglacial epochs of
relatively mild climate, as proved by the occurrences of beds of coal
between certain of the layers of glacial materials in Australia, South
Africa, and Brazil.

During much of Permian time the climate was arid over large areas as,
for example, much of the western interior of the United States, from
Ireland to central Germany, and in eastern Russia, as proved by great
deposits of salt, gypsum, and red sediments. During late Permian time
the greatest salt beds in the world were deposited in northern Germany,
a well near Berlin having penetrated a practically solid body of salt
associated with certain potash and magnesia salts to a depth of about
4,000 feet without reaching the bottom.

The occurrence of some coal beds, especially in the earlier Permian
rocks shows that, temporarily at least, climatic conditions must have
favored luxurious growths of coal-forming plants in South Africa,
Brazil, Australia, and our own Appalachian district.

From the above facts we see that the Permian represents a remarkable
combination of very extensive glaciation, widespread aridity, and
warmth and moisture favorable to prolific plant growth all in a single
period of geologic time.

The Permian period, and, therefore, the great Paleozoic era, was
brought to a close by one of the most profound physical disturbances
in the known history of North America. This has been called the
Appalachian Revolution because at that time the Appalachian Mountain
range was born out of the sea by folding and upheaval of the strata.
In fact, "the Appalachian Revolution was one of the most critical
periods in the history of the earth, and may have been the greatest
of them all in its results." (C. Schuchert). Mountains were brought
forth in all the continents, including Australia. All of the mountains
which were formed late in the Paleozoic have since been profoundly
affected by erosion, and the only ones (e.g., Appalachians) which now
show considerable altitudes are those which have been rejuvenated by
relatively (geologically) recent earth movements.

We shall now turn our attention to the origin of the Appalachian
Range. All through the vast time (probably fully 20,000,000 years) of
the Paleozoic era a large land mass was remarkably persistent along
the eastern side of North America. This land, which has been called
"Appalachia," had its western boundary approximately along the eastern
border of the sites of the Appalachian Range and the western part of
New England. It extended east of the present coast line at least to the
border of the continental shelf from 100 to 200 miles out. Concerning
the actual altitude and topography of Appalachia we know little or
nothing, but the tremendous quantities of sediment derived from its
erosion show that it was high enough during nearly all of its history
to undergo vigorous erosion.

Barring certain minor oscillations of level, the region just west
of Appalachia was mostly occupied by sea water throughout much of
Paleozoic time, and sediments derived from the erosion of Appalachia
were laid down layer upon layer as strata upon that sea bottom. In
general, the coarsest and greatest thickness of sediments accumulated
relatively near the land, while finer materials, in thinner sheets,
deposited well out over much of the eastern Mississippi Valley area
in the shallow seas which were there so commonly present. By actual
measurement we know that the thickness of strata deposited over the
site of the Appalachians was at least 25,000 feet. Since these latter
strata are mostly of comparatively shallow sea-water origin, as proved
by coarseness of grain of material, ripple marks, fossil coral reefs,
etc., we are forced to conclude that this marginal sea bottom gradually
sank while the process of sedimentation was in progress. Otherwise
we cannot possibly explain the great pile of strata of shallow water
origin. The very weight of accumulating strata may either have aided or
actually caused the sinking of the long, relatively narrow trough.

Finally, toward the close of the Paleozoic era, sinking of the marginal
sea floor and deposition of sediments gave way to a yielding of the
earth's crust by a great force of lateral compression, causing the
strata to be thrown into folds well below the surface and more or less
fractured in their upper portion. Thus, along the eastern side of the
site of the great interior Paleozoic sea, the Appalachian Mountains
rose out of what for millions of years had been a long, narrow, sinking
sea floor. There was more or less folding from the Gulf of St. Lawrence
to central Alabama. Figure 24 diagrammatically represents the principal
stages in the history of the Appalachian Range.

While the most pronounced earth disturbance occurred through the
long Appalachian belt, the whole eastern side of the continent was
profoundly affected. Thus the Mississippi Valley area east of the Great
Plains was considerably upraised never again to be submerged except
along the Gulf Coast, and an eastern interior sea has never since
overspread the region which was repeatedly sea-covered during Paleozoic
time.



CHAPTER XV

MEDIEVAL EARTH HISTORY

(_Mesozoic Era_)


What was the condition of North America during the first or Triassic
period of the Mesozoic era, approximately 8 or 10 million years ago?
As a result of the Appalachian Revolution the sea was excluded from
all the land except along much of the western side from southern
California to parts of Alaska. On this western side of the continent
the Appalachian Revolution had little or no effect and the Permian
conditions continued, essentially without change through the Triassic.
The Triassic strata up to 4,000 feet thick are there of typical marine
origin. In British Columbia and Alaska there was much igneous activity.

Throughout much of the Rocky Mountains and Great Plains region of the
western United States there are extensive deposits of red sediments
(so-called "Red Beds"), containing layers of salt and gypsum, from 200
to 1,000 or more feet thick. These strata commonly rest in regular
order on Permian Red Beds, so that conditions of deposition of Permian
time continued through Triassic time, that is continental deposits
formed mostly in salt lakes, fresh lakes, along stream courses, and on
land in part by the action of wind.

[Illustration: Fig. 40.--Map showing the general relations of land
and water in North America during the Triassic period. Lined areas
represent land; vertical-lined areas, basins in which continental
deposits formed. (Based upon map by Willis; courtesy of the Journal of
Geology.)]

In the eastern half of North America there is no record of accumulation
of any marine strata whatever, because, as a result of the Appalachian
Revolution, the land was brought well above sea level. There was,
however, deposition of a remarkable series of nonmarine strata in
several long, narrow, troughlike depressions whose trend was parallel
to, and just east of, the main axis of the newly formed Appalachian
Range. These troughs lay between the Appalachians and the very
persistent old land mass called Appalachia which we have already
described. The facts that these troughs are truly down-warps; that
they so perfectly follow the trend of the Appalachian Mountain folds;
and that the strata in them are of late Triassic Age, make it certain
that they were formed by a great lateral pressure which must have been
a continuation of the Appalachian Revolution. Thus the Appalachian
Mountains continued to grow well into the Triassic period, and, while
the Paleozoic strata were being folded, the surface of old Appalachia
(including part of the Taconic Mountain region) was down-warped to
form the troughs in which the late Triassic strata accumulated. One
trough extended through the Connecticut Valley; another (the largest)
from southeastern New York through northern New Jersey, southeastern
Pennsylvania, Maryland, and into Virginia; while several smaller ones
occurred in Virginia and North Carolina.

The down-warps or troughlike basins were very favorably situated
for rapid accumulation of thick sedimentary deposits because of
their position just between large, high land masses which were being
vigorously eroded. The sediments derived from the erosion of the young
Appalachians were especially abundant because of the vigorous wearing
down of the newly formed high mountains. A thickness of from 5,000
to fully 15,000 feet of mostly red sandstones and shales accumulated
in these down-warps, the character and great thickness of the strata
strongly pointing to gradual down-warping as the deposition of the
sediments went on. It is often stated that these strata were formed in
estuaries, but, in the northern areas, at least from Massachusetts to
Maryland, many of the layers show ripple marks, sun cracks, rain-drop
pits, fossil plants, and fossil bones and tracks of land reptiles. Such
strata may well have formed in very shallow water, such as river-flood
plains or temporary lakes, where changing conditions frequently allowed
the surface layers to lie exposed to the sun.

[Illustration: Fig. 41.--Block diagram of the region westward from
New York City and vicinity, showing the main relief features, the
underground relations of rocks of widely different ages, and the
relation of the relief to the rock formations. (Part of larger drawing
by A. K. Lobeck.)]

During the time of the accumulation of the late Triassic strata in the
down-warp basins there was considerable igneous activity, as proved by
the occurrence of sheets of igneous rock within the body of strata. In
some cases true lava flows with cindery tops were forced out on the
surface and then buried under later sediments, while in other cases
the sheets of molten rock were forced up either between the strata or
obliquely through them, thus proving their intrusive character. As a
result of subsequent erosion, these very resistant lava masses often
stand out conspicuously as relief features. Perhaps the most noteworthy
example is the great layer of such intrusive igneous rock, part of
which outcrops for seventy miles mostly as a bold cliff forming the
famous Palisades of the Hudson, near New York City. During the process
of cooling and solidification of the molten mass there was contraction
which expressed itself by breaking the rock mass into great, crude,
nearly vertical columns, and hence the origin of the name "Palisades."
The cliff character of the outcrop is due to the fact that the lava
is much more resistant to erosion than the sandstone above and below
it. In the Connecticut Valley of Massachusetts a layer of lava several
hundred feet thick boldly outcrops, forming the crest of the well-known
Mount Tom-Mount Holyoke Range.

The close of the Triassic period was marked by enough uplift to leave
the whole eastern two-thirds of the continent dry land undergoing
erosion. The Triassic deposits of the Atlantic Coast are much broken up
into large fault blocks, and this faulting probably took place as a
result of the crustal disturbances toward the end of the period. In the
west the Triassic conditions seem to have continued without much change
into the next (or Jurassic) period.

During the Jurassic period the relations of land and water in North
America were very simple. In the earlier Jurassic all was dry land
except portions of the western fringe of the continent from southern
California to Alaska, where marine strata 2,000 to 10,000 feet thick
accumulated. Late in the period the conditions were the same, except
for a long, narrow arm of the sea or mediterranean which extended from
the Arctic Ocean southward across the site of the Rocky Mountains to
Arizona. There is no evidence for the existence of anything like real
mountains anywhere on the continent during the period.

[Illustration: Fig. 42.--Structure section showing profile and
underground relations of rocks across the Connecticut Valley (through
Mount Tom) of Massachusetts. Js and Jl are sandstone strata, with
included lava sheets (in black) resting upon Paleozoic rocks on either
side. The rocks have been notably tilted and faulted. (After Emerson,
U. S. Geological Survey.)] Profound crustal disturbances marked the
close of the Jurassic period in the western part of the continent.
Strata which had accumulated to great thickness during millions
of years of time, mainly over the sites of the Sierra Nevada and
Cascade Mountains, finally yielded to a tremendous force of lateral
compression, especially in the Sierra region, and were folded,
crumpled, and upraised. Thus the Sierra-Cascade district was originally
built up into a high mountain range. Since that time the Sierras
have been much cut down by erosion and they have been rejuvenated
by faulting and tilting of the great earth block. The Cascade Range
from northern California into British Columbia was apparently not so
profoundly raised, and its present height is mainly due to subsequent
volcanic activity. The rocks of the Klamath Mountains of northwestern
California, and of the Humboldt Range of Nevada, were also folded at
that time.

During the mountain-making disturbances on the western side of the
continent great quantities of molten granite were forced up into the
lower portions of the folding strata. Because of profound subsequent
erosion this granite is now widely exposed as, for example, in the
great walls of the Yosemite Valley.

During the earlier half of the last period (the Cretaceous) of the
Mesozoic era, sea water spread from Mississippi northwestward to the
site of Denver and southward over Texas and much of Mexico. At the
same time much of the western margin of the continent from Alaska to
California was submerged. All the rest of the continent was land.
During this time sediments accumulated on low lands just east of the
site of the present Rocky Mountains, and also east of the Appalachians,
as proved by the numerous fossils of land plants found in these
deposits.

[Illustration: Fig. 43.--Map Showing the general rotations of land
and water in North America during later Cretaceous time, several
million years ago. Lined areas represent land; vertical lines, mainly
continental deposits. (Principal data from a map by Willis, published
in the Journal of Geology.)]

As Cretaceous time went on the marine waters gradually spread until
the whole Atlantic and Gulf coastal plain regions from Long Island,
New York, to Mexico became submerged under marine water, and a wide
arm of the sea, or great mediterranean, spread from Texas north to the
mouth of the Mackenzie River. The Gulf of Mexico was thus directly
connected with the Arctic Ocean. This great interior sea was nowhere
connected with the Pacific Ocean, though portions of the Pacific
border of the continent were submerged. This vast interior sea was not
only the largest of any which reached well into the continent since
the Mississippian period of the Paleozoic era, but it was the last
body of marine water which ever extended well into the continent. It
should be stated that the later Cretaceous was also a time of unusually
widespread submergence of the continents, when most of southern Europe
and southeastern Asia, as well as about one-half of both Africa and
South America were submerged. Over much of the site of the Rocky
Mountains during the late Cretaceous there were low lands receiving
continental deposits, and extensive marshes supporting prolific
vegetation were common. Much of this vegetable matter became buried,
and has since been converted into workable coal.

The maximum thickness of strata accumulated during all of Cretaceous
time over the Atlantic coastal plain area was about 1,700 feet; over
the Gulf coastal plain region fully 7,500 feet; over the western
interior 10,000 to 15,000; and over parts of the Pacific border 25,000
to 30,000 feet, as in California. The last-named figures are truly
phenomenal, representing a thickness about equal to the total thickness
of all the strata accumulated during the whole Paleozoic era (seven
periods) and piled up in the Appalachian Mountain region. This great
deposit of strata of mostly early Cretaceous Age is readily accounted
for when we realize that these sediments, which accumulated in the
marginal sea bottom, were derived from the very rapidly eroding, newly
formed lofty Sierra Nevada Range.

[Illustration: Fig. 44.--Sketch of a mountain range along Skolat Creek,
Alaska, showing Tertiary lava beds resting upon deeply eroded tilted
limestones and lavas of late Paleozoic (Carboniferous) Age. The present
topography has been produced by erosion since the Tertiary lavas flowed
out. (After U. S. Geological Survey.)]

Especially in Alabama and Texas the Cretaceous system is remarkable for
its richness in chalk deposits. In Alabama a widespread formation of
late Cretaceous Age, about 1,000 feet thick, contains much nearly pure
white chalk, and in Texas a similarly constituted formation of early
middle Cretaceous Age is from 1,000 to 5,000 feet thick. These chalk
deposits consist almost wholly of carbonate of lime shells or very
tiny single-celled animals which accumulated under exceptionally clear
sea water which spread over those parts of Alabama and Texas where the
chalk now occurs. Here again we have a bit of evidence supporting the
fact of very long geologic time. Think of how long it must have taken
for the tiny (even microscopic) shells to form a widespread layer of
chalk nearly a mile thick!

The close of the Cretaceous period, or what is the same, the close
of the Mesozoic era, was marked by some of the grandest crustal
disturbances in the known history of the earth. In fact, it is not
known that the western hemisphere was ever affected by more profound
and widespread mountain-making disturbances than those which took
place toward the close of the Mesozoic era, and continued into the
succeeding Tertiary period. These disturbances were of three kinds:
folding of strata, volcanic activity, and renewed uplift of old
mountains without folding of the rocks. Greatest of all was the "Rocky
Mountain Revolution," during which the thick strata, which accumulated
during the Paleozoic and Mesozoic eras over the site of the Rockies,
yielded to vigorous deformation when they were more or less folded
and dislocated from Alaska to Central America. This was in truth the
birth of the Rocky Mountains, although their existing altitude and
configuration have, to a very considerable degree, resulted from later
uplift and erosion. In the northern United States and southern Canada
the Rocky Mountain strata, up to over 40,000 feet thick, were most
severely folded and fractured, forming a range which quite certainly
was fully 20,000 feet high. In this district a great thrust fault,
hundreds of miles long, developed, and rocks as old as the Proterozoic
were shoved at least seven miles, and probably as much as twenty
miles, westward, over Cretaceous and other rocks much later than the
Proterozoic. At the same time the Andes Mountains throughout South
America were notably upraised and the rocks folded.

The second type of physical disturbance was volcanic activity which
took place on a tremendous scale, and which appears to have started as
a direct accompaniment of the Rocky Mountain Revolution. This igneous
activity took place not only in the Rocky Mountains but also westward
to and in the Sierra-Cascade Range, as well as in the mountains of
western British Columbia and Alaska. This activity continued well into
the succeeding Cenozoic era, and it is more fully considered in the
next chapter.

The third type of crustal disturbance took place on a large scale when
the Appalachian Mountains, which had been almost wholly planed away by
erosion during Mesozoic time, were reelevated from 1,000 to 3,000 feet
by an uplifting force not accompanied by folding. All or nearly all
of New York and New England, as well as much of southeastern Canada,
were similarly upraised at the same time. This notable uplift of so
much of eastern North America is a matter of great importance because
the major relief features of that area have been produced by erosion
or dissection of the upraised surface since late Mesozoic or early
Cenozoic time. In view of the fact that this work of erosion took
place almost wholly during the Cenozoic era, it will be discussed in
the next chapter.

In conclusion, brief mention may be made of the kind of climate of the
Mesozoic era. As shown by the character and distribution of fossil
plants and animals, the Mesozoic climate was in general mild and rather
uniform over the earth, but with some distinction of climatic zones.
Such distinction of climatic zones is unknown for the Paleozoic era,
while it was notably less than at present.



CHAPTER XVI

MODERN EARTH HISTORY

(_Cenozoic Era_)


Since the Cenozoic era is the last one of geologic time, it will be of
particular interest to trace out the main events which have led up to
the present day conditions, especially in North America. Both because
of the recency of the time and the unusual accessibility of the rocks,
which are mostly at or near the surface, our knowledge of the Cenozoic
era is exceptionally detailed and accurate. It will, therefore, be more
necessary than ever to select only the very significant features of
this history for our brief discussion.

During the first half of the Tertiary period portions only of the
Atlantic coastal plain were submerged under shallow water, but soon
after the middle of the period (Miocene epoch) the sea spread over
practically the whole Atlantic coastal plain area from Martha's
Vineyard south to and including Florida. During the late Tertiary the
marine waters had become greatly restricted, and by the close of the
period the sea was entirely excluded from the Atlantic seaboard. The
total thickness of these Tertiary strata is less than 1,000 feet, and
they all tilt downward gently toward the sea. The strata consist mostly
of unconsolidated sands, gravels, clays, marls, etc.

The Gulf coastal plain area from Florida through Texas and south
through eastern Mexico was largely overspread by the sea during most
of Tertiary time, except the latest. During early Tertiary time an arm
of the Gulf reached north to the mouth of the Ohio River. Late in the
period but little of the Gulf Plain was submerged, and at its close sea
water was wholly excluded. On the Gulf Coast the Tertiary strata from
2,000 to 4,000 feet thick are also mainly sands, gravels, clays, and
marls. They are commonly rich in fossils, and they show a gentle tilt
downward toward the Gulf.

Throughout Tertiary time local portions of the Pacific border of
the continent were submerged, this having been especially true of
portions of California, Oregon, and Washington. In spite of the very
restricted marine waters, the Tertiary strata of the Pacific Coast,
especially in California, are remarkably thick, 10,000 to 20,000 feet
being common, while the maximum thickness is fully 30,000 feet. Such
great thicknesses are readily explained when we realize that erosion
was notably speeded up by pronounced uplifts resulting from crustal
disturbances toward the close of the preceding period, and again in the
midst of the Tertiary period itself.

To summarize the Tertiary relations of sea and land for North America
we may say that only local portions of the continental border ever
became submerged, and that, by late Tertiary time, practically the
whole continent was a land area. At the close of the period the
continent was, as we shall see, even larger than now because the
continental shelves of the ocean were then also largely above water.

[Illustration: Fig. 45.--Map showing the general relations of land
and water in North America during part of the middle Tertiary period.
(After Willis, courtesy of the Journal of Geology.)]

The whole of the Cenozoic era, including both the Tertiary and
Quaternary periods, has been a time of profound crustal disturbances
throughout much of the continent, certain of these movements having
continued right up to the present time, with positive evidence that
some of them are still continuing. These great movements have included
notable foldings of strata, uplifts without folding, faulting, and
igneous activity, the whole effect having been to greatly increase
the general altitude and ruggedness of the continent. In fact, North
America is not known ever to have been at once higher, broader, and
more rugged than it was very late in the Tertiary, or early in the
Quaternary, period. Since that time the only notable change (barring
the great Ice Age and its effects) has been a restriction of the area
of the continent to its present size by spreading of sea waters over
the borders of the continent, that is over the continental shelves.

We shall now rather systematically consider the more profound earth
changes which have affected the continent, producing the existing major
relief features, from west to east.

The "Coast Range Revolution" took place in the midst of the Tertiary
period. Over the site of the Coast Ranges, strata had accumulated,
especially during Cretaceous and earlier Tertiary times, to a
thickness of thousands of feet. In middle Tertiary time these strata
were subjected to a mountain-making force of compression and more or
less folded, faulted (fractured), and uplifted into the Coast Range
Mountains. Some portions of the range were intensely folded and faulted
and upraised many thousands of feet, while other portions were only
moderately folded and uplifted. It is an interesting fact that the
great San Francisco earthquake rift or fault originated at this time.
It was a renewed, sudden movement of a few feet along this fault which
caused the disastrous earthquake of 1906. Still other considerable
earth movements took place in the Coast Range region during late
Tertiary and Quaternary times, as, for example, uplift without folding,
as proved by distinct sea-cut terraces at altitudes of more than a
thousand feet, like those north of San Francisco and south of Los
Angeles. A moderate amount of still later subsidence has caused the
development of San Francisco Bay. The large islands off the coast
of southern California have in very recent geologic time (probably
Quaternary) been cut off from the mainland by sinking of the land.

[Illustration: Plate 11.--(_a_) Part of the Mammoth Hot Springs Terrace
in the Yellowstone Park. The view shows the deposit with boiling water
flowing over it. The water enters the earth back on the mountain,
travels underground in contact with hot lava, rises through limestone,
from which the boiling water takes into solution much carbonate of
lime which is deposited when the water reaches the surface. (_Photo by
Jackson, U. S. Geological Survey._)]

[Illustration: Plate 11.--(_b_) View Across Part of Crater Lake,
Oregon. This great hole, 3,000 to 4,000 feet deep and 6 miles in
diameter and now partly filled with a lake 2,000 feet deep, was formed
by a subsidence of the top of a once great cone-shaped volcano fully
14,000 feet high above the sea. The high rock in the distance rises
2,000 feet above the lake which is over 6,000 feet above sea level. The
island is a small volcano of recent origin. (_Photo by Russell, U. S.
Geological Survey._)]

[Illustration: Plate 12.--(_a_) Detailed View of Part of the Very
Oldest Known (Archeozoic) Rock Formation of the Earth. The rock is
distinctly stratified and represents sands and muds deposited layer
upon layer upon a sea floor at least 50,000,000 years ago. The sands
and muds first consolidated into sandstone and shale below the earth's
surface. Then, under conditions of heat, moisture, and pressure, they
were notably altered, mainly by crystallization of minerals, and raised
high above sea level. Finally the strata were laid bare by erosion.
(_Photo by the author._)]

[Illustration: Plate 12.--(_b_) A Twisted Mass of Stratified Archeozoic
Limestone Surrounded by Granite in Northern New York. The limestone was
enveloped in the granite while it was being forced in molten condition
into the earth's crust. (_Photo by the author._)]

The Sierra Nevada Range, which originated by intense folding of rocks
late in the Jurassic period, underwent profound erosion until about
the middle of the Tertiary period, by which time it had been cut down
to a range of hills or low mountains. Then the great fault (fracture)
previously described began to develop along the eastern side. As a
result of many sudden movements along this fault, which is hundreds of
miles long, the vast earth block has been tilted westward with a very
steep eastern face and a long, more gradual western slope, the crest of
the fault block forming the summit of the range. The amount of nearly
vertical displacement along this fault has been commonly from 10,000
to 20,000 feet, and, in spite of considerable erosion of the top of
the fault block and accumulation of sediment at its eastern base, the
modified fault face now usually stands out boldly from 2,000 to 10,000
feet high. As an evidence that this movement of faulting has not yet
ceased we may cite the Inyo earthquake of 1872, when there was a sudden
renewal of movement of ten to twenty-five feet along this fault for
many miles. Since the great Sierra block began to tilt, the many mighty
canyons, like Yosemite, Hetch-Hetchy, King's River, and Feather River,
have been carved out by the action of streams, in some cases aided by
former glaciers. King's River canyon has been sunk to a maximum depth
of 6,900 feet in solid granite solely by the erosive action of the
river!

The Cascade Mountains, too, were reduced to nearly a peneplain
condition by late Tertiary time when they began to be rejuvenated by
arching or bowing of the surface unaccompanied by great faulting or
fracturing, and many canyons, like that of the Columbia River, have
since been carved out.

Mention should now be made of the vigorous volcanic activity which took
place in the Cascade and Sierra Nevada Ranges. Most of this activity
occurred during Tertiary time (particularly in the latter part) and it
has continued with diminishing force practically to the present time.
In California streams of lava buried many gold-bearing river gravels
which have yielded rich mines. Many well-known mountain peaks, such
as Shasta, Lassen, Pitt, Hood, and Rainier, from northern California
to Washington, are great volcanic cones which date from Tertiary
time, and which are now mostly inactive. That this volcanic activity
has not yet altogether ceased is shown by renewed eruptions of Mount
Lassen (or Lassen Peak, altitude 10,437 feet) in northern California.
Since the beginning of this renewed activity in 1914, several hundred
outbursts have occurred. No molten rock has flowed out, but large
quantities of rock fragments, dust and steam have been erupted, in many
cases forming great clouds two or three miles high over the top of the
mountain (Plate 10). At this writing (October, 1920), Mount Lassen is
still showing vigorous activity. At Cinder Cone, only ten miles from
Mount Lassen, there were two eruptions of cinders and a considerable
outpouring of lava within the last 200 years. Still other very recent
cinder cones occur in southeastern California and Arizona.

[Illustration: Fig. 46.--Sketch map showing the distribution of
volcanic rocks of Cenozoic (mostly Tertiary) Age in western North
America. Only one volcano (Mount Lassen, California) is now active
in the United States proper, but a number are more or less active
in Mexico and Central America. (Data from Willis, U. S. Geological
Survey.)]

One of the greatest lava fields in the world forms the Columbian
Plateau between western Wyoming (including the Yellowstone National
Park) and the Cascade Mountains from northeastern California to
northern Washington. It covers fully 200,000 square miles and is really
considerably larger than shown on the map because the lava in parts of
the plateau region are covered by very recent sedimentary materials.

The great lava fields of the Deccan, India, and of the plateau region
of western Mexico are comparable in size to the Columbian field and
these lava fields are all of the same age. In the Columbian Plateau
most of the lava was poured out during later Tertiary time. Sheets of
molten rock, averaging fifty to one hundred feet in thickness, spread
out over various parts of the region and piled up by overlapping
layers one over another until the lava plateau more than a mile high
was built up. Many hills and low mountains were completely buried
under the molten floods, and in other places the liquid rock masses
flowed against the higher mountains. "For thousands of square miles
the surface is a lava plain which meets the boundary mountains as a
lake or sea meets a rugged and deeply indented coast.... The plateau
was long in building. Between the layers are found in places old soil
beds and forest grounds and the sediments of lakes.... So ancient are
the latest floods in the Columbia River Basin that they have weathered
to a residual yellow clay from thirty to sixty feet in depth, and
marvelously rich in the mineral substances on which plants feed. In
the Snake River Valley the latest lavas are much younger (Quaternary).
Their surfaces are so fresh and undecayed that here the effusive
eruptions may have continued to within the period of human history."
(W. H. Norton.) Many of the lava layers are plainly visible where the
Columbia River has cut its great gorge or canyon. The Snake River in
places has sunk its channel several thousand feet into the lava plateau
without reaching underlying rock.

Both north and south of the Columbian Plateau there was also much
volcanic activity in the Rocky Mountain region during Tertiary time. A
single formation in Colorado consists mostly of volcanic "ash" or dust
over 2,000 feet thick. There was also much volcanic activity over the
Colorado Plateau area of southern Utah, New Mexico, and Arizona. The
volcanoes there exhibit all stages from those which are very recent and
practically unaffected by erosion to others which have been completely
cut away with the exception of the cores or "volcanic necks."

During the second half of the Tertiary period the whole region known
as the Great Basin, between the Sierra Nevada Mountains of California
and the Wasatch Mountains of Utah, began to be affected by profound
faulting or fracturing and tilting of portions of the earth's crust.
The two largest faults, one on the western side of the Wasatch Range
and the other on the eastern side of the Sierra Range, are each
hundreds of miles long. Each of these ranges owes most of its present
altitude to the uptilting of great fault blocks, and most of the many
nearly north-south Basin Ranges of Nevada and Utah are in reality
recently tilted fault blocks.

