path: root/40404-0.txt

*** START OF THE PROJECT GUTENBERG EBOOK 40404 ***

[Transcriber's Note:

   _Text_ and =Text= represent italic and bold text respectively.

   Subscripts are displayed as an underscore followed by the number
     or text in braces: SiO_{2}.]


       *       *       *       *       *


    [Illustration: A Valley with Rocky Ledges cut in the Horizontal
       Strata, Scotland]




THE ELEMENTS OF GEOLOGY

BY

WILLIAM HARMON NORTON

PROFESSOR OF GEOLOGY IN CORNELL COLLEGE




GINN & COMPANY

BOSTON * NEW YORK * CHICAGO * LONDON




Copyright, 1905, 1921, by

WILLIAM HARMON NORTON

ALL RIGHTS RESERVED

5511


The Atheneum Press

GINN & COMPANY PROPRIETORS

BOSTON * U.S.A.




PREFACE


Geology is a science of such rapid growth that no apology is expected
when from time to time a new text-book is added to those already in the
field. The present work, however, is the outcome of the need of a
text-book of very simple outline, in which causes and their consequences
should be knit together as closely as possible,--a need long felt by the
author in his teaching, and perhaps by other teachers also. The author
has ventured, therefore, to depart from the common usage which
subdivides geology into a number of departments,--dynamical, structural,
physiographic, and historical,--and to treat in immediate connection
with each geological process the land forms and the rock structures
which it has produced.

It is hoped that the facts of geology and the inferences drawn from
them have been so presented as to afford an efficient discipline in
inductive reasoning. Typical examples have been used to introduce many
topics, and it has been the author's aim to give due proportion to
both the wide generalizations of our science and to the concrete facts
on which they rest.

There have been added a number of practical exercises such as the
author has used for several years in the class room. These are not
made so numerous as to displace the problems which no doubt many
teachers prefer to have their pupils solve impromptu during the
recitation, but may, it is hoped, suggest their use.

In historical geology a broad view is given of the development of the
North American continent and the evolution of life upon the planet.
Only the leading types of plants and animals are mentioned, and
special attention is given to those which mark the lines of descent of
forms now living.

By omitting much technical detail of a mineralogical and
palæontological nature, and by confining the field of view almost
wholly to our own continent, space has been obtained to give to what
are deemed for beginners the essentials of the science a fuller
treatment than perhaps is common.

It is assumed that field work will be introduced with the commencement
of the study. The common rocks are therefore briefly described in the
opening chapters. The drift also receives early mention, and teachers
in the northern states who begin geology in the fall may prefer to
take up the chapter on the Pleistocene immediately after the chapter
on glaciers.

Simple diagrams have been used freely, not only because they are often
clearer than any verbal statement, but also because they readily lend
themselves to reproduction on the blackboard by the pupil. The text
will suggest others which the pupil may invent. It is hoped that the
photographic views may also be used for exercises in the class room.

The generous aid of many friends is recognized with special pleasure.
To Professor W. M. Davis of Harvard University there is owing a large
obligation for the broad conceptions and luminous statements of
geologic facts and principles with which he has enriched the
literature of our science, and for his stimulating influence in
education. It is hoped that both in subject-matter and in method the
book itself makes evident this debt. But besides a general obligation
shared by geologists everywhere, and in varying degrees by perhaps all
authors of recent American text-books in earth science, there is owing
a debt direct and personal. The plan of the book, with its use of
problems and treatment of land forms and rock structures in immediate
connection with the processes which produce them, was submitted to
Professor Davis, and, receiving his approval, was carried into effect,
although without the sanction of precedent at the time. Professor
Davis also kindly consented to read the manuscript throughout, and his
many helpful criticisms and suggestions are acknowledged with sincere
gratitude.

Parts of the manuscript have been reviewed by Dr. Samuel Calvin and
Dr. Frank M. Wilder of the State University of Iowa; Dr. S. W. Beyer
of the Iowa College of Agriculture and Mechanic Arts; Dr. U. S. Grant
of Northwestern University; Professor J. A. Udden of Augustana
College, Illinois; Dr. C. H. Gordon of the New Mexico State School of
Mines; Principal Maurice Ricker of the High School, Burlington, Iowa;
and the following former students of the author who are engaged in the
earth sciences: Dr. W. C. Alden of the United States Geological Survey
and the University of Chicago; Mr. Joseph Sniffen, instructor in the
Academy of the University of Chicago, Morgan Park; Professor Martin
Iorns, Fort Worth University, Texas; Professor A. M. Jayne, Dakota
University; Professor G. H. Bretnall, Monmouth College, Illinois;
Professor Howard E. Simpson, Colby College, Maine; Mr. E. J. Cable,
instructor in the Iowa State Normal College; Principal C. C. Gray of
the High School, Fargo, North Dakota; and Mr. Charles Persons of the
High School, Hannibal, Missouri. A large number of the diagrams of the
book were drawn by Mr. W. W. White of the Art School of Cornell
College. To all these friends, and to the many who have kindly
supplied the illustrations of the text, whose names are mentioned in
an appended list, the writer returns his heartfelt thanks.

WILLIAM HARMON NORTON

Cornell College, Mount Vernon, Iowa

July, 1905




INTRODUCTORY NOTE


During the preparation of this book Professor Norton has
frequently discussed its plan with me by correspondence, and we
have considered together the matters of scope, arrangement, and
presentation.

As to scope, the needs of the young student and not of the expert
have been our guide; the book is therefore a text-book, not a
reference volume.

In arrangement, the twofold division of the subject was chosen
because of its simplicity and effectiveness. The principles of
physical geology come first; the several chapters are arranged in
what is believed to be a natural order, appropriate to the
greatest part of our country, so that from a simple beginning a
logical sequence of topics leads through the whole subject. The
historical view of the science comes second, with many specific
illustrations of the physical processes previously studied, but
now set forth as part of the story of the earth, with its many
changes of aspect and its succession of inhabitants. Special
attention is here given to North America, and care is taken to
avoid overloading with details.

With respect to method of presentation, it must not be forgotten
that the text-book is only one factor in good teaching, and that
in geology, as in other sciences, the teacher, the laboratory, and
the local field are other factors, each of which should play an
appropriate part. The text suggests observational methods, but it
cannot replace observation in field or laboratory; it offers
certain exercises, but space cannot be taken to make it a
laboratory manual as well as a book for study; it explains many
problems, but its statements are necessarily more terse than the
illustrative descriptions that a good and experienced teacher
should supply. Frequent use is made of induction and inference in
order that the student may come to see how reasonable a science is
geology, and that he may avoid the too common error of thinking
that the opinions of "authorities" are reached by a private road
that is closed to him. The further extension of this method of
presentation is urged upon the teacher, so that the young
geologist may always learn the evidence that leads to a
conclusion, and not only the conclusion itself.

W. M. DAVIS

Harvard University, Cambridge, Mass.

July, 1905




ACKNOWLEDGMENT OF ILLUSTRATIONS


    Adams, Professor F. D., McGill University, Canada, 241.
    Alden, Dr. W. C., Washington, D.C., 353.
    American Museum of Natural History, New York, 344.
    Ash, H. C., Galesburg, Ill., 133.
    Beyer, Dr. S. W., Iowa College of Agriculture, 363.
    Calvin, Dr. Samuel, Iowa State University, 45, 295, 317, 325, 371.
    Carney, Frank, Ithaca, N.Y., 356.
    Clark, Dr. Wm. B., Maryland Geological Survey, 43.
    Borne, Dr. Georg v. d., Jena, Germany, 5, 6.
    Daly, Dr. R. A., Ottawa, Canada, 164.
    Defieux, C. A., Liverpool, England, 154.
  * Detroit Photographic Co., 235, 236.
  * Ellis, W. M., Edna, Kan., 13.
    Fairchild, Professor H. L., University of Rochester, 141, 357.
    Field Columbian Museum, Chicago, 87.
    Forster, Dr. A. E., University of Vienna, 32.
    Gardner, J. L., Boston, 12, 140, 352.
    Geological Survey of Canada, 256.
    Gilbert, Dr. G. K., by courtesy of the American Book Company, 39.
  * Haines, Ben, New Albany, Ind., 33.
  * Haynes, F. J., St. Paul, Minn., 52, 95, 233.
    Henderson, Judge Julius, Boulder, Col., 94.
    James, George Wharton, Pasadena, Cal., 16, 127, 215, 229.
    Johnston-Lavis, Professor H. J., Beaulieu, France, 216.
    King, J. Harding, Stourbridge, England, 119.
    Lawson, Dr. Andrew C., University of California, 113.
    Le Conte, Professor J. N., University of California, 8.
    Libbey, Dr. William, Princeton University, 92.
  * McAllister, T. H., New York, 242.
  * Meyers, H. C., Boise, Id., 19.
    Mills, Professor H. A., Cornell College, 208, 304.
    Norton, Professor W. H., Cornell College, 14, 35, 59, 88, 128,
       183, 226, 234, 255, 340, 364, 367.
  * Notman, Wm. & Son, Montreal, Canada, 98, 181.
    Obrutschew, Dr. W., Tomsk Technological Institute, Siberia, 73.
    Oldham, Dr. R. D., Geological Survey of India, 120.
  * Peabody, H. C., Pasadena, Cal., 54.
  * Pierce, C. C. & Co., Los Angeles, Cal., 15.
    Pillsbury, Arthur, San Francisco, Cal., .115.
  * Rau, Wm., Philadelphia, 18, 21, 122, 123, 218.
    Reusch, Dr. Hans, Geological Survey of Norway, 112.
    Reynolds, Professor S. H., University College, Bristol, England, 202.
    Ricker, Principal Maurice, Burlington, Iowa, 48, 89.
  * Shepard, E. A., Minneapolis, Minn., 105.
    Smith, W. S. Tangier, Los Gatos, Cal., 186.
  * Soule Photographic Co., Boston, 131.
    U. S. Geological Survey, 3, 4, 23, 25, 34, 41, 63, 69, 78, 79,
       80, 110, 111, 114, 125, 126, 129, 130, 142, 151, 153, 169,
       172,177, 178, 188, 211, 212, 214, 228, 237, 238, 239, 243, 244,
       254, 257, 340, 341, 353, 355.
    U. S. National Museum, 149, 220, 221, 222, 225, 332.
  * Valentine & Sons, Dundee, Scotland, 40, 136, 227.
    Vroman, A. C., Pasadena, Cal., 17.
  * Ward's Natural Science Establishment, Rochester, N.Y., 152.
  * Welch, R., Belfast, Ireland, 1, 37.
  * Westgate, Dr. L. G., Ohio Wesleyan University, 66.
    Whymper, Edward, London, England, 106.
  * Wilcox, W. D., Washington, D.C., 20.
  * Wilson, Dr. A. W. G., McGill University, Canada, 68.
  * Wilson, G. W., & Co., Aberdeen, Scotland, 82, 213.
  * Worsley-Benison, F. H., Cheapstow, England, 170.

  * Dealer in photographs or lantern slides.




CONTENTS


                                                                Page

   Introduction.--The Scope And Aim Of Geology                      1

  PART I

  EXTERNAL GEOLOGICAL AGENCIES

  Chapter
      I.    The Work Of The Weather                                 5
     II.    The Work Of Ground Water                               39
    III.    Rivers And Valleys                                     54
     IV.    River Deposits                                         93
      V.    The Work Of Glaciers                                  113
     VI.    The Work Of The Wind                                  144
    VII.    The Sea And Its Shores                                155
   VIII.    Offshore And Deep-Sea Deposits                        174

  PART II

  INTERNAL GEOLOGICAL AGENCIES

     IX.    Movements Of The Earth's Crust                        195
      X.    Earthquakes                                           233
     XI.    Volcanoes                                             238
    XII.    Underground Structures Of Igneous Origin              265
   XIII.    Metamorphism And Mineral Veins                        281

  PART III

  HISTORICAL GEOLOGY

    XIV.    The Geological Record                                 291
     XV.    The Pre-Cambrian Systems                              304
    XVI.    The Cambrian                                          315
   XVII.    The Ordovician And Silurian                           327
  XVIII.    The Devonian                                          341
    XIX.    The Carboniferous                                     350
     XX.    The Mesozoic                                          368
    XXI.    The Tertiary                                          394
   XXII.    The Quaternary                                        416

            INDEX                                                 451




THE ELEMENTS OF GEOLOGY




INTRODUCTION


THE SCOPE AND AIM OF GEOLOGY

Geology deals with the rocks of the earth's crust. It learns from
their composition and structure how the rocks were made and how they
have been modified. It ascertains how they have been brought to their
present places and wrought to their various topographic forms, such as
hills and valleys, plains and mountains. It studies the vestiges which
the rocks preserve of ancient organisms which once inhabited our
planet. Geology is the history of the earth and its inhabitants, as
read in the rocks of the earth's crust.

To obtain a general idea of the nature and method of our science
before beginning its study in detail, we may visit some valley, such
as that illustrated in the frontispiece, on whose sides are rocky
ledges. Here the rocks lie in horizontal layers. Although only their
edges are exposed, we may infer that these layers run into the upland
on either side and underlie the entire district; they are part of the
foundation of solid rock which everywhere is found beneath the loose
materials of the surface.

The ledges of the valley of our illustration are of sandstone. Looking
closely at the rock we see that it is composed of myriads of grains of
sand cemented together. These grains have been worn and rounded. They
are sorted also, those of each layer being about of a size. By some
means they have been brought hither from some more ancient source.
Surely these grains have had a history before they here found a
resting place,--a history which we are to learn to read.

The successive layers of the rock suggest that they were built one
after another from the bottom upward. We may be as sure that each
layer was formed before those above it as that the bottom courses of
stone in a wall were laid before the courses which rest upon them.

We have no reason to believe that the lowest layers which we see here
were the earliest ever formed. Indeed, some deep boring in the
vicinity may prove that the ledges rest upon other layers of rock
which extend downward for many hundreds of feet below the valley
floor. Nor may we conclude that the highest layers here were the
latest ever laid; for elsewhere we may find still later layers lying
upon them.

A short search may find in the rock relics of animals, such as the
imprints of shells, which lived when it was deposited; and as these
are of kinds whose nearest living relatives now have their home in the
sea, we infer that it was on the flat sea floor that the sandstone was
laid. Its present position hundreds of feet above sea level proves
that it has since emerged to form part of the land; while the flatness
of the beds shows that the movement was so uniform and gentle as not
to break or strongly bend them from their original attitude.

The surface of some of these layers is ripple-marked. Hence the sand
must once have been as loose as that of shallow sea bottoms and sea
beaches to-day, which is thrown into similar ripples by movements of
the water. In some way the grains have since become cemented into firm
rock.

Note that the layers on one side of the valley agree with those on the
other, each matching the one opposite at the same level. Once they
were continuous across the valley. Where the valley now is was once a
continuous upland built of horizontal layers; the layers now show
their edges, or _outcrop_, on the valley sides because they have been
cut by the valley trench.

The rock of the ledges is crumbling away. At the foot of each step of
rock lie fragments which have fallen. Thus the valley is slowly
widening. It has been narrower in the past; it will be wider in the
future.

Through the valley runs a stream. The waters of rains which have
fallen on the upper parts of the stream's basin are now on their way
to the river and the sea. Rock fragments and grains of sand creeping
down the valley slopes come within reach of the stream and are washed
along by the running water. Here and there they lodge for a time in
banks of sand and gravel, but sooner or later they are taken up again
and carried on. The grains of sand which were brought from some
ancient source to form these rocks are on their way to some new goal.
As they are washed along the rocky bed of the stream they slowly rasp
and wear it deeper. The valley will be deeper in the future; it has
been less deep in the past.

In this little valley we see slow changes now in progress. We find
also in the composition, the structure, and the attitude of the rocks,
and the land forms to which they have been sculptured, the record of a
long succession of past changes involving the origin of sand grains
and their gathering and deposit upon the bottom of some ancient sea,
the cementation of their layers into solid rock, the uplift of the
rocks to form a land surface, and, last of all, the carving of a
valley in the upland.

Everywhere, in the fields, along the river, among the mountains, by the
seashore, and in the desert, we may discover slow changes now in
progress and the record of similar changes in the past. Everywhere we
may catch glimpses of a process of gradual change, which stretches
backward into the past and forward into the future, by which the forms
and structures of the face of the earth are continually built and
continually destroyed. The science which deals with this long process is
geology. Geology treats of the natural changes now taking place upon the
earth and within it, the agencies which produce them, and the land forms
and rock structures which result. It studies the changes of the present
in order to be able to read the history of the earth's changes in the
past.

The various agencies which have fashioned the face of the earth may.
be divided into two general classes. In Part I we shall consider those
which work upon the earth from without, such as the weather, running
water, glaciers, the wind, and the sea. In Part II we shall treat of
those agencies whose sources are within the earth, and among whose
manifestations are volcanoes and earthquakes and the various movements
of the earth's crust. As we study each agency we shall notice not only
how it does its work, but also the records which it leaves in the rock
structures and the land forms which it produces. With this preparation
we shall be able in Part III to read in the records of the rocks the
history of our planet and the successive forms of life which have
dwelt upon it.




Part I

EXTERNAL GEOLOGICAL AGENCIES


CHAPTER I

THE WORK OF THE WEATHER


In our excursion to the valley with sandstone ledges we witnessed a
process which is going forward in all lands. Everywhere the rocks are
crumbling away; their fragments are creeping down hillsides to the
stream ways and are carried by the streams to the sea, where they are
rebuilt into rocky layers. When again the rocks are lifted to form
land the process will begin anew; again they will crumble and creep
down slopes and be washed by streams to the sea. Let us begin our
study of this long cycle of change at the point where rocks
disintegrate and decay under the action of the weather. In studying
now a few outcrops and quarries we shall learn a little of some common
rocks and how they weather away.

=Stratification and jointing.= At the sandstone ledges we saw that the
rock was divided into parallel layers. The thicker layers are known as
_strata_, and the thin leaves into which each stratum may sometimes be
split are termed _laminæ_. To a greater or less degree these layers
differ from each other in fineness of grain, showing that the material
has been sorted. The planes which divide them are called _bedding
planes_.

Besides the bedding planes there are other division planes, which cut
across the strata from top to bottom. These are found in all rocks and
are known as _joints_ (Fig. 1). Two sets of joints, running at about
right angles to each other, together with the bedding planes, divide
the sandstone into quadrangular blocks.

   [Illustration: Fig. 1. Cliff of Sandstone, Ireland]

=Sandstone.= Examining a piece of sandstone we find it composed of
grains quite like those of river sand or of sea beaches. Most of the
grains are of a clear glassy mineral called quartz. These quartz
grains are very hard and will scratch the steel of a knife blade. They
are not affected by acid, and their broken surfaces are irregular like
those of broken glass.

The grains of sandstone are held together by some cement. This may be
_calcareous_, consisting of soluble carbonate of lime. In brown
sandstones the cement is commonly _ferruginous_,--hydrated iron oxide,
or iron rust, forming the bond, somewhat as in the case of iron nails
which have rusted together. The strongest and most lasting cement is
_siliceous_, and sand rocks whose grains are closely cemented by
silica, the chemical substance of which quartz is made, are known as
quartzites.

We are now prepared to understand how sandstone is affected by the
action of the weather. On ledges where the rock is exposed to view its
surface is more or less discolored and the grains are loose and may be
rubbed off with the finger. On gentle slopes the rock is covered with
a soil composed of sand, which evidently is crumbled sandstone, and
dark carbonaceous matter derived from the decay of vegetation. Clearly
it is by the dissolving of the cement that the rock thus breaks down
to loose sand. A piece of sandstone with calcareous cement, or a bit
of old mortar, which is really an artificial stone also made of sand
cemented by lime, may be treated in a test tube with hydrochloric acid
to illustrate the process.

   [Illustration: Fig. 2. Section of Limestone Quarry

   Scale, 1 in. = 30 ft. _a_, red residual clay; _mn_, pitted
   surface of rotted limestone; _bb_, limestone divided into thin
   layers; _c_, thick layers of laminated limestone, the laminæ
   being firmly cemented together; _j_, _j_, _j_, joints. Is _bb_
   thin-layered because originally so laid, or because it has been
   broken up by weathering, although once like _c_ thick-layered?]

A limestone quarry. Here also we find the rock stratified and jointed
(Fig. 2). On the quarry face the rock is distinctly seen to be altered
for some distance from its upper surface. Below the altered zone the
rock is sound and is quarried for building; but the altered upper
layers are too soft and broken to be used for this purpose. If the
limestone is laminated, the laminae here have split apart, although
below they hold fast together. Near the surface the stone has become
rotten and crumbles at the touch, while on the top it has completely
broken down to a thin layer of limestone meal, on which rests a fine
reddish clay.

Limestone is made of minute grains of carbonate of lime all firmly
held together by a calcareous cement. A piece of the stone placed in a
test tube with hydrochloric acid dissolves with brisk effervescence,
leaving the insoluble impurities, which were disseminated through it,
at the bottom of the tube as a little clay.

We can now understand the changes in the upper layers of the quarry.
At the surface of the rock the limestone has completely dissolved,
leaving the insoluble residue as a layer of reddish clay. Immediately
below the clay the rock has disintegrated into meal where the cement
between the limestone grains has been removed, while beneath this the
laminae are split apart where the cement has been dissolved only along
the planes of lamination where the stone is more porous. As these
changes in the rock are greatest at the surface and diminish downward,
we infer that they have been caused by agents working downward from
the surface.

At certain points these agencies have been more effective than
elsewhere. The upper rock surface is pitted. Joints are widened as
they approach the surface, and along these seams we may find that the
rock is altered even down to the quarry floor.

=A shale pit.= Let us now visit some pit where shale--a laminated and
somewhat hardened clay--is quarried for the manufacture of brick. The
laminae of this fine-grained rock may be as thin as cardboard in
places, and close joints may break the rock into small rhombic blocks.
On the upper surface we note that the shale has weathered to a clayey
soil in which all traces of structure have been destroyed. The clay
and the upper layers of the shale beneath it are reddish or yellow,
while in many cases the color of the unaltered rock beneath is blue.

=The sedimentary rocks.= The three kinds of layered rocks whose
acquaintance we have made--sandstone, limestone, and shale--are the
leading types of the great group of stratified, or sedimentary, rocks.
This group includes all rocks made of sediments, their materials
having settled either in water upon the bottoms of rivers, lakes, or
seas, or on dry land, as in the case of deposits made by the wind and
by glaciers. Sedimentary rocks are divided into the fragmental
rocks--which are made of fragments, either coarse or fine--and the far
less common rocks which are constituted of chemical precipitates.

   [Illustration: Fig. 3. Conglomerate]

The sedimentary rocks are divided according to their composition into
the following classes:

1. The arenaceous, or quartz rocks, including beds of loose sand and
gravel, sandstone, quartzite, and conglomerate (a rock made of
cemented rounded gravel or pebbles).

2. The calcareous, or lime rocks, including limestone and a soft white
rock formed of calcareous powder known as chalk.

3. The argillaceous, or clay rocks, including muds, clays, and shales.
These three classes pass by mixture into one another. Thus there are
limy and clayey sandstones, sandy and clayey limestones, and sandy and
limy shales.

=Granite.= This familiar rock may be studied as an example of the
second great group of rocks,--_the unstratified_, or _igneous rocks_.
These are not made of cemented sedimentary grains, but of interlocking
crystals which have crystallized from a molten mass. Examining a piece
of granite, the most conspicuous crystals which meet the eye are those
of feldspar. They are commonly pink, white, or yellow, and break along
smooth cleavage planes which reflect the light like tiny panes of
glass. Mica may be recognized by its glittering plates, which split
into thin elastic scales. A third mineral, harder than steel, breaking
along irregular surfaces like broken glass, we identify as quartz.

How granite alters under the action of the weather may be seen in
outcrops where it forms the bed rock, or country rock, underlying the
loose formations of the surface, and in many parts of the northern
states where granite bowlders and pebbles more or less decayed may be
found in a surface sheet of stony clay called the drift. Of the
different minerals composing granite, quartz alone remains unaltered.
Mica weathers to detached flakes which have lost their elasticity. The
feldspar crystals have lost their luster and hardness, and even have
decayed to clay. Where long-weathered granite forms the country rock,
it often may be cut with spade or trowel for several feet from the
surface, so rotten is the feldspar, and here the rock is seen to break
down to a clayey soil containing grains of quartz and flakes of mica.

These are a few simple illustrations of the surface changes which some
of the common kinds of rocks undergo. The agencies by which these
changes are brought about we will now take up under two
divisions,--_chemical agencies_ producing rock decay and _mechanical
agencies_ producing rock disintegration.


The Chemical Work Of Water

As water falls on the earth in rain it has already absorbed from the
air carbon dioxide (carbonic acid gas) and oxygen. As it sinks into
the ground and becomes what is termed ground water, it takes into
solution from the soil humus acids and carbon dioxide, both of which
are constantly being generated there by the decay of organic matter.
So both rain and ground water are charged with active chemical agents,
by the help of which they corrode and rust and decompose all rocks to
a greater or less degree. We notice now three of the chief chemical
processes concerned in weathering,--solution, the formation of
carbonates, and oxidation.

=Solution.= Limestone, although so little affected by pure water that
five thousand gallons would be needed to dissolve a single pound, is
easily dissolved in water charged with carbon dioxide. In limestone
regions well water is therefore "hard." On boiling the water for some
time the carbon dioxide gas is expelled, the whole of the lime
carbonate can no longer be held in solution, and much of it is thrown
down to form a crust or "scale" in the kettle or in the tubes of the
steam boiler. All waters which flow over limestone rocks or soak
through them are constantly engaged in dissolving them away, and in
the course of time destroy beds of vast extent and great thickness.

   [Illustration: Fig. 4. Surface of Limestone furrowed by
     Weathering, Montana]

The upper surface of limestone rocks becomes deeply pitted, as we saw
in the limestone quarry, and where the mantle of waste has been
removed it may be found so intricately furrowed that it is difficult
to traverse (Fig. 4).

Beds of _rock salt_ buried among the strata are dissolved by seeping
water, which issues in salt springs. _Gypsum_, a mineral composed of
hydrated sulphate of lime, and so soft that it may be scratched with
the finger nail, is readily taken up by water, giving to the water of
wells and springs a peculiar hardness difficult to remove.

The dissolving action of moisture may be noted on marble tombstones of
some age, marble being a limestone altered by heat and pressure and
composed of crystalline grains. By assuming that the date on each
monument marks the year of its erection, one may estimate how many
years on the average it has taken for weathering to loosen fine grains
on the polished surface, so that they may be rubbed off with the
finger, to destroy the polish, to round the sharp edges of tool marks
in the lettering, and at last to open cracks and seams and break down
the stone. We may notice also whether the gravestones weather more
rapidly on the sunny or the shady side, and on the sides or on the
top.

The weathered surface of granular limestone containing shells shows
them standing in relief. As the shells are made of crystalline
carbonate of lime, we may infer whether the carbonate of lime is less
soluble in its granular or in its crystalline condition.

=The formation of carbonates.= In attacking minerals water does more
than merely take them into solution. It decomposes them, forming new
chemical compounds of which the carbonates are among the most
important. Thus feldspar consists of the insoluble silicate of
alumina, together with certain alkaline silicates which are broken up
by the action of water containing carbon dioxide, forming alkaline
carbonates. These carbonates are freely soluble and contribute potash
and soda to soils and river waters. By the removal of the soluble
ingredients of feldspar there is left the silicate of alumina, united
with water or hydrated, in the condition of a fine plastic clay which,
when white and pure, is known as _kaolin_ and is used in the
manufacture of porcelain. Feldspathic rocks which contain no iron
compounds thus weather to whitish crusts, and even apparently sound
crystals of feldspar, when ground to thin slices and placed under the
microscope, may be seen to be milky in color throughout because an
internal change to kaolin has begun.

   [Illustration: Fig. 5. Bowlder split by Heat and Cold,
     Western Texas]

=Oxidation.= Rocks containing compounds of iron weather to reddish
crusts, and the seams of these rocks are often lined with rusty films.
Oxygen and water have here united with the iron, forming hydrated iron
oxide. The effects of oxidation may be seen in the alteration of many
kinds of rocks and in red and yellow colors of soils and subsoils.

_Pyrite_ is a very hard mineral of a pale brass color, found in
scattered crystals in many rocks, and is composed of iron and sulphur
(iron sulphide). Under the attack of the weather it takes up oxygen,
forming iron sulphate (green vitriol), a soluble compound, and
insoluble hydrated iron oxide, which as a mineral is known as
limonite. Several large masses of iron sulphide were placed some years
ago on the lawn in front of the National Museum at Washington. The
mineral changed so rapidly to green vitriol that enough of this
poisonous compound was washed into the ground to kill the roots of the
surrounding grass.


Agents Of Mechanical Disintegration

=Heat and cold.= Rocks exposed to the direct rays of the sun become
strongly heated by day and expand. After sunset they rapidly cool and
contract. When the difference in temperature between day and night is
considerable, the repeated strains of sudden expansion and contraction
at last become greater than the rocks can bear, and they break, for
the same reason that a glass cracks when plunged into boiling water
(Fig. 5).

Rocks are poor conductors of heat, and hence their surfaces may become
painfully hot under the full blaze of the sun, while the interior
remains comparatively cool. By day the surface shell expands and tends
to break loose from the mass of the stone. In cooling in the evening
the surface shell suddenly contracts on the unyielding interior and in
time is forced off in scales (Fig. 6).

   [Illustration: Fig. 6. Bowlders scaling off under Heat and Cold,
     Western Texas]

Many rocks, such as granite, are made up of grains of various minerals
which differ in color and in their capacity to absorb heat, and which
therefore contract and expand in different ratios. In heating and
cooling these grains crowd against their neighbors and tear loose from
them, so that finally the rock disintegrates into sand.

The conditions for the destructive action of heat and cold are most
fully met in arid regions when vegetation is wanting for lack of
sufficient rain. The soil not being held together by the roots of
plants is blown away over large areas, leaving the rocks bare to the
blazing sun in a cloudless sky. The air is dry, and the heat received
by the earth by day is therefore rapidly radiated at night into space.
There is a sharp and sudden fall of temperature after sunset, and the
rocks, strongly heated by day, are now chilled perhaps even to the
freezing point.

In the Sahara the thermometer has been known to fall 131° F. within a
few hours. In the light air of the Pamir plateau in central Asia a
rise of 90° F. has been recorded from seven o'clock in the morning to
one o'clock in the afternoon. On the mountains of southwestern Texas
there are frequently heard crackling noises as the rocks of that arid
region throw off scales from a fraction of an inch to four inches in
thickness, and loud reports are made as huge bowlders split apart.
Desert pebbles weakened by long exposure to heat and cold have been
shivered to fine sharp-pointed fragments on being placed in sand
heated to 180 degrees F. Beds half a foot thick, forming the floor of
limestone quarries in Wisconsin, have been known to buckle and arch
and break to fragments under the heat of the summer sun.

=Frost.= By this term is meant the freezing and thawing of water
contained in the pores and crevices of rocks. All rocks are more or
less porous and all contain more or less water in their pores. Workers
in stone call this "quarry water," and speak of a stone as "green"
before the quarry water has dried out. Water also seeps along joints
and bedding planes and gathers in all seams and crevices. Water
expands in freezing, ten cubic inches of water freezing to about
eleven cubic inches of ice. As water freezes in the rifts and pores of
rocks it expands with the irresistible force illustrated in the
freezing and breaking of water pipes in winter. The first rift in the
rock, perhaps too narrow to be seen, is widened little by little by
the wedges of successive frosts, and finally the rock is broken into
detached blocks, and these into angular chip-stone by the same
process.

It is on mountain tops and in high latitudes that the effects of frost
are most plainly seen. "Every summit" says Whymper, "amongst the rock
summits upon which I have stood has been nothing but a piled-up heap
of fragments" (Fig. 7). In Iceland, in Spitzbergen, in Kamchatka, and
in other frigid lands large areas are thickly strewn with sharp-edged
fragments into which the rock has been shattered by frost.

   [Illustration: Fig. 7. Rocks broken by Frost, Summit of the
     Eggischhorn, Switzerland]

=Organic agents.= We must reckon the roots of plants and trees among
the agents which break rocks into pieces. The tiny rootlet in its
search for food and moisture inserts itself into some minute rift, and
as it grows slowly wedges the rock apart. Moreover, the acids of the
root corrode the rocks with which they are in contact. One may
sometimes find in the soil a block of limestone wrapped in a mesh of
roots, each of which lies in a little furrow where it has eaten into
the stone.

Rootless plants called _lichens_ often cover and corrode rocks as yet
bare of soil; but where lichens are destroying the rock less rapidly
than does the weather, they serve in a way as a protection.

=Conditions favoring disintegration and decay.= The disintegration of
rocks under frost and temperature changes goes on most rapidly in cold
and arid climates, and where vegetation is scant or absent. On the
contrary, the decay of rocks under the chemical action of water is
favored by a warm, moist climate and abundant vegetation. Frost and
heat and cold can only act within the few feet from the surface to
which the necessary temperature changes are limited, while water
penetrates and alters the rocks to great depths.

The pupil may explain.

In what ways the presence of joints and bedding planes assists in the
breaking up and decay of rocks under the action of the weather.

Why it is a good rule of stone masons never to lay stones on edge, but
always on their natural bedding planes.

Why stones fresh from the quarry sometimes go to pieces in early
winter, when stones which have been quarried for some months remain
uninjured.

Why quarrymen in the northern states often keep their quarry floors
flooded during winter.

Why laminated limestone should not be used for curbstone.

Why rocks composed of layers differing in fineness of grain and in
ratios of expansion do not make good building stone.

Fine-grained rocks with pores so small that capillary attraction keeps
the water which they contain from readily draining away are more apt
to hold their pores ten elevenths full of water than are rocks whose
pores are larger. Which, therefore, are more likely to be injured by
frost?

Which is subject to greater temperature changes, a dark rock or one of
a light color? the north side or the south side of a valley?


The Mantle of Rock Waste

We have seen that rocks are everywhere slowly wasting away. They are
broken in pieces by frost, by tree roots, and by heat and cold. They
dissolve and decompose under the chemical action of water and the
various corrosive substances which it contains, leaving their
insoluble residues as residual clays and sands upon the surface. As a
result there is everywhere forming a mantle of rock waste which covers
the land. It is well to imagine how the country would appear were this
mantle with its soil and vegetation all scraped away or had it never
been formed. The surface of the land would then be everywhere of bare
rock as unbroken as a quarry floor.

=The thickness of the mantle.= In any locality the thickness of the
mantle of rock waste depends as much on the rate at which it is
constantly being removed as on the rate at which it is forming. On the
face of cliffs it is absent, for here waste is removed as fast as it
is made. Where waste is carried away more slowly than it is produced,
it accumulates in time to great depth.

The granite of Pikes Peak is disintegrated to a depth of twenty feet.
In the city of Washington granite rock is so softened to a depth of
eighty feet that it can be removed with pick and shovel. About
Atlanta, Georgia, the rocks are completely rotted for one hundred feet
from the surface, while the beginnings of decay may be noticed at
thrice that depth. In places in southern Brazil the rock is decomposed
to a depth of four hundred feet.

In southwestern Wisconsin a reddish residual clay has an average depth
of thirteen feet on broad uplands, where it has been removed to the
least extent. The country rock on which it rests is a limestone with
about ten per cent of insoluble impurities. At least how thick, then,
was that portion of the limestone which has rotted down to the clay?

=Distinguishing characteristics of residual waste.= We must learn to
distinguish waste formed in place by the action of the weather from
the products of other geological agencies. Residual waste is
unstratified. It contains no substances which have not been derived
from the weathering of the parent rock. There is a gradual transition
from residual waste into the unweathered rock beneath. Waste resting
on sound rock evidently has been shifted and was not formed in place.

In certain regions of southern Missouri the land is covered with a
layer of broken flints and red clay, while the country rock is
limestone. The limestone contains nodules of flint, and we may infer
that it has been by the decay and removal of thick masses of limestone
that the residual layer of clay and flints has been left upon the
surface. Flint is a form of quartz, dull-lustered, usually gray or
blackish in color, and opaque except on thinnest edges, where it is
translucent.

Over much of the northern states there is spread an unstratified stony
clay called the _drift_. It often rests on sound rocks. It contains
grains of sand, pebbles, and bowlders composed of many different
minerals and rocks that the country rock cannot furnish. Hence the
drift cannot have been formed by the decay of the rock of the region.
A shale or limestone, for example, cannot waste to a clay containing
granite pebbles. The origin of the drift will be explained in
subsequent chapters.

The differences in rocks are due more to their soluble than to their
insoluble constituents. The latter are few in number and are much the
same in rocks of widely different nature, being chiefly quartz,
silicate of alumina, and iron oxide. By the removal of their soluble
parts very many and widely different rocks rot down to a residual clay
gritty with particles of quartz and colored red or yellow with iron
oxide.

In a broad way the changes which rocks undergo in weathering are an
adaptation to the environment in which they find themselves at the
earth's surface,--an environment different from that in which they
were formed under sea or under ground. In open air, where they are
attacked by various destructive agents, few of the rock-making
minerals are stable compounds except quartz, the iron oxides, and the
silicate of alumina; and so it is to one or more of these
comparatively insoluble substances that most rocks are reduced by long
decay.

Which produces a mantle of finer waste, frost or chemical decay? which
a thicker mantle? In what respects would you expect that the mantle of
waste would differ in warm humid lands like India, in frozen countries
like Alaska, and in deserts such as the Sahara?

=The soil.= The same agencies which produce the mantle of waste are
continually at work upon it, breaking it up into finer and finer
particles and causing its more complete decay. Thus on the surface,
where the waste has weathered longest, it is gradually made fine
enough to support the growth of plants, and is then known as _soil_.
The coarser waste beneath is sometimes spoken of as subsoil. Soil
usually contains more or less dark, carbonaceous, decaying organic
matter, called humus, and is then often termed the _humus layer_. Soil
forms not only on waste produced in place from the rock beneath, but
also on materials which have been transported, such as sheets of
glacial drift and river deposits.

Until rocks are reduced to residual clays the work of the weather is
more rapid and effective on the fragments of the mantle of waste than
on the rocks from which waste is being formed. Why?

Any fresh excavation of cellar or cistern, or cut for road or railway,
will show the characteristics of the humus layer. It may form only a
gray film on the surface, or we may find it a layer a foot or more
thick, dark, or even black, above, and growing gradually lighter in
color as it passes by insensible gradations into the subsoil. In some
way the decaying vegetable matter continually forming on the surface
has become mingled with the material beneath it.

=How humus and the subsoil are mingled.= The mingling of humus and the
subsoil is brought about by several means. The roots of plants
penetrate the waste, and when they die leave their decaying substance
to fertilize it. Leaves and stems falling on the surface are turned
under by several agents. Earthworms and other animals whose home is in
the waste drag them into their burrows either for food or to line
their nests. Trees overthrown by the wind, roots and all, turn over
the soil and subsoil and mingle them together. Bacteria also work in
the waste and contribute to its enrichment. The animals living in the
mantle do much in other ways toward the making of soil. They bring the
coarser fragments from beneath to the surface, where the waste
weathers more rapidly. Their burrows allow air and water to penetrate
the waste more freely and to affect it to greater depths.

=Ants.= In the tropics the mantle of waste is worked over chiefly by
ants. They excavate underground galleries and chambers, extending
sometimes as much as fourteen feet below the surface, and build mounds
which may reach as high above it. In some parts of Paraguay and
southern Brazil these mounds, like gigantic potato hills, cover tracts
of considerable area.

In search for its food--the dead wood of trees--the so-called white
ant constructs runways of earth about the size of gas pipes, reaching
from the base of the tree to the topmost branches. On the plateaus of
central Africa explorers have walked for miles through forests every
tree of which was plastered with these galleries of mud. Each grain of
earth used in their construction is moistened and cemented by slime as
it is laid in place by the ant, and is thus acted on by organic
chemical agents. Sooner or later these galleries are beaten down by
heavy rains, and their fertilizing substances are scattered widely by
the winds.

=Earthworms.= In temperate regions the waste is worked over largely by
earthworms. In making their burrows worms swallow earth in order to
extract from it any nutritive organic matter which it may contain.
They treat it with their digestive acids, grind it in their stony
gizzards, and void it in castings on the surface of the ground. It was
estimated by Darwin that in many parts of England each year, on every
acre, more than ten tons of earth pass through the bodies of
earthworms and are brought to the surface, and that every few years
the entire soil layer is thus worked over by them.

In all these ways the waste is made fine and stirred and enriched.
Grain by grain the subsoil with its fresh mineral ingredients is
brought to the surface, and the rich organic matter which plants and
animals have taken from the atmosphere is plowed under. Thus Nature
plows and harrows on "the great world's farm" to make ready and ever
to renew a soil fit for the endless succession of her crops.

The world processes by which rocks are continually wasting away are
thus indispensable to the life of plants and animals. The organic
world is built on the ruins of the inorganic, and because the solid
rocks have been broken down into soil men are able to live upon the
earth.

=Solar energy.= The source of the energy which accomplishes all this
necessary work is the sun. It is the radiant energy of the sun which
causes the disintegration of rocks, which lifts vapor into the
atmosphere to fall as rain, which gives life to plants and animals.
Considering the earth in a broad way, we may view it as a globe of
solid rock,--_the lithosphere_,--surrounded by two mobile envelopes:
the envelope of air,--_the atmosphere_; and the envelope of
water,--_the hydrosphere_. Under the action of solar energy these
envelopes are in constant motion. Water from the hydrosphere is
continually rising in vapor into the atmosphere, the air of the
atmosphere penetrates the hydrosphere,--for its gases are dissolved in
all waters,--and both air and water enter and work upon the solid
earth. By their action upon the lithosphere they have produced a third
envelope,--the mantle of rock waste.

This envelope also is in movement, not indeed as a whole, but particle
by particle. The causes which set its particles in motion, and the
different forms which the mantle comes to assume, we will now proceed
to study.


Movements of the Mantle of Rock Waste

At the sandstone ledges which we first visited we saw not only that
the rocks were crumbling away, but also that grains and fragments of
them were creeping down the slopes of the valley to the stream and
were carried by it onward toward the sea. This process is going on
everywhere. Slowly it may be, and with many interruptions, but surely,
the waste of the land moves downward to the sea. We may divide its
course into two parts,--the path to the stream, which we will now
consider, and its carriage onward by the stream, which we will defer
to a later chapter.

=Gravity.= The chief agent concerned in the movement of waste is
gravity. Each particle of waste feels the unceasing downward pull of
the earth's mass and follows it when free to do so. All agencies which
produce waste tend to set its particles free and in motion, and
therefore coöperate with gravity. On cliffs, rocks fall when wedged
off by frost or by roots of trees, and when detached by any other
agency. On slopes of waste, water freezes in chinks between stones,
and in pores between particles of soil, and wedges them apart. Animals
and plants stir the waste, heat expands it, cold contracts it, the
strokes of the raindrops drive loose particles down the slope and the
wind lifts and lets them fall. Of all these movements, gravity assists
those which are downhill and retards those which are uphill. On the
whole, therefore, the downhill movements prevail, and the mantle of
waste, block by block and grain by grain, creeps along the downhill
path.

A slab of sandstone laid on another of the same kind at an angle of
17° and left in the open air was found to creep down the slope at the
rate of a little more than a millimeter a month. Explain why it did
so.

=Rain.= The most efficient agent in the carriage of waste to the
streams is the rain. It moves particles of soil by the force of the
blows of the falling drops, and washes them down all slopes to within
reach of permanent streams. On surfaces unprotected by vegetation, as
on plowed fields and in arid regions, the rain wears furrows and
gullies both in the mantle of waste and in exposures of unaltered rock
(Fig. 17).

At the foot of a hill we may find that the soil has accumulated by
creep and wash to the depth of several feet; while where the hillside
is steepest the soil may be exceedingly thin, or quite absent, because
removed about as fast as formed. Against the walls of an abbey built
on a slope in Wales seven hundred years ago, the creeping waste has
gathered on the uphill side to a depth of seven feet. The slow-flowing
sheet of waste is often dammed by fences and walls, whose uphill side
gathers waste in a few years so as to show a distinctly higher surface
than the downhill side, especially in plowed fields where the movement
is least checked by vegetation.

=Talus.= At the foot of cliffs there is usually to be found a slope of
rock fragments which clearly have fallen from above (Fig. 8). Such a
heap of waste is known as _talus_. The amount of talus in any place
depends both on the rate of its formation and the rate of its removal.
Talus forms rapidly in climates where mechanical disintegration is
most effective, where rocks are readily broken into blocks because
closely jointed and thinly bedded rather than massive, and where they
are firm enough to be detached in fragments of some size instead of in
fine grains. Talus is removed slowly where it decays slowly, either
because of the climate or the resistance of the rock. It may be
rapidly removed by a stream flowing along its base.

   [Illustration: Fig. 8. Talus at Foot of Granite Cliffs, Sierra
     Nevada Mountains]

In a moist climate a soluble rock, such as massive limestone, may form
talus little if any faster than the talus weathers away. A
loose-textured sandstone breaks down into incoherent sand grains,
which in dry climates, where unprotected by vegetation, may be blown
away as fast as they fall, leaving the cliff bare to the base. Cliffs
of such slow-decaying rocks as quartzite and granite when closely
jointed accumulate talus in large amounts.

   [Illustration: Fig. 9. Diagram Illustrating Retreat of Cliff,
     _c_, and Talus, _t_]

Talus slopes may be so steep as to reach _the angle of repose_, i.e.
the steepest angle at which the material will lie. This angle varies
with different materials, being greater with coarse and angular
fragments than with fine rounded grains. Sooner or later a talus
reaches that equilibrium where the amount removed from its surface
just equals that supplied from the cliff above. As the talus is
removed and weathers away its slope retreats together with the retreat
of the cliff, as seen in Figure 9.

=Graded slopes.= Where rocks weather faster than their waste is
carried away, the waste comes at last to cover all rocky ledges. On
the steeper slopes it is coarser and in more rapid movement than on
slopes more gentle, but mountain sides and hills and plains alike come
to be mantled with sheets of waste which everywhere is creeping toward
the streams. Such unbroken slopes, worn or built to the least
inclination at which the waste supplied by weathering can be urged
onward, are known as _graded slopes_.

Of far less importance than the silent, gradual creep of waste, which
is going on at all times everywhere about us, are the startling local
and spasmodic movements which we are now to describe.

=Avalanches.= On steep mountain sides the accumulated snows of winter
often slip and slide in avalanches to the valleys below. These rushing
torrents of snow sweep their tracks clean of waste and are one of
Nature's normal methods of moving it along the downhill path.

   [Illustration: Fig. 10. A Landslide, Quebec]

=Landslides.= Another common and abrupt method of delivering waste to
streams is by slips of the waste mantle in large masses. After long
rains and after winter frosts the cohesion between the waste and the
sound rock beneath is loosened by seeping water underground. The waste
slips on the rock surface thus lubricated and plunges down the
mountain side in a swift roaring torrent of mud and stones.

   [Illustration: Fig. 11. Diagram Illustrating Conditions favorable
     to a Landslide

   _lm_, limestone dipping toward valley of river, _r_; _sh_, shale]

We may conveniently mention here a second type of landslide, where
masses of solid rock as well as the mantle of waste are involved in
the sudden movement. Such slips occur when valleys have been rapidly
deepened by streams or glaciers and their sides have not yet been
graded. A favorable condition is where the strata dip (i.e. incline
downwards) towards the valley (Fig. 11), or are broken by joint planes
dipping in the same direction. The upper layers, including perhaps the
entire mountain side, have been cut across by the valley trench and
are left supported only on the inclined surface of the underlying
rocks. Water may percolate underground along this surface and loosen
the cohesion between the upper and the underlying strata by converting
the upper surface of a shale to soft wet clay, by dissolving layers of
a limestone, or by removing the cement of a sandstone and converting
it into loose sand. When the inclined surface is thus lubricated the
overlying masses may be launched into the valley below. The solid
rocks are broken and crushed in sliding and converted into waste
consisting, like that of talus, of angular unsorted fragments, blocks
of all sizes being mingled pell-mell with rock meal and dust. The
principal effects of landslides may be gathered from the following
examples.

At Gohna, India, in 1893, the face of a spur four thousand feet high,
of the lower ranges of the Himalayas, slipped into the gorge of the
headwaters of the Ganges River in successive rock falls which lasted
for three days. Blocks of stone were projected for a mile, and clouds
of limestone dust were spread over the surrounding country. The débris
formed a dam one thousand feet high, extending for two miles along the
valley. A lake gathered behind this barrier, gradually rising until it
overtopped it in a little less than a year. The upper portion of the
dam then broke, and a terrific rush of water swept down the valley in
a wave which, twenty miles away, rose one hundred and sixty feet in
height. A narrow lake is still held by the strong base of the dam.

In 1896, after forty days of incessant rain, a cliff of sandstone
slipped into the Yangtse River in China, reducing the width of the
channel to eighty yards and causing formidable rapids.

   [Illustration: Fig. 12. Bowlders of Weathering, Granite Quarry,
     Cape Ann, Massachusetts]

At Flims, in Switzerland, a prehistoric landslip flung a dam eighteen
hundred feet high across the headwaters of the Rhine. If spread evenly
over a surface of twenty-eight square miles, the material would cover
it to a depth of six hundred and sixty feet. The barrier is not yet
entirely cut away, and several lakes are held in shallow basins on its
hummocky surface.

A slide from the precipitous river front of the citadel hill of
Quebec, in 1889, dashed across Champlain Street, wrecking a number of
houses and causing the death of forty-five persons. The strata here
are composed of steeply dipping slate (Fig. 10).

In lofty mountain ranges there may not be a single valley without its
traces of landslides, so common there is this method of the movement
of waste, and of building to grade over-steepened slopes.


Rock Sculpture By Weathering

We are now to consider a few of the forms into which rock masses are
carved by the weather.

   [Illustration: Fig. 13. Differential Weathering on a Monument,
     Colorado]

=Bowlders of weathering.= In many quarries and outcrops we may see
that the blocks into which one or more of the uppermost layers have
been broken along their joints and bedding planes are no longer
angular, as are those of the layers below. The edges and corners of
these blocks have been worn away by the weather. Such rounded cores,
known as bowlders of weathering, are often left to strew the surface.

=Differential weathering.= This term covers all cases in which a rock
mass weathers differently in different portions. Any weaker spots or
layers are etched out on the surface, leaving the more resistant in
relief. Thus massive limestones become pitted where the weather drills
out the weaker portions. In these pits, when once they are formed,
moisture gathers, a little soil collects, vegetation takes root, and
thus they are further enlarged until the limestone may be deeply
honeycombed.

   [Illustration: Fig. 14. Honeycombed Limestone, Iowa]

   [Illustration: Fig. 15. Cliffs and Slopes on North Wall of the
     Grand Canyon of the Colorado River, Arizona]

On the sides of canyons, and elsewhere where the edges of strata are
exposed, the harder layers project as cliffs, while the softer weather
back to slopes covered with the talus of the harder layers above them.
It is convenient to call the former _cliff makers_ and the latter
_slope makers_ (Fig. 15).