Turning now to the Colorado Plateau, studies have shown that region
to have been more or less periodically raised fully 20,000 feet since
the beginning of Tertiary time, but because of profound erosion in
the meantime its present altitude is only 6,000 to 9,000 feet. During
late Tertiary time the land stood at a much lower level than to-day,
so that, practically during the last period (Quaternary) of geologic
time, the region has been elevated to its present position. As a
direct result of this profound rejuvenation the Colorado River has
had its erosive activity tremendously increased, and it has carved
out the mightiest of all existing canyons--the Grand Canyon. The work
of deepening and widening the canyon is still proceeding at a rapid
geologic rate.

As we have learned, the Rocky Mountains and many of its subsidiary
ranges were formed by folding and uplift of strata toward the close
of the Mesozoic era (Cretaceous period). During much of Tertiary time
the newly formed mountains had been considerably reduced by erosion.
Then, late in the Tertiary period, much of the Rocky Mountain region,
as well as much of the Great Plains area just east of the mountains,
became rejuvenated by differential uplift without any notable folding
of strata. We can tell that this general uplift amounted to at least
several thousand feet because definite formations of relatively late
Tertiary strata, originally horizontally deposited under inland bodies
of water, gradually rise so that at the base of the Front Range of
the Rockies they are fully 3,000 feet higher than they are 200 miles
or more farther east. Thus, the original folding and faulting of the
Rockies, Tertiary volcanic activity, late Tertiary rejuvenation, and
subsequent erosion account for the present altitude and relief features
of the great Rocky Mountain system.

Portions of the rejuvenated Great Plains region have been notably
dissected by erosion since the late Tertiary, this being particularly
true of the so-called "Bad Lands," especially in parts of Wyoming and
South Dakota, where mostly relatively soft Tertiary strata have been
cut to pieces.

Turning our attention now to the eastern half of the continent we find
that all, or nearly all, of it was more or less raised toward the
close of the Tertiary period. Practically the whole Mississippi Valley
east of the Great Plains, as well as much of the country to the north
in Canada, was elevated some hundreds of feet and the streams have
since the late Tertiary uplift (except where the land was ice-covered
during the Ice Age) been at work sinking their channels below the newly
upraised surface.

As already pointed out, the lowlands of the Atlantic and Gulf Coastal
Plains were mostly submerged under the sea during early middle Tertiary
time. By the close of the period they had emerged practically to their
present positions, and they have been only moderately affected by
erosion.

We have still to explain the existing topography or relief of a large
and important part of eastern North America, including the whole of
the Appalachian Mountains, Allegheny Plateau, Piedmont Plateau, New
York, New England, and the Canadian region to the north. As a starting
point in this discussion we should recall the fact that, after the
great Appalachian Mountain Revolution toward the close of the Paleozoic
era, the predominant geologic process which affected the region under
consideration was erosion throughout the succeeding Mesozoic era. By
about the close of the Mesozoic (Cretaceous period) the whole region,
with some local exceptions, has been worn down to a comparatively
smooth plain (peneplain) not far above sea level. Local exceptions
were mainly in the New York and New England region as, for example,
some of the higher parts of the Adirondack and White Mountains, Mount
Monadnock in southern New Hampshire, and Mount Greylock in western
Massachusetts. These and other masses rose rather conspicuously above
the general level of the great plain of erosion commonly called the
"Cretaceous peneplain" because it is believed to have been well
developed by the close of that period.

The uplift of the vast Cretaceous peneplain about the beginning of
the Cenozoic era (Tertiary period) was an event of prime importance
in the recent geological history of eastern North America because
it was literally the initial step in bringing about nearly all of
the existing major relief features of the Appalachian-New York-New
England-St. Lawrence region. The amount of uplift (unaccompanied by
folding) of the peneplain was commonly from a few hundred to a few
thousand feet with the greatest amount in general along the main trend
of the Appalachians. The fact should be emphasized that nearly all the
principal topographic features of the great upraised region have been
produced by dissection (erosion) of the uplifted peneplain surface.
Thus nearly all the valleys, small and large, including those of the
St. Lawrence, Hudson, Mohawk, Connecticut, and Susquehanna, have been
carved out by streams since the uplift of the great peneplain.

The streams which flowed upon the old low-lying peneplain surface
meandered sluggishly over deep alluvial or flood-plain deposits, and
their courses were little if any determined by the character and
structure of the underlying rocks, because, with few exceptions, all
rocks were worn down to the general plain level. The uplift of the
peneplain, however, caused great revival of activity of erosive power
by the streams, the larger ones of which soon cut through the loose
superficial alluvial deposits and then into the underlying bedrock.
Thus the large, original streams had their courses well determined in
the overlying deposits, and when the underlying rocks were reached
the same courses had to be pursued entirely without reference to the
underlying rock character and structure. Such streams are said to be
"superimposed" because they have, so to speak, been let down upon and
into the underlying rock masses. As Professor Berkey has well said:
"The larger rivers, the great master streams, of the superimposed
drainage system, in some cases were so efficient in the corrosion of
their channels that the discovery of discordant structures (in the
underlying rocks) has not been of sufficient influence to displace
them, or reverse them, or even to shift them very far from their
original direct course to the sea. They cut directly across mountain
ridges because they flowed over the plain out of which these ridges
have been carved, and because their own erosive and transporting power
have exceeded those of any of their tributaries or neighbors."

Fine examples of such superimposed streams which are now entirely
out of harmony with the structure of regions through which they flow
are the Susquehanna, Delaware, and Hudson. Thus the Susquehanna cuts
across a whole succession of Appalachian ridges while, in accordance
with the same explanation, the Delaware cuts through the Kittatiny
range or ridge at the famous Delaware Water Gap. The ridges are
explained as follows: while the great master streams were cutting deep
trenches or channels in hard and soft rock alike, numerous side streams
(tributaries) came into existence and naturally mostly developed
along belts of weak, easily eroded rock parallel to geologic (folded)
structure. Thus the Appalachian valleys have been, and are being,
formed, while the ridges represent the more resistant rock formations
which have more effectually stood out against erosion. The lower Hudson
River flows at a considerable angle across folded formations above the
Highlands, after which it passes though a deep gorge which it has cut
into the hard granite and other rocks of the Highlands. The simple
explanation is that the Hudson had its course determined upon the
surface of the upraised Cretaceous peneplain, and that it has been able
to keep that course in spite of discordant structure and character of
the underlying rocks. In a similar manner we may readily account for
the passage of the Connecticut River through a great gap in the Holyoke
ridge or range of hard lava in western Massachusetts.

Before leaving this part of our discussion we shall briefly present
some evidence showing that the New York-New England-St. Lawrence region
at least must have been considerably higher shortly before the Ice Age
(Quaternary period). An old channel of the Hudson River has been traced
about 100 miles eastward beyond the present mouth of the river and it
forms a distinct trench under the shallow sea in the continental shelf.
Even in the Hudson Valley, many miles above New York City, the bedrock
bottom of the river lies hundreds of feet (near West Point, 800 feet)
below sea level. Obviously this submerged channel must have been cut
when the land in the general vicinity of New York City was fully 1,000
feet higher than at present. That the land thus stood higher late in
the Tertiary and possibly early in the Quaternary periods is proved as
follows: (1) because most of Tertiary time must have been needed for
the river to erode such a deep valley after the initial uplift of the
peneplain about the beginning of the period; and (2) because glacial
deposits of Quaternary age filled the former channel to a considerable
depth. The valleys of the coast of Maine, and the submerged lower St.
Lawrence Valley (Gulf of St. Lawrence), in a similar way lead us to
conclude that the region farther north was also notably higher just
before the Ice Age.

In the eastern hemisphere early in the Tertiary period a great
submergence set in and marine waters spread over much of western and
southern Europe, northern Africa, and southern Asia. The sites of the
Himalayas, Alps, Pyrenees, Apennines and other mountains were then
mostly submerged. A very remarkable marine deposit, made up almost
wholly of carbonate of lime shells of a single-celled animal called
Nummulites, formed on the floor of this vastly expanded early Tertiary
mediterranean. This rock attains a thickness of several thousand feet.
It is doubtful if any other single formation made up almost entirely of
the shells of but one species is at once so widespread and thick. In
the Alps this remarkable marine deposit may be seen 10,000 feet above
sea level, and in Tibet fully 20,000 feet. Much of the rock in the
Egyptian pyramids was quarried from this formation.

Later in the Tertiary in Eurasia and Africa the marine waters
gradually became very restricted, so that by the close of the period
the relations of land and sea were not strikingly different from
the present, although northwestern Europe, like northeastern North
America, was notably higher just before the Ice Age than it is to-day.

Eurasia witnessed tremendous crustal disturbances during the middle and
later Tertiary time when, due to intense folding and uplift of great
zones, the Himalayas, Caucasus, Alps, Pyrenees, Apennines, and other
great ranges were formed. The crustal disturbance was most remarkable
in the region of the Alps, where the movement resulted in "elevating
and folding the Tertiary and older strata into overturned, recumbent,
and nearly horizontal folds, and pushing the southern or Lepontine Alps
about sixty miles (over a low angle fault fracture) to the northward
into the Helvetic region. Erosion has since carved up these overthrust
sheets, leaving remnants lying on foundations which belong to a more
northern portion of the ancient (early Tertiary) sea. Most noted of
these residuals of overthrust masses is the Matterhorn, a mighty
mountain without roots, a stranger in a foreign geologic environment."
(C. Schuchert.)

The last period of geological time--the Quaternary--was ushered in
by the spreading of vast sheets of ice over much of northern North
America and northern Europe, and this ranks among the most interesting
and remarkable events of known geological time. On first thought the
former existence of such vast ice sheets seems unbelievable, but the
Ice Age occurred so short a time ago that the records of the event
are perfectly clear and conclusive. The fact of this great Ice Age
was discovered by Louis Agassiz in 1837, and fully announced before
the British Scientific Association in 1840. For some years the idea
was opposed, especially by advocates of the so-called iceberg theory.
Now, however, no important event of earth history is more firmly
established, and no student of the subject ever questions the fact of
the Quaternary Ice Age.

Some of the proofs of the former presence of the great ice sheet
are as follows: (1) polished and striated rock surfaces which are
precisely like those produced by existing glaciers, and which could not
possibly have been produced by any other agency; (2) glacial bowlders
or "erratics" which are often somewhat rounded and scratched, and
which have often been transported many miles from their parent rock
ledges (Plate 20); (3) true glacial moraines, especially terminal
moraines, like that which extends the full length of Long Island and
marks the southernmost limit of the great ice sheet; and (4) the
generally widespread distribution over most of the glaciated area of
heterogeneous glacial débris, both unstratified and stratified, which
is clearly transported material and typically rests upon the bedrock by
sharp contact.

The best known existing great ice sheets are those of Greenland and
Antarctica, especially the former, which covers about 500,000 square
miles. This glacier is so large and deep that only an occasional high
rocky mountain projects above its surface, and the ice is known to
be slowly moving outward in all directions from the interior to the
margins of Greenland. Along the margins, where melting is more rapid,
some land is exposed, and often the ice flows out into the ocean where
it breaks off to form large icebergs.

[Illustration: Fig. 47.--Map of North America showing the area buried
under ice during the Great Ice Age of the Quaternary period; the three
great glacial centers; and the extent of mountain glaciers in the west.
(After U. S. Geological Survey.)]

The accompanying map shows the area of nearly 4,000,000 square miles
of North America covered by ice at the time of maximum glaciation, and
also the three great centers of accumulation and dispersal of the ice.
The directions of flow from these centers have been determined by the
study of the directions of many thousands of glacial scratches on rock
ledges. The Labradorean (or Laurentide) glacier spread out 1,600 miles
to the south to Long Island and near the mouth of the Ohio River. The
vast Keewatin glacier sent a great lobe of ice nearly as far south,
that is into northern Missouri. "One of the most marvelous features of
the ice dispersion was the great extension of the Keewatin sheet from
a low flat center westward and southward over what is now a semiarid
plain, rising in the direction in which the ice moved, while the
mountain glaciers on the west (Cordilleran region), where now known,
pushed eastward but little beyond the foot-hills." (Chamberlin and
Salisbury.)

The Labradorean and Keewatin ice sheets everywhere coalesced except in
two places. One of these is an area of about 10,000 square miles mostly
in southwestern Wisconsin. In spite of several ice invasions during
the Ice Age, this area, hundreds of miles north of the southern limit
of the ice sheets, was never ice-covered. There is a total absence
of records of glaciation within this area, and so we here have an
excellent sample of the kind of topography which prevailed over the
northern Mississippi Valley just before the advent of the ice. A much
smaller, nonglaciated area occurs in northeastern Missouri near the
southern limit of ice extension.

The Cordilleran ice sheet was the smallest of the three, and it was
probably not such a continuous mass of ice, the higher mountains
projecting above its surface. A surprising fact is that neither
this ice sheet nor any other overspread northern Alaska, which is
well within the Arctic Circle, during the Ice Age. More than likely
the temperature was low enough, but precipitation of snow was not
sufficient to permit the building up of a great glacier.

At the same time that nearly 4,000,000 square miles of North America
were ice-covered, about 600,000 square miles of northern Europe
were buried under ice which spread from the one great center over
Scandinavia southwest, south, and southeast over most of the British
Isles, well into Germany, and well into Russia.

In both North America and Europe the high mountains, well south of the
great glacier limits, especially the Sierras, Rockies, Alps, Pyrenees,
and Caucasus, supported many large local glaciers in valleys which now
contain none at all or only relatively small ones.

Records of glaciation, such as glacial scratches, bowlders, lakes,
etc., occur high up in the White and Green Mountains, Adirondacks,
Catskills, and the Berkshire Hills, thus proving that the ice must
have been at least some thousands of feet thick over New England
and New York. We have good reason to believe that even the highest
summits, except possibly in the Catskills, from 4,000 to over 6,000
feet above sea level, were completely submerged under the ice. On top
of a mountain of Archeozoic granite nearly 4,000 feet in altitude,
facing the St. Lawrence Valley in northern New York, the writer has
found many fragments of sandstone which were picked off by the ice in
the low valley, moved southward a good many miles, and uphill several
thousand feet to the top of the mountain. The reader may wonder how a
great glacier at least a mile thick in northern New York could have
thinned out to disappearance within the short distance to the southern
border of the State, but observations on existing large glaciers show
that it is quite the habit for them to thin out very rapidly near their
margins, thus producing steep ice fronts.

The fact that glacial ice flows as though it were a viscous substance
is well known from studies of valley glaciers in the Alps and Alaska,
and the great ice sheet of Greenland. A common assumption, either that
the land at one of the great centers of ice accumulation during the Ice
Age must have been many thousands of feet higher, or that the ice must
there have been immensely thick, in order to permit ice flowage so far
out from the center, is not necessary. Viscous tar slowly poured upon
a level surface will gradually flow out in all directions, and at no
time need the tar at the center of accumulation be very much thicker
than elsewhere. The movement of glacial ice from the great centers of
dispersal during the Ice Age was much the same in principle, only in
the case of the glaciers the accumulations of snow and ice were by no
means confined to the immediate centers.

The fronts of the vast ice sheets, like those of ordinary valley
glaciers, must have undergone many advances and retreats of greater
or less consequence. In the northern Mississippi Valley, and also in
Europe, there is positive proof for five or six important advances and
retreats of the ice which gave rise to the true interglacial stages.
The strongest evidence is the presence of successive layers of glacial
(morainic) débris piled one upon another, a given layer often having
been oxidized, eroded, and even covered with plant life before the
next or overlying layer was deposited. Such is the condition of things
throughout much of Iowa, where wells sunk into the glacial deposits
commonly pass through layers of partly decomposed vegetable matter
at depths of from 100 to 300 feet. Near Toronto, Canada, the finding
of warm climate plants between two glacial deposits proves that the
climate there during an interglacial stage was much like that of the
southern States to-day. During the great interglacial stages the vast
glaciers were notably restricted in size, and in some or possibly all,
cases they may have wholly disappeared from the continent.

In former years there was a tendency to ascribe mighty erosive power
to the vast slow-moving ice sheets, but to-day scarcely any geologist
would hold that the ice really produced large valleys solely by ice
erosion, or that mountains were notably cut down. Throughout the
glaciated region, especially toward the north, the deep preglacial
residual soils and rotten rocks were nearly all scoured off by the
passage of the ice. That the ice, where properly shod with rock
fragments, actually eroded to at least little depths into hard and
fresh rocks is well known, but the evidence is clear and conclusive
that the preglacial hills and mountains, and most of the valleys
(including all the large ones), were rarely more than a little modified
in shape and size.

One of the principal effects of the Ice Age is the widespread
distribution of glacial deposits, and other deposits which were formed
under water in direct association with the ice. Such materials have
been described in the chapter on "Glaciers and Their Work."

As a direct result of the Ice Age, many thousands of lakes came into
existence throughout the glaciated region where few, if any, previously
existed. Many of these lasted only while the ice was present because
their waters were held up by walls of ice acting as dams. Thousands
of others still persist, most of these having their water levels
maintained by dams of glacial débris left by the ice across valleys.
Good examples of lakes of both types, including a summary of the
remarkable history of the Great Lakes, are considered in the chapter on
"A Study of Lakes."

Many drainage changes, gorges, and waterfalls have also directly
resulted from the great Ice Age. In fact it is not too much to say
that practically all true gorges and waterfalls of the glaciated
region have originated as a direct result of the Ice Age. The most
remarkable combination of waterfall and gorge thus produced is that of
the world-famous Niagara, described in the chapter on "Stream Work."
Not only are Niagara Falls and gorge of postglacial origin but there
was no Niagara River as such before the Ice Age. In New York the
well-known Ausable Chasm, Trenton Falls Gorge, and Watkins Glen are all
excellent examples of gorges cut since the Ice Age by streams which,
because their old valleys were filled with glacial débris, have been
forced to take new courses. A gorge of very special interest is that at
Little Falls in central New York. This gorge, two miles long, with its
precipitous walls hundreds of feet high, is the most important gateway
for traffic between the Atlantic border and the Great Lakes region.
The bottom of this defile contains six tracks of the New York Central
and West Shore Railroads, the Barge Canal, an important highway, and
the Mohawk River. Before the Ice Age there was a stream divide instead
of a gorge, several hundred feet above the present river level. During
a late stage of the Ice Age, when the Great Lakes drained through the
Mohawk Valley, a tremendous volume of water passed over the divide and
cut it down to form nearly all of the gorge except the inner or bottom
trench which has since been eroded by the Mohawk River.

[Illustration: Fig. 48.--Sketch map of the region between Lake George
and Schenectady, New York, showing how certain of the main drainage
courses have been revolutionized by the great retreating ice sheet and
the deposits it left. Preglacial courses shown by dotted lines only
where essentially different from the present streams. (By the author,
as published by New York State Museum.)]

Only a few of the numerous stream changes directly due to the Ice Age
will be briefly referred to. Certain of the principles involved are
exceptionally well illustrated in the general vicinity of Saratoga
Springs and Lake George, New York. During the retreat of the great
glacier a lobe of ice occupied the Lake George Valley and forced the
Hudson River west over a divide at Stony Creek. Then, because of heavy
glacial deposits near Corinth, the Hudson could not continue south
through what had been the preglacial valley of Luzerne River, but it
was forced eastward over a divide in a low mountain ridge to Glens
Falls. The remarkable shift of the Sacandaga River from its preglacial
channel was caused by the building up of a great morainic ridge across
the valley in the vicinity of Broadalbin.

The drainage of the basin of the upper Ohio River has also been
revolutionized as a result of the glaciation. All the drainage of
western Pennsylvania passed northward into Lake Erie just before the
Ice Age instead of southwestward through the Ohio River as at present.

Rivers as large as the Mississippi and the Missouri were also more
or less locally deflected from their preglacial courses. Thus the
Missouri, which in preglacial time followed the James River Valley of
eastern South Dakota, was forced, by a great lobe of retreating ice, to
find its present course many miles farther west.

How long ago did the Ice Age end? In seeking an answer to this question
we should bear in mind not only the fact that the Ice Age ended at
different times, according to latitude, the more southern districts
having been first freed from ice, but also the fact that approximately
4,000,000 square miles of the polar regions are now ice-covered, so
that in a real sense those portions of the earth are still in an Ice
Age. Some of the best estimates of the length of postglacial time for
a given place are based upon the rate of recession of Niagara Falls,
the average of the estimates being about 25,000 years. The evidence for
this conclusion is briefly set forth in Chapter III. A careful study of
the rate of recession of St. Anthony Falls, Minnesota, has led to the
conclusion that the last retreat of the ice occurred there from 10,000
to 16,000 years ago. Certain clays deposited under tidewater since
the last withdrawal of ice in Sweden show a remarkable succession of
alternating layers thought to represent seasonal changes. By counting
the layers it has been estimated that Stockholm was freed from ice only
9,000 years ago.

Although the actual duration of the Ice Age is by no means accurately
known, we can be quite sure that the time represented is far longer
than that of postglacial time. That it must have lasted fully 500,000
years seems certain when due consideration is given to amount of time
necessary to bring about the repeated changes of climate between the
glacial and interglacial stages; the amount of plant accumulation
during the interglacial stages; the amount of weathering and erosion
of the various layers of glacial deposits. Some estimates run as high
as 1,500,000 years for the duration of the Ice Age, and an average is
about 1,000,000 years, which probably indicates, at least roughly, the
order of magnitude of the time involved.

When it is considered not only that the fact of the great Ice Age
was not even thought of until 1837, but also that many factors enter
into the general problem of the climate of geologic time, it is not
surprising that the cause (or causes) of the glacial climate is still
not definitely known. A few of the various hypotheses which have been
advocated to account for the glacial climate will now be very briefly
referred to. One is that the increased cold (not more than 10 to 15
degrees for the yearly average) was brought about by the notably
increased altitudes of late Tertiary and early Quaternary times in
northern North America and Europe. In this connection it is interesting
to note that the four times of real glaciation during geologic time
(mid-Proterozoic, early Paleozoic, late Paleozoic, and early Cenozoic)
did occur directly after great crustal disturbances and notable uplifts
of land. According to this hypothesis the interglacial stages would
have to be explained by a rather unreasonable assumption of repeated
rising and sinking of the glaciated lands.

Another hypothesis, long held in favor, is based upon certain
astronomical considerations. Thus we now have winter in the northern
hemisphere when the earth is nearest the sun, but in about 10,500
years, due to wobbling of the earth on its axis, our winter will occur
when the earth is farthest from the sun, thus making the winters longer
and colder, and the summers shorter and hotter. After a much longer
period of time the earth will be millions of miles farther from the
sun in winter than in summer and this would still further accentuate
the length and coldness of the winters. The interglacial stages
represent the 10,500 year periods when the earth in winter (northern
hemisphere) is nearest the sun. A difficulty in the way of accepting
this hypothesis is that it is inconceivable that each glacial and
interglacial stage lasted only 10,500 years. Another objection to the
hypothesis as an explanation of Ice Ages is that it is directly opposed
by the fact of widespread glaciation at low latitudes either side of
the equator during the late Paleozoic Ice Age.

Another hypothesis is based upon variations in quantity of carbonic
acid gas and water vapor in the air. Increase or decrease of these
constituents causes increase or decrease of temperature because they
have high capacities for absorbing heat. "The great elevation of the
land at the close of the Tertiary seems to afford conditions favorable
both for the consumption of carbon dioxide in large quantities (by
weathering of rocks) and for the reduction of the water content of
the air. Depletion of these heat-absorbing elements was equivalent
to the thinning of the thermal blanket which they constitute. If it
was thinned, the temperature was reduced.... By variations in the
consumption of carbon dioxide, especially in its absorption and escape
from the ocean, the hypothesis attempts to explain the periodicity of
glaciation (i.e., glacial and interglacial stages)." (Chamberlin and
Salisbury.)

Still another suggested explanation is based upon variability of amount
of heat radiated by the sun. Slight variations are now known to take
place, and possibly in the past during certain periods of time these
variations may have been sufficiently great to cause a glacial climate
with interglacial stages.

Here, as in the case of so many other great natural phenomena, a
single, simple explanation does not seem sufficient to account for all
the features of the several well-known glacial epochs of geologic time.
Two or more hypotheses, or parts of hypotheses, must more than likely
be combined to explain a particular Ice Age.



CHAPTER XVII

EVOLUTION OF PLANTS


Have we any knowledge regarding the beginning of life on our planet?
Our answer to this question must be decidedly in the negative. We can,
however, be very positive in regard to two important matters concerning
life in early geological time, namely, that plants must have existed
before animals, and that the very oldest known (Archeozoic) rocks of
the earth contain vestiges of organisms. We may be sure that plants
preceded animals because animal life ultimately depends upon plants
for its food supply or, in other words, all animals could never have
been carnivorous. Now, if we can prove that organisms existed during
Archeozoic time, it is evident that plants at least must have lived in
that oldest known era of earth history. That living things did then
exist is proved by the common occurrence of graphite, a crystallized
form of carbon, in the oldest known of the Archeozoic rocks. The facts
that flakes of graphite are abundantly scattered through many layers of
strata of Archeozoic Age, and that adjacent layers of strata contain
such varying amounts of graphite, render it practically certain that
such graphite represents the carbon of organisms. Graphite existing
under such conditions could not be of igneous origin. Carbonaceous
or bituminous strata, so called because they contain more or less
decomposed organic matter, would, when crystallized under conditions
of metamorphism, yield graphite-bearing rocks exactly like those of
Archeozoic Age, and there is every reason to believe that this was
their origin. But, since only graphite (carbon) of the Archeozoic
organisms remains, the rest having disappeared through chemical change
or decomposition, it is impossible to say whether much or all of it
represents original plants or animals. In any case we can be very sure
about the existence of plants (probably very simple or primitive types)
in Archeozoic time, but the presence of any form of animal life has not
been proved.

In the next, or Proterozoic era, some plants and animals of definite
types are known to have existed and, from here on in the present
chapter, it is our purpose to consider the salient points in the
geological history of plants, taking up the main types in the regular
order of their appearance from the remote Proterozoic days to the
present. The very oldest known definitely determinable fossils of any
kind are the more or less rounded masses of crudely concentric layers
of carbonate of lime from one to fifteen inches in diameter found in
middle Proterozoic limestone of western Ontario, Canada. Similar forms
are abundant in late Proterozoic strata of Montana. They occur in large
numbers as layers or reefs, in many cases repeating themselves through
hundreds or even thousands of feet of strata. Careful studies have
shown that these forms are the limey secretions of some of the very
simplest types of plants, that is thallophytes (e.g., seaweeds), which
lived in water.

Before proceeding to describe the plants of Paleozoic and later time,
the reader should be impressed with the important fact that plants of
higher and higher types came into existence throughout geological time
in almost exactly the botanical order of their classification, that
is to say, from the very simplest types (thallophytes) of Proterozoic
time there were gradually evolved, through the long geological ages,
higher and higher plant forms reaching a climax in the complex
and highly organized plants of the present time. This is the most
significant general fact in regard to the geological history of plants.
For the convenience of the reader the largest subdivisions in the
classification of plants are here given.


OUTLINE CLASSIFICATION OF PLANTS

  I.   Cryptogams      {1. Thallophytes (e.g., seaweeds, mushrooms)
       (seedless and   {2. Bryophytes (e.g., mosses)
         flowerless)   {3. Pteridophytes (e.g., "club mosses,"
                             "horsetails," ferns)

  II.  Pteridosperms   {
       (seed-bearing,  { (e.g., seed ferns--wholly extinct)
         flowerless)   {

  III. Phanerogams     {1. Gymnosperms (e.g., cycads, conifers)
       (seed-bearing,  {2. Angiosperms (e.g., grasses, lilies,
         flowering)    {     oaks, roses)

Throughout the first two periods--Cambrian and Ordovician--of the
Paleozoic era, plant life appears to have made little or no progress
toward higher forms. The very simple Thallophytes (e.g., seaweeds)
continued to secrete concentric layers of carbonate of lime in almost
exactly the same way as during the middle and late Proterozoic era.
Remarkable reefs of such forms occur in the late Cambrian limestone
near Saratoga Springs, New York, where one locality has been set aside
as a state park. During the Ordovician there were seaweeds of the more
familiar branching types without carbonate of lime supports, and these
have left very perfect impressions in some of the Ordovician strata.

During the Silurian period seaweeds continued, as, in fact, they did
throughout succeeding geologic time to the present. The Silurian strata
seem to contain some vestiges of the first-known land plants, though
the records are meager and some of the specimens are of a doubtful
character. Most interesting of all is a fern or fernlike plant found
in France. When we consider the profusion of land plants (all of
relatively simple types) of the next or Devonian period, it seems
certain that their progenitors must have existed in the Silurian, and
their remains may very likely be discovered.

Beginning with the Devonian period of the Paleozoic era the records
show that important advances had taken place in the evolution of the
plant kingdom. Among the very simple Thallophyte plants some seaweeds
of unusually large size occur in fossil form, but the important
fact is that all the principal subdivisions of the typical higher
non-flowering plants (Pteridophytes) as well as Pteridosperms, and even
some primitive representatives of the lower order flowering plants
(gymnosperms) were well represented in the Devonian. Our knowledge
of land plants earlier than the Devonian amounts to almost nothing
and they certainly could not have been at all prominent, but the
fossil records make it very clear that many Devonian land areas were
clad with rich and diversified plant life. There were even forests,
probably the first on earth, but they were far different, both in
general and in particular, from those of to-day because the trees
were all of exceptionally low organization types. During the next two
periods--Mississippian and Pennsylvanian--there was no really important
progress in the evolution of plants, and since these remarkable
types of land plants have left such wonderfully preserved records in
strata of the Pennsylvanian or great Coal Age, we shall proceed to
descriptions of the main types of that time, especially those which
contributed to the formation of beds of coal.

As shown by the abundant records, the land plant life of Pennsylvanian
time must have been not only prolific but exceedingly varied. Thousands
of species have been unearthed from the coal-bearing formations
alone, and these must represent only a fraction of all species of
plants which lived during the period. Most prominent of all were
the giant Lycopods constituting the lowest main subdivision of the
Pteridophytes (see above classification). These great, non-flowering
plants were at once the biggest, most common and conspicuous trees of
the extensive swamp forests, and they were the greatest contributors
to the formation of coal (Plate 15). Many species have been described.
They commonly attained heights of 50 to 100 feet and diameters of 2 to
6 feet. In one important type the fairly numerous branches bristled
with stiff, needle-shaped leaves. When the leaves dropped off the
older or trunk portions, scars were left spirally arranged around the
trunks of the trees. In another important type the leaf scars were
vertically arranged on the lower portions of the tree trunks. The upper
portions of the trunks (rarely branched) were thickly set with long,
slender leaves, which in some species were two or three feet long. An
interesting fact is that the inner parts of the trunks of the great
Lycopods were filled with soft, pithy material. This explains why the
fossil trees are nearly always flattened out, as a result of burial
within the earth. The nonbranching type of Lycopod has been totally
extinct for millions of years, while the branching type is to-day
represented only by small, mostly delicate, trailing plants familiarly
known as "club mosses" and "ground pines." The most conspicuous
trees of the great Pennsylvanian lowlands and swamps have, indeed,
left meager modern representatives, and here we have an excellent
illustration of a once prominent group of plants which has dwindled
away almost to extinction.

Another common type of Pennsylvanian vegetation was the so-called
"horsetail" plant or giant rush. The much smaller scouring rush,
represented by several species to-day, is the direct descendant of
this type which, during later Paleozoic time, grew to be 50 to 90 feet
high and 1 to 2 feet in diameter. The long, slender trunks filled with
pith were segmented with variously shaped leaves arranged in whorls
around the joints. A fine, vertical-fluted structure without leaf scars
characterized the surfaces of the trunk.

Recent study has shown that many of the Pennsylvanian plants, long
classed as true ferns, were really "seed ferns," as described below.
Many of the true ferns grew to be real trees up to fifty or sixty feet
high, but all Paleozoic types were primitive in structure as compared
with modern ferns.

Very remarkable among the later Paleozoic plants were the
Pteridosperms, represented by the so-called "seed ferns." These now
wholly extinct plants seem to have formed the connecting link between
the seedless, flowerless plants (Cryptogams) and the seed-bearing,
flowering plants (Phanerogams), because they bore seeds but not
flowers. Many of them were small and herbaceous, but others were tall
trees, in general appearance resembling the tree ferns. "Seed ferns,"
which play such an important part in the evolution of plants, are not
known to have existed after Paleozoic time.

During the latter half of the Paleozoic era some very primitive types
of flowering plants (Gymnosperms) existed. Most abundant of these were
the so-called Cordaites, which were the tallest trees of the time,
some having reached heights of over 100 feet. The upper portions only
bore numerous branches supplied with many simple, parallel-veined,
strap-shaped leaves up to six feet long and six inches wide. Excepting
the pithy cores the trunks of these trees were of real wood covered
with thick bark. Trees of this kind became extinct in the early
Mesozoic era.

Very late in the Paleozoic (Permian period) two other types of the
simple flowering plants (Gymnosperms) made their appearance. These were
the cycads and conifers, which were the most conspicuous trees during
the first two periods of the Mesozoic era. The cycads reached their
culmination in the Jurassic period, but they still exist in modified
form in some parts of the world. The short, stout trunk was crowned
with long, stiff, palm-like leaves. In fact, the cycads are distantly
related to the palms, which belong to a higher group of plants. Some
specimens of cycads, especially from the Mesozoic strata of South
Dakota, are so wonderfully fossilized that even the detailed structures
of trunks, leaves, flowers, and seeds are so perfectly preserved that
almost as much is known about these plants of millions of years ago as
though they were living forms.

The conifers, with which are classed present-day pines, spruces, and
many other evergreen trees, gradually took on a more modern aspect, so
that late in the Mesozoic era they were much like those now living.
Among the most interesting trees were the sequoias, to which the
living "big trees" and red-woods of California belong. These began in
relatively late Mesozoic time, reached their climax in numbers, variety
of species, and widespread distribution in the early Cenozoic era;
and are now almost extinct, being represented by only two species in
local portions of California. Cordaites, trees which were so large and
abundant in later Paleozoic time, were reduced to extinction in the
early Mesozoic era.

During Mesozoic time the Thallophytes, represented by seaweeds, were
common. Among the Pteridophytes the ferns and "horsetail" plants
were fairly common, but the very large forms gradually gave way to
much smaller ones during Mesozoic time. The giant Lycopods of later
Paleozoic time dwindled almost to extinction even in early Mesozoic
time, so that from that time to the present they have been very small
and relatively insignificant.

Tens of millions of years of earth history had passed before the
true flowering plants--the Angiosperms--appeared upon the earth. The
Cretaceous period marks their advent. So far as known, these plants
originated along the eastern side of North America, and very soon after
their establishment they spread over the earth with amazing rapidity
and dominated the vegetation as they do to-day, more than half of the
existing species of plants being Angiosperms. Among the common types
which have been unearthed from Cretaceous strata are palms, grasses,
maples, oaks, elms, figs, magnolias, willows, beeches, chestnuts, and
poplars.

[Illustration: Plate 13.--A Slab of Very Early Paleozoic (Cambrian)
Rock, Covered with Some of the Oldest Known Definitely Determinable
Animal Remains. These creatures lived in a sea which overspread the
site of the Rocky Mountains of southern Canada fully 25,000,000 years
ago. Most of the fossils are trilobites (including some very small
ones) and other related crustacean forms (lighter portions). (_After C.
D. Wolcott, Smithsonian Institution, Washington, D. C._)]

[Illustration: Plate 14.--(_a_) Photographs of Small Slabs of
Ordovician Strata Full of Fossils. These slabs are actual bits of
sea bottom at least 20,000,000 years old. The left picture shows
"stone-lily" stems, so-called "sea mosses," brachiopods. Right picture
shows various species of brachiopods. (_Photo by the author._)]

[Illustration: Plate 14.--(_b_) An Outcrop of Middle Ordovician
Stratified Limestone in Northern New York. This ledge is full of
fossils similar to those above. The material was deposited on the floor
of the Ordovician sea which overspread much of the continent. (_Photo
by the author._)]

The introduction of the higher flowering plants (Angiosperms) "was,
perhaps, the most important and far-reaching event in the whole
history of vegetation, not only because they almost immediately became
dominant, but also because of their influence upon the animal life of
the succeeding periods. Hardly had flowers appeared, before a great
horde of insects, which fed upon their honey or pollen, seem to have
sprung into existence. The nutritious grasses and the various nuts,
seeds, and fruits afforded a better food for noncarnivores than ever
before in the history of the world. It was to be expected, therefore,
that some new type of animal life would be developed to take advantage
of this superior food supply. As we shall see in the discussion of
the Tertiary (next chapter), the mammals, which kept a subordinate
position throughout the Mesozoic, rapidly took on bulk and variety and
acquired possession of the earth as soon as they became adapted to this
new food, quickly supplanting the great reptiles of the Mesozoic."
(Cleland.)

During the present or Cenozoic era vegetation gradually took on a more
and more modern aspect until the existing species were developed. The
grasses especially developed and spread rapidly, but the cereals did
not evolve until late in the era. Certain single-celled plants, called
diatoms, may be especially mentioned, for they must have literally
swarmed in some of the Tertiary seas which spread over parts of the
present lands. "The microscopic plants which form siliceous shells,
called diatoms, make extensive deposits in some places. One stratum
near Richmond, Virginia, is thirty feet thick and is many miles in
extent; another, near Monterey, California, is fifty feet thick, and
the material is as white and fine as chalk, which it resembles in
appearance; another, near Bilin, in Bohemia, is fourteen feet thick....
Ehrenberg has calculated that a cubic inch of the fine, earthy
rock contains about forty-one thousand millions of organisms. Such
accumulations of diatoms are made both in fresh waters and salt, and in
those of the ocean at all depths." (J. D. Dana.)



CHAPTER XVIII

GEOLOGICAL HISTORY OF ANIMALS (EXCLUDING VERTEBRATES)


A study of the animals of the past is not only of great interest in
itself, but also it furnishes a mainstay of the great doctrine of
organic evolution. At the very outset of our discussion the reader
should have already in mind at least the main subdivisions of the
animal kingdom in order to reasonably well understand where the
important animal types of the different geological ages fit in, and
how those types bear upon the doctrine of evolution. The accompanying,
very brief, general classification includes the usually recognized
subkingdoms with special reference to representatives of those which
are of most geological and evolutionary significance. Reading downward
in this classification, the degree of complexity of organization
steadily increases from single-celled animals to man himself.

    I. Protozoans,       e.g. foraminifers (with lime carbonate shells)

                              {Sponges
   II. Coelenterates,  e.g. {So-called "jellyfishes," graptolites
                              {Corals

                              {So-called "sea lilies"
  III. Echinoderms,      e.g. {So-called "starfishes"
                              {So-called "sea urchins"

   IV. Worms,

    V. Molluscoids,      e.g. {So-called "sea mosses"
                              {Brachiopods

                              {Clams, oysters
                              {
                              {Snails            {Pearly nautilus,
    VI. Mollusks,        e.g. {                  {  ammonites
                              {
                              {Cephalopods, e.g. {So-called
                              {                  {  "cuttle fishes"

                              {Trilobites
   VII. Arthropods,      e.g. {Crabs, lobsters
                              {So-called "sea scorpions"
                              {Insects

                              {Ostracoderms
                              {Fishes
  VIII. Vertebrates,     e.g. {Amphibians
                              {Reptiles
                              {Birds
                              {Mammals (including man)

Before entering into a brief but rather systematic discussion of some
of the most important types of animals which lived during geological
time, it may be well for the reader to have in mind some of the most
important conclusions which have been reached as a result of the study
of the fossil animal records. These conclusions may be summarized as
follows:

1. Animal life existed many millions of years ago.

2. Not only the animals of to-day, but also those of any given
geological period, directly descended from those of preceding
geological periods.

3. Animal life has undergone continuous change since its introduction
upon the earth, so that each group of strata, representing a particular
geological age, contains a characteristic assemblage of fossil animals.

4. Many of the changes in the history of animals have been progressive
or evolutionary, so that strata of early geological time contain
distinctly more primitive or lower order forms than the strata of late
geological time. But, while the line of evolution has been maintained
without a break, culminating in man, there have been many offshoots of
a retrogressive nature.

5. Even as far along in geological time as the early Paleozoic era,
the highest subkingdom--vertebrates--had no representative whatever.
In other words, all the important subdivisions of animal life from a
little below fishes to man have been evolved since about the close of
the Ordovician period.

6. Any species of animal which ever became extinct has never been known
to reappear, and literally tens of thousands of species are known to
have become extinct.

7. No species like those now living are found in the more ancient
strata, such being confined to the strata of relatively recent
geological dates.

8. While more and more highly organized animals have continuously
been evolved, many of the earlier and simpler types have persisted,
a remarkable case in point being the single-celled animals called
foraminifers which may be traced, without very notable change, through
the tens of millions of years of geological time from the late
Proterozoic era to the present day.

9. Many species have been able to maintain themselves practically
without change through long stretches of geological time, while others
have had only very brief existence.

When did animal life begin on the earth, and what were the first forms
like? We can only partially answer the first question by saying that
animals have existed for tens of millions of years, certainly as early
at least as Proterozoic time. Up to the present time we are utterly
in the dark as to what the earliest animal forms looked like, but we
have positive knowledge that the oldest forms found as fossils in the
rocks represent creatures which were far more primitive and lower in
organization than many animals of to-day, and that since those oldest
known forms lived, the animal kingdom has undergone various profound
alterations. In view of the above statements, and also the fact that
the oldest known plant forms were extremely simple or single-celled,
it is more than likely that the first animal life of the earth was
single-celled. In harmony with this view is the fact that fossil
single-celled animals are found in the very oldest (Proterozoic) rocks
which contain any definitely determinable fossil animals.

Do the most ancient known rocks show that animal life existed during
Archeozoic time? In the preceding chapter we pointed out the fact that
the carbon (in the form of graphite), so commonly present in those most
ancient known strata, proves the existence of life of some kind during
Archeozoic time. But because nothing like definitely determinable
fossil forms have thus far been discovered in those rocks, we cannot be
sure whether the carbon represents plant or animal life or both, though
certainly plants of very low order at least must have existed. Because
of the intense alteration (metamorphism) of those very old strata,
all definite forms have long since been obliterated as such. We may,
however, in the light of the vast evolution which took place through
succeeding geological time, be very sure that any animals which may
have existed during Archeozoic time were in general much simpler forms
than those of even early Paleozoic time.

The early and middle Proterozoic strata throw no more light upon the
early history of animal life than do the Archeozoic strata. The upper
or later Proterozoic rocks, however, contain the oldest recognizable
animal fossils. Very recently fossil remains of single-celled,
shell-bearing Protozoans have been found in northern France, while
the upper Proterozoic strata of the Rocky Mountains in Montana, and
the Grand Canyon of Arizona have yielded worm tracks, a Molluscoid
(brachiopod) and fragments of lower forms of Arthropods. This record,
although very meager, clearly proves that animal life was so well
advanced by late Proterozoic time, that next to the highest subkingdom
was actually represented (see above classification), and that there
must have been a long line of simpler and simpler ancestors, probably
extending far back into the Archeozoic era. When we stop to consider
that Archeozoic and Proterozoic time was fully as long as all
succeeding geological time, it is not so surprising that fairly highly
developed animals (except Vertebrates) had been evolved before the
close of the Proterozoic era.

In regard to abundance of fossil animals the oldest (Cambrian)
Paleozoic strata stand out in marked contrast to the Proterozoic. Many
hundreds of species of animal fossils have been described from Cambrian
strata, and a great many others yet remain to be discovered. Cambrian
fossils are remarkably numerous, varied in species, and complex
in organization (Plate 13). All subkingdoms of animals except the
Vertebrates were represented, though usually only by the simpler types
in each subkingdom. It is quite generally agreed that no less than 50
per cent of animal evolution had taken place before the beginning of
the Cambrian period. The reader should, however, clearly bear in mind
that tremendous advances in evolution have taken place since early
Cambrian time when not only all forms from lower scale Arthropods to
the highest mammals (including man) have evolved, but also when many
thousands of species of lower subkingdom animals developed.

Why are the very early Paleozoic strata so rich in fossils, while the
immediately preceding Proterozoic rocks show so few? The seemingly
sudden appearance of so many highly developed animals in earliest
Paleozoic (Cambrian) time is one of the most important considerations
in the history of animal life, and it is by no means definitely
understood. The following statements bear directly upon the problem:
The early animal forms were probably soft or gelatinous without shells
and lived mostly in the open sea where food (seaweeds, etc.) was
abundant. Such animals were very unfavorable for preservation in fossil
form. Then, late in Proterozoic time or very early in the Paleozoic,
a severe struggle for existence set in, probably due to crowding
along shores, and hard parts began to develop both for support and
defensive purposes. Such hard parts or shells were commonly favorable
for fossilization. This view is strongly supported by the fact that
very thin shells only are known from late Proterozoic rocks, and mostly
very thin shells from the earliest Cambrian, the heavier shells having
been evolved later. A fact of importance to bear in mind in this
connection is that just at the critical time (late Proterozoic) in
shell development, the lands of the earth were undergoing widespread
and deep erosion as pointed out early in the chapter on "Ancient Earth
History." The earliest Cambrian strata, therefore, nearly everywhere
rest upon the deeply eroded surface of the Proterozoic rocks so that
the transition strata--the very ones which would contain most fossils
of the early shell development stage--are nearly everywhere missing.
Finally, mention should be made of the fact, that all Archeozoic
strata are profoundly altered (metamorphosed), and so are nearly
all Proterozoic strata, except the later. Fossils once present in
those rocks would of course have been obliterated by the process of
metamorphism, but the fact remains that very considerable thicknesses
of practically unaltered Proterozoic strata show few if any animal
fossils.

We shall now proceed to a rather systematic consideration of the most
interesting and significant types of creatures which have inhabited
the earth since the beginning of Paleozoic time at least twenty-five
million years ago. It is our purpose to bring out the salient features
in the history of each subkingdom of animals, beginning with the lowest
or simplest, and taking up in turn the higher and higher subkingdoms.
By this method the reader may easily follow the main thread of organic
evolution or progressive change which runs through most of the known
history of animal life of our planet, and which is so important in the
science of geology.

Protozoans, which include all the tiny single-celled animals, are
known in fossil form even in late Proterozoic rocks and, as proved by
the fossil records, they have been more or less abundant ever since,
even now swarming in large portions of the surface sea waters. One of
the most remarkable facts in the history of animal life is, that such
exceedingly simple creatures persisted almost without change through
the tens of millions of years when such profound and even revolutionary
changes took place in the animal kingdom in general. The only fossil
Protozoans are those which developed delicate shells either of
carbonate of lime (the foraminifers) or silica. Special mention should
be made of the Cretaceous period when foraminifers must have been
exceedingly profuse in clear sea waters which spread over the Gulf
Coastal Plain of the United States, parts of southern England, much of
France, and other areas, as proved by their accumulated shells which
make up formations of chalk hundreds of feet in thickness and many
miles in extent.

[Illustration: Fig. 49.--A compound colony of fossil graptolites
characteristic of late Ordovician time, fully 20,000,000 years ago.
Each little prong once held a tiny individual living graptolite which
was a very simple type of animal belonging to the subkingdom called
"Coelenterates." (Modified after Ruedemann.)]

The Coelenterates, which comprise the simplest of the many-celled
animals, are saclike forms with mouth openings, but with few other
differentiations of parts. All are marine animals. Of these the sponges
are porous, and the other types (including corals) have tentacles
around their mouths. Sponges have been more or less common from early
Paleozoic time to the present, and they have undergone relatively
little change. "Jellyfishes," which are in truth not fishes at all,
are wholly soft or gelatinous Coelenterates which have left some very
remarkable impressions and casts in strata of very early Paleozoic age,
those very ancient forms evidently having been almost exactly like
those of to-day. Graptolites were slender, plumelike, delicate forms
consisting of colonies of tiny individuals, in many cases in branching
or radiating combinations. They existed only during the first half of
the Paleozoic era. Both because they floated in the open sea, thus
permitting widespread distribution, and because they underwent many
distinct species changes during short geologic intervals, they are
among the most useful fossils for separating the various subdivisions
of strata of the earlier Paleozoic.

[Illustration: Fig. 50.--Corals, representing the very simple
subkingdom of animals called "Coelenterates": a, fossil shell of an
individual "cup coral" found only in Paleozoic strata; b, a compound or
"chain coral" skeleton found only in relatively old Paleozoic strata;
and c, part of a modern coral colony showing living corals.]

Corals comprise another important branch of the Coelenterates. During
the Cambrian period there were corallike sponges and possibly simple
corals, but from the early Ordovician to the present true corals
have been common, especially in the clearer, warmer seas. Their
carbonate of lime skeletons have accumulated to help build up great
limestone formations representing almost every geologic age from early
Paleozoic time to the present. Paleozoic corals were in general
notably different from those of later time. There were three main
types including the compound "honeycomb" and "chain" types, and the
solitary or compound "cup" type. They all had four, or multiples of
four, radiating partitions; were rarely branched; and were generally
large, some individual cup corals ranging in length from half an inch
to a foot or more. Modern corals (beginning with the Mesozoic) have six
or eight partitions; are nearly all profusely branched; and are mostly
tiny individuals.

Echinoderms are all marine animals, including the so-called
"starfishes," which are not really fishes. They have body cavity, with
digestive canal, low order nervous system, and a water circulatory
system. Most of them have radially segmented shells or skeletons. The
oldest fossil forms are found in Cambrian strata, these being very
simple or primitive types, with a bladderlike head set on the end of a
segmented stem, both head and stem having been supported by carbonate
of lime. Such forms lived only to middle Paleozoic time. Ordovician
strata contain representatives of all the main types of Echinoderms in
well-fossilized forms.

[Illustration: Fig. 51.--Fossil Echinoderms or so-called "starfishes":
a, simple type known as the "stone lily" with head, stem, and roots
intact from Silurian strata; b and c, irregular and regular higher type
Echinoderms called "sea urchins" from Cretaceous strata.]

A stemmed Echinoderm of special interest, first known from the
Ordovician, has persisted to the present day. It is the so-called
"sea lily" or "stone lily," consisting of a complex, headlike portion
attached to the sea bottom by a long segmented stem, the whole being
supported by lime carbonate. They were very numerous during the
Silurian, but they seem to have culminated in variety of species and
numbers of individuals during the Mississippian period when they were
exceedingly profuse. Hundreds of species of "stone lilies" are known
from Mississippian strata alone, and in certain localities, as at
Crawfordsville, Ind., and Burlington, Ia., the "stone lily" remains are
so numerous that when living they must have literally forested parts of
the sea bottom. From Mississippian time to middle Mesozoic time they
occupied a relatively subordinate position when they again developed in
great profusion. The Mesozoic forms were distinctly more like those
of to-day, and it scarcely seems credible that any creature could have
contained such a multiplicity of hard parts, more than 600,000 segments
having been counted in a single fossil from Jurassic strata. The "sea
lilies" of to-day are relatively unimportant.

The familiar five-pointed "starfishes," so common along our seacoasts,
are first known from the Ordovician, and they persisted through the
many millions of years to the present time with remarkably little
change. The so-called "sea urchins" live in rounded, segmented
lime-carbonate shells bristling with movable spines. "Sea urchins"
are first known from the Ordovician, but they did not become abundant
and diversified until Mesozoic time, when many of them took on a very
modern aspect.

Worms are known to have existed ever since late Proterozoic time,
as proved by the occurrence of tracks, borings and more rarely
delicate impressions on rock surfaces. Because of their softness they
have rarely been well fossilized and are, therefore, of no great
evolutionary or geological importance.

[Illustration: Fig. 52.--Fossil brachiopods belonging to the subkingdom
of animals known as "Molluscoids": a, b, c, forms characteristic of the
Ordovician, Devonian, and Triassic periods, respectively.]

The subkingdom Molluscoids has been richly represented by both the
so-called "sea mosses" and brachiopods. The "sea mosses" form colonies
of tiny mosslike tufts, resembling corals outwardly, though they are
much more highly organized. They have been common from Ordovician
time to the present, their carbonate of lime skeletons often having
contributed to the building of limestone formations. Brachiopods always
have two external shells or valves, in most cases working on a hinge,
and also a pair of long, spiral-fringed arms associated with the soft
part of the animal inside the shells. They differ from the other type
of bivalve (e.g., clam, oyster) in that they are symmetrical with
reference to a plane passed through the middle of the shells at right
angles to the hinge line. They have rarely grown to be more than a few
inches long. A few scant brachiopod remains are known from the late
Proterozoic, but throughout known geologic time they reached their
greatest development in the Paleozoic era, more especially in the
Devonian period. Combining number of species and number of individuals,
the brachiopods probably hold the record of all important groups of
fossil animals, more than 7,000 species being known. Many layers of
rock are filled with their shells (Plate 14). Since the close of the
Paleozoic they have fallen off notably, and are now represented by
relatively few small forms. From the standpoint of evolution it is
interesting to note that in very early Paleozoic time the brachiopods
were mostly small, of relatively simple organization, and their thin
shells were not joined by hinges. Later they became larger and more
complex and their thicker shells worked on hinges. Nearly all the
Paleozoic forms had long, straight hinge lines, which made it difficult
for their enemies to open them. Along with the change to narrower,
curved hinge lines came the decline of the tribe. They have been of
great value to the geologist in subdividing the geological column of
strata into its many formations.

The Mollusks, which are more highly organized than the Molluscoids,
have more or less distinctly developed heads and locomotive organs.
Many thousands of species are now extinct, the classes of most
geological importance being represented by clams, snails, and the
pearly nautilus. Most of them have shells and gills for breathing. The
members of the simplest group, well represented by the clam tribe,
possess two similar shells working on hinges, so that in this regard
they are much like brachiopods, but, unlike the latter, they are not
symmetrical with reference to a plane at right angles to the hinge
line. Cambrian strata contain the oldest known of the fossil forms
where they are small, relatively thin-shelled, and rare. In marked
contrast to the brachiopods these bivalves have rather steadily
increased in numbers of species and individuals to the present time,
now being represented by thousands of forms. During the Mesozoic era
they greatly out-numbered the brachiopod bivalves and took on a more
distinctly modern aspect, when the oyster tribe and closely related
types were prominently developed. Culmination in size and thickness of
shell seem to have been reached in early Cenozoic time, strata of that
age in certain places, for example in Georgia and southern California,
being filled with oyster shells 10 to 20 inches long and 4 to 6 inches
thick! In addition to their gigantic size and thickness, many of the
shells were fluted or ribbed, and so they represented an extreme type
of defensive armor among the lower animals.

Snails have existed from the earliest Paleozoic era to the present
time, and the outstanding fact of interest concerning them is that
they furnish one of the finest illustrations of an important class
of animals which has undergone practically no conspicuous change or
evolution during all those millions of years of time.

[Illustration: Fig. 53.--Sketches of chambered cephalopods showing
the main steps in the evolution of the shell forms and compartment
partitions: a, b, the only kinds in Cambrian time; c, d, forms added
in the Ordovician; e, added in the Devonian; f, added in the late
Paleozoic; g, h, characteristic of the Mesozoic era; and i, a living
form (pearly nautilus) cut through. (Drawn by the author.)]

We shall now turn our attention to the highest order of Mollusks--the
cephalopods. These creatures, whose heads are armed with powerful
tentacles and supplied with complex eyes, propel themselves by forcible
ejection of water. One general type--the chambered cephalopod--has a
shell divided into compartments (e.g., modern pearly nautilus) which
are successively built up and abandoned by the animal as it grows
larger. These chamber-shelled cephalopods constitute one of the most
remarkable and instructive illustrations of evolutionary change within
any important subgroup of invertebrate animals, ranging from early
Paleozoic to the present. Both because of the abundance of fossil
forms in rocks of all these periods of geological times, and because
certain of the evolutionary changes are so clearly expressed in the
well preserved shell portions, they are specially adapted for study. In
the late Cambrian only straight and slightly curved forms with smooth,
nearly straight chamber partitions existed. Notable advance took place
during the next (Ordovician) period when there were straight, curved,
open-coiled, and even close-coiled forms. All had simple partitions,
and the straighter forms predominated. "The size attained by the
Ordovician cephalopods was probably never surpassed by representatives
of the class. Some of the (straight) shells were twelve to fifteen feet
in length, and a foot in diameter. From this great size they ranged
down to or below the size of a pipe stem." (Chamberlin and Salisbury.)
They were more than likely the undisputed masters of the Ordovician
seas. Silurian time marked no important change in their structures,
but the coiled forms predominated for the first time. During the second
half of the Paleozoic era all preceding types with simple partitions
persisted, but in some forms the simple partitions gradually became
angled and finally rather complexly curved. During the Mesozoic era
the partition lines of the close-coiled forms evolved until a most
remarkable degree of complexity was attained, comparable, indeed, to
the sutures of the human skull plates. These remarkable forms called
ammonites, of which more than 2,000 species are known, began with the
Mesozoic, reached their climax, and passed out of existence toward the
close of the same era. Certain strata of Jurassic age are literally
filled with ammonites, some shells being several feet in diameter.
Various eccentric changes took place in the ammonites shortly before
their extinction. Some shells became uncoiled and even straight, thus
outwardly at least showing reversion to the original early Paleozoic
ancestors, but with retention of the complex partitions. Others assumed
spiral shapes and still others became curved or coiled at each end.
While these extraordinary evolutionary changes were going on among
the chambers of cephalopods during Mesozoic time, some of the ancient
close-coiled forms with very simple partitions managed to persist.
In fact this simple type, almost exactly like its early Paleozoic
ancestor, has been the only one out of this whole remarkable class of
animals to persist to the present time, being now barely represented by
the well-known pearly nautilus of the Indian Ocean.