Differential weathering plays a large part in the sculpture of the
land. Areas of weak rock are wasted to plains, while areas of hard
rock adjacent are still left as hills and mountain ridges, as in the
valleys and mountains of eastern Pennsylvania. But in such instances
the lowering of the surface of the weaker rock is also due to the wear
of streams, and especially to the removal by them from the land of the
waste which covers and protects the rocks beneath.

   [Illustration: Fig. 16. Taverlone Mesa, New Mexico]

Rocks owe their weakness to several different causes. Some, such as
beds of loose sand, are soft and easily worn by rains; some, as
limestone and gypsum for example, are soluble. Even hard insoluble
rocks are weak under the attack of the weather when they are closely
divided by joints and bedding planes and are thus readily broken up
into blocks by mechanical agencies.

   [Illustration: Fig. 17. Monuments, Arizona

    Note the rain furrows on the slope at the foot of the monuments.
    In the foreground are seen fragments of petrified trunks of trees,
    composed of silica and extremely resistant to the weather. On the
    removal of the rock layers in which these fragments were imbedded
    they are left to strew the surface in the same way as are the
    residual flints of southern Missouri.]

=Outliers and monuments.= As cliffs retreat under the attack of the
weather, portions are left behind where the rock is more resistant or
where the attack for any reason is less severe. Such remnant masses,
if large, are known as outliers. When flat-topped, because of the
protection of a resistant horizontal capping layer, they are termed
_mesas_ (Fig. 16),--a term applied also to the flat-topped portions of
dissected plateaus (Fig. 129). Retreating cliffs may fall back a
number of miles behind their outliers before the latter are finally
consumed.

   [Illustration: Fig. 18. Undercut Monuments, Colorado]

Monuments are smaller masses and may be but partially detached from
the cliff face. In the breaking down of sheets of horizontal strata,
outliers grow smaller and smaller and are reduced to massive
rectangular monuments resembling castles (Fig. 17). The rock castle
falls into ruin, leaving here and there an isolated tower; the tower
crumbles to a lonely pillar, soon to be overthrown. The various and
often picturesque shapes of monuments depend on the kind of rock, the
attitude of the strata, and the agent by which they are chiefly
carved. Thus pillars may have a capital formed of a resistant stratum.
Monuments may be undercut and come to rest on narrow pedestals,
wherever they weather more rapidly near the ground, either because of
the greater moisture there, or--in arid climates--because worn at
their base by drifting sands.

Stony clays disintegrating under the rain often contain bowlders
which protect the softer material beneath from the vertical blows
of raindrops, and thus come to stand on pedestals of some height
(Fig. 19). One may sometimes see on the ground beneath dripping eaves
pebbles left in the same way, protecting tiny pedestals of sand.

=Mountain peaks and ridges.= Most mountains have been carved out of
great broadly uplifted folds and blocks of the earth's crust. Running
water and glacier ice have cut these folds and blocks into masses
divided by deep valleys; but it is by the weather, for the most part,
that the masses thus separated have been sculptured to the present
forms of the individual peaks and ridges.

   [Illustration: Fig. 19. Roosevelt Column, Idaho

   An erosion pillar 70 feet high. How was it produced? Why
   quadrangular? What does it show as to the recent height of the
   hillside surface?]

Frost and heat and cold sculpture high mountains to sharp, tusklike
peaks and ragged, serrate crests, where their waste is readily removed
(Fig. 8).

The Matterhorn of the Alps is a famous example of a mountain peak
whose carving by the frost and other agents is in active progress. On
its face "scarcely a rock anywhere is firmly attached," and the fall
of loosened stones is incessant. Mountain climbers who have camped at
its base tell how huge rocks from time to time come leaping down its
precipices, followed by trains of dislodged smaller fragments and rock
dust; and how at night one may trace the course of the bowlders by the
sparks which they strike from the mountain walls. Mount Assiniboine,
Canada (Fig. 20), resembles the Matterhorn in form and has been carved
by the same agencies.

"The Needles" of Arizona are examples of sharp mountain peaks in a
warm arid region sculptured chiefly by temperature changes.

Chemical decay, especially when carried on beneath a cover of waste
and vegetation, favors the production of rounded knobs and dome-shaped
mountains.

=The weather curve.= We have seen that weathering reduces the angular
block quarried by the frost to a rounded bowlder by chipping off its
corners and smoothing away its edges. In much the same way weathering
at last reduces to rounded hills the earth blocks cut by streams or
formed in any other way. High mountains may at first be sculptured by
the weather to savage peaks (Fig. 181), but toward the end of their
life history they wear down to rounded hills (Fig. 182). The weather
curve, which may be seen on the summits of low hills (Fig. 21), is
convex upward.

   [Illustration: Fig. 20. Mount Assiniboine, Canada]

   [Illustration: Fig. 21. Big Round Top and Little Round Top,
     Gettysburg, Pennsylvania]

In Figure 22, representing a cubic block of stone whose faces are a
yard square, how many square feet of surface are exposed to the
weather by a cubic foot at a corner _a_; by one situated in the middle
of an edge _b_; by one in the center of a side _c_? How much faster
will _a_ and _b_ weather than _c_, and what will be the effect on the
shape of the block?

   [Illustration: Fig. 22]

=The coöperation of various agencies in rock sculpture.= For the sake
of clearness it is necessary to describe the work of each geological
agent separately. We must not forget, however, that in Nature no agent
works independently and alone; that every result is the outcome of a
long chain of causes. Thus, in order that the mountain peak may be
carved by the agents of disintegration, the waste must be rapidly
removed,--a work done by many agents, including some which we are yet
to study; and in order that the waste may be removed as fast as
formed, the region must first have been raised well above the level of
the sea, so that the agents of transportation could do their work
effectively. The sculpture of the rocks is accomplished only by the
coöperation of many forces.

The constant removal of waste from the surface by creep and wash and
carriage by streams is of the highest importance, because it allows
the destruction of the land by means of weathering to go on as long as
any land remains above sea level. If waste were not removed, it would
grow to be so thick as to protect the rock beneath from further
weathering, and the processes of destruction which we have studied
would be brought to an end. The very presence of the mantle of waste
over the land proves that on the whole rocks weather more rapidly than
their waste is removed. The destruction of the land is going on as
fast as the waste can be carried away.

We have now learned to see in the mantle of waste the record of the
destructive action of the agencies of weathering on the rocks of the
land surface. Similar records we shall find buried deeply among the
rocks of the crust in old soils and in rocks pitted and decayed,
telling of old land surfaces long wasted by the weather. Ever since
the dry land appeared these agencies have been as now quietly and
unceasingly at work upon it, and have ever been the chief means of the
destruction of its rocks. The vast bulk of the stratified rocks of the
earth's crust is made up almost wholly of the waste thus worn from
ancient lands.

   [Illustration: Fig. 23. Mount Sneffels, Colorado

   Describe and account for what you see in this view. What
   changes may the mountain be expected to undergo in the future
   from the agencies now at work upon it?]

In studying the various geological agencies we must remember the
almost inconceivable times in which they work. The slowest process
when multiplied by the immense time in which it is carried on produces
great results. The geologist looks upon the land forms of the earth's
surface as monuments which record the slow action of weathering and
other agents during the ages of the past. The mountain peak, the
rounded hill, the wide plain which lies where hills and mountains once
stood, tell clearly of the great results which slow processes will
reach when given long time in which to do their work. We should
accustom ourselves also to think of the results which weathering will
sooner or later bring to pass. The tombstone and the bowlder of the
field, which each year lose from their surfaces a few crystalline
grains, must in time be wholly destroyed. The hill whose rocks are
slowly rotting underneath a cover of waste must become lower and lower
as the centuries and millenniums come and go, and will finally
disappear. Even the mountains are crumbling away continually, and
therefore are but fleeting features of the landscape.




CHAPTER II

THE WORK OF GROUND WATER


=Land waters.= We have seen how large is the part that water plays at
and near the surface of the land in the processes of weathering and in
the slow movement of waste down all slopes to the stream ways. We now
take up the work of water as it descends beneath the ground,--a
corrosive agent still, and carrying in solution as its load the
invisible waste of rocks derived from their soluble parts.

Land waters have their immediate source in the rainfall. By the heat
of the sun water is evaporated from the reservoir of the ocean and
from moist surfaces everywhere. Mingled as vapor with the air, it is
carried by the winds over sea and land, and condensed it returns to
the earth as rain or snow. That part of the rainfall which descends on
the ocean does not concern us, but that which falls on the land
accomplishes, as it returns to the sea, the most important work of all
surface geological agencies.

The rainfall may be divided into three parts: the first _dries up_,
being discharged into the air by evaporation either directly from the
soil or through vegetation; the second _runs off_ over the surface to
flood the streams; the third _soaks in_ the ground and is henceforth
known as _ground_ or _underground water_.

=The descent of ground water.= Seeping through the mantle of waste,
ground water soaks into the pores and crevices of the underlying rock.
All rocks of the upper crust of the earth are more or less porous, and
all drink in water. _Impervious rocks_, such as granite, clay, and
shale, have pores so minute that the water which they take in is held
fast within them by capillary attraction, and none drains through.
_Pervious rocks_, on the other hand, such as many sandstones, have
pore spaces so large that water filters through them more or less
freely. Besides its seepage through the pores of pervious rocks, water
passes to lower levels through the joints and cracks by which all
rocks, near the surface are broken.

Even the closest-grained granite has a pore space of 1 in 400, while
sandstone may have a pore space of 1 in 4. Sand is so porous that it
may absorb a third of its volume of water, and a loose loam even as
much as one half.

   [Illustration: Fig. 24. Diagram Illustrating the Relation of the
      Ground-Water Surface to the Surface of the Ground

   The dotted line represents the ground-water surface, and the
   arrows indicate the direction of the movements of ground-water.
   _m_, marsh; _w_, well; _r_, river]

=The ground-water surface= is the name given the upper surface of
ground water, the level below which all rocks are saturated. In dry
seasons the ground-water surface sinks. For ground water is constantly
seeping downward under gravity, it is evaporated in the waste and its
moisture is carried upward by capillarity and the roots of plants to
the surface to be evaporated in the air. In wet seasons these constant
losses are more than made good by fresh supplies from that part of the
rainfall which soaks into the ground, and the ground-water surface
rises.

In moist climates the ground-water surface (Fig. 24) lies, as a rule,
within a few feet of the land surface and conforms to it in a general
way, although with slopes of less inclination than those of the hills
and valleys. In dry climates permanent ground water may be found only
at depths of hundreds of feet. Ground water is held at its height by
the fact that its circulation is constantly impeded by capillarity and
friction. If it were as free to drain away as are surface streams, it
would sink soon after a rain to the level of the deepest valleys of
the region.

=Wells and springs.= Excavations made in permeable rocks below the
ground-water surface fill to its level and are known as wells. Where
valleys cut this surface permanent streams are formed, the water
either oozing forth along ill-defined areas or issuing at definite
points called springs, where it is concentrated by the structure of
the rocks. A level tract where the ground-water surface coincides with
the surface of the ground is a swamp or marsh.

By studying a spring one may learn much of the ways and work of ground
water. Spring water differs from that of the stream into which it
flows in several respects. If we test the spring with a thermometer
during successive months, we shall find that its temperature remains
much the same the year round. In summer it is markedly cooler than the
stream; in winter it is warmer and remains unfrozen while the latter
perhaps is locked in ice. This means that its underground path must
lie at such a distance from the surface that it is little affected by
summer's heat and winter's cold.

While the stream is often turbid with surface waste washed into it by
rains, the spring remains clear; its water has been filtered during
its slow movement through many small underground passages and the
pores of rocks. Commonly the spring differs from the stream in that it
carries a far larger load of dissolved rock. Chemical analysis proves
that streams contain various minerals in solution, but these are
usually in quantities so small that they are not perceptible to the
taste or feel. But the water of springs is often well charged with
soluble minerals; in its slow, long journey underground it has
searched out the soluble parts of the rocks through which it seeps and
has dissolved as much of them as it could. When spring water is boiled
away, the invisible load which it has carried is left behind, and in
composition is found to be practically identical with that of the
soluble ingredients of the country rock. Although to some extent the
soluble waste of rocks is washed down surface slopes by the rain, by
far the larger part is carried downward by ground water and is
delivered to streams by springs.

In limestone regions springs are charged with calcium carbonate (the
carbonate of lime), and where the limestone is magnesian they contain
magnesium carbonate also. Such waters are "hard"; when used in
washing, the minerals which they contain combine with the fatty acids
of soap to form insoluble curdy compounds. When springs rise from
rocks containing gypsum they are hard with calcium sulphate. In
granite regions they contain more or less soda and potash from the
decay of feldspar.

The flow of springs varies much less during the different seasons of
the year than does that of surface streams. So slow is the movement of
ground water through the rocks that even during long droughts large
amounts remain stored above the levels of surface drainage.

=Movements of ground water.= Ground water is in constant movement
toward its outlets. Its rate varies according to many conditions, but
always is extremely slow. Even through loose sands beneath the beds of
rivers it sometimes does not exceed a fifth of a mile a year.

   [Illustration: Fig. 26. Geological Conditions favorable to
      Strong Springs

   _a_, limestone; _b_, shale; _c_, coarse sandstone; _d_,
   limestone; _e_, sandstone; _f_, fissure. The strata dip toward
   the South, _S_. Redraw the diagram, marking the points at which
   strong springs (_ss_) may be expected.]

In any region two zones of flow may be distinguished. The _upper zone
of flow_ extends from the ground-water surface downward through the
waste mantle and any permeable rocks on which the mantle rests, as far
as the first impermeable layer, where the descending movement of the
water is stopped. The =deep zones of flow= occupy any pervious rocks
which may be found below the impervious layer which lies nearest to
the surface. The upper zone is a vast sheet of water saturating the
soil and rocks and slowly seeping downward through their pores and
interstices along the slopes to the valleys, where in part it
discharges in springs and often unites also in a wide underflowing
stream which supports and feeds the river (Fig. 24).

   [Illustration: Fig. 27. Diagram of Well which goes dry in
      Drought, _a_, and of of Unfailing Well, _b_

   Redraw the diagram, showing by dotted line the normal
   ground-water surface and by broken line the ground-water
   surface at times of drought]

   [Illustration: Fig. 28. Diagram of Wet Weather Stream, _a_, and
      of Permanent Stream, _b_

   Redraw the diagram, showing ground-water surface by dotted line]

A city in a region of copious rains, built on the narrow flood plain
of a river, overlooked by hills, depends for its water supply on
driven wells, within the city limits, sunk in the sand a few yards
from the edge of the stream. Are these wells fed by water from the
river percolating through the sand, or by ground water on its way to
the stream and possibly contaminated with the sewage of the town?

At what height does underground water stand in the wells of your
region? Does it vary with the season? Have you ever known wells to go
dry? It may be possible to get data from different wells and to draw a
diagram showing the ground-water surface as compared with the surface
of the ground.

=Fissure springs and artesian wells.= The _deeper zones of flow_ lie
in pervious strata which are overlain by some impervious stratum. Such
layers are often carried by their dip to great depths, and water may
circulate in them to far below the level of the surface streams and
even of the sea. When a fissure crosses a water-bearing stratum, or
_aquifer, water is forced upward by the pressure of the weight of
the water contained in the higher parts of the stratum, and may reach
the surface as a fissure spring. A boring which taps such an aquifer
is known as an artesian well, a name derived from a province in France
where wells of this kind have been long in use. The rise of the water
in artesian wells, and in fissure springs also, depends on the
following conditions illustrated in Figure 29. The aquifer dips toward
the region of the wells from higher ground, where it outcrops and
receives its water. It is inclosed between an impervious layer above
and water-tight or water-logged layers beneath. The weight of the
column of water thus inclosed in the aquifer causes water to rise in
the well, precisely as the weight of the water in a standpipe forces
it in connected pipes to the upper stories of buildings.

   [Illustration: Fig. 29. Section across South Dakota from the
      Black Hills to Sioux Falls (S), Illustrating the Conditions
      of Artesian Wells

   _a_, crystalline impervious rocks; _b_, sedimentary rocks,
   shales, limestones, and sandstones; _c_, pervious sandstone,
   the aquifer; _d_, impervious shales; _w_, _w_, _w_, artesian wells.]

Which will supply the larger region with artesian wells, an aquifer
whose dip is steep or one whose dip is gentle? Which of the two
aquifers, their thickness being equal, will have the larger outcrop
and therefore be able to draw upon the larger amount of water from the
rainfall? Illustrate with diagrams.

=The zone of solution.= Near the surface, where the circulation of
ground water is most active, it oxidizes, corrodes, and dissolves the
rocks through which it passes. It leaches soils and subsoils of their
lime and other soluble minerals upon which plants depend for their
food. It takes away the soluble cements of rocks; it widens fissures
and joints and opens winding passages along the bedding planes; it may
even remove whole beds of soluble rocks, such as rock salt, limestone,
or gypsum. The work of ground water in producing landslides has
already been noticed. The zone in which the work of ground water is
thus for the most part destructive we may call the zone of solution.

   [Illustration: Fig. 30. Diagram of Caverns and Sink Holes]

=Caves.= In massive limestone rocks, ground water dissolves channels
which sometimes form large caves (Fig. 30). The necessary conditions
for the excavation of caves of great size are well shown in central
Kentucky, where an upland is built throughout of thick horizontal beds
of limestone. The absence of layers of insoluble or impervious rock in
its structure allows a free circulation of ground water within it by
the way of all natural openings in the rock. These water ways have
been gradually enlarged by solution and wear until the upland is
honeycombed with caves. Five hundred open caverns are known in one
county.

Mammoth Cave, the largest of these caverns, consists of a labyrinth of
chambers and winding galleries whose total length is said to be as
much as thirty miles. One passage four miles long has an average width
of about sixty feet and an average height of forty feet. One of the
great halls is three hundred feet in width and is overhung by a solid
arch of limestone one hundred feet above the floor. Galleries at
different levels are connected by well-like pits, some of which
measure two hundred and twenty-five feet from top to bottom. Through
some of the lowest of these tunnels flows Echo River, still at work
dissolving and wearing away the rock while on its dark way to appear
at the surface as a great spring.

=Natural bridges.= As a cavern enlarges and the surface of the land
above it is lowered by weathering, the roof at last breaks down and
the cave becomes an open ravine. A portion of the roof may for a while
remain, forming a "natural bridge."

=Sink holes.= In limestone regions channels under ground may become so
well developed that the water of rains rapidly drains away through
them. Ground water stands low and wells must be sunk deep to find it.
Little or no surface water is left to form brooks.

   [Illustration: Fig. 31. Sink Holes in the Karst, Austria]

Thus across the limestone upland of central Kentucky one meets but
three surface streams in a hundred miles. Between their valleys
surface water finds its way underground by means of sink holes. These
are pits, commonly funnel shaped, formed by the enlargement of crevice
or joint by percolating water, or by the breakdown of some portion of
the roof of a cave. By clogging of the outlet a sink hole may come to
be filled by a pond.

Central Florida is a limestone region with its drainage largely
subterranean and in part below the level even of the sea. Sink holes
are common, and many of them are occupied by lakelets. Great springs
mark the point of issue of underground streams, while some rise from
beneath the sea. Silver Spring, one of the largest, discharges from a
basin eight hundred feet wide and thirty feet deep a little river
navigable for small steamers to its source. About the spring there are
no surface streams for sixty miles.

   [Illustration: Fig. 32. Underground Stream Issuing from Base of
      Cliff, the Karst, Austria]

=The Karst.= Along the eastern coast of the Adriatic, as far south as
Montenegro, lies a belt of limestone mountains singularly worn and
honeycombed by the solvent action of water. Where forests have been
cut from the mountain sides and the red soil has washed away, the
surface of the white limestone forms a pathless desert of rock where
each square rod has been corroded into an intricate branch work of
shallow furrows and sharp ridges. Great sink holes, some of them six
hundred feet deep and more, pockmark the surface of the land. The
drainage is chiefly subterranean. Surface streams are rare and a
portion of their courses is often under ground. Fragmentary valleys
come suddenly to an end at walls of rock where the rivers which occupy
the valleys plunge into dark tunnels to reappear some miles away.
Ground water stands so far below the surface that it cannot be reached
by wells, and the inhabitants depend on rain water stored for
household uses. The finest cavern of Europe, the Adelsberg Grotto, is
in this region. Karst, the name of a part of this country, is now used
to designate any region or landscape thus sculptured by the chemical
action of surface and ground water. We must remember that Karst
regions are rare, and striking as is the work of their subterranean
streams, it is far less important than the work done by the sheets of
underground water slowly seeping through all subsoils and porous rocks
in other regions.

Even when gathered into definite channels, ground water does not have
the erosive power of surface streams, since it carries with it little
or no rock waste. Regions whose underground drainage is so perfect
that the development of surface streams has been retarded or prevented
escape to a large extent the leveling action of surface running
waters, and may therefore stand higher than the surrounding country.
The hill honeycombed by Luray Cavern, Virginia, has been attributed to
this cause.

   [Illustration: Fig. 33. Stalactites and Stalagmites, Marengo
      Cavern, Indiana]

=Cavern deposits.= Even in the zone of solution water may under
certain circumstances deposit as well as erode. As it trickles from
the roof of caverns, the lime carbonate which it has taken into
solution from the layers of limestone above is deposited by
evaporation in the air in icicle-like pendants called _stalactites_.
As the drops splash on the floor there are built up in the same way
thicker masses called _stalagmites_, which may grow to join the
stalactites above, forming pillars. A stalagmitic crust often seals
with rock the earth which accumulates in caverns, together with
whatever relics of cave dwellers, either animals or men, it may
contain.

Can you explain why slender stalactites formed by the drip of single
drops are often hollow pipes?

=The zone of cementation.= With increasing depth subterranean water
becomes more and more sluggish in its movements and more and more
highly charged with minerals dissolved from the rocks above. At such
depths it deposits these minerals in the pores of rocks, cementing
their grains together, and in crevices and fissures, forming mineral
veins. Thus below the zone of solution where the work of water is to
dissolve, lies the zone of cementation where its work is chemical
deposit. A part of the invisible load of waste is thus transferred
from rocks near the surface to those at greater depths.

As the land surface is gradually lowered by weathering and the work of
rain and streams, rocks which have lain deep within the zone of
cementation are brought within the zone of solution. Thus there are
exposed to view limestones, whose cracks were filled with calcite
(crystallized carbonate of lime), with quartz or other minerals, and
sandstones whose grains were well cemented many feet below the
surface.

=Cavity filling.= Small cavities in the rocks are often found more or
less completely filled with minerals deposited from solution by water
in its constant circulation underground. The process may be
illustrated by the deposit of salt crystals in a cup of evaporating
brine, but in the latter instance the solution is not renewed as in
the case of cavities in the rocks. A cavity thus lined with
inward-pointing crystals is called a _geode_.

=Concretions.= Ground water seeping through the pores of rocks may
gather minerals disseminated throughout them into nodular masses
called concretions. Thus silica disseminated through limestone is
gathered into nodules of flint. While geodes grow from the outside
inwards, concretions grow outwards from the center. Nor are they
formed in already existing cavities as are geodes. In soft clays
concretions may, as they grow, press the clay aside. In many other
rocks concretions are made by the process of _replacement_. Molecule
by molecule the rock is removed and the mineral of the concretion
substituted in its place. The concretion may in this way preserve
intact the lamination lines or other structures of the rock (Fig. 34).
Clays and shales often contain concretions of lime carbonate, of iron
carbonate, or of iron sulphide. Some fossil, such as a leaf or shell,
frequently forms the nucleus around which the concretion grows.

Why are building stones more easily worked when "green" than after
their quarry water has dried out?

   [Illustration: Fig. 34. Concretions in Sandstone, Wyoming]

=Deposits of ground water in arid regions.= In arid lands where ground
water is drawn by capillarity to the surface and there evaporates, it
leaves as surface incrustations the minerals held in solution. White
limy incrustations of this nature cover considerable tracts in
northern Mexico. Evaporating beneath the surface, ground water may
deposit a limy cement in beds of loose sand and gravel. Such firmly
cemented layers are not uncommon in western Kansas and Nebraska, where
they are known as "mortar beds."

=Thermal springs.= While the lower limit of surface drainage is sea
level, subterranean water circulates much below that depth, and is
brought again to the surface by hydrostatic pressure. In many
instances springs have a higher temperature than the average annual
temperature of the region, and are then known as thermal springs. In
regions of present or recent volcanic activity, such as the
Yellowstone National Park, we may believe that the heat of thermal
springs is derived from uncooled lavas, perhaps not far below the
surface. But when hot springs occur at a distance of hundreds of miles
from any volcano, as in the case of the hot springs of Bath, England,
it is probable that their waters have risen from the heated rocks
of the earth's interior. The springs of Bath have a temperature of
120° F., 70° above the average annual temperature of the place. If
we assume that the rate of increase in the earth's internal heat is
here the average rate, 1° F. to every sixty feet of descent, we may
conclude that the springs of Bath rise from at least a depth of
forty-two hundred feet.

Water may descend to depths from which it can never be brought back by
hydrostatic pressure. It is absorbed by highly heated rocks deep below
the surface. From time to time some of this deep-seated water may be
returned to open air in the steam of volcanic eruptions.

   [Illustration: Fig. 35. Calcareous Deposits from Hot Springs,
      Yellowstone National Park]

=Surface deposits of springs.= Where subterranean water returns to the
surface highly charged with minerals in solution, on exposure to the
air it is commonly compelled to lay down much of its invisible load in
chemical deposits about the spring. These are thrown down from
solution either because of cooling, evaporation, the loss of carbon
dioxide, or the work of algae.

Many springs have been charged under pressure with carbon dioxide from
subterranean sources and are able therefore to take up large
quantities of lime carbonate from the limestone rocks through which
they pass. On reaching the surface the pressure is relieved, the gas
escapes, and the lime carbonate is thrown down in deposits called
_travertine_. The gas is sometimes withdrawn and the deposit produced
in large part by the action of algae and other humble forms of plant
life.

At the Mammoth Hot Springs in the valley of the Gardiner River,
Yellowstone National Park, beautiful terraces and basins of travertine
(Fig. 35) are now building, chiefly by means of algae which cover the
bottoms, rims, and sides of the basins and deposit lime carbonate upon
them in successive sheets. The rock, snow-white where dry, is coated
with red and orange gelatinous mats where the algae thrive in the
over-flowing waters.

Similar terraces of travertine are found to a height of fourteen
hundred feet up the valley side. We may infer that the springs which
formed these ancient deposits discharged near what was then the bottom
of the valley, and that as the valley has been deepened by the river
the ground water of the region has found lower and lower points of
issue.

In many parts of the country calcareous springs occur which coat with
lime carbonate mosses, twigs, and other objects over which their
waters flow. Such are popularly known as petrifying springs, although
they merely incrust the objects and do not convert them into stone.

Silica is soluble in alkaline waters, especially when these are hot.
Hot springs rising through alkaline siliceous rocks, such as lavas,
often deposit silica in a white spongy formation known as _siliceous
sinter_, both by evaporation and by the action of algae which secrete
silica from the waters. It is in this way that the cones and mounds of
the geysers in the Yellowstone National Park and in Iceland have been
formed (Fig. 234).

Where water oozes from the earth one may sometimes see a rusty deposit
on the ground, and perhaps an iridescent scum upon the water. The scum
is often mistaken for oil, but at a touch it cracks and breaks, as oil
would not do. It is a film of hydrated iron oxide, or _limonite_, and
the spring is an iron, or chalybeate, spring. Compounds of iron have
been taken into solution by ground water from soil and rocks, and are
now changed to the insoluble oxide on exposure to the oxygen of the
air.

In wet ground iron compounds leached by ground water from the soil
often collect in reddish deposits a few feet below the surface, where
their downward progress is arrested by some impervious clay. At the
bottom of bogs and shallow lakes iron ores sometimes accumulate to a
depth of several feet.

Decaying organic matter plays a large part in these changes. In its
presence the insoluble iron oxides which give color to most red and
yellow rocks are decomposed, leaving the rocks of a gray or bluish
color, and the soluble iron compounds which result are readily leached
out,--effects seen where red or yellow clays have been bleached about
some decaying tree root.

The iron thus dissolved is laid down as limonite when oxidized, as
about a chalybeate spring; but out of contact with the air and in the
presence of carbon dioxide supplied by decaying vegetation, as in a
peat bog, it may be deposited as iron carbonate, or _siderite_.

=Total amount of underground waters.= In order to realize the vast work
in solution and cementation which underground waters are now doing and
have done in all geological ages, we must gain some conception of their
amount. At a certain depth, estimated at about six miles, the weight of
the crust becomes greater than the rocks can bear, and all cavities and
pores in them must be completely closed by the enormous pressure which
they sustain. Below a depth, therefore, water cannot go. Above it all
rocks are water-soaked, up to the limit of their capacity, to within a
few feet of the surface. Estimating the average pore space of the rocks
above a depth of six miles at from two and a half per cent to five per
cent of their volume, it is found that the total amount of ground water
may be great enough to cover the entire surface of the earth to a depth
of from eight hundred to sixteen hundred feet.




CHAPTER III

RIVERS AND VALLEYS


=The run-off.= We have traced the history of that portion of the
rainfall which soaks into the ground; let us now return to that part
which washes along the surface and is known as the _run-off_. Fed by
rains and melting snows, the run-off gathers into courses, perhaps but
faintly marked at first, which join more definite and deeply cut
channels, as twigs their stems. In a humid climate the larger ravines
through which the run-off flows soon descend below the ground-water
surface. Here springs discharge along the sides of the little valleys
and permanent streams begin. The water supplied by the run-off here
joins that part of the rainfall which had soaked into the soil, and
both now proceed together by way of the stream to the sea.

=River floods.= Streams vary greatly in volume during the year. At
stages of flood they fill their immediate banks, or overrun them and
inundate any low lands adjacent to the channel; at stages of low water
they diminish to but a fraction of their volume when at flood.

At times of flood, rivers are fed chiefly by the run-off; at times of
low water, largely or even wholly by springs.

How, then, will the water of streams differ at these times in
turbidity and in the relative amount of solids carried in solution?

In parts of England streams have been known to continue flowing after
eighteen months of local drought, so great is the volume of water
which in humid climates is stored in the rocks above the drainage
level, and so slowly is it given off in springs.

In Illinois and the states adjacent, rivers remain low in winter and a
"spring freshet" follows the melting of the winter's snows. A "June
rise" is produced by the heavy rains of early summer. Low water
follows in July and August, and streams are again swollen to a
moderate degree under the rains of autumn.

=The discharge of streams.= The per cent of rainfall discharged by
rivers varies with the amount of rainfall, the slope of the drainage
area, the texture of the rocks, and other factors. With an annual
rainfall of fifty inches in an open country, about fifty per cent is
discharged; while with a rainfall of twenty inches only fifteen per
cent is discharged, part of the remainder being evaporated and part
passing underground beyond the drainage area. Thus the Ohio discharges
thirty per cent of the rainfall of its basin, while the Missouri
carries away but fifteen per cent. A number of the streams of the
semi-arid lands of the West do not discharge more than five per cent
of the rainfall.

Other things being equal, which will afford the larger proportion of
run-off, a region underlain with granite rock or with coarse
sandstone? grass land or forest? steep slopes or level land? a
well-drained region or one abounding in marshes and ponds? frozen or
unfrozen ground? Will there be a larger proportion of run-off after
long rains or after a season of drought? after long and gentle rains,
or after the same amount of precipitation in a violent rain? during
the months of growing vegetation, from June to August, or during the
autumn months?

   [Illustration: Fig. 36. Rise of Ground-Water Surface (broken
      line) beneath Valley (_V_) in Arid Region]

=Desert streams.= In arid regions the ground-water surface lies so low
that for the most part stream ways do not intersect it. Streams
therefore are not fed by springs, but instead lose volume as their
waters soak into the thirsty rocks over which they flow. They
contribute to the ground water of the region instead of being
increased by it. Being supplied chiefly by the run-off, they wither at
times of drought to a mere trickle of water, to a chain of pools, or
go wholly dry, while at long intervals rains fill their dusty beds
with sudden raging torrents. Desert rivers therefore periodically
shorten and lengthen their courses, withering back at times of drought
for scores of miles, or even for a hundred miles from the point
reached by their waters during seasons of rain.

=The geological work of streams.= The work of streams is of three
kinds,--transportation, erosion, and deposition. Streams _transport_
the waste of the land; they wear, or _erode_, their channels both on
bed and banks; and they _deposit_ portions of their load from time to
time along their courses, finally laying it down in the sea. Most of
the work of streams is done at times of flood.


Transportation

=The invisible load of streams.= Of the waste which a river transports
we may consider first the invisible load which it carries in solution,
supplied chiefly by springs but also in part by the run-off and from
the solution of the rocks of its bed. More than half the dissolved
solids in the water of the average river consists of the carbonates of
lime and magnesia; other substances are gypsum, sodium sulphate
(Glauber's salts), magnesium sulphate (Epsom salts), sodium chloride
(common salt), and even silica, the least soluble of the common
rock-making minerals. The amount of this invisible load is
surprisingly large. The Mississippi, for example, transports each year
113,000,000 tons of dissolved rock to the Gulf.

=The visible load of streams.= This consists of the silt which the
stream carries in suspension, and the sand and gravel and larger
stones which it pushes along its bed. Especially in times of flood one
may note the muddy water, its silt being kept from settling by the
rolling, eddying currents; and often by placing his ear close to the
bottom of a boat one may hear the clatter of pebbles as they are
hurried along. In mountain torrents the rumble of bowlders as they
clash together may be heard some distance away. The amount of the load
which a stream can transport depends on its velocity. A current of two
thirds of a mile per hour can move fine sand, while one of four miles
per hour sweeps along pebbles as large as hen's eggs. The transporting
power of a stream varies as the sixth power of its velocity. If its
velocity is multiplied by two, its transporting power is multiplied by
the sixth power of two: it can now move stones sixty-four times as
large as it could before.

Stones weigh from two to three times as much as water, and in water
lose the weight of the volume of water which they displace. What
proportion, then, of their weight in air do stones lose when
submerged?

=Measurement of stream loads.= To obtain the total amount of waste
transported by a river is an important but difficult matter. The
amount of water discharged must first be found by multiplying the
number of square feet in the average cross section of the stream by
its velocity per second, giving the discharge per second in cubic
feet. The amount of silt to a cubic foot of water is found by
filtering samples of the water taken from different parts of the
stream and at different times in the year, and drying and weighing the
residues. The average amount of silt to the cubic foot of water,
multiplied by the number of cubic feet of water discharged per year,
gives the total load carried in suspension during that time. Adding to
this the estimated amount of sand and gravel rolled along the bed,
which in many swift rivers greatly exceeds the lighter material held
in suspension, and adding also the total amount of dissolved solids,
we reach the exceedingly important result of the total load of waste
discharged by the river. Dividing the volume of this load by the area
of the river basin gives another result of the greatest geological
interest,--the rate at which the region is being lowered by the
combined action of weathering and erosion, or the rate of denudation.

=The rate of denudation of river basins.= This rate varies widely. The
Mississippi basin may be taken as a representative land surface
because of the varieties of surface, altitude and slope, climate, and
underlying rocks which are included in its great extent. Careful
measurements show that the Mississippi basin is now being lowered at a
rate of one four-thousandth of a foot a year, or one foot in four
thousand years. Taking this as the average rate of denudation for the
land surfaces of the globe, estimates have been made of the length of
time required at this rate to wash and wear the continents to the
level of the sea. As the average elevation of the lands of the globe
is reckoned at 2411 feet, this result would occur in nine or ten
million years, if the present rate of denudation should remain
unchanged. But even if no movements of the earth's crust should lift
or depress the continents, the rate of wear and the removal of waste
from their surfaces will not remain the same. It must constantly
decrease as the lands are worn nearer to sea level and their slopes
become more gentle. The length of time required to wear them away is
therefore far in excess of that just stated.

The drainage area of the Potomac is 11,000 square miles. The silt
brought down in suspension in a year would cover a square mile to the
depth of four feet. At what rate is the Potomac basin being lowered
from this cause alone?

It is estimated that the Upper Ganges is lowering its basin at the
rate of one foot in 823 years, and the Po one foot in 720 years. Why
so much faster than the Potomac and the Mississippi?

=How streams get their loads.= The load of streams is derived from a
number of sources, the larger part being supplied by the weathering of
valley slopes. We have noticed how the mantle of waste creeps and
washes to the stream ways. Watching the run-off during a rain, as it
hurries muddy with waste along the gutter or washes down the hillside,
we may see the beginning of the route by which the larger part of
their load is delivered to rivers. Streams also secure some of their
load by wearing it from their beds and banks,--a process called
erosion.


Erosion

Streams erode their beds chiefly by means of their bottom load,--the
stones of various sizes and the sand and even the fine mud which they
sweep along. With these tools they smooth, grind, and rasp the rock of
their beds, using them in much the fashion of sandpaper or a file.

   [Illustration: Fig. 37. Pothole in Bed of Stream, Ireland]

=Weathering of river beds.= The erosion of stream beds is greatly
helped by the work of the weather. Especially at low water more or
less of the bed is exposed to the action of frost and heat and cold,
joints are opened, rocks are pried loose and broken up and made ready
to be swept away by the stream at time of flood.

=Potholes.= In rapids streams also drill out their rocky beds. Where
some slight depression gives rise to an eddy, the pebbles which gather
in it are whirled round and round, and, acting like the bit of an
auger, bore out a cylindrical pit called a pothole. Potholes sometimes
reach a depth of a score of feet. Where they are numerous they aid
materially in deepening the channel, as the walls between them are
worn away and they coalesce.

=Waterfalls.= One of the most effective means of erosion which the
river possesses is the waterfall. The plunging water dislodges stones
from the face of the ledge over which it pours, and often undermines
it by excavating a deep pit at its base. Slice after slice is thus
thrown down from the front of the cliff, and the cataract cuts its way
upstream leaving a gorge behind it.

   [Illustration: Fig. 38. Map of the Gorge of the Niagara River]

=Niagara Falls.= The Niagara River flows from Lake Erie at Buffalo in
a broad channel which it has cut but a few feet below the level of the
region. Some thirteen miles from the outlet it plunges over a ledge
one hundred and seventy feet high into the head of a narrow gorge
which extends for seven miles to the escarpment of the upland in which
the gorge is cut. The strata which compose the upland dip gently
upstream and consist at top of a massive limestone, at the Falls about
eighty feet thick, and below of soft and easily weathered shale.
Beneath the Falls the underlying shale is cut and washed away by the
descending water and retreats also because of weathering, while the
overhanging limestone breaks down in huge blocks from time to time.

Niagara is divided by Goat Island into the Horseshoe Falls and the
American Falls. The former is supplied by the main current of the
river, and from the semicircular sweep of its rim a sheet of water in
places at least fifteen or twenty feet deep plunges into a pool a
little less than two hundred feet in depth. Here the force of the
falling water is sufficient to move about the fallen blocks of
limestone and use them in the excavation of the shale of the bed. At
the American Falls the lesser branch of the river, which flows along
the American side of Goat Island, pours over the side of the gorge and
breaks upon a high talus of limestone blocks which its smaller volume
of water is unable to grind to pieces and remove.

A series of surveys have determined that from 1842 to 1890 the
Horseshoe Falls retreated at the rate of 2.18 feet per year, while the
American Falls retreated at the rate of 0.64 feet in the same period.
We cannot doubt that the same agency which is now lengthening the
gorge at this rapid rate has cut it back its entire length of seven
miles.

While Niagara Falls have been cutting back a gorge seven miles long
and from two hundred to three hundred feet deep, the river above the
Falls has eroded its bed scarcely below the level of the upland on
which it flows. Like all streams which are the outlets of lakes, the
Niagara flows out of Lake Erie clear of sediment, as from a settling
basin, and carries no tools with which to abrade its bed. We may infer
from this instance how slight is the erosive power of clear water on
hard rock.

   [Illustration: Fig. 39. Longitudinal Section of Niagara Gorge

   Black, water; _F_, falls; _R_, rapids; _W_, whirlpool;
   _E_, escarpment; _N_, north; _S_, south]

Assuming that the rate of recession of the combined volumes of the
American and Horseshoe Falls was three feet a year below Goat Island,
and _assuming that this rate has been uniform in the past_, how long
is it since the Niagara River fell over the edge of the escarpment
where now is the mouth of the present gorge?

The profile of the bed of the Niagara along the gorge (Fig. 39) shows
alternating deeps and shallows which cannot be accounted for, except
in a single instance, by the relative hardness of the rocks of the
river bed. The deeps do not exceed that at the foot of the Horseshoe
Falls at the present time. When the gorge was being cut along the
shallows, how did the Falls compare in excavating power, in force, and
volume with the Niagara of to-day? How did the rate of recession at
those times compare with the present rate? Is the assumption made
above that the rate of recession has been uniform correct?

The first stretch of shallows below the Falls causes a tumultuous
rapid impossible to sound. Its depth has been estimated at thirty-five
feet. From what data could such an estimate be made?

Suggest a reason why the Horseshoe Falls are convex upstream.

At the present rate of recession which will reach the head of Goat
Island the sooner, the American or the Horseshoe Falls? What will be
the fate of the Falls left behind when the other has passed beyond the
head of the island?

The rate at which a stream erodes its bed depends in part upon the
nature of the rocks over which it flows. Will a stream deepen its
channel more rapidly on massive or on thin-bedded and close-jointed
rocks? on horizontal strata or on strata steeply inclined?

   [Illustration: Fig. 40. A Stream in Scotland

   In what ways is the bed now being deepened?]


Deposition

While the river carries its invisible load of dissolved rock on
without stop to the sea, its load of visible waste is subject to many
delays en route. Now and again it is laid aside, to be picked up later
and carried some distance farther on its way. One of the most striking
features of the river therefore is the waste accumulated along its
course, in bars and islands in the channel, beneath its bed, and in
flood plains along its banks. All this _alluvium_, to use a general
term for river deposits, with which the valley is cumbered is really
en route to the sea; it is only temporarily laid aside to resume its
journey later on. Constantly the river is destroying and rebuilding
its alluvial deposits, here cutting and there depositing along its
banks, here eroding and there building a bar, here excavating its bed
and there filling it up, and at all times carrying the material picked
up at one point some distance on downstream before depositing it at
another.

   [Illustration: Fig. 41. Sand Bar deposited by Stream, showing
      Cross Bedding]

These deposits are laid down by slackening currents where the velocity
of the stream is checked, as on the inner side of curves, and where
the slope of the bed is diminished, and in the lee of islands, bridge
piers and projecting points of land. How slight is the check required
to cause a current to drop a large part of its load may be inferred
from the law of the relation of the transporting power to the
velocity. If the velocity is decreased one half, the current can move
fragments but one sixty-fourth the size of those which it could move
before, and must drop all those of larger size.

Will a river deposit more at low water or at flood? when rising or
when falling?

=Stratification.= River deposits are stratified, as may be seen in any
fresh cut in banks or bars. The waste of which they are built has been
sorted and deposited in layers, one above another; some of finer and
some of coarser material. The sorting action of running water depends
on the fact that its transporting power varies with the velocity. A
current whose diminishing velocity compels it to drop coarse gravel,
for example, is still able to move all the finer waste of its load,
and separating it from the gravel, carries it on downstream; while at
a later time slower currents may deposit on the gravel bed layers of
sand, and, still later, slack water may leave on these a layer of mud.
In case of materials lighter than water the transporting power does
not depend on the velocity, and logs of wood, for instance, are
floated on to the sea on the slowest as well as on the most rapid
currents.

   [Illustration: Fig. 42. Longitudinal Section of a River Bar]

=Cross bedding.= A section of a bar exposed at low water may show that
it is formed of layers of sand, or coarser stuff, inclined downstream
as steeply often as the angle of repose of the material. From a boat
anchored over the lower end of a submerged sand bar we may observe the
way in which this structure, called cross bedding, is produced. Sand
is continually pushed over the edge of the bar at _b_ (Fig. 42) and
comes to rest in successive layers on the sloping surface. At the same
time the bar may be worn away at the upper end, _a_, and thus slowly
advance down stream. While the deposit is thus cross bedded, it
constitutes as a whole a stratum whose upper and lower surfaces are
about horizontal. In sections of river banks one may often see a
vertical succession of cross-bedded strata, each built in the way
described.

=Water wear.= The coarser material of river deposits, such as
cobblestones, gravel, and the larger grains of sand, are _water worn_,
or rounded, except when near their source. Rolling along the bottom
they have been worn round by impact and friction as they rubbed
against one another and the rocky bed of the stream.

Experiments have shown that angular fragments of granite lose nearly
half their weight and become well rounded after traveling fifteen
miles in rotating cylinders partly filled with water. Marbles are
cheaply made in Germany out of small limestone cubes set revolving in
a current of water between a rotating bed of stone and a block of oak,
the process requiring but about fifteen minutes. It has been found
that in the upper reaches of mountain streams a descent of less than a
mile is sufficient to round pebbles of granite.

    [Illustration: Fig. 43. Water-Worn Pebbles, Upper Potomac River,
      Maryland]


Land Forms Due To River Erosion

=River valleys.= In their courses to the sea, rivers follow valleys of
various forms, some shallow and some deep, some narrow and some wide.
Since rivers are known to erode their beds and banks, it is a fair
presumption that, aided by the weather, they have excavated the
valleys in which they flow.

Moreover, a bird's-eye view or a map of a region shows the significant
fact that the valleys of a system unite with one another in a branch
work, as twigs meet their stems and the branches of a tree its trunk.
Each valley, from that of the smallest rivulet to that of the master
stream, is proportionate to the size of the stream which occupies it.
With a few explainable exceptions the valleys of tributaries join that
of the trunk stream at a level; there is no sudden descent or break in
the bed at the point of juncture. These are the natural consequences
which must follow if the land has long been worked upon by streams,
and no other process has ever been suggested which is competent to
produce them. We must conclude that valley systems have been formed by
the river systems which drain them, aided by the work of the weather;
they are not gaping fissures in the earth's crust, as early observers
imagined, but are the furrows which running water has drawn upon the
land.

As valleys are made by the slow wear of streams and the action of the
weather, they pass in their development through successive stages,
each of which has its own characteristic features. We may therefore
classify rivers and valleys according to the stage which they have
reached in their life history from infancy to old age.


Young River Valleys

=Infancy.= The Red River of the North. A region in northwestern
Minnesota and the adjacent portions of North Dakota and Manitoba was
so recently covered by the waters of an extinct lake, known as Lake
Agassiz, that the surface remains much as it was left when the lake
was drained away. The flat floor, spread smooth with lake-laid silts,
is still a plain, to the eye as level as the sea. Across it the Red
River of the North and its branches run in narrow, ditch-like
channels, steep-sided and shallow, not exceeding sixty feet in depth,
their gradients differing little from the general slopes of the
region. The trunk streams have but few tributaries; the river system,
like a sapling with few limbs, is still undeveloped. Along the banks
of the trunk streams short gullies are slowly lengthening headwards,
like growing twigs which are sometime to become large branches.

   [Illustration: Fig. 44. A Young Lacustrine Plain; the Red River
      of the North

   Scale 5 inches = about 11 miles. Contour interval, 20 feet]

The flat interstream areas are as yet but little scored by drainage
lines, and in wet weather water lingers in ponds in any initial
depressions on the plain.

   [Illustration: Fig. 45. A Young River, Iowa

   Note that it has hardly begun to cut in the plain of glacial
   drift on which it flows]

=Contours.= In order to read the topographic maps of the text-book and
the laboratory the student should know that contours are lines drawn
on maps to represent relief, all points on any given contour being of
equal height above sea level. The _contour interval_ is the uniform
vertical distance between two adjacent contours and varies on
different maps. To express regions of faint relief a contour interval
of ten or twenty feet is commonly selected; while in mountainous
regions a contour interval of two hundred and fifty, five hundred, or
even one thousand feet may be necessary in order that the contours may
not be too crowded for easy reading.

Whether a river begins its life on a lake plain, as in the example
just cited, or upon a coastal plain lifted from beneath the sea or on
a spread of glacial drift left by the retreat of continental ice
sheets, such as covers much of Canada and the northeastern parts of
the United States, its infantile stage presents the same
characteristic features,--a narrow and shallow valley, with
undeveloped tributaries and undrained interstream areas. Ground water
stands high, and, exuding in the undrained initial depressions, forms
marshes and lakes.

   [Illustration: Fig. 46. A Young Drift Region in Wisconsin

    Describe this area. How high are the hills? Are they such in form
    and position as would be left by stream erosion? Consult a map of
    the entire state and notice that the Fox River finds its way to Lake
    Michigan, while the Wisconsin empties into the Mississippi. Describe
    that portion of the divide here shown between the Mississippi and
    the St. Lawrence systems. Which is the larger river, the Wisconsin
    or the Fox? Other things being equal, which may be expected to
    deepen its bed the more rapidly? What changes are likely to occur
    when one of these rivers comes to flow at a lower level than the
    other? Why have not these changes occurred already?]

=Lakes.= Lakes are perhaps the most obvious of these fleeting features
of infancy. They are short-lived, for their destruction is soon
accomplished by several means. As a river system advances toward
maturity the deepening and extending valleys of the tributaries lower
the ground-water surface and invade the undrained depressions of the
region. Lakes having outlets are drained away as their basin rims are
cut down by the outflowing streams,--a slow process where the rim is
of hard rock, but a rapid one where it is of soft material such as
glacial drift.

Lakes are effaced also by the filling of their basins. Inflowing
streams and the wash of rains bring in waste. Waves abrade the shore
and strew the débris worn from it over the lake bed. Shallow lakes are
often filled with organic matter from decaying vegetation.

Does the outflowing stream, from a lake carry sediment? How does this
fact affect its erosive power on hard rock? on loose material?

Lake Geneva is a well-known example of a lake in process of
obliteration. The inflowing Rhone has already displaced the waters of
the lake for a length of twenty miles with the waste brought down from
the high Alps. For this distance there extends up the Rhone Valley an
alluvial plain, which has grown lakeward at the rate of a mile and a
half since Roman times, as proved by the distance inland at which a
Roman port now stands.

   [Illustration: Fig. 47. A Small Lake being broadened and shoaled
      by Wave Wear

   _ls_, lake surface; dotted line, initial shore;
   _b_, fill made of material taken from _a_]

How rapidly a lake may be silted up under exceptionally favorable
conditions is illustrated by the fact that over the bottom of the
artificial lake, of thirty-five square miles, formed behind the great
dam across the Colorado River at Austin, Texas, sediments thirty-nine
feet deep gathered in seven years.

Lake Mendota, one of the many beautiful lakes of southern Wisconsin,
is rapidly cutting back the soft glacial drift of its shores by means
of the abrasion of its waves. While the shallow basin is thus
broadened, it is also being filled with the waste; and the time is
brought nearer when it will be so shoaled that vegetation can complete
the work of its effacement.