During the Mesozoic era the highest type of cephalopod, represented by
modern squids and so-called "cuttlefishes," branched off and developed
in great profusion. These had slender internal shells, but no external
chambered shells. An inky black liquid secreted in a bag was forced
out to cloud the water when the animal was escaping its enemy, thus
antedating by millions of years the principle of smoke screen so
effectively used by ships during the World War. Some Jurassic species
got to be over two feet long, and a few specimens of that age have
been found in such perfect state of preservation that drawings of the
fossils have actually been made with the ink (after moistening) taken
from their own ink bags.

[Illustration: Fig. 54.--A fossil nonchambered cephalopod of Jurassic
age. It was closely related to the modern squid, and its ink bag is
well shown just to the left of the middle. (Modified after Mantell.)]

Before concluding this chapter we shall take up the salient points
in the geological history of Arthropods which constitute the highest
subkingdom of all animals except the Vertebrates. They are now very
abundant and varied, familiar examples being crabs and insects. A few
scant remains of simpler forms are known from the Proterozoic, but
since very early Paleozoic time they have been very common and have
undergone great evolutionary changes. A few striking examples only will
be dwelt upon. Among the most common and interesting of all Paleozoic
animals were the trilobites, distantly related to modern lobsters and
crabs.

[Illustration: Fig. 55.--Restorations of trilobites based upon actual
fossils characteristic of earlier Paleozoic time: a, Cambrian; b,
Ordovician; c, Devonian; b shows the appendages.]

Some of these grew to be two feet long, but usually they were only
one or two inches long. First known from the earliest Paleozoic, they
reached their culmination relatively early in the era and then dwindled
away to utter extinction before its close. "They were characteristic
of the Paleozoic era, beginning in great variety in the Lower Cambrian
and dominating the seas of the Cambrian (300 species) and Ordovician
(950 species). In the Silurian, though they were still common, the
trilobites were nevertheless on the decline (485 species), and this
ebbing of their vital force is seemingly shown in many picturesque
forms replete with protuberances, spines, and exaggeration of parts.
As a rule, in evolution, one finds that when an organic stock is
losing its vital force there arises in it an exaggeration of parts,
as if heroic efforts were being made to maintain the race. Spinosity
in animals is often the prophecy of tribal death. In the Devonian,
the variety and number of the trilobites were greatly reduced
(105 species), at a time when the ancient types of fishes, which
undoubtedly fed on these crustaceans (trilobites), began to be common
in the seas. In the later Paleozoic seas, the trilobites were relics,
or animals surviving from a time better suited to their needs, and one
by one they vanished, until a little before the close of the Paleozoic
era none were left." (Schuchert.)

[Illustration: Fig. 56.--A giant, sea scorpion of Devonian time. Length
nearly 3 feet. (After Clarke and Ruedemann, New York State Museum.)]

An extraordinary type of Arthropod which ranged throughout Paleozoic
time and became extinct at its close was the so-called "sea scorpion,"
closely related to the modern scorpion. Their five or six pairs of
appendages all came out from the head portion, one pair in some cases
having been developed as powerful pincers. Their culmination in size
was reached during the Devonian when some forms grew to the astonishing
length of over eight feet! Such gigantic creatures must have been
tyrants of the seas until they were subdued by the oncoming powerful
fishes. True scorpions are known from rocks as old as the Silurian.
Lobsters and crabs made their appearance during the Mesozoic era.

Since insects constitute the highest subdivision of Arthropods, they
include the very highest forms of animal life except the Vertebrates.
The oldest known fossil insects are from Pennsylvanian strata, more
than 1,000 species having been described from rocks of that age.
They were all simple or primitive types like cockroaches and dragon
flies, and were remarkable for size. Giant cockroaches got to be four
inches long. One form of dragon fly, with a spread of wing of over two
feet, was probably the largest insect which ever lived (Plate 15).
Development of insect life was especially favored during the great
Coal Age because of the prolific vegetation, but more than likely
insects originated somewhat earlier. Early in the Mesozoic era a great
progressive change began to come over insect life and higher forms
gradually evolved until by the close of the era many of the highest
types like flies, ants, and bees were common. As might be expected, the
highest insects did not develop until after the appearance of the true
flowering plants in later Mesozoic time, butterflies apparently not
having evolved until early in Cenozoic time. Many of the thousands of
known species of fossil insects are from strata of Tertiary age during
which time they may have been even more numerous than to-day, although
there are about 400,000 species now living. An almost incredible case
is a Tertiary stratum only a few feet thick in Switzerland from which
nearly 1,000 species of insects have been unearthed. Another famous
locality is Florissant, Colorado, where during early Tertiary time
there was a small lake into which showers of fine volcanic dust fell
and entombed vast numbers of insects, more than 2,000 species having
been unearthed. Still another extraordinary occurrence is along the
shores of the southern Baltic Sea where more than 2,000 species of
insects have been found in a fossil resin called amber. The insects
were caught in the still soft sticky resin while it was exuding from
the trees, and thus we have the insects, fully two or three million
years old, literally embalmed and marvelously preserved, often in
beautifully transparent amber.



CHAPTER XIX

GEOLOGICAL HISTORY OF VERTEBRATE ANIMALS (INCLUDING MAN)


Vertebrates comprise the highest subkingdom of all animals with man
himself at the very top. They are characterized by the possession of
a vertebral column, which, in all but the very simple or primitive
forms, is an ossified backbone. Their main subdivisions are given
in the classification table near the beginning of the preceding
chapter. The oldest known Vertebrates, found in fossil form in middle
Ordovician strata, were represented by curious and bizarre creatures
called ostracoderms, or more popularly "armor fishes." They were not
true fishes because they were really somewhat lower in the scale of
organization than fishes. Some were distinctly fishlike in appearance,
and others notably resembled certain of the Arthropods, so that some
students consider them to have formed the connecting link between
the highest Invertebrates (Arthropods) and low order fishes of the
Vertebrates. The vertebral column always consisted of cartilage or
gristle and, in most forms, it extended through tail fin. None had true
side fins like fishes, but many were provided with a pair of jointed
flappers or paddles. The jawlike portions of the heads moved over each
other sidewise as, for example, in beetles and not up and down in true
Vertebrate fashion. Two eyes were always very close together. One of
the most striking features was the protection of the head and fore part
of the body by an armor of bony plates, while the rest of the body had
scales. They seldom grew to be more than six or seven inches long.
Beginning in the Devonian, they remained rare during the Silurian, and
then in the Devonian period they reached their climax of development
only to become extinct at its close. Many species were abundantly
represented in many parts of the world. By some the Ostracoderms are
thought to have been a primitive (sharklike) fish development in the
wrong direction, and hence they became extinct.

[Illustration: Fig. 57.--Two restored forms of very primitive and
ancient (Devonian) types of Vertebrates called "ostracoderms." They
were lower in organization than true fishes. (After Dean-Woodward and
British Museum, respectively.)]

Fishes, represented only by very primitive sharks, are known to
have existed as early as the Silurian period, but the remains are
scant. During the Devonian period, however, they showed a marvelous
development into many species and countless myriads of individuals.
The Devonian is, therefore, commonly called the "Age of Fishes."
These very ancient (Devonian) primitive (fish) types of Vertebrate
animal life are of profound significance in organic evolution because
they were the direct progenitors of the great groups of still higher
Vertebrates which since later Paleozoic time gradually increased in
diversity and complexity of structure through amphibians, reptiles,
birds, and mammals finally to man himself.

[Illustration: Fig. 58.--Restorations of characteristic Devonian
fishes, based upon actual fossils: a, a "lung fish" with leglike fins
(after Huasakof); b, a "ganoid." (After Nicholson.)]

In marked contrast to the most typical and highly organized fishes
so abundant to-day, all Devonian fishes were of simple types with
cartilaginous skeletons and vertebrated tails. Many of them were also
generalized types, that is, associated with their clearly defined fish
characters were others connecting them with certain higher Vertebrates,
as, for example, amphibians and reptiles. Thus all their tail fins
were vertebrated as in reptiles; their labyrinthine, internal tooth
structure was to be an amphibian feature when those creatures evolved;
many had protective armor or bony scales like most early amphibians and
many modern reptiles; and many had paired fins which were something
like jointed legs. Most abundant and highly organized of the Devonian
fishes were "ganoids," characterized by a covering of small plates or
bony scales set together but not overlapping like in typical modern
fishes. Their intricate tooth structure and limblike fins strongly
suggest the amphibians of later Paleozoic time. The skeleton of
cartilage gradually became somewhat ossified during succeeding geologic
periods. From their great profusion and diversity in the Devonian
period the ganoids have steadily fallen away until they now have very
few descendants like the gar pike.

Another important group of remarkable fishes, now totally extinct,
but common in Devonian and somewhat later time, had heavy, bony armor
plates over the fore part of the body. Those which grew to be fifteen
to twenty-five feet long were probably the rulers of the middle
Paleozoic seas. Another remarkable Devonian fish was able to breathe in
both water and air because, like their few modern descendants, they had
both gills and lungs. Because of their leglike fins and lung sac, it
is commonly believed that they were progenitors of the later Paleozoic
amphibians. The simplest of all fishes, the sharks, began in the
Silurian, underwent no important change through the millions of years
since, and are now of course well represented. During early Cenozoic
time the sharks seem to have reached culmination in size--sixty to
eighty feet long, with teeth five or six inches long.

Among modern fishes the most abundant by far, and the most highly
organized, are the true bony fishes, called the "teleosts," which
made their first appearance in the middle of the Mesozoic era. Those
earliest forms clearly show their descent from the ganoids. Apparently
they have not yet passed their prime.

We shall now consider the next higher group of Vertebrates, the
amphibians, which breathe by gills when young and later develop
lungs. Many live both on land and in water like the frogs. Unlike
fishes they have legs with toes and not fins. Beginning probably in
the Devonian as a branch of the fishes, amphibians showed a marvelous
development during later Paleozoic and very early Mesozoic times when
they reached their climax, after which they fell off remarkably, being
now relatively unimportant like the frogs and salamanders. They are of
special significance because they were the first of all the back-boned
animals (Vertebrates) to inhabit the land which they dominated only
until the great rise of reptiles of Mesozoic time. The reptiles in fact
evolved from the amphibians in the late Paleozoic when many transition
forms occurred. (Plate 15.) During those ancient days the numerous and
very diversified amphibians were like giant salamanders, commonly five
to eight feet long, with one Triassic form fifteen to twenty feet long,
and with heavily armored skulls two to four feet long.

Turning now to the reptiles we find that they are much more distinctly
land animals than the preceding types of Vertebrates. Reptilian life
of the earth began in late Paleozoic time as an evolutionary branch
of the amphibians. The earliest forms were in many ways much like the
amphibians, but gradually they diversified and progressed so that
before the close of the Mesozoic era, which has long been called the
"Age of Reptiles," they were the rulers of the world. "They covered
the land with gigantic herbivorous and carnivorous forms; they swarmed
in the sea, and, as literal dragons, they dominated the air." (Scott.)
Mesozoic reptiles are of special interest and significance not only in
themselves, but also because from one of their branches the birds were
evolved, and from another the mammals. "In advancing from the amphibian
to the reptile the evolution of the Vertebrates was far from finished.
The cold-blooded, clumsy and sluggish, small-brained and unintelligent
reptile is as far inferior to the higher mammals, whose day was still
to come, as it is superior to the amphibian and the fish." (Norton.)

Since the reptiles of the Mesozoic era constitute one of the few most
remarkable and diversified classes of animals which ever inhabited the
earth, we shall attempt to give the reader a fair idea of the most
typical groups which have been totally extinct since the close of the
Mesozoic era some millions of years ago. Of the swimming reptiles
which lived in the seas many types are known and only a few will be
described. Among these one important type was the ichthyosaur, a
fishlike form which not uncommonly grew to be twenty to even forty feet
long (Plate 18). The large head, sometimes four or five feet long,
contained as many as 200 big sharp teeth and enormous eyes up to a foot
in diameter. The body was heavy set, and the neck very short. There
were four short, stout swimming paddles, and the tail was vertebrated.
Some specimens of ichthyosaurs have been so perfectly preserved in
Mesozoic strata that even the unborn young are plainly seen in the
bodies! In some cases it is actually possible to tell what was the last
meal of a particular ichthyosaur those millions of years ago; in one
specimen, for example, remains of 200 creatures of the "cuttle-fish"
tribe having been found in the exact position of the stomach.

[Illustration: Fig. 59.--Chart showing the main branches in the history
of Vertebrate (back-boned) animal life reaching its culmination in man.
(By the author, in part after Cleland.)]

The mosasaurs of the late Mesozoic were the only real sea serpents of
the geologic ages. They were something like the ichthyosaurs, but with
smaller heads and much longer, more slender, serpentlike bodies. Some
grew to be thirty or forty feet long.

Plesiosaurs were perhaps the strangest of all the Mesozoic marine
reptiles. They grew to be forty to fifty feet long, with stout body,
very long, slender neck, small head, short tail, and four long,
powerful swimming paddles which were distinctly leglike. These and the
mosasaurs were both flesh eaters, as shown by the sharp teeth.

[Illustration: Plate 15.--(_a_) Restoration of a Late Paleozoic (Coal
Age) Landscape. Showing the main kinds of plants which have entered
into the making of most of our coal. Giant "club mosses" both with
and without branches, in the left background; giant "horsetail" (or
"scouring rush") plants on the right; and seed ferns in the left
foreground. A primitive reptile in the water; two large amphibians or
giant salamanders, called "stegocephalians," on the land; and a great
"dragon fly," two feet wide, in the air. (_From a drawing by Prof.
Williston. Courtesy of D. Van Nostrand Co._)]

[Illustration: Plate 15.--(_b_) Photograph of a Fossil Fern or Seed
Fern Frond on a Piece of Shale Millions of Years Old. The specimen
is of the Pennsylvanian Age and was taken from the coal fields of
Pennsylvania. (_After White, U. S. Geological Survey._)]

[Illustration: Plate 16.--Restoration Showing the General Appearance
of Some of the Largest Animals Which Ever Trod the Earth. A mounted
skeleton in the American Museum of Natural History is sixty-seven feet
long, and the skeleton of a similar creature in the Carnegie Museum,
Pittsburgh, is eighty-seven feet long. They lived millions of years ago
during the middle and late Mesozoic era. (_After C. R. Knight. Courtesy
of the American Museum of Natural History, New York._)]

The most remarkable walking reptiles of all time were the dinosaurs
or "terrible lizards." We shall describe enough types of these unique
creatures to give the reader a fair idea of their appearance and
habits. Most astonishing of all were the sauropods including the
largest animals which ever trod the earth. They grew to be as much
as sixty to ninety feet or more in length. Remarkably well preserved
skeletons have been found, one from Utah, eighty-seven feet long, being
mounted in the Carnegie Museum of Pittsburgh. The largest of these
brutes stood fifteen to twenty feet high and they must have weighed
thirty to fifty tons. The very long, serpentlike neck and tail, and
very small head were grotesque features. Considering the structure of
the dinosaurs, the kind of strata in which they are embedded, and the
associated fossil remains, it seems clear that they mostly lived in
and near fresh water and on near-by lowlands. The character of their
teeth shows that they fed entirely on soft plants which they must have
habitually bolted because their teeth were not well adapted to grinding
food. It is difficult to believe that a single huge beast could have
consumed less than a few hundred pounds of vegetable matter per day,
and, on account of the very small size of the head, he must have spent
most of his time eating. Also the comparatively very small size of
the brain, and its simplicity of structure, render it certain that
they were extremely stupid creatures. "To make up for this they had an
enormous enlargement of the spinal cord in the sacral region (i.e.,
over the hind legs). This sacral brain--if we may so call it--was ten
to twenty times bigger than the cranial brain. It was necessary in
order to work the powerful hind legs and tail." (Le Conte.)

[Illustration: Fig. 60.--Skeleton of a great four-legged (sauropod)
dinosaur. A mounted skeleton in the American Museum of Natural History,
New York, is sixty-seven feet long. This creature lived millions of
years ago during the Jurassic period. (After Marsh.)]

Another dinosaur, in some respects like the sauropod, was the stegosaur
which grew to be twenty to thirty feet long, and heavier than the
elephant. Unlike the sauropod, it had a short neck and was armored with
a double row of great plates over its back, and sharp spines (one to
three feet long) toward the end of the tail. The excessive stupidity of
the creature is proved by the fact that its very simple brain weighed
less than three ounces! Stegosaurs were plant eaters as indicated by
the tooth structure, and, though they looked ferocious, they were
probably not fighters, certainly at least nothing like the carnivorous
types of dinosaurs we shall soon describe.

[Illustration: Fig. 61.--Skeleton of the curious kind of dinosaur
(stegosaur) of Mesozoic Age with great bony plates over the back.
Length about thirty feet. (After Marsh.)]

The ferocious dinosaurs of Mesozoic time were carnivorous, or flesh
eaters, as shown by their numerous sharp teeth in relatively large
heads. The largest known type is the tyrannosaur, an almost perfect
skeleton of which, 40 feet long and 16 feet high, is mounted in the
American Museum of Natural History in New York (Plate 17). So far as
known, this was the largest carnivorous animal which ever walked on
the earth. It is evident from the structure that it walked on its
hind legs, the front ones having been much shorter and used something
like arms. There were also various other smaller forms of two-legged
flesh-eating dinosaurs, many of the wonderfully preserved tracks in
the Triassic sandstones of the Connecticut River Valley having been
made by such creatures when they walked around over soft, sandy mud
flats at least eight or ten million years ago. The sandy mud with its
tracks became somewhat hardened and then deeply buried under much more
sediment which, through the ages, has been eroded off, thus exposing to
view certain of the layers covered with tracks. Some bones of dinosaurs
have also been found in the Connecticut Valley.

[Illustration: Fig. 62.--Skeleton (restored) of a great two-legged
dinosaur of the Mesozoic era. This type of plant eater grew to be fully
twenty-five feet long. (After Marsh.)]

Another remarkable type of two-legged dinosaur was much like the flesh
eaters just described, but they were plant eaters. The largest of these
grew to be 30 feet long and 15 to 20 feet high, comparable, therefore,
to the tyrannosaur in size. A wonderful collection of almost perfect
skeletons may be seen in the museum in Brussels, Belgium. In mining
coal 1,000 feet below the surface in Belgium, twenty-two complete
skeletons and several partial skeletons were found in an ancient river
deposit of Cretaceous Age. A marvelously preserved specimen of one
of these two-legged plant eaters found in Wyoming, has been called a
"dinosaur mummy" because the skin and much of the flesh of the creature
had shriveled down upon its bones. The minutest details of the texture
of its skin are almost perfectly preserved.

[Illustration: Fig. 63.--Skeleton of a dinosaur (triceratops) with a
large remarkable head. This creature grew to be twenty-five feet long
during Cretaceous time. (After Marsh.)]

Another type of dinosaur, so different from the others, should be
briefly described. This was triceratops, or the "three-horned face"
beast, so named because of the three powerful horns which projected
forward from the top of the very large, flattened skull. It grew to be
twenty to twenty-five feet long. Skulls six to eight feet long have
been unearthed. Just where the brain might have developed, the skull
dished downward, and so one authority considers triceratops to have had
the largest head and smallest brain of all the great reptiles.

It is well known that dinosaurs of many types lived during the great
"Age of Reptiles," though by no means all types ranged through the
whole era. No dinosaurs are definitely known to have crossed the line
into the Cenozoic era. One of the most astonishing facts in the history
of animal life is the extinction of the mighty dinosaurs, but no very
satisfactory explanation has yet been offered. Probably their great
size was a contributing factor, for it is well known "that while very
large animals spend nearly all their time in eating, small animals
spend a small proportion of theirs, and most of it in other activities.
Now, as long as food is abundant, the larger animals of a race have the
better chances, but if a scarcity of food ensues, the larger animals
may all be suddenly swept out of existence." (Matthew.) Whatever may
have been the real reason for dinosaur extinction we can at least be
sure "that with the extensive changes in the elevation of land areas
(Rocky Mountain Revolution) which mark the close of the Mesozoic, came
the withdrawing of the great inland Cretaceous seas along the low-lying
shores of which the dinosaurs had their home, and with the consequent
restriction of old haunts, came the blotting out of a heroic race.
Their career was not a brief one, for the duration of their recorded
evolution was twice that of the subsequent mammalian (Cenozoic) age.
They do not represent a futile attempt on the part of nature to people
the world with creatures of insignificant moment, but are comparable
in majestic rise, slow culmination, and dramatic fall to the greatest
nations of antiquity." (Schuchert.)

Among the most extraordinary animals not only of the Mesozoic, but also
of all time, were the flying reptiles or literal dragons of the air.
Some were very small, while others were the largest creatures which
ever flew, with a spread of wing of twenty to twenty-five feet--twice
that of any modern bird. Unlike birds they had no feathers, but the
two wings consisted of large membranes (batlike) supported by one
enormously elongated finger of each front limb. The other fingers
were armed with sharp claws. The early Mesozoic flying reptiles had
sharp teeth, while the later ones were mostly entirely toothless, but
all were carnivorous. Their short bodies were supplied with tails of
varying lengths, one long-tailed species having a rudder at the end.
Their heads were fairly large, but of light build. The creature called
"pteranodon" was not only the largest of the flying reptiles, but
also probably the most highly specialized creature which ever lived,
everything possible apparently having been sacrificed to facilitate
flight (Plate 18). The hollow bones were so wonderfully light and
strong that it has been estimated that the living animal, with
twenty-five foot spread of wing, and head four feet long, could not
have weighed more than twenty-five pounds! The rear portions of the
body and hind limbs were very weak.

[Illustration: Fig. 64.--A small carnivorous flying reptile of Mesozoic
time. Spread of wings about two feet. (Restored by Marsh.)]

It should not be thought that the above-described groups of reptiles
were the only ones which existed during Mesozoic time. There were also
certain groups still living, like turtles, lizards, and crocodiles, but
they were doubtless mostly completely under the dominance of certain
of the now long-extinct types above described. The oldest-known fossil
snakes are from very late Mesozoic rocks, where they are small and
comparatively rare. More than likely they evolved from lizards by
deterioration of the legs. Poisonous snakes were not evolved until
early in the next (Cenozoic) era.

We shall now turn our attention to next to the highest class of
Vertebrate animals--the birds. They and the mammals are the only
warm-blooded animals. What is their ancestry? From what original stock
did they branch off? The oldest-known bird lived during the Jurassic
period, and it was so decidedly reptilian in character as to render it
practically certain that birds are specialized descendants of certain
Mesozoic reptiles, though not, as might be supposed, of the flying
reptiles. The few known specimens of the Jurassic birds were found
in the famous lithographic limestone quarries of Bavaria. At least
two of the specimens are in a marvelous state of preservation, with
practically the whole skeleton intact and almost perfect impressions of
the feathers on the rock. That the creature was really a bird is proved
not only by its feathers, but also its beak, brain, limb bones, and
feet. Among the reptilian characters are its long, vertebrated tail,
teeth set in sockets, and long claws on the wings. This reptilian bird
was about the size of a small crow.

By late Cretaceous time the birds made notable evolutionary progress
and they became diversified, more than thirty species being known from
Cretaceous rocks. These were distinctly more modern in structure
and appearance than the Jurassic bird. The only important reptilian
characteristic still retained was the possession of teeth. The tail
had become much shortened and the brain was still relatively smaller
than in modern birds. One type, about nine inches high, was a powerful
flier, as shown by the strong keel and wing bones. Another important
Cretaceous type was almost wholly a water dweller, with powerfully
developed legs used in swimming. Its teeth were set in grooves instead
of in sockets, thus indicating degeneration of tooth structure. This
type was notable for its size--five to six feet in length.

[Illustration: Fig. 65.--An early type of bird with teeth. This bird
grew to a height of about nine inches in Cretaceous time, millions of
years ago. (Restored by Marsh.)]

During the early part of the Cenozoic era birds became still more
advanced and numerous, with many modern groups represented. Some of
the more primitive types were, however, still left over during the
Tertiary, as, for example, a toothed bird, in which the teeth were
merely dentations of the bill, thus being the most degenerate of all
types of tooth structure.

Mammals comprise the highest class of all animals. They are, of course,
all warm blooded and characterized by suckling their young. So far
as known, mammal life began in the early Mesozoic era as a branch of
primitive reptiles, but they made little progress throughout the era
when they occupied a very subordinate position in the animal world.
They were few in number, small, and primitive in structure. There is
no evidence for the Mesozoic existence of any of the higher forms of
mammals, that is, those which give birth to well-formed young which
are prenatally attached to the mother by the so-called placentum.
"During the eons of the Mesozoic, from late Triassic time until its
close, the mammals (including the remote progenitors of humanity) were
in existence, but held in such effective check (by reptiles) that
their evolutionary progress was practically insignificant. This curb
is strikingly illustrated by the wonderful series of tiny jaws and
teeth of these diminutive creatures found in the Comanchian (early
Cretaceous) of Wyoming, in actual association with the single tooth of
a carnivorous dinosaur, many times the bulk of the largest mammalian
jaw. The removal of this check resulted (in the Tertiary period) in the
speedy evolution of the archaic mammals." (Schuchert.)

The phenomenal development of mammals during the Tertiary period forms
one of the most wonderful chapters in the whole evolution of organisms.
Even very early in the Tertiary, many important higher (placental)
types of mammals had evolved, and the simpler, more primitive Mesozoic
forms became very subordinate. By the close of the Tertiary the higher
types of mammals had become marvelously differentiated into most of
the present-day groups or types. A very significant feature of the
evolution was the steady increase in relative size of brain. The vast
numbers of fossil skeletons and bones of mammals found in Tertiary
strata is scarcely believable. In our brief discussion we can do no
more than describe a few representative examples of the Cenozoic
evolution of mammals.

The great diversity of modern placental animals may be suggested by
a few examples, as the tiger, dog, horse, camel, elephant, squirrel,
hedgehog, whale, monkey, and man. Forms like these, traced back
through their ancestors to the very early part of the Tertiary period,
gradually become less and less distinct until they cannot be at all
distinguished as separate groups, but rather there are ancestral
generalized forms which show combinations of features of the later
groups. Those early Tertiary generalized placental mammals had four
feet of primitive character, with five toes on each foot; the whole
foot, which from toe to heel touched the ground, was not adapted to
swift running; the teeth were simple (primitive) in type and of full
original number (forty-four); the toes were supplied with nails which
were about intermediate between real claws and hoofs in structure; and
the brain was relatively much smaller and simpler in structure than in
most modern mammals.

[Illustration: Fig. 66.--Chart showing the main features of interest
in the evolution of the horse family through several million years of
the present (Cenozoic) era of geologic time. (After Matthew, American
Museum of Natural History.)]

The history of the horse family furnishes an excellent illustration of
certain evolutionary changes among mammals. Skeletons of many species,
ranging from the early Tertiary to the present, have been found in
remarkable state of preservation representing every important change
in the history of the horse family. A study of the chart will make
clear some of the most striking changes which have taken place. The
oldest member of the horse family represented on the chart was about
the size of a small fox, with four toes and a degenerated fifth toe
(splint) on the front foot, and three toes and splint on the hind
foot. Since the chart was made a still more primitive form, even more
closely resembling the original five-toed ancestor, has been found.
Gradually the middle toe enlarged, while the others disappeared except
the two splints or very degenerate toes still left in the modern horse.
Increase in size of the animal and brain capacity accompanied these
changes. Also the teeth underwent notable change, and two originally
separate bones (radius and ulna) of the foreleg became consolidated
into a single stronger bone.

The even-toed hoofed mammals of to-day, like the deer, pig, and camel,
are also the product of evolution much like that of the horse, except
that two of the original five toes have been equally developed,
while the others have either greatly degenerated, as in the pig, or
disappeared entirely, as in the camel.

The elephants, or trunk-bearing animals, illustrate a very different
kind of evolution. They seem to have reached their climax of
development in the late Tertiary when they grew to be as much as 14
feet high, and were more abundant and widespread over the earth than at
any other time. The modern elephant, like the horse, has been traced
back through many intermediate forms to its primitive early Tertiary
ancestry. Some of the most important evolutionary changes took place
in the head portion. The trunk is a highly developed form of snout,
the earliest form of which was much like that of the modern tapir. The
tusks are highly specialized and elongated teeth. During the earlier
history the chin was very long and supported short tusks, so that there
were then four tusks.

Carnivorous mammals, like tigers and wolves, and gnawers, like rats and
squirrels, may also be traced back to generalized early Tertiary types.

Another kind of evolution is well illustrated by certain mammals which,
even in early Tertiary time, so thoroughly adapted themselves to a
water environment as to become whales, porpoises, etc.

[Illustration: Fig. 67.--Comparison of feet of monkeys and man.]

The primates include the highest group of all Vertebrates, and
therefore of all animals. Monkeys, apes, and man belong to the
primates. There is no evidence whatever for the appearance of even the
simplest and most primitive forms before the opening of the Cenozoic
era, but even very early in Tertiary time, lemurs and primitive types
of monkeys existed. Later in the Tertiary true monkeys and apes were
common, and by the close of the period some apes were highly enough
developed to strongly resemble certain of the oldest and most primitive
types of man. We have, however, no positive knowledge of the existence
of man in even the latest Tertiary. In the light of much evidence in
regard to the antiquity of man, it seems improbable that true human
fossils will ever be found in rocks older than the Quaternary, though
if we are willing to descend (far enough in the human scale toward
apes) it is not unlikely that man-apes may be discovered in very late
Tertiary rocks. The difficulty comes in the classification. Where
are we to draw the line between the higher apes and the lowest forms
of man? But this very difficulty is one of the strongest arguments
in favor of the organic evolution of man because practically all
intermediate forms between true man and certain other high-grade
primates are known from the strata. The following tabular summary of
the geological history of man is based upon the work of most of the
ablest students of the subject.

  ==========================+===============================+==============
  3. Homo sapiens           |Historic (bronze and iron) age.|Modern
     (e.g., modern man)     |                               |
                            |Neolithic ("recent stone") age |Postglacial
                            |  (carefully shaped and        |but
                            |  polished stone implements)   |prehistoric
  --------------------------+-------------------------------+--------------
  2. Homo primigenius       |Upper Paleolithic ("ancient    |Late Glacial
     (e.g., Neanderthal     |  stone") age (rough bone      |
     man)                   |  and stone implements, cave   |
                            |  frescoes, bone carvings,     |
                            |  etc.)                        |
                            |Lower Paleolithic ("ancient    |Middle Glacial
                            |  stone") age (rude stone      |
                            |  implements of so-called      |
                            |  "river man")                 |
  --------------------------+-------------------------------+--------------
  1. Early ancestral forms  |Possibly some very crude       |Early Glacial
     (e.g., Pithecanthropus |  stone implements             |and possibly
     erectus)               |                               |late Tertiary
  --------------------------+-------------------------------+--------------

Of the early ancestral forms, that is, those which were rather
distinctly man-apes, two will be very briefly referred to. One of
these, known as _Pithecanthropus erectus_, was a remarkable creature
whose partial skeleton, consisting of the upper part of a skull, lower
jaw, several teeth, and a thigh bone, was found in early Quaternary
deposits in Java in 1891. It was certainly a man-ape or possibly
ape-man of low order, about 5-1/2 feet high. The skull has a low crown,
very receding forehead, and prominent brow ridges, but the brain
capacity is 850 cubic centimeters, as compared to 500 cubic centimeters
in ordinary higher apes, and nearly 1,500 cubic centimeters in the
average modern man. The very recently extinct very low-type aborigines
of Tasmania had a skull capacity of 1,199 cubic centimeters.

In 1907 the lower jaw of an anthropoid or manlike ape set with rather
human teeth was found associated with very crude stone implements
seventy-five feet below the surface in river-deposited sand in Germany.
It is of either early or middle Glacial time and quite certainly
represents a lower order creature than the oldest Paleolithic man as
described below.

Many bones and implements of Paleolithic man (see above table) have
been found mainly in river gravels and caves. The relative ages
of Paleolithic human bones and implements are best determined by
the associated fossil animals. Thus the most ancient truly human
fossils are found directly associated with bones of very old types of
elephants, rhinoceroses, and hippopotamuses which are definitely known
to have lived during middle or early middle Glacial (Quaternary) time
corresponding to early Paleolithic time. A very conservative estimate
would make the age of such very old human remains at least 150,000 to
250,000 years because the Ice Age was at least 500,000 years long. In a
later human stage there are many associations with extinct animals like
an older type of mammoth, cave bear, cave hyena, and others of later
Glacial time estimated at 50,000 to 150,000 years ago. Last of all was
the latest Paleolithic stage corresponding to the close of the Ice Age,
the human remains of which are found associated with reindeer and the
latest mammoths which roamed in great numbers across Europe. This was
probably not more than 30,000 to 50,000 years ago.

Paleolithic man is so called because he fashioned stone weapons and
implements. The structure of skull and skeleton shows him to have
been a low-type savage, something over five feet high on the average,
with a forward stooping carriage. The average Paleolithic brain was
not greatly inferior in size to that of modern civilized man, but
it was not so highly organized and occupied a thick skull with much
lower forehead and heavy brow ridges. The bushmen of Australia and
the recently extinct Tasmanians are the nearest modern resemblances.
Many fine specimens of Paleolithic man have been found, especially in
cave deposits. That he was an expert hunter is proved by the great
accumulation of bones of now extinct animals found in and about his
haunts or camps, bones representing at least 100,000 horses having been
found around a single camp site!

[Illustration: Fig. 68.--Comparison of skulls: a, Paleolithic
(Neanderthal) man; b, modern man. (After Woodward, British Museum.)]

Only two among the many known Paleolithic man localities will be
briefly described. In the Perigord district of southwestern France a
number of caves contain human relics ranging in age from early to late
Paleolithic. Of special interest among these relics are fishhooks
made of bone, and crude sketches of animals such as the mammoth and
reindeer now extinct in that region. The Aurignac cave, also in France,
was no doubt a family or tribal burial place. Seventeen Paleolithic
human skeletons, associated with bones of extinct animals and crude
art works, were found in the cave. Near the entrance there were ashes
and charcoal mixed with burned and split bones of extinct animals.
Certain of the caves occupied by late Paleolithic man have their
walls decorated with sketches and even colored pictures. These are,
therefore, the oldest known art galleries. An excellent example is the
cave at Altamira in northern Spain. "As we glance at the pictures one
of the first things to impress us is the excellence of the drawing, the
proportions and postures being unusually good.... The next observation
may be that, in spite of this perfection of technique, there is no
perspective composition--that is, no attempt to combine or group the
figures.... It is also clear that the work of many different artists
is represented, covering a considerable period of time. The walls show
traces of many other paintings that were erased to make way for new
work." (Wissler.)

[Illustration: Fig. 69.--Sketch of a painting by Paleolithic man found
in a cave in west-central France. Various animals, including the
extinct mammoth elephant, are represented. (Courtesy of American Museum
of Natural History.)]

The Neolithic, or "recent stone" age was a gradual development from
the late Paleolithic, and man was then more highly developed and more
similar in structure to modern man. His stone implements were more
perfectly made, and often more or less polished and ground at the
edges. "The remains of Neolithic man are found, much as are those of
the North American Indians, upon or near the surface, in burial mounds,
in shell heaps (the refuse heaps of their settlements), in peat bogs,
caves, recent flood-plain deposits, and in beds of lakes near shore
where they sometimes built their dwellings upon piles.... Neolithic man
in Europe had learned to make pottery, to spin and weave linen, to hew
timber, and build boats, and to grow wheat and barley. The dog, horse,
ox, sheep, goat, and hog had been domesticated." (Norton.)

"Man is linked to the past through the system of life, of which he is
the last, the completing creation. But, unlike other species of that
closing system of the past, he, through his spiritual nature, is more
intimately connected with the opening future." (J. D. Dana.)



CHAPTER XX

MINERALOGY


We are more or less familiar with the division of all materials of
nature into the animal, vegetable, and mineral kingdoms. With slight
exceptions minerals are the materials which make up the known part
of the earth. In a very real sense, then, mineralogy is the most
fundamental of the various branches of the great science of geology
because the events of earth history, as interpreted by the geologist,
are recorded in the mineral matter (including most rocks) of the
earth. When we examine the rocky material or mineral matter of the
earth in any region we find that it consists of various kinds of
substances each of which may be recognized by certain characteristics.
Each definite substance (barring those of organic origin) is called
a mineral. Or, more specifically, a mineral is a natural, inorganic,
homogeneous substance of definite chemical composition. According to
this definition a mineral must be found ready made in nature, must not
be a product of life, must be of the same nature throughout, and its
composition must be so definite that it can be expressed by a chemical
formula. All artificial substances, such as laboratory and furnace
products, are excluded from the category of minerals. Coal is not a
mineral because it is both organic and of indefinite composition. A
few examples of very common substances which perfectly satisfy the
definition of a mineral are quartz, feldspar, mica, calcite, and
magnetite. Only two substances--water and mercury--are ordinarily
liquid minerals. There are nearly a thousand distinct mineral species,
and to them and their varieties several thousand names have been
applied.

It is a surprising fact that of the eighty or more chemical elements,
that is substances which cannot be subdivided into simpler ones, only
eight make up more than 98 per cent of the weight of the crust of the
earth, though, with one very slight exception, none of the eight exist
as such in mineral form. The eight elements are oxygen (nearly 50 per
cent), silicon (over 25 per cent), aluminum (over 7 per cent), iron
(over 5 per cent), calcium (or "lime"), magnesium (or "magnesia"),
sodium (or "soda"), and potassium (or "potash").

Certain rock formations are made up essentially of but one mineral
in the form of numerous grains as, for example, limestone, which
consists of calcite (carbonate of lime). Most of the ordinary rocks
are, however, made up of two or more minerals mechanically bound
together. Thus, in a specimen of granite on the author's desk several
distinct mineral substances are distinguishable by the naked eye.
These mineral grains are from one to five millimeters across. Most
common among them are hard, clear, glassy grains called quartz; nearly
white, hard grains, with smooth faces, called feldspar; small, silvery
white plates, easily separable into very thin flakes, called mica;
and small, hard, black grains, called magnetite. It is the business
of the mineralogist to learn the characters of each mineral, how they
may be distinguished from each other, how they may be classified, how
they are found in nature, and what economic value they may have. It
is an important part of the business of the geologist to learn what
individual minerals combine to form the many kinds of rocks, how such
rocks originate, what changes they have undergone, and what geological
history they record. It is thus clear that the great science of geology
is much broader in its scope than mineralogy.

One of the most remarkable facts about minerals is that most of them
by far have a crystalline structure, that is they are built up of tiny
particles known as molecules. Such crystalline minerals are often more
or less regular solid forms bounded by plane faces and sharp angles,
such forms being known as "crystals." How do crystals develop such
regularity of form? Any solid is considered to be made up of many very
tiny (submicroscopic) molecules held together by an attractive force
called cohesion. In liquids the molecules may more or less freely roll
over each other, thus altering the shape of the mass without disrupting
it. In gases the molecules are considered to be relatively long
distances apart and moving rapidly. During the process of change of a
substance from the condition of a liquid or gas to that of a solid, due
to lowering of temperature or evaporation, the cohesive force pulls
the particles (molecules) together into a rigid mass. Under favorable
conditions such a solid has a regular polyhedral form. "This results
from the fact that the particles or molecules of the substance which,
while it was liquid or gaseous, rolled about on one another, have
been in some way arranged, grouped and built up. To illustrate this,
suppose a quantity of small shot to be poured into a glass: the shot
will represent the molecules of a substance in the liquid state, as
for example a solution of alum. If, now, we suppose these same shot to
be coated with varnish or glue so that they will adhere to each other,
and imagine them grouped as shown in Figure 70a, they will represent
the arrangement of the molecules of the alum after it has become solid
or crystallized. This arranging, grouping, and piling up of molecules
is called crystallization, and the solid formed in this way is called
a crystal. Figures 70b and 70c show the shot arranged to reproduce two
common forms of crystals (e.g., fluorite and calcite)." (Whitlock.)

[Illustration: Fig. 70.--Piles of shot arranged to give some idea of
the manner in which molecules are bound together in various crystal
forms. (After Whitlock, New York Museum.)]

A combination of certain facts regarding crystals furnish all but
absolute proof of some sort of regularity of arrangement of particles
within them. Among such facts are the following: (1) the wonderful
regularity of arrangement of faces upon crystals is practically
impossible to account for except as the outward manifestation of
regularity of structure or systematic network arrangement of the
interior; (2) most crystals split or cleave more or less perfectly
in one or more directions presumably in accordance with certain
layered structure of the constituent particles; (3) all of the many
known forms of crystals can be accurately grouped in regard to their
effects upon the passage of light (especially polarized light) through
them, each kind or type of network structure presumably producing a
different effect upon light; and (4) X-ray photographs have proved that
particles, or at least groups of particles, are very systematically
arranged within crystals.

It will be instructive for us to make a comparison between the
growth of crystals and organisms. Both really grow, but each species
of organism is rather definitely limited in size while there is no
known limit to the size which may be attained by a crystal so long as
material is supplied to it under proper conditions. As a matter of
fact crystals vary in size from microscopic to several feet in length,
those less than an inch in length being most abundant by far. Organisms
mostly grow from within, while crystals grow from material externally
added. It is an astonishing fact that in crystals as well as organisms
growth takes most rapidly on a wound or broken place. Thus if a crystal
is removed from the solution in which it is growing and put back
after a corner has been broken off, the fractured surface will build
up more rapidly than the rest. Finally, crystals are not necessarily
limited in age like organisms. Under certain natural conditions, as,
for example, weathering, crystals may decay or be broken up; but where
they are protected as constituent parts of rock formations well below
the earth's surface they may remain unchanged for indefinite millions
of years. Thus in a ledge of the most ancient known or Archeozoic rock
only recently laid bare by erosion one may see crystals which are
precisely as they were when they crystallized many millions of years
ago.

One of the most remarkable properties of a crystal is its symmetry,
by which is meant the greater or less degree of regularity in the
arrangement of its faces, edges, and vertices. A given substance may,
according to circumstances, crystallize in a variety of forms or
combinations of forms, but, with very few exceptions, all crystals of
a given substance exhibit the same kind or grade of symmetry. There
are three kinds of crystal symmetry, namely, in respect to a plane,
a line or axis, and a point or center. A plane of symmetry divides a
crystal into halves in such a way that for every point on one side of
the plane there is a corresponding point directly opposite on the other
side. Crystals may be cut into halves along various surfaces which
are not symmetry planes. An axis of symmetry is a line about which a
complete rotation (or in a few cases rotation combined with reflection)
brings the crystal into the same relative position two, three, four
or six times, these being called two, three, four, and sixfold axes
of symmetry--no others being possible. A crystal has a center of
symmetry when any line passing through it encounters corresponding
points at equal distances from it on opposite sides. There are just
32 classes or combinations of the symmetry elements among crystals
and just 232 definite crystal forms. Not only is it demonstrable that
no more can exist, but actual experience with crystals of hundreds
of species of minerals has never revealed any more. Obviously, then,
symmetry furnishes us with a very scientific basis of classification
of crystals, all of the 232 crystal forms constituting the 32 symmetry
classes being in turn referable to seven fundamental crystal systems.
To bring out the relations of the faces of a crystal and further aid
in classification, prominent, straight lines or directions passing
through the center of a crystal are chosen as crystallographic axes.
Such axes may or may not coincide with symmetry axes. Basing our
definitions upon both symmetry axes and crystallographic axes, the
seven systems are as follows:

1. Isometric. There must be at least four threefold axes of symmetry,
while the highest grade symmetry class of the five in the system
includes three fourfold, four threefold, and six twofold axes of
symmetry; nine planes of symmetry; and a center of symmetry. There are
three interchangeable crystallographic axes at right angles to each
other.

[Illustration: Fig. 71.--Figures showing, a, crystal axes of Isometric
system; b, points of emergence of the nine axes of symmetry in a cube
of the Isometric system; c, nine planes of symmetry in a cubic crystal.
(After Whitlock, New York State Museum.)]

2. Tetragonal. There must be one and only one fourfold symmetry axis,
while the highest of its seven symmetry classes contains also four
twofold axes of symmetry; five planes; and a center. Characterized by
three crystallographic axes at right angles to each other, only two of
them interchangeable.

3. Trigonal. Characterized by one and only one threefold symmetry axis,
the highest of the five classes having also three twofold axes; four
planes; and a center. Crystallographic axes as for hexagonal.

4. Hexagonal. One and only one sixfold axis of symmetry must
be present, but the highest of the seven classes also has six
twofold axes; seven planes; and a center. Characterized by four
crystallographic axes, one vertical and three interchangeable
horizontal axes making angles of 60 degrees with each other.

5. Orthorhombic. There must be no axis of symmetry higher than a
twofold and three prominent directions (i.e., parallel to important
faces) at right angles to each other, the highest grade of the three
classes having three twofold axes; three planes; and a center. There
are three noninterchangeable crystallographic axes at right angles.

6. Monoclinic. There is no axis of symmetry higher than a twofold
and only two prominent directions at right angles to each other, the
highest of the three classes having one twofold axis; one plane; and a
center. There are three noninterchangeable crystallographic axes, only
two of which are at right angles.

7. Triclinic. There is no axis of symmetry of any kind, and there
are no prominent directions at right angles. One of the two classes
has a center of symmetry only, and the other no symmetry at all.
Characterized by three noninterchangeable crystallographic axes, none
at right angles.

A fact which should be strongly emphasized is that crystals only, of
all the objects of nature, can be definitely referred to the above
seven systems comprising the 32 classes of symmetry, and 232 crystal
forms. Since there are about 1,000 mineral species and only 232
fundamental forms, it necessarily follows that two or more species may
crystallize in the same form within a class, so that it is not always
possible to tell the species of mineral merely by its crystal form.
It is, however, a remarkable fact that, where two or more substances
crystallize in the same class (i.e., show the same grade of symmetry)
each substance almost invariably exhibits "crystal habit" which is a
pronounced tendency to crystallize in certain relatively few forms or
combinations of forms out of the many possibilities. It is clear, then,
that grade of symmetry combined with "habit" are of great practical
value in determining crystallized minerals, because, on the basis of
symmetry, a crystal is referred to a certain definite symmetry class in
which only a limited number of substances crystallize, and then, by its
characteristic "habit," the particular substance can be told.

[Illustration: Fig. 72.--Figures illustrating three crystal forms
with exactly the same symmetry elements; a and b are separate forms,
and c is a combination of the two. The mineral "garnet" nearly always
crystallizes in one of these forms.]

From the above discussion it should not be presumed that crystals
always develop with perfect geometric symmetry. As a matter of fact
such is seldom the case because, due to variations of conditions or
interference of surrounding crystals in liquids (ordinary or molten), a
crystal usually grows more rapidly (by building out faces) in certain
directions than in others. Under such conditions actual crystals are
said to become distorted because they are not geometrically perfect.

Whether geometrically perfect or not, all crystals respond to the law
of constancy of interfacial angles which means that on all crystals of
the same substances the angles between similar (corresponding) faces
are always equal. This is one of the most fundamental and remarkable
laws of minerals. That it must be true follows from the fact that the
crystal faces merely outwardly express in definite form the definite
internal structure or arrangement of particles which have built up the
crystal. In other words, the real structural symmetry of a crystal
never varies no matter how much its geometric symmetry may vary.
The practical application of the law of constancy of interfacial
angles lies in the fact that in many cases a mineral may actually be
identified merely by measuring the interfacial angles of its crystal
form.

The relative lengths of the crystallographic axes is a very important
feature of all crystals except those of the isometric system in which
the axes are always of equal length so that the ratio is 1:1:1. In
all the other systems, however, at least one axis differs in length
from the others and, since the amount of difference is absolutely
characteristic of each substance, the axial ratio of a crystal, when
carefully determined by measurement of the angles between the different
faces, affords a never-failing method of determining the mineral
for all systems except the isometric. By way of illustration, the
tetragonal crystal of the mineral zircon, with only one axis different
in length, shows the very definite axial ratio 1:1:0.64, while the
orthorhombic crystal of sulphur, with all three axes of different
lengths, has an axial ratio 0.813:1:1.903. These ratios of course
always hold true no matter what the size or particular outward form of
the crystal.

As might be expected from the above discussion of the remarkable
structure of crystals, experience has proved that the relative lengths
of all intercepts (or distances from the center) of all faces upon
any crystal can be expressed by whole numbers, definite fractions, or
infinity. It necessarily follows that the ratios between the intercepts
of the faces of any face on a crystal to those of any other face on
the same crystal may always be expressed by rational numbers, and this
is known as the law of definite mathematical ratio. It is a remarkable
fact that very small whole numbers or fractions, or infinity or zero,
will always express the intercepts of any crystal face.

Thus far our discussion has centered about crystals as individuals,
but, in most cases by far, they form groups or aggregates. Most
commonly crystal grouping is very irregular, but by no means rare
is parallel grouping where whole crystals, or more usually parts of
crystals, have all corresponding parts exactly parallel. But most
remarkable of all are the twin crystals in which two or more crystals
intergrown or in contact have all corresponding parts in exactly
reverse order. The conditioning circumstances under which twin crystals
develop are unknown.

In the light of the facts and principles above explained, the reader
will more than likely agree with the author that crystals rank very
high among nature's most wonderful objects. But there are still other
characteristic features of crystals naturally resulting from their
marvelous structure. Some of these will now be briefly referred to.

[Illustration: Fig. 73.--Figures illustrating twin crystals: a, gypsum
(Monoclinic system); b, fluorite (Isometric system); c, cassiterite
(Tetragonal system). (After New York State Museum Bulletin.)]

Many crystals and crystalline substances exhibit the important property
known as cleavage which is the marked tendency to break easily in
certain directions yielding more or less smooth plane surfaces. As
would be expected, a cleavage surface is always parallel to an actual,
or at least a possible, crystal face, and it takes place along the
surfaces of weaker molecular cohesion. The degree of cleavage varies
from almost perfect, as in mica, to very poor or none at all, as in
quartz. The number of cleavage directions exhibited by common minerals
is illustrated as follows: mica, one; feldspar, two; calcite, three;
and fluorite, four.

It is a striking fact that when a crystal or cleavage piece is placed
in a solvent, the action proceeds with different velocities in
crystallographically different directions and little pits or cavities,
called etching figures, are developed on some or all of the faces.
Since the symmetry of these etching figures and their arrangement upon
the faces are directly related to, and natural effects of the crystal
symmetry, the figures often furnish an important method of placing
a doubtful crystal or even merely a cleavage fragment in its proper
symmetry class.

Another marvelous property of crystals and crystalline substances
is their effect upon light. Since the study of the passage of light
through crystals has really become a large separate branch of
mineralogical study, we can no more than state a few fundamental facts
and principles in the short space at our disposal. Light is caused by
vibrations of the so-called "ether," and always travels in straight
lines. The vibration directions are at right angles to the direction
of transmission of the light. When a ray of light enters a crystal
or crystalline mineral representing any crystal system except the
isometric it is doubly refracted (i.e., broken into two rays), each
of the two rays is polarized (i.e., made to vibrate in a single plane
only), and one ray vibrates almost at right angles to the other. Double
refraction is strikingly shown by placing a piece of clear calcite
(Iceland spar) over a dot on paper when two dots instead of one are
visible. The amount of double refraction varies with the substance, and
in some degree according to the direction of passage of light through
a crystal. Isometric crystals only are singly refracting and hence a
ray of light is not affected in passing through them. Crystals of all
the other six systems doubly refract and polarize light and in three
systems--tetragonal, hexagonal, and trigonal--one direction (coincident
with the main axis of symmetry) produces single refraction only,
while in the remaining three systems--orthorhombic, monoclinic, and
triclinic--there are always two directions of single refraction whose
positions vary with the substance. Many crystals outside the isometric
system also exhibit a remarkable tendency to absorb light differently
in different crystallographic directions, thus producing two or three
color tints, which vary according to the substance. After gaining a
practical knowledge of the above and many other optical properties
of crystals, it is possible by the aid of a specially constructed
(polarizing) microscope, to recognize (with few exceptions) each one of
the many mineral species. This method is of great value in determining
the various minerals which are aggregated in the form of a rock, in
which case a very thin slice of the rock is studied with the microscope.

An important criterion for the recognition of minerals is hardness,
by which is meant the resistance of a smooth surface to abrasion or
scratching. The generally adopted scale of hardness follows:

  1.--Soft, greasy feel, and easily scratched by the finger nail (e.g.,
        talc).

  2.--Just scratched by the finger nail (e.g., gypsum).

  3.--Just scratched by a copper coin (e.g., calcite).

  4.--Easily cut by a knife, but does not cut glass (e.g., fluorite).

  5.--Just scratches soft glass, and is cut by a knife (e.g., apatite).

  6.--Harder than steel, and scratches glass easily (e.g., orthoclase).

  7, 8, 9, and 10.--Harder than any ordinary substance and
        represented in order by quartz, topaz, corundum, and diamond.

[Illustration: Plate 17.--Skeleton of the Great Two-Legged, Carnivorous
Dinosaur Reptile, Called "Tyrannosaurus," Which Lived During Cretaceous
Time. (_Courtesy American Museum of Natural History._)

Small Picture.--Restoration of the Earliest Known Bird of Which Several
Nearly Perfect Skeletons Have Been Found. This feathered creature with
reptilian characteristics lived at least 5,000,000 years ago. It had a
long vertebrated tail, claws on the ends of the wings, and teeth. (_By
E. W. Berry._)]

Plate 18.--(_a_) Skeleton of the Largest Known Creature That Ever Flew.
It was a flying reptile with spread wings of nearly twenty-five feet,
and lived during the Cretaceous period several million years ago.
(_Courtesy of the American Museum of Natural History._)

Plate 18.--(_b_) Skeleton of a Remarkable Swimming Reptile of the
Mesozoic Era. Length about twelve feet. Parts of skeletons of unborn
young are seen. (_Courtesy of the American Museum of Natural History._)

Minerals also show a great variety of colors. Many of them like quartz
and calcite are colorless or white, others like galena (steel-gray)
and pyrite (brass-yellow) show inherently characteristic colors, while
still others like amethyst (purple) and sapphire (blue) are colored by
impurities.

There is also a great range in relative weights or density of minerals,
commonly called the specific gravity, which range from less than one
for ice to 21.5 for platinum, and even somewhat higher. The average
specific gravity of all minerals of the earth is about 2.6.

In the light of the above discussion of the general properties of
minerals, we shall now proceed to name and briefly describe some of the
minerals which are either very common, or of special interest, or of
special economic importance. Only those features are listed by which
the mineral species may be recognized at sight, or by the aid of very
simple nonchemical tests.

[Illustration: Fig. 74.--Drawings showing forms of crystals of common
minerals: a and b, garnet (Isometric); c and d, feldspars (Monoclinic);
e, f, and g, quartz (Trigonal); h, i, and j, calcite (Hexagonal); k,
augite (Monoclinic); l. hornblende (Monoclinic); m, pyrite (Isometric).]

Amphibole. A number of species closely related in composition, crystal
form, and properties are here included. They are silicates of lime
and magnesia usually with aluminum and iron. Most common by far are
those which crystallize in the monoclinic system with prismatic faces
and two good prismatic cleavages meeting at about 24 degrees. Color,
commonly brown to black, but sometimes green or white. Hardness varies
from 5 to 6, and specific gravity from 3 to 3.4. _Hornblende_, the
most common species, is a dark colored silicate of lime, magnesia,
aluminum, and iron. It is one of the few most common of all mineral
species, especially in igneous and metamorphic rocks. _Tremolite_ is
a white to light gray silicate of lime and magnesia found especially
in metamorphic limestones. _Actinolite_ is a green silicate of lime,
magnesia, and iron especially common in certain metamorphic rocks. One
kind of jade is an amphibole similar to tremolite and actinolite in
composition, while the other kind is a pyroxene (see below). _Jade_ is
and has been highly prized in the east (especially in China) where it
has been carved into many objects of exceptional variety and beauty.
Jade is probably the toughest (not hardest) of all minerals because of
its wonderful microscopically fibrous structure. In color it is white,
gray, and green.