   [Illustration: Fig. 48. A Lake well-nigh effaced, Montana

   By what means is the lake bed being filled?]

Along the margin of a shallow lake mosses, water lilies, grasses, and
other water-loving plants grow luxuriantly. As their decaying remains
accumulate on the bottom, the ring of marsh broadens inwards, the lake
narrows gradually to a small pond set in the midst of a wide bog, and
finally disappears. All stages in this process of extinction may be
seen among the countless lakelets which occupy sags in the recent
sheets of glacial drift in the northern states; and more numerous than
the lakes which still remain are those already thus filled with
carbonaceous matter derived from the carbon dioxide of the atmosphere.
Such fossil lakes are marked by swamps or level meadows underlain with
muck.

   [Illustration: Fig. 49. A Level Meadow, Scotland

   Explain its origin. What will be its future?]

=The advance to maturity.= The infantile stage is brief. As a river
advances toward maturity the initial depressions, the lake basins of
its area, are gradually effaced. By the furrowing action of the rain
wash and the head ward lengthening, of tributaries a branchwork of
drainage channels grows until it covers the entire area, and not an
acre is left on which the fallen raindrop does not find already cut
for it an uninterrupted downward path which leads it on by way of
gully, brook, and river to the sea. The initial surface of the land,
by whatever agency it was modeled, is now wholly destroyed; the region
is all reduced to valley slopes.

   [Illustration: Fig. 50. Drainage Maps

   _A_, an area in its infancy, Buena Vista County, Iowa;
   _B_, an area in its maturity, Ringgold County, Iowa]

   [Illustration: Fig. 51. Successive Longitudinal Profiles of a
      Stream

   _am_, initial profile, with waterfall at _w_, and basins at _l_
   and _l´_, which at first are occupied by lakes and later are
   filled or drained; _b_, _c_, _d_, and _e_, profiles established
   in succession as the stream advances from infancy toward old
   age. Note that these profiles are concave toward the sky. This
   is the _erosion curve_. What contrasting form has the weather
   weather curve (p. 34)?]

=The longitudinal profile of a stream.= This at first corresponds with
the initial surface of the region on which the stream begins to flow,
although its way may lead through basins and down steep descents. The
successive profiles to which it reduces its bed are illustrated in
Figure 51. As the gradient, or rate of descent of its bed, is lowered,
the velocity of the river is decreased until its lessening energy is
wholly consumed in carrying its load and it can no longer erode its
bed. The river is now _at grade_, and its capacity is just equal to
its load. If now its load is increased the stream deposits, and thus
builds up, or _aggrades_, its bed. On the other hand, if its load is
diminished it has energy to spare, and resuming its work of erosion,
_degrades_ its bed. In either case the stream continues aggrading or
degrading until a new gradient is found where the velocity is just
sufficient to move the load, and here again it reaches grade.

   [Illustration: Fig. 52. A V-Valley,--the Canyon of the
      Yellowstone

   Note the steep sides. What processes are at work upon them? How
   wide is the valley at the base compared with the width of the
   stream? Do you see any river deposits along the banks? Is the
   stream flowing swiftly over a rock bed, or quietly over a bed
   which it has built up? Is it graded or ungraded? Note that the
   canyon walls project in interlocking spurs]

=V-Valleys.= Vigorous rivers well armed with waste make short work of
cutting their beds to grade, and thus erode narrow, steep-sided gorges
only wide enough at the base to accommodate the stream. The steepness
of the valley slopes depends on the relative rates at which the bed is
cut down by the stream and the sides are worn back by the weather. In
resistant rock a swift, well-laden stream may saw out a gorge whose
sides are nearly or even quite vertical, but as a rule young valleys
whose streams have not yet reached grade are V-shaped; their sides
flare at the top because here the rocks have longest been opened up to
the action of the weather. Some of the deepest canyons may be found
where a rising land mass, either mountain range or plateau, has long
maintained by its continued uplift the rivers of the region above
grade.

   [Illustration: Fig. 53. Section of the Yellowstone Canyon

   This canyon is 100 feet deep, 2500 feet wide at the top, and
   about 250 feet wide at the bottom. Neglecting any cutting of the
   river against the banks, estimate what part of the excavation
   of the canyon is due to the vertical erosion of its bed by the
   river and what to weathering and rain wash on the canyon sides]

In the northern hemisphere the north sides of river valleys are
sometimes of more gentle slope than the south sides. Can you suggest a
reason?

=The Grand Canyon of the Colorado River in Arizona.= The Colorado
River trenches the high plateau of northern Arizona with a colossal
canyon two hundred and eighteen miles long and more than a mile in
greatest depth (Fig. 15). The rocks in which the canyon is cut are for
the most part flat-lying, massive beds of limestones and sandstones,
with some shales, beneath which in places harder crystalline rocks are
disclosed. Where the canyon is deepest its walls have been profoundly
dissected. Lateral ravines have widened into immense amphitheaters,
leaving between them long ridges of mountain height, buttressed
and rebuttressed with flanking spurs and carved into majestic
architectural forms. From the extremity of one of these promontories
it is two miles or more across the gulf to the point of the one
opposite, and the heads of the amphitheaters are thirteen miles apart.

   [Illustration: Fig. 54. Grand Canyon of the Colorado River,
      Arizona]

The lower portion of the canyon is much narrower (Fig. 54) and its
walls of dark crystalline rock sink steeply to the edge of the river,
a swift, powerful stream a few hundred feet wide, turbid with reddish
silt, by means of which it continually rasps its rocky bed as it
hurries on. The Colorado is still deepening its gorge. In the Grand
Canyon its gradient is seven and one half feet to the mile, but, as in
all ungraded rivers, the descent is far from uniform. Graded reaches
in soft rock alternate with steeper declivities in hard rock, forming
rapids such as, for example, a stretch of ten miles where the fall
averages twenty-one feet to the mile. Because of these dangerous
rapids the few exploring parties who have traversed the Colorado
canyon have done so at the hazard of their lives.

The canyon has been shaped by several agencies. Its depth is due to
the river which has sawed its way far toward the base of a lofty
rising plateau. Acting alone this would have produced a slitlike gorge
little wider than the breadth of the stream. The impressive width of
the canyon and the magnificent architectural masses which fill it are
owing to two causes. Running water has gulched the walls and
weathering has everywhere attacked and driven them back. The
horizontal harder beds stand out in long lines of vertical cliffs,
often hundreds of feet in height, at whose feet talus slopes conceal
the outcrop of the weaker strata (Fig. 15). As the upper cliffs have
been sapped and driven back by the weather, broad platforms are left
at their bases and the sides of the canyon descend to the river by
gigantic steps. Far up and down the canyon the eye traces these
horizontal layers, like the flutings of an elaborate molding,
distinguishing each by its contour as well as by its color and
thickness.

   [Illustration: Fig. 55. Diagrams illustrating Conditions which
      produce Falls or Rapids

   _A_, vertical succession of harder and softer rocks;
   _B_, horizontal succession of the same. In _A_ the stream _ab_
   in sinking its bed through a mass of strata of different degrees
   of hardness has discovered the weak layer _s_ beneath the hard
   layer _h_. It rapidly cuts its way in _s_, while in _A_ its
   work is delayed. Thus the profile _afb´_ is soon reached, with
   falls at _f_. In _B_ the initial profile is shown by dotted
   line.]

The Grand Canyon of the Colorado is often and rightly cited as an
example of the stupendous erosion which may be accomplished by a
river. And yet the Colorado is a young stream and its work is no more
than well begun. It has not yet wholly reached grade, and the great
task of the river and its tributaries--the task of leveling the lofty
plateau to a low plain and of transporting it grain by grain to the
sea--still lies almost entirely in the future.

   [Illustration: Fig. 56. Longitudinal Section of Yellowstone
      River at Lower Fall, _F_, and Upper Fall, _F´_, Yellowstone
      National Park

   _la_, lava deeply decayed through action of thermal waters; _m_
   and _m´_, masses of decayed lavas to whose hardness the falls
   are due. Which fall will be worn away the sooner? How far
   upstream will each fall migrate? Draw profile of the river when
   one fall has disappeared]

   [Illustration: Fig. 57. Diagram illustrating Migration of a
      Fall due to a Hard Layer _H_, in the Midst of Soft Layers
      _S_ and _S_, all dipping upstream

   _a_, _b_, _c_, _d_, and _e_, successive positions of the fall;
   _r_, rapid to which the fall is reduced. Draw diagram showing
   migration of fall in strata dipping _downstream_. Under what
   conditions of inclination of the strata will a fall migrate the
   farthest and have the longest life? Under what conditions will
   it migrate the least distance and soonest be destroyed?]

=Waterfalls and rapids.= Before the bed of a stream is reduced to
grade it may be broken by abrupt descents which give rise to
waterfalls and rapids. Such breaks in a river's bed may belong to the
initial surface over which it began its course; still more commonly
are they developed in the rock mass through which it is cutting its
valley. Thus, wherever a stream leaves harder rocks to flow over
softer ones the latter are quickly worn below the level of the former,
and a sharp change in slope, with a waterfall or rapid, results.

At time of flood young tributaries with steeper courses than that of
the trunk stream may bring down stones and finer waste, which the
gentler current cannot move along, and throw them as a dam across its
way. The rapids thus formed are also ephemeral, for as the gradient of
the tributaries is lowered the main stream becomes able to handle the
smaller and finer load which they discharge.

A rare class of falls is produced where the minor tributaries of a
young river are not able to keep pace with their master stream in the
erosion of their beds because of their smaller volume, and thus join
it by plunging over the side of its gorge. But as the river approaches
grade and slackens its down cutting, the tributaries sooner or later
overtake it, and effacing their falls, unite with it on a level.

   [Illustration: Fig. 58. Maturely Dissected Plateau near
      Charleston, West Virginia

   Compare the number of streams in any given number of square
   miles with the number on an area of the same size in the Red
   River Valley (Fig. 44). What is the shape of the ridges? Are
   their summits broad or narrow? Are their crests even or broken
   by knobs and cols (the depressions on the crest line)? If the
   latter, how deeply have the cols been worn beneath the summits
   of the knobs?]

Waterfalls and rapids of all kinds are evanescent features of a
river's youth. Like lakes they are soon destroyed, and if any long
time had already elapsed since their formation they would have been
obliterated already.

=Local baselevels.= That balanced condition called grade, where a
river neither degrades its bed by erosion nor aggrades it by
deposition, is first attained along reaches of soft rocks, ungraded
outcrops of hard rocks remaining as barriers which give rise to rapids
or falls. Until these barriers are worn away they constitute local
baselevels, below which level the stream, up valley from them, cannot
cut. They are eroded to grade one after another, beginning with the
least strong, or the one nearest the mouth of the stream. In a similar
way the surface of a lake in a river's course constitutes for all
inflowing streams a local baselevel, which disappears when the basin
is filled or drained.

   [Illustration: Fig. 59. A Maturity Dissected Region of Slight
      Relief, Iowa]


Mature And Old Rivers

Maturity is the stage of a river's complete development and most
effective work. The river system now has well under way its great task
of wearing down the land mass which it drains and carrying it particle
by particle to the sea. The relief of the land is now at its greatest;
for the main channels have been sunk to grade, while the divides
remain but little worn below their initial altitudes. Ground water now
stands low. The run-off washes directly to the streams, with the least
delay and loss by evaporation in ponds and marches; the discharge of
the river is therefore at its height. The entire region is dissected
by stream ways. The area of valley slopes is now largest and sheds to
the streams a heavier load of waste than ever before. At maturity the
river system is doing its greatest amount of work both in erosion and
in the carriage of water and of waste to the sea.

   [Illustration: Fig. 60. Successive Stages, _A_, _B_, _C_, and
      _D_, in Valley-Widening by Planation

   Describe valley _A_. What changes have taken place in _B_, _C_,
   and _D_? Do the river bends remain stationary or move up or
   down valley? With what effect on the projecting spurs of the
   valley sides? Draw diagrams showing a still later stage than _D_]

=Lateral erosion.= On reaching grade a river ceases to scour its bed,
and it does not again begin to do so until some change in load or
volume enables it to find grade at a lower level. On the other hand, a
stream erodes its banks at all stages in its history, and with graded
rivers this process, called lateral erosion, or _planation_, is
specially important. The current of a stream follows the outer side of
all curves or bends in the channel, and on this side it excavates its
bed the deepest and continually wears and saps its banks. On the inner
side deposition takes place in the more shallow and slower-moving
water. The inner bank of bends is thus built out while the outer bank
is worn away. By swinging its curves against the valley sides a graded
river continually cuts a wider and wider floor. The V-valley of youth
is thus changed by planation to a flat-floored valley with flaring
sides which gradually become subdued by the weather to gentle slopes.
While widening their valleys streams maintain a constant width of
channel, so that a wide-floored valley does not signify that it ever
was occupied by a river of equal width.

=The gradient.= The gradients of graded rivers differ widely. A large
river with a light load reaches grade on a faint slope, while a
smaller stream heavily burdened with waste requires a steep slope to
give it velocity sufficient to move the load.

The Platte, a graded river of Nebraska with its headwaters in the
Rocky Mountains, is enfeebled by the semi-arid climate of the Great
Plains and surcharged with the waste brought down both by its branches
in the mountains and by those whose tracks lie over the soft rocks of
the plains. It is compelled to maintain a gradient of eight feet to
the mile in western Nebraska. The Ohio reaches grade with a slope of
less than four inches to the mile from Cincinnati to its mouth, and
the powerful Mississippi washes along its load with a fall of but
three inches per mile from Cairo to the Gulf.

Other things being equal, which of graded streams will have the
steeper gradient, a trunk stream or its tributaries? a stream supplied
with gravel or one with silt?

Other factors remaining the same, what changes would occur if the
Platte should increase in volume? What changes would occur if the load
should be increased in amount or in coarseness?


   [Illustration: Fig. 61. Successive Cross Sections of a Region as
      it advances from Infancy _a_, to Old Age _e_]

_The old age of rivers._ As rivers pass their prime, as denudation
lowers the relief of the region, less waste and finer is washed over
the gentler slopes of the lowering hills. With smaller loads to carry,
the rivers now deepen their valleys and find grade with fainter
declivities nearer the level of the sea. This limit of the level of
the sea beneath which they cannot erode is known as _baselevel_.[1] As
streams grow old they approach more and more closely to baselevel,
although they are never able to attain it. Some slight slope is needed
that water may flow and waste be transported over the land. Meanwhile
the relief of the land has ever lessened. The master streams and their
main tributaries now wander with sluggish currents over the broad
valley floors which they have planed away; while under the erosion of
their innumerable branches and the wear of the weather the divides
everywhere are lowered and subdued to more and more gentle slopes.
Mountains and high plateaus are thus reduced to rolling hills, and at
last to plains, surmounted only by such hills as may still be
unreduced to the common level, because of the harder rocks of which
they are composed or because of their distance from the main erosion
channels. Such regions of faint relief, worn down to near base level
by subaërial agencies, are known as _peneplains_ (almost plains).
Any residual masses which rise above them are called _monadnocks_,
from the name of a conical peak of New Hampshire which overlooks the
now uplifted peneplain of southern New England.

   [1] The term "baselevel" is also used to designate the close
       approximation to sea level to which streams are able to
       subdue the land.

In its old age a region becomes mantled with thick sheets of fine and
weathered waste, slowly moving over the faint slopes toward the water
ways and unbroken by ledges of bare rock. In other words, the waste
mantle also is now graded, and as waterfalls have been effaced in the
river beds, so now any ledges in the wide streams of waste are worn
away and covered beneath smooth slopes of fine soil. Ground water
stands high and may exude in areas of swamp. In youth the land mass
was roughhewn and cut deep by stream erosion. In old age the faint
reliefs of the land dissolve away, chiefly under the action of the
weather, beneath their cloak of waste.

   [Illustration: Fig. 62. Peneplain surrounded by Monadnocks,
      Piedmont Belt, Virginia

   From Davis' _Elementary Physical Geography]

=The cycle of erosion.= The successive stages through which a land
mass passes while it is being leveled to the sea constitute together a
cycle of erosion. Each stage of the cycle from infancy to old age
leaves, as we have seen, its characteristic records in the forms
sculptured on the land, such as the shapes of valleys and the contours
of hills and plains. The geologist is thus able to determine by the
land forms of any region the stage in the erosion cycle to which it
now belongs, and knowing what are the earlier stages of the cycle, to
read something of the geological history of the region.

=Interrupted cycles.= So long a time is needed to reduce a land mass
to baselevel that the process is seldom if ever completed during a
single uninterrupted cycle of erosion. Of all the various
interruptions which may occur the most important are gradual movements
of the earth's crust, by which a region is either depressed or
elevated relative to sea level.

   [Illustration: Fig. 63. Young Inner Gorge in Wide Older Valley,
      Alaska]

The _depression_ of a region hastens its old age by decreasing the
gradient of streams, by destroying their power to excavate their beds
and carry their loads to a degree corresponding to the amount of the
depression, and by lessening the amount of work they have to do. The
slackened river currents deposit their waste in Hood plains which
increase in height as the subsidence continues. The lower courses of
the rivers are invaded by the sea and become estuaries, while the
lower tributaries are cut off from the trunk stream.

_Elevation_, on the other hand, increases the activity of all agencies
of weathering, erosion, and transportation, restores the region to its
youth, and inaugurates a new cycle of erosion. Streams are given a
steeper gradient, greater velocity, and increased energy to carry
their loads and wear their beds. They cut through the alluvium of
their flood plains, leaving it on either bank as successive terraces,
and intrench themselves in the underlying rock. In their older and
wider valleys they cut narrow, steep-walled inner gorges, in which
they flow swiftly over rocky floors, broken here and there by falls
and rapids where a harder layer of rock has been discovered. Winding
streams on plains may thus incise their meanders in solid rock as the
plains are gradually uplifted. Streams which are thus restored to
their youth are said to be _revived_.

   [Illustration: Fig. 64. Incised Meanders of Oneota River, Iowa]

As streams cut deeper and the valley slopes are steepened, the mantle
of waste of the region undergoing elevation is set in more rapid
movement. It is now removed particle by particle faster than it forms.
As the waste mantle thins, weathering attacks the rocks of the region
more energetically until an equilibrium is reached again; the rocks
waste rapidly and their waste is as rapidly removed.

=Dissected peneplains.= When a rise of the land brings one cycle to an
end and begins another, the characteristic land forms of each cycle
are found together and the topography of the region is composite until
the second cycle is so far advanced that the land forms of the first
cycle are entirely destroyed. The contrast between the land surfaces
of the later and the earlier cycles is most striking when the earlier
had advanced to age and the later is still in youth. Thus many
peneplains which have been elevated and dissected have been recognized
by the remnants of their ancient erosion surfaces, and the length of
time which has elapsed since their uplift has been measured by the
stage to which the new cycle has advanced.

   [Illustration: Fig. 65.

   Describe the valley of stream _a_. Is it young or old? How does
   the valley of _b_ differ from that of _a_? Compare as to form
   and age the inner valley of _b_ with the outer valley and with
   the valley of _a_. Account for the inner valley. Why does it
   not extend to the upper portion of the course of _b_? Will it
   ever do so? Draw longitudinal profile of _b_, showing the
   different gradient of upper and lower portions of its course
   not here seen. As the inner valley of tributary _c_ extends
   headward it may invade the valley of _a_ before the inner
   valley of _a_ has worked upstream to the area seen in the
   diagram. With what results?]

=The Piedmont Belt.= As an example of an ancient peneplain uplifted
and dissected we may cite the Piedmont Belt, a broad upland lying
between the Appalachian Mountains and the Atlantic coastal plain. The
surface of the Piedmont is gently rolling. The divides, which are
often smooth areas of considerable width, rise to a common plane, and
from them one sees in every direction an even sky line except where in
places some lone hill or ridge may lift itself above the general level
(Fig. 62). The surface is an ancient one, for the mantle of residual
waste lies deep upon it, soils are reddened by long oxidation, and the
rocks are rotted to a depth of scores of feet.

At present, however, the waste mantle is not forming so rapidly as it
is being removed. The streams of the upland are actively engaged in
its destruction. They flow swiftly in narrow, rock-walled valleys over
rocky beds. This contrast between the young streams and the aged
surface which they are now so vigorously dissecting can only be
explained by the theory that the region once stood lower than at
present and has recently been upraised. If now we imagine the valleys
refilled with the waste which the streams have swept away, and the
upland lowered, we restore the Piedmont region to the condition in
which it stood before its uplift and dissection,--a gently rolling
plain, surmounted here and there by isolated hills and ridges.

   [Illustration: Fig. 66. Dissected Peneplain of Southern New
      England]

The surface of the ancient Piedmont plain, as it may be restored from
the remnants of it found on the divides, is not in accordance with the
structures of the country rocks. Where these are exposed to view they
are seen to be far from horizontal. On the walls of river gorges they
dip steeply and in various directions and the streams flow over their
upturned edges. As shown in Figure 67, the rocks of the Piedmont have
been folded and broken and tilted.

   [Illustration: Fig. 67. Section in Piedmont Belt
      _M_, a monadnock]

It is not reasonable to believe that when the rocks of the Piedmont
were thus folded and otherwise deformed the surface of the region was
a plain. The upturned layers have not always stopped abruptly at the
even surface of the Piedmont plain which now cuts across them. They
are the bases of great folds and tilted blocks which must once have
risen high in air. The complex and disorderly structures of the
Piedmont rocks are those seen in great mountain ranges, and there is
every reason to believe that these rocks after their deformation rose
to mountain height.

   [Illustration: Fig. 68. The area of the Laurentian Peneplain
      (shaded)]

The ancient Piedmont plain cuts across these upturned rocks as
independently of their structure as the even surface of the sawed
stump of some great tree is independent of the direction of its
fibers. Hence the Piedmont plain as it was before its uplift was not a
coastal plain formed of strata spread in horizontal sheets beneath the
sea and then uplifted; nor was it a structural plain, due to the
resistance to erosion of some hard, flat-lying layer of rock. Even
surfaces developed on rocks of discordant structure, such as the
Piedmont shows, are produced by long denudation, and we may consider
the Piedmont as a peneplain formed by the wearing down of mountain
ranges, and recently uplifted.

=The Laurentian peneplain.= This is the name given to a denuded
surface on very ancient rocks which extends from the Arctic Ocean to
the St. Lawrence River and Lake Superior, with small areas also in
northern Wisconsin and New York. Throughout this U-shaped area, which
incloses Hudson Bay within its arms, the country rocks have the
complicated and contorted structures which characterize mountain
ranges (see Fig. 179, P. 211). But the surface of the area is by no
means mountainous. The sky line when viewed from the divides is
unbroken by mountain peaks or rugged hills. The surface of the arm
west of Hudson Bay is gently undulating and that of the eastern arm
has been roughened to low-rolling hills and dissected in places by
such deep river gorges as those of the Ottawa and Saguenay. This
immense area may be regarded as an ancient peneplain truncating the
bases of long-vanished mountains and dissected after elevation.

In the examples cited the uplift has been a broad one and to
comparatively little height. Where peneplains have been uplifted to
great height and have since been well dissected, and where they have
been upfolded and broken and uptilted, their recognition becomes more
difficult. Yet recent observers have found evidences of ancient
lowland surfaces of erosion on the summits of the Allegheny ridges,
the Cascade Mountains (Fig. 69), and the western slope of the Sierra
Nevadas.

   [Illustration: Fig. 69. View in the Cascade Mountains, Washington

   The general level to which these ridges rise may be accounted
   for by the uplift and dissection of a once low-lying peneplain]

=The southern Appalachian region.= We have here an example of an area
the latter part of whose geological history may be deciphered by means
of its land forms. The generalized section of Figure 70, which passes
from west to east across a portion of the region in eastern Tennessee,
shows on the west a part of the broad Cumberland plateau. On the east
is a roughened upland platform, from which rise in the distance the
peaks of the Great Smoky Mountains. The plateau, consisting of strata
but little changed from their original flat-lying attitude, and the
platform, developed on rocks of disordered structure made crystalline
by heat and pressure, both stand at the common level of the line AB.
They are separated by the Appalachian valley, forty miles wide, cut in
strata which have been folded and broken into long narrow blocks. The
valley is traversed lengthwise by long, low ridges, the outcropping
edges of the harder strata, which rise to about the same level,--that
of the line _cd_. Between these ridges stretch valley lowlands at the
level _ef_ excavated in the weaker rocks, while somewhat below them lie
the channels of the present streams now busily engaged in deepening
their beds.

_The valley lowlands._ Were they planed by graded or ungraded streams?
Have the present streams reached grade? Why did the streams cease
widening the floors of the valley lowlands? How long since? When will
they begin anew the work of lateral planation? What effect will this
have on the ridges if the present cycle of erosion continues long
uninterrupted?

   [Illustration: Fig. 70. Generalized Section of the Southern
      Appalachian Region in Eastern Tennessee]

_The ridges of the Appalachian valley._ Why do they stand above the
valley lowlands? Why do their summits lie in about the same plane?
Refilling the valleys intervening between these ridges with the
material removed by the streams, what is the nature of the surface
thus restored? Does this surface _cd_ accord with the rock structures
on which it has been developed? How may it have been made? At what
height did the land stand then, compared with its present height? What
elevations stood above the surface _cd_? Why? What name may you use to
designate them? How does the length of time needed to develop the
surface _cd_ compare with that needed to develop the valley lowlands?

_The Platform And Plateau._ Why do they stand at a common level ab? Of
what surface may they be remnants? Is it accordant with the rock
structure? How was it produced? What unconsumed masses overlooked it?
Did the rocks of the Appalachian valley stand above this surface when
it was produced? Did they then stand below it? Compare the time needed
to develop this surface with that needed to develop _cd_. Which surface
is the older?

How many cycles of erosion are represented here? Give the erosion
history of the region by cycles, beginning with the oldest, the work
done in each and the work left undone, what brought each cycle to a
close, and how long relatively it continued.




CHAPTER IV

RIVER DEPOSITS


The characteristic features of river deposits and the forms which they
assume may be treated under three heads: (1) valley deposits, (2)
basin deposits, and (3) deltas.


Valley Deposits

=Flood plains.= The deposits which streams build along their courses
at times of flood are known as flood plains. A swift current then
sweeps along the channel, while a shallow sheet of water moves slowly
over the flood plain, spreading upon it a thin layer of sediment. It
has been estimated that each inundation of the Nile leaves a layer of
fertilizing silt three hundredths of an inch thick over the flood
plain of Egypt.

Flood plains may consist of a thin spread of alluvium over the flat
rock floor of a valley which is being widened by the lateral erosion
of a graded stream (Fig. 60). Flood-plain deposits of great thickness
may be built by aggrading rivers even in valleys whose rock floors
have never been thus widened (Fig. 368).

   [Illustration: Fig. 71. Cross Section of a Flood Plain]

A cross section of a flood plain (Fig. 71) shows that it is highest
next the river, sloping gradually thence to the valley sides. These
wide natural embankments are due to the fact that the river deposit is
heavier near the bank, where the velocity of the silt-laden channel
current is first checked by contact with the slower-moving overflow.

   [Illustration: Fig. 72. Waste-filled Valley and Braided
     Channels of the Upper Mississippi]

Thus banked off from the stream, the outer portions of a flood plain
are often ill-drained and swampy, and here vegetal deposits, such as
peat, may be interbedded with river silts.

A map of a wide flood plain, such as that of the Mississippi or the
Missouri (Fig. 77), shows that the courses of the tributaries on
entering it are deflected downstream. Why?

The aggrading streams by which flood plains are constructed gradually
build their immediate banks and beds to higher and higher levels, and
therefore find it easy at times of great floods to break their natural
embankments and take new courses over the plain. In this way they
aggrade each portion of it in turn by means of their shifting
channels.

=Braided channels.= A river actively engaged in aggrading its valley
with coarse waste builds a flood plain of comparatively steep gradient
and often flows down it in a fairly direct course and through a
network of braided channels. From time to time a channel becomes
choked with waste, and the water no longer finding room in it breaks
out and cuts and builds itself a new way which reunites down valley
with the other channels. Thus there becomes established a network of
ever-changing channels inclosing low islands of sand and gravel.

   [Illustration: Fig. 73. Terraced Valley of River in Central Asia]

   [Illustration: Fig. 74. Terraces carved in Alluvial Deposits]

   Which is older, the rock floor of the valley or the river
   deposits which fill it? What are the relative ages of terraces
   _a_, _b_, _c_, and _e_? It will be noted that the remnants of
   the higher flood plains have not been swept away by the
   meandering river, as it swung from side to side of the valley
   at lower levels, because they have been defended by ledges of
   hard rock in the projecting spurs of the initial valley. The
   stream has encountered such defending ledges at the point
   marked _d_]

   [Illustration: Fig. 75. River Terraces of Rock covered with
      Alluvium

   _c_, recent flood plain of the river. To what processes is it
   due? Account for the alluvium at _a_ and _b_ and on the
   opposite side of the valley at the same levels. Which is the
   older? Account for the flat rock floors on which these deposits
   of alluvium rest. Give the entire history which may be read in
   the section]

=Terraces.= While aggrading streams thus tend to shift their channels,
degrading streams, on the contrary, become more and more deeply
intrenched in their valleys. It often occurs that a stream, after
having built a flood plain, ceases to aggrade its bed because of a
lessened load or for other reasons, such as an uplift of the region,
and begins instead to degrade it. It leaves the original flood plain
out of reach of even the highest floods. When again it reaches grade
at a lower level it produces a new flood plain by lateral erosion in
the older deposits, remnants of which stand as terraces on one or both
sides of the valley. In this way a valley may be lined with a
succession of terraces at different levels, each level representing an
abandoned flood plain.

   [Illustration: Fig. 76. Development of a Meander

   The dotted line in _a_, _b_, and _c_ shows the stage preceding   that indicate by the unbroken line]

=Meanders.= Valleys aggraded with fine waste form well-nigh level
plains over which streams wind from side to side of a direct course in
symmetric bends known as meanders, from the name of a winding river of
Asia Minor. The giant Mississippi has developed meanders with a radius
of one and one half miles, but a little creek may display on its
meadow as perfect curves only a rod or so in radius. On the flood
plain of either river or creek we may find examples of the successive
stages in the development of the meander, from its beginning in the
slight initial bend sufficient to deflect the current against the
outer side. Eroding here and depositing on the inner side of the bend,
it gradually reaches first the open bend (Fig. 76, _a_) whose width
and length are not far from equal, and later that of the horseshoe
meander (Fig. 76, _b_) whose diameter transverse to the course of the
stream is much greater than that parallel with it. Little by little
the neck of land projecting into the bend is narrowed, until at last
it is cut through and a "cut-off" is established. The old channel is
now silted up at both ends and becomes a crescentic lagoon (Fig. 76,
_c_), or oxbow lake, which fills gradually to an arc-shaped shallow
depression.

   [Illustration: Fig. 77. Map of a portion of the Flood Plain of
        the Missouri River

   Each small square represents one square mile. How wide is the
   flood plain of the Missouri? How wide is the flood plain of the
   Big Sioux? Why is the latter river deflected down valley on
   entering the flood plain of the master stream? How do the
   meanders of the two rivers compare in size? How does the width
   of each flood plain compare with the width of the belt occupied
   by the meanders of the river? Do you find traces of any former
   channels?]

=Flood plains characteristic of mature rivers.= On reaching grade a
stream planes a flat floor for its continually widening valley. Ever
cutting on the outer bank of its curves, it deposits on the inner bank
scroll-like flood-plain patches (Fig 60). For a while the valley bluffs
do not give its growing meanders room to develop to their normal size,
but as planation goes on, the bluffs are driven back to the full width
of the meander belt and still later to a width which gives room for
broad stretches of flood plain on either side (Fig. 77).

Usually a river first attains grade near its mouth, and here first sinks
its bed to near baselevel. Extending its graded course upstream by
cutting away barrier after barrier, it comes to have a widened and
mature valley over its lower course, while its young headwaters are
still busily eroding their beds. Its ungraded branches may thus bring
down to its lower course more waste than it is competent to carry on to
the sea, and here it aggrades its bed and builds a flood plain in order
to gain a steeper gradient and velocity enough to transport its load.

As maturity is past and the relief of the land is lessened, a smaller
and smaller load of waste is delivered to the river. It now has energy
to spare and again degrades its valley, excavating its former flood
plains and leaving them in terraces on either side, and at last in its
old age sweeping them away.

   [Illustration: Fig. 78. Alluvial Cones, Wyoming]

=Alluvial cones and fans.= In hilly and mountainous countries one often
sees on a valley side a conical or fan-shaped deposit of waste at the
mouth of a lateral stream. The cause is obvious: the young branch has
not been able as yet to wear its bed to accordant level with the already
deepened valley of the master stream. It therefore builds its bed to
grade at the point of juncture by depositing here its load of waste,--a
load too heavy to be carried along the more gentle profile of the trunk
valley.

   [Illustration: Fig. 79. Tributaries and Distributaries of a
      Fan-Building Stream]

Where rivers descend from a mountainous region upon the plain they may
build alluvial fans of exceedingly gentle slope. Thus the rivers of
the western side of the Sierra Nevada Mountains have spread fans with
a radius of as much as forty miles and a slope too slight to be
detected without instruments, where they leave the rock-cut canyons in
the mountains and descend upon the broad central valley of California.

As a river flows over its fan it commonly divides into a branchwork of
shifting channels called _distributaries_, since they lead off the
water from the main stream. In this way each part of the fan is
aggraded and its symmetric form is preserved.

=Piedmont plains.= Mountain streams may build their confluent fans
into widespread piedmont (foot of the mountain) alluvial plains. These
are especially characteristic of arid lands, where the streams wither
as they flow out upon the thirsty lowlands and are therefore compelled
to lay down a large portion of their load. In humid climates
mountain-born streams are usually competent to carry their loads of
waste on to the sea, and have energy to spare to cut the lower
mountain slopes into foothills. In arid regions foothills are commonly
absent and the ranges rise, as from pedestals, above broad, sloping
plains of stream-laid waste.

   [Illustration: Fig. 80. Section from the Rocky Mountains Eastward
      River deposits dotted]

=The High Plains.= The rivers which flow eastward from the Rocky
Mountains have united their fans in a continuous sheet of waste which
stretches forward from the base of the mountains for hundreds of miles
and in places is five hundred feet thick (Fig. 80). That the deposit
was made in ancient times on land and not in the sea is proved by the
remains which it contains of land animals and plants of species now
extinct. That it was laid by rivers and not by fresh-water lakes is
shown by its structure. Wide stretches of flat-lying, clays and sands
are interrupted by long, narrow belts of gravel which mark the
channels of the ancient streams. Gravels, and sands are often cross
bedded, and their well worn pebbles may be identified with the rocks
of the mountains. After building this sheet of waste the streams
ceased to aggrade and began the work of destruction. Large uneroded
remnants, their surfaces flat as a floor, remain as the High Plains of
western Kansas and Nebraska.

=River deposits in subsiding troughs.= To a geologist the most
important river deposits are those which gather in areas of gradual
subsidence; they are often of vast extent and immense thickness, and
such deposits of past geological ages have not infrequently been
preserved, with all their records of the times in which they were
built, by being carried below the level of the sea, to be brought to
light by a later uplift. On the other hand, river deposits which
remain above baselevels of erosion are swept away comparatively soon.

=The Great Valley Of California= is a monotonously level plain of
great fertility, four hundred miles in length and fifty miles in
average width, built of waste swept down by streams from the mountain
ranges which inclose it,--the Sierra Nevada on the east and the Coast
Range on the west. On the waste slopes at the foot of the bordering
hills coarse gravels and even bowlders are left, while over the
interior the slow-flowing streams at times of flood spread wide sheets
of silt. Organic deposits are now forming by the decay of vegetation
in swampy tule (reed) lands and in shallow lakes which occupy
depressions left by the aggrading streams.

Deep borings show that this great trough is filled to a depth of at
least two thousand feet below sea level with recent unconsolidated
sands and silts containing logs of wood and fresh-water shells. These
are land deposits, and the absence of any marine deposits among them
proves that the region has not been invaded by the sea since the
accumulation began. It has therefore been slowly subsiding and its
streams, although continually carried below grade, have yet been able
to aggrade the surface as rapidly as the region sank, and have
maintained it, as at present, slightly above sea level.

=The Indo-Gangetic Plain=, spread by the Brahmaputra, the Ganges, and
the Indus river systems, stretches for sixteen hundred miles along the
southern base of the Himalaya Mountains and occupies an area of three
hundred thousand square miles (Fig. 342). It consists of the flood
plains of the master streams and the confluent fans of the tributaries
which issue from the mountains on the north. Large areas are subject
to overflow each season of flood, and still larger tracts mark
abandoned flood plains below which the rivers have now cut their beds.
The plain is built of far-stretching beds of clay, penetrated by
streaks of sand, and also of gravel near the mountains. Beds of impure
peat occur in it, and it contains fresh-water shells and the bones of
land animals of species now living in northern India. At Lucknow an
artesian well was sunk to one thousand feet below sea level without
reaching the bottom of these river-laid sands and silts, proving a
slow subsidence with which the aggrading rivers have kept pace.

=Warped valleys.= It is not necessary that an area should sink below
sea level in order to be filled with stream-swept waste. High valleys
among growing mountain ranges may suffer warping, or may be blockaded
by rising mountain folds athwart them. Where the deformation is rapid
enough, the river may be ponded and the valley filled with lake-laid
sediments. Even when the river is able to maintain its right of way it
may yet have its declivity so lessened that it is compelled to aggrade
its course continually, filling the valley with river deposits which
may grow to an enormous thickness.

Behind the outer ranges of the Himalaya Mountains lie several
waste-filled valleys, the largest of which are Kashmir and Nepal, the
former being an alluvial plain about as large as the state of
Delaware. The rivers which drain these plains have already cut down
their outlet gorges sufficiently to begin the task of the removal of
the broad accumulations which they have brought in from the
surrounding mountains. Their present flood plains lie as much as some
hundreds of feet below wide alluvial terraces which mark their former
levels. Indeed, the horizontal beds of the Hundes Valley have been
trenched to the depth of nearly three thousand feet by the Sutlej
River. These deposits are recent or subrecent, for there have been
found at various levels the remains of land plants and land and
fresh-water shells, and in some the bones of such animals as the hyena
and the goat, of species or of genera now living. Such soft deposits
cannot be expected to endure through any considerable length of future
time the rapid erosion to which their great height above the level of
the sea will subject them.

   [Illustration: Fig. 81. Cross Section of Aggraded Valley,
      showing Structure of River Deposits]

=Characteristics of river deposits.= The examples just cited teach
clearly the characteristic features of extensive river deposits. These
deposits consist of broad, flat-lying sheets of clay and fine sand
left by the overflow at time of flood, and traversed here and there by
long, narrow strips of coarse, cross-bedded sands and gravels thrown
down by the swifter currents of the shifting channels. Occasional beds
of muck mark the sites of shallow lakelets or fresh-water swamps. The
various strata also contain some remains of the countless myriads of
animals and plants which live upon the surface of the plain as it is
in process of building. River shells such as the mussel, land shells
such as those of snails, the bones of fishes and of such land animals
as suffer drowning at times of flood or are mired in swampy places,
logs of wood, and the stems and leaves of plants are examples of the
variety of the remains of land and fresh-water organisms which are
entombed in river deposits and sealed away as a record of the life of
the time, and as proof that the deposits were laid by streams and not
beneath the sea.


Basin Deposits

=Deposits in dry basins.= On desert areas without outlet to the sea,
as on the Great Basin of the United States and the deserts of central
Asia, stream-swept waste accumulates indefinitely. The rivers of the
surrounding mountains, fed by the rains and melting snows of these
comparatively moist elevations, dry and soak away as they come down
upon the arid plains. They are compelled to lay aside their entire
load of waste eroded from the mountain valleys, in fans which grow to
enormous size, reaching in some instances thousands of feet in
thickness.

The monotonous levels of Turkestan include vast alluvial tracts now in
process of building by the floods of the frequently shifting channels
of the Oxus and other rivers of the region. For about seven hundred
miles from its mouth in Aral Lake the Oxus receives no tributaries,
since even the larger branches of its system are lost in a network of
distributaries and choked with desert sands before they reach their
master stream. These aggrading rivers, which have channels but no
valleys, spread their muddy floods--which in the case of the Oxus
sometimes equal the average volume of the Mississippi--far and wide
over the plain, washing the bases of the desert dunes.

=Playas.= In arid interior basins the central depressions may be
occupied by playas,--plains of fine mud washed forward from the
margins. In the wet season the playa is covered with a thin sheet of
muddy water, a playa lake, supplied usually by some stream at flood.
In the dry season the lake evaporates, the river which fed it
retreats, and there is left to view a hard, smooth, level floor of
sun-baked and sun-cracked yellow clay utterly devoid of vegetation.

In the Black Rock desert of Nevada a playa lake spreads over an area
fifty miles long and twenty miles wide. In summer it disappears; the
Quinn River, which feeds it, shrinks back one hundred miles toward its
source, leaving an absolutely barren floor of clay, level as the sea.

=Lake deposits.= Regarding lakes as parts of river systems, we may now
notice the characteristic features of the deposits in lake basins.
Soundings in lakes of considerable size and depth show that their
bottoms are being covered with tine clays. Sand and gravel are found
along; their margins, being brought in by streams and worn by waves
from the shore, but there are no tidal or other strong currents to
sweep coarse waste out from shore to any considerable distance. Where
fine clays are now found on the land in even, horizontal layers
containing the remains of fresh-water animals and plants, uncut by
channels tilled with cross-bedded gravels and sands and bordered by
beach deposits of coarse waste, we may safely infer the existence of
ancient lakes.

=Marl.= Marl is a soft, whitish deposit of carbonate of lime, mingled
often with more or less of clay, accumulated in small lakes whose
feeding springs are charged with carbonate of lime and into which
little waste is washed from the land. Such lakelets are not infrequent
on the surface of the younger drift sheets of Michigan and northern
Indiana, where their beds of marl--sometimes as much as forty feet
thick--are utilized in the manufacture of Portland cement. The deposit
results from the decay of certain aquatic plants which secrete lime
carbonate from the water, from the decomposition of the calcareous
shells of tiny mollusks which live in countless numbers on the lake
floor, and in some cases apparently from chemical precipitation.

=Peat.= We have seen how lakelets are extinguished by the decaying
remains of the vegetation which they support. A section of such a
fossil lake shows that below the growing mosses and other plants of
the surface of the bog lies a spongy mass composed of dead vegetable
tissue, which passes downward gradually into _peat_,--a dense, dark
brown carbonaceous deposit in which, to the unaided eye, little or no
trace of vegetable structure remains. When dried, peat forms a fuel of
some value and is used either cut into slabs and dried or pressed into
bricks by machinery.

   [Illustration: Fig. 82. Digging Peat, Scotland]

When vegetation decays in open air the carbon of its tissues, taken
from the atmosphere by the leaves, is oxidized and returned to it in
its original form of carbon dioxide. But decomposing in the presence
of water, as in a bog, where the oxygen of the air is excluded, the
carbonaceous matter of plants accumulates in deposits of peat.

Peat bogs are numerous in regions lately abandoned by glacier ice,
where river systems are so immature that the initial depressions left
in the sheet of drift spread over the country have not yet been
drained. One tenth of the surface of Ireland is said to be covered
with peat, and small bogs abound in the drift-covered area of New
England and the states lying as far west as the Missouri River. In
Massachusetts alone it has been reckoned that there are fifteen
billion cubic feet of peat, the largest bog occupying several thousand
acres.

Much larger swamps occur on the young coastal plain of the Atlantic
from New Jersey to Florida. The Dismal Swamp, for example, in Virginia
and North Carolina is forty miles across. It is covered with a dense
growth of water-loving trees such as the cypress and black gum. The
center of the swamp is occupied by Lake Drummond, a shallow lake seven
miles in diameter, with banks of pure-peat, and still narrowing from
the encroachment of vegetation along its borders.

=Salt lakes.= In arid climates a lake rarely receives sufficient
inflow to enable it to rise to the basin rim and find an outlet.
Before this height is reached its surface becomes large enough to
discharge by evaporation into the dry air the amount of water that is
supplied by streams. As such a lake has no outlet, the minerals in
solution brought into it by its streams cannot escape from the basin.
The lake water becomes more and more heavily charged with such
substances as common salt and the sulphates and carbonates of lime, of
soda, and of potash, and these are thrown down from solution one after
another as the point of saturation for each mineral is reached.
Carbonate of lime, the least soluble and often the most abundant
mineral brought in, is the first to be precipitated. As concentration
goes on, gypsum, which is insoluble in a strong brine, is deposited,
and afterwards common salt. As the saltness of the lake varies with
the seasons and with climatic changes, gypsum and salt are laid in
alternate beds and are interleaved with sedimentary clays spread from
the waste brought in by streams at times of flood. Few forms of life
can live in bodies of salt water so concentrated that chemical
deposits take place, and hence the beds of salt, gypsum, and silt of
such lakes are quite barren of the remains of life. Similar deposits
are precipitated by the concentration of sea water in lagoons and arms
of the sea cut off from the ocean.

   [Illustration: Fig. 83. Map of Lake Bonneville and Lahontan

   From Davis' _Physical Geography_]

=Lakes Bonneville and Lahontan.= These names are given to extinct
lakes which once occupied large areas in the Great Basin, the former
in Utah, the latter in northwestern Nevada. Their records remain in
old horizontal beach lines which they drew along their mountainous
shores at the different levels at which they stood, and in the
deposits of their beds. At its highest stage Lake Bonneville, then one
thousand feet deep, overflowed to the north and was a fresh-water
lake. As it shrank below the outlet it became more and more salty, and
the Great Salt Lake, its withered residue, is now depositing salt
along its shores. In its strong brine lime carbonate is insoluble, and
that brought in by streams is thrown down at once in the form of
travertine.

   [Illustration: Fig. 84. Section of Deposits in Beds of Lakes
      Bonneville and Lahontan]

Lake Lahontan never had an outlet. The first chemical deposits to be
made along its shores were deposits of travertine, in places eighty
feet thick. Its floor is spread with fine clays, which must have been
laid in deep, still water, and which are charged with the salts
absorbed by them as the briny water of the lake dried away. These
sedimentary clays are in two divisions, the upper and lower, each
being about one hundred feet thick (_a_ and _c_, Fig. 84). They are
separated by heavy deposits of well-rounded, cross-bedded gravels and
sands (_b_, Fig. 84), similar to those spread at the present time by
the intermittent streams of arid regions. A similar record is shown in
the old floors of Lake Bonneville. What conclusions do you draw from
these facts as to the history of these ancient lakes?


Deltas

In the river deposits which are left above sea level particles of
waste are allowed to linger only for a time. From alluvial fans and
flood plains they are constantly being taken up and swept farther on
downstream. Although these land forms may long persist, the particles
which compose them are ever changing. We may therefore think of the
alluvial deposits of a valley as a stream of waste fed by the waste
mantle as it creeps and washes down the valley sides, and slowly
moving onwards to the sea.

In basins waste finds a longer rest, but sooner or later lakes and dry
basins are drained or filled, and their deposits, if above sea level,
resume their journey to their final goal. It is only when carried
below the level of the sea that they are indefinitely preserved.

On reaching this terminus, rivers deliver their load to the ocean. In
some cases the ocean is able to take it up by means of strong tidal
and other currents, and to dispose of it in ways which we shall study
later. But often the load is so large, or the tides are so weak, that
much of the waste which the river brings in settles at its mouth,
there building up a deposit called the _delta_, from the Greek letter
(D) of that name, whose shape it sometimes resembles.

Deltas and alluvial fans have many common characteristics. Both owe
their origin to a sudden check in the velocity of the river,
compelling a deposit of the load; both are triangular in outline, the
apex pointing upstream; and both are traversed by distributaries which
build up all parts in turn.

In a delta we may distinguish deposits of two distinct kinds,--the
submarine and the subaërial. In part a delta is built of waste
brought down by the river and redistributed and spread by waves and
tides over the sea bottom adjacent to the river's mouth. The origin of
these deposits is recorded in the remains of marine animals and plants
which they contain.

   [Illustration: Fig. 85. Delta of the Mississippi River]

As the submarine delta grows near to the level of the sea the
distributaries of the river cover it with subaërial deposits
altogether similar to those of the flood plain, of which indeed the
subaërial delta is the prolongation. Here extended deposits of peat
may accumulate in swamps, and the remains of land and fresh-water
animals and plants swept down by the stream are imbedded in the silts
laid at times of flood.

Borings made in the deltas of great rivers such as the Mississippi,
the Ganges, and the Nile, show that the subaërial portion often
reaches a surprising thickness. Layers of peat, old soils, and forest
grounds with the stumps of trees are discovered hundreds of feet below
sea level. In the Nile delta some eight layers of coarse gravel were
found interbedded with river silts, and in the Ganges delta at
Calcutta a boring nearly five hundred feet in depth stopped in such a
layer.

The Mississippi has built a delta of twelve thousand three hundred
square miles, and is pushing the natural embankments of its chief
distributaries into the Gulf at a maximum rate of a mile in sixteen
years. Muddy shoals surround its front, shallow lakes, e.g. lakes
Pontchartrain and Borgne, are formed between the growing delta and the
old shore line, and elongate lakes and swamps are inclosed between the
natural embankments of the distributaries.

The delta of the Indus River, India, lies so low along shore that a
broad tract of country is overflowed by the highest tides. The
submarine portion of the delta has been built to near sea level over
so wide a belt offshore that in many places large vessels cannot come
even within sight of land because of the shallow water.

   [Illustration: Fig. 86. Radial Section of a Delta

   This section of a delta illustrates the structure of the
   platform which swift streams well loaded with coarse waste
   build in the water bodies into which they empty. Three members
   may be distinguished: the _bottom set beds_, _a_: the _fore set
   beds_, _b_; and the _top set beds_, _c_. Account for the slope
   of each of these. Why are the bottom set beds of the finer
   material and why do they extend beyond the others? How does the
   profile of this delta differ from that of an alluvial cone and
   why?]

A former arm of the sea, the Rann of Cutch, adjoining the delta on the
east has been silted up and is now an immense barren flat of sandy mud
two hundred miles in length and one hundred miles in greatest breadth.
Each summer it is flooded with salt water when the sea is brought in
by strong southwesterly monsoon winds, and the climate during the
remainder of the year is hot and dry. By the evaporation of sea water
the soil is thus left so salty that no vegetation can grow upon it,
and in places beds of salt several feet in thickness have accumulated.
Under like conditions salt beds of great thickness have been formed in
the past and are now found buried among the deposits of ancient
deltas.

=Subsidence of great deltas.= As a rule great deltas are slowly
sinking. In some instances upbuilding by river deposits has gone on as
rapidly as the region has subsided. The entire thickness of the Ganges
delta, for example, so far as it has been sounded, consists of
deposits laid in open air. In other cases interbedded limestones and
other sedimentary rocks containing marine fossils prove that at times
subsidence has gained on the upbuilding and the delta has been covered
with the sea.