Apatite. Crystallizes in the hexagonal system with a six-sided prism
usually capped at each end by a six-sided pyramid (see Figure 75g).
Composition, a phosphate of lime. Color variable, but mostly white,
green, or brown. Hardness of 5, or just enough to scratch soft glass.
Specific gravity, 3.2. No good cleavage. Tiny crystals are widely
disseminated through many common rocks--igneous, metamorphic, and
sedimentary. In certain metamorphic limestones excellent crystals
a foot or more in length have been found. Apatite, mostly in
uncrystallized form, is the source of most of our phosphate fertilizers.

Azurite. An azure-blue hydrous carbonate of copper which crystallizes
commonly in small monoclinic crystals. Hardness, nearly 4, and specific
gravity, nearly 4. Commonly occurs in veins deposited by underground
water. One of the great ores of copper, especially in Arizona, Chile,
and Australia.

Barite. A sulphate of barium crystallizing in orthorhombic prisms
usually of tabular habit. White to light color shades. Hardness, 3.5;
specific gravity, 4.5, which is notably higher than the average of
light-colored minerals. Three good cleavages parallel to principal
crystal faces. A common and widely distributed mineral, especially in
many vein deposits associated with certain ores. Used in ground form to
give weight to certain kinds of paper and cloth, and a barium compound
used for refining sugar is made from it.

[Illustration: Fig. 75.--Crystal forms cf common minerals: a, galena
(Isometric); b, sphalerite (Isometric); c, beryl (Hexagonal);
d, hematite (Hexagonal); e, magnetite (Isometric); f, barite
(Orthorhombic); g, apatite (Hexagonal); h. sulphur (Monoclinic);
i, gypsum (Monoclinic); j, chalcopyrite (Tetragonal); k, fluorite
(Isometric); l, zircon (Tetragonal); m, tourmaline (Trigonal); n,
corundum (Hexagonal).]

Beryl. A silicate of aluminum and the rare chemical element beryllium.
Hexagonal crystals usually of very simple six-sided prismatic habit
(see Figure 75c). Color white, green, blue, or yellow. Specific
gravity, 2.8. Cleavage practically absent. It is a very exceptionally
hard mineral, being 8 in the scale. Very large crystals have been
found, as, for example, in New Hampshire, where single crystals several
feet long weigh a ton or more. Beryl is also of special interest
because two of its varieties--_emerald_ (green) and _aquamarine_
(blue)--are well-known gem stones, the emerald being one of the most
highly prized gems. The colors are due to slight impurities. Beryl most
commonly occurs in dikes of coarse granite called pegmatite, but also
in certain metamorphic and sedimentary rocks.

Calcite. Commonly called "calc spar." A carbonate of lime. Hexagonal
crystals in a great variety of forms, but all with crystal faces
arranged in sixes around the principal or vertical axis forming
rhombohedrons, prisms, or double-pointed pyramids. The principal axis
of symmetry is sixfold by a combination of rotation and reflection.
Very perfect cleavages in three directions yielding fragments whose
faces make angles of 75 and 105 degrees. Color, white when pure, but
variously colored when impure. Hardness, 3 (very easily scratched by
a knife); specific gravity, 2.7. Calcite is a very common mineral,
especially in limestone (including _chalk_) and marble which are
usually largely made up of it. Also commonly found in veins, and as
spring and cave deposits (stalactites). A porous, stringy variety,
called _travertine_, is deposited by certain hot springs, as at Mammoth
Hot Springs in Yellowstone Park. A very transparent crystalline variety
is called _Iceland spar_. Calcite is a very useful mineral. Limestone
and marble are widely used as a building stone, and for decorative
purposes, statuary, etc. Limestone is burned for quicklime, used as a
flux in smelting certain ores, in glass making, etc.

Cassiterite. The one great ore of tin whose composition is oxide of
tin. Tetragonal crystallization (Figure 73c). Hardness greater than
steel, being over 6 in the scale. Specific gravity 7, which is notably
high. Color, brown to nearly black. Cleavage, practically absent.
Fairly widespread in small amounts, and in commercial quantities in
only a few localities, usually in veins in granite or metamorphic rocks
near granite, as at Cornwall, England, also in the form of rounded
masses in gravel deposition as in the Malay region.

Chalcocite. Crystallizes in the orthorhombic system, usually in tabular
form, but crystals not common. A black sulphide of copper with metallic
luster. Hardness, nearly 3; specific gravity, nearly 6. No cleavage.
Chalcocite occurs in vein deposits as one of the important copper ores,
especially at Butte, Montana.

Chalcopyrite. Known as "copper pyrites," (Figure 75j). A deep
brass-yellow sulphide of iron and copper. Seldom crystallized in
tetragonal forms. Hardness, 3.5; specific gravity, over 4. No cleavage.
Metallic luster. Widely distributed in vein deposits associated
with other metal-bearing minerals. A very important ore of copper,
especially at Rio Tinto, Spain.

Chlorite. A soft, green mineral, usually in small tabular crystals,
in general appearance much like mica (see below), but unlike mica,
the almost perfect cleavage leaves are not elastic, though they are
flexible. Composition, a silicate of aluminum and magnesia. Always of
secondary origin as a result of chemical alteration of certain other
minerals, such as biotite-mica, pyroxene or amphibole.

Cinnabar. A vermilion-red sulphide of mercury. An extra soft metallic
mineral, only 2.5 in the scale. Specific gravity over 8, which is
notably high. Completely vaporizes on being heated. Small trigonal
crystals rare. Cinnabar is the one great ore of mercury, occurring in
veins, especially in California and Spain.

Copper. Copper as such (so-called "native copper") is widely
distributed in veins, usually in small amounts with other copper
minerals, but in the great mines of northern Michigan it occurs in
immense quantities as the only important ore. It is readily recognized
by its color, softness (less than 3), and notable weight (specific
gravity, nearly 9). Isometric crystals uncommon.

Corundum. An oxide of aluminum of hexagonal crystallization, usually
in six-sided prisms, capped by very steep pyramidal faces (see Figure
75n). It is next to the hardest of all known minerals (9 in the scale),
the diamond only exceeding it. Specific gravity about 4. Three good
cleavages making angles of nearly 90 degrees with each other. The
color of corundum is usually brown, but it varies greatly. Two of the
most highly prized of all precious stones--_ruby_ (red) and _sapphire_
(blue)--are nearly transparent varieties of corundum, colored by
certain impurities. _Oriental topaz_ (yellow), _oriental emerald_
(green), and _oriental amethyst_ (purple) are also clear varieties of
corundum. It occurs in various igneous and metamorphic rocks, and in
some stream gravels. The finest rubies, associated with some sapphires,
occur in gravels in Burma, Siam, and Ceylon. _Emery_ is a fine-grained
mixture of corundum and other minerals, especially magnetite.

Diamond. This mineral is remarkable not only because it is the king
of precious stones, but also because it is easily the hardest known
substance (10 in the scale). Specific gravity, 3.5. Very brilliant
luster. Crystals of usually octahedral habit in the isometric system.
Usually colorless, but often variously tinted. Composition, pure
carbon. Burns completely away at high temperature. The greatest mines
in the world are in South Africa, where the diamonds occur in masses of
rather soft (decomposed) igneous rock, evidently having crystallized
during the cooling of the molten masses. In Brazil and India diamonds
are found in stream gravels.

Feldspar Group. The feldspars are by far the most abundant of all
minerals in the crust of the earth. (Figures 74c, 74d.) There are
several important species or varieties of feldspar with certain
features in common as follows: crystal forms, either monoclinic
or triclinic (closely resembling monoclinic), in prismatic forms
whose faces usually meet at or near 90 or 120 degrees; two good
cleavages at or near 90 degrees, hardness at or near 6; specific
gravity, a little over 2.5; color, usually white, gray, or pink; and
composition, silicate of aluminum with potash, soda, or lime. The two
potash feldspars are _orthoclase_ and _microcline_, the former being
monoclinic, with cleavages at exactly 90 degrees, and the latter
triclinic, with cleavages a little less than 90 degrees. A kind of
green microcline is known as _Amazon stone_. The soda-lime feldspars
go by the general name _plagioclase_. They are triclinic, with
cleavages meeting at approximately 86 degrees. Very commonly one of
the cleavage faces exhibits characteristic, well defined striations
or fine parallel lines caused by multiple twinning during crystal
growth. Some of the common plagioclases are _albite_, a white soda
feldspar, including most so-called _moonstone_; _oligoclase_, a usually
greenish-white to reddish-gray soda-lime feldspar including _sunstone_;
and _labradorite_, a lime-soda feldspar, usually gray to greenish-gray
with a beautiful play of colors. The feldspars occur in all three great
groups of rocks, but they have most commonly crystallized during the
cooling of molten masses of igneous rocks. Where many sedimentary rocks
have undergone great change (metamorphism) under conditions of heat,
pressure, and moisture, feldspars have very commonly formed. Orthoclase
and microcline feldspar are used in the manufacture of porcelain and
chinaware. Some special varieties of feldspar are cut or polished for
semiprecious stones or decorative purposes.

Fluorite. A common mineral whose composition is fluoride of lime.
(Figure 73b.) Isometric crystals, usually cubes with edges modified,
are common. Twinned cubes are also common. Easily scratched by a
knife (hardness, 4), and specific gravity a little over 3. Clear
and colorless when pure, but variously colored, especially green,
blue, yellow, and brown, due to impurities in solution during
crystallization. Remarkable because of its four good cleavages meeting
at such angles as to permit good cleavage octahedrons to be broken out
of crystals. Fluorite is widely distributed, most commonly in vein
deposits, often associated with metallic ores. Occurs also as crystals
in some limestones and igneous rocks. Some fissure veins of fluorite
in limestone in southern Illinois are twenty to forty feet wide. Used
mostly as a flux in the manufacture of certain steel, in glass making,
and in making enamel ware.

Galena. Commonly as isometric crystals either as cubes or combinations
of cubes and octahedrons. Composition, sulphide of lead. (Figure 75a.)
Color, lead-gray with metallic luster. Hardness, 2.5; specific gravity
high, 7.5. Very brittle. Three excellent cleavages at right angles and
parallel to the crystal faces of the cube. Nearly all of the lead of
commerce comes from the smelting of galena. It is mined in many parts
of the world where it nearly always occurs in typical vein deposits
often associated with sphalerite (see below).

Garnet Group. The members of this very interesting mineral group very
commonly occur in isometric crystallized forms, mostly twelve and
twenty-four faced figures or both combined, as shown by Figure 72. All
the six species of garnets are silicates, mostly of aluminum usually
with either lime, magnesia, or iron. Cleavage, very imperfect or
absent. Hardness great, 6.5 to 7.5, and specific gravity 3.1 to 4.3,
varying according to species. Color also varies with composition, but
most commonly red, brown, and more rarely yellow, black, and green.
Garnets are most common as crystals embedded in metamorphic rocks,
especially highly altered strata. Also occurs in many igneous rocks and
in some sands. Commonly used as a semiprecious stone, and also ground
for use as an abrasive, especially in making a kind of sand (or garnet)
paper.

Gold. Gold as such ("native gold") is, in small amounts, really a very
widely distributed mineral. It is characterized by its yellow color,
softness (less than 3 in the scale), great weight (specific gravity,
over 19), and extreme malleability. Most of the commercial gold occurs
in river gravels (so-called "placer deposits"), and in veins associated
with the very common mineral quartz.

Graphite. Commonly called "black lead," but it is not lead at all. Its
composition is pure carbon--the same as that of the diamond. We here
have a very remarkable example of a single substance (carbon) which,
according to circumstances, crystallizes in two distinctly different
systems (diamond in isometric, and graphite in hexagonal) yielding very
thin, flexible flakes; greasy in feel; and easily rubs off on paper. It
weighs less than the average mineral (specific gravity, a little over
2). Good crystals of hexagonal tabular form are rare. The most natural
home of graphite is in the metamorphic rocks, especially certain of
the highly altered strata, where it occurs in the form of more or less
abundant flakes, having originated from organic matter. Some also
occurs in igneous rocks and in veins. Large quantities are made at
Niagara Falls from anthracite by electricity.

Gypsum. Monoclinic crystals common, usually of simple forms, as shown
by Figure 75i. Sometimes twin crystals. Composition, sulphate of lime.
Colorless or white when pure. Can be scratched by the finger nail
(hardness, 2). Specific gravity, 2.3. Three good cleavages, especially
the prismatic, yielding cleavage plates with angles of 66 and 114
degrees. Thin cleavage layers, moderately flexible. There are several
varieties: (1) _selenite_, which is clear, crystalline; (2) _satin
spar_, fibrous with silky luster; (3) _alabaster_, fine-grained and
compact crystalline; and (4) _rock gypsum_, massive granular or earthy.
Gypsum is common and widespread especially among stratified rocks
often as thick beds which have mostly resulted from evaporation of
bodies of water containing it in solution, and often associated with
salt beds. Also occurs as scattering crystals in shales and clays, and
in some veins. In greatest quantities it is burned to make plaster
of Paris. Satin spar and alabaster are often cut and polished for
ornaments, etc. (See Figure 73a.)

Halite. Common salt. Composition, chloride of soda. Isometric crystals,
nearly always in cubes with three good cleavages at right angles, and
parallel to the faces of the cube. Hardness, 2.5; specific gravity,
2.5. Colorless to white when pure. Characteristic salty taste. Abundant
and widespread, often as extensive strata in rocks of nearly all ages,
having resulted from evaporation of inland bodies of salt water. Also
in vast quantities in solution in salt lakes and the sea. Halite
has many uses, as for example, cooking and preservative purposes,
indirectly in glass making and soap making, glazing pottery, and in
many ore-smelting and chemical processes.

Hematite.--One of the common and important iron oxides with less iron
than magnetite and no water as has limonite. Crystallizes in hexagonal
forms. Color, black, with metallic luster, when crystalline, otherwise
usually dull red. Hardness, about 6; specific gravity, about 5. No
cleavage. Red streak when rubbed on rough porcelain. Hematite is
extremely widespread in rocks of all ages, especially in metamorphic
and sedimentary rocks. Some occurs as crystals in igneous rocks, and
some in vein deposits. It is the greatest ore of iron in the United
States, especially in Minnesota, Michigan, Wisconsin, and Alabama.

Kaolin. Commonly called "China clay." Composition, a hydrous silicate
of aluminum. Crystallizes in scalelike monoclinic forms, but usually
forms compact claylike masses. Hardness, a little over 2; specific
gravity, 2.6. Color when pure, white. Usually feels smooth and plastic.
Very abundant and widespread, especially forming the main body of clay
and of much shale. Always of secondary origin, generally resulting from
the decomposition of feldspar. It is the main constituent of chinaware,
pottery, porcelain, tiles, bricks, etc.

Limonite. An important oxide of iron in composition like hematite
except for its variable water content. Never crystallized. Hardness,
about 5; specific gravity, nearly 4. Color, light to dark brown to
nearly black. Leaves a characteristic yellowish-brown streak when
rubbed on rough porcelain. Exceedingly common and widely distributed,
always as a mineral of secondary origin as a product of weathering
of various iron-bearing minerals. Where accumulated in considerable
deposits it is an iron ore of some importance.

Magnetite. One of the three important oxides of iron containing no
water, and richer in iron than hematite. (See Figure 75e.) Commonly
crystallizes in isometric octahedral forms alone or combined with
twelve-faced forms. Hardness, 6; specific gravity, 5. Color, black
with metallic luster. Leaves black streaks on rough porcelain.
Characteristically highly magnetic. Wide-spread as crystals in nearly
all kinds of igneous rocks, and as large segregation masses in certain
igneous rocks. Also very common in metamorphic rocks, in many cases
forming lenses and beds as ore deposits. Occurs in some strata and
sands. It is an important ore of iron.

Malachite. A light-green hydrous carbonate of copper. In almost every
way, except difference in color and slight difference in composition,
it is very much like azurite (see above).

Mica Group. The micas rank high in abundance among the most common
minerals of the earth. All of the several species are silicates of
aluminum combined with other chemical elements according to the
species. All crystallize in monoclinic six-sided prisms whose angles
are nearly 120 degrees. These prisms closely approach true hexagonal
forms. All are characterized by one exceedingly good cleavage at right
angles to the prismatic faces, yielding very thin elastic cleavage
sheets. Hardness, 2 to 2.5; specific gravity, 2.7 to 3. The various
species or varieties are not always sharply separated from each other.
Most common are: _muscovite_, or so-called _isinglass_, a potash mica
which is colorless and transparent in thin sheets when pure; _biotite_,
an iron-magnesia mica, black to dark green; and _phlogopite_, a brown
magnesia mica.

Olivine. Often called _chrysolite_. A silicate of iron and magnesia.
Orthorhombic crystals, usually in stout prismatic form. Color, usually
yellowish green. Hardness, nearly 7; specific gravity, 3.3. Transparent
to translucent. No real cleavage. Its hardness, color, and crystal form
generally characterize it. It is a fairly common mineral found mainly
as crystalline grains in certain dark-colored igneous rocks. A clear
green variety, called _peridot_, is used as a gem stone.

Opal. An oxide of silicon, like quartz in composition except that it
is combined with a varying amount of water. It never crystallizes,
probably because of its rather indefinite composition. Hardness 5.5
to 6.5 (softer than quartz); specific gravity, about 2. Varieties
variously colored. _Common opal_, usually translucent with greasy
luster. _Precious opal_, translucent with beautiful play of colors,
used as a gem. _Fire opal_, with bright red to orange internal
reflections. _Hyalite_, colorless and transparent in small rounded
masses. _Wood opal_, wood petrified by opal. _Geyserite_, a white,
porous, stringy variety deposited by certain hot springs like
the Yellowstone geysers. _Tripolite_, fine-grained, chalklike in
appearance, consisting of tiny siliceous shells of very simple plants
called diatoms.

Platinum. This mineral occurs as an impure native metal, usually
alloyed with certain other metals. Native platinum, hardness, 4.5
(exceptionally high for a metal); specific gravity as usually alloyed,
14 to 19. Pure platinum, specific gravity, over 21, or one of the very
heaviest known substances. Color, light steel-gray, with metallic
luster. Very malleable and ductile. A rare metal found commercially
mostly in gravel or "placer" deposits mostly in the Ural Mountains,
also as grains in certain dark igneous rocks. Used for many scientific
instruments, in the electrical industry, as jewelry, etc.

Pyrite. Commonly called "iron pyrites." Sometimes called "fool's gold."
(See Figure 74m.) A sulphide of iron which commonly crystallizes in
the isometric system mostly as cubes, twelve-faced pyritohedrons,
octahedrons, or combinations of these. Color, light brass-yellow, with
metallic luster. Cleavage, practically absent. Hardness, greater than
that of steel (over 6 in the scale); specific gravity, about 5. Leaves
greenish black streak when rubbed on rough porcelain. Differs from
chalcopyrite by paler color and much greater hardness. It is a common
and very widely disseminated mineral in rocks of all kinds and ages,
but especially in metamorphic rocks as veins, and banded or lenslike
deposits. Most igneous rocks contain small scattering grains of pyrite.
Many deposits of commercial value are known. Great quantities are
burned for the manufacture of sulphuric acid ("oil of vitriol") which
is one of the most important of all chemicals.

Pyroxene Group. Along with quartz and feldspars, the pyroxenes rank
among the most common of all minerals. (See Figure 74k.) Composition,
very similar to amphibole (see above). Pyroxenes crystallizing in the
monoclinic system are the most important. These crystals are prismatic
in habit, with prism faces making angles of nearly 45 or 90 degrees
instead of about 124 degrees as in the monoclinic amphiboles which
the monoclinic pyroxenes greatly resemble. Two fairly good prismatic
cleavages cross at an angle of nearly 90 degrees, instead of at
about 124 degrees as in the monoclinic amphiboles. Hardness, 5 to 6;
specific gravity, 3.2 to 3.6. Color, variable according to species.
The most common variety of pyroxene is _augite_, a dark-green to black
silicate of aluminum, iron, lime, and magnesia. Certain pyroxenes also
crystallize in the orthorhombic system. Pyroxene is most abundantly
represented as crystals in many kinds of igneous and metamorphic rocks.
It is practically useless except as one kind of _jade_.

Quartz. Next to the feldspars, quartz is probably the most common
of all minerals, especially at and near the earth's surface. (See
Figures 74e, 74f, and 74g.) Composition, oxide of silicon. Often
crystallizes in the trigonal system almost always as six-sided prisms
capped by six-sided pyramids, which are really combined three-sided
forms, often with alternate corners modified by small faces. These
small modifying faces, etching figures, and microscopic tests show that
quartz is really trigonal in spite of the common occurrence of simple
six-sided outward forms. The pyramidal faces make different angles
than those of either apatite or beryl, both of which are somewhat
like quartz in crystal form. Hardness, 7 (distinctly high, cannot be
scratched by the knife); specific gravity, 2.6 (about average for
all minerals). Cleavage, practically absent, and breaks like glass.
Colorless when pure, but varieties exhibit many colors. A few only of
the many varieties will be briefly described. Among the distinctly
crystalline varieties are: _rock crystal_, pure colorless; _amethyst_,
purple; _rose quartz_, pink; _milky quartz_, white; and _smoky quartz_,
dark--due to tiny inclusions of carbon. Among the fine-grained, compact
more or less indistinctly crystalline or noncrystalline varieties,
usually translucent with a waxy luster, are: _chalcedony_, bluish
gray, waxy looking, usually in small rounded masses; _carnelian_,
red; _prase_, green; _agate_, with parallel bands, usually variously
colored; _flint_ and _jasper_, opaque to translucent, dark to red.

Quartz is exceedingly abundant in all the great groups of rocks. It
constitutes the main bulk of sandstones, is common in shales, and
occurs in certain other strata. In many igneous rocks, like granite, it
is a very prominent constituent. Most of the metamorphic rocks contain
its crystalline forms in greater or less amounts. Quartz is the most
common of all vein minerals, in many cases associated with valuable
ores. Various varieties are widely used for ornamental purposes. Used
in making sandpaper, glass, porcelain, mortar, concrete, and in certain
ore-smelting processes. Sandstone is widely used as a building stone.

Serpentine. A hydrous silicate of magnesia never in distinct
crystals as such, but shown to be monoclinic under the microscope.
Hardness variable, 2.5 to 5; specific gravity, about 2.6. Mostly of
variegated green or yellowish green color with waxy luster, except
a fibrous variety (_asbestos_) which is light green to white. The
fibrous variety of serpentine is the principal source of asbestos, an
amphibole asbestos being less common. Ordinary serpentine (sometimes
miscalled "green marble") is widely used as a building and decorative
stone. Serpentine is common and widespread, especially in igneous and
metamorphic rocks, but never as a really original mineral. It always
results from alteration of certain other magnesia-bearing silicate
minerals, such as pyroxene, amphibole, olivine, etc.

Silver. Native silver is not a very rare mineral and it is mined in
certain parts of the world, but most of the metal is obtained from
certain silver-bearing minerals, especially sulphides and a chloride.
Silver crystallizes rather rarely in the isometric system. More
commonly it occurs as irregular masses, plates, and wirelike forms.
Characterized by its color, metallic luster, softness (less than 3 in
the scale), and exceptional weight (specific gravity, 10.5). Usually
occurs in vein deposits, commonly associated with other metals or
metal-bearing minerals, especially copper.

Sphalerite. A sulphide of zinc commonly in crystalline form belonging
in the isometric system, especially in tetrahedral combination forms
(see Figure 75b). Color, usually brown, yellow or nearly black with
resinous luster. Hardness, nearly 4; specific gravity, 4. Several good
cleavages, yielding fragments whose faces meet at 90 and 120 degrees.
Sphalerite is a fairly common and widespread mineral, occurring nearly
always in veins in most kinds of rocks. It is very often associated
with other ores, particularly the great ore of lead (galena).
Sphalerite is by far the greatest ore of zinc.

Sulphur. Native sulphur. Crystallization, orthorhombic, usually in
combination pyramidal forms. (See Figure 75h.) Characterized by
yellow color, resinous luster, softness (about 2 in the scale),
low specific gravity (about 2), and very poor cleavages. It has
most commonly resulted from alteration of certain sulphur-bearing
minerals, especially gypsum, the decomposition of which has yielded
vast deposits. Some also of volcanic origin. Great quantities are
used in making sulphuric acid, matches, gunpowder, fireworks, and for
vulcanizing and bleaching rubber goods.

Talc. Often called _steatite_. Monoclinic crystals rare. One perfect
cleavage, yielding very thin, flexible leaves. Very soft (hardness,
1). Feels greasy, and looks waxy to pearly. Color, white, gray, to
light green. Specific gravity, 2.8. Composition, a hydrous silicate of
magnesia, much like that of serpentine. Talc is always of secondary
origin, generally derived by chemical alteration of various common
minerals rich in silicate of magnesia. _Soapstone_ is a common variety
resulting from alteration of whole rock masses. Soapstone has many
practical uses as for washtubs, table tops, electrical switchboards,
hearthstones, stove and furnace linings, blackboards, gas tips, etc.
Talc proper is used as a lubricant, to weight paper, in soap, as
dustless crayon, talcum powder, etc.

Topaz. A silicate of aluminum and fluorine. Orthorhombic crystals
common, usually prisms capped at one end by pyramided faces and
abruptly terminated at the other. Colorless when pure, but often
variously colored due to impurities. Very exceptionally hard (8 in
the scale); specific gravity, 3.5. One good cleavage across the prism
zone; usually found as crystals in, and in cavities in, igneous rocks.
Appears always to have formed from highly heated vapors or liquids
given off by cooling molten rock masses. Topaz is one of the more
highly prized of the gem stones.

Tourmaline. Composition, very complex, but chiefly a silicate of boron
and several metals and semimetals. Commonly as crystals in the trigonal
system in both long and short prismatic forms, as shown by Figure 75m,
with opposite ends not unlike. Extra hard (7 in the scale); specific
gravity, about 3. Color, widely various, but brown and black are most
common. Practically no cleavage. Tourmaline probably always originated
as a high temperature mineral, especially as crystals in granites
and related rocks and in certain metamorphic rocks which have been
subjected to high temperature and pressure. Certain transparent colored
varieties of tourmaline rank high among the semiprecious stones.

Turquoise. A hydrous phosphate of aluminum. Massive noncrystalline,
blue to green, waxy luster, mostly opaque, hardness of 6, and specific
gravity of about 2.7. Turquoise is a high temperature mineral found in
veins and cavities in certain igneous rocks. It is a rare mineral used
as a gem stone.

Zircon. A silicate of zirconium usually crystallized in the tetragonal
system as simple four-sided prisms capped by four-sided pyramids. (See
Figure 75l.) Very poor cleavages. Color usually brown. Hardness, 7.5
(extra high); specific gravity, nearly 4.7. Brilliant luster. Zircon
is very commonly present as scattering crystals of varying size in
most igneous rocks. Also common as crystals in various metamorphosed
stratified rocks, and less common in some sand and gravel deposits.
Certain transparent varieties, especially the brown and pink ones
called _hyacinth_, are used as gem stones. Zircon is also the source
of oxide of zirconium used in making mantles for certain incandescent
lights.



CHAPTER XXI

ECONOMIC GEOLOGY


In this chapter it is our purpose to briefly consider geology in its
direct relations to the arts and industries. When we realize that the
value of strictly geologic products taken from the earth each year
in the United States alone amounts to billions of dollars, we can
better appreciate the practical application of geological science.
Such products include coal, petroleum, natural gas, many valuable
metal-bearing minerals, and many nonmetalliferous minerals and rocks.
In most cases these valuable products of nature have been slowly
accumulated or concentrated at many times and under widely varying
conditions throughout the millions of years of known geological time.
To trace the extent of, and most advantageously remove, such deposits
for the use of man is always invariably impossible unless geological
knowledge is brought to bear. In many cases the problems involved
are intricate, and only the trained geologist is able to at all
successfully cope with them. In such cases it is necessary not only
to have a thorough knowledge of minerals and rocks as such, but also
of their origin and structure. Much of the practical application of
geology is carried out by the mining engineer who should have, above
all, a thorough knowledge of the great principles of geology.

Our plan of discussion is to consider, first, coal, petroleum, and
natural gas; then the most important metalliferous deposits of ores;
and finally nonmetalliferous minerals and rocks of exceptional
commercial importance. Underground waters have already been discussed
from the practical standpoint in the chapter on "Waters Within the
Earth." Certain minerals have already been sufficiently considered from
the economic standpoint in the chapter on "Mineralogy."