It is by gradual depression that delta deposits attain enormous
thickness, and, being lowered beneath the level of the sea, are safely
preserved from erosion until a movement of the earth's crust in the
opposite direction lifts them to form part of the land. We shall read
later in the hard rocks of our continent the records of such ancient
deltas, and we shall not be surprised to find them as thick as are
those now building at the mouths of great rivers.

=Lake deltas.= Deltas are also formed where streams lose their
velocity on entering the still waters of lakes. The shore lines of
extinct lakes, such as Lake Agassiz and Lakes Bonneville and Lahontan,
may be traced by the heavy deposits at the mouths of their tributary
streams.

       *       *       *       *       *

We have seen that the work of streams is to drain the lands of the
water poured upon them by the rainfall, to wear them down, and to
carry their waste away to the sea, there to be rebuilt by other agents
into sedimentary rocks. The ancient strata of which the continents are
largely made are composed chiefly of material thus worn from still
more ancient lands--lands with their hills and valleys like those of
to-day--and carried by their rivers to the ocean. In all geological
times, as at the present, the work of streams has been to destroy the
lands, and in so doing to furnish to the ocean the materials from
which the lands of future ages were to be made. Before we consider how
the waste of the land brought in by streams is rebuilt upon the ocean
floor, we must proceed to study the work of two agents, glacier ice
and the wind, which coöperate with rivers in the denudation of the
land.

   [Illustration: Fig. 87. Section of Undifferentiated Drift near
      Chicago]




CHAPTER V

THE WORK OF GLACIERS


=The drift.= The surface of northeastern North America, as far south
as the Ohio and Missouri rivers, is generally covered by the drift,--a
formation which is quite unlike any which we have so far studied. A
section of it, such as that illustrated in Figure 87, shows that for
the most part it is unstratified, consisting of clay, sand, pebbles,
and even large bowlders, all mingled pell-mell together. The agent
which laid the drift is one which can carry a load of material of all
sizes, from the largest bowlder to the finest clay, and deposit it
without sorting.

   [Illustration: Fig. 88. Characteristic Pebbles from the Drift

   No. 1 has six facets; No. 4, originally a rounded river
   pebble, has been nibbled down to one flat face; Nos. 3
   and 5 are battered subangular fragments on one side only]

The stones of the drift are of many kinds. The region from which it
was gathered may well have been large in order to supply these many
different varieties of rocks. Pebbles and bowlders have been left far
from their original homes, as may be seen in southern Iowa, where the
drift contains nuggets of copper brought from the region about Lake
Superior. The agent which laid the drift is one able to gather its
load over a large area and carry it a long way.

   [Illustration: Fig. 89. Smoothed and Scored Rock Surface exposed
      to View by the Removal of Overlying Drift, Iowa]

The pebbles of the drift are unlike those rounded by running water or
by waves. They are marked with scratches. Some are angular, many have
had their edges blunted, while others have been ground flat and smooth
on one or more sides, like gems which have been faceted by being held
firmly against the lapidary's wheel (Fig. 88). In many places the
upper surface of the country rock beneath the drift has been swept
clean of residual clays and other waste. All rock rotten has been
planed away, and the ledges of sound rock to which the surface has
been cut down have been rubbed smooth and scratched with long,
straight, parallel lines (Fig. 89). The agent which laid the drift can
hold sand and pebbles firmly in its grasp and can grind them against
the rock beneath, thus planing it down and scoring it, while faceting
the pebbles also.

Neither water nor wind can do these things. Indeed, nothing like the
drift is being formed by any process now at work anywhere in the
eastern United States. To find the agent which has laid this extensive
formation we must go to a region of different climatic conditions.

   [Illustration: Fig. 90. Map of Greenland

   Glacier ice covers all but the areas shaded]

=The inland ice of Greenland.= Greenland is about fifteen hundred
miles long and nearly seven hundred miles in greatest width. With the
exception of a narrow fringe of mountainous coast land, it is
completely buried beneath a sheet of ice, in shape like a vast white
shield, whose convex surface rises to a height of nine thousand feet
above the sea. The few explorers who have crossed the ice cap found it
a trackless desert destitute of all life save such lowly forms as the
microscopic plant which produces the so-called "red snow." On the
smooth plain of the interior no rock waste relieves the snow's
dazzling whiteness; no streams of running water are seen; the silence
is broken only by howling storm winds and the rustle of the surface
snow which they drive before them. Sounding with long poles, explorers
find that below the powdery snow of the latest snowfall lie successive
layers of earlier snows, which grow more and more compact downward,
and at last have altered to impenetrable ice. The ice cap formed by
the accumulated snows of uncounted centuries may well be more than a
mile in depth. Ice thus formed by the compacting of snow is
distinguished when in motion as _glacier ice_.

   [Illustration: Fig. 91. Hypothetical Cross Section of Greenland]

The inland ice of Greenland moves. It flows with imperceptible
slowness under its own weight, like, a mass of some viscous or plastic
substance, such as pitch or molasses candy, in all directions outward
toward the sea. Near the edge it has so thinned that mountain peaks
are laid bare, these islands in the sea of ice being known as
_nunataks_. Down the valleys of the coastal belt it drains in separate
streams of ice, or _glaciers_. The largest of these reach the sea at
the head of inlets, and are therefore called _tide glaciers_. Their
fronts stand so deep in sea water that there is visible seldom more
than three hundred feet of the wall of ice, which in many glaciers
must be two thousand and more feet high. From the sea walls of tide
glaciers great fragments break off and float away as icebergs. Thus
snows which fell in the interior of this northern land, perhaps many
thousands of years ago, are carried in the form of icebergs to melt at
last in the North Atlantic.

Greenland, then, is being modeled over the vast extent of its interior
not by streams of running water, as are regions in warm and humid
climates, nor by currents of air, as are deserts to a large extent,
but by a sheet of flowing ice. What the ice sheet is doing in the
interior we may infer from a study of the separate glaciers into which
it breaks at its edge.

=The smaller Greenland glaciers.= Many of the smaller glaciers of
Greenland do not reach the sea, but deploy on plains of sand and
gravel. The edges of these ice tongues are often as abrupt as if
sliced away with a knife (Fig. 92), and their structure is thus
readily seen. They are stratified, their layers representing in part
the successive snowfalls of the interior of the country. The upper
layers are commonly white and free from stones; but the lower layers,
to the height of a hundred feet or more, are dark with debris which is
being slowly carried on. So thickly studded with stones is the base of
the ice that it is sometimes difficult to distinguish it from the rock
waste which has been slowly dragged beneath the glacier or left about
its edges. The waste beneath and about the glacier is unsorted. The
stones are of many kinds, and numbers of them have been ground to flat
faces. Where the front of the ice has retreated the rock surface is
seen to be planed and scored in places by the stones frozen fast in
the sole of the glacier.

   [Illustration: Fig. 92. A Greenland Glacier]

We have now found in glacier ice an agent able to produce the drift of
North America. The ice sheet of Greenland is now doing what we have
seen was done in the recent past in our own land. It is carrying for
long distances rocks of many kinds gathered, we may infer, over a
large extent of country. It is laying down its load without assortment
in unstratified deposits. It grinds down and scores the rock over
which it moves, and in the process many of the pebbles of its load are
themselves also ground smooth and scratched. Since this work can be
done by no other agent, we must conclude that the northeastern part of
our own continent was covered in the recent past by glacier ice, as
Greenland is to-day.


Valley Glaciers

The work of glacier ice can be most conveniently studied in the
separate ice streams which creep down mountain valleys in many regions
such as Alaska, the western mountains of the United States and Canada,
the Himalayas, and the Alps. As the glaciers of the Alps have been
studied longer and more thoroughly than any others, we shall describe
them in some detail as examples of valley glaciers in all parts of the
world.

=Conditions of glacier formation.= The condition of the great
accumulation of snow to which glaciers are due--that more or less of
each winter's snow should be left over unmelted and unevaporated to
the next--is fully met in the Alps. There is abundant moisture brought
by the winds from neighboring seas. The currents of moist air driven
up the mountain slopes are cooled by their own expansion as they rise,
and the moisture which they contain is condensed at a temperature at
or below 32° F., and therefore is precipitated in the form of snow.
The summers are cool and their heat does not suffice to completely
melt the heavy snow of the preceding winter. On the Alps the _snow
line_--the lower limit of permanent snow--is drawn at about eight
thousand five hundred feet above sea level. Above the snow line on the
slopes and crests, where these are not too steep, the snow lies the
year round and gathers in valley heads to a depth of hundreds of feet.

   [Illustration: Fig. 93. Glaciers heading in Snow-filled
      Amphitheaters, the Alps]

   [Illustration: Fig. 94. Bergschrund of a Glacier in Colorado]

This is but a small fraction of the thickness to which snow would be
piled on the Alps were it not constantly being drained away. Below the
snow fields which mantle the heights the mountain valleys are occupied
by glaciers which extend as much as a vertical mile below the snow
line. The presence in the midst of forests and meadows and cultivated
fields of these tongues of ice, ever melting and yet from year to year
losing none of their bulk, proves that their loss is made good in the
only possible way. They are fed by snow fields above, whose surplus of
snow they drain away in the form of ice. The presence of glaciers
below the snow line is a clear proof that, rigid and motionless as
they appear, glaciers really are in constant motion down valley.

=The névé field.= The head of an Alpine valley occupied by a glacier
is commonly a broad amphitheater deeply filled with snow (Fig. 93).
Great peaks tower above it, and snowy slopes rise on either side on
the flanks of mountain spurs. From these heights fierce winds drift
the snows into the amphitheater, and avalanches pour in their torrents
of snow and waste. The snow of the amphitheater is like that of drifts
in late winter after many successive thaws and freezings. It is made
of hard grains and pellets and is called _névé_. Beneath the surface
of the névé field and at its outlet the granular névé has been
compacted to a mass of porous crystalline ice. Snow has been changed
to névé, and névé to glacial ice, both by pressure, which drives the
air from the interspaces of the snowflakes, and also by successive
meltings and freezings, much as a snowball is packed in the warm hand
and becomes frozen to a ball of ice.

   [Illustration: Fig. 95. Sea Wall of the Muir Glacier, Alaska]

=The bergschrund.= The névé is in slow motion. It breaks itself loose
from the thinner snows about it, too shallow to share its motion, and
from the rock rim which surrounds it, forming a deep fissure called
the bergschrund, sometimes a score and more feet wide (Fig. 94).

=Size of glaciers.= The ice streams of the Alps vary in size according
to the amount of precipitation and the area of the névé fields which
they drain. The largest of Alpine glaciers, the Aletsch, is nearly ten
miles long and has an average width of about a mile. The thickness of
some of the glaciers of the Alps is as much as a thousand feet. Giant
glaciers more than twice the length of the longest in the Alps occur
on the south slope of the Himalaya Mountains, which receive frequent
precipitations of snow from moist winds from the Indian Ocean. The
best known of the many immense glaciers of Alaska, the Muir, has an
area of about eight hundred square miles (Fig. 95).

   [Illustration: Fig. 96. Diagram showing Movement of Row of

   Stakes _a_, set in a direct line across the surface of a glacier;
   _b_, _c_, and _d_, successive later positions of the stakes]

   [Illustration: Fig. 97. Diagram showing Movement of Vertical
      Row of Stakes _a_, set on side of glacier]

=Glacier motion.= The motion of the glaciers of the Alps seldom
exceeds one or two feet a day. Large glaciers, because of the enormous
pressure of their weight and because of less marginal resistance, move
faster than small ones. The Muir advances at the rate of seven feet a
day, and some of the larger tide glaciers of Greenland are reported to
move at the exceptional rate of fifty feet and more in the same time.
Glaciers move faster by day than by night, and in summer than in
winter. Other laws of glacier motion may be discovered by a study of
Figures 96 and 97. It is important to remember that glaciers do not
slide bodily over their beds, but urged by gravity move slowly down
valley in somewhat the same way as would a stream of thick mud.
Although small pieces of ice are brittle, the large mass of granular
ice which composes a glacier acts as a viscous substance.

   [Illustration: Fig. 98. Crevasses of a Glacier, Canada]

=Crevasses.= Slight changes of slope in the glacier bed, and the
different rates of motion in different parts, produce tensions under
which the ice cracks and opens in great fissures called crevasses. At
an abrupt descent in the bed the ice is shattered into great
fragments, which unite again below the icefall. Crevasses are opened
on lines at right angles to the direction of the tension. _Transverse
crevasses_ are due to a convexity in the bed which stretches the ice
lengthwise (Fig. 99). _Marginal crevasses_ are directed upstream and
inwards; _radial crevasses_ are found where the ice stream deploys
from some narrow valley and spreads upon some more open space. What is
the direction of the tension which causes each and to what is it due?
(Figs. 100 and 101).

   [Illustration: Fig. 99. Longitudinal Section of a Portion of a
      Glacier, showing Traverse Crevasses]

   [Illustration: Fig. 100. Map view of Marginal Crevasses]

   [Illustration: Fig. 101. The Rhone Glacier, showing Radial
      Crevasses, the Alps]

   [Illustration: Fig. 102. Map View of the Junction of Two
      Branches of a Glacier

   The moraines are represented by broken lines]

=Lateral and medial moraines.= The surface of a glacier is striped
lengthwise by long dark bands of rock debris. Those in the center are
called the medial moraines. The one on either margin is a lateral
moraine, and is clearly formed of waste which has fallen on the edge
of the ice from the valley slopes. A medial moraine cannot be formed
in this way, since no rock fragments can fall so far out from the
sides. But following it up the glacial stream, one finds that a medial
moraine takes its beginning at the junction of the glacier and some
tributary and is formed by the union of their two adjacent lateral
moraines (Fig. 102). Each branch thus adds a medial moraine, and by
counting the number of medial moraines of a trunk stream one may learn
of how many branches it is composed.

   [Illustration: Fig. 103. Cross Section of a Glacier showing
      Lateral Moraines

   _l_, _l_, and Medial Moraines _m_, _m_]

Surface moraines appear in the lower course of the glacier as ridges,
which may reach the exceptional height of one hundred feet. The bulk
of such a ridge is ice. It has been protected from the sun by the
veneer of moraine stuff; while the glacier surface on either side has
melted down at least the distance of the height of the ridge. In
summer the lowering of the glacial surface by melting goes on rapidly.
In Swiss glaciers it has been estimated that the average lowering of
the surface by melting and evaporation amounts to ten feet a year. As
a moraine ridge grows higher and more steep by the lowering of the
surface of the surrounding ice, the stones of its cover tend to slip
down its sides. Thus moraines broaden, until near the terminus of a
glacier they may coalesce in a wide field of stony waste.

   [Illustration: Fig. 104. Glacier with Medial Moraines, the Alps

   Is the ice moving from or towards the observer?]

=Englacial drift.= This name is applied to whatever debris is carried
within the glacier. It consists of rock waste fallen on the névé and
there buried by accumulations of snow, and of that engulfed in the
glacier where crevasses have opened beneath a surface moraine. As the
surface of the glacier is lowered by melting, more or less englacial
drift is brought again to open air, and near the terminus it may help
to bury the ice from view beneath a sheet of debris.

=The ground moraine.= The drift dragged along at the glacier's base
and lodged beneath it is known as the ground moraine. Part of the
material of it has fallen down deep crevasses and part has been torn
and worn from the glacier's bed and banks. While the stones of the
surface moraines remain as angular as when they lodged on the ice,
many of those of the ground moraine have been blunted on the edges and
faceted and scratched by being ground against one another and the
rocky bed.

In glaciers such as those of Greenland, whose basal layers are well
loaded with drift and whose surface layers are nearly clean, different
layers have different rates of motion, according to the amount of
drift with which they are clogged. One layer glides over another, and
the stones inset in each are ground and smoothed and scratched.
Usually the sides of glaciated pebbles are more worn than the ends,
and the scratches upon them run with the longer axis of the stone.
Why?

=The terminal moraine.= As a glacier is in constant motion, it brings
to its end all of its load except such parts of the ground moraine as
may find permanent lodgment beneath the ice. Where the glacier front
remains for some time at one place, there is formed an accumulation of
drift known as the terminal moraine. In valley glaciers it is shaped
by the ice front to a crescent whose convex side is downstream. Some
of the pebbles of the terminal moraine are angular, and some are
faceted and scored, the latter having come by the hard road of the
ground moraine. The material of the dump is for the most part
unsorted, though the water of the melting ice may find opportunity to
leave patches of stratified sands and gravels in the midst of the
unstratified mass of drift, and the finer material is in places washed
away.

   [Illustration: Fig. 105. Terminal Moraine of a Glacier in Montana

   The ice has melted back from the morainic ridge on the left and
   is building another on the right. The hollow between the ridges
   is occupied by a lakelet.]

=Glacier drainage.= The terminal moraine is commonly breached by a
considerable stream, which issues from beneath the ice by a tunnel
whose portal has been enlarged to a beautiful archway by melting
in the sun and the warm air (Fig. 107). The stream is gray with
silt and loaded with sand and gravel washed from the ground
moraine. "Glacier milk" the Swiss call this muddy water, the gray
color of whose silt proves it rock flour freshly ground by the ice
from the unoxidized sound rock of its bed, the mud of streams
being yellowish when it is washed from the oxidized mantle of
waste. Since glacial streams are well loaded with waste due to
vigorous ice erosion, the valley in front of the glacier is
commonly aggraded to a broad, flat floor. These outwash deposits
are known as _valley drift_.

   [Illustration: Fig. 106. Heavy Moraine about the Terminus of a
      Glacier in the Rocky Mountains of Canada

   Account for the fact that the morainic ridge rises considerably
   above the surface of the ice]

The sand brought out by streams from beneath a glacier differs from
river sand in that it consists of freshly broken angular grains. Why?

The stream derives its water chiefly from the surface melting of the
glacier. As the ice is touched by the rays of the morning sun in
summer, water gathers in pools, and rills trickle and unite in
brooklets which melt and cut shallow channels in the blue ice. The
course of these streams is short. Soon they plunge into deep wells cut
by their whirling waters where some crevasse has begun to open across
their path. These wells lead into chambers and tunnels by which sooner
or later their waters find way to the rock floor of the valley and
there unite in a subglacial stream.

   [Illustration: Fig. 107. Subglacial Stream Issuing from Tunnel
      in the Ice, Norway]

=The lower limit of glaciers.= The glaciers of a region do not by any
means end at a uniform height above sea level. Each terminates where
its supply is balanced by melting. Those therefore which are fed by
the largest and deepest névés and those also which are best protected
from the sun by a northward exposure or by the depth of their
inclosing valleys flow to lower levels than those whose supply is less
and whose exposure to the sun is greater.

A series of cold, moist years, with an abundant snowfall, causes
glaciers to thicken and advance; a series of warm, dry years causes
them to wither and melt back. The variation in glaciers is now
carefully observed in many parts of the world. The Muir glacier has
retreated two miles in twenty years. The glaciers of the Swiss Alps
are now for the most part melting back, although a well-known glacier
of the eastern Alps, the Vernagt, advanced five hundred feet in the
year 1900, and was then plowing up its terminal moraine.

How soon would you expect a glacier to advance after its névé fields
have been swollen with unusually heavy snows, as compared with the
time needed for the flood of a large river to reach its mouth after
heavy rains upon its headwaters?

   [Illustration: Fig. 108. A Glacier Table]

On the surface of glaciers in summer time one may often see large
stones supported by pillars of ice several feet in height (Fig. 108).
These "glacier tables" commonly slope more or less strongly to the
south, and thus may be used to indicate roughly the points of the
compass. Can you explain their formation and the direction of their
slope? On the other hand, a small and thin stone, or a patch of dust,
lying on the ice, tends to sink a few inches into it. Why?

In what respects is a valley glacier like a mountain stream which
flows out upon desert plains?

Two confluent glaciers do not mingle their currents as do two
confluent rivers. What characteristics of surface moraines prove this
fact?

What effect would you expect the laws of glacier motion to have on the
slant of the sides of transverse crevasses?

   [Illustration: Fig. 109. Map of Malaspina Glacier, Alaska]

A trunk glacier has four medial moraines. Of how many tributaries is
it composed? Illustrate by diagram.

State all the evidences which you have found that glaciers move.

If a glacier melts back with occasional pauses up a valley, what
records are left of its retreat?

   [Illustration: Fig. 110. Outwash Plain, the Delta of the Yahtse
      River, Alaska]


Piedmont Glaciers

=The Malaspina glacier.= Piedmont (foot of the mountain) glaciers are,
as the name implies, ice fields formed at the foot of mountains by the
confluence of valley glaciers. The Malaspina glacier of Alaska, the
typical glacier of this kind, is seventy miles wide and stretches for
thirty miles from the foot of the Mount Saint Elias range to the shore
of the Pacific Ocean. The valley glaciers which unite and spread to
form this lake of ice lie above the snow line and their moraines are
concealed beneath névé. The central area of the Malaspina is also
free from debris; but on the outer edge large quantities of englacial
drift are exposed by surface melting and form a belt of morainic waste
a few feet thick and several miles wide, covered in part with a
luxuriant forest, beneath which the ice is in places one thousand feet
in depth. The glacier here is practically stagnant, and lakes a few
hundred yards across, which could not exist were the ice in motion and
broken with crevasses, gather on their beds sorted waste from the
moraine. The streams which drain the glacier have cut their courses in
englacial and subglacial tunnels; none flow for any distance on the
surface. The largest, the Yahtse River, issues from a high archway in
the ice,--a muddy torrent one hundred feet wide and twenty feet deep,
loaded with sand and stones which it deposits in a broad outwash plain
(Fig. 110). Where the ice has retreated from the sea there is left a
hummocky drift sheet with hollows filled with lakelets. These deposits
help to explain similar hummocky regions of drift and similar plains
of coarse, water-laid material often found in the drift-covered area
of the northeastern United States.


The Geological Work Of Glacier Ice

The sluggish glacier must do its work in a different way from the
agile river. The mountain stream is swift and small, and its channel
occupies but a small portion of the valley. The glacier is slow and
big; its rate of motion may be less than a millionth of that of
running water over the same declivity, and its bulk is proportionately
large and fills the valley to great depth. Moreover, glacier ice is a
solid body plastic under slowly applied stresses, while the water of
rivers is a nimble fluid.

=Transportation.= Valley glaciers differ from rivers as carriers in
that they float the major part of their load upon their surface,
transporting the heaviest bowlder as easily as a grain of sand; while
streams push and roll much of their load along their beds, and their
power of transporting waste depends solely upon their velocity. The
amount of the surface load of glaciers is limited only by the amount
of waste received from the mountain slopes above them. The moving
floor of ice stretched high across a valley sweeps along as lateral
moraines much of the waste which a mountain stream would let
accumulate in talus and alluvial cones.

While a valley glacier carries much of its load on top, an ice sheet,
such as that of Greenland, is free from surface debris, except where
moraines trail away from some nunatak. If at its edge it breaks into
separate glaciers which drain down mountain valleys, these tongues of
ice will carry the selvages of waste common to valley glaciers. Both
ice sheets and valley glaciers drag on large quantities of rock waste
in their ground moraines.

Stones transported by glaciers are sometimes called erratics. Such are
the bowlders of the drift of our northern states. Erratics may be set
down in an insecure position on the melting of the ice.

=Deposit.= Little need be added here to what has already been said of
ground and terminal moraines. All strictly glacial deposits are
unstratified. The load laid down at the end of a glacier in the
terminal moraine is loose in texture, while the drift lodged beneath
the glacier as ground moraine is often an extremely dense, stony clay,
having been compacted under the pressure of the overriding ice.

=Erosion.= A glacier erodes its bed and banks in two ways,--by
abrasion and by plucking.

The rock bed over which a glacier has moved is seen in places to have
been abraded, or ground away, to smooth surfaces which are marked by
long, straight, parallel scorings aligned with the line of movement of
the ice and varying in size from hair lines and coarse scratches to
exceptional furrows several feet deep. Clearly this work has been
accomplished by means of the sharp sand, the pebbles, and the larger
stones with which the base of the glacier is inset, and which it holds
in a firm grasp as running water cannot. Hard and fine-grained rocks,
such as granite and quartzite, are often not only ground down to a
smooth surface but are also highly polished by means of fine rock
flour worn from the glacier bed.

In other places the bed of the glacier is rough and torn. The rocks
have been disrupted and their fragments have been carried away,--a
process known as _plucking_. Moving under immense pressure the ice
shatters the rock, breaks off projections, presses into crevices and
wedges the rocks apart, dislodges the blocks into which the rock is
divided by joints and bedding planes, and freezing fast to the
fragments drags them on. In this work the freezing and thawing of
subglacial waters in any cracks and crevices of the rock no doubt play
an important part. Plucking occurs especially where the bed rock is
weak because of close jointing. The product of plucking is bowlders,
while the product of abrasion is fine rock flour and sand.

Is the ground moraine of Figure 87 due chiefly to abrasion or to
plucking?

   [Illustration: Fig. 111. Roches Moutonnés, Bronx Park, New York]

=Roches moutonnées and rounded hills.= The prominences left between
the hollows due to plucking are commonly ground down and rounded on
the stoss side,--the side from which the ice advances,--and sometimes
on the opposite, the lee side, as well. In this way the bed rock often
comes to have a billowy surface known as roches moutonnées (sheep
rocks). Hills overridden by an ice sheet often have similarly rounded
contours on the stoss side, while on the lee side they may be craggy,
either because of plucking or because here they have been less worn
from their initial profile (Fig. 112).

=The direction of glacier movement.= The direction of the flow of
vanished glaciers and ice sheets is recorded both in the differences
just mentioned in the profiles of overridden hills and also in the
minute details of the glacier trail.

Flint nodules or other small prominences in the bed rock are found
more worn on the stoss than on the lee side, where indeed they may
have a low cone of rock protected by them from abrasion. Cavities, on
the other hand, have their edges worn on the lee side and left sharp
upon the stoss.

Surfaces worn and torn in the ways which we have mentioned are said to
be glaciated. But it must not be supposed that a glacier everywhere
glaciates its bed. Although in places it acts as a rasp or as a pick,
in others, and especially where its pressure is least, as near the
terminus, it moves over its bed in the manner of a sled. Instances are
known where glaciers have advanced over deposits of sand and gravel
without disturbing them to any notable degree. Like a river, a glacier
does not everywhere erode. In places it leaves its bed undisturbed and
in places aggrades it by deposits of the ground moraine.

   [Illustration: Fig. 112. A Glaciated Hill, Norway. Sharp
      Weathered Mountain Peaks in the Distance]

=Cirques.= Valley glaciers commonly head as we have seen, in broad
amphitheaters deeply filled with snow and ice. On mountains now
destitute of glaciers, but whose glaciation shows that they have
supported glaciers in the past, there are found similar crescentic
hollows with high, precipitous walls and glaciated floors. Their
floors are often basined and hold lakelets whose deep and quiet waters
reflect the sheltering ramparts of rugged rock which tower far above
them. Such mountain hollows are termed _cirques_. As a powerful spring
wears back a recess in the valley side where it discharges, so the
fountain head of a glacier gradually wears back a cirque. In its slow
movement the névé field broadly scours its bed to a flat or basined
floor. Meanwhile the sides of the valley head are steepened and driven
back to precipitous walls. For in winter the crevasse of the
bergschrund which surrounds the névé field is filled with snow and the
névé is frozen fast to the rocky sides of the valley. In early summer
the névé tears itself free, dislodging and removing any loosened
blocks, and the open fissure of the bergschrund allows frost and other
agencies of weathering to attack the unprotected rock. As cirques are
thus formed and enlarged the peaks beneath which they lie are
sharpened, and the mountain crests are scalloped and cut back from
either side to knife-edged ridges (Figs. 113 and 93).

   [Illustration: Fig. 113. Cirques, Sierra Nevada Mountains]

In the western mountains of the United States many cirques, now empty
of névé and glacier ice, and known locally as "basins," testify to the
fact that in recent times the snow line stood beneath the levels of
their floors, and thus far below its present altitude.

   [Illustration: Fig. 114. A Glacier Trough, Montana]

=Glacier troughs.= The channel worn to accommodate the big and clumsy
glacier differs markedly from the river valley cut as with a saw by
the narrow and flexible stream and widened by the weather and the wash
of rains. The valley glacier may easily be from one thousand to three
thousand feet deep and from one to three miles wide. Such a ponderous
bulk of slowly moving ice does not readily adapt itself to sharp turns
and a narrow bed. By scouring and plucking all resisting edges it
develops a fitting channel with a wide, flat floor, and steep, smooth
sides, above which are seen the weathered slopes of stream-worn
mountain valleys. Since the trunk glacier requires a deeper channel
than do its branches, the bed of a branch glacier enters the main
trough at some distance above the floor of the latter, although the
surface of the two ice streams may be accordant. Glacier troughs can
be studied best where large glaciers have recently melted completely
away, as is the case in many valleys of the mountains of the western
United States and of central and northern Europe (Fig. 114). The
typical glacier trough, as shown in such examples, is U-shaped, with a
broad, flat floor, and high, steep walls. Its walls are little broken
by projecting spurs and lateral ravines. It is as if a V-valley cut by
a river had afterwards been gouged deeper with a gigantic chisel,
widening the floor to the width of the chisel blade, cutting back the
spurs, and smoothing and steepening the sides. A river valley could
only be as wide-floored as this after it had long been worn down to
grade.

   [Illustration: Fig. 115 Lynn Canal, Alaska, a Fjord]

But the floor of a glacier trough may not be graded; it is often
interrupted by irregular steps perhaps hundreds and even a thousand
feet in height, over which the stream that now drains the valley
tumbles in waterfalls. Reaches between the steps are often basined.
Lakelets may occupy hollows excavated in solid rock, and other lakes
may be held behind terminal moraines left as dams across the valley at
pauses in the retreat of the glacier.

=Fjords= are glacier troughs now occupied in part or wholly by the
sea, either because they were excavated by a tide glacier to their
present depth below sea level, or because of a submergence of the
land. Their characteristic form is that of a long, deep, narrow bay
with steep rock walls and basined floor (Fig. 115). Fjords are found
only in regions which have suffered glaciation, such as Norway and
Alaska.

   [Illustration: Fig. 116. _A_, V-River Valley, with Valley of
      Tributary joining it a Accordant Level; _B_, the Same changed
      after Long Glaciation to a Trough with Hanging Valley]

=Hanging valleys.= These are lateral valleys which open on their main
valley some distance above its floor. They are conspicuous features of
glacier troughs from which the ice has vanished; for the trunk glacier
in widening and deepening its channel cut its bed below the bottoms of
the lateral valleys (Fig. 116).

Since the mouths of hanging valleys are suspended on the walls of the
glacier trough, their streams are compelled to plunge down its steep,
high sides in waterfalls. Some of the loftiest and most beautiful
waterfalls of the world leap from hanging valleys,--among them the
celebrated Staubbach of the Lauterbrunnen valley of Switzerland, and
those of the fjords of Norway and Alaska (Fig. 117).

   [Illustration: Fig. 117. Hanging Valley on the Wall of a Fjord,
      Norway]

Hanging valleys are found also in river gorges where the smaller
tributaries have not been able to keep pace with a strong master
stream in cutting down their beds. In this case, however, they are a
mark of extreme youth; for, as the trunk stream approaches grade and
its velocity and power to erode its bed decrease, the side streams
soon cut back their falls and wear their beds at their mouths to a
common level with that of the main river. The Grand Canyon of the
Colorado must be reckoned a young valley. At its base it narrows to
scarcely more than the width of the river, and yet its tributaries,
except the very smallest, enter it at a common level.

Why could not a wide-floored valley, such as a glacier trough, with
hanging valleys opening upon it, be produced in the normal development
of a river valley?

=The troughs of young and of mature glaciers.= The features of a
glacier trough depend much on the length of time the preexisting
valley was occupied with ice. During the infancy of a glacier, we may
believe, the spurs of the valley which it fills are but little blunted
and its bed is but little broken by steps. In youth the glacier
develops icefalls, as a river in youth develops waterfalls, and its
bed becomes terraced with great stairs. The mature glacier, like the
mature river, has effaced its falls and smoothed its bed to grade. It
has also worn back the projecting spurs of its valley, making itself a
wide channel with smooth sides. The bed of a mature glacier may form a
long basin, since it abrades most in its upper and middle course,
where its weight and motion are the greatest. Near the terminus, where
weight and motion are the least, it erodes least, and may instead
deposit a sheet of ground moraine, much as a river builds a flood
plain in the same part of its course as it approaches maturity. The
bed of a mature glacier thus tends to take the form of a long,
relatively narrow basin, across whose lower end may be stretched the
dam of the terminal moraine. On the disappearance of the ice the basin
is rilled with a long, narrow lake, such as Lake Chelan in Washington
and many of the lakes in the Highlands of Scotland.

Piedmont glaciers apparently erode but little. Beneath their lake-like
expanse of sluggish or stagnant ice a broad sheet of ground moraine is
probably being deposited.

Cirques and glaciated valleys rapidly lose their characteristic forms
after the ice has withdrawn. The weather destroys all smoothed,
polished, and scored surfaces which are not protected beneath glacial
deposits. The over-steepened sides of the trough are graded by
landslips, by talus slopes, and by alluvial cones. Morainic heaps of
drift are dissected and carried away. Hanging valleys and the
irregular bed of the trough are both worn down to grade by the streams
which now occupy them. The length of time since the retreat of the ice
from a mountain valley may thus be estimated by the degree to which
the destruction of the characteristic features of the glacier trough
has been carried.

In Figure 104 what characteristics of a glacier trough do you notice?
What inference do you draw as to the former thickness of the glacier?

Name all the evidences you would expect to find to prove the fact that
in the recent geological past the valleys of the Alps contained far
larger glaciers than at present, and that on the north of the Alps the
ice streams united in a piedmont glacier which extended across the
plains of Switzerland to the sides of the Jura Mountains.

=The relative importance of glaciers and of rivers.= Powerful as
glaciers are, and marked as are the land forms which they produce, it
is easy to exaggerate their geological importance as compared with
rivers. Under present climatic conditions they are confined to lofty
mountains or polar lands. Polar ice sheets are permanent only so long
as the lands remain on which they rest. Mountain glaciers can stay
only the brief time during which the ranges continue young and high.
As lofty mountains, such as the Selkirks and the Alps, are lowered by
frost and glacier ice, the snowfall will decrease, the line of
permanent snow will rise, and as the mountain hollows in which snow
may gather are worn beneath the snow line, the glaciers must
disappear. Under present climatic conditions the work of glaciers is
therefore both local and of short duration.

   [Illustration: Fig. 118. Longitudinal Section of a Tide Glacier
        occupying a Fjord and discharging Icebergs
      Dotted Line, sea level]

Even the glacial epoch, during which vast ice sheets deposited drift
over northeastern North America, must have been brief as well as
recent, for many lofty mountains, such as the Rockies and the Alps,
still bear the marks of great glaciers which then filled their
valleys. Had the glacial epoch been long, as the earth counts time,
these mountains would have been worn low by ice; had the epoch been
remote, the marks of glaciation would already have been largely
destroyed by other agencies.

On the other hand, rivers are well-nigh universally at work over the
land surfaces of the globe, and ever since the dry land appeared they
have been constantly engaged in leveling the continents and in
delivering to the seas the waste which there is built into the
stratified rocks.

=Icebergs.= Tide glaciers, such as those of Greenland and Alaska, are
able to excavate their beds to a considerable distance below sea
level. From their fronts the buoyancy of sea water raises and breaks
away great masses of ice which float out to sea as icebergs. Only
about one seventh of a mass of glacier ice floats above the surface,
and a berg three hundred feet high may be estimated to have been
detached from a glacier not less than two thousand feet thick where it
met the sea.

Icebergs transport on their long journeys whatever drift they may have
carried when part of the glacier, and scatter it, as they melt, over
the ocean floor. In this way pebbles torn by the inland ice from the
rocks of the interior of Greenland and glaciated during their carriage
in the ground moraine are dropped at last among the oozes of the
bottom of the North Atlantic.




CHAPTER VI

THE WORK OF THE WIND


   [Illustration: Fig. 119. A sandy Region in a Desert, the Sahara]

We are now to study the geological work of the currents of the
atmosphere, and to learn how they erode, and transport and deposit
waste as they sweep over the land. Illustrations of the wind's work
are at hand in dry weather on any windy day.

Clouds of dust are raised from the street and driven along by the
gale. Here the roadway is swept bare; and there, in sheltered places,
the dust settles in little windrows. The erosive power of waste-laden
currents of air is suggested as the sharp grains of flying sand sting
one's face or clatter against the window. In the country one sometimes
sees the dust whirled in clouds from dry, plowed fields in spring and
left in the lee of fences in small drifts resembling in form those of
snow in winter.

=The essential conditions= for the wind's conspicuous work are
illustrated in these simple examples; they are aridity and the absence
of vegetation. In humid climates these conditions are only rarely and
locally met; for the most part a thick growth of vegetation protects
the moist soil from the wind with a cover of leaves and stems and a
mattress of interlacing roots. But in arid regions either vegetation
is wholly lacking, or scant growths are found huddled in detached
clumps, leaving interspaces of unprotected ground (Fig. 119). Here,
too, the mantle of waste, which is formed chiefly under the action of
temperature changes, remains dry and loose for long periods. Little or
no moisture is present to cause its particles to cohere, and they are
therefore readily lifted and drifted by the wind.


Transportation By The Wind

In the desert the finer waste is continually swept to and fro by the
ever-shifting wind. Even in quiet weather the air heated by contact
with the hot sands rises in whirls, and the dust is lifted in stately
columns, sometimes as much as one thousand feet in height, which march
slowly across the plain. In storms the sand is driven along the ground
in a continuous sheet, while the air is tilled with dust. Explorers
tell of sand storms in the deserts of central Asia and Africa, in
which the air grows murky and suffocating. Even at midday it may
become dark as night, and nothing can be heard except the roar of the
blast and the whir of myriads of grains of sand as they fly past the
ear.

Sand storms are by no means uncommon in the arid regions of the
western United States. In a recent year, six were reported from Yuma,
Arizona. Trains on transcontinental railways are occasionally
blockaded by drifting sand, and the dust sifts into closed passenger
coaches, covering the seats and floors. After such a storm thirteen
car loads of sand were removed from the platform of a station on a
western railway.

=Dust falls.= Dust launched by upward-whirling winds on the swift
currents of the upper air is often blown for hundreds of miles beyond
the arid region from which it was taken. Dust falls from western
storms are not unknown even as far east as the Great Lakes. In 1896 a
"black snow" fell in Chicago, and in another dust storm in the same
decade the amount of dust carried in the air over Rock Island, Ill.,
was estimated at more than one thousand tons to the cubic mile.

   [Illustration: Fig. 120. A Tract of Rocky Desert, Arabia
      By what process have these rocks been broken up?
      Why is finer waste here absent?]

In March, 1901, a cyclonic storm carried vast quantities of dust from
the Sahara northward across the Mediterranean to fall over southern
and central Europe. On March 8th dust storms raged in southern
Algeria; two days later the dust fell in Italy; and on the 11th it had
reached central Germany and Denmark. It is estimated that in these few
days one million eight hundred thousand tons of waste were carried
from northern Africa and deposited on European soil.

We may see from these examples the importance of the wind as an agent
of transportation, and how vast in the aggregate are the loads which
it carries. There are striking differences between air and water as
carriers of waste. Rivers flow in fixed and narrow channels to
definite goals. The channelless streams of the air sweep across broad
areas, and, shifting about continually, carry their loads back and
forth, now in one direction and now in another.


Wind Deposits

The mantle of waste of deserts is rapidly sorted by the wind. The
coarser rubbish, too heavy to be lifted into the air, is left to strew
wide tracts with residual gravels (Fig. 120). The sand derived from
the disintegration of desert rocks gathers in vast fields. About one
eighth of the surface of the Sahara is said to be thus covered with
drifting sand. In desert mountains, as those of Sinai, it lies like
fields of snow in the high valleys below the sharp peaks. On more
level tracts it accumulates in seas of sand, sometimes, as in the
deserts of Arabia, two hundred and more feet deep.

   [Illustration: Fig. 121. Longitudinal Dunes, Desert of
        Northwestern India
      Scale, 1 in = 3 miles]

=Dunes.= The sand thus accumulated by the wind is heaped in wavelike
hills called dunes. In the desert of northwestern India, where the
prevalent wind is of great strength, the sand is laid in longitudinal
dunes, i.e. in stripes running parallel with the direction of the
wind; but commonly dunes lie, like ripple marks, transverse to the
wind current. On the windward side they show a long, gentle slope, up
which grains of sand can readily be moved; while to the lee their
slope is frequently as great as the angle of repose (Fig. 122). Dunes
whose sands are not fixed by vegetation travel slowly with the wind;
for their material is ever shifted forward as the grains are driven up
the windward slope and, falling over the crest, are deposited in
slanting layers in the quiet of the lee.

   [Illustration: Fig. 122. A Transverse Dune, Seven Mile Beach,
        New Jersey
      Account for the difference of slope in the two sides of the
      dune. Is the dune marching? In what direction? With what
      effect? Do the ridges of the ripple marks upon the dune extend
      along it or athwart it? Why?]

Like river deposits, wind-blown sands are stratified, since they are
laid by currents of air varying in intensity, and therefore in
transporting power, which carry now finer and now coarser materials
and lay them down where their velocity is checked (Fig. 123). Since
the wind varies in direction, the strata dip in various directions.
They also dip at various angles, according to the inclination of the
surface on which they were laid.

   [Illustration: Fig. 123. Stratified Wind-Blown Sands, Bermuda
        Islands
      These islands are made wholly of limestone, the product of
      reef-building corals, and of lime from the sea water. The
      limestone sand of the beaches has been blown up into great
      dunes, some more than two hundred feet in height. Much of the
      loose dune sand has been changed to firm rock by percolating
      waters, which have dissolved some of the limestone and
      deposited it again as a cement between the grains]

Dunes occur not only in arid regions, but also wherever loose sand
lies unprotected by vegetation from the wind. From the beaches of sea
and lake shores the wind drives inland the surface sand left dry
between tides and after storms, piling it in dunes which may invade
forests and fields and bury villages beneath their slowly advancing
waves. On flood plains during summer droughts river deposits are often
worked over by the wind; the sand is heaped in hummocks and much of
the fine silt is caught and held by the forests and grassy fields of
the bordering hills.

   [Illustration: Fig. 124. Cross Section of Transverse Dune after
      Reversal of Wind

   Redraw diagram, showing by dotted line the original outline of
   the dune]

The sand of shore dunes differs little in composition and the shape of
its grains from that of the beach from which it was derived. But in
deserts, by the long wear of grain on grain as they are blown hither
and thither by the wind, all soft minerals are ground to powder and
the sand comes to consist almost wholly of smooth round grams of hard
quartz.

   [Illustration: Fig. 125. Dune Sands, Shore of Lake Michigan

   Account for the dead forest, for its leaning tree trunks. Is
   the lake shore to the right or left? What has been the history
   of the landscape?]

Some marine sandstones, such as the St. Peter sandstone of the upper
Mississippi valley, are composed so entirely of polished spherules of
quartz that it has been believed by some that their grains were long
blown about in ancient deserts before they were deposited in the sea.

   [Illustration: Fig. 126. Crescentic Sand Dunes, Valley of the
      Columbia River

   Did the wind which shaped them blow from the left or from the
   right?]

=Dust deposits.= As desert sands are composed almost wholly of quartz,
we may ask what has become of the softer minerals of which the rocks
whose disintegration has supplied the sand were in part, and often in
large part, composed. The softer minerals have been ground to powder,
and little by little the quartz sand also is worn by attrition to fine
dust. Yet dust deposits are scant and few in great deserts such as the
Sahara. The finer waste is blown beyond its limits and laid in
adjacent oceans, where it adds to the muds and oozes of their floors,
and on bordering steppes and forest lands, where it is bound fast by
vegetation and slowly accumulates in deposits of unstratified loose
yellow earth. The fine waste of the Sahara has been identified in
dredgings from the bottom of the Atlantic Ocean, taken hundreds of
miles from the coast of Africa.

=Loess.= In northern China an area as large as France is deeply
covered with a yellow pulverulent earth called loess (German, loose),
which many consider a dust deposit blown from the great Mongolian
desert lying to the west. Loess mantles the recently uplifted
mountains to the height of eight thousand feet and descends on the
plains nearly to sea level. Its texture and lack of stratification
give it a vertical cleavage; hence it stands in steep cliffs on the
sides of the deep and narrow trenches which have been cut in it by
streams.

On loess hillsides in China are thousands of villages whose eavelike
dwellings have been excavated in this soft, yet firm, dry loam. While
dust falls are common at the present time in this region, the loess is
now being rapidly denuded by streams, and its yellow silt gives name
to the muddy Hwang-ho (Yellow River), and to the Yellow Sea, whose
waters it discolors for scores of miles from shore.

Wind deposits both of dust and of sand may be expected to contain the
remains of land shells, bits of wood, and bones of land animals,
testifying to the fact that they were accumulated in open air and not
in the sea or in bodies of fresh water.


Wind Erosion

   [Illustration: Fig. 127. Wind-Carved Rocks, Arizona]

Sand-laden currents of air abrade and smooth and polish exposed rock
surfaces, acting in much the same way as does the jet of steam fed
with sharp sand, which is used in the manufacture of ground glass.
Indeed, in a single storm at Cape Cod a plate glass of a lighthouse
was so ground by flying sand that its transparency was destroyed and
its removal made necessary.

   [Illustration: Fig. 128. A Wind-Carved Pebble, Cape Cod]

Telegraph poles and wires whetted by wind-blown sands are destroyed
within a few years. In rocks of unequal resistance the harder parts
are left in relief, while the softer are etched away. Thus in the pass
of San Bernardino, Cal., through which strong winds stream from the
west, crystals of garnet are left projecting on delicate rock fingers
from the softer rock in which they were imbedded.

Wind-carved pebbles are characteristically planed, the facets meeting
along a summit ridge or at a point like that of a pyramid. We may
suppose that these facets were ground by prevalent winds from certain
directions, or that from time to time the stone was undermined and
rolled over as the sand beneath it was blown away on the windward
side, thus exposing fresh surfaces to the driving sand. Such
wind-carved pebbles are sometimes found in ancient rocks and may be
accepted as evidence that the sands of which the rocks are composed
were blown about by the wind.

=Deflation.= In the denudation of an arid region, wind erosion is
comparatively ineffective as compared with deflation (Latin, _de_,
from; _flare_, to blow),--a term by which is meant the constant
removal of waste by the wind, leaving the rocks bare to the continuous
attack of the weather. In moist climates denudation is continually
impeded by the mantle of waste and its cover of vegetation, and the
land surface can be lowered no faster than the waste is removed by
running water. Deep residual soils come to protect all regions of
moderate slope, concealing from view the rock structure, and the
various forms of the land are due more to the agencies of erosion and
transportation than to differences in the resistance of the underlying
rocks.

   [Illustration: Fig. 129. Mesa Verde, Colorado

   In the distance on the left are high volcanic mountains. On the
   extreme right are seen outliers of strata which once covered
   the region of the mesa]

But in arid regions the mantle is rapidly removed, even from well-nigh
level plains and plateaus, by the sweep of the wind and the wash of
occasional rains. The geological structure of these regions of naked
rock can be read as far as the eye can see, and it is to this
structure that the forms of the land are there largely due. In a land
mass of horizontal strata, for example, any softer surface rocks wear
down to some underlying, resistant stratum, and this for a while forms
the surface of a level plateau (Fig. 129). The edges of the capping
layer, together with those of any softer layers beneath it, wear back
in steep cliffs, dissected by the valleys of wet-weather streams and
often swept bare to the base by the wind. As they are little protected
by talus, which commonly is removed about as fast as formed, these
escarpments and the walls of the valleys retreat indefinitely,
exposing some hard stratum beneath which forms the floor of a widening
terrace.

The high plateaus of northern Arizona and southern Utah (Fig. 130),
north of the Grand Canyon of the Colorado River, are composed of
stratified rocks more than ten thousand feet thick and of very gentle
inclination northward. From the broad plat form in which the canyon
has been cut rises a series of gigantic stairs, which are often more
than one thousand feet high and a score or more of miles in breadth.
The retreating escarpments, the cliffs of the mesas and buttes which
they have left behind as outliers, and the walls of the ravines are
carved into noble architectural forms--into cathedrals, pyramids,
amphitheaters, towers, arches, and colonnades--by the processes of
weathering aided by deflation. It is thus by the help of the action of
the wind that great plateaus in arid regions are dissected and at last
are smoothed away to waterless plains, either composed of naked rock,
or strewed with residual gravels, or covered with drifting residual
sand.

   [Illustration: Fig. 130. North-South Section, Eighty-Five Miles
      Long, across the Plateau North of the Grand Canyon of the
      Colorado River, Arizona, showing Retreating Escarpments

   _O_, outliers; _V_, canyon of the Colorado; _A-H_, rock systems
   from the Archean to the Tertiary; _P_, platform of the plateau
   from which the once overlying rocks have been stripped; dotted
   lines indicate probable former extension of the strata. How
   thick is the mass of strata which has been removed from over
   the platform? Has this work been accomplished while the
   Colorado River has been cutting its present canyon?]

The specific gravity of air is 1/823 that of water. How does this fact
affect the weight of the material which each can carry at the same
velocity?

If the rainfall should lessen in your own state to from five to ten
inches a year, what changes would take place in the vegetation of the
country? in the soil? in the streams? in the erosion of valleys? in
the agencies chiefly at work in denuding the land?

In what way can a wind-carved pebble be distinguished from a
river-worn pebble? from a glaciated pebble?




CHAPTER VII

THE SEA AND ITS SHORES


   [Illustration: Fig. 131. Sea Cliff and Rock Bench Cut in Chalk,
      Dover, England]

We have already seen that the ocean is the goal at which the waste of
the land arrives. The mantle of rock waste, creeping down slopes, is
washed to the sea by streams, together with the material which the
streams have worn from their beds and that dissolved by underground
waters. In arid regions the winds sweep waste either into bordering
oceans or into more humid regions where rivers take it up and carry it
on to the sea. Glaciers deliver the load of their moraines either
directly to the sea or leave it for streams to transport to the same
goal. All deposits made on the land, such as the flood plains of
rivers, the silts of lake beds, dune sands, and sheets of glacial
drift, mark but pauses in the process which is to bring all the
materials of the land now above sea level to rest upon the ocean bed.

But the sea is also at work along all its shores as an agent of
destruction, and we must first take up its work in erosion before we
consider how it transports and deposits the waste of the land.


Sea Erosion

=The sea cliff and the rock bench.= On many coasts the land fronts the
ocean in a line of cliffs (Fig. 131). To the edge of the cliffs there
lead down valleys and ridges, carved by running water, which, if
extended, would meet the water surface some way out from shore.
Evidently they are now abruptly cut short at the present shore line
because the land has been cut back.

   [Illustration: Fig. 132. Diagram of Sea Cliff _sc_, and Rock Bench _rb_

   The broken line indicates the former extent of the land.]

Along the foot of the cliff lies a gently shelving bench of rock, more
or less thickly veneered with sand and shingle. At low tide its inner
margin is laid bare, but at high tide it is covered wholly, and the
sea washes the base of the cliffs. A notch, of which the _sea cliff_
and the _rock bench_ are the two sides, has been cut along the shore
(Fig. 132).

=Waves.= The position of the rock bench, with its inner margin
slightly above low tide, shows that it has been cut by some agent
which acts like a horizontal saw set at about sea level. This agent is
clearly the surface agitation of the water; it is the wind-raised
wave.

As a wave comes up the shelving bench the crest topples forward and
the wave "breaks," striking a blow whose force is measured by the
momentum of all its tons of falling water (Fig. 133). On the coast of
Scotland the force of the blows struck by the waves of the heaviest
storms has sometimes exceeded three tons to the square foot. But even
a calm sea constantly chafes the shore. It heaves in gentle
undulations known as the ground swell, the result of storms perhaps a
thousand miles distant, and breaks on the shore in surf.

   [Illustration: Fig. 133. Breaking Wave, Lake Superior]

The blows of the waves are not struck with clear water only, else they
would have little effect on cliffs of solid rock. Storm waves arm
themselves with the sand and gravel, the cobbles, and even the large
bowlders which lie at the base of the cliff, and beat against it with
these hammers of stone.

Where a precipice descends sheer into deep water, waves swash up and
down the face of the rocks but cannot break and strike effective
blows. They therefore erode but little until the talus fallen from the
cliff is gradually built up beneath the sea to the level at which the
waves drag bottom upon it and break.

Compare the ways in which different agents abrade. The wind lightly
brushes sand and dust over exposed surfaces of rock. Running water
sweeps fragments of various sizes along its channels, holding them
with a loose hand. Glacial ice grinds the stones of its ground moraine
against the underlying rock with the pressure of its enormous weight.
The wave hurls fragments of rock against the sea cliff, bruising and
battering it by the blow. It also rasps the bench as it drags sand and
gravel to and fro upon it.

=Weathering of sea cliffs.= The sea cliff furnishes the weapons for
its own destruction. They are broken from it not only by the wave but
also by the weather. Indeed the sea cliff weathers more rapidly, as a
rule, than do rock ledges inland. It is abundantly wet with spray.
Along its base the ground water of the neighboring land finds its
natural outlet in springs which under mine it. Moreover, it is
unprotected by any shield of talus. Fragments of rock as they fall
from its face are battered to pieces by the waves and swept out to
sea. The cliff is thus left exposed to the attack of the weather, and
its retreat would be comparatively rapid for this reason alone.

   [Illustration: Fig. 134. Sea Caves, La Jolla, California

   Copyright, 1899, by the Detroit Photography Company]

Sea cliffs seldom overhang, but commonly, as in Figure 134, slope
seaward, showing that the upper portion has retreated at a more rapid
rate than has the base. Which do you infer is on the whole the more
destructive agent, weathering or the wave?

Draw a section of a sea cliff cut in well jointed rocks whose joints
dip toward the land. Draw a diagram of a sea cliff where the joints
dip toward the sea.

=Sea caves.= The wave does not merely batter the face of the cliff.
Like a skillful quarryman it inserts wedges in all natural fissures,
such as joints, and uses explosive forces. As a wave flaps against a
crevice it compresses the air within with the sudden stroke; as it
falls back the air as suddenly expands. On lighthouses heavily barred
doors have been burst outward by the explosive force of the air
within, as it was released from pressure when a partial vacuum was
formed by the refluence of the wave. Where a crevice is filled with
water the entire force of the blow of the wave is transmitted by
hydraulic pressure to the sides of the fissure. Thus storm waves
little by little pry and suck the rock loose, and in this way, and by
the blows which they strike with the stones of the beach, they quarry
out about a joint, or wherever the rock may be weak, a recess known as
a _sea cave_, provided that the rock above is coherent enough to form
a roof. Otherwise an open chasm results.

   [Illustration: Fig. 135. A Sea Arch, California

   Copyright, 1899, by the Detroit Photography Company]

=Blowholes and sea arches.= As a sea cave is drilled back into the
rock, it may encounter a joint or crevice opened to the surface by
percolating water. The shock of the waves soon enlarges this to a
blowhole, which one may find on the breezy upland, perhaps a hundred
yards and more back from the cliff's edge. In quiet weather the
blowhole is a deep well; in storm it plays a fountain as the waves
drive through the long tunnel below and spout their spray high in air
in successive jets. As the roof of the cave thus breaks down in the
rear, there may remain in front for a while a sea arch, similar to the
natural bridges of land caverns (Fig. 135).

   [Illustration: Fig. 136. Chasms worn by Waves, Coast of Scotland]

=Stacks and wave-cut islands.= As the sea drives its tunnels and open
drifts into the cliff, it breaks through behind the intervening
portions and leaves them isolated as stacks, much as monuments are
detached from inland escarpments by the weather; and as the sea cliff
retreats, these remnant masses may be left behind as rocky islets.
Thus the rock bench is often set with stacks, islets in all stages of
destruction, and sunken reefs,--all wrecks of the land testifying to
its retreat before the incessant attack of the waves.

   [Illustration: Fig. 137. A Stack, Scotland]

   [Illustration: Fig. 138. Wave-Cut Islands, Scotland

   How far did the land once extend?]

=Coves.= Where zones of soft or closely jointed rock outcrop along a
shore, or where minor water courses conic down to the sea and aid in
erosion, the shore is worn back in curved reëntrants called coves;
while the more resistant rocks on either hand are left projecting as
headlands (Fig. 139). After coves are cut back a short distance by the
waves, the headlands come to protect them, as with breakwaters, and
prevent their indefinite retreat. The shore takes a curve of
equilibrium, along which the hard rock of the exposed headland and the
weak rock of the protected cove wear back at an equal rate.

   [Illustration: Fig. 139. Coves formed in Softer Strata _S_, _S_;
     while the Harder Strata _H_, _H_, are left as Headlands]

=Rate of recession.= The rate at which a shore recedes depends on
several factors. In soft or incoherent rocks exposed to violent storms
the retreat is so rapid as to be easily measured. The coast of
Yorkshire, England, whose cliffs are cut in glacial drift, loses seven
feet a year on the average, and since the Norman conquest a strip a
mile wide, with farmsteads and villages and historic seaports, has
been devoured by the sea. The sandy south shore of Martha's Vineyard
wears back three feet a year. But hard rocks retreat so slowly that
their recession has seldom been measured by the records of history.

   [Illustration: Fig. 140. A Pebble Beach, Cape Ann, Massachusetts]


Shore Drift

=Bowlder and pebble beaches.= About as fast as formed the waste of the
sea cliff is swept both along the shore and out to sea. The road of
waste along shore is the _beach_. We may also define the beach as the
exposed edge of the sheet of sediment formed by the carriage of land
waste out to sea. At the foot of sea cliffs, where the waves are
pounding hardest, one commonly finds the rock bench strewn on its
inner margin with large stones, dislodged by the waves and by the
weather and somewhat worn on their corners and edges. From this
_bowlder beach_ the smaller fragments of waste from the cliff and the
fragments into which the bowlders are at last broken drift on to more
sheltered places and there accumulate in a _pebble beach_, made of
pebbles well rounded by the wear which they have suffered. Such
beaches form a mill whose raw material is constantly supplied by the
cliff. The breakers of storms set it in motion to a depth of several
feet, grinding the pebbles together with a clatter to be heard above
the roar of the surf. In such a rock crusher the life of a pebble is
short. Where ships have stranded on our Atlantic coast with cargoes of
hard-burned brick or of coal, a year of time and a drift of five miles
along the shore have proved enough to wear brick and coal to powder.
At no great distance from their source, therefore, pebble beaches give
place to beaches of sand, which occupy the more sheltered reaches of
the shore.

=Sand beaches.= The angular sand grains of various minerals into which
pebbles are broken by the waves are ground together under the beating
surf and rounded, and those of the softer minerals are crushed to
powder. The process, however, is a slow one, and if we study these
sand grains under a lens we may be surprised to see that, though their
corners and edges have been blunted, they are yet far from the
spherical form of the pebbles from which they were derived. The grains
are small, and in water they have lost about half their weight in
air; the blows which they strike one another are therefore weak.
Besides, each grain of sand of the wet beach is protected by a cushion
of water from the blows of its neighbors.

The shape and size of these grains and the relative proportion of
grains of the softer minerals which still remain give a rough measure
of the distance in space and time which they have traveled from their
source. The sand of many beaches, derived from the rocks of adjacent
cliffs or brought in by torrential streams from neighboring highlands,
is dark with grains of a number of minerals softer than quartz. The
white sand of other beaches, as those of the east coast of Florida, is
almost wholly composed of quartz grains; for in its long travel down
the Atlantic coast the weaker minerals have been worn to powder and
the hardest alone survive.

How does the absence of cleavage in quartz affect the durability of
quartz sand?

=How shore drift migrates.= It is under the action of waves and
currents that shore drift migrates slowly along a coast. Where waves
strike a coast obliquely they drive the waste before them little by
little along the shore. Thus on a north-south coast, where the
predominant storms are from the northeast, there will be a migration
of shore drift southwards.

All shores are swept also by currents produced by winds and tides.
These are usually far too gentle to transport of themselves the coarse
materials of which beaches are made. But while the wave stirs the
grains of sand and gravel, and for a moment lifts them from the
bottom, the current carries them a step forward on their way. The
current cannot lift and the wave cannot carry, but together the two
transport the waste along the shore. The road of shore drift is
therefore the zone of the breaking waves.

   [Illustration: Fig. 141. A Bay Bar, Lake Ontario]

=The bay-head beach.= As the waste derived from the wear of waves and
that brought in by streams is trailed along a coast it assumes, under
varying conditions, a number of distinct forms. When swept into the
head of a sheltered bay it constitutes the bay-head beach. By the
highest storm waves the beach is often built higher than the ground
immediately behind it, and forms a dam inclosing a shallow pond or
marsh.

=The bay bar.= As the stream of shore drift reaches the mouth of a bay
of some size it often occurs that, instead of turning in, it sets
directly across toward the opposite headland. The waste is carried out
from shore into the deeper waters of the bay mouth; where it is no
longer supported by the breaking waves, and sinks to the bottom. The
dump is gradually built to the surface as a stubby spur, pointing
across the bay, and as it reaches the zone of wave action current and
wave can now combine to carry shore drift along it, depositing their
load continually at the point of the spur. An embankment is thus
constructed in much the same manner as a railway fill, which, while it
is building, serves as a roadway along which the dirt from an adjacent
cut is carted to be dumped at the end. When the embankment is
completed it bridges the bay with a highway along which shore drift
now moves without interruption, and becomes a bay bar.

   [Illustration: Fig. 142. A Hook, Lake Michigan]

=Incomplete bay bars.= Under certain conditions the sea cannot carry
out its intention to bridge a bay. Rivers discharging in bays demand
open way to the ocean. Strong tidal currents also are able to keep
open channels scoured by their ebb and flow. In such cases the most
that land waste can do is to build spits and shoals, narrowing and
shoaling the channel as much as possible. Incomplete bay bars
sometimes have their points recurved by currents setting at right
angles to the stream of shore drift and are then classified as _hooks_
(Fig. 142).

   [Illustration: Fig. 143. Cross Section of Sand Reef _sr_, and
      Lagoon; _sl_, Sea Level]

=Sand reefs.= On low coasts where shallow water extends some distance
out, the highway of shore drift lies along a low, narrow ridge, termed
the sand reef, separated from the land by a narrow stretch of shallow
water called the _lagoon_ (Fig. 143). At intervals the reef is held
open by _inlets_,--gaps through which the tide flows and ebbs, and by
which the water of streams finds way to the sea.

   [Illustration: Fig. 144. Sand Reef and Lagoon, Texas]

No finer example of this kind of shore line is to be found in the
world than the coast of Texas. From near the mouth of the Rio Grande a
continuous sand reef draws its even curve for a hundred miles to
Corpus Christi Pass, and the reefs are but seldom interrupted by
inlets as far north as Galveston Harbor. On this coast the tides are
variable and exceptionally weak, being less than one foot in height,
while the amount of waste swept along the shore is large. The lagoon
is extremely shallow, and much of it is a mud flat too shoal for even
small boats. On the coast of New Jersey strong tides are able to keep
open inlets at intervals of from two to twenty miles in spite of a
heavy alongshore drift.

Sand reefs are formed where the water is so shallow near shore that
storm waves cannot run in it and therefore break some distance out
from land. Where storm waves first drag bottom they erode and deepen
the sea floor, and sweep in sediment as far as the line where they
break. Here, where they lose their force, they drop their load and
beat up the ridge which is known as the sand reef when it reaches the
surface.


Shores of Elevation and Depression

Our studies have already brought to our notice two distinct forms of
strand lines,--one the high, rocky coast cut back to cliffs by the
attack of the waves, and the other the low, sandy coast where the
waves break usually upon the sand reef. To understand the origin of
these two types we must know that the meeting place of sea and land is
determined primarily by movements of the earth's crust. Where a coast
land emerges the--shore line moves seaward; where it is being
submerged the shore line advances on the land.

=Shores of elevation.= The retreat of the sea, either because of a
local uplift of the land or for any other reason, such as the lowering
of any portion of ocean bottom, lays bare the inner margin of the sea
floor. Where the sea floor has long received the waste of the land it
has been built up to a smooth, subaqueous plain, gently shelving from
the land. Since the new shore line is drawn across this even surface
it is simple and regular, and is bordered on the one side by shallow
water gradually deepening seaward, and on the other by low land
composed of material which has not yet thoroughly consolidated to firm
rock. A sand reef is soon beaten up by the waves, and for some time
conditions will favor its growth. The loss of sand driven into the
lagoon beyond, and of that ground to powder by the surf and carried
out to sea, is more than made up by the stream of alongshore drift,
and especially by the drag of sediments to the reef by the waves as
they deepen the sea floor on its seaward side.

Meanwhile the lagoon gradually fills with waste from the reef and from
the land. It is invaded by various grasses and reeds which have
learned to grow in salt and brackish water; the marsh, laid bare only
at low tide, is built above high tide by wind drift and vegetable
deposits, and becomes a meadow, soldering the sand reef to the
mainland.

While the lagoon has been filling, the waves have been so deepening
the sea floor off the sand reef that at last they are able to attack
it vigorously. They now wear it back, and, driving the shore line
across the lagoon or meadow, cut a line of low cliffs on the mainland.
Such a shore is that of Gascony in southwestern France,--a low,
straight, sandy shore, bordered by dunes and unprotected by reefs from
the attack of the waves of the Bay of Biscay.

   [Illustration: Fig. 145. Map of New Jersey, with that Portion of
      the State one Hundred Feet and more above Sea Level shaded

   Describe the coast line which the state would have if depressed
   one hundred feet. Compare it with the present coastline]

We may say, then, that on shores of elevation the presence of sand
reefs and lagoons indicates the stage of youth, while the absence of
these features and the vigorous and unimpeded attack by the sea upon
the mainland indicate the stage of maturity. Where much waste is
brought in by rivers the maturity of such a coast may be long delayed.
The waste from the land keeps the sea shallow offshore and constantly
renews the sand reef. The energy of the waves is consumed in handling
shore drift, and no energy is left for an effective attack upon the
land. Indeed, with an excessive amount of waste brought down by
streams the land may be built out and encroach temporarily upon the
sea; and not until long denudation has lowered the land, and thus
decreased the amount of waste from it, may the waves be able to cut
through the sand reef and thus the coast reach maturity.


Shores of Depression

Where a coastal region is undergoing submergence the shore line moves
landward. The horizontal plane of the sea now intersects an old land
surface roughened by subaërial denudation. The shore line is irregular
and indented in proportion to the relief of the land and the amount of
the submergence which the land has suffered. It follows up partially
submerged valleys, forming bays, and bends round the divides, leaving
them to project as promontories and peninsulas. The outlines of shores
of depression are as varied as are the forms of the land partially
submerged. We give a few typical illustrations.

   [Illustration: Fig. 146. Chesapeake Bay

   Draw a sketch of this area before its depression]

The characteristics of the coast of Maine are due chiefly to the fact
that a mountainous region of hard rocks, once worn to a peneplain, and
after a subsequent elevation deeply dissected by north-south valleys,
has subsided, the depression amounting on its southern margin to as
much as six hundred feet below sea level. Drowned valleys penetrate
the land in long, narrow bays, and rugged divides project in long,
narrow land arms prolonged seaward by islands representing the high
portions of their extremities. Of this exceedingly ragged shore there
are said to be two thousand miles from the New Brunswick boundary as
far west as Portland,--a straight-line distance of but two hundred
miles. Since the time of its greatest depression the land is known to
have risen some three hundred feet; for the bays have been shortened,
and the waste with which their floors were strewn is now in part laid
bare as clay plains about the bay heads and in narrow selvages about
the peninsulas and islands.

The coast of Dalmatia, on the Adriatic Sea, is characterized by long
land arms and chains of long and narrow islands, all parallel to the
trend of the coast. A region of parallel mountain ranges has been
depressed, and the longitudinal valleys which lie between them are
occupied by arms of the sea.

Chesapeake Bay is a branching bay due to the depression of an ancient
coastal plain which, after having emerged from the sea, was channeled
with broad, shallow valleys. The sea has invaded the valley of the
trunk stream and those of its tributaries, forming a shallow bay whose
many branches are all directed toward its axis (Fig. 146).

Hudson Bay, and the North, the Baltic, and the Yellow seas are
examples where the sinking of the land has brought the sea in over low
plains of large extent, thus deeply indenting the continental
outline. The rise of a few hundred feet would restore these submerged
plains to the land.

=The cycle of shores of depression.= In its _infantile stage_ the
outline of a shore of depression depends almost wholly on the previous
relief of the land, and but little on erosion by the sea. Sea cliffs
and narrow benches appear where headlands and outlying islands have
been nipped by the waves. As yet, little shore waste has been formed.
The coast of Maine is an example of this stage.

In _early youth_ all promontories have been strongly cliffed, and under
a vigorous attack of the sea the shore of open bays may be cut back
also. Sea stacks and rocky islets, caves and coves, make the shore
minutely ragged. The irregularity of the coast, due to depression, is
for a while increased by differential wave wear on harder and softer
rocks. The rock bench is still narrow. Shore waste, though being
produced in large amounts, is for the most part swept into deeper
water and buried out of sight. Examples of this stage are the east
coast of Scotland and the California coast near San Francisco.

_Later youth_ is characterized by a large accumulation of shore waste.
The rock bench has been cut back so that it now furnishes a good
roadway for shore drift. The stream of alongshore drift grows larger
and larger, filling the heads of the smaller bays with beaches,
building spits and hooks, and tying islands with sand bars to the
mainland. It bridges the larger bays with bay bars, while their length
is being reduced as their inclosing promontories are cut back by the
waves. Thus there comes to be a straight, continuous, and easy road,
no longer interrupted by headlands and bays, for the transportation of
waste alongshore. The Baltic coast of Germany is in this stage.

   [Illustration: Fig. 147. Portion of the Northwest Coast of France]

All this while streams have been busy filling with delta deposits the
bays into which they empty. By these steps a coast gradually advances
to _maturity_, the stage when the irregularities due to depression
have been effaced, when outlying islands formed by subsidence have
been planed away, and when the shore line has been driven back behind
the former bay heads. The sea now attacks the land most effectively
along a continuous and fairly straight line of cliffs. Although the
first effect of wave wear was to increase the irregularities of the
shore, it sooner or later rectifies it, making it simple and smooth.
The northwest coast of France is often cited as an example of a coast
which has reached this stage of development (Fig. 147).

In the _old age_ of coasts the rock bench is cut back so far that the
waves can no longer exert their full effect upon the shore. Their
energy is dissipated in moving shore drift hither and thither and in
abrading the bench when they drag bottom upon it. Little by little the
bench is deepened by tidal currents and the drag of waves; but this
process is so slow that meanwhile the sea cliffs melt down under the
weather, and the bench becomes a broad shoal where waves and tides
gradually work over the waste from the land to greater fineness and
sweep it out to sea.

   [Illustration: Fig. 148. The South Shore of Martha's Vineyard

   The land is shaded. To what class of coasts does this belong?
   What stage has it reached, and by what process? What changes
   will take place in the future?]

=Plains of marine abrasion.= While subaërial denudation reduces the
land to baselevel, the sea is sawing its edges to _wave base_, i.e.
the lowest limit of the wave's effective wear. The widened rock bench
forms when uplifted a plain of marine abrasion, which like the
peneplain bevels across strata regardless of their various
inclinations and various degrees of hardness.

How may a plain of marine abrasion be expected to differ from a
peneplain in its mantle of waste?

Compared with subaërial denudation, marine abrasion is a comparatively
feeble agent. At the rate of five feet per century--a higher rate than
obtains on the youthful rocky, coast of Britain--it would require more
than ten million years to pare a strip one hundred miles wide from the
margin of a continent, a time sufficient, at the rate at which the
Mississippi valley is now being worn away, for subaërial denudation to
lower the lands of the globe to the level of the sea.

Slow submergence favors the cutting of a wide rock bench. The water
continually deepens upon the bench; storm waves can therefore always
ride in to the base of the cliffs and attack them with full force;
shore waste cannot impede the onset of the waves, for it is
continually washed out in deeper water below wave base.

=Basal conglomerates.= As the sea marches across the land during a
slow submergence, the platform is covered with sheets of sea-laid
sediments. Lowest of these is a conglomerate,--the bowlder and pebble
beach, widened indefinitely by the retreat of the cliffs at whose base
it was formed, and preserved by the finer deposits laid upon it in
the constantly deepening water as the land subsides. Such basal
conglomerates are not uncommon among the ancient rocks of the land,
and we may know them by their rounded pebbles and larger stones,
composed of the same kind of rock as that of the abraded and evened
surface on which they lie.




CHAPTER VIII

OFFSHORE AND DEEP-SEA DEPOSITS


The alongshore deposits which we have now studied are the exposed edge
of a vast subaqueous sheet of waste which borders the continents and
extends often for as much as two or three hundred miles from land.
Soundings show that offshore deposits are laid in belts parallel to
the coast, the coarsest materials lying nearest to the land and the
finest farthest out. The pebbles and gravel and the clean, coarse sand
of beaches give place to broad stretches of sand, which grows finer
and finer until it is succeeded by sheets of mud. Clearly there is an
offshore movement of waste by which it is sorted, the coarser being
sooner dropped and the finer being carried farther out.


Offshore Deposits

The debris torn by waves from rocky shores is far less in amount than
the waste of the land brought down to the sea by rivers, being only
one thirty-third as great, according to a conservative estimate. Both
mingle alongshore in all the forms of beach and bar that have been
described, and both are together slowly carried out to sea. On the
shelving ocean floor waste is agitated by various movements of the
unquiet water,--by the undertow (an outward-running bottom current
near the shore), by the ebb and flow of tides, by ocean currents where
they approach the land, and by waves and ground swells, whose effects
are sometimes felt to a depth of six hundred feet. By all these means
the waste is slowly washed to and fro, and as it is thus ground finer
and finer and its soluble parts are more and more dissolved, it drifts
farther and farther out from land. It is by no steady and rapid
movement that waste is swept from the shore to its final resting
place. Day after day and century after century the grains of sand and
particles of mud are shifted to and fro, winnowed and spread in
layers, which are destroyed and rebuilt again and again before they
are buried safe from further disturbance.

These processes which are hidden from the eye are among the most
important of those with which our science has to do; for it is they
which have given shape to by far the largest part of the stratified
rocks of which the land is made.

=The continental delta.= This fitting term has been recently suggested
for the sheet of waste slowly accumulating along the borders of the
continents. Within a narrow belt, which rarely exceeds two or three
hundred miles, except near the mouths of muddy rivers such as the
Amazon and Congo, nearly all the waste of the continent, whether worn
from its surface by the weather, by streams, by glaciers, or by the
wind, or from its edge by the chafing of the waves, comes at last to
its final resting place. The agencies which spread the material of the
continental delta grow more and more feeble as they pass into deeper
and more quiet water away from shore. Coarse materials are therefore
soon dropped along narrow belts near land. Gravels and coarse sands
lie in thick, wedge-shaped masses which thin out seaward rapidly and
give place to sheets of finer sand.

=Sea muds.= Outermost of the sediments derived from the waste of the
continents is a wide belt of mud; for fine clays settle so slowly,
even in sea water,--whose saltness causes them to sink much faster
than they would in fresh water,--that they are wafted far before they
reach a bottom where they may remain undisturbed. Muds are also found
near shore, carpeting the floors of estuaries, and among stretches of
sandy deposits in hollows where the more quiet water has permitted the
finer silt to rest.

Sea muds are commonly bluish and consolidate to bluish shales; the red
coloring matter brought from land waste--iron oxide--is altered to
other iron compounds by decomposing organic matter in the presence of
sea water. Yellow and red muds occur where the amount of iron oxide in
the silt brought down to the sea by rivers is too great to be reduced,
or decomposed, by the organic matter present.

Green muds and green sand owe their color to certain chemical changes
which take place where waste from the land accumulates on the sea
floor with extreme slowness. A greenish mineral called _glauconite_--a
silicate of iron and alumina--is then formed. Such deposits, known as
_green sand_, are now in process of making in several patches off the
Atlantic coast, and are found on the coastal plain of New Jersey among
the offshore deposits of earlier geological ages.

=Organic deposits.= Living creatures swarm along the shore and on the
shallows out from land as nowhere else in the ocean. Seaweed often
mantles the rock of the sea cliff between the levels of high and low
tide, protecting it to some degree from the blows of waves. On the
rock bench each little pool left by the ebbing tide is an aquarium
abounding in the lowly forms of marine life. Below low-tide level
occur beds of molluscous shells, such as the oyster, with countless
numbers of other humble organisms. Their harder parts--the shells of
mollusks, the white framework of corals, the carapaces of crabs and
other crustaceans, the shells of sea urchins, the bones and teeth of
fishes--are gradually buried within the accumulating sheets of
sediment, either whole or, far more often, broken into fragments by
the waves.

By means of these organic remains each layer of beach deposits and
those of the continental delta may contain a record of the life of the
time when it was laid. Such a record has been made ever since living
creatures with hard parts appeared upon the globe. We shall find it
sealed away in the stratified rocks of the continents,--parts of
ancient sea deposits now raised to form the dry land. Thus we have in
the traces of living creatures found in the rocks, i.e. in fossils, a
history of the progress of life upon the planet.

   [Illustration: Fig. 149. Coquina, Florida]

=Molluscous shell deposits.= The forms of marine life of importance in
rock making thrive best in clear water, where little sediment is being
laid, and where at the same time the depth is not so great as to
deprive them of needed light, heat, and of sufficient oxygen absorbed
by sea water from the air. In such clear and comparatively shallow
water there often grow countless myriads of animals, such as mollusks
and corals, whose shells and skeletons of carbonate of lime gradually
accumulate in beds of limestone.

A shell limestone made of broken fragments cemented together is
sometimes called _coquina_, a local term applied to such beds recently
uplifted from the sea along the coast of Florida (Fig. 149).

_Oölitic_ limestone (_öon_, an egg; _lithos_, a stone) is so named
from the likeness of the tiny spherules which compose it to the roe of
fish. Corals and shells have been pounded by the waves to calcareous
sand, and each grain has been covered with successive concentric
coatings of lime carbonate deposited about it from solution.

The impalpable powder to which calcareous sand is ground by the waves
settles at some distance from shore in deeper and quieter water as a
limy silt, and hardens into a dense, fine-grained limestone in which
perhaps no trace of fossil is found to suggest the fact that it is of
organic origin.

From Florida Keys there extends south to the trough of Florida Straits
a limestone bank covered by from five hundred and forty to eighteen
hundred feet of water. The rocky bottom consists of limestone now
slowly building from the accumulation of the remains of mollusks,
small corals, sea urchins, worms with calcareous tubes, and
lime-secreting seaweed, which live upon its surface.

Where sponges and other silica-secreting organisms abound on limestone
banks, silica forms part of the accumulated deposit, either in its
original condition, as, for example, the spicules of sponges, or
gathered into concretions and layers of flint.

Where considerable mud is being deposited along with carbonate of lime
there is in process of making a clayey limestone or a limy shale;
where considerable sand, a sandy limestone or a limy sandstone.

=Consolidation of offshore deposits.= We cannot doubt that all these
loose sediments of the sea floor are being slowly consolidated to
solid rock. They are soaked with water which carries in solution lime
carbonate and other cementing substances. These cements are deposited
between the fragments of shells and corals, the grains of sand and
the particles of mud, binding them together into firm rock. Where
sediments have accumulated to great thickness the lower portions tend
also to consolidate under the weight of the overlying beds. Except in
the case of limestones, recent sea deposits uplifted to form land are
seldom so well cemented as are the older strata, which have long been
acted upon by underground waters deep below the surface within the
zone of cementation, and have been exposed to view by great erosion.

   [Illustration: Fig. 150. Ripple Marks on Layers of Ancient
      Sandstone, Wisconsin]

=Ripple marks, sun cracks, etc.= The pulse of waves and tidal currents
agitates the loose material of offshore deposits, throwing it into
fine parallel ridges called ripple marks. One may see this beautiful
ribbing imprinted on beach sands uncovered by the outgoing tide, and
it is also produced where the water is of considerable depth. While
the tide is out the surface of shore deposits may be marked by the
footprints of birds and other animals, or by the raindrops of a
passing shower (Fig. 153). The mud of flats, thus exposed to the sun
and dried, cracks in a characteristic way (Figs. 151 and 152). Such
markings may be covered over with a thin layer of sediment at the next
flood tide and sealed away as a lasting record of the manner and place
in which the strata were laid. In Figure 150 we have an illustration
of a very ancient ripple-marked sand consolidated to hard stone,
uplifted and set on edge by movements of the earth's crust, and
exposed to open air after long erosion.

   [Illustration: Fig. 151. Sun Cracks]

=Stratification.= For the most part the sheet of sea-laid waste is
hidden from our sight. Where its edge is exposed along the shore we
may see the surface markings which have just been noticed. Soundings
also, and the observations made in shallow waters by divers, tell
something of its surface; but to learn more of its structures we must
study those ancient sediments which have been lifted from the sea and
dissected by subaërial agencies. From them we ascertain that sea
deposits are stratified. They lie in distinct layers which often
differ from one another in thickness, in size of particles, and
perhaps in color. They are parted by bedding planes, each of which
represents either a change in material or a pause during which
deposition ceased and the material of one layer had time to settle and
become somewhat consolidated before the material of the next was laid
upon it. Stratification is thus due to intermittently acting forces,
such as the agitation of the water during storms, the flow and ebb of
the tide, and the shifting channels of tidal currents. Off the mouths
of rivers, stratification is also caused by the coarser and more
abundant material brought down at time of floods being laid on the
finer silt which is discharged during ordinary stages.

   [Illustration: Fig. 152. The Under Side of a Layer deposited
      upon a Sun-Cracked Surface, showing Casts of the Cracks]

   [Illustration: Fig. 153. Rain Prints]

How stratified deposits are built up is well illustrated in the flats
which border estuaries, such as the Bay of Fundy. Each advance of the
tide spreads a film of mud, which dries and hardens in the air during
low water before another film is laid upon it by the next incoming
tidal flood. In this way the flats have been covered by a clay which
splits into leaves as thin as sheets of paper.

It is in fine material, such as clays and shales and limestones, that
the thinnest and most uniform layers, as well as those of widest
extent, occur. On the other hand, coarse materials are commonly laid
in thick beds, which soon thin out seaward and give place to deposits
of finer stuff. In a general way strata are laid in well-nigh
horizontal sheets, for the surface on which they are laid is generally
of very gentle inclination. Each stratum, however, is lenticular, or
lenslike, in form, having an area where it is thickest, and thinning
out thence to its edges, where it is overlapped by strata similar in
shape.

   [Illustration: Fig. 154. Cross Bedding in Sandstone, England]

=Cross bedding.= There is an apparent exception to this rule where
strata whose upper and lower surfaces may be about horizontal are made
up of layers inclined at angles which may be as high as the angle of
repose. In this case each stratum grew by the addition along its edge
of successive layers of sediment, precisely as does a sand bar in a
river, the sand being pushed continuously over the edge and coming to
rest on a sloping surface. Shoals built by strong and shifting tidal
currents often show successive strata in which the cross bedding is
inclined in different directions.

=Thickness of sea deposits.= Remembering the vast amount of material
denuded from the land and deposited offshore, we should expect that
with the lapse of time sea deposits would have grown to an enormous
thickness. It is a suggestive fact that, as a rule, the profile of the
ocean bed is that of a soup plate,--a basin surrounded by a flaring
rim. On the _continental shelf_, as the rim is called, the water is
seldom more than six hundred feet in depth at the outer edge, and
shallows gradually towards shore. Along the eastern coast of the
United States the continental shelf is from fifty to one hundred and
more miles in width; on the Pacific coast it is much narrower. So far
as it is due to upbuilding, a wide continental shelf, such as that of
the Atlantic coast, implies a massive continental delta thousands of
feet in thickness. The coastal plain of the Atlantic states may be
regarded as the emerged inner margin of this shelf, and borings made
along the coast probe it to the depth of as much as three thousand
feet without finding the bottom of ancient offshore deposits.
Continental shelves may also be due in part to a submergence of the
outer margin of a continental plateau and to marine abrasion.

=Deposition of sediments and subsidence.= The stratified rocks of the
land show in many places ancient sediments which reach a thickness
which is measured in miles, and which are yet the product of well-nigh
continuous deposition. Such strata may prove by their fossils and by
their composition and structure that they were all laid offshore in
shallow water. We must infer that, during the vast length of time
recorded by the enormous pile, the floor of the sea along the coast
was slowly sinking, and that the trough was constantly being filled,
foot by foot, as fast as it was depressed. Such gradual, quiet
movements of the earth's crust not only modify the outline of coasts,
as we have seen, but are of far greater geological importance in that
they permit the making of immense deposits of stratified rock.

A slow subsidence continued during long time is recorded also in the
succession of the various kinds of rock that come to be deposited in
the same area. As the sea transgresses the land, i.e. encroaches upon
it, any given part of the sea bottom is brought farther and farther
from the shore. The basal conglomerate formed by bowlder and pebble
beaches comes to be covered with sheets of sand, and these with layers
of mud as the sea becomes deeper and the shore more remote; while
deposits of limestone are made when at last no waste is brought to the
place from the now distant land, and the water is left clear for the
growth of mollusks and other lime-secreting organisms.

   [Illustration: Fig. 155. Succession of Deposits recording a
      Transgressing Sea

   _c_, conglomerate; _ss_, sandstone; _sh_, shale; _lm_, limestone]

=Rate of deposition.= As deposition in the sea corresponds to
denudation on the land, we are able to make a general estimate of the
rate at which the former process is going on. Leaving out of account
the soluble matter removed, the Mississippi is lowering its basin at
the rate of one foot in five thousand years, and we may assume this as
the average rate at which the earth's land surface of fifty-seven
million square miles is now being denuded by the removal of its
mechanical waste. But sediments from the land are spread within a zone
but two or three hundred miles in width along the margin of the
continents, a line one hundred thousand miles long. As the area of
deposition--about twenty-five million square miles--is about one half
the area of denudation, the average rate of deposition must be twice
the average rate of denudation, i.e. about one foot in twenty-five
hundred years. If some deposits are made much more rapidly than this,
others are made much more slowly. If they were laid no faster than the
present average rate, the strata of ancient sea deposits exposed in a
quarry fifty feet deep represent a lapse of at least one hundred and
twenty-five thousand years, and those of a formation five hundred feet
thick required for their accumulation one million two hundred and
fifty thousand years.

   [Illustration: Fig. 156. Thick Offshore Deposits of Coarse Waste
      recording the Presence of a Young Mountain Range near Shore]

=The sedimentary record and the denudation cycle.= We have seen that
the successive stages in a cycle of denudation, such as that by which
a land mass of lofty mountains is worn to low plains, are marked each
by its own peculiar land forms, and that the forms of the earlier
stages are more or less completely effaced as the cycle draws toward
an end. Far more lasting records of each stage are left in the
sedimentary deposits of the continental delta. Thus, in the youth of
such a land mass as we have mentioned, torrential streams flowing down
the steep mountain sides deliver to the adjacent sea their heavy loads
of coarse waste, and thick offshore deposits of sand and gravel (Fig.
156) record the high elevation of the bordering land. As the land is
worn to lower levels, the amount and coarseness of the waste brought
to the sea diminishes, until the sluggish streams carry only a fine
silt which settles on the ocean floor near to land in wide sheets of
mud which harden into shale. At last, in the old age of the region
(Fig. 157), its low plains contribute little to the sea except the
soluble elements of the rocks, and in the clear waters near the land
lime-secreting organisms flourish and their remains accumulate in beds
of limestone. When long-weathered lands mantled with deep,
well-oxidized waste are uplifted by a gradual movement of the earth's
crust, and the mantle is rapidly stripped off by the revived streams,
the uprise is recorded in wide deposits of red and yellow clays and
sands upon the adjacent ocean floor.

Where the waste brought in is more than the waves can easily
distribute, as off the mouths of turbid rivers which drain highlands
near the sea, deposits are little winnowed, and are laid in rapidly
alternating, shaly sandstones and sandy shales.

   [Illustration: Fig. 157. Offshore Deposits recording Old Age of
      the Adjacent Land

   _ss_, sandstone; _sh_, shale; _lm_, limestone]

Where the highlands are of igneous rock, such as granite, and
mechanical disintegration is going on more rapidly than chemical
decay, these conditions are recorded in the nature of the deposits
laid offshore. The waste swept in by streams contains much feldspar
and other minerals softer and more soluble than quartz, and where the
waves have little opportunity to wear and winnow it, it comes to rest
in beds of sandstone in which grains of feldspar and other soft
minerals are abundant. Such feldspathic sandstones are known as
_arkose_.

On the other hand, where the waste supplied to the sea comes chiefly
from wide, sandy, coastal plains, there are deposited offshore clean
sandstones of well-worn grains of quartz alone. In such coastal plains
the waste of the land is stored for ages. Again and again they are
abandoned and invaded by the sea as from time to time the land slowly
emerges and is again submerged. Their deposits are long exposed to the
weather, and sorted over by the streams, and winnowed and worked over
again and again by the waves. In the course of long ages such deposits
thus become thoroughly sorted, and the grains of all minerals softer
than quartz are ground to mud.

   [Illustration: Fig. 158. Globigerina Ooze under the Microscope]


Deep-Sea Oozes and Clays

=Globigerina ooze.= Beyond the reach of waste from the land the bottom
of the deep sea is carpeted for the most part with either chalky ooze
or a fine red clay. The surface waters of the warm seas swarm with
minute and lowly animals belonging to the order of the _Foraminifera_,
which secrete shells of carbonate of lime. At death these tiny white
shells fall through the sea water like snowflakes in the air, and,
slowly dissolving, seem to melt quite away before they can reach
depths greater than about three miles. Near shore they reach bottom,
but are masked by the rapid deposit of waste derived from the land. At
intermediate depths they mantle the ocean floor with a white, soft
lime deposit known as _Globigerina ooze_, from a genus of the
Foraminifera which contributes largely to its formation.

=Red clay.= Below depths of from fifteen to eighteen thousand feet the
ocean bottom is sheeted with red or chocolate colored clay. It is the
insoluble residue of seashells, of the debris of submarine volcanic
eruptions, of volcanic dust wafted by the winds, and of pieces of
pumice drifted by ocean currents far from the volcanoes from which
they were hurled. The red clay builds up with such inconceivable
slowness that the teeth of sharks and the hard ear bones of whales may
be dredged in large numbers from the deep ocean bed, where they have
lain unburied for thousands of years; and an appreciable part of the
clay is also formed by the dust of meteorites consumed in the
atmosphere,--a dust which falls everywhere on sea and land, but which
elsewhere is wholly masked by other deposits.

The dark, cold abysses of the ocean are far less affected by change
than any other portion of the surface of the lithosphere. These vast,
silent plains of ooze lie far below the reach of storms. They know no
succession of summer and winter, or of night and day. A mantle of deep
and quiet water protects them from the agents of erosion which
continually attack, furrow, and destroy the surface of the land. While
the land is the area of erosion, the sea is the area of deposition.
The sheets of sediment which are slowly spread there tend to efface
any inequalities, and to form a smooth and featureless subaqueous
plain.

With few exceptions, the stratified rocks of the land are proved by
their fossils and composition to have been laid in the sea; but in the
same way they are proved to be offshore, shallow-water deposits, akin
to those now making on continental shelves. Deep-sea deposits are
absent from the rocks of the land, and we may therefore infer that the
deep sea has never held sway where the continents now are,--that the
continents have ever been, as now, the elevated portions of the
lithosphere, and that the deep seas of the present have ever been its
most depressed portions.


The Reef-Building Corals

In warm seas the most conspicuous of rock-making organisms are the
corals known as the reef builders. Floating in a boat over a coral
reef, as, for example, off the south coast of Florida or among the
Bahamas, one looks down through clear water on thickets of branching
coral shrubs perhaps as much as eight feet high, and hemispherical
masses three or four feet thick, all abloom with countless minute
flowerlike coral polyps, gorgeous in their colors of yellow, orange,
green, and red. In structure each tiny polyp is little more than a
fleshy sac whose mouth is surrounded with petal-like tentacles, or
feelers. From the sea water the polyps secrete calcium carbonate and
build it up into the stony framework which supports their colonies.
Boring mollusks, worms, and sponges perforate and honeycomb this
framework even while its surface is covered with myriads of living
polyps. It is thus easily broken by the waves, and white fragments of
coral trees strew the ground beneath. Brilliantly colored fishes live
in these coral groves, and countless mollusks, sea urchins, and other
forms of marine life make here their home. With the debris from all
these sources the reef is constantly built up until it rises to
low-tide level. Higher than this the corals cannot grow, since they
are killed by a few hours' exposure to the air.

   [Illustration: Fig. 159. Patch of Growing Corals exposed at an
      Exceptionally Low Tide, Great Barrier Reef, Australia]

When the reef has risen to wave base, the waves abrade it on the
windward side and pile to leeward coral blocks torn from their
foundation, filling the interstices with finer fragments. Thus they
heap up along the reef low, narrow islands (Fig. 160).

Reef building is a comparatively rapid progress. It has been estimated
that off Florida a reef could be built up to the surface from a depth
of fifty feet in about fifteen hundred years.

   [Illustration: Fig. 160. Wave-Built Island on Coral Reef

   _r_, reef; _sl_, sea level]

=Coral limestones.= Limestones of various kinds are due to the reef
builders. The reef rock is made of corals in place and broken
fragments of all sizes, cemented together with calcium carbonate from
solution by infiltrating waters. On the island beaches coral sand is
forming oolitic limestone, and the white coral mud with which the sea
is milky for miles about the reef in times of storm settles and
concretes into a compact limestone of finest grain. Corals have been
among the most important limestone builders of the sea ever since they
made their appearance in the early geological ages.

The areas on which coral limestone is now forming are large. The Great
Barrier Reef of Australia, which lies off the northeastern coast, is
twelve hundred and fifty miles long, and has a width of from ten to
ninety miles. Most of the islands of the tropics are either skirted
with coral reefs or are themselves of coral formation.

=Conditions of coral growth.= Reef-building corals cannot live except
in clear salt water less, as a rule, than one hundred and fifty feet
in depth, with a winter temperature not lower than 68° F. An important
condition also is an abundant food supply, and this is best secured in
the path of the warm oceanic currents.

Coral reefs may be grouped in three classes,--fringing reefs, barrier
reefs, and atolls.

=Fringing reefs.= These take their name from the fact that they are
attached as narrow fringes to the shore. An example is the reef which
forms a selvage about a mile wide along the northeastern coast of
Cuba. The outer margin, indicated by the line of white surf, where the
corals are in vigorous growth, rises from about forty feet of water.
Between this and the shore lies a stretch of shoal across which one
can wade at low water, composed of coral sand with here and there a
clump of growing coral.

=Barrier reefs.= Reefs separated from the shore by a ship channel of
quiet water, often several miles in width and sometimes as much as
three hundred feet in depth, are known as barrier reefs. The seaward
face rises abruptly from water too deep for coral growth. Low islands
are cast up by the waves upon the reef, and inlets give place for the
ebb and flow of the tides. Along the west coast of the island of New
Caledonia a barrier reef extends for four hundred miles, and for a
length of many leagues seldom approaches within eight miles of the
shore.

=Atolls.= These are ring-shaped or irregular coral islands, or
island-studded reefs, inclosing a central lagoon. The narrow zone of
land, like the rim of a great bowl sunken to the water's edge, rises
hardly more than twenty feet at most above the sea, and is covered
with a forest of trees such as the cocoanut, whose seeds can be
drifted to it uninjured from long distances. The white beach of coral
sand leads down to the growing reef, on whose outer margin the surf is
constantly breaking. The sea face of the reef falls off abruptly,
often to depths of thousands of feet, while the lagoon varies in depth
from a few feet to one hundred and fifty or two hundred, and
exceptionally measures as much as three hundred and fifty feet.

=Theories of coral reefs.= Fringing reefs require no explanation,
since the depth of water about them is not greater than that at which
coral can grow; but barrier reefs and atolls, which may rise from
depths too great for coral growth demand a theory of their origin.

   [Illustration: Fig. 161. Diagram illustrating the Subsidence
      Theory of Coral Reefs]

Darwin's theory holds that barrier reefs and atolls are formed from
fringing reefs by _subsidence_. The rate of sinking cannot be greater
than that of the upbuilding of the reef, since otherwise the corals
would be carried below their depth and drowned. The process is
illustrated in Figure 161, where v represents a volcanic island in mid
ocean undergoing slow depression, and _ss_ the sea level before the
sinking began, when the island was surrounded by a fringing reef. As
the island slowly sinks, the reef builds up with equal pace. It rears
its seaward face more steep than the island slope, and thus the
intervening space between the sinking, narrowing land and the outer
margin of the reef constantly widens. In this intervening space the
corals are more or less smothered with silt from the outer reef and
from the land, and are also deprived in large measure of the needful
supply of food and oxygen by the vigorous growth of the corals on the
outer rim. The outer rim thus becomes a barrier reef and the inner
belt of retarded growth is deepened by subsidence to a ship channel,
_s´s´_ representing sea level at this time. The final stage, where the
island has been carried completely beneath the sea and overgrown by
the contracting reef, whose outer ring now forms an atoll, is
represented by _s´´s´´_.

   [Illustration: Fig. 162. Barrier Reef formed without Subsidence

   _a_, zone of coral growth; _f_, former fringing reef; _t_,
   talus; _b_, barrier reef]

In very many instances, however, atolls and barrier reefs may be
explained without subsidence. Thus a barrier reef may be formed by the
seaward growth of a fringing reef upon the talus of its sea face. In
Figure 162, _f_ is a fringing reef whose outer wall rises from about
one hundred and fifty feet, the lower limit of the reef-building
species. At the foot of this submarine cliff a talus of fallen blocks
t accumulates, and as it reaches the zone of coral growth becomes the
foundation on which the reef is steadily extended seaward. As the reef
widens, the polyps of the circumference flourish, while those of the
inner belt are retarded in their growth and at last perish. The coral
rock of the inner belt is now dissolved by sea water and scoured out
by tidal currents until it gives place to a gradually deepening ship
channel, while the outer margin is left as a barrier reef.

   [Illustration: Fig. 163. Section of Atoll on a Shoal which has
      been built up to near the Surface by Organic Deposits upon a
      Submarine Volcanic Peak

   _v_, volcano; _f_, foraminiferal deposits; _m_, molluscous shell
   deposits; _c_, coral reef; _sl_, sea level]

In much the same way atolls may be built on any shoal which lies
within the zone of coral growth. Such shoals may be produced when
volcanic islands are leveled by waves and ocean currents, and when
submarine plateaus, ridges, and peaks are built up by various organic
agencies, such as molluscous and foraminiferal shell deposits (Fig.
163). The reef-building corals, whose eggs are drifted widely over the
tropic seas by ocean currents, colonize such submarine foundations
wherever the conditions are favorable for their growth. As the reef
approaches the surface the corals of the inner area are smothered by
silt and starved, and their Submarine Volcanic Peak hard parts are
dissolved and scoured away; while those of the circumference, with
abundant food supply, nourish and build the ring of the atoll. Atolls
may be produced also by the backward drift of sand from either end of
a crescentic coral reef or island, the spits uniting in the quiet
water of the lee to inclose a lagoon. In the Maldive Archipelago all
gradations between crescent-shaped islets and complete atoll rings
have been observed.

In a number of instances where coral reefs have been raised by
movements of the earth's crust, the reef formation is found to be a
thin veneer built upon a foundation of other deposits. Thus Christmas
Island, in the Indian Ocean, is a volcanic pile rising eleven hundred
feet above sea level and fifteen thousand five hundred feet above the
bottom of the sea. The summit is a plateau surrounded by a rim of
hills of reef formation, which represent the ring of islets of an
ancient atoll. Beneath the reef are thick beds of limestone, composed
largely of the remains of foraminifers, which cover the lavas and
fragmental materials of the old submarine volcano.

Among the ancient sediments which now form the stratified rocks of the
land there occur many thin reef deposits, but none are known of the
immense thickness which modern reefs are supposed to reach according
to the theory of subsidence.

Barrier and fringing reefs are commonly interrupted off the mouths of
rivers. Why?

=Summary.= We have seen that the ocean bed is the goal to which the
waste of the rocks of the land at last arrives. Their soluble parts,
dissolved by underground waters and carried to the sea by rivers, are
largely built up by living creatures into vast sheets of limestone.
The less soluble portions--the waste brought in by streams and the
waste of the shore--form the muds and sands of continental deltas. All
of these sea deposits consolidate and harden, and the coherent rocks
of the land are thus reconstructed on the ocean floor. But the
destination is not a final one. The stratified rocks of the land are
for the most part ancient deposits of the sea, which have been lifted
above sea level; and we may believe that the sediments now being laid
offshore are the "dust of continents to be," and will some time emerge
to form additions to the land. We are now to study the movements of
the earth's crust which restore the sediments of the sea to the light
of day, and to whose beneficence we owe the habitable lands of the
present.




PART II

INTERNAL GEOLOGICAL AGENCIES


CHAPTER IX

MOVEMENTS OF THE EARTH'S CRUST


The geological agencies which we have so far studied--weathering,
streams, underground waters, glaciers, winds, and the ocean--all work
upon the earth from without, and all are set in motion by an energy
external to the earth, namely, the radiant energy of the sun. All,
too, have a common tendency to reduce the inequalities of the earth's
surface by leveling the lands and strewing their waste beneath the
sea.