COAL, PETROLEUM, AND NATURAL GAS

Coal. Most valuable of all geological products is coal. Although it is
not, strictly speaking, a mineral, both because of its organic origin
and lack of definite chemical composition, coal is generally classed
among our mineral resources. Some idea of the national importance of
coal in the United States may be gained when we realize that the energy
derived from a single year's output is equivalent to that of hundreds
of millions of men working full time through the year. The uses of coal
are too well known to need mention here.

Coal is, beyond question, of organic (plant) origin as shown by its
very composition; perfect gradations between plant deposits like peat
and true coal; and the presence of microscopic plant remains and spores
in the coal. An excellent summary of just what happens during the
transition of ordinary vegetable matter into coal has been given by
D. White as follows: "All coal was laid down in beds analogous to the
peat beds of to-day. All kinds of plants, especially such species as
were adapted to the particular region where the deposit was located, in
whole or in part went into the deposit.

"Plants are composed chiefly of cellulose and proteins. The former,
comprising by far the larger bulk, constitute the framework, whereas
the latter are concerned in the vital functions. With these are
associated many other substances, among which are chiefly starch,
sugars, and fats and oils, constituting reserve foodstuffs; waxes,
resin waxes, resins, and higher fats, performing mainly protective
functions.... These components differ widely in their resistance to
various agencies. Those substances involved in the life function
and the support of the plant are relatively very stable under the
conditions imposed upon them.

"At the death of the plants, governed by conditions imposed in the
bog, a partial decomposition, maceration, elimination, and chemical
reduction begins, brought about by various agencies, chiefly organic,
mainly fungi at first and bacteria later. The most labile are removed
first, the more resistant next, and so on, as the conditions require,
leaving the most resistant behind in a residue called peat.

"The process of decomposition, elimination, and chemical reduction
begun in peat, chiefly by biochemical means, is taken up and continued
by dynamochemical means into and through the various successive
later stages, and results in the various grades of coal, as lignite,
sub-bituminous, and cannel coal, and anthracite."

The principal chemical elements involved in the changes which take
place are carbon, oxygen, and hydrogen, as shown by the following
analyses of about average samples of each member of the so-called "coal
series."

=========================+========+========+==========+=========
    The "coal series"  | Carbon | Oxygen | Hydrogen | Nitrogen
-------------------------+--------+--------+----------+---------
Wood (cellulose)         |   50   |   43   |    6     |     1
Peat                     |   59   |   33   |    6     |     2
Lignite                  |   69   |   25   |    5.5   |   0.8
Bituminous coal          |   82   |   13   |    5     |   0.8
Anthracite coal          |   95   |   2.5  |    2.5   | trace
Graphite                 |  100   |   ..   |    ..    |    ..
-------------------------+--------+--------+----------+---------

From this table it is seen that the oxygen relatively diminishes while
the carbon relatively increases, though, of course, all three elements
actually decrease during the chemical change from cellulose to coal.
These three elements disappear mainly in the form of gases, such as
water vapor, marsh gas, and carbonic acid gas. The final or graphite
stage is almost reached by the graphitic anthracite of Rhode Island,
which is so nearly pure carbon as to be really useless as coal.

The conditions under which successive layers of vegetable matter (later
turned into coal) become embedded in the earth's crust have been
outlined in the chapter on the "Evolution of Plants." The most perfect
conditions for prolific plant growth, and accumulation as great beds
in the earth's crust, were during the Pennsylvanian period of the late
Paleozoic era in many parts of the world, but especially in the United
States, China, Great Britain, and Germany. Most of the world's great
supply of coal comes from rocks of Pennsylvanian Age, while next in
importance are Cretaceous rocks, and some comes from strata of other
ages later than the Pennsylvanian, even as late as the Tertiary.

The United States not only has the greatest known coal fields, but
it also produces far more coal than any other country. In 1918 the
production was 678,000,000 tons, the greatest in our history, or
enough, if loaded into cars of forty tons capacity, to fill a train
which would reach around the earth at the equator about six times!
Equally amazing is the fact that this coal was nearly all consumed by
this one nation! In 1919 the production fell to 544,000,000 tons. Is
there real danger that our supply of coal will soon run out? Hardly so
when we consider, first, the fact that probably not more than 1 per
cent of the readily available coal has thus far been removed, and,
second, the high probability that rate of increase in coal production
for the last twenty years will not continue. In fact, during the last
two or three years the production has fallen off considerably. But even
so, coal, which is our greatest natural resource, and which can never
be replaced, should be scientifically conserved. In the case of the
very restricted anthracite coal fields what might be called a crisis
has already been reached, because a very considerable part of the
available supply has been taken out.

Something like 350,000 square miles of the United States are underlain
with one or more beds of workable coal (not including lignite)--in
some areas five to twenty or more beds one above the other. There are
also about 150,000 square miles of country underlain with the more or
less imperfect coal called lignite. It has been estimated that there
are more than a trillion tons of easily accessible coal, and another
trillion tons accessible with some difficulty in the principal coal
fields of the United States.

The greatest production of coal by far is from the Appalachian Mountain
and Allegheny Plateau districts, from the western half of Pennsylvania
to Alabama, where all the coal is bituminous of Pennsylvanian Age.
Here as well as elsewhere the coal beds are interstratified with
various kinds of sedimentary rocks, most commonly with shales and
sandstones. In the Appalachian field the strata including coal beds are
more or less folded toward the east, while they are nearly horizontal
toward the west. The famous Pittsburgh coal bed is probably the most
extensive important single coal bed known. It covers an area of over
12,000 square miles and is workable, with a thickness of five to
fifteen feet, over an area of 6,000 square miles of parts of western
Pennsylvania, Ohio, and West Virginia.

[Illustration: Fig. 76.--Map of the United States, showing the
principal coal fields. Cross-lined areas represent lignitic coals.
(After U. S. Geological Survey.)]

The greatest production of anthracite coal by far is from
central-eastern Pennsylvania, where strata of Pennsylvania Age,
including a number of anthracite beds, are mostly highly folded. Most
remarkable of all in this district is the so-called "mammoth bed" of
anthracite, nearly everywhere present, with a thickness up to as much
as fifty or sixty feet. Less than 500 square miles are there underlain
by workable anthracite coal.

Next to the greatest production of coal in the United States is from
the two large areas in the middle of the Mississippi Valley. It is
all bituminous coal, associated with nearly horizontal strata of
Pennsylvanian Age.

The scattering areas through the Rocky Mountains yield all types of
coal--anthracite, bituminous, and lignite. In some of these areas the
coal beds have been but little disturbed from their original horizontal
position, but usually they are more or less folded along with the
inclosing strata, the crustal disturbances affecting the coal beds
having taken place late in the Mesozoic era and early in the Cenozoic
era. Practically all of these coals are of Cretaceous and Tertiary
Ages, the best being Cretaceous. Very little of the Rocky Mountain coal
is anthracite.

On the Pacific Coast coal production is relatively very small. The
coals are there bituminous to lignitic of Tertiary Age, usually folded
in with the strata.

In Alaska there are widely distributed, relatively small coal fields,
but they have been little developed. Alaskan coals range in age from
Pennsylvanian to Tertiary, and in kind from anthracite to lignite.

Petroleum. Crude oil or petroleum is an organic substance consisting
of a mixture of hydrocarbons, that is, it is made up very largely of
the two chemical elements carbon and hydrogen, in rather complex and
variable combinations. It is practically certain that petroleum has
been derived by a sort of slow process of distillation from organic
matter--animal or vegetable or both--in stratified rocks within the
earth. Many strata, as for example carbonaceous shales, are more or
less charged with dark-colored decomposing organic matter. The chemical
composition itself, the kinds of rocks with which it is associated, and
certain optical (microscopic) tests all point to the organic origin of
petroleum. In southern California at least, certain of the oils have
quite certainly been derived from the very tiny oily plants called
diatoms which fill many of the strata.

[Illustration: Fig. 77.--Profile and structure section showing folding
of strata, with included coal beds, across one of the anthracite coal
fields of eastern Pennsylvania. Length of section, a little over 2
miles. (After U. S. Geological Survey.)]

During the last twenty years petroleum has come to be one of the most
important and useful natural products. Among the many substances
artificially derived from petroleum are kerosene, gasoline, naphtha,
benzine, vaseline, and paraffine. The United States leads in the
production of petroleum, while southern Russia and Mexico are very
important producers. In the United States the principal areas
underlain with petroleum-bearing strata are the northern Appalachian
field (through western Pennsylvania to central West Virginia); the
Ohio-Indiana field (central Indiana to northwestern Ohio); the
mid-continental field (southeastern Kansas and northeastern Oklahoma);
the southeastern Texas-Louisiana field; and the southwestern California
field. The total areas underlain with oil total about 10,000 square
miles. In the Appalachian, Ohio-Indiana, and mid-continental fields the
strata carrying oil range in age from Ordovician to Pennsylvanian, and
they are mostly but little disturbed from their original horizontal
position. The Texas-Louisiana oils come mainly from Cretaceous and
Tertiary strata which gently downtilt under the Coastal Plain toward
the Gulf. In California the oil-bearing strata are of Tertiary Age and
generally considerably disturbed and folded.

Under proper conditions below the earth's surface the derived oil
accumulates in porous or fractured rocks. There must, of course, be a
source from which the petroleum is derived or distilled; a porous or
fractured rock formation to take it up; a cap rock or impervious layer
to hold it in; and a proper geologic structure to favor accumulation.
The most common porous (containing) rock is sandstone, and the most
common cap rock is shale. Oil is rarely found without gas, and
saline water is likewise often present. If the containing strata are
horizontal, the oil and gas are usually irregularly scattered, but
if tilted or folded, and the beds porous throughout, they appear
to collect at the highest point possible. It was the result of
observations along this line that led I. C. White to develop what is
known as the "anticlinal theory." According to this theory, in folded
areas the gas collects at the summit of the fold (anticline), with the
oil immediately below, on either side, followed by the water. It is,
of course, necessary that the oil-bearing stratum shall be capped by a
practically impervious one.

[Illustration: Fig. 78.--Map showing the principal petroleum and
natural gas fields of the United States. (After U. S. Geological
Survey.)]

[Illustration: Fig. 79.--A vertical (structure) section showing a very
common type of oil-bearing structure. In this anticline, water, oil,
and gas arranged in order of their specific gravities. Removal of the
gas would allow the oil and water to rise higher toward the apex of the
porous layer. (After Indiana Geological Survey.)]

[Illustration: Plate 19.--General View in the Appalachian Mountains
Along New River, Virginia. This is a typical portion of the great area
which, during Mesozoic time, was reduced by erosion to the condition
of a low-lying plain ("peneplain"). Since early Cenozoic time the
peneplain has been upraised and New River has carved out its V-shaped
valley to its present depth, while tributary streams have carved out a
series of valleys along belts of weak rocks nearly at right angles to
the main valley. The remarkably even sky line marks approximately the
old peneplain surface. (_Photo by Hillers, U. S. Geological Survey._)]

[Illustration: Plate 20.--(_a_) A Big Glacial Bowlder of Plutonic
Igneous Rock Carried Miles from Its Parent Ledge by the Ice Sheet Which
Passed Over the Adirondack Mountains During the Ice Age. (_Photo by the
author._)]

[Illustration: Plate 20.--(_b_) A Long, Winding Ridge of Sand and
Gravel (Called an "Esker") Deposited by a Stream in a Channel in the
Ice Near the Margin of the Great Glacier During Its Retreat from the
Adirondack Mountains. (_Photo by the author._)]

"If the rocks are dry, then the chief points of accumulation of the oil
will be at or near the bottom of the syncline (downfold), or lowest
portion of the porous bed. If the rocks are partially saturated with
water, then the oil accumulates at the upper level of saturation. In a
tilted bed, which is locally porous, and not so throughout, the oil,
gas, and water may arrange themselves according to their gravity in
this porous part." (Ries.)

Although the term "oil pool" is commonly used, there is really no
actual pool or underground lake of oil, but rather porous rock
saturated with oil. It has been estimated that in an oil field of
average productiveness a cubic foot of the porous rock contains from
six to twelve pints of oil. The life of a well drilled into an "oil
pool" varies from a few months to twenty or thirty years, or sometimes
even more, but a heavy producer (especially a "gusher") almost
invariably falls off very notably in production in a few months, or at
most a few years. The typical Pennsylvanian oil well is said to last
about seven years. The fact that the United States is still able to
increase oil output is because new fields are found and developed, the
most recent being in the interior and northern parts of Texas. It is
practically certain, however, that the climax of oil production in the
United States will be reached before many years--long before that of
bituminous coal.

It is a well-known fact that oil, as well as natural gas, is usually
under more or less pressure within the earth. The pressure is so
great in some cases that where, in the course of drilling, oil or
gas accumulated under proper conditions, as for example those shown
by Figure 79, are encountered the pressure may be hundreds of pounds
per square inch, or enough to blow to pieces much of the drilling
outfit. It is under such conditions that great "gushers" are struck.
A wonderful case in point was the famous Lakeview gusher, struck
in California in 1910. "Within a few days the well was far beyond
control. It continued to flow (for a time shooting high into the air)
for eighteen months, finally stopping after it had produced over
8,000,000 barrels of oil, about 6,000,000 of which had been saved.
The daily production of the well varied greatly, reaching a maximum
of 65,000 barrels." (Pack.) One very common cause of oil pressure is
the expansive force of the associated imprisoned gas which steadily
increases as the gas is generated. Another cause which is seemingly
applicable in many cases is hydrostatic pressure, where under certain
structural conditions the pressure of water in a long-tilted layer is
exerted against oil accumulated toward the top of an anticline (or
upbend) in the strata.

The world's output of petroleum for 1917 was nearly 559,000,000
barrels, of which the United States produced nearly 338,000,000
barrels, Mexico 87,000,000 barrels, and Russia 34,000,000 barrels of 42
gallons.

Natural Gas. The most perfect fuel with which nature has provided
us is natural gas. Not only is it easily transported even long
distances through pipes, but also as a fuel it is easily regulated,
leaves no refuse, and is less damaging to boilers than coal. It is
a colorless, odorless, free-burning gas, consisting very largely
of the simple hydrocarbon called marsh gas or fire damp. Petroleum
nearly always has more or less natural gas associated with it, but in
some cases considerable quantities of gas may exist alone. Natural
gas, like petroleum, is of organic origin--a product of slow natural
distillation of vegetable or animal matter, or both, within the
earth's crust.

One of the most common modes of occurrence of gas is at the top of an
anticline (upfold) in porous rock (like sandstone) between impervious
layers (like shale). Figure 79 well illustrates the principle, the
gas lying above the oil, and the oil above the water; that is, the
three substances are arranged according to specific gravity. Gas may
also exist in considerable quantities in irregular bodies of porous or
fractured rocks. Natural gas is nearly always under pressure within
the earth, hundreds of pounds per square inch being common, while more
exceptionally, as in certain West Virginia wells, pressures of over
1,000 pounds have been registered.

The United States is by far the greatest world producer of natural
gas, the output for 1918 having been 720,000,000,000 cubic feet. West
Virginia easily headed the list, with Oklahoma and Pennsylvania next in
order. Areas underlain with natural gas are, in the main, the same as
for petroleum, and they total more than 10,000 square miles. During the
last forty years the waste of natural gas in the United States has been
appalling. In many cases wells in quest of oil have encountered gas and
often such abandoned wells have been allowed to play millions of cubic
feet of gas daily into the air for years. A striking example was the
Murraysville well of western Pennsylvania, which shot 20,000,000 cubic
feet of gas per day into the air for six years!


METAL-BEARING (ORE) DEPOSITS

Iron. Without question the most useful of all metals is iron. As
such it is rare in nature, but in chemical combination with other
substances it is extremely widespread and very common. Iron makes up
about 5 per cent of the weight of the earth's crust, but in the form
of ore (i.e., a metal-bearing mineral or rock of sufficient value to
be mined) it is notably restricted in occurrence. The three great ores
of iron are the minerals hematite, magnetite, and limonite, whose
composition and characteristic properties the reader will find stated
in the preceding chapter on "Mineralogy."

One of the worst impurities in iron ore is phosphorus, which makes
iron "cold short," i.e., brittle when cold. Ore for the manufacture of
Bessemer steel must contain very little phosphorus (less than 1/1000 of
the metallic iron content of the ore). Sulphur as an impurity in the
ore tends to make the iron "hot short." Silica (quartz) is bad because
it necessitates the use of more lime for flux in the furnace.

Iron ores occur in rocks of most of the great geologic ages, but in
the United States principally in the pre-Paleozoic and Paleozoic. The
United States is by far the greatest producer of iron ore in the world,
the output for 1917 having been about 75,288,000 tons, the greatest in
the history of this or any other country. This one year's output loaded
into cars of 40 tons capacity would have made a train about 15,000
miles long! All but about 5,000,000 tons of this tremendous production
was hematite ore. In 1919 the output of iron ore dropped to about
60,000,000 tons.

[Illustration: Fig. 80.--Drawing showing details of part of an
ore-bearing vein at Pinos Altos, New Mexico. The chalcopyrite and
sphalerite are the ores. Somewhat reduced in size. (After Paige, U. S.
Geological Survey.)]


We shall now very briefly consider several of the greatest iron-mining
districts of the United States, giving some idea of the modes of
occurrence and origin of the ores. Greatest of all is the Lake Superior
region, not far west and south of the lake in Minnesota, Michigan,
and Wisconsin. Considerably more than one-half the iron ore mined
in the United States comes from the single State of Minnesota, and
about one-fourth of it from Michigan. Most of the Minnesota ore by
far is obtained from the so-called "Mesaba Range," which in 1917
produced 41,000,000 tons of hematite ore. The ore deposits are there
of irregular shape, lying at or near the surface (usually covered only
by glacial deposits). None of them extend downward more than a few
hundred feet. The soft, high-grade ore is removed by steam shovels in
great open pits. In the several districts of northern Michigan and
Wisconsin the ores (nearly all hematite) are associated with more or
less highly folded rocks at considerable depths. The Lake Superior iron
ores all occur in rocks of Archeozoic and Proterozoic Ages. According
to the best explanation of their origin the iron of the ores was once
part of a sedimentary series of rocks in the form of iron carbonate and
silicate, interstratified with layers of a flintlike rock associated
with slate, quartzite, etc. After these rocks were raised into land and
subjected to weathering the old iron compounds were altered to oxides,
mainly hematite, and somewhat concentrated. Further concentration of
the ore was caused by dissolving out the flintlike layers of the old
rocks.

The Birmingham, Ala., region is the second most important iron ore
producer in the United States, with an output of nearly 6,000,000 tons
in 1918. The ore is hematite, forming part of the famous Clinton iron
ore deposits of Silurian Age. This deposit, named from Clinton, N.
Y., extends through central New York and in more or less interrupted
parallel bands through the Appalachian Mountains to near Birmingham
where the richest deposits occur. This ore appears to be an original
bed (or locally several beds) of fairly rich iron ore deposited on the
shallow Silurian sea bottom and then covered by other strata. At the
time of the Appalachian Mountain revolution the iron ore was more or
less highly folded in with other strata throughout the Appalachians. A
remarkable fact regarding the Birmingham district is that in the near
vicinity of the ore there are both coal for fuel and limestone flux for
smelting the ores.

The next most important mining region of the United States is the
Adirondack Mountain region of northern New York, where about 1,000,000
tons of ore are obtained yearly. Magnetite is the ore, and it occurs
in more or less irregular lenses and bands in granite and closely
associated rocks of pre-Paleozoic Age. One view regarding the origin
of this ore is that it segregated during the process of cooling of the
molten granite, and another view (recently advocated by the author) is
that it was derived from an older iron-rich igneous formation by either
the molten granite or very hot solutions from it and concentrated into
the ores. About 2,500,000 tons of magnetite were mined in the United
States in 1916, nearly one-half of it in the Adirondacks.

The third important iron ore is limonite, nearly 2,000,000 tons of
which were produced in the United States in 1916. Most of it came from
the Appalachian Mountains. All of this limonite is of secondary origin;
that is, it has been derived from certain early Paleozoic iron-bearing
limestones either by weathering or solution, and concentrated into ore
deposits.

Copper. This is one of the most useful of all metals. Several of its
very important uses are as a conductor of electricity in the form
of wire; in making alloys such as brass and bronze; in copperplate
engraving; and in roofing and plumbing. Various minerals containing
copper are found in many parts of the world, but only about six of them
are really important as ores. These are native copper, chalcopyrite,
chalcocite, azurite, malachite, and cuprite, most of which are
described in the chapter on "Mineralogy." The number of places where
they may be profitably mined as ore is distinctly limited. Fifteen or
twenty countries produce more or less copper, but the United States is
by far the greatest producer, with an output of nearly 2,000,000,000
pounds of copper in 1916, the output having fallen off some in 1918.
This was two-thirds of the world's output and ten times as much as
the nearest competitor. The other leading countries are Japan, Chile,
Mexico, Spain, and Canada. In 1918 the four leading States in order
were Arizona, Montana, Michigan, and Utah, with production ranging from
nearly 765,000,000 pounds to about 230,000,000 pounds.

In Arizona several great copper-mining districts lie in the
southeastern one-fourth of the State. Almost invariably the ores are
directly associated with limestone and an igneous rock (granite), both
of late Paleozoic Age. The ores are almost always near the border
between the two rocks, mostly as great irregular deposits within the
limestone, and less commonly as veins within the granite. The original
ores were carried in solution and deposited by hot liquids (or vapors)
from the cooling granite. At lower levels the ores are mainly sulphides
of copper (e.g., chalcopyrite and chalcocite), while at higher levels
they are mostly carbonates (malachite and azurite) and oxides (e.g.
cuprite). The difference is due to the fact that the ores nearer
the surface have been subjected to weathering and altered from their
original condition.

The region around Butte, Mont., is next to the greatest copper
producer. Nearly all the ores are sulphides of copper (mainly
chalcocite) which occur with quartz in a great system of nearly
parallel veins in granite of Tertiary Age. "It is supposed that in the
copper veins the hot ore-bearing solutions ascended the fractures in
the granite, replacing the rock by ore, and resulting in an intense
alteration of the walls." (Ries.)

Third in rank among the copper-producing States is Michigan, the mines
being located on Keweenaw Peninsula, which extends into Lake Superior.
For fully fifty years this district has been one of the most famous and
important copper producers in the world. A unique feature is that the
ore is native copper, associated with some native silver. The rocks
containing the ore are steeply tilted lava sheets and conglomerate
(cemented gravel) strata of Proterozoic Age. Openings in porous
lava and spaces between the conglomerate pebbles have been filled
by metallic copper, which was carried off in hot solutions from the
cooling lavas. Certain of the mining shafts have been sunk more than
5,000 feet below the surface, these being next to the deepest in the
world.

Utah ranks fourth among the copper producers, the greatest mining
district being at Bingham Canyon, southwest of Salt Lake City. The
rocks are late Paleozoic strata, pierced by a large body of igneous
rock. Some of the sulphide ores (mainly chalcopyrite) occur in veins
in the igneous rock and some in large tabular masses in the adjacent
limestone. Hot solutions from lower portions of the uncooled igneous
rock carried the ore in solution into the limestone and into cracks in
the upper cooled igneous rock.

Lead. Lead must surely be counted among the five or six most useful
metals. As in the case of nearly all the other most important natural
resources, the United States is the world's greatest producer of
lead, the output of metallic lead having been 552,000 tons in 1916
and somewhat less in 1918. Most of this came from Missouri, Idaho,
Utah, and Colorado. The leading other countries are in order--Spain,
Germany, Mexico, and Australia. Nearly all the lead comes from the
mineral galena (a sulphide of lead), which is described in the chapter
on "Mineralogy." Among the many uses of lead are the following:
manufacture of certain high-grade paints from lead compounds; making
alloys such as pewter, type metal, solder, babbit metal; in plumbing;
in glass making; and in the manufacture of shot.

The greatest lead-mining district is in the vicinity of Joplin, Mo.,
where the ore (galena), associated with much zinc ore, occurs as veins
and great irregular deposits in limestone of early Paleozoic Age. It
is generally agreed that underground waters dissolved the ores out of
the limestone in which they were disseminated as tiny particles and
deposited them in concentrated form at lower levels.

In the famous Coeur d'Alene district of northern Idaho the great output
of lead is really obtained from a lead-silver ore; that is, galena
rich in silver. This ore is in composition a lead-silver sulphide. It
occurs in great fissure veins, mostly following fault fractures in
highly folded strata of Proterozoic Age. Igneous rocks cut through the
strata, and it is believed that hot ore-bearing solutions given off
from the highly heated igneous rocks rose in the fissures and deposited
the ores.

The Park City and Tintic districts of Utah are great producers of lead.
The lead ore (galena) is usually rich in silver. It occurs mainly in
veins and irregular deposits in limestone of Paleozoic Age closely
associated with certain igneous rocks.

One of the most famous mining districts in the world is that around
Leadville, Col., where ores of four metals--gold, silver, lead, and
zinc--have been extensively mined. The salient points in the rather
complex geology are the following: Paleozoic strata, including much
limestone, rest upon a foundation of pre-Paleozoic granite. Sheets
of igneous rock are interbedded with the strata and many dikes of
igneous rocks cut through the whole combination. After the last igneous
activity all the rocks were somewhat folded and notably faulted in
many places. The ores were dissolved out of the igneous rock and
deposited in large masses mostly in the limestone and in fissure veins,
especially along and near the fault zones.

Zinc. Another of the few most useful metals is zinc. It never occurs
in metallic form in nature, but most of it by far is obtained from the
ore mineral sphalerite (sulphide of zinc) described in the chapter on
"Mineralogy." A red oxide of zinc ore, called zincite, assumes great
economic importance in New Jersey. In 1917 the United States produced
686,000 tons of metallic zinc and was easily the world's leader. Since
1917 the production has fallen notably. The four greatest producing
States are Missouri, Montana, New Jersey, and Colorado. Germany and
Belgium are the greatest foreign producers.

Most important of all in the United States is the district around
Joplin, Mo., where the ore is closely associated with lead ore. The
mode of occurrence and origin of these ores are above referred to in
the discussion of lead.

In Montana some of the great east-west fissure veins in granite are
rich in silver ores in the upper levels, and in zinc ores (mainly
sphalerite) at depths of from some hundreds of feet to nearly 2,000
feet, that is as far down as they have been mined. They, like the
great copper veins of the same general district, were carried by
hot solutions which rose from the lower still very hot granite and
deposited the ores in fissures of the same cooler rock higher up.

Two great ore bodies in the general vicinity of Franklin, N. J., are of
unique interest, because they are mostly the red oxide of zinc called
zincite. The ore deposits occur in white limestone along or close to
its contact with metamorphosed (altered) strata and granite of early
Paleozoic Age. It is not definitely known how the ore originated, but
it was probably derived in solution from the hot granite and deposited
in the limestone by replacement of the latter.

In Colorado the principal zinc mines are around Leadville, where lead
ore is nearly always directly associated with the zinc ore. This
district is above described in the discussion of lead.

Among many uses of zinc are for galvanizing; for making certain
high-grade paints; brass and white metal; and for roofing and plumbing.

Gold. This precious metal has been used and highly prized by man for
thousands of years. The discovery of gold in California in 1848 was
one of the most important events in the history of the mining world.
As early as 1852 that State reached its climax of production with an
output of at least $81,000,000 worth of the metal. The Transvaal region
of South Africa has for two decades been the world's greatest gold
producer. Though long known, the metal has there been worked only since
1886. In 1915 the peak of gold production in the world ($468,700,000)
was reached and nearly maintained in 1916, but since that time there
has been a great falling off. In 1916 South Africa produced gold to
the value of about $200,000,000; the United States over $90,000,000;
Australia over $40,000,000; Russia over $26,000,000; and Canada over
$19,000,000.

In tiny amounts gold is really very widespread. It occurs in many
stream gravels where so-called "color" may be obtained by washing
gravel, and it is even dissolved in sea water. Gold-mining localities
are also numerous in many parts of the world, but relatively few of
them only have ever paid. The total amount of money spent in actual
gold-mining operations; in hopeless but honest operations; and for
stock in fake gold mines has no doubt exceeded the actual value of gold
produced. In many a case acceptance of a report based upon a very brief
examination of the ground by a competent geologist would have saved
the cost of hopeless expenditure of money. Some one in nearly every
community has a so-called "gold mine."