But despite the unceasing efforts of these external agencies, they
have not destroyed the continents, which still rear their broad plains
and great plateaus and mountain ranges above the sea. Either, then,
the earth is very young and the agents of denudation have not yet had
time to do their work, or they have been opposed successfully by other
forces.

We enter now upon a department of our science which treats of forces
which work upon the earth from within, and increase the inequalities
of its surface. It is they which uplift and recreate the lands which
the agents of denudation are continually destroying; it is they which
deepen the ocean bed and thus withdraw its waters from the shores. At
times also these forces have aided in the destruction of the lands by
gradually lowering them and bringing in the sea. Under the action of
forces resident within the earth the crust slowly rises or sinks; from
time to time it has been folded and broken; while vast quantities of
molten rock have been pressed up into it from beneath and outpoured
upon its surface. We shall take up these phenomena in the following
chapters, which treat of upheavals and depressions of the crust,
foldings and fractures of the crust, earthquakes, volcanoes, the
interior conditions of the earth, mineral veins, and metamorphism.


Oscillations of the Crust

Of the various movements of the crust due to internal agencies we will
consider first those called oscillations, which lift or depress large
areas so slowly that a long time is needed to produce perceptible
changes of level, and which leave the strata in nearly their original
horizontal attitude. These movements are most conspicuous along
coasts, where they can be referred to the datum plane of sea level; we
will therefore take our first illustrations from rising and sinking
shores.

=New Jersey.= Along the coasts of New Jersey one may find awash at
high tide ancient shell heaps, the remains of tribal feasts of
aborigines. Meadows and old forest grounds, with the stumps still
standing, are now overflowed by the sea, and fragments of their turf
and wood are brought to shore by waves. Assuming that the sea level
remains constant, it is clear that the New Jersey coast is now
gradually sinking. The rate of submergence has been estimated at about
two feet per century.

On the other hand, the wide coastal plain of New Jersey is made of
stratified sands and clays, which, as their marine fossils show, were
outspread beneath the sea. Their present position above sea level
proves that the land now subsiding emerged in the recent past.

The coast of New Jersey is an example of the slow and tranquil
oscillations of the earth's unstable crust now in progress along many
shores. Some are emerging from the sea, some are sinking beneath it;
and no part of the land seems to have been exempt from these changes
in the past.

=Evidences of changes of level.= Taking the surface of the sea as a
level of reference, we may accept as proofs of relative upheaval
whatever is now found in place above sea level and could have been
formed only at or beneath it, and as proofs of relative subsidence
whatever is now found beneath the sea and could only have been formed
above it.

Thus old strand lines with sea cliffs, wave-cut rock benches, and
beaches of wave-worn pebbles or sand, are striking proofs of recent
emergence to the amount of their present height above tide. No less
conclusive is the presence of sea-laid rocks which we may find in the
neighboring quarry or outcrop, although it may have been long ages
since they were lifted from the sea to form part of the dry land.

Among common proofs of subsidence are roads and buildings and other
works of man, and vegetal growths and deposits, such as forest grounds
and peat beds, now submerged beneath the sea. In the deltas of many
large rivers, such as the Po, the Nile, the Ganges, and the
Mississippi, buried soils prove subsidences of hundreds of feet; and
in several cases, as in the Mississippi delta, the depression seems to
be now in progress.

Other proofs of the same movement are drowned land forms which are
modeled only in open air. Since rivers cannot cut their valleys
farther below the baselevel of the sea than the depths of their
channels, _drowned valleys_ are among the plainest proofs of
depression. To this class belong Narragansett, Delaware, Chesapeake,
Mobile, and San Francisco bays, and many other similar drowned valleys
along the coasts of the United States. Less conspicuous are the
_submarine channels_ which, as soundings show, extend from the mouths
of a number of rivers some distance out to sea. Such is the submerged
channel which reaches from New York Bay southeast to the edge of the
continental shelf, and which is supposed to have been cut by the
Hudson River when this part of the shelf was a coastal plain.

=Warping.= In a region undergoing changes of level the rate of
movement commonly varies in different parts. Portions of an area may
be rising or sinking, while adjacent portions are stationary or moving
in the opposite direction. In this way a land surface becomes
_warped_. Thus, while Nova Scotia and New Brunswick are now rising
from the level of the sea, Prince Edward Island and Cape Breton Island
are sinking, and the sea now flows over the site of the famous old
town of Louisburg destroyed in 1758.

Since the close of the glacial epoch the coasts of Newfoundland and
Labrador have risen hundreds of feet, but the rate of emergence has
not been uniform. The old strand line, which stands at five hundred
and seventy-five feet above tide at St. John's, Newfoundland, declines
to two hundred and fifty feet near the northern point of Labrador
(Fig. 164).

   [Illustration: Fig. 164. Warped Strand Line from St. John's,
      Newfoundland, to Nachvak, Labrador]

=The Great Lakes= is now undergoing perceptible warping. Rivers enter
the lakes from the south and west with sluggish currents and deep
channels resembling the estuaries of drowned rivers; while those that
enter from opposite directions are swift and shallow. At the western
end of Lake Erie are found submerged caves containing stalactites, and
old meadows and forest grounds are now under water. It is thus seen
that the water of the lakes is rising along their southwestern shores,
while from their northeastern shores it is being withdrawn. The
region of the Great Lakes is therefore warping; it is rising in the
northeast as compared with the southwest.

From old bench marks and records of lake levels it has been estimated
that _the rate of warping_ amounts to five inches a century for every
one hundred miles. It is calculated that the water of Lake Michigan is
rising at Chicago at the rate of nine or ten inches per century. The
divide at this point between the tributaries of the Mississippi and
Lake Michigan is but eight feet above the mean stage of the lake. If
the canting of the region continues at its present rate, in a thousand
years the waters of the lake will here overflow the divide. In three
thousand five hundred years all the lakes except Ontario will
discharge by this outlet, via the Illinois and Mississippi rivers,
into the Gulf of Mexico. The present outlet by the Niagara River will
be left dry, and the divide between the St. Lawrence and the
Mississippi systems will have shifted from Chicago to the vicinity of
Buffalo.

=Physiographic effects of oscillations.= We have already mentioned
several of the most important effects of movements of elevation and
depression, such as their effects on rivers, the mantle of waste (pp.
85, 86), and the forms of coasts (p. 166). Movements of
elevation--including uplifts by folding and fracture of the crust to
be noticed later--are the necessary conditions for erosion by whatever
agent. They determine the various agencies which are to be chiefly
concerned m the wear of any land,--whether streams or glaciers,
weathering or the wind,--and the degree of their efficiency. The lands
must be uplifted before they can be eroded, and since they must be
eroded before their waste can be deposited, movements of elevation are
a prerequisite condition for sedimentation also. Subsidence is a
necessary condition for deposits of great thickness, such as those of
the Great Valley of California and the Indo-Gangetic plain (p. 101),
the Mississippi delta (p. 109), and the still more important
formations of the continental delta in gradually sinking troughs (p.
183). It is not too much to say that the character and thickness of
each formation of the stratified rocks depend primarily on these
crustal movements.

Along the Baltic coast of Sweden, bench marks show that the sea is
withdrawing from the land at a rate which at the north amounts to
between three and four feet per century; Towards the south the rate
decreases. South of Stockholm, until recent years, the sea has gained
upon the land, and here in several seaboard towns streets by the shore
are still submerged. The rate of oscillation increases also from the
coast inland. On the other hand, along the German coast of the Baltic
the only historic fluctuations of sea level are those which may be
accounted for by variations due to changes in rainfall. In 1730
Celsius explained the changes of level of the Swedish coast as due to
a lowering of the Baltic instead of to an elevation of the land. Are
the facts just stated consistent with his theory?

   [Illustration: Fig. 165. Old Strand Lines, Tadousac, Quebec]

At the little town of Tadousac--where the Saguenay River empties into
the St. Lawrence--there are terraces of old sea beaches, some almost
as fresh as recent railway fills, the highest standing two hundred and
thirty feet above the river (Fig. 165). Here the Saguenay is eight
hundred and forty feet in depth, and the tide ebbs and flows far up
its stream. Was its channel cut to this depth by the river when the
land was at its present height? What oscillations are here recorded,
and to what amount?

   [Illustration: Fig. 166. Diagram showing Ruins of Temple, North
      of Naples

   _C_, ancient sea cliff; _m_, marble pillars, dotted where bored
   by mollusks; _sl_, sea level]

A few miles north of Naples, Italy, the ruins of an ancient Roman
temple lie by the edge of the sea, on a narrow plain which is
overlooked in the rear by an old sea cliff (Fig. 166). Three marble
pillars are still standing. For eleven feet above their bases these
columns are uninjured, for to this height they were protected by an
accumulation of volcanic ashes; but from eleven to nineteen feet they
are closely pitted with the holes of boring marine mollusks. From
these facts trace the history of the oscillations of the region.

   [Illustration: Fig. 167. Section in a Region of Folded Rocks]


Foldings of the Crust

The oscillations which we have just described leave the strata not far
from their original horizontal attitude. Figure 167 represents a
region in which movements of a very different nature have taken place.
Here, on either side of the valley _v_, we find outcrops of layers
tilted at high angles. Sections along the ridge _r_ show that it is
composed of layers which slant inward from either side. In places the
outcropping strata stand nearly on edge, and on the right of the
valley they are quite overturned; a shale _sh_ has come to overlie a
limestone _lm_ although the shale is the older rock, whose original
position was beneath the limestone.

   [Illustration: Fig. 168. Dip and Strike]

It is not reasonable to suppose that these rocks were deposited in the
attitude in which we find them now; we must believe that, like other
stratified rocks, they were outspread in nearly level sheets upon the
ocean floor. Since that time they must have been deformed. Layers of
solid rock several miles in thickness have been crumpled and folded
like soft wax in the hand, and a vast denudation has worn away the
upper portions of the folds, in part represented in our section by
dotted lines.

=Dip and strike.= In districts where the strata have been disturbed it
is desirable to record their attitude. This is most easily done by
taking the angle at which the strata are inclined and the compass
direction in which they slant. It is also convenient to record the
direction in which the outcrop of the strata trends across the
country.

   [Illustration: Fig. 169. An Anticline, Maryland]

The inclination of a bed of rocks to the horizon is its _dip_ (Fig.
168). The amount of the dip is the angle made with a horizontal plane.
The dip of a horizontal layer is zero, and that of a vertical layer
is 90°. The direction of the dip is taken with the compass. Thus a
geologist's notebook in describing the attitude of outcropping strata
contains many such entries as these: dip 32° north, or dip 8° south 20°
west,--meaning in the latter case that the amount of the dip is 8° and
the direction of the dip bears 20° west of south.

The line of intersection of a layer with the horizontal plane is the
_strike_. The strike always runs at right angles to the dip.

Dip and strike may be illustrated by a book set aslant on a shelf. The
dip is the acute angle made with the shelf by the side of the book,
while the strike is represented by a line running along the book's
upper edge. If the dip is north or south, the strike runs east and
west.

   [Illustration: Fig. 170. Folded Strata, Coast of England

   A syncline in the center, with an anticline on either side]

=Folded structures.= An upfold, in which the strata dip away from a
line drawn along the crest and called the axis of the fold, is known
as an _anticline_ (Fig. 169). A downfold, where the strata dip from
either side toward the axis of the trough, is called a _syncline_
(Fig. 170). There is sometimes seen a downward bend in horizontal or
gently inclined strata, by which they descend to a lower level. Such a
single flexure is a _monocline_ (Fig. 171).

   [Illustration: Fig. 171. A Monocline]

=Degrees of folding.= Folds vary in degree from broad, low swells,
which can hardly be detected, to the most highly contorted and
complicated structures. In _symmetric_ folds (Figs. 169 and 180) the
dips of the rocks on each side the axis of the fold are equal. In
_unsymmetrical_ folds one limb is steeper than the other, as in the
anticline in Figure 167. In _overturned_ folds (Figs. 167 and 172) one
limb is inclined beyond the perpendicular. _Fan folds_ have been so
pinched that the original anticlines are left broader at the top than
at the bottom (Fig. 173).

   [Illustration: Fig. 172. Overturned Fold, Vermont]

In folds where the compression has been great the layers are often
found thickened at the crest and thinned along the limbs (174). Where
strong rocks such as heavy limestones are folded together with weak
rocks such as shales, the strong rocks are often bent into great
simple folds, while the weak rocks are minutely crumpled.

   [Illustration: Fig. 173. Fan Folds, the Alps]

=Systems of folds.= As a rule, folds occur in systems. Over the
Appalachian mountain belt, for example, extending from northeastern
Pennsylvania to northern Alabama and Georgia, the earth's crust has
been thrown into a series of parallel folds whose axes run from
northeast to southwest (Fig. 175). In Pennsylvania one may count a
score or more of these earth waves,--some but from ten to twenty miles
in length, and some extending as much as two hundred miles before they
die away. On the eastern part of this belt the folds are steeper and
more numerous than on the western side.

   [Illustration: Fig. 174. Folds with Layers thickened at the
      Crest and thinned along the Limbs]

=Cause and conditions of folding.= The sections which we have studied
suggest that rocks are folded by lateral pressure. While a single,
simple fold might be produced by a heave, a series of folds, including
overturns, fan folds, and folds thickened on their crests at the
expense of their limbs, could only be made in one way,--by pressure
from the side. Experiment has reproduced all forms of folds by
subjecting to lateral thrust layers of plastic material such as wax.

Vast as the force must have been which could fold the solid rocks of
the crust as one may crumple the leaves of a magazine in the fingers,
it is only under certain conditions that it could have produced the
results which we see. Rocks are brittle, and it is only when under a
_heavy load_ and by _great pressure slowly applied_, that they can
thus be folded and bent instead of being crushed to pieces. Under
these conditions, experiments prove that not only metals such as
steel, but also brittle rocks such as marble, can be deformed and
molded and made to flow like plastic clay.

   [Illustration: Fig. 175. Relief Map of the Northern Appalachian
      Region

   From Bingham's _Geographic Influences in American History_]

=Zone of flow, zone of flow and fracture, and zone of fracture.= We
may believe that at depths which must be reckoned in tens of thousands
of feet the load of overlying rocks is so great that rocks of all
kinds yield by folding to lateral pressure, and flow instead of
breaking. Indeed, at such profound depths and under such inconceivable
weight no cavity can form, and any fractures would be healed at once
by the welding of grain to grain. At less depths there exists a zone
where soft rocks fold and flow under stress, and hard rocks are
fractured; while at and near the surface hard and soft rocks alike
yield by fracture to strong pressure.


Structures developed in Compressed Rocks

Deformed rocks show the effects of the stresses to which they have
yielded, not only in the immense folds into which they have been
thrown but in their smallest parts as well. A hand specimen of slate,
or even a particle under the microscope, may show plications similar
in form and origin to the foldings which have produced ranges of
mountains. A tiny flake of mica in the rocks of the Alps may be
puckered by the same resistless forces which have folded miles of
solid rock to form that lofty range.

=Slaty cleavage.= Rocks which have yielded to pressure often split
easily in a certain direction across the bedding planes. This cleavage
is known as slaty cleavage, since it is most perfectly developed in
fine-grained, homogeneous rocks, such as slates, which cleave to the
thin, smooth-surfaced plates with which we are familiar in the slates
used in roofing and for ciphering and blackboards. In coarse-grained
rocks, pressure develops more distant partings which separate the
rocks into blocks.

Slaty cleavage cannot be due to lamination, since it commonly crosses
bedding planes at an angle, while these planes have been often
well-nigh or quite obliterated. Examining slate with a microscope, we
find that its cleavage is due to the grain of the rock. Its particles
are flattened and lie with their broad faces in parallel planes, along
which the rock naturally splits more easily than in any other
direction. The irregular grains of the mud which has been altered to
slate have been squeezed flat by a pressure exerted at right angles to
the plane of cleavage. Cleavage is found only in folded rocks, and, as
we may see in Figure 176, the strike of the cleavage runs parallel to
the strike of the strata and the axis of the folds. The dip of the
cleavage is generally steep, hence the pressure was nearly horizontal.
The pressure which has acted at right angles to the cleavage, and to
which it is due, is the same lateral pressure which has thrown the
strata into folds.

   [Illustration: Fig. 176. Slaty Cleavage]

We find additional proof that slates have undergone compression at
right angles to their cleavage in the fact that any inclusions in
them, such as nodules and fossils, have been squeezed out of shape and
have their long diameters lying in the planes of cleavage.

That pressure is competent to cause cleavage is shown by experiment.
Homogeneous material of fine grain, such as beeswax, when subjected to
heavy pressure cleaves at right angles to the direction of the
compressing force.

=Rate of folding.= All the facts known with regard to rock deformation
agree that it is a secular process, taking place so slowly that, like
the deepening of valleys by erosion, it escapes the notice of the
inhabitants of the region. It is only under stresses slowly applied
that rocks bend without breaking. The folds of some of the highest
mountains have risen so gradually that strong, well-intrenched rivers
which had the right of way across the region were able to hold to
their courses, and as a circular saw cuts its way through the log
which is steadily driven against it, so these rivers sawed their
gorges through the fold as fast as it rose beneath them. Streams which
thus maintain the course which they had antecedent to a deformation of
the region are known as _antecedent_ streams. Examples of such are the
Sutlej and other rivers of India, whose valleys trench the outer
ranges of the Himalayas and whose earlier river deposits have been
upturned by the rising ridges. On the other hand, mountain crests are
usually divides, parting the head waters of different drainage
systems. In these cases the original streams of the region have been
broken or destroyed by the uplift of the mountain mass across their
paths.

On the whole, which have worked more rapidly, processes of deformation
or of denudation?

   [Illustration: Fig. 177. An Unroofed Anticline]


Land Forms due to Folding

As folding goes on so slowly, it is never left to form surface
features unmodified by the action of other agencies. An anticlinal
fold is attacked by erosion as soon as it begins to rise above the
original level, and the higher it is uplifted, and the stronger are
its slopes, the faster is it worn away. Even while rising, a young
upfold is often thus unroofed, and instead of appearing as a long,
Smooth, boat-shaped ridge, it commonly has had opened along the rocks
of the axis, when these are weak, a valley which is overlooked by the
infacing escarpments of the hard layers of the sides of the fold (Fig.
177). Under long-continued erosion, anticlines may be degraded to
valleys, while the synclines of the same system may be left in relief
as ridges (Fig. 167).

=Folded mountains.= The vastness of the forces which wrinkle the crust
is best realized in the presence of some lofty mountain range. All
mountains, indeed, are not the result of folding. Some, as we shall
see, are due to upwarps or to fractures of the crust; some are piles
of volcanic material; some are swellings caused by the intrusion of
molten matter beneath the surface; some are the relicts left after the
long denudation of high plateaus.

   [Illustration: Fig. 178. Mountain Peaks carved in Folded
      Strata, Rocky Mountains, Montana]

But most of the mountain ranges of the earth, and some of the
greatest, such as the Alps and the Himalayas, were originally
mountains of folding. The earth's crust has wrinkled into a fold; or
into a series of folds, forming a series of parallel ridges and
intervening valleys; or a number of folds have been mashed together
into a vast upswelling of the crust, in which the layers have been so
crumpled and twisted, overturned and crushed, that it is exceedingly
difficult to make out the original structure.

The close and intricate folds seen in great mountain ranges were
formed, as we have seen, deep below the surface, within the zone of
folding. Hence they may never have found expression in any individual
surface features. As the result of these deformations deep under
ground the surface was broadly lifted to mountain height, and the
crumpled and twisted mountain structures are now to be seen only
because erosion has swept away the heavy cover of surface rocks under
whose load they were developed.

   [Illustration: Fig. 179. Section of a Portion of the Alps]

When the structure of mountains has been deciphered it is possible to
estimate roughly the amount of horizontal compression which the region
has suffered. If the strata of the folds of the Alps were smoothed
out, they would occupy a belt seventy-four miles wider than that to
which they have been compressed, or twice their present width. A
section across the Appalachian folds in Pennsylvania shows a
compression to about two thirds the original width; the belt has been
shortened thirty-five miles in every hundred.

Considering the thickness of their strata, the compression which
mountains have undergone accounts fully for their height, with enough
to spare for all that has been lost by denudation.

The Appalachian folds involve strata thirty thousand feet in
thickness. Assuming that the folded strata rested on an unyielding
foundation, and that what was lost in width was gained in height, what
elevation would the range have reached had not denudation worn it as
it rose?

=The life history of mountains.= While the disturbance and uplift of
mountain masses are due to deformation, their sculpture into ridges
and peaks, valleys and deep ravines, and all the forms which meet the
eye in mountain scenery, excepting in the very youngest ranges, is due
solely to erosion. We may therefore classify mountains according to
the degree to which they have been dissected. The Juras are an example
of the stage of early youth, in which the anticlines still persist as
ridges and the synclines coincide with the valleys; this they owe as
much to the slight height of their uplift as to the recency of its
date (Fig. 180).

   [Illustration: Fig. 180. Section of a Portion of the Jura
      Mountains]

The Alps were upheaved at various times (Fig. 399), the last uplift
being later than the uplift of the Juras, but to so much greater
height that erosion has already advanced them well on towards
maturity. The mountain mass has been cut to the core, revealing
strange contortions of strata which could never have found expression
at the surface. Sharp peaks, knife-edged crests, deep valleys with
ungraded slopes subject to frequent landslides, are all features of
Alpine scenery typical of a mountain range at this stage in its life
history. They represent the survival of the hardest rocks and the
strongest structures, and the destruction of the weaker in their long
struggle for existence against the agents of erosion. Although miles
of rock have been removed from such ranges as the Alps, we need not
suppose that they ever stood much, if any, higher than at present. All
this vast denudation may easily have been accomplished while their
slow upheaval was going on; in several mountain ranges we have
evidence that elevation has not yet ceased.

   [Illustration: Fig. 181. Young Mountains, Rocky Mountains of
      Canada]

Under long denudation mountains are subdued to the forms
characteristic of old age. The lofty peaks and jagged crests of their
earlier life are smoothed down to low domes and rounded crests. The
southern Appalachians and portions of the Hartz Mountains in Germany
(Fig. 182) are examples of mountains which have reached this stage.

   [Illustration: Fig. 182. Subdued Mountains, the Hartz
      Mountains, Germany]

There are numerous regions of upland and plains in which the rocks are
found to have the same structure that we have seen in folded
mountains; they are tilted, crumpled, and overturned, and have clearly
suffered intense compression. We may infer that their folds were once
lifted to the height of mountains and have since been wasted to
low-lying lands. Such a section as that of Figure 67 illustrates how
ancient mountains may be leveled to their roots, and represents the
final stage to which even the Alps and the Himalayas must sometime
arrive. Mountains, perhaps of Alpine height, once stood about Lake
Superior; a lofty range once extended from New England and New Jersey
southwestward to Georgia along the Piedmont belt. In our study of
historic geology we shall see more clearly how short is the life of
mountains as the earth counts time, and how great ranges have been
lifted, worn away, and again upheaved into a new cycle of erosion.

=The sedimentary history of folded mountains.= We may mention here
some of the conditions which have commonly been antecedent to great
foldings of the crust.

1. Mountain ranges are made of belts of enormously and exceptionally
thick sediments. The strata of the Appalachians are thirty thousand
feet thick, while the same formations thin out to five thousand feet
in the Mississippi valley. The folds of the Wasatch Mountains involve
strata thirty thousand feet thick, which thin to two thousand feet in
the region of the Plains.

2. The sedimentary strata of which mountains are made are for the most
part the shallow-water deposits of continental deltas. Mountain ranges
have been upfolded along the margins of continents.

3. Shallow-water deposits of the immense thickness found in mountain
ranges can be laid only in a gradually sinking area. A profound
subsidence, often to be reckoned in tens of thousands of feet,
precedes the upfolding of a mountain range.

Thus the history of mountains of folding is as follows: For long ages
the sea bottom off the coast of a continent slowly subsides, and the
great trough, as fast as it forms, is filled with sediments, which at
last come to be many thousands of feet thick. The downward movement
finally ceases. A slow but resistless pressure sets in, and gradually,
and with a long series of many intermittent movements, the vast mass
of accumulated sediments is crumpled and uplifted into a mountain
range.


Fractures and Dislocations of the Crust

Considering the immense stresses to which the rocks of the crust are
subjected, it is not surprising to find that they often yield by
fracture, like brittle bodies, instead of by folding and flowing, like
plastic solids. Whether rocks bend or break depends on the character
and condition of the rocks, the load of overlying rocks which they
bear, and the amount of the force and the slowness with which it is
applied.

=Joints.= At the surface, where their load is least, we find rocks
universally broken into blocks of greater or less size by partings
known as joints. Under this name are included many division planes
caused by cooling and drying; but it is now generally believed that
the larger and more regular joints, especially those which run
parallel to the dip and strike of the strata, are fractures due to
up-and-down movements and foldings and twistings of the rocks.

   [Illustration: Fig. 183. Joints utilized by a River in widening
      its Valley, Iowa]

Joints are used to great advantage in quarrying, and we have seen how
they are utilized by the weather in breaking up rock masses, by rivers
in widening their valleys, by the sea in driving back its cliffs, by
glaciers in plucking their beds, and how they are enlarged in soluble
rocks to form natural passageways for underground waters. The ends of
the parted strata match along both sides of joint planes; in. joints
there has been little or no displacement of the broken rocks.

   [Illustration: Fig. 184. A Normal Fault]

=Faults.= In Figure 184 the rocks have been both broken and dislocated
along the plane _ff´_. One side must have been moved up or down past
the other. Such a dislocation is called a fault. The amount of the
displacement, as measured by the vertical distance between the ends of
a parted layer, is the _throw_ (_cd_). The angle (_ff´v_) which the
fault plane makes with the vertical is the _hade_. In Figure 184 the
right side has gone down relatively to the left; the right is the side
of the downthrow, while the left is the side of the upthrow. Where the
fault plane is not vertical the surfaces on the two sides may be
distinguished as the _hanging wall_ (that on the right of Figure 184)
and the _foot wall_ (that on the left of the same figure). Faults
differ in throw from a fraction of an inch to many thousands of feet.

=Slickensides.= If we examine the walls of a fault, we may find
further evidence of movement in the fact that the surfaces are
polished and grooved by the enormous friction which they have suffered
as they have ground one upon the other. These appearances, called
slickensides, have sometimes been mistaken for the results of glacial
action.

=Normal faults.= Faults are of two kinds,--normal faults and thrust
faults. Normal faults, of which Figure 184 is an example, hade to the
downthrow; the hanging wall has gone down. The total length of the
strata has been increased by the displacement. It seems that the
strata have been stretched and broken, and that the blocks have
readjusted themselves under the action of gravity as they settled.

=Thrust faults.= Thrust faults hade to the upthrow; the hanging wall
has gone up. Clearly such faults, where the strata occupy less space
than before, are due to lateral thrust. Folds and thrust faults are
closely associated. Under lateral pressure strata may fold to a
certain point and then tear apart and fault along the surface of least
resistance. Under immense pressure strata also break by shear without
folding. Thus, in Figure 185, the rigid earth block under lateral
thrust has found it easier to break along the fault plane than to
fold. Where such faults are nearly horizontal they are distinguished
as _thrust planes_.

   [Illustration: Fig. 185. A Thrust Fault]

In all thrust faults one mass has been pushed over another, so as to
bring the underlying and older strata upon younger beds; and when the
fault planes are nearly horizontal, and especially when the rocks have
been broken into many slices which have slidden far one upon another,
the true succession of strata is extremely hard to decipher.

In the Selkirk Mountains of Canada the basement rocks of the region
have been driven east for seven miles on a thrust plane, over rocks
which originally lay thousands of feet above them.

Along the western Appalachians, from Virginia to Georgia, the mountain
folds are broken by more than fifteen parallel thrust planes, running
from northeast to southwest, along which the older strata have been
pushed westward over the younger. The longest continuous fault has
been traced three hundred and seventy-five miles, and the greatest
horizontal displacement has been estimated at not less than eleven
miles.

=Crush breccia.= Rocks often do not fault with a clean and simple
fracture, but along a zone, sometimes several yards in width, in which
they are broken to fragments. It may occur also that strata which as a
whole yield to lateral thrust by folding include beds of brittle
rocks, such as thin-layered limestones, which are crushed to pieces by
the strain. In either case the fragments when recemented by
percolating waters form a rock known as a _crush breccia_ (pronounced
_bretcha_)(Fig. 186).

   [Illustration: Fig. 186. Breccia]

Breccia is a term applied to any rock formed of cemented _angular_
fragments. This rock may be made by the consolidation of volcanic
cinders, of angular waste at the foot of cliffs, or of fragments of
coral torn by the waves from coral reefs, as well as of strata crushed
by crustal movements.


Surface Features due to Dislocations

=Fault scarps.= A fault of recent date may be marked at surface by a
scarp, because the face of the upthrown block has not yet been worn to
the level of the downthrow side.

After the upthrown block has been worn down to this level,
differential erosion produces fault scarps wherever weak rocks and
resistant rocks are brought in contact along the fault plane; and the
harder rocks, whether on the upthrow or the downthrow side, emerge in
a line of cliffs. Where a fault is so old that no abrupt scarps
appear, its general course is sometimes marked by the line of division
between highland and lowland or hill and plain. Great faults have
sometimes brought ancient crystalline rocks in contact with weaker and
younger sedimentary rocks, and long after erosion has destroyed all
fault scarps the harder crystallines rise in an upland of rugged or
mountainous country which meets the lowland along the line of
faulting.

   [Illustration: Fig. 187. A Concealed Fault

   This fault may be inferred from the changes in strata in
   passing along the strike, as from _b_ to _a´_ and from
   _c_ to _b´_]

The vast majority of faults give rise to no surface features. The
faulted region may be old enough to have been baseleveled, or the
rocks on both sides of the line of dislocation may be alike in their
resistance to erosion and therefore have been worn down to a common
slope. The fault may be entirely concealed by the mantle of waste, and
in such cases it can be inferred from abrupt changes in the character
or the strike and dip of the strata where they may outcrop near it
(Fig. 187).

   [Illustration: Fig. 188. East-West Section across the Broken
      Plateau north of the Grand Canyon of the Colorado River,
      Arizona]

The plateau trenched by the Grand Canyon of the Colorado River
exhibits a series of magnificent fault scarps whose general course is
from north to south, marking the edges of the great crust blocks into
which the country has been broken. The highest part of the plateau is
a crust block ninety miles long and thirty-five miles in maximum
width, which has been hoisted to nine thousand three hundred feet
above, sea level. On the east it descends four thousand feet by a
monoclinal fold, which passes into a fault towards the north. On the
west it breaks down by a succession of terraces faced by fault scarps.
The throw of these faults varies from seven hundred feet to more than
a mile. The escarpments, however, are due in a large degree to the
erosion of weaker rock on the downthrow side.

   [Illustration: Fig. 189. The Fault separating the Highlands and
      the Lowlands, Scotland]

The Highlands of Scotland (Fig. 189) meet the Lowlands on the south
with a bold front of rugged hills along a line of dislocation which
runs across the country from sea to sea. On the one side are hills of
ancient crystalline rocks whose crumpled structures prove that they
are but the roots of once lofty mountains; on the other lies a lowland
of sandstone and other stratified rocks formed from the waste of those
long-vanished mountain ranges. Remnants of sandstone occur in places
on the north of the great fault, and are here seen to rest on the worn
and fairly even surface of the crystallines. We may infer that these
ancient mountains were reduced along their margins to low plains,
which were slowly lowered beneath the sea to receive a cover of
sedimentary rocks. Still later came an uplift and dislocation. On the
one side erosion has since stripped off the sandstones for the most
part, but the hard crystalline rocks yet stand in bold relief. On the
other side the weak sedimentary rocks have been worn down to lowlands.

=Rift valleys.= In a broken region undergoing uplift or the unequal
settling which may follow, a slice inclosed between two fissures may
sink below the level of the crust blocks on either side, thus forming
a linear depression known as a rift valley, or valley of fracture.

   [Illustration: Fig. 190. Section from the Mountains of
      Palestine to the Mountains of Moab across the Dead Sea

   _a_, ancient schists; _b_, Carboniferous strata; _c_, _d_, and
   _e_, Cretaceous strata]

One of the most striking examples of this rare type of valley is the
long trough which runs straight from the Lebanon Mountains of Syria on
the north to the Red Sea on the south, and whose central portion is
occupied by the Jordan valley and the Dead Sea. The plateau which it
gashes has been lifted more than three thousand feet above sea level,
and the bottom of the trough reaches a depth of two thousand six
hundred feet below that level in parts of the Dead Sea. South of the
Dead Sea the floor of the trough rises somewhat above sea level, and
in the Gulf of Akabah again sinks below it. This uneven floor could be
accounted for either by the profound warping of a valley of erosion or
by the unequal depression of the floor of a rift valley. But that the
trough is a true valley of fracture is proved by the fact that on
either side it is bounded by fault scarps and monoclinal folds. The
keystone of the arch has subsided. Many geologists believe that the
Jordan-Akabah trough, the long narrow basin of the Red Sea, and the
chain of down-faulted valleys which in Africa extends from the strait
of Bab-el-Mandeb as far south as Lake Nyassa--valleys which contain
more than thirty lakes--belong to a single system of dislocation.

Should you expect the lateral valleys of a rift valley at the time of
its formation to enter it as hanging valleys or at a common level?

=Block mountains.= Dislocations take place on so grand a scale that by
the upheaval of blocks of the earth's crust or the downfaulting of
the blocks about one which is relatively stationary, mountains known
as block mountains are produced. A tilted crust block may present a
steep slope on the side upheaved and a more gentle descent on the side
depressed.

   [Illustration: Fig. 191. Block Mountains, Southern Oregon]

=The Basin ranges.= The plateaus of the United States bounded by the
Rocky Mountains on the east, and on the west by the ranges which
front the Pacific, have been profoundly fractured and faulted. The
system of great fissures by which they are broken extends north and
south, and the long, narrow, tilted crust blocks intercepted between
the fissures give rise to the numerous north-south ranges of the
region. Some of the tilted blocks, as those of southern Oregon, are as
yet but moderately carved by erosion, and shallow lakes lie on the
waste that has been washed into the depressions between them (Fig.
191). We may therefore conclude that their displacement is somewhat
recent. Others, as those of Nevada, are so old that they have been
deeply dissected; their original form has been destroyed by erosion,
and the intermontane depressions are occupied by wide plains of waste.

=Dislocations and river valleys.= Before geologists had proved that
rivers can by their own unaided efforts cut deep canyons, it was
common to consider any narrow gorge as a gaping fissure of the crust.
This crude view has long since been set aside. A map of the plateaus
of northern Arizona shows how independent of the immense faults of the
region is the course of the Colorado River. In the Alps the tunnels on
the Saint Gotthard railway pass six times beneath the gorge of the
Reuss, but at no point do the rocks show the slightest trace of a
fault.

   [Illustration: Fig. 192. Fault crossing Valley in Japan]

=Rate of dislocation.= So far as human experience goes, the earth
movements which we have just studied, some of which have produced
deep-sunk valleys and lofty mountain ranges, and faults whose throw is
to be measured in thousands of feet, are slow and gradual. They are
not accomplished by a single paroxysmal effort, but by slow creep and
a series of slight slips continued for vast lengths of time.

In the Aspen mining district in Colorado faulting is now going on at a
comparatively rapid rate. Although no sudden slips take place, the
creep of the rock along certain planes of faulting gradually bends out
of shape the square-set timbers in horizontal drifts and has closed
some vertical shafts by shifting the upper portion across the lower.
Along one of the faults of this region it is estimated that there has
been a movement of at least four hundred feet since the Glacial epoch.
More conspicuous are the instances of active faulting by means of
sudden slips. In 1891 there occurred along an old fault plane in Japan
a slip which produced an earth rent traced for fifty miles (Fig. 192).
The country on one side was depressed in places twenty feet below that
on the other, and also shifted as much as thirteen feet horizontally
in the direction of the fault line.

In 1872 a slip occurred for forty miles on the great line of
dislocation which runs along the eastern base of the Sierra Nevada
Mountains. In the Owens valley, California, the throw amounted to
twenty-five feet in places, with a horizontal movement along the fault
line of as much as eighteen feet. Both this slip and that in Japan
just mentioned caused severe earthquakes.

For the sake of clearness we have described oscillations, foldings,
and fractures of the crust as separate processes, each giving rise to
its own peculiar surface features, but in nature earth movements are
by no means so simple,--they are often implicated with one another:
folds pass into faults; in a deformed region certain rocks have bent,
while others under the same strain, but under different conditions of
plasticity and load, have broken; folded mountains have been worn to
their roots, and the peneplains to which they have been denuded have
been upwarped to mountain height and afterwards dissected,--as in the
case of the Allegheny ridges, the southern Carpathians, and other
ranges,--or, as in the case of the Sierra Nevada Mountains, have been
broken and uplifted as mountains of fracture.

Draw the following diagrams, being careful to show the direction
in which the faulted blocks have moved, by the position of the two
parts of some well-defined layer of limestone, sandstone, or
shale, which occurs on each side of the fault plane, as in Figure
184.

1. A normal fault with a hade of 15°, the original fault
scarp remaining.

2. A normal fault with a hade of 50°, the original fault
scarp worn away, showing cliffs caused by harder strata on the
downthrow side.

3. A thrust fault with a hade of 30°, showing cliffs due to
harder strata outcropping on the downthrow.

4. A thrust fault with a hade of 80°, with surface
baseleveled.

5. In a region of normal faults a coal mine is being worked along
the seam of coal _AB_ (Fig. 193). At _B_ it is found broken by a fault
f which hades toward _A_. To find the seam again, should you advise
tunneling up or down from _B_?

   [Illustration: Fig. 193]

6. In a vertical shaft of a coal mine the same bed of coal is
pierced twice at different levels because of a fault. Draw a
diagram to show whether the fault is normal or a thrust.

   [Illustration: Fig. 194. Ridges to be explained by Faulting]

7. Copy the diagram in Figure 194, showing how the two ridges may
be accounted for by a single resistant stratum dislocated by a
fault. Is the fault a _strike fault_, i.e. one running parallel with
the strike of the strata, or a _dip fault_, one running parallel
with the direction of the dip?

   [Illustration: Fig. 195. Earth Block of Tilted Strata, with
      Included Seam of Coal _cc_]

8. Draw a diagram of the block in Figure 195 as it would appear if
dislocated along the plane _efg_ by a normal fault whose throw equals
one fourth the height of the block. Is the fault a strike or a dip
fault? Draw a second diagram showing the same block after denudation
has worn it down below the center of the upthrown side. Note that the
outcrop of the coal seam is now deceptively repeated. This exercise
may be done in blocks of wood instead of drawings.

   [Illustration: Fig. 196. _A_ and _B_. Repeated Outcrops of Same
      Strata]

9. Draw diagrams showing by dotted lines the conditions both of _A_
and _B_, Figure 196, after deformation had given the strata their
present attitude.

   [Illustration: Fig. 197. A Block Mountain]

10. What is the attitude of the strata of this earth block, Figure
197? What has taken place along the plane _baf_? When did the
dislocation occur compared with the folding of the strata? With the
erosion of the valleys on the right-hand side of the mountain? With
the deposition of the sediments _efg_? Do you find any remnants of the
original surface _baf_ produced by the dislocation? From the left-hand
side of the mountain infer what was the relief of the region before
the dislocation. Give the complete history recorded in the diagram
from the deposition of the strata to the present.

   [Illustration: Fig. 198. A Faulted Lava Flow _aa´_]

11. Which is the older fault, in Figure 198, _f_ or _f´_? When did the
lava flow occur? How long a time elapsed between the formation of the
two faults as measured in the work done in the interval? How long a
time since the formation of the later fault?

   [Illustration: Fig. 199. Measurement of the Thickness of
      Inclined Strata]

12. Measure by the scale the thickness _bc_ of the coal-bearing strata
outcropping from _a_ to _b_ in Figure 199. On any convenient scale
draw a similar section of strata with a dip of 30° outcropping along a
horizontal line normal to the strike one thousand feet in length, and
measure the thickness of the strata by the scale employed. The
thickness may also be calculated by trigonometry.

   [Illustration: Fig. 200. Unconformity between Parallel Strata]

   [Illustration: Fig. 201. Unconformity between Non-parallel
      Strata]


Unconformity

Strata deposited one upon, another in an unbroken succession are said
to be _conformable_. But the continuous deposition of strata is often
interrupted by movements of the earth's crust, Old sea floors are
lifted to form land and are again depressed beneath the sea to receive
a cover of sediments only after an interval during which they were
carved by subaërial erosion. An erosion surface which thus parts older
from younger strata is known as an _unconformity_, and the strata
above it are said to be _unconformable_ with the rocks below, or to
rest unconformably upon them. An unconformity thus records movements
of the crust and a consequent break in the deposition of the strata.
It denotes a period of land erosion of greater or less length, which
may sometimes be roughly measured by the stage in the erosion cycle
which the land surface had attained before its burial. Unconformable
strata may be _parallel_, as in Figure 200, where the record includes
the deposition of strata _a_, their emergence, the erosion of the land
surface _ss_, a submergence and the deposit of the strata _b_, and
lastly, emergence and the erosion of the present surface _s´s´_.

   [Illustration: Fig. 202. Carboniferous Limestone resting
      unconformably on Early Silurian Slates, Yorkshire, England]

Often the earth movements to which the uplift or depression was due
involved tilting or folding of the earlier strata, so that the strata
are now nonparallel as well as unconformable. In Figure 201, for
example, the record includes deposition, uplift, and _tilting_ of _a_;
erosion, depression, the deposit of _b_; and finally the uplift which
has brought the rocks to open air and permitted the dissection by
which the unconformity is revealed.

From this section we infer that during early Silurian times the area
was sea, and thick sea muds were laid upon it. These were later
altered to hard slates by pressure and upfolded into mountains. During
the later Silurian and the Devonian the area was land and suffered
vast denudation. In the Carboniferous period it was lowered beneath
the sea and received a cover of limestone.

   [Illustration: Fig. 203. Diagram Illustrating how the Age of
      Mountains is determined]

=The age of mountains.= It is largely by means of unconformities that
we read the history of mountain making and other deformations and
movements of the crust. In Figure 203, for example, the deformation
which upfolded the range of mountains took place after the deposit of
the series of strata a of which the mountains are composed, and before
the deposit of the stratified rocks, which rest unconformably on a and
have not shared their uplift.

   [Illustration: Fig. 204. Section of Mountain Range showing
      repeated Uplifts

   _a_, strata whose folding formed a mountain range; on,
   baseleveled surface produced by long denudation of the
   mountains; _b_, tilted strata resting unconformably on _a_;
   _c_, horizontal strata parted from _b_ by the unconformity
   _u´u´_. The first uplift of the range preceded the period of
   time when _b_ was deposited. The and uplift, to which the
   present mountains owe their height, was later than this period
   but earlier than the period when strata _c_ were laid]

Most great mountain ranges, like the Sierra Nevada and the Alps, mark
lines of weakness along which the earth's crust has yielded again and
again during the long ages of geological time. The strata deposited at
various times about their flanks have been infolded by later
crumplings with the original mountain mass, and have been repeatedly
crushed, inverted, faulted, intruded with igneous rocks, and denuded.
The structure of great mountain ranges thus becomes exceedingly
complex and difficult to read. A comparatively simple case of repeated
uplift is shown in Figure 204. In the section of a portion of the Alps
shown in Figure 179 a far more complicated history may be deciphered.

   [Illustration: Fig. 205. Unconformity showing Buried Valleys

   _lm_, limestone; _sh_, shale; _r_, _r´_, and _r´´_, river silts
   filling eroded valleys in the limestone. The upper surface of
   the limestone is evidently a land surface developed by erosion.
   The valleys which trench it are narrow and steep-sided; hence
   the land surface had not reached maturity. The sands and muds,
   now hardened to firm rock, which fill these valleys, _r_, _r´_,
   and _r´´_, contain no relics of the sea, but Instead the remains
   of land animals and plants. They are river deposits, and we may
   infer that owing to a subsidence the young rivers ceased to
   degrade their channels and slowly filled their gorges with
   sands and silts. The overlying shale records a further
   depression which brought the lanes below the level of the sea.
   A section similar to this is to be seen in the coal mines of
   Bernissant, Belgium, where a gorge twice as deep as that of
   Niagara was discovered within whose ancient river deposits were
   found entombed the skeletons of more than a score of the huge
   reptiles characteristic of the age when the gorge was cut and
   filled]

   [Illustration: Fig. 206. Unconformity showing Buried Mountains,
      Scotland

   _gn_, ancient crystalline rocks; _ss_, marine sandstones. The
   surface _bb_ of the ancient crystalline rocks is mountainous,
   with peaks rising to a height of as much as three thousand
   feet. It is one of the most ancient land surfaces on the planet
   and is covered unconformably with pre-Cambrian sandstones
   thousands of feet in thickness, in which the Torridonian
   Mountains of Scotland have been carved. What has been the
   history of the region since the mountainous surface _bb_ was
   produced by erosion?]

=Unconformities in the Colorado Canyon, Arizona.= How geological
history may be read in unconformities is further illustrated in
Figures 207 and 208>. The dark crystalline rocks _a_ at the bottom of
the canyon are among the most ancient known, and are overlain
unconformably by a mass of tilted coarse marine sandstones _b_, whose
total thickness is not seen in the diagram and measures twelve
thousand feet perpendicularly to the dip. Both _a_ and _b_ rise to a
common level _nn´_ and upon them rest the horizontal sea-laid strata
_c_, in which the upper portion of the canyon has been cut.

   [Illustration: Fig. 207. Diagram of Wall of the Colorado
      Canyon, Arizona, showing Unconformities]

Note that the crystalline rocks a have been crumpled and crushed.
Comparing their structure with that of folded mountains, what do you
infer as to their relief after their deformation? To which surface
were they first worn down, _mm´_ or _nm_? Describe and account for the
surface _mm´_. How does it differ from the surface of the crystalline
rocks seen in the Torridonian Mountains (Fig. 206), and why? This
surface _mm´_ is one of the oldest land surfaces of which any vestige
remains. It is a bit of fossil geography buried from view since the
earliest geological ages and recently brought to light by the erosion
of the canyon.

   [Illustration: Fig. 208. View of the North Wall of the Grand
      Canyon of the Colorado River, Arizona, showing the
      Unconformities illustrated in Figure 207]

How did the surface _mm´_ come to receive its cover of sandstones _b_?
From the thickness and coarseness of these sediments draw inferences
as to the land mass from which they were derived. Was it rising or
subsiding? high or low? Were its streams slow or swift? Was the amount
of erosion small or great?

Note the strong dip of these sandstones _b_. Was the surface _mm´_
tilted as now when the sandstones were deposited upon it? When was it
tilted? Draw a diagram showing the attitude of the rocks after this
tilting occurred, and their height relative to sea level.

The surface _nn´_ is remarkably even, although diversified by some low
hills which rise into the bedded rocks of _c_, and it may be traced
for long distances up and down the canyon. Were the layers of _b_ and
the surface _mm´_ always thus cut short by _nn´_ as now? What has made
the surface _nn´_ so even? How does it come to cross the hard
crystalline rocks a and the weaker sandstones _b_ at the same
impartial level? How did the sediments of _c_ come to be laid upon it?
Give now the entire history recorded in the section, and in addition
that involved in the production of the platform _P_, shown in Figure
130, and that of the cutting of the canyon. How does the time involved
in the cutting of the canyon compare with that required for the
production of the surfaces _mm´_, _nn´_, and _P_?




CHAPTER X

EARTHQUAKES


Any sudden movement of the rocks of the crust, as when they tear apart
when a fissure is formed or extended, or slip from time to time along
a growing fault, produces a jar called an earthquake, which spreads in
all directions from the place of disturbance.

=The Charleston earthquake.= On the evening of August 31, 1886, the
city of Charleston, S.C., was shaken by one of the greatest
earthquakes which has occurred in the United States. A slight tremor
which rattled the windows was followed a few seconds later by a roar,
as of subterranean thunder, as the main shock passed beneath the city.
Houses swayed to and fro, and their heaving floors overturned
furniture and threw persons off their feet as, dizzy and nauseated,
they rushed to the doors for safety. In sixty seconds a number of
houses were completely wrecked, fourteen thousand chimneys were
toppled over, and in all the city scarcely a building was left without
serious injury. In the vicinity of Charleston railways were twisted
and trains derailed. Fissures opened in the loose superficial
deposits, and in places spouted water mingled with sand from shallow
underlying aquifers.

The point of origin, or _focus_, of the earthquake was inferred from
subsequent investigations to be a rent in the rocks about twelve miles
beneath the surface. From the center of greatest disturbance, which
lay above the focus, a few miles northwest of the city, the surface
shock traveled outward in every direction, with decreasing effects, at
the rate of nearly two hundred miles per minute. It was felt from
Boston to Cuba, and from eastern Iowa to the Bermudas, over a circular
area whose diameter was a thousand miles.

An earthquake is transmitted from the focus through the elastic rocks
of the crust, as a wave, or series of waves, of compression and
rarefaction, much as a sound wave is transmitted through the elastic
medium of the air. Each earth particle vibrates with exceeding
swiftness, but over a very short path. The swing of a particle in firm
rock seldom exceeds one tenth of an inch in ordinary earthquakes, and
when it reaches one half an inch and an inch, the movement becomes
dangerous and destructive.

   [Illustration: Fig. 210. Block of the Earth's Crust shaken by
      an Earthquake

   _x_, focus; _a_, _b_, _c_, _d_, successive spheroidal waves in
   the crust; _a´_, _b´_, _c´_, _d´_, successive surface waves
   produced by the outcropping of _a_, _b_, _c_, and _d_]

The velocity of earthquake waves, like that of all elastic waves,
varies with the temperature and elasticity of the medium. In the deep,
hot, elastic rocks they speed faster than in the cold and broken rocks
near the surface. The deeper the point of origin and the more violent
the initial shock, the faster and farther do the vibrations run.

Great earthquakes, caused by some sudden displacement or some violent
rending of the rocks, shake the entire planet. Their waves run through
the body of the earth at the rate of about three hundred and fifty
miles a minute, and more slowly round its circumference, registering
their arrival at opposite sides of the globe on the exceedingly
delicate instruments of modern earthquake observatories.

=Geological effects.= Even great earthquakes seldom produce geological
effects of much importance. Landslides may be shaken down from the
sides of mountains and hills, and cracks may be opened in the surface
deposits of plains; but the transient shiver, which may overturn
cities and destroy thousands of human lives, runs through the crust
and leaves it much the same as before.

=Earthquakes attending great displacements.= Great earthquakes
frequently attend the displacement of large masses of the rocks of the
crust. In 1822 the coast of Chile was suddenly raised three or four
feet, and the rise was five or six feet a mile inland. In 1835 the
same region was again upheaved from two to ten feet. In each instance
a destructive earthquake was felt for one thousand miles along the
coast.

The great California earthquake of 1906.= A sudden dislocation
occurred in 1906 along an ancient fault plane which extends for 300
miles through western California. The vertical displacement did not
exceed four feet, while the horizontal shifting reached a maximum of
twenty feet. Fences, rows of trees, and roads which crossed the fault
were broken and offset. The latitude and longitude of all points over
thousands of square miles were changed. On each side of the fault the
earth blocks moved in opposite directions, the block on the east
moving southward and that on the west moving northward and to twice
the distance. East and west of the fault the movements lessened with
increasing distance from it.

This sudden slip set up an earthquake lasting sixty-five seconds,
followed by minor shocks recurring for many days. In places the jar
shook down the waste on steep hillsides, snapped off or uprooted
trees, and rocked houses from their foundations or threw down their
walls or chimneys. The water mains of San Francisco were broken, and
the city was thus left defenseless against a conflagration which
destroyed $500,000,000 worth of property. The destructive effects
varied with the nature of the ground. Buildings on firm rock suffered
least, while those on deep alluvium were severely shaken by the
undulations, like water waves, into which the loose material was
thrown. Well-braced steel structures, even of the largest size, were
earthquake proof, and buildings of other materials, when honestly
built and intelligently designed to withstand earthquake shocks,
usually suffered little injury. The length of the intervals between
severe earthquakes in western California shows that a great
dislocation so relieves the stresses of the adjacent earth blocks that
scores of years may elapse before the stresses again accumulate and
cause another dislocation.

Perhaps the most violent earthquake which ever visited the United
States attended the depression, in 1812, of a region seventy-five
miles long and thirty miles wide, near New Madrid, Mo. Much of the
area was converted into swamps and some into shallow lakes, while a
region twenty miles in diameter was bulged up athwart the channel of
the Mississippi. Slight quakes are still felt in this region from time
to time, showing that the strains to which the dislocation was due
have not yet been fully relieved.

=Earthquakes originating beneath the sea.= Many earthquakes originate
beneath the sea, and in a number of examples they seem to have been
accompanied, as soundings indicate, by local subsidences of the ocean
bottom. There have been instances where the displacement has been
sufficient to set the entire Pacific Ocean pulsating for many hours.
In mid ocean the wave thus produced has a height of only a few feet,
while it may be two hundred miles in width. On shores near the point
of origin destructive waves two or three score feet in height roll in,
and on coasts thousands of miles distant the expiring undulations may
be still able to record themselves on tidal gauges.

=Distribution of earthquakes.= Every half hour some considerable area
of the earth's surface is sensibly shaken by an earthquake, but
earthquakes are by no means uniformly distributed over the globe. As
we might infer from what we know as to their causes, earthquakes are
most frequent in regions now undergoing deformation. Such are young
rising mountain ranges, fault lines where readjustments recur from
time to time, and the slopes of suboceanic depressions whose steepness
suggests that subsidence may there be in progress.

Earthquakes, often of extreme severity, frequently visit the lofty and
young ranges of the Andes, while they are little known in the subdued
old mountains of Brazil. The Highlands of Scotland are crossed by a
deep and singularly straight depression called the Great Glen, which
has been excavated along a very ancient line of dislocation. The
earthquakes which occur from time to time in this region, such as the
Inverness earthquake in 1891, are referred to slight slips along this
fault plane.

In Japan, earthquakes are very frequent. More than a thousand are
recorded every year, and twenty-nine world-shaking earthquakes
occurred in the three years ending with 1901. They originate, for the
most part, well down on the eastern flank of the earth fold whose
summit is the mountainous crest of the islands, and which plunges
steeply beneath the sea to the abyss of the Tuscarora Deep.

=Minor causes of earthquakes.= Since any concussion within the crust
sets up an earth jar, there are several minor causes of earthquakes,
such as volcanic explosions and even the collapse of the roofs of
caves. The earthquakes which attend the eruption of volcanoes are
local, even in the case of the most violent volcanic paroxysms known.
When the top of a volcano has been blown to fragments, the
accompanying earth shock has sometimes not been felt more than
twenty-five miles away.

=Depth of focus.= The focus of the Charleston earthquake, estimated at
about twelve miles below the surface, was exceptionally deep. Volcanic
earthquakes are particularly shallow, and probably no earthquakes
known have started at a greater depth than fifteen or twenty miles.
This distance is so slight compared with the earth's radius that we
may say that earthquakes are but skin-deep.

Should you expect the velocity of an earthquake to be greater in a
peneplain or in a river delta?

After an earthquake, piles on which buildings rested were found driven
into the ground, and chimneys crushed at base. From what direction did
the shock come?

Chimneys standing on the south walls of houses toppled over on the
roof. Should you infer that the shock in this case came from the north
or south?

How should you expect a shock from the east to affect pictures hanging
on the east and the west walls of a room? how the pictures hanging on
the north and the south walls?

In parts of the country, as in southwestern Wisconsin, slender erosion
pillars, or "monuments," are common. What inference could you draw as
to the occurrence in such regions of severe earthquakes in the recent
past?




CHAPTER XI

VOLCANOES


Connected with movements of the earth's crust which take place so
slowly that they can be inferred only from their effects is one of the
most rapid and impressive of all geological processes,--the extrusion
of molten rock from beneath the surface of the earth, giving rise to
all the various phenomena of volcanoes.

In a volcano, molten rock from a region deep below, which we may call
its reservoir, ascends through a pipe or fissure to the surface. The
materials erupted may be spread over vast areas, or, as is commonly
the case, may accumulate about the opening, forming a conical pile
known as the volcanic cone. It is to this cone that popular usage
refers the word _volcano_; but the cone is simply a conspicuous part
of the volcanic mechanism whose still more important parts, the
reservoir and the pipe, are hidden from view.

Volcanic eruptions are of two types,--_effusive_ eruptions, in which
molten rock wells up from below and flows forth in streams of _lava_
(a comprehensive term applied to all kinds of rock emitted from
volcanoes in a molten state), and _explosive_ eruptions, in which the
rock is blown out in fragments great and small by the expansive force
of steam.


Eruptions of the Effusive Type

=The Hawaiian volcanoes.= The Hawaiian Islands are all volcanic
in origin, and have a linear arrangement characteristic of many
volcanic groups in all parts of the world. They are strung along a
northwest-southeast line, their volcanoes standing in two parallel
rows as if reared along two adjacent lines of fracture or folding. In
the northwestern islands the volcanoes have long been extinct and are
worn low by erosion. In the southeastern island. Hawaii, three
volcanoes are still active and in process of building. Of these Mauna
Loa, the monarch of volcanoes, with a girth of two hundred miles and a
height of nearly fourteen thousand feet above sea level, is a lava
dome the slope of whose sides does not average more than five degrees.
On the summit is an elliptical basin ten miles in circumference and
several hundred feet deep. Concentric cracks surround the rim, and
from time to time the basin is enlarged as great slices are detached
from the vertical walls and engulfed.

Such a volcanic basin, formed by the insinking of the top of the cone,
is called a _caldera_.

    [Illustration: Fig. 211. Mauna Loa]

    [Illustration: Fig. 212. Caldera of Mauna Loa]

On the flanks of Mauna Loa, four thousand feet above sea level, lies the
caldera of Kilauea, an independent volcano whose dome has been joined to
the larger mountain by the gradual growth of the two. In each caldera
the floor, which to the eye is a plain of black lava, is the congealed
surface of a column of molten rock. At times of an eruption lakes of
boiling lava appear which may be compared to air holes in a frozen
river. Great waves surge up, lifting tons of the fiery liquid a score of
feet in air, to fall back with a mighty plunge and roar, and
occasionally the lava rises several hundred feet in fountains of
dazzling brightness. The lava lakes may flood the floor of the basin,
but in historic times have never been known to fill it and overflow the
rim. Instead, the heavy column of lava breaks way through the sides of
the mountain and discharges in streams which flow down the mountain
slopes for a distance sometimes of as much as thirty-five miles. With
the drawing off of the lava the column in the duct of the volcano
lowers, and the floor of the caldera wholly or in part subsides. A black
and steaming abyss marks the place of the lava lakes (Fig. 213). After a
time the lava rises in the duct, the floor is floated higher, and the
boiling lakes reappear.

    [Illustration: Fig. 213. Portion of Caldera of Kilauea after
       Collapse following an Eruption]

The eruptions of the Hawaiian volcanoes are thus of the effusive type.
The column of lava rises, breaks through the side of the mountain, and
discharges in lava streams. There are no explosions, and usually no
earthquakes, or very slight ones, accompany the eruptions. The lava in
the calderas boils because of escaping steam, but the vapor emitted is
comparatively little, and seldom hangs above the summits in heavy
clouds. We see here in its simplest form the most impressive and
important fact in all volcanic action, molten rock has been driven
upward to the surface from some deep-lying source.

=Lava flows.= As lava issues from the side of a volcano or overflows
from the summit, it flows away in a glowing stream resembling molten
iron drawn white-hot from an iron furnace. The surface of the stream
soon cools and blackens, and the hard crust of nonconducting rock may
grow thick and firm enough to form a tunnel, within which the fluid
lava may flow far before it loses its heat to any marked degree. Such
tunnels may at last be left as caves by the draining away of the lava,
and are sometimes several miles in length.

    [Illustration: Fig. 214. Pahoehoe Lava, Hawaii]

=Pahoehoe and aa.= When the crust of highly fluid lava remains unbroken
after its first freezing, it presents a smooth, hummocky, and ropy
surface known by the Hawaiian term _pahoehoe_ (Fig. 214). On the other
hand, the crust of a viscid flow may be broken and splintered as it is
dragged along by the slowly moving mass beneath. The stream then appears
as a field of stones clanking and grinding on, with here and there from
some chink a dull red glow or a wisp of steam. It sets to a surface
called _aa_, of broken, sharp-edged blocks, which is often both
difficult and dangerous to traverse (Fig. 215).

    [Illustration: Fig. 215. Lava Flow of the _Aa_ Type, Cinder
       Cones in the Distance, Arizona]

=Fissure eruptions.= Some of the largest and most important outflows
of lava have not been connected with volcanic cones, but have been
discharged from fissures, flooding the country far and wide with
molten rock. Sheet after sheet of molten rock has been successively
outpoured, and there have been built up, layer upon layer, plateaus of
lava thousands of feet in thickness and many thousands of square miles
in area.

=Iceland.= This island plateau has been rent from time to time by
fissures from which floods of lava have outpoured. In some instances
the lava discharges along the whole length of the fissure, but more
often only at certain points upon it. The Laki fissure, twenty miles
long, was in eruption in 1783 for seven months. The inundation of
fluid rock which poured from it is the largest of historic record,
reaching a distance of forty-seven miles and covering two hundred and
twenty square miles to an average depth of a hundred feet. At the
present time the fissure is traced by a line of several hundred
insignificant mounds of fragmental materials which mark where the lava
issued (Fig. 216).

The distance to which the fissure eruptions of Iceland flow on slopes
extremely gentle is noteworthy. One such stream is ninety miles in
length, and another seventy miles long has a slope of little more than
one half a degree.

Where lava is emitted at one point and flows to a less distance there
is gradually built up a dome of the shape of an inverted saucer with
an immense base but comparatively low. Many _lava domes_ have been
discovered in Iceland, although from their exceedingly gentle slopes,
often but two or three degrees, they long escaped the notice of
explorers.

The entire plateau of Iceland, a region as large as Ohio, is composed
of volcanic products,--for the most part of successive sheets of lava
whose total thickness falls little short of two miles. The lava sheets
exposed to view were outpoured in open air and not beneath the sea;
for peat bogs and old forest grounds are interbedded with them, and
the fossil plants of these vegetable deposits prove that the plateau
has long been building and is very ancient. On the steep sea cliffs of
the island, where its structure is exhibited, the sheets of lava are
seen to be cut with many _dikes_,--fissures which have been filled by
molten rock,--and there is little doubt that it was through these
fissures that the lava outwelled in successive flows which spread far
and wide over the country and gradually reared the enormous pile of
the plateau.


Eruptions of the Explosive Type

In the majority of volcanoes the lava which rises in the pipe is at
least in part blown into fragments with violent explosions and shot
into the air together with vast quantities of water vapor and various
gases. The finer particles into--which the lava is exploded are called
_volcanic dust_ or _volcanic ashes_, and are often carried long
distances by the wind before they settle to the earth. The coarser
fragments fall about the vent and there accumulate in a steep,
conical, volcanic mountain. As successive explosions keep open the
throat of the pipe, there remains on the summit a cup-shaped
depression called the _crater_.

=Stromboli.= To study the nature of these explosions we may visit
Stromboli, a low volcano built chiefly of fragmental materials, which
rises from the sea off the north coast of Sicily and is in constant
though moderate action.

Over the summit hangs a cloud of vapor which strikingly resembles the
column of smoke puffed from the smokestack of a locomotive, in that it
consists of globular masses, each the product of a distinct explosion.
At night the cloud of vapor is lighted with a red glow at intervals of
a few minutes, like the glow on the trail of smoke behind the
locomotive when from time to time the fire box is opened. Because of
this intermittent light flashing thousands of feet above the sea,
Stromboli has been given the name of the Lighthouse of the
Mediterranean.

Looking down into the crater of the volcano, one sees a viscid lava
slowly seething. The agitation gradually increases. A great bubble
forms. It bursts with an explosion which causes the walls of the
crater to quiver with a miniature earthquake, and an outrush of steam
carries the fragments of the bubble aloft for a thousand feet to fall
into the crater or on the mountain side about it. With the explosion
the cooled and darkened crust of the lava is removed, and the light of
the incandescent liquid beneath is reflected from the cloud of vapor
which overhangs the cone.

At Stromboli we learn the lesson that the explosive force in volcanoes
is that of steam. The lava in the pipe is permeated with it much as is
a thick boiling porridge. The steam in boiling porridge is unable to
escape freely and gathers into bubbles which in breaking spurt out
drops of the pasty substance; in the same way the explosion of great
bubbles of steam in the viscid lava shoots clots and fragments of it
into the air.

=Krakatoa.= The most violent eruption of history, that of Krakatoa, a
small volcanic island in the strait between Sumatra and Java, occurred
in the last week of August, 1883. Continuous explosions shot a column
of steam and ashes. seventeen miles in air. A black cloud, beneath
which was midnight darkness and from which fell a rain of ashes and
stones, overspread the surrounding region to a distance of one hundred
and fifty miles. Launched on the currents of the upper air, the dust
was swiftly carried westward to long distances. Three days after the
eruption it fell on the deck of a ship sixteen hundred miles away, and
in thirteen days the finest impalpable powder from the volcano had
floated round the globe. For many months the dust hung over Europe and
America as a faint lofty haze illuminated at sunrise and sunset with
brilliant crimson. In countries nearer the eruption, as in India and
Africa, the haze for some time was so thick that it colored sun and
moon with blue, green, and copper-red tints and encircled them with
coronas.

At a distance of even a thousand miles the detonations of the eruption
sounded like the booming of heavy guns a few miles away. In one
direction they were audible for a distance as great as that from San
Francisco to Cleveland. The entire atmosphere was thrown into
undulations under which all barometers rose and fell as the air waves
thrice encircled the earth. The shock of the explosions raised sea
waves which swept round the adjacent shores at a height of more than
fifty feet, and which were perceptible halfway around the globe.

At the close of the eruption it was found that half the mountain had
been blown away, and that where the central part of the island had
been the sea was a thousand feet deep.

=Martinique and St. Vincent.= In 1902 two dormant volcanoes of the
West Indies, Mt. Pelee in Martinique and Soufrière in St. Vincent,
broke into eruption simultaneously. No lava was emitted, but there
were blown into the air great quantities of ashes, which mantled the
adjacent parts of the islands with a pall as of gray snow. In early
stages of the eruption lakes which occupied old craters were
discharged and swept down the ash-covered mountain valleys in torrents
of boiling mud.

On several occasions there was shot from the crater of each volcano a
thick and heavy cloud of incandescent ashes and steam, which rushed
down the mountain side like an avalanche, red with glowing stones and
scintillating with lightning flashes. Forests and buildings in its
path were leveled as by a tornado, wood was charred and set on fire by
the incandescent fragments, all vegetation was destroyed, and to
breathe the steam and hot, suffocating dust of the cloud was death to
every living creature. On the morning of the 8th of May, 1902, the
first of these peculiar avalanches from Mt. Pelee fell on the city of
St. Pierre and instantly destroyed the lives of its thirty thousand
inhabitants.

   [Illustration: Fig. 219. An Eruption of Vesuvius, 1872]

The eruptions of many volcanoes partake of both the effusive and the
explosive types: the molten rock in the pipe is in part blown into the
air with explosions of steam, and in part is discharged in streams of
lava over the lip of the crater and from fissures in the sides of the
cone. Such are the eruptions of Vesuvius, one of which is illustrated
in Figure 219.

=Submarine eruptions.= The many volcanic islands of the ocean and the
coral islands resting on submerged volcanic peaks prove that eruptions
have often taken place upon the ocean floor and have there built up
enormous piles of volcanic fragments and lava. The Hawaiian volcanoes
rise from a depth of eighteen thousand feet of water and lift their
heads to about thirty thousand feet above the ocean bed. Christmas
Island (see p. 194), built wholly beneath the ocean, is a coral-capped
volcanic peak, whose total height, as measured from the bottom of the
sea, is more than fifteen thousand feet. Deep-sea soundings have
revealed the presence of numerous peaks which fail to reach sea level
and which no doubt are submarine volcanoes. A number of volcanoes on
the land were submarine in their early stages, as, for example, the
vast pile of Etna, the celebrated Sicilian volcano, which rests on
stratified volcanic fragments containing marine shells now uplifted
from the sea.

Submarine outflows of lava and deposits of volcanic fragments become
covered with sediments during the long intervals between eruptions.
Such volcanic deposits are said to be _contemporaneous_, because they
are formed during the same period as the strata among which they are
imbedded. Contemporaneous lava sheets may be expected to bake the
surface of the stratum on which they rest, while the sediments
deposited upon them are unaltered by their heat. They are among the
most permanent records of volcanic action, far outlasting the greatest
volcanic mountains built in open air.

From upraised submarine volcanoes, such as Christmas Island, it is
learned that lava flows which are poured out upon the bottom of the
sea do not differ materially either in composition or texture from
those of the land.


Volcanic Products

Vast amounts of steam are, as we have seen, emitted from volcanoes,
and comparatively small quantities of other vapors, such as various
acid and sulphurous gases. The rocks erupted from volcanoes differ
widely in chemical composition and in texture.

   [Illustration: Fig. 220. Cellular Lava]

=Acidic and basic lavas.= Two classes of volcanic rocks may be
distinguished,--those containing a large proportion of silica (silicic
acid, SiO_{2}) and therefore called _acidic_, and those containing less
silica and a larger proportion of the bases (lime, magnesia, soda,
etc.) and therefore called _basic_. The acidic lavas, of which
_rhyolite_ and _thrachyte_ are examples, are comparatively light in
color and weight, and are difficult to melt. The basic lavas, of which
_basalt_ is a type, are dark and heavy and melt at a lower
temperature.

=Scoria and pumice.= The texture of volcanic rocks depends in part on
the degree to which they were distended by the steam which permeated
them when in a molten state. They harden into compact rock where the
steam cannot expand. Where the steam is released from pressure, as on
the surface of a lava stream, it forms bubbles (steam blebs) of
various sizes, which give the hardened rock a cellular structure
(Fig. 220), In this way are formed the rough slags and clinkers called
_scoria_, which are found on the surface of flows and which are also
thrown out as clots of lava in explosive eruptions.

On the surface of the seething lava in the throat of the volcano there
gathers a rock foam, which, when hurled into the air, is cooled and
falls as _pumice_,--a spongy gray rock so light that it floats on
water.

   [Illustration: Fig. 221. Amygdules in Lava]

=Amygdules.= The steam blebs of lava flows are often drawn out from a
spherical to an elliptical form resembling that of an almond, and
after the rock has cooled these cavities are gradually filled with
minerals deposited from solution by underground water. From their
shape such casts are called amygdules (Greek, _amygdalon_, an almond).
Amygdules are commonly composed of silica. Lavas contain both silica
and the alkalies, potash and soda, and after dissolving the alkalies,
percolating water is able to take silica also into solution. Most
_agates_ are banded amygdules in which the silica has been laid in
varicolored, concentric layers (Fig. 222).

   [Illustration: Fig. 222. Polished Section of an Agate]

   [Illustration: Fig. 223. Microsection showing the Beginnings of
      Crystal Growth in Glassy Lava]

=Glassy and stony lavas.= Volcanic rocks differ in texture according
also to the rate at which they have solidified. When rapidly cooled,
as on the surface of a lava flow, molten rock chills to a glass,
because the minerals of which it is composed have not had time to
separate themselves from the fused mixture and form crystals. Under
slow cooling, as in the interior of the flow, it becomes a stony mass
composed of crystals set in a glassy paste. In thin slices of volcanic
glass one may see under the microscope the beginnings of crystal
growth in filaments and needles and feathery forms, which are the
rudiments of the crystals of various minerals.

Spherulites, which also mark the first changes of glassy lavas toward
a stony condition, are little balls within the rock, varying from
microscopic size to several inches in diameter, and made up of
radiating fibers.

Perlitic structure, common among glassy lavas, consists of microscopic
curving and interlacing cracks, due to contraction.

   [Illustration: Fig. 224. Perlitic Structure and Spherulites,
       _a_, _a_]

   [Illustration: Fig. 225. Flow Lines in Lava]

=Flow lines= are exhibited by volcanic rocks both to the naked eye and
under the microscope. Steam blebs, together with crystals and their
embryonic forms, are left arranged in lines and streaks by the
currents of the flowing lava as it stiffened into rock.

   [Illustration: Fig. 226. Porphyritic Structure]

=Porphyritic structure.= Rocks whose ground mass has scattered through
it large conspicuous crystals (Fig. 226) are said to be _porphyritic_,
and it is especially among volcanic rocks that this structure occurs.
The ground mass of porphyries either may be glassy or may consist in
part of a felt of minute crystals; in either case it represents the
consolidation of the rock after its outpouring upon the surface. On
the other hand, the large crystals of porphyry have slowly formed deep
below the ground at an earlier date.

=Columnar structure.= Just as wet starch contracts on drying to
prismatic forms, so lava often contracts on cooling to a mass of
close-set, prismatic, and commonly six-sided columns, which stand at
right angles to the cooling surface. The upper portion of a flow, on
rapid cooling from the surface exposed to the air, may contract to a
confused mass of small and irregular prisms; while the remainder forms
large and beautifully regular columns, which have grown upward by slow
cooling from beneath (Fig. 227).


Fragmental Materials

Rocks weighing many tons are often thrown from a volcano at the
beginning of an outburst by the breaking up of the solidified floor of
the crater; and during the progress of an eruption large blocks may be
torn from the throat of the volcano by the outrush of steam. But the
most important fragmental materials are those derived from the lava
itself. As lava rises in the pipe, the steam which permeates it is
released from pressure and explodes, hurling the lava into the air in
fragments of all sizes,--large pieces of scoria, _lapilli_ (fragments
the size of a pea or walnut), volcanic "sand" and volcanic "ashes."
The latter resemble in appearance the ashes of wood or coal, but they
are not in any sense, like them, a residue after combustion.

   [Illustration: Fig. 227. Columnar Structure in Basaltic Lava,
      Scotland]

Volcanic ashes are produced in several ways: lava rising in the
volcanic duct is exploded into fine dust by the steam which permeates
it; glassy lava, hurled into the air and cooled suddenly, is brought
into a state of high strain and tension, and, like Prince Rupert's
drops, flies to pieces at the least provocation. The clash of rising
and falling projectiles also produces some dust, a fair sample of
which may be made by grating together two pieces of pumice.

Beds of volcanic ash occur widely among recent deposits in the western
United States. In Nebraska ash beds are found in twenty counties, and
are often as white as powdered pumice. The beds grow thicker and
coarser toward the southwestern part of the state, where their
thickness sometimes reaches fifty feet. In what direction would you
look for the now extinct volcano whose explosive eruptions are thus
recorded?

=Tuff.= This is a convenient term designating any rock composed of
volcanic fragments. Coarse tuffs of angular fragments are called
_volcanic breccia_, and when the fragments have been rounded and sorted
by water the rock is termed a _volcanic conglomerate_. Even when
deposited in the open air, as on the slopes of a volcano, tuffs may be
rudely bedded and their fragments more or less rounded, and unless
marine shells or the remains of land plants and animals are found as
fossils in them, there is often considerable difficulty in telling
whether they were laid in water or in air. In either case they soon
become consolidated. Chemical deposits from percolating waters fill
the interstices, and the bed of loose fragments is cemented to hard
rock.

The materials of which tuffs are composed are easily recognized as
volcanic in their origin. The fragments are more or less cellular,
according to the degree to which they were distended with steam when
in a molten state, and even in the finest dust one may see the glass
or the crystals of lava from which it was derived. Tuffs often contain
_volcanic bombs_,--balls of lava which took shape while whirling in
the air, and solidified before falling to the ground.

   [Illustration: Fig. 228. Volcanic Bombs, Cinder Cone, California]

   [Illustration: Fig. 229. A Volcanic Cone, Arizona]

=Ancient volcanic rocks.= It is in these materials and structures
which we have described that volcanoes leave some of their most
enduring records. Even the volcanic rocks of the earliest geological
ages, uplifted after long burial beneath the sea and exposed to view
by deep erosion, are recognized and their history read despite the
many changes which they may have undergone. A sheet of ancient lava
may be distinguished by its composition from the sediments among which
it is imbedded. The direction of its flow lines may be noted. The
cellular and slaggy surface where the pasty lava was distended by
escaping steam is recognized by the amygdules which now fill the
ancient steam blebs. In a pile of successive sheets of lava each flow
may be distinguished and its thickness measured; for the surface of
each sheet is glassy and scoriaceous, while beneath its upper portions
the lava of each flow is more dense and stony. The length of time
which elapsed before a sheet was buried beneath the materials of
succeeding eruptions may be told by the amount of weathering which it
had undergone, the depth of ancient soil--now baked to solid
rock--upon it, and the erosion which it had suffered in the interval.

If the flow occurred from some submarine volcano, we may recognize the
fact by the sea-laid sediments which cover it, filling the cracks and
crevices of its upper surface and containing pieces of lava washed
from it in their basal layers.

Long-buried glassy lavas devitrify, or pass to a stony condition,
under the unceasing action of underground waters; but their flow lines
and perlitic and spherulitic structures remain to tell of their
original state.

Ancient tuffs are known by the fragmental character of their volcanic
material, even though they have been altered to firm rock. Some
remains of land animals and plants may be found imbedded to tell that
the beds were laid in open air; while the remains of marine organisms
would prove as surely that the tuffs were deposited in the sea.

In these ways ancient volcanoes have been recognized near Boston, in
southeastern Pennsylvania, about Lake Superior, and in other regions
of the United States.


The Life History of a Volcano

The invasion of a region by volcanic forces is attended by movements
of the crust heralded by earthquakes. A fissure or a pipe is opened
and the building of the cone or the spreading of wide lava sheets is
begun.

=Volcanic cones.= The shape of a volcanic cone depends chiefly on the
materials erupted. Cones made of fragments may have sides as steep as
the angle of repose, which in the case of coarse scoria is sometimes
as high as thirty or forty degrees. About the base of the mountain the
finer materials erupted are spread in more gentle slopes, and are also
washed forward by rains and streams. The normal profile is thus a
symmetric cone with a flaring base.

   [Illustration: Fig. 230. Sarcoui, a Trachyte Dome, France]

Cones built of lava vary in form according to the liquidity of the
lava. Domes of gentle slope, as those of Hawaii, for example, are
formed of basalt, which flows to long distances before it congeals.
When superheated and emitted from many vents, this easily melted lava
builds great plateaus, such as that of Iceland. On the other hand,
lavas less fusible, or poured out at a lower temperature, stiffen when
they have flowed but a short distance, and accumulate in a steep cone.
Trachyte has been extruded in a state so viscid that it has formed
steep-sided domes like that of Sarcoui (Fig. 230).

Most volcanoes are built, like Vesuvius, both of lava flows and of
tuffs, and sections show that the structure of the cone consists of
outward-dipping, alternating layers of lava, scoria, and ashes.

   [Illustration: Fig. 231. Section of Vesuvius

    _V_, Vesuvius; _S_, Somma, a mountainous rampart half encircling
    Vesuvius, and like it built of outward-dipping sheets of tuff and
    lava; _a_, crystalline rocks; _b_, marine strata; _c_, tuffs
    containing seashells. Which is the older mountain, Vesuvius or
    Somma? Of what is Somma a remnant? Draw a diagram showing its
    original outline. Suggest what processes may have brought it to its
    present form. What record do you find of the earliest volcanic
    activity? What do you infer as to the beginnings of the volcano?]

From time to time the cone is rent by the violence of explosions and
by the weight of the column of lava in the pipe. The fissures are
filled with lava and some discharge on the sides of the mountain,
building parasitic cones, while all form dikes, which strengthen the
pile with ribs of hard rock and make it more difficult to rend.

Great catastrophes are recorded in the shape of some volcanoes which
consist of a circular rim perhaps miles in diameter, inclosing a vast
crater or a caldera within which small cones may rise. We may infer
that at some time the top of the mountain has been blown off, or has
collapsed and been engulfed because some reservoir beneath had been
emptied by long-continued eruptions (Fig. 230).

The cone-building stage may be said to continue until eruptions of
lava and fragmental materials cease altogether. Sooner or later the
volcanic forces shift or die away, and no further eruptions add to the
pile or replace its losses by erosion during periods of repose. Gases
however are still emitted, and, as sulphur vapors are conspicuous
among them, such vents are called _solfataras_. Mount Hood, in Oregon,
is an example of a volcano sunk to this stage. From a steaming rift on
its side there rise sulphurous fumes which, half a mile down the wind,
will tarnish a silver coin.

   [Illustration: Fig. 232. Crater Lake, Oregon

   How wide and deep is the basin which holds the lake? The
   mountain walls which enclose it are made of outward-dipping
   sheets of lava. Draw a diagram restoring the volcano of which
   they are the remnant. No volcanic fragments of the same nature
   as the materials of which the volcano is built are found about
   the region. What theory of the destruction of the cone does
   this fact favor? _W´_, Wizard Island, is a cinder cone. When was
   it built?]

=Geysers and hot springs.= The hot springs of volcanic regions are
among the last vestiges of volcanic heat. Periodically eruptive
boiling springs are termed geysers. In each of the geyser regions of
the earth--the Yellowstone National Park, Iceland, and New
Zealand--the ground water of the locality is supposed to be heated by
ancient lavas that, because of the poor conductivity of the rock,
still remain hot beneath the surface.

   [Illustration: Fig. 233. Old Faithful Geyser in Eruption,
      Yellowstone National Park]

=Old Faithful=, one of the many geysers of the Yellowstone National
Park, plays a fountain of boiling water a hundred feet in air; while
clouds of vapor from the escaping steam ascend to several times that
height. The eruptions take place at intervals of from seventy to
ninety minutes. In repose the geyser is a quiet pool, occupying a
craterlike depression in a conical mound some twelve feet high. The
conduit of the spring is too irregular to be sounded. The mound is
composed of porous silica deposited by the waters of the geyser.

Geysers erupt at intervals instead of continuously boiling, because
their long, narrow, and often tortuous conduits do not permit a free
circulation of the water. After an eruption the tube is refilled and
the water again gradually becomes heated. Deep in the tube where it is
in contact with hot lavas the water sooner or later reaches the
boiling point, and bursting into steam shoots the water above it high
in air.

   [Illustration: Fig. 234. Terrace and Cones of Siliceous Sinter
      deposited by Geysers, Yellowstone National Park]

=Carbonated springs.= After all the other signs of life have gone, the
ancient volcano may emit carbon dioxide as its dying breath. The
springs of the region may long be charged with carbon dioxide, or
carbonated, and where they rise through limestone may be expected to
deposit large quantities of travertine. We should remember, however,
that many carbonated springs, and many hot springs, are wholly
independent of volcanoes.

   [Illustration: Fig. 235. Mount Shasta, California]

   [Illustration: Fig. 236. Mount Hood, Oregon]

=The destruction of the cone.= As soon as the volcanic cone ceases to
grow by eruptions the agents of erosion begin to wear it down, and the
length of time that has elapsed since the period of active growth may
be roughly measured by the degree to which the cone has been
dissected. We infer that Mount Shasta, whose conical shape is still
preserved despite the gullies one thousand feet deep which trench its
sides (Fig. 235), is younger than Mount Hood, which erosive agencies
have carved to a pyramidal form (Fig. 236). The pile of materials
accumulated about a volcanic vent, no matter how vast in bulk, is at
last swept entirely away. The cone of the volcano, active or extinct,
is not old as the earth counts time; volcanoes are short-lived
geological phenomena.

   [Illustration: Fig. 237. Crandall Volcano]

=Crandall volcano.= This name is given to a dissected ancient volcano
in the Yellowstone National Park, which once, it is estimated, reared
its head thousands of feet above the surrounding country and greatly
exceeded in bulk either Mount Shasta or Mount Etna. Not a line of the
original mountain remains; all has been swept away by erosion except
some four thousand feet of the base of the pile. This basal wreck now
appears as a rugged region about thirty miles in diameter, trenched by
deep valleys and cut into sharp peaks and precipitous ridges. In the
center of the area is found the nucleus (_N_, Fig. 237),--a mass of
coarsely crystalline rock that congealed deep in the old volcanic
pipe. From it there radiate in all directions, like the spokes of a
wheel, long dikes whose rock grows rapidly finer of grain as it leaves
the vicinity of the once heated core. The remainder of the base of the
ancient mountain is made of rudely bedded tuffs and volcanic breccia,
with occasional flows of lava, some of the fragments of the breccia
measuring as much as twenty feet in diameter. On the sides of canyons
the breccia is carved by rain erosion to fantastic pinnacles. At
different levels in the midst of these beds of tuff and lava are many
old forest grounds. The stumps and trunks of the trees, now turned to
stone, still in many cases stand upright where once they grew on the
slopes of the mountain as it was building (Fig. 238). The great size
and age of some of these trees indicate, the lapse of time between the
eruption whose lavas or tuffs weathered to the soil on which they grew
and the subsequent eruption which buried them beneath showers of
stones and ashes.

Near the edge of the area lies Death Gulch, in which carbon dioxide is
given off in such quantities that in quiet weather it accumulates in a
heavy layer along the ground and suffocates the animals which may
enter it.

   [Illustration: Fig. 238. Fossil Tree Trunks, Yellowstone National Park]




CHAPTER XII

UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN


It is because long-continued erosion lays bare the innermost anatomy
of an extinct volcano, and even sweeps away the entire pile with much
of the underlying strata, thus leaving the very roots of the volcano
open to view, that we are able to study underground volcanic
structures. With these we include, for convenience, intrusions of
molten rock which have been driven upward into the crust, but which
may not have succeeded in breaking way to the surface and establishing
a volcano. All these structures are built of rock forced when in a
fluid or pasty state into some cavity which it has found or made, and
we may classify them therefore, according to the shape of the molds in
which the molten rock has congealed, as (1) dikes, (2) volcanic necks,
(3) intrusive sheets, and (4) intrusive masses.

=Dikes.= The sheet of once molten rock with which a fissure has been
filled is known as a dike. Dikes are formed when volcanic cones are
rent by explosions or by the weight of the lava column in the duct,
and on the dissection of the pile they appear as radiating vertical
ribs cutting across the layers of lava and tuff of which the cone is
built. In regions undergoing deformation rocks lying deep below the
ground are often broken and the fissures are filled with molten rock
from beneath, which finds no outlet to the surface. Such dikes are
common in areas of the most ancient rocks, which have been brought to
light by long erosion.

In exceptional cases dikes may reach the length of fifty or one
hundred miles. They vary in width from a fraction of a foot to even as
much as three hundred feet.

   [Illustration: Fig. 239. Dikes, Spanish Peaks, Colorado]

Dikes are commonly more fine of grain on the sides than in the center,
and may have a glassy and crackled surface where they meet the
inclosing rock. Can you account for this on any principle which you
have learned?

   [Illustration: Fig. 240. A Dissected Volcanic Cone

   _N_, volcanic neck; _l_, _l_, lava-topped table mountains;
   _t_, _t_, beds of tuff; _d_, _d_, dikes; dotted lines indicate
   the initial profile]

=Volcanic necks.= The pipe of a volcano rises from far below the base
of the cone,--from the deep reservoir from which its eruptions are
supplied. When the volcano has become extinct this great tube remains
filled with hardened lava. It forms a cylindrical core of solid rock,
except for some distance below the ancient crater, where it may
contain a mass of fragments which had fallen back into the chimney
after being hurled into the air.

   [Illustration: Fig. 241. Mount Johnson, a Volcanic Neck near
      Montreal]

As the mountain is worn down, this central column known as the
_volcanic neck_ is left standing as a conical hill (Fig. 240). Even
when every other trace of the volcano has been swept away, erosion
will not have passed below this great stalk on which the volcano was
borne as a fiery flower whose site it remains to mark. In volcanic
regions of deep denudation volcanic necks rise solitary and abrupt
from the surrounding country as dome-shaped hills. They are marked
features in the landscape in parts of Scotland and in the St. Lawrence
valley about Montreal (Fig. 241).

   [Illustration: Fig. 242. The Palisades of the Hudson, New Jersey]

=Intrusive sheets.= Sheets of igneous rocks are sometimes found
interleaved with sedimentary strata, especially in regions where the
rocks have been deformed and have suffered from volcanic action. In
some instances such a sheet is seen to be _contemporaneous_ (p. 248).
In other instances the sheet must be _intrusive_. The overlying
stratum, as well as that beneath, has been affected by the heat of the
once molten rock. We infer that the igneous rock when in a molten
state was forced between the strata, much as a card may be pushed
between the leaves of a closed book. The liquid wedged its way between
the layers, lifting those above to make room for itself. The source of
the intrusive sheet may often be traced to some dike (known therefore
as the _feeding dike_), or to some mass of igneous rock.

Intrusive sheets may extend a score and more of miles, and, like the
longest surface flows, the most extensive sheets consist of the more
fusible and fluid lavas,--those of the basic class of which basalt is
an example. Intrusive sheets are usually harder than the strata in
which they lie and are therefore often left in relief after long
denudation of the region (Fig. 315).

   [Illustration: Fig. 243. Diagram of the Palisades of the Hudson

   _i_, intrusive sheet; _s_, sandstone; _d_, feeding dike;
   _HR_, Hudson River]

On the west bank of the Hudson there extends from New York Bay north
for thirty miles a bold cliff several hundred feet high,--the
_Palisades of the Hudson_. It is the outcropping edge of a sheet of
ancient igneous rock, which rests on stratified sandstones and is
overlain by strata of the same series. Sandstones and lava sheet
together dip gently to the west and the latter disappears from view
two miles back from the river.

It is an interesting question whether the Palisades sheet is
_contemporaneous_ or _intrusive_. Was it outpoured on the sandstones
beneath it when they formed the floor of the sea, and covered
forthwith by the sediments of the strata above, or was it intruded
among these beds at a later date?

   [Illustration: Fig. 244. Section of Electric Peak. E. and Gray
      Peak, G, Yellowstone National Park

   Intrusive sheets and masses of igneous rock are drawn in black]

The latter is the case: for the overlying stratum is intensely baked
along the zone of contact. At the west edge of the sheet is found the
dike in which the lava rose to force its way far and wide between the
strata.

_Electric Peak_, one of the prominent mountains of the Yellowstone
National Park, is carved out of a mass of strata into which many
sheets of molten rock have been intruded. The western summit consists
of such a sheet several hundred feet thick. Studying the section of
Figure 244, what inference do you draw as to the source of these
intrusive sheets?

   [Illustration: Fig. 245. Stone Mountain, Georgia, a Granite Boss]


Intrusive Masses

=Bosses.= This name is generally applied to huge irregular masses of
coarsely crystalline igneous rock lying in the midst of other
formations. Bosses vary greatly in size and may reach scores of miles
in extent. Seldom are there any evidences found that bosses ever had
connection with the surface. On the other hand, it is often proved
that they have been driven, or have melted their way, upward into the
formations in which they lie; for they give off dikes and intrusive
sheets, and have profoundly altered the rocks about them by their
heat.

   [Illustration: Fig. 246. Map of Granite Bosses near Baltimore
      (areas horizontally Lined)

The texture of the rock of bosses proves that consolidation proceeded
slowly and at great depths, and it is only because of vast denudation
that they are now exposed to view. Bosses are commonly harder than the
rocks about them, and stand up, therefore, as rounded hills and
mountainous ridges long after the surrounding country has worn to a
low plain (Fig. 245).

Figure 246 exhibits a few small bosses of granite near Baltimore as
examples of numerous areas of igneous rock within the Piedmont Belt
which represent bodies of molten rock which solidified deep below the
surface.

The _Spanish Peaks_ of southeastern Colorado were formed by the
upthrust of immense masses of igneous rock, bulging and breaking the
overlying strata. On one side of the mountains the throw of the fault
is nearly a mile, and fragments of deep-lying beds were dragged upward
by the rising masses. The adjacent rocks were altered by heat to a
distance of several thousand feet. No evidence appears that the molten
rock ever reached the surface, and if volcanic eruptions ever took
place either in lava flows or fragmental materials, all traces of them
have been effaced. The rock of the intrusive masses is coarsely
crystalline, and no doubt solidified slowly under the pressure of vast
thicknesses of overlying rock, now mostly removed by erosion.

A magnificent system of dikes radiates from the Peaks to a distance of
fifteen miles, some now being left by long erosion as walls a hundred
feet in height (Fig. 239). Intrusive sheets fed by the dikes penetrate
the surrounding strata, and their edges are cut by canyons as much as
twenty-five miles from the mountain. In these strata are valuable beds
of lignite, an imperfect coal, which the heat of dikes and sheets has
changed to coke.

   [Illustration: Fig. 247. Section of a Laccolith]

=Laccoliths.= The laccolith (Greek laccos, cistern; lithos, stone) is
a variety of intrusive masses in which molten rock has spread between
the strata, and, lifting the strata above it to a dome-shaped form,
has collected beneath them in a lens-shaped body with a flat base.

The _Henry Mountains_, a small group of detached peaks in southern
Utah, rise from a plateau of horizontal rocks. Some of the peaks are
carved wholly in separate domelike uplifts of the strata of the
plateau. In others, as Mount Hillers, the largest of the group, there
is exposed on the summit a core of igneous rock from which the
sedimentary rocks of the flanks dip steeply outward in all directions.
In still others erosion has stripped off the covering strata and has
laid bare the core to its base; and its shape is here seen to be that
of a plano-convex lens or a baker's bun, its flat base resting on the
undisturbed bedded rocks beneath. The structure of Mount Hillers is
shown in Figure 248. The nucleus of igneous rock is four miles in
diameter and more than a mile in depth.

   [Illustration: Fig. 248. Section of Mount Hillers]

=Regional intrusions.= These vast bodies of igneous rock, which may
reach hundreds of miles in diameter, differ little from bosses except
in their immense bulk. Like bosses, regional intrusions give off dikes
and sheets and greatly change the rocks about them by their heat. They
are now exposed to view only because of the profound denudation which
has removed the upheaved dome of rocks beneath which they slowly
cooled. Such intrusions are accompanied--whether as cause or as
effect is still hardly known--by deformations, and their masses of
igneous rock are thus found as the core of many great mountain ranges.
The granitic masses of which the Bitter Root Mountains and the Sierra
Nevadas have been largely carved are each more than three hundred
miles in length. Immense regional intrusions, the cores of once lofty
mountain ranges, are found upon the Laurentian peneplain.

=Physiographic effects of intrusive masses.= We have already seen
examples of the topographic effects of intrusive masses in Mount
Hillers, the Spanish Peaks, and in the great mountain ranges mentioned
in the paragraph on regional intrusions, although in the latter
instances these effects are entangled with the effects of other
processes. Masses of igneous rock cannot be intruded within the crust
without an accompanying deformation on a scale corresponding to the
bulk of the intruded mass. The overlying strata are arched into hills
or mountains, or, if the molten material is of great extent, the
strata may conceivably be floated upward to the height of a plateau.
We may suppose that the transference of molten matter from one region
to another may be among the causes of slow subsidences and elevations.
Intrusions give rise to fissures, dikes, and intrusive sheets, and
these dislocations cannot fail to produce earthquakes. Where intrusive
masses open communication with the surface, volcanoes are established
or fissure eruptions occur such as those of Iceland.


The Intrusive Rocks

The igneous rocks are divided into two general classes,--the
_volcanic_ or _eruptive_ rocks, which have been outpoured in open air
or on the floor of the sea, and the _intrusive_ rocks, which have been
intruded within the rocks of the crust and have solidified below the
surface. The two classes are alike in chemical composition and may be
divided into acidic and basic groups. In texture the intrusive rocks
differ from the volcanic rocks because of the different conditions
under which they have solidified. They cooled far more slowly beneath
the cover of the rocks into which they were pressed than is permitted
to lava flows in open air. Their constituent minerals had ample
opportunity to sort themselves and crystallize from the fluid mixture,
and none of that mixture was left to congeal as a glassy paste.

They consolidated also under pressure. They are never scoriaceous, for
the steam with which they were charged was not allowed to expand and
distend them with steam blebs. In the rocks of the larger intrusive
masses one may see with a powerful microscope exceedingly minute
cavities, to be counted by many millions to the cubic inch, in which
the gaseous water which the mass contained was held imprisoned under
the immense pressure of the overlying rocks.

Naturally these characteristics are best developed in the intrusives
which cooled most slowly, i.e. in the deepest-seated and largest
masses; while in those which cooled more rapidly, as in dikes and
sheets, we find gradations approaching the texture of surface flows.

=Varieties of the intrusive rocks.= We will now describe a few of the
varieties of rocks of deep-seated intrusions. All are even grained,
consisting of a mass of crystalline grains formed during one
continuous stage of solidification, and no porphyritic crystals appear
as in lavas.

_Granite_, as we have learned already, is composed of three
minerals,--quartz, feldspar, and mica. According to the color of the
feldspar the rock may be red, or pink, or gray. Hornblende--a black or
dark green mineral, an iron-magnesian silicate, about as hard as
feldspar--is sometimes found as a fourth constituent, and the rock is
then known as _hornblendic granite_. Granite is an acidic rock
corresponding to rhyolite in chemical composition. We may believe that
the same molten mass which supplies this acidic lava in surface flows
solidifies as granite deep below ground in the volcanic reservoir.

_Syenite_, composed of feldspar and mica, has consolidated from a less
siliceous mixture than has granite.

_Diorite_, still less siliceous, is composed of hornblende and
feldspar,--the latter mineral being of different variety from the
feldspar of granite and syenite.

_Gabbro_, a typical basic rock, corresponds to basalt in chemical
composition. It is a dark, heavy, coarsely crystalline aggregate of
feldspar and _augite_ (a dark mineral allied to hornblende). It often
contains _magnetite_ (the magnetic black oxide of iron) and _olivine_
(a greenish magnesian silicate).

In the northern states all these types, and many others also of the
vast number of varieties of intrusive rocks, can be found among the
rocks of the drift brought from the areas of igneous rock in Canada
and the states of our northern border.

   [Illustration: Fig. 249. Ground Plan of Dikes in Granite.
      (Scale 80 feet to the inch)

   What is the relative age of the dikes _aa_, _bb_, and _cc_?]

   [Illustration: Fig. 250. _A_ and _B_. Mountains of coarsely
      Crystalline Igneous _i_, surrounded by Sedimentary Strata _s_
      and _s´_

   Copy each diagram and complete it, so as to show whether the
   mass of igneous rock is a volcanic neck, a boss, or a laccolith]

=Summary.= The records of geology prove that since the earliest of
their annals tremendous forces have been active in the earth. In all
the past, under pressures inconceivably great, molten rock has been
driven upward into the rocks of the crust. It has squeezed into
fissures forming dikes; it has burrowed among the strata as intrusive
sheets; it has melted the rocks away or lifted the overlying strata,
filling the chambers which it has made with intrusive masses. During
all geological ages molten rock has found way to the surface, and
volcanoes have darkened the sky with clouds of ashes and poured
streams of glowing lava down their sides. The older strata,--the
strata which have been most deeply buried,--and especially those which
have suffered most from folding and from fracture, show the largest
amount of igneous intrusions. The molten rock which has been driven
from the earth's interior to within the crust or to the surface during
geologic time must be reckoned in millions of cubic miles.

   [Illustration: Fig. 251.

   1, limestone; 2, tuff; 3, 5, 7, shale with marine shells; 4, 6,
   lava, dotted portions scoriaceous. Give the history recorded in
   this section]

   [Illustration: Fig. 252.

   _a_, sedimentary strata with intrusive sheets; _b_, sedimentary
   strata; _c_, lava flow; _d_, dike. Give the succession of
   events recorded in this section]

   [Illustration: Fig. 253.

   Which of the lava sheets of this section are contemporaneous
   anti which intrusive,--_A_, whose upper surface is overlain
   with a conglomerate of rolled lava pebbles; _B_, the cracks and
   seams of whose upper surface are filled with the material of
   the overlying sandstone; _C_, which breaks across the strata in
   which it is imbedded; _D_, which includes fragments of both the
   underlying and overlying strata and penetrates their crevices
   and seams?]

   [Illustration: Fig. 254. Mato Tepee, Wyoming

   This magnificent tower of igneous rock three hundred feet in
   height has been called by some a volcanic neck. Is the
   direction of the columns that which would obtain in the
   cylindrical pipe of a volcano? The tower is probably the
   remnant of a small laccolith, an outlying member of a group of
   laccoliths situated not far distant]


The Interior Condition of the Earth and Causes of Vulcanism and
Deformation

The problems of volcanoes and of deformation are so closely connected
with that of the earth's interior that we may consider them together.
Few of these problems are solved, and we may only state some known
facts and the probable conclusions which may be drawn as inferences
from them.

=The interior of the earth is hot.= Volcanoes prove that in many parts
of the earth there exist within reach of the surface regions of such
intense heat that the rock is in a molten condition. Deep wells and
mines show everywhere an increase in temperature below the surface
shell affected by the heat of summer and the cold of winter,--a shell
in temperate latitudes sixty or seventy feet thick. Thus in a boring
more than a mile deep at Schladebach, Germany, the earth grows warmer
at the rate of 1° F. for every sixty-seven feet as we descend. Taking
the average rate of increase at one degree for every sixty feet of
descent, and assuming that this rate, observed at the moderate
distances open to observation, continues to at least thirty-five
miles, the temperature at that depth must be more than three thousand
degrees,--a temperature at which all ordinary rocks would melt at the
earth's surface. The rate of increase in temperature probably lessens
as we go downward, and it may not be appreciable below a few hundred
miles. But there is no reason to doubt that _the interior of the earth
is intensely hot_. Below a depth of one or two score miles we may
imagine the rocks everywhere glowing with heat.

Although the heat of the interior is great enough to melt all rocks at
atmospheric pressure, it does not follow that the interior is fluid.
Pressure raises the fusing point of rocks, and the weight of the crust
may keep the interior in what may be called a solid state, although so
hot as to be a liquid or a gas were the pressure to be removed.

=The interior of the earth is dense and heavy.= The earth behaves as a
globe more rigid than glass under the strains to which it is subjected
by the attractions of the sun and moon and other heavenly bodies. The
jar of world-shaking earthquakes passes through the earth's interior
with nearly twice the velocity with which it would traverse solid steel,
and since the speed of elastic waves depends on the density and
elasticity of the medium, it follows that the globe is as a whole more
dense and rigid than steel. _The interior of the earth is extremely
dense and rigid._

The common rocks of the crust are about two and a half times heavier
than water, while the earth as a whole weighs five and six-tenths
times as much as a globe of water of the same size. _The interior is
therefore much more heavy than the crust._ This may be caused in part
by compression of the interior under the enormous weight of the crust,
and in part also by an assortment of material, the heavier substances,
such as the heavy metals, having gravitated towards the center.

Between the crust, which is solid because it is cool, and the
interior, which is hot enough to melt were it not for the pressure
which keeps it dense and rigid, there may be an intermediate zone in
which heat and pressure are so evenly balanced that here rock
liquefies whenever and wherever the pressure upon it may be relieved
by movements of the crust. It is perhaps from such a subcrustal layer
that the lava of volcanoes is supplied.

=The causes of volcanic action.= It is now generally believed that the
_heat_ of volcanoes is that of the earth's interior. Other causes,
such as friction and crushing in the making of mountains and the
chemical reactions between oxidizing agents of the crust and the
unoxidized interior, have been suggested, but to most geologists they
seem inadequate.

There is much difference of opinion as to the _force_ which causes
molten rock to rise to the surface in the ducts of volcanoes. Steam is
so evidently concerned in explosive eruptions that many believe that
lava is driven upward by the expansive force of the steam with which
it is charged, much as a viscid liquid rises and boils over in a test
tube or kettle.

But in quiet eruptions, and still more in the irruption of intrusive
sheets and masses, there is little if any evidence that steam is the
driving force. It is therefore believed by many geologists that it is
_pressure due to crustal movements and internal stresses_ which
squeezes molten rock from below into fissures and ducts in the crust.
It is held by some that where considerable water is supplied to the
rising column of lava, as from the ground water of the surrounding
region, and where the lava is viscid so that steam does not readily
escape, the eruption is of the explosive type; when these conditions
do not obtain, the lava outwells quietly, as in the Hawaiian
volcanoes. It is held by others not only that volcanoes are due to the
outflow of the earth's deep-seated heat, but also that the steam and
other emitted gases are for the most part native to the earth's
interior and never have had place in the circulation of atmospheric
and ground waters.

=Volcanic action and deformation.= Volcanoes do not occur on wide
plains or among ancient mountains. On the other hand, where movements
of the earth's crust are in progress in the uplift of high plateaus,
and still more in mountain making, molten rock may reach the surface,
or may be driven upward toward it forming great intrusive masses. Thus
extensive lava flows accompanied the upheaval of the block mountains
of western North America and the uplift of the Colorado plateau. A
line of recent volcanoes may be traced along the system of rift
valleys which extends from the Jordan and Dead Sea through eastern
Africa to Lake Nyassa. The volcanoes of the Andes show how conspicuous
volcanic action may be in young rising ranges. Folded mountains often
show a core of igneous rock, which by long erosion has come to form
the axis and the highest peaks of the range, as if the molten rock had
been squeezed up under the rising upfolds. As we decipher the records
of the rocks in historical geology we shall see more fully how, in all
the past, volcanic action has characterized the periods of great
crustal movements, and how it has been absent when and where the
earth's crust has remained comparatively at rest.

=The causes of deformation.= As the earth's interior, or nucleus, is
highly heated it must be constantly though slowly losing its heat by
conduction through the crust and into space; and since the nucleus is
cooling it must also be contracting. The nucleus has contracted also
because of the extrusion of molten matter, the loss of constituent
gases given off in volcanic eruptions, and (still more important) the
compression and consolidation of its material under gravity. As the
nucleus contracts, it tends to draw away from the cooled and solid
crust, and the latter settles, adapting itself to the shrinking
nucleus much as the skin of a withering apple wrinkles down upon the
shrunken fruit. The unsupported weight of the spherical crust develops
enormous tangential pressures, similar to the stresses of an arch or
dome, and when these lateral thrusts accumulate beyond the power of
resistance the solid rock is warped and folded and broken.

Since the planet attained its present mass it has thus been lessening
in volume. Notwithstanding local and relative upheavals the earth's
surface on the whole has drawn nearer and nearer to the center. The
portions of the lithosphere which have been carried down the farthest
have received the waters of the oceans, while those portions which
have been carried down the least have emerged as continents.

Although it serves our convenience to refer the movements of the crust
to the sea level as datum plane, it is understood that this level is
by no means fixed. Changes in the ocean basins increase or reduce
their capacity and thus lower or raise the level of the sea. But since
these basins are connected, the effect of any change upon the water
level is so distributed that it is far less noticeable than a
corresponding change would be upon the land.




CHAPTER XIII

METAMORPHISM AND MINERAL VEINS


Under the action of internal agencies rocks of all kinds may be
rendered harder, more firmly cemented, and more crystalline. These
processes are known as _metamorphism_, and the rocks affected, whether
originally sedimentary or igneous, are called _metamorphic rocks_. We
may contrast with metamorphism the action of external agencies in
weathering, which render rocks less coherent by dissolving their
soluble parts and breaking down their crystalline grains.

=Contact metamorphism.= Rocks beneath a lava flow or in contact with
igneous intrusions are found to be metamorphosed to various degrees by
the heat of the cooling mass. The adjacent strata may be changed only
in color, hardness, and texture. Thus, next to a dike, bituminous coal
may be baked to coke or anthracite, and chalk and limestone to
crystalline marble. Sandstone may be converted into quartzite, and
shale into _argillite_, a compact, massive clay rock. New minerals may
also be developed. In sedimentary rocks there may be produced crystals
of mica and of _garnet_ (a mineral as hard as quartz, commonly
occurring in red, twelve-sided crystals). Where the changes are most
profound, rocks may be wholly made over in structure and mineral
composition.

In contact metamorphism, thin sheets of molten rock produce less
effect than thicker ones. The strongest heat effects are naturally
caused by bosses and regional intrusions, and the zone of change about
them may be several miles in width. In these changes heated waters and
vapors from the masses of igneous rocks undoubtedly play a very
important part.

Which will be more strongly altered, the rocks about a closed dike in
which lava began to cool as soon as it filled the fissure, or the
rocks about a dike which opened on the surface and through which the
molten rock flowed for some time?

Taking into consideration the part played by heated waters, which will
produce the most far-reaching metamorphism, dikes which cut across the
bedding planes or intrusive sheets which are thrust between the
strata?

=Regional metamorphism.= Metamorphic rocks occur widespread in many
regions, often hundreds of square miles in area, where such extensive
changes cannot be accounted for by igneous intrusions. Such are the
dissected cores of lofty mountains, as the Alps, and the worn-down
bases of ancient ranges, as in New England, large areas in the
Piedmont Belt, and the Laurentian peneplain.

In these regions the rocks have yielded to immense pressure. They have
been folded, crumpled, and mashed, and even their minute grains, as
one may see with a microscope, have often been puckered, broken, and
crushed to powder. It is to these mechanical movements and strains
which the rocks have suffered in every part that we may attribute
their metamorphism, and the degree to which they have been changed is
in direct proportion to the degree to which they have been deformed
and mashed.

Other factors, however, have played important parts. Rock crushing
develops heat, and allows a freer circulation of heated waters and
vapors. Thus chemical reactions are greatly quickened; minerals are
dissolved and redeposited in new positions, or their chemical
constituents may recombine in new minerals, entirely changing the
nature of the rock, as when, for example, feldspar recrystallizes as
quartz and mica.

Early stages of metamorphism are seen in _slate_. Pressure has
hardened the marine muds, the arkose (p. 186), or the volcanic ash
from which slates are derived, and has caused them to cleave by the
rearrangement of their particles.

Under somewhat greater pressure, slate becomes _phyllite_, a clay
slate whose cleavage surfaces are lustrous with flat-lying mica
flakes. The same pressure which has caused the rock to cleave has set
free some of its mineral constituents along the cleavage planes to
crystallize there as mica.

   [Illustration: Fig. 255. A Foliated Rock]

=Foliation.= Under still stronger pressure the whole structure of the
rock is altered. The minerals of which it is composed, and the new
minerals which develop by heat and pressure, arrange themselves along
planes of cleavage or of shear in rudely parallel leaves, or _folia_.
Of this structure, called _foliation_, we may distinguish two
types,--a coarser feldspathic type, and a fine type in which other
minerals than feldspar predominate.

_Gneiss_ is the general name under which are comprised coarsely
foliated rocks banded with irregular layers of feldspar and other
minerals. The gneisses appear to be due in many cases to the crushing
and shearing of deep-seated igneous rocks, such as granite and gabbro.

_The crystalline schists_, representing the finer types of foliation,
consist of thin, parallel, crystalline leaves, which are often
remarkably crumpled. These folia can be distinguished from the laminae
of sedimentary rocks by their lenticular form and lack of continuity,
and especially by the fact that they consist of platy, crystalline
grains, and not of particles rounded by wear.

_Mica schist_, the most common of schists, and in fact of all
metamorphic rocks, is composed of mica and quartz in alternating wavy
folia. All gradations between it and phyllite may be traced, and in
many cases we may prove it due to the metamorphism of slates and
shales. It is widespread in New England and along the eastern side of
the Appalachians. _Talc schist_ consists of quartz and _talc_, a
light-colored magnesian mineral of greasy feel, and so soft that it
can be scratched with the thumb nail.

_Hornblende schist_, resulting in many cases from the foliation of
basic igneous rocks, is made of folia of hornblende alternating with
bands of quartz and feldspar. Hornblende schist is common over large
areas in the Lake Superior region.

_Quartz schist_ is produced from quartzite by the development of fine
folia of mica along planes of shear. All gradations may be found
between it and unfoliated quartzite on the one hand and mica schist on
the other.

Under the resistless pressure of crustal movements almost any rocks,
sandstones, shales, lavas of all kinds, granites, diorites, and
gabbros may be metamorphosed into schists by crushing and shearing.
Limestones, however, are metamorphosed by pressure into _marble_, the
grains of carbonate of lime recrystallizing freely to interlocking
crystals of calcite.

These few examples must suffice of the great class of metamorphic
rocks. As we have seen, they owe their origin to the alteration of
both of the other classes of rocks--the sedimentary and the
igneous--by heat and pressure, assisted usually by the presence of
water. The fact of change is seen in their hardness arid cementation,
their more or less complete recrystallization, and their foliation;
but the change is often so complete that no trace of their original
structure and mineral composition remains to tell whether the rocks
from which they were derived were sedimentary or igneous, or to what
variety of either of these classes they belonged.

   [Illustration: Fig. 256. Contorted Gneiss, the Ottawa River,
      Canada]

   [Illustration: Fig. 257. Quartz Veins in Slate]

In many cases, however, the early history of a metamorphic rock can be
deciphered. Fossils not wholly obliterated may prove it originally
water-laid. Schists may contain rolled-out pebbles, showing their
derivation from a conglomerate. Dikes of igneous rocks may be followed
into a region where they have been foliated by pressure. The most
thoroughly metamorphosed rocks may sometimes be traced out into
unaltered sedimentary or igneous rocks, or among them may be found
patches of little change where their history maybe read.

Metamorphism is most common among rocks of the earlier geological
ages, and most rare among rocks of recent formation. No doubt it is
now in progress where deep-buried sediments are invaded by heat either
from intrusive igneous masses or from the earth's interior, or are
suffering slow deformation under the thrust of mountain-making forces.

Suggest how rocks now in process of metamorphism may sometimes be
exposed to view. Why do metamorphic rocks appear on the surface
to-day?


Mineral Veins

In regions of folded and broken rocks fissures are frequently found to
be filled with sheets of crystalline minerals deposited from solution
by underground water, and fissures thus filled are known as _mineral
veins_. Much of the importance of mineral veins is due to the fact
that they are often metalliferous, carrying valuable native metals and
metallic ores disseminated in fine particles, in strings, and
sometimes in large masses in the midst of the valueless nonmetallic
minerals which make up what is known as the _vein stone_.

The most common vein stones are _quartz_ and _calcite_. _fluorite_
(calcium fluoride), a mineral harder than calcite and crystallizing in
cubes of various colors, and _barite_ (barium sulphate), a heavy white
mineral, are abundant in many veins.

   [Illustration: Fig. 258. Placer Deposits in California

   _g_, gold-bearing gravels in present river beds; _g´_, ancient
   gold-bearing river gravels; _a_, _a_, lava flows capping table
   mountains; _s_, slate. Draw a diagram showing by dotted lines
   conditions before the lava flows occurred. What changes have
   since taken place?]

The gold-bearing quartz veins of California traverse the metamorphic
slates of the Sierra Nevada Mountains. Below the zone of solution (p.
45) these veins consist of a vein stone of quartz mingled with pyrite
(p. 13), the latter containing threads and grains of native gold. But
to the depth of about fifty feet from the surface the pyrite of the
vein has been dissolved, leaving a rusty, cellular quartz with grains
of the insoluble gold scattered through it.

The _placer deposits_ of California and other regions are gold-bearing
deposits of gravel and sand in river beds. The heavy gold is apt to be
found mostly near or upon the solid rock, and its grains, like those
of the sand, are always rounded. How the gold came in the placers we
may leave the pupil to suggest.

Copper is found in a number of ores, and also in the native metal.
Below the zone of surface changes the ore of a copper vein is often a
double sulphide of iron and copper called _chalcopyrite_, a mineral
softer than pyrite--it can easily be scratched with a knife--and
deeper yellow in color. For several score of feet below the ground the
vein may consist of rusty quartz from which the metallic ores have
been dissolved; but at the base of the zone of solution we may find
exceedingly rich deposits of copper ores,--copper sulphides, red and
black copper oxides, and green and blue copper carbonates, which have
clearly been brought down in solution from the leached upper portion
of the vein.

=Origin of mineral veins.= Both vein stones and ores have been
deposited slowly from solution in water, much as crystals of salt are
deposited on the sides of a jar of saturated brine. In our study of
underground water we learned that it is everywhere circulating through
the permeable rocks of the crust, descending to profound depths under
the action of gravity and again driven to the surface by hydrostatic
pressure. Now fissures, wherever they occur, form the trunk channels
of the underground circulation. Water descends from the surface along
these rifts; it moves laterally from either side to the fissure plane,
just as ground water seeps through the surrounding rocks from every
direction to a well; and it ascends through these natural water ways
as in an artesian well, whenever they intersect an aquifer in which
water is under hydrostatic pressure.

The waters which deposit vein stones and ores are commonly hot, and in
many cases they have derived their heat from intrusions of igneous
rock still uncooled within the crust. The solvent power of the water
is thus greatly increased, and it takes up into solution various
substances from the igneous and sedimentary rocks which it traverses.
For various reasons these substances stances are deposited in the vein
as ores and vein stones. On rising through the fissure the water cools
and loses pressure, and its capacity to hold minerals in solution is
therefore lessened. Besides, as different currents meet in the
fissure, some ascending, some descending, and some coming in from the
sides, the chemical reaction of these various weak solutions upon one
another and upon the walls of the vein precipitates the minerals of
vein stuffs and ores.

As an illustration of the method of vein deposits we may cite the case
of a wooden box pipe used in the Comstock mines, Nevada, to carry the
hot water of the mine from one level to another, which in ten years
was lined with calcium carbonate more than half an inch thick.

The Steamboat Springs, Nevada, furnish examples of mineral veins in
process of formation. The steaming water rises through fissures in
volcanic rocks and is now depositing in the rifts a vein stone of
quartz, with metallic ores of iron, mercury, lead, and other metals.

=Reconcentration.= Near the base of the zone of solution veins are
often stored with exceptionally large and valuable ore deposits. This
local enrichment of the vein is due to the reconcentration of its
metalliferous ores. As the surface of the land is slowly lowered by
weathering and running water, the zone of solution is lowered at an
equal rate and encroaches constantly on the zone of cementation. The
minerals of veins are therefore constantly being dissolved along their
upper portions and carried down the fissures by ground water to lower
levels, where they are redeposited.

Many of the richest ore deposits are thus due to successive
concentrations: the ores were leached originally from the rocks to a
large extent by laterally seeping waters; they were concentrated in
the ore deposits of the vein chiefly by ascending currents; they have
been reconcentrated by descending waters in the way just mentioned.

=The original source of the metals.= It is to the igneous rocks that
we may look for the original source of the metals of veins. Lavas
contain minute percentages of various metallic compounds, and no doubt
this was the case also with the igneous rocks which formed the
original earth crust. By the erosion of the igneous rocks the metals
have been distributed among sedimentary strata, and even the sea has
taken into solution an appreciable amount of gold and other metals,
but in this widely diffused condition they are wholly useless to man.
The concentration which has made them available is due to the
interaction of many agencies. Earth movements fracturing deeply the
rocks of the crust, the intrusion of heated masses, the circulation of
underground waters, have all coöperated in the concentration of the
metals of mineral veins.

While fissure veins are the most important of mineral veins, the
latter term is applied also to any water way which has been filled by
similar deposits from solution. Thus in soluble rocks, such as
limestones, joints enlarged by percolating water are sometimes filled
with metalliferous deposits, as, for example, the lead and zinc
deposits of the upper Mississippi valley. Even a porous aquifer may be
made the seat of mineral deposits, as in the case of some
copper-bearing and silver-bearing sandstones of New Mexico.

       *       *       *       *       *

   [Illustration: Fig. 260. Geological Map of the United states
      and Part of Canada]

       *       *       *       *       *




PART III

HISTORICAL GEOLOGY


CHAPTER XIV

THE GEOLOGICAL RECORD


=What a formation records.= We have already learned that each
individual body of stratified rock, or formation, constitutes a record
of the time when it was laid. The structure and the character of the
sediments of each formation tell whether the area was land or sea at
the time when they were spread; and if the former, whether the land
was river plain, or lake bed, or was covered with wind-blown sands, or
by the deposits of an ice sheet. If the sediments are marine, we may
know also whether they were laid in shoal water near the shore or in
deeper water out at sea, and whether during a period of emergence, or
during a period of subsidence when the sea transgressed the land. By
the same means each formation records the stage in the cycle of
erosion of the land mass from which its sediments were derived (p.
185). An unconformity between two marine formations records the fact
that between the periods when they were deposited in the sea the area
emerged as land and suffered erosion (p. 227). The attitude and
structure of the strata tell also of the foldings and fractures,
the deformation and the metamorphism, which they have suffered; and
the igneous rocks associated with them as lava flows and igneous
intrusions add other details to the story. Each formation is thus a
separate local chapter in the geological history of the earth, and its
strata are its leaves. It contains an authentic record of the physical
conditions--the geography--of the time and place when and where its
sediments were laid.

=Past cycles of erosion.= These chapters in the history of the planet
are very numerous, although much of the record has been destroyed in
various ways. A succession of different formations is usually seen in
any considerable section of the crust, such as a deep canyon or where
the edges of upturned strata are exposed to view on the flanks of
mountain ranges; and in any extensive area, such as a state of the
Union or a province of Canada, the number of formations outcropping on
the surface is large.

It is thus learned that our present continent is made up for the most
part of old continental deltas. Some, recently emerged as the strata
of young coastal plains, are the records of recent cycles of erosion;
while others were deposited in the early history of the earth, and in
many instances have been crumpled into mountains, which afterwards
were leveled to their bases and lowered beneath the sea to receive a
cover of later sediments before they were again uplifted to form land.

The cycle of erosion now in progress and recorded in the layers of
stratified rock being spread beneath the sea in continental deltas has
therefore been preceded by many similar cycles. Again and again
movements of the crust have brought to an end one cycle--sometimes
when only well under way, and sometimes when drawing toward its
close--and have begun another. Again and again they have added to the
land areas which before were sea, with all their deposition records of
earlier cycles, or have lowered areas of land beneath the sea to
receive new sediments.

=The age of the earth.= The thickness of the stratified rocks now
exposed upon the eroded surface of the continents is very great. In
the Appalachian region the strata are seven or eight miles thick, and
still greater thicknesses have been measured in several other mountain
ranges. The aggregate thickness of all the formations of the
stratified rocks of the earth's crust, giving to each formation its
maximum thickness wherever found, amounts to not less than forty
miles. Knowing how slowly sediments accumulate upon the sea floor
(p. 184), we must believe that the successive cycles which the earth
has seen stretch back into a past almost inconceivably remote, and
measure tens of millions and perhaps even hundreds of millions of
years.

=How the formations are correlated and the geological record made up.=
Arranged in the order of their succession, the formations of the
earth's crust would constitute a connected record in which the
geological history of the planet may be read, and therefore known as
the _geological record_. But to arrange the formations in their
natural order is not an easy task. A complete set of the volumes of
the record is to be found in no single region. Their leaves and
chapters are scattered over the land surface of the globe. In one area
certain chapters may be found, though perhaps with many missing
leaves, and with intervening chapters wanting, and these absent parts
perhaps can be supplied only after long search through many other
regions.

Adjacent strata in any region are arranged according to the _law of
superposition_, i.e. any stratum is younger than that on which it was
deposited, just as in a pile of paper, any sheet was laid later than
that on which it rests. Where rocks have been disturbed, their
original attitude must be determined before the law can be applied.
Nor can the law of superposition be used in identifying and comparing
the strata of different regions where the formations cannot be traced
continuously from one region to the other.

The formations of different regions are arranged in their true order
by the _law of included organisms_; i.e. formations, however widely
separated, which contain a similar assemblage of fossils are
equivalent and belong to the same division of geological time.

The correlation of formations by means of fossils may be explained by
the formations now being deposited about the north Atlantic.
Lithologically they are extremely various. On the continental shelf of
North America limestones of different kinds are forming off Florida,
and sandstones and shales from Georgia northward. Separated from them
by the deep Atlantic oozes are other sedimentary deposits now
accumulating along the west coast of Europe. If now all these offshore
formations were raised to open air, how could they be correlated?
Surely not by lithological likeness, for in this respect they would be
quite diverse. All would be similar, however, in the fossils which
they contain. Some fossil species would be identical in all these
formations and others would be closely allied. Making all due
allowance for differences in species due to local differences in
climate and other physical causes, it would still be plain that plants
and animals so similar lived at the same period of time, and that the
formations in which their remains were imbedded were contemporaneous
in a broad way. The presence of the bones of whales and other marine
mammals would prove that the strata were laid after the appearance of
mammals upon earth, and imbedded relics of man would give a still
closer approximation to their age. In the same way we correlate the
earlier geological formations.

For example, in 1902 there were collected the first fossils ever found
on the antarctic continent. Among the dozen specimens obtained were
some fossil ammonites (a family of chambered shells) of genera which
are found on other continents in certain formations classified as the
Cretaceous system, and which occur neither above these formations nor
below them. On the basis of these few fossils we may be confident that
the strata in which they were found in the antarctic region were laid
in the same period of geologic time as were the Cretaceous rocks of
the United States and Canada.

=The record as a time scale.= By means of the law of included
organisms and the law of superposition the formations of different
countries and continents are correlated and arranged in their natural
order. When the geological record is thus obtained it may be used as a
universal time scale for geological history. Geological time is
separated into divisions corresponding to the times during which the
successive formations were laid. The largest assemblages of formations
are known as groups, while the corresponding divisions of time are
known as eras. Groups are subdivided into systems, and systems into
series. Series are divided into stages and substages,--subdivisions
which do not concern us in this brief treatise. The corresponding
divisions of time are given in the following table.

  _Strata_       _Time_

   Group          Era
   System         Period
   Series         Epoch

The geologist is now prepared to read the physical history--the
geographical development--of any country or of any continent by means
of its formations, when he has given each formation its true place in
the geological record as a time scale.

The following chart exhibits the main divisions of the record, the
name given to each being given also to the corresponding time
division. Thus we speak of the _Cambrian system_, meaning a certain
succession of formations which are classified together because of
broad resemblances in their included organisms; and of the _Cambrian
period_, meaning the time during which these rocks were deposited.

  _Group and Era_    _System and Period_  _Series and Epoch_

                                          { Recent
                     { Quaternary . . . . { Pleistocene
                     {
  Cenozoic . . . .   {                    { Pliocene
                     { Tertiary   . . . . { Miocene
                                          { Eocene

                     { Cretaceous
  Mesozoic . . . .   { Jurassic
                     { Triassic

                                          { Permian
                     { Carboniferous  . . { Pennsylvanian
                     {                    { Mississippian
  Paleozoic . . . .  { Devonian
                     { Silurian
                     { Ordovician
                     { Cambrian

  Algonkian
  Archean


Fossils and what they teach

The geological formations contain a record still more important than
that of the geographical development of the continents; the fossils
imbedded in the rocks of each formation tell of the kinds of animals
and plants which inhabited the earth at that time, and from these
fossils we are therefore able to construct the history of life upon
the earth.

=Fossils.= These remains of organisms are found in the strata in all
degrees of perfection, from trails and tracks and fragmentary
impressions, to perfectly preserved shells, wood, bones, and complete
skeletons. As a rule, it is only the hard parts of animals and plants
which have left any traces in the rocks. Sometimes the original hard
substance is preserved, but more often it has been replaced by some
less soluble material. Petrifaction, as this process of slow
replacement is called, is often carried on in the most exquisite
detail. When wood, for example, is undergoing petrifaction, the woody
tissue may be replaced, particle by particle, by silica in solution
through the action of underground waters, even the microscopic
structures of the wood being perfectly reproduced. In shells
originally made of _aragonite_, a crystalline form of carbonate of
lime, that mineral is usually replaced by _calcite_, a more stable
form of the same substance. The most common petrifying materials are
calcite, silica, and pyrite.

Often the organic substance has neither been preserved nor replaced,
but the _form_ has been retained by means of molds and casts.
Permanent impressions, or molds, may be made in sediments not only by
the hard parts of organisms, but also by such soft and perishable
parts as the leaves of plants, and, in the rarest instances, by the
skin of animals and the feathers of birds. In fine-grained limestones
even the imprints of jellyfish have been retained.

The different kinds of molds and casts may be illustrated by means of
a clam shell and some moist clay, the latter representing the
sediments in which the remains of animals and plants are entombed.
Imbedding the shell in the clay and allowing the clay to harden, we
have a _mold of the exterior_ of the shell, as is seen on cutting the
clay matrix in two and removing the shell from it. Filling this mold
with clay of different color, we obtain a _cast of the exterior_,
which represents accurately the original form and surface markings of
the shell. In nature, shells and other relics of animals or plants are
often removed by being dissolved by percolating waters, and the molds
are either filled with sediments or with minerals deposited from
solution.

Where the fossil is hollow, a _cast of the interior_ is made in the
same way. Interior casts of shells reproduce any markings on the
inside of the valves, and casts of the interior of the skulls of
ancient vertebrates show the form and size of their brains.

=Imperfection of the life record.= At the present time only the
smallest fraction of the life on earth ever gets entombed in rocks now
forming. In the forest great fallen tree trunks, as well as dead
leaves, decay, and only add a little to the layer of dark vegetable
mold from which they grew. The bones of land animals are, for the most
part, left unburied on the surface and are soon destroyed by chemical
agencies. Even where, as in the swamps of river, flood plains and in
other bogs, there are preserved the remains of plants, and sometimes
insects, together with the bones of some animal drowned or mired, in
most cases these swamp and bog deposits are sooner or later destroyed
by the shifting channels of the stream or by the general erosion of
the land.

In the sea the conditions for preservation are more favorable than on
land; yet even here the proportion of animals and plants whose hard
parts are fossilized is very small compared with those which either
totally decay before they are buried in slowly accumulating sediments
or are ground to powder by waves and currents.

We may infer that during each period of the past, as at the present,
only a very insignificant fraction of the innumerable organisms of sea
and land escaped destruction and left in continental and oceanic
deposits permanent records of their existence. Scanty as these
original life records must have been, they have been largely destroyed
by metamorphism of the rocks in which they were imbedded, by solution
in underground waters, and by the vast denudation under which the
sediments of earlier periods have been eroded to furnish materials for
the sedimentary records of later times. Moreover, very much of what
has escaped destruction still remains undiscovered. The immense bulk
of the stratified rocks is buried and inaccessible, and the records of
the past which it contains can never be known. Comparatively few
outcrops have been thoroughly searched for fossils. Although new
species are constantly being discovered, each discovery may be
considered as the outcome of a series of happy accidents,--that the
remains of individuals of this particular species happened to be
imbedded and fossilized, that they happened to escape destruction
during long ages, and that they happened to be exposed and found.

=Some inferences from the records of the history of life upon the
planet.= Meager as are these records, they set forth plainly some
important truths which we will now briefly mention.

1. Each series of the stratified rocks, except the very deepest,
contains vestiges of life. Hence _the earth was tenanted by living
creatures for an uncalculated length of time before human history
began_.

2. _Life on the earth has been ever-changing._ The youngest strata hold
the remains of existing species of animals and plants and those of
species and varieties closely allied to them. Strata somewhat older
contain fewer existing species, and in strata of a still earlier, but
by no means an ancient epoch, no existing species are to be found; the
species of that epoch and of previous epochs have vanished from the
living world. During all geological time since life began on earth old
species have constantly become extinct and with them the genera and
families to which they belong, and other species, genera, and families
have replaced them. The fossils of each formation differ on the whole
from those of every other. The assemblage of animals and plants (the
_fauna-flora_) of each epoch differs from that of every other epoch.

In many cases the extinction of a type has been gradual; in other
instances apparently abrupt. There is no evidence that any organism
once become extinct has ever reappeared. The duration of a species in
time, or its "vertical range" through the strata, varies greatly. Some
species are limited to a stratum a few feet in thickness; some may
range through an entire formation and be found but little modified in
still higher beds. A formation may thus often be divided into zones,
each characterized by its own peculiar species. As a rule, the simpler
organisms have a longer duration as species, though not as
individuals, than the more complex.

3. _The larger zoölogical and botanical groupings survive longer than
the smaller._ Species are so short-lived that a single geological
epoch may be marked by several more or less complete extinctions of
the species of its fauna-flora and their replacement by other species.
A genus continues with new species after all the species with which it
began have become extinct. Families survive genera, and orders
families. Classes are so long-lived that most of those which are known
from the earliest formations are represented by living forms, and no
subkingdom has ever become extinct.

Thus, to take an example from the stony corals,--the
_zoantharia_,--the particular characters--which constituted a certain
_species_--_Facosites niagarensis_--of the order are confined to the
Niagara series. Its _generic_ characters appeared in other species
earlier in the Silurian and continued through the Devonian. Its
_family_ characters, represented in different genera and species,
range from the Ordovician to the close of the Paleozoic; while the
characters which it shares with all its order, the Zoantharia, began
in the Cambrian and are found in living species.

4. _The change in organisms has been gradual._ The fossils of each
life zone and of each formation of a conformable series closely
resemble, with some explainable exceptions, those of the beds
immediately above and below. The animals and plants which tenanted the
earth during any geological epoch are so closely related to those of
the preceding and the succeeding epochs that we may consider them to
be the descendants of the one and the ancestors of the other, thus
accounting for the resemblance by heredity. It is therefore believed
that the species of animals and plants now living on the earth are the
descendants of the species whose remains we find entombed in the
rocks, and that the chain of life has been unbroken since its
beginning.

5. _The change in species has been a gradual differentiation._ Tracing
the lines of descent of various animals and plants of the present
backward through the divisions of geologic time, we find that these
lines of descent converge and unite in simpler and still simpler
types. The development of life may be represented by a tree whose
trunk is found in the earliest ages and whose branches spread and
subdivide to the growing twigs of present species.

6. _The change in organisms throughout geologic time has been a
progressive change._ In the earliest ages the only animals and plants
on the earth were lowly forms, simple and generalized in structure;
while succeeding ages have been characterized by the introduction of
types more and more specialized and complex, and therefore of higher
rank in the scale of being. Thus the Algonkian contains the remains of
only the humblest forms of the invertebrates. In the Cambrian,
Ordovician, and Silurian the invertebrates were represented in all
their subkingdoms by a varied fauna. In the Devonian, fishes--the
lowest of the vertebrates--became abundant. Amphibians made their
entry on the stage in the Carboniferous, and reptiles came to rule the
world in the Mesozoic. Mammals culminated in the Tertiary in strange
forms which became more and more like those of the present as the long
ages of that era rolled on; and latest of all appeared the noblest
product of the creative process, man.

Just as growth is characteristic of the individual life, so gradual,
progressive change, or evolution, has characterized the history of
life upon the planet. The evolution of the organic kingdom from its
primitive germinal forms to the complex and highly organized
fauna-flora of to-day may be compared to the growth of some noble oak
as it rises from the acorn, spreading loftier and more widely extended
branches as it grows.

7. While higher and still higher types have continually been evolved,
until man, the highest of all, appeared, _the lower and earlier types
have generally persisted_. Some which reached their culmination early
in the history of the earth have since changed only in slight
adjustments to a changing environment. Thus the brachiopods, a type of
shellfish, have made no progress since the Paleozoic, and some of
their earliest known genera are represented by living forms hardly to
be distinguished from their ancient ancestors. The lowest and earliest
branches of the tree of life have risen to no higher levels since they
reached their climax of development long ago.

8. A strange parallel has been found to exist between the evolution of
organisms and the development of the individual. In the embryonic
stages of its growth the individual passes swiftly through the
successive stages through which its ancestors evolved during the
millions of years of geologic time. _The development of the individual
recapitulates the evolution of the race._

       *       *       *       *       *

The frog is a typical amphibian. As a tadpole it passes through a
stage identical in several well-known features with the maturity of
fishes; as, for example, its aquatic life, the tail by which it swims,
and the gills through which it breathes. It is a fair inference that
the tadpole stage in the life history of the frog represents a stage
in the evolution of its kind,--that the Amphibia are derived from
fishlike ancestral forms. This inference is amply confirmed in the
geological record; fishes appeared before Amphibia and were connected
with them by transitional forms.

=The great length of geologic time inferred from the slow change of
species.= Life forms, like land forms, are thus subject to change
under the influence of their changing environment and of forces acting
from within. How slowly they change may be seen in the apparent
stability of existing species. In the lifetime of the observer and
even in the recorded history of man, species seem as stable as the
mountain and the river. But life forms and land forms are alike
variable, both in nature and still more under the shaping hand of man.
As man has modified the face of the earth with his great engineering
works, so he has produced widely different varieties of many kinds of
domesticated plants and animals, such as the varieties of the dog and
the horse, the apple and the rose, which may be regarded in some
respects as new species in the making. We have assumed that land forms
have changed in the past under the influence of forces now in
operation. Assuming also that life forms have always changed as they
are changing at present, we come to realize something of the immensity
of geologic time required for the evolution of life from its earliest
lowly forms up to man.

It is because the onward march of life has taken the same general
course the world over that we are able to use it as a _universal time
scale_ and divide geologic time into ages and minor subdivisions
according to the ruling or characteristic organisms then living on the
earth. Thus, since vertebrates appeared, we have in succession the Age
of Fishes, the Age of Amphibians, the Age of Reptiles, and the Age of
Mammals.

The chart given on page 295 is thus based on the law of superposition
and the law of the evolution of organisms. The first law gives the
succession of the formations in local areas. The fossils which they
contain demonstrate the law of the progressive appearance of
organisms, and by means of this law the formations of different
countries are correlated and set each in its place in a universal time
scale and grouped together according to the affinities of their
imbedded organic remains.

=Geologic time divisions compared with those of human history.= We may
compare the division of geologic time into eras, periods, and other
divisions according to the dominant life of the time, to the
ill-defined ages into which human history is divided according to the
dominance of some nation, ruler, or other characteristic feature. Thus
we speak of the _Dark Ages_, the _Age of Elizabeth_, and the _Age of
Electricity_. These crude divisions would be of much value if, as in
the case of geologic time, we had no exact reckoning of human history
by years.

And as the course of human history has flowed in an unbroken stream
along quiet reaches of slow change and through periods of rapid change
and revolution, so with the course of geologic history. Periods of
quiescence, in which revolutionary forces are perhaps gathering head,
alternate with periods of comparatively rapid change in physical
geography and in organisms, when new and higher forms appear which
serve to draw the boundary line of new epochs. Nevertheless,
geological history is a continuous progress; its periods and epochs
shade into one another by imperceptible gradations, and all our
subdivisions must needs be vague and more or less arbitrary.

=How fossils tell of the geography of the past.= Fossils are used not
only as a record of the development of life upon the earth, but also
in testimony to the physical geography of past epochs. They indicate
whether in any region the climate was tropical, temperate, or arctic.
Since species spread slowly from some center of dispersion where they
originate until some barrier limits their migration farther, the
occurrence of the same species in rocks of the same system in
different countries implies the absence of such barriers at the
period. Thus in the collection of antarctic fossils referred to on
page 294 there were shallow-water marine shells identical in species
with Mesozoic shells found in India and in the southern extremity of
South America. Since such organisms are not distributed by the
currents of the deep sea and cannot migrate along its bottom, we infer
a shallow-water connection in Mesozoic times between India, South
America, and the antarctic region. Such a shallow-water connection
would be offered along the marginal shelf of a continent uniting these
now widely separated countries.




CHAPTER XV

THE PRE-CAMBRIAN SYSTEMS


=The earth's beginnings.= The geological record does not tell us of
the beginnings of the earth. The history of the planet, as we have
every reason to believe, stretches far back beyond the period of the
oldest stratified rocks, and is involved in the history of the solar
system and of the nebula,--the cloud of glowing gases or of cosmic
dust,--from which the sun and planets are believed to have been
derived.

=The nebular hypothesis.= It was long held that the earth began as
a vaporous, shining sphere, formed by the gathering together of the
material of a gaseous ring which had been detached from a cooling
and shrinking nebula. Such a vaporous sphere would condense to a
liquid fiery globe, whose surface would become cold and solid, while
the interior would long remain intensely hot because of the slow
conductivity of the crust. Under these conditions the primeval
atmosphere of the earth must have contained in vapor the water now
belonging to the earth's crust and surface. It also held all the oxygen
since locked up in rocks by their oxidation, and all the carbon dioxide
which has since been laid away in limestones, besides that corresponding
to the carbon of carbonaceous deposits, such as peat, coal, and
petroleum. On this hypothesis the original atmosphere was dense, dark,
and noxious, and enormously heavier than the atmosphere at present.

=The accretion hypothesis.= On the other hand, it has been recently
suggested that the earth may have grown to its present size by the
gradual accretion of meteoritic masses. Such cold, stony bodies might
have come together at so slow a rate that the heat caused by their
impact would not raise sensibly the temperature of the growing planet.
Thus the surface of the earth may never have been hot and luminous; but
as the loose aggregation of stony masses grew larger and was more and
more compressed by its own gravitation, the heat thus generated raised
the interior to high temperatures, while from time to time molten rock
was intruded among the loose, cold meteoritic masses of the crust and
outpoured upon the surface.

Such a spiral nebula might be formed by the close approach of one star
to another,--of a passing star to our own sun, for example, before the
birth of the solar system. As the pull of the moon raises the tides on
opposite sides of the earth, so, it is supposed, the pull of the
passing star released the explosive forces of the sun, and two streams
of matter were flung out from it. The knots in the arms formed the
nuclei of the planets. The gaseous matter scattered outside the knots
cooled into small stony masses, revolving about a central mass and
hence called planetesimals (little planets). Like the meteorites which
still fall upon the earth, the planetesimals were gradually gathered
in by the nuclear knots, which thus grew to the present planets.

It is supposed that the meteorites of which the earth was built
brought to it, as meteorites do now, various gases shut up within
their pores. As the heat of the interior increased, these gases
transpired to the surface and formed the primitive atmosphere and
hydrosphere. The atmosphere has therefore grown slowly from the
smallest beginnings. Gases emitted from the interior in volcanic
eruptions and in other ways have ever added to it, and are adding to
it now. On the other hand, the atmosphere has constantly suffered
loss, as it has been robbed of oxygen by the oxidation of rocks in
weathering, and of carbon dioxide in the making of limestones and
carbonaceous deposits.

While all hypotheses of the earth's beginnings are as yet unproved
speculations, they serve to bring to mind one of the chief lessons
which geology has to teach,--that the duration of the earth in time,
like the extension of the universe in space, is vastly beyond the
power of the human mind to realize. Behind the history recorded in the
rocks, which stretches back for many million years, lies the long
unrecorded history of the beginnings of the planet; and still farther
in the abysses of the past are dimly seen the cycles of the evolution
of the solar system and of the nebula which gave it birth.

We pass now from the dim realm of speculation to the earliest era of
the recorded history of the earth, where some certain facts may be
observed and some sure inferences from them may be drawn.


The Archean

The oldest known sedimentary strata, wherever they are exposed by uplift
and erosion, are found to be involved with a mass of crystalline rocks
which possesses the same characteristics in all parts of the world. It
consists of foliated rocks, gneisses, and schists of various kinds,
which have been cut with dikes and other intrusions of molten rock, and
have been broken, crumpled, and crushed, and left in interlocking masses
so confused that their true arrangement can usually be made out only
with the greatest difficulty if at all. The condition of this body of
crystalline rocks is due to the fact that they have suffered not only
from the faultings, foldings, and igneous intrusions of their time, but
necessarily, also, from those of all later geological ages.

At present three leading theories are held as to the origin of these
basal crystalline rocks.

1. They are considered by perhaps the majority of the geologists who
have studied them most carefully to be igneous rocks intruded in a
molten state among the sedimentary rocks involved with them. In many
localities this relation is proved by the phenomena of contact (p. 268);
but for the most part the deformations which the rocks have since
suffered again and again have been sufficient to destroy such evidence
if it ever existed.

2. An older view regards them as profoundly altered sedimentary strata,
the most ancient of the earth.

3. According to a third theory they represent portions of the earth's
original crust; not, indeed, its original surface, but deeper portions
uncovered by erosion and afterwards mantled with sedimentary deposits.
All these theories agree that the present foliated condition of these
rocks is due to the intense metamorphism which they have suffered.

It is to this body of crystalline rocks and the stratified rocks
involved with it, which form a very small proportion of its mass,
that the term _Archean_ (Greek, arche, beginning) is applied by
many geologists.


The Algonkian

In some regions there rests unconformably on the Archean an immense
body of stratified rocks, thousands and in places even scores
of thousands of feet thick, known as the _Algonkian_. Great
unconformities divide it into well-defined systems, but as only
the scantiest traces of fossils appear here and there among its strata,
it is as yet impossible to correlate the formations of different
regions and to give them names of more than local application. We
will describe the Algonkian rocks of two typical areas.

=The Grand Canyon of the Colorado.= We have already studied a very
ancient peneplain whose edge is exposed to view deep on the walls of
the Colorado Canyon (_nu´_, Fig. 207). The formation of flat-lying
sandstone which covers this buried land surface is proved by its
fossils to belong to the Cambrian,--the earliest period of the
Paleozoic era. The tilted rocks (_b_, Fig. 207). on whose upturned
edges the Cambrian sandstone rests are far older, for the physical
break which separates them from it records a time interval during
which they were upheaved to mountainous ridges and worn down to a low
plain. They are therefore classified as Algonkian. They comprise two
immense series. The upper is more than five thousand feet thick and
consists of shales and sandstones with some limestones. Separated from
it by an unconformity which does not appear in Figure 207, the lower
division, seven thousand feet thick, consists chiefly of massive
reddish sandstones with seven or more sheets of lava interbedded. The
lowest member is a basal conglomerate composed of pebbles derived from
the erosion of the dark crumpled schists beneath,--schists which are
supposed to be Archean. As shown in Figure 207, a strong unconformity
(_nm´_, Fig. 207) parts the schists and the Algonkian. The floor on
which the Algonkian rests is remarkably even, and here again is proved
an interval of incalculable length, during which an ancient land mass
of Archean rocks was baseleveled before it received the cover of the
sediments of the later age.

=The Lake Superior region.= In eastern Canada an area of pre-Cambrian
rocks, Archean and Algonkian, estimated at two million square miles,
stretches from the Great Lakes and the St. Lawrence River northward to
the confines of the continent, inclosing Hudson Bay in the arms of a
gigantic U. This immense area, which we have already studied as the
Laurentian peneplain (p. 89), extends southward across the Canadian
border into northern Minnesota, Wisconsin, and Michigan. The rocks of
this area are known to be pre-Cambrian; for the Cambrian strata,
wherever found, lie unconformably upon them.

   [Illustration: Fig. 262. Ideal Section in the Lake Superior
      Region]

The general relations of the formations of that portion of the area
which lies about Lake Superior are shown in Figure 262. Great
unconformities, _UU´_ separate the Algonkian both from the Archean and
from the Cambrian, and divide it into three distinct systems,--the
_Lower Huronian_, the _Upper Huronian_, and the _Keweenawan_. The
Lower and the Upper Huronian consist in the main of old sea muds and
sands and limy oozes now changed to gneisses, schists, marbles,
quartzites, slates, and other metamorphic rocks. The Keweenawan is
composed of immense piles of lava, such as those of Iceland, overlain
by bedded sandstones. What remains of these rock systems after the
denudation of all later geologic ages is enormous. The Lower Huronian
is more than a mile thick, the Upper Huronian more than two miles
thick, while the Keweenawan exceeds nine miles in thickness. The vast
length of Algonkian time is shown by the thickness of its marine
deposits and by the cycles of erosion which it includes. In Figure 262
the student may read an outline of the history of the Lake Superior
region, the deformations which it suffered, their relative severity,
the times when they occurred, and the erosion cycles marked by the
successive unconformities.

=Other pre-Cambrian areas in North America.= Pre-Cambrian rocks are
exposed in various parts of the continent, usually by the erosion of
mountain ranges in which their strata were infolded. Large areas occur
in the maritime provinces of Canada. The core of the Green Mountains
of Vermont is pre-Cambrian, and rocks of these systems occur in
scattered patches in western Massachusetts. Here belong also the
oldest rocks of the Highlands of the Hudson and of New Jersey. The
Adirondack region, an outlier of the Laurentian region, exposes
pre-Cambrian rocks, which have been metamorphosed and tilted by the
intrusion of a great boss of igneous rock out of which the central
peaks are carved. The core of the Blue Ridge and probably much of the
Piedmont Belt are of this age. In the Black Hills the irruption of an
immense mass of granite has caused or accompanied the upheaval of
pre-Cambrian strata and metamorphosed them by heat and pressure into
gneisses, schists, quartzites, and slates. In most of these
mountainous regions the lowest strata are profoundly changed by
metamorphism, and they can be assigned to the pre-Cambrian only where
they are clearly overlain unconformably by formations proved to be
Cambrian by their fossils. In the Belt Mountains of Montana, however,
the Cambrian is underlain by Algonkian sediments twelve thousand feet
thick, and but little altered.

=Mineral wealth of the pre-Cambrian rocks.= The pre-Cambrian rocks are
of very great economic importance, because of their extensive
metamorphism and the enormous masses of igneous rock which they
involve. In many parts of the country they are the source of supply of
granite, gneiss, marble, slate, and other such building materials.
Still more valuable are the stores of iron and copper and other metals
which they contain.

At the present time the pre-Cambrian region about Lake Superior leads
the world in the production of iron ore, its output for 1903 being
more than five sevenths of the entire output of the whole United
States, and exceeding that of any foreign country. The ore bodies
consist chiefly of the red oxide of iron (hematite) and occur in
troughs of the strata, underlain by some impervious rock. A theory
held by many refers the ultimate source of the iron to the igneous
rocks of the Archean. When these rocks were upheaved and subjected to
weathering, their iron compounds were decomposed. Their iron was
leached out and carried away to be laid in the Algonkian water bodies
in beds of iron carbonate and other iron compounds. During the later
ages, after the Algonkian strata had been uplifted to form part of the
continent, a second concentration has taken place. Descending
underground waters charged with oxygen have decomposed the iron
carbonate and deposited the iron, in the form of iron oxide, in
troughs of the strata where their downward progress was arrested by
impervious floors.

The pre-Cambrian rocks of the eastern United States also are rich in
iron. In certain districts, as in the Highlands of New Jersey, the
black oxide of iron (magnetite) is so abundant in beds and
disseminated grains that the ordinary surveyor's compass is useless.

The pre-Cambrian copper mines of the Lake Superior region are among
the richest on the globe. In the igneous rocks copper, next to iron,
is the most common of all the useful metals, and it was especially
abundant in the Keweenawan lavas. After the Keweenawan was uplifted to
form land, percolating waters leached out much of the copper diffused
in the lava sheets and deposited it within steam blebs as amygdules of
native copper, in cracks and fissures, and especially as a cement, or
matrix, in the interbedded gravels which formed the chief aquifers of
the region. The famous Calumet and Hecla mine follows down the dip of
the strata to the depth of nearly a mile and works such an ancient
conglomerate whose matrix is pure copper.

   [Illustration: Fig. 263. Successive Stages in the Development
      of the Ovum to the Gastrula Stage]

=The appearance of life.= Sometime during the dim ages preceding the
Cambrian, whether in the Archean or in the Algonkian we know not,
occurred one of the most important events in the history of the earth.
Life appeared for the first time upon the planet. Geology has no
evidence whatever to offer as to whence or how life came. All
analogies lead us to believe that its appearance must have been
sudden. Its earliest forms are unknown, but analogy suggests that as
every living creature has developed from a single cell, so the
earliest organisms upon the globe--the germs from which all later life
is supposed to have been evolved--were tiny, unicellular masses of
protoplasm, resembling the amoeba of to-day in the simplicity of their
structure.

Such lowly forms were destitute of any hard parts and could leave no
evidence of their existence in the record of the rocks. And of their
supposed descendants we find so few traces in the pre-Cambrian strata
that the first steps in organic evolution must be supplied from such
analogies in embryology as the following. The fertilized ovum, the
cell with which each animal begins its life, grows and multiplies by
cell division, and develops into a hollow globe of cells called the
_blastosphere_. This stage is succeeded by the stage of the
_gastrula_,--an ovoid or cup-shaped body with a double wall of cells
inclosing a body cavity, and with an opening, the primitive mouth.
Each of these early embryological stages is represented by living
animals,--the undivided cell by the _protozoa_, the blastosphere by
some rare forms, and the gastrula in the essential structure of the
_coelenterates_,--the subkingdom to which the fresh-water hydra and
the corals belong. All forms of animal life, from the coelenterates to
the mammals, follow the same path in their embryological development
as far as the gastrula stage, but here their paths widely diverge,
those of each subkingdom going their own separate ways.

We may infer, therefore, that during the pre-Cambrian periods organic
evolution followed the lines thus dimly traced