Most of the commercially valuable gold occurs in nature as native
gold, either mixed with gravel and sand (i.e., placer deposits) along
existing or ancient stream beds, or in veins mechanically held in the
mineral pyrite (described in the preceding chapter) in submicroscopic
form, or visibly mixed with quartz in vein deposits. Another kind of
ore which assumes considerable importance, as in parts of Colorado, is
in the form of telluride of gold always found in veins. In deep vein
deposits it is quite the rule to find free or native gold mechanically
and visibly mixed with quartz in the upper levels, while deeper down
the gold is mechanically, but invisibly, held in combination usually
in pyrite, which latter is associated with quartz. This difference is
due to the fact that the lower level ores are now just as they were
formed, while in the upper levels the ores have been weathered, and the
gold set free and often more or less further concentrated by solutions.
Vein deposits, including also telluride ores, are found in many kinds
of rocks--igneous, sedimentary, and metamorphic--of nearly all ages
generally directly associated with igneous rocks. In nearly all cases
the best evidence indicates that the vein fillings were formed by hot
ore-bearing solutions from the igneous rock, which solutions deposited
the ore plus quartz in fissures in either the igneous or adjacent
rocks. Among the many localities where fissure veins of the kind just
described are of great economic importance are the "Mother Lode" belt
of the Sierra Nevada Mountains of California; Cripple Creek (telluride
ore), Georgetown and the San Juan region of Colorado; Goldfield,
Tonopah, and Comstock Lode of Nevada; and near Juneau, Alaska.

Placer deposits, that is, free gold mixed with gravel and sand, also
yield much gold. They are most prominently developed in California
and Alaska. These gold-bearing "gravels represent the more resistant
products of weathering, such as quartz and native gold, which have
been washed down from the hills on whose slopes the gold-bearing
quartz veins outcrop, and were too heavy to be carried any distance,
unless the grade was steep. They have consequently settled down in the
stream channels, the gold, on account of its higher specific gravity,
collecting usually in the lower part of the gravel (placer) deposit."
(Ries.) Such gold occurs as grains, flakes, or nuggets. When a chunk of
gold-bearing vein quartz, with crevices filled by thin plates of the
metal, is carried downstream pieces are gradually broken away, and the
tough, very malleable gold bends or welds together into a single mass
called a "nugget." Nuggets varying in weight up to over 2,000 ounces
have been found. Many placer deposits are along existing drainage
channels, while others occur in abandoned and even buried former
channels.

Most of the gold of South Africa comes from Witwatersrand district
where the native metal occurs in a unique manner in beds or layers
of conglomerate associated with other strata, all the rocks being
considerably folded and somewhat faulted. Some of the mines are more
than a mile deep (vertically), the deepest in the world. The gold
either accumulated in placer form with gravel which later consolidated
into conglomerate, or it was introduced into spaces between the pebbles
subsequently by ore-bearing solutions.

Silver. For many years the United States and Mexico have been the
world's greatest silver producers, sometimes one and sometimes the
other leading, with Canada third, and Australasia fourth. In 1918 the
United States produced nearly 68,000,000 ounces of silver and Mexico
over 62,000,000 ounces. In the United States in 1918 the four leading
States were Montana, Utah, Idaho and Nevada with outputs ranging from
over 10,000,000 to over 15,000,000 ounces each.

In Montana most of the silver is in the native form, more especially
in the upper portions of the great veins rich in copper and zinc ores
near Butte. These ores and their origin are described above under the
captions "Copper" and "Zinc."

The two greatest silver districts of Nevada are Tonopah and Comstock
Lode where silver and gold minerals are associated as ores in Tertiary
igneous rocks, the ores having been deposited in veins by hot
ore-bearing solutions from the igneous rocks.

In Idaho the Coeur d'Alene district produces most of the silver, the
ore there being a silver-bearing lead ore (galena). The nature and
origin of these deposits are described above under the caption "Lead."

In Utah the silver is also obtained from silver-bearing galena
especially in the Tintic, Cottonwood Canyon, and Bingham Canyon
districts where the ores occur mainly as irregular deposits and
in fissure veins in Paleozoic strata (chiefly limestone) directly
associated with igneous rocks, hot ore-bearing solutions from the
igneous rocks having furnished the ores.

Tin. Production of tin in the United States has never amounted to much,
a little mining having been carried on from time to time in South
Carolina, Black Hills of South Dakota, and southern California. About
one-half of the world's supply of tin (121,000 long tons 1918) comes
from the Malay Peninsula and two small islands near by. The only other
great producer is Bolivia, though a number of other countries produce
from 1,000 to 9,000 tons each.

The only important ore of tin is the mineral cassiterite (oxide of tin)
described above in the chapter on "Mineralogy." In the Malay region
the ore all occurs in placer deposits and is, therefore, of secondary
origin, the source of the ore not being known. In Bolivia the tin ore
occurs in veins in and close to granite, the ore having been carried
by very hot vapors or liquids which were derived from the still highly
heated granite.

Tin is used chiefly in the making of tin plate, bronze, pewter, gun
metal, and bell metal.

Aluminum. The mineral called bauxite (a hydrous oxide of aluminum)
is the great ore from which aluminum is obtained by an electrical
process. Bauxite is noncrystalline, relatively light in weight, white
to yellowish in color, and in the form of rounded grains, or earthy or
claylike masses. The United States and France are the only two great
producers of bauxite, most of which is treated for metallic aluminum.
In 1918 the United States produced more than 100,000 tons of aluminum.
In the United States the principal deposits are in Georgia, Alabama,
and Arkansas. Bauxite is probably always a secondary mineral formed
by decomposition of igneous rocks rich in certain aluminum silicate
minerals. In some cases, as in the Georgia-Alabama region, the bauxite
appears to have been formed and concentrated in deposits by hot
solutions from uncooled igneous rocks.

Aluminum is most used in the manufacture of wire for electric current
transmission. It is also mixed with certain other metals like copper,
zinc, magnesium, and tungsten to form special types of alloys, some of
which possess remarkable tensile strength up to nearly 50,000 pounds
per square inch. Aluminum is used in powdered form to generate very
high temperatures in certain welding processes. It is also made into
many kinds of utensils and instruments.

Mercury. This metal, commonly known as "quicksilver," is of special
interest because it is the only one which exists in liquid form at
ordinary temperatures. The metal occurs in only small quantities in
nature, most of it by far being obtained from the red mineral cinnabar
described in the chapter on "Mineralogy." In order of importance the
greatest quicksilver producing countries in 1916 were Italy, United
States, Austria, and Spain. In the United States, California is by far
the leading State, while Texas and Nevada are the only other important
producers.

In California most of the ore occurs in veins and irregular deposits
in metamorphosed strata of Mesozoic and Cenozoic ages usually closely
associated with igneous rocks. There, as well as in other parts of the
world, hot vapors from igneous rocks carried the volatile ore upward
and deposited it in fissures.

Among the many uses of mercury are in making fulminate for explosives;
making certain drugs and chemicals, pigments, electrical and physical
apparatus; silvering mirrors; and in the amalgamation process of
extracting gold and silver.


OTHER ECONOMIC PRODUCTS

Building Stones. Some of the principal features which should be
considered in building stones are power to resist weathering, power to
withstand heat, color, hardness, and density, and crushing strength.
Building stones representing rocks of nearly all important geologic
ages are widely distributed throughout the world.

_Granite_, including certain other closely related rocks, is one of
the oldest and most useful building stones. The New England States
are the greatest producers, while the Piedmont Plateau district (east
of the Appalachians) from Philadelphia to Alabama also contains
important granite quarries. In the Adirondack Mountains, in Wisconsin
and Minnesota, through the Rocky Mountains, and the Sierra Nevada
Mountains there are extensive areas of granite which are relatively
little quarried. The granite occurs only in regions of highly disturbed
rocks, usually in mountains or hills, where great volumes of the molten
rock were forced into the earth's crust, cooled, and later laid bare by
erosion.

_Marble_, according to geological definition, is a metamorphosed
limestone, that is a limestone which has been crystallized under
conditions of heat, pressure, and moisture within the earth. More
loosely in trade any limestone which takes a polish may be called
marble. The greatest marble-producing districts of the United States
are western New England (especially Vermont) and the Piedmont Plateau
and Appalachian Mountains in rocks of Paleozoic age. In northern New
York and the mountains of the west there are relatively few marble
quarries.

Ordinary _limestones_ are widely distributed in many States where they
range in age from early Paleozoic to Tertiary. Most of the quarries
supply stone for near-by markets. The so-called Bedford limestone
of Indiana has, for many years, been perhaps the most widely used
limestone for building purposes in the United States.

_Sandstones_, which are stratified rocks consisting mainly of
rounded quartz grains cemented together, are widely used in building
operations. Like limestones, they are very widespread in formations
of all ages except the very old. There are many sandstone quarries
supplying more or less local markets throughout the country. Two of
the best known and most widely used sandstones are the so-called
brown-stone of Triassic Age extending interruptedly from the
Connecticut Valley of Massachusetts to North Carolina, and the Berea,
Ohio, sandstone of light gray color and uniform texture.

_Slate_ is mostly a metamorphosed shale, that is a shale which has been
subjected to great pressure within the earth so that the stratification
has been obliterated and a well defined cleavage has been developed
at right angles to the direction of application of the pressure.
Good slate is fine-grained, dense, and splits readily into wide thin
plates. It occurs only where mountain making pressure and metamorphism
have been brought to bear upon the strata. Most of our great slate
quarries are located in early Paleozoic rocks from New England through
the Piedmont Plateau. Some quarries are also located in Arkansas,
Minnesota, and westward to California.

Clay. "Clay, which is one of the most widely distributed materials
and one of the most valuable, commercially, may be defined as a
fine-grained mixture of the mineral kaolinite with fragments of other
minerals, such as silicates, oxides, and hydrates, and also often
organic compounds, the mass possessing plasticity when wet and
becoming rock-hard when burned to at least a temperature of redness."
(Ries.)

Most clays originate by the weathering of rocks, particularly igneous
and metamorphic rocks rich in the mineral feldspar. As a result of the
decomposition of the feldspar, much clay is formed, the main substance
of which is kaolin. Both feldspar and kaolin are described in the
preceding chapter. When the resulting clay rests upon the rock from
which it has been derived it is called residual clay. Much of the clay
is, however, carried away, mainly by streams, and deposited in lakes or
the sea, or on river flood plains. Some clay deposits are of wind-blown
origin, and still others are formed by the grinding action of glaciers.
Clays are very widespread, and they are directly associated with rocks
of all geologic ages.

Among the many important uses of clay are the following: manufacture of
common brick, fire brick, pottery, chinaware, porcelain ware, tiles,
terra cotta, and Portland cement.

Lime and Cement. Limestone, which is one of the most common and
widespread of all stratified rocks, forms the basis for the manufacture
of the important substances lime (or "quicklime") and Portland cement.
Lime results when pure limestone (carbonate of lime) is "burned" or
heated to a temperature high enough to drive off the carbonic acid
gas. The greatest use of lime is for mixing with water and sand to
make mortar. A few of its other numerous uses are in plastering;
whitewashing; purifying certain steel; in making gas, paper, and soap;
and as a fertilizer.

Certain limestones containing clay of the right kind and proportion
are called natural cement rocks because, after being "burned," they
develop the property of "setting," like cement when mixed with water.
The "setting" of a cement is due to the fact that certain chemical
compounds formed during the heating crystallize when mixed with water,
and the hard, tiny interlocking crystals of the newly formed silicate
minerals give rigidity to the mass. Of recent years Portland cement
has largely superseded the natural rock cements. "Portland cement is
the product obtained by burning a finely ground artificial mixture
consisting essentially of lime, silica, alumina, and some iron oxide,
these substances being present in certain definite proportions."
(Ries.) The necessary ingredients are generally obtained by grinding
and burning carefully selected mixtures of limestone in some form,
and clay or shale. The great and growing uses of cement need not be
detailed here.

Salt. Most of the common salt (the mineral "halite") of commercial
value occurs in nature in sea or salt lake water; or in beds or strata
of rock salt associated with other strata; or as natural brine in
openings or pores in certain rocks. Considerable salt is obtained by
evaporation of tidewater, as around San Francisco Bay, and of salt lake
water, as at Great Salt Lake, Utah. It has been estimated that the
Great Salt Lake, whose area is about 2,000 square miles and greatest
depth 50 feet, contains several hundred million tons of common salt.
This salt has been washed out of the rocks of the surrounding country
and gradually accumulated in the lake because it has no outlet.

Most important of all sources of salt is the rock salt which occurs
in the form of strata within the earth's crust. Such strata are found
in rocks of nearly all ages from the early Paleozoic to the present.
They resulted from the evaporation of salt lakes or salty more or
less cut-off arms of the sea, after which other strata accumulated on
top of them. Thus in the Silurian system of strata underlying all of
southwestern New York State there occur almost universally from one to
seven beds of salt. The strata including the salt dip gently southward
so that at Ithaca, New York, seven salt beds were struck in a well
at a depth of about 2,200 feet. Northward the salt comes nearer and
nearer the surface. One well penetrated a layer of solid salt 325 feet
thick. Some of this salt is being mined much like coal, but most of it
is obtained by running water into deep wells to dissolve the salt, the
resulting brine being pumped out and evaporated.

Under portions of southern Michigan there are both salt beds and
natural brines charging certain porous rock layers. Both the salt beds
(of Silurian Age) and the brines (of Mississippian Age) supply great
quantities of salt from brines pumped out and evaporated.

In 1918 the United States produced 51,000,000 barrels (280 lbs. each)
of salt. Michigan (17,000,000 barrels) and New York were the leading
States, followed by Kansas, Ohio, West Virginia, and California. Some
of the uses of common salt are given in the description of halite in
the preceding chapter.

Gypsum. The composition and properties of this common and useful
mineral are given in the chapter on "Mineralogy." Rock gypsum is
the variety of great commercial importance. It is widespread, being
quarried in many States, and occurs interstratified with rocks of many
ages where it has originated by evaporation or partial evaporation of
salt water lakes or more or less cut-off arms of the sea. Salt beds
are often associated with gypsum.

For about ten years the average yearly production of gypsum in the
United States has been approximately 2,500,000 tons, or about ten
times that of the nearest foreign competitor (Canada). New York, Iowa,
Michigan, and Ohio are the chief producers. In New York the rock gypsum
(usually four to ten feet thick) lies between shale and limestone
strata of Silurian age, and it is quarried from the central to the
western part of the State. In Michigan the rock gypsum beds, commonly
five to twenty feet thick, lie in Mississippian strata in the southern
portion of the State. A great bed of exceptionally pure rock gypsum
underlies about twenty-five square miles of Webster County, Iowa, in
rocks of late Paleozoic Age. The Kansas gypsum deposits extend across
the central part of the State in rocks of Permian Age.

Rock gypsum is mainly used in making "plaster of Paris," as a retarder
in cement, and as a fertilizer (so-called "land plaster").



GLOSSARY OF COMMON

GEOLOGICAL TERMS


Names of subkingdoms and important classes of fossil plants and
animals, and mineral species, are not included; these being briefly and
systematically discussed in chapters 17, 18, 19, and 20, respectively.
By using the index the reader can quickly locate the page where any
one of these names is discussed. Some definitions in this glossary are
taken from U. S. Survey Bulletin No. 613.

  _Anticline._--A kind of folded structure in which strata have been
     bent upward or arched.

  _Archeozoic._--The earliest known era of geologic time.

  _Basalt._--A common lava of dark color and of great fluidity when
     molten. Basalt is less siliceous than granite and rhyolite, and
     contains much more iron, calcium, and magnesium.

  _Base-level._--The lowest level to which a stream can cut (erode) its
     channel. A whole region may be base-leveled by erosion.

  _Cambrian._--The first or earliest period of the Paleozoic era of
     geologic time.

  _Cenozoic._--The present era of geologic time. It began at least
     several million years ago.

  _Chalk._--A soft, fine-grained, white limestone consisting mainly of
     tiny shells.

  _Conglomerate._--A sedimentary rock consisting of consolidated or
     cemented gravel. Often sandy.

  _Cretaceous._--The last period of the Mesozoic era of geologic time.

  _Crystal._--A regular polyhedral form, possessing a definite internal
     molecular structure, which is assumed by a substance in passing
     from the state of a liquid or gas to that of a solid. Nearly
     every mineral, under proper conditions, will crystallize.

  _Crystalline Rock._--A rock composed of closely fitting mineral
     crystals that have formed in the rock substance, as contrasted
     with one made up of cemented grains of sand or other materials, or
     with a volcanic glass.

  _Crystallography._--The study of crystals.

  _Devonian._--The middle one of the seven periods of the Paleozoic era
     of geologic time.

  _Dike._--A mass of igneous rock that has solidified in a fissure or
     crack in the earth's crust.

  _Drift._--Commonly called glacial drift. The rock fragments--soil,
     gravel, and silt--carried by a glacier. Drift includes the
     unassorted material known as till (ground moraine) and deposits
     made by streams flowing from a glacier.

  _Drowned River Valley._--When a land surface sinks enough to
     permit tidewater to enter the lower ends of its valleys to form
     estuaries, a good example being the lower Hudson Valley.

  _Era._--A name applied to one of the broadest subdivisions of
     geologic time (e.g. Paleozoic era).

  _Erosion._--The wearing away and transportation of materials at and
     near the earth's surface by weathering and solution, and the
     mechanical action of running water, waves, moving ice, or winds
     which use rock fragments as tools or abrasives.

  _Exfoliation._--The splitting off of sheets of rock of various sizes
     and shapes due to changes of temperature. It is a process of
     weathering.

  _Fault._--A fracture in the earth's crust accompanied by movement of
     the rock on one side of the break past that on the other. If the
     fracture is inclined and the rock on one side appears to have slid
     down the slope of the fracture the fault is termed a normal fault.
     If, on the other hand, the rock on one side appears to have been
     shoved up the inclined plane of the break, the fault is termed a
     reverse or thrust fault.

  _Fault-block._--A part of the earth's crust bounded wholly or in part
     by faults.

  _Fault-scarp._--The cliff formed by a fault. Most fault scarps have
     been modified by erosion since the faulting.

  _Fissure._--A crack, break, or fracture in the earth's crust or in a
     mass of rock.

  _Flood-plain._--The nearly level land that borders a stream and is
     subject to occasional overflow. Flood-plains are built up by
     sediment left by such overflows.

  _Fold._--A bend in rock layers or beds. Anticlines and synclines are
     the common types of folds.

  _Formation._--A rock layer, or a series of continuously deposited
     layers grouped together, regarded by the geologist as a unit for
     purposes of description and mapping. A formation is usually named
     from some place where it is exposed in its typical character.

  _Fossil._--The whole or any part of an animal or plant that has been
     preserved in the rocks or the impression left on rock by a plant
     or animal. Preservation is invariably accompanied by some change
     in substance, and from some fossils the original substance has all
     been removed.

  _Geography._--The study of the distribution of the earth's physical
     features, in their relation to each other to the life of sea and
     land, and human life and culture.

  _Geology._--The science which deals with the history of the earth and
     its inhabitants as revealed in the rocks.

  _Glacier._--A body of ice which slowly spreads or moves over the land
     from its place of accumulation.

  _Gneiss_ (pronounced nice).--A metamorphic, crystalline rock with
     mineral grains arranged with long axes more or less parallel,
     giving the rock a banded appearance. Derived from either igneous
     or stratified rocks well within the earth under conditions of
     pressure, and usually also heat and moisture.

  _Igneous Rocks._--Rocks formed by the cooling and solidification
     of a hot liquid material, known as magma, that has originated
     at unknown depths within the earth. Those that have solidified
     beneath the surface are known as intrusive rocks, or if the
     cooling has taken place slowly at great depth, as plutonic rocks,
     e.g. granite. Those that have flowed out over the surface are
     known as effusive rocks, extrusive rocks, or lavas, e.g., basalt.
     Volcanic rocks include not only lavas, but bombs, pumice, tuff,
     volcanic ash, and other fragmental materials or ejecta thrown out
     from volcanoes.

  _Joints._--Nearly all rocks, except very loose surface materials, are
     separated into blocks of varying size and shape by a system of
     cracks called joints. They may be caused by earth-crust movements,
     contraction during solidification of molten rocks, or contraction
     during drying out of sediments.

  _Jurassic._--The middle one of the three periods of the Mesozoic era
     of geologic time.

  _Lava._--An igneous rock which in molten condition has poured out
     upon or close to the earth's surface, e.g. basalt.

  _Limestone._--A sedimentary rock consisting essentially of carbonate
     of lime which generally represents accumulation of shells of
     organisms, but in some cases precipitates from solution. Often
     impure.

  _Loess_ (pronounced lurse with the r obscure).--A fine homogeneous
     silt or loam showing usually no division into layers and forming
     thick and extensive deposits in the Mississippi Valley and in
     China. It is generally regarded as in part at least a deposit of
     wind-blown dust.

  _Marble._--A crystalline limestone, usually a metamorphic rock, the
     limestone having been altered by heat, pressure, and moisture
     within the earth.

  _Meander._--To flow in serpentine curves. A loop in a stream. Most
     streams in flowing across plains develop meanders.

  _Mesa._--A flat-topped hill or mountain left isolated during the
     general erosion or cutting down of a region.

  _Mesozoic._--Next to the present era of geologic time. _ Metamorphic
     Rock._--Any igneous or sedimentary rock which has undergone
     metamorphism, that is notable alteration from its original
     condition. (See Metamorphism.)

  _Metamorphism._--Any change in rocks effected in the earth by heat,
     pressure, solutions, or gases. A common cause of the metamorphism
     of rocks is the intrusion into them of igneous rocks. Rocks that
     have been so changed are termed metamorphic. Marble, for example,
     is metamorphosed limestone.

  _Mineral._--An inorganic substance of definite chemical composition
     found ready made in nature, e.g. calcite, quartz.

  _Mississippian._--A period of the Paleozoic era of geologic time--in
     order of age, the third from the last of the era.

  _Moraine._--Glacial drift carried on, within, or under a glacier and
     deposited at the end, along the sides, or under the glacier.

  _Oil-pool._--An accumulation or body of oil in sedimentary rock that
     yields petroleum on drilling. The oil occurs in the pores of the
     rock and is not a pool or pond in the ordinary sense of these
     words.

  _Ordovician._--Next to the earliest period of the Paleozoic era of
     geologic time.

  _Ore._--A metal-bearing mineral or rock of sufficient value to be
     mined.

  _Outcrop._--That part of a rock formation which appears at the
     surface. The appearance of a rock at the surface or its projection
     above the soil. Often called an exposure.

  _Paleontology._--The study of the world's (geologically) ancient
     life, either plant or animal, by means of fossils.

  _Paleozoic._--An old era of geologic time--third back from the
     present.

  _Peneplain._--A region reduced almost to a plain by the
     long-continued normal erosion of a land surface. It should be
     distinguished from a plain produced by the attack of waves along a
     coast or the built-up flood plain of a river.

  _Pennsylvanian._--Next to the last period of the Paleozoic era of
     geologic time.

  _Period._--A name applied to one of the subdivisions of an era of
     geologic time, e.g. Cambrian period.

  _Permian._--The last period of the Paleozoic era of geologic time.

  _Petrology._--The study of rocks, including igneous, sedimentary, and
     metamorphic rocks.

  _Physiography._--The study of the relief features of the earth and
     how they were produced.

  _Placer Deposit._--A mass of gravel, sand, or similar material
     resulting from the crumbling and erosion of solid rocks and
     containing particles or nuggets of gold, platinum, tin, or other
     valuable minerals, which have been derived from rocks or veins.

  _Plutonic Rock._--An igneous rock solidified from a molten condition
     well within the earth. (See Igneous Rocks.)

  _Proterozoic._--Next to the earliest known era of geologic time.

  _Quartzite._--A metamorphic rock composed of sand grains cemented by
     silica into an extremely hard mass.

  _Quaternary._--The later of the two periods of the Cenozoic era of
     geologic time.

  _Rejuvenated._--Any region which has been subjected to erosion for
     a greater or less length of time and then reelevated so that the
     streams are renewed in activity.

  _Rock._--Any extensive constituent of the crust of the earth, usually
     consisting of a mechanical mixture of two or more minerals, e.g.
     granite, shale. Less commonly a rock consists of a single mineral
     (e.g. pure marble), or of organic matter (e.g. coal).

  _Sandstone._--A sedimentary rock consisting of consolidated or
     cemented sand. Often shaly or limy.

  _Schist._--A rock that by subjection to heat and pressure and usually
     moisture within the earth has undergone a change in the character
     of the particles or minerals that compose it and has these
     minerals arranged in such a way that the rock splits more easily
     in certain directions than in others. It is a metamorphic rock
     derived from either sedimentary or igneous rock, more commonly the
     former.

  _Sedimentary Rocks._--Rocks formed by the accumulation of sediment
     in water (aqueous deposits) or from air (eolian deposits). The
     sediment may consist of rock fragments or particles of various
     sizes (conglomerate, sandstone, shale); of the remains or products
     of animals or plants (certain limestones and coal); of the product
     of chemical action or of evaporation (salt, gypsum, etc.); or of
     mixtures of these materials. Some sedimentary deposits (tuffs)
     are composed of fragments blown from volcanoes and deposited on
     land or in water. A characteristic feature of sedimentary deposits
     is a layered structure known as bedding or stratification. Each
     layer is a bed or stratum. Sedimentary beds as deposited lie flat
     or nearly flat, but subsequently they have often been deformed by
     folding and faulting.

  _Shale._--A sedimentary rock consisting of hardened thin layers of
     fine mud.

  _Silurian._--A period of the Paleozoic era of geologic time--in order
     of age, the third from the beginning of the era.

  _Slate._--A rock that by subjection to pressure within the earth has
     acquired the property of splitting smoothly into thin plates. The
     cleavage is smoother and more regular than the splitting of schist
     along its grain. It is a metamorphic rock nearly always derived
     from shale.

  _Soil._--The mantle of loose material resting upon bedrock, either in
     its place of origin or transported by water, wind, or ice.

  _Strata_ (or stratified rocks).--Sedimentary rocks which, by the
     sorting power of water (less often by wind), are arranged in
     more or less definite layers or beds separated by stratification
     surfaces.

  _Stratification._--The separation of sedimentary rocks into more or
     less parallel layers or beds.

  _Stratigraphy._--The branch of geologic science that deals with the
     order and relations of the strata of the earth's crust.

  _Structure._--In geology, the forms assumed by sedimentary beds and
     igneous rocks that have been moved from their original position by
     forces within the earth, or the forms taken by intrusive masses
     of igneous rock in connection with effects produced mechanically
     on neighboring rocks by the intrusion. Folds (anticlines and
     synclines) and faults are the principal mechanical effects
     considered under structure. Schistosity and cleavage are also
     structural features.

  _Syncline._--A kind of folded structure in which strata have been
     bent downward. It is an inverted arch--the opposite of an
     anticline.

  _Talus_ (pronounced t[=a]y'lus).--The mass of loose rock fragments
     that accumulates at the base of a cliff or steep slope.

  _Terrace._--A steplike bench on a hillside. Most terraces along
     rivers are remnants of valley bottoms formed when the stream
     flowed at higher levels. Other terraces have been formed by waves.
     Some terraces have been cut in solid rock, others have been built
     up of sand and gravel, and still others have been partly cut and
     partly built up.

  _Tertiary._--The earlier of the two periods of the Cenozoic era of
     geologic time.

  _Triassic._--The earliest period of the Mesozoic Era of geologic time.

  _Unconformity._--A break in the regular succession of sedimentary
     rocks, indicated by the fact that one bed rests on the eroded
     surface of one or more beds which may have a distinctly different
     dip from the bed above. An unconformity may indicate that the beds
     below it have at some time been raised above the sea and have been
     eroded. In some places beds thousands of feet thick have been
     washed away before the land again became submerged and the first
     bed above the surface of unconformity was deposited. If beds of
     rock may be regarded as leaves in the volume of geologic history,
     an unconformity marks a gap in the record.

  _Vein._--A mass of mineral material that has been deposited in or
     along a fissure in the rocks. A vein differs from a dike in that
     the vein material was introduced gradually by deposition from
     solution, whereas a dike was intruded in a molten condition.
     Quartz and calcite are very common vein minerals.

  _Volcanic Rocks._--Igneous rocks erupted at or near the earth's
     surface, including lavas, tuffs, volcanic ashes, and like material.

  _Weathering._--The group of processes, such as the chemical action of
     air and rain water, and of plants and bacteria, and the mechanical
     action of changes of temperature, whereby rocks on exposure to the
     weather change in character, decay, and finally crumble into soil.



Transcriber Note


All illustrations splitting paragraphs were moved before or after the
paragraph. All simple typos were corrected (i.e., Reudemann to Ruedemann,
pryoxene to pyroxene).





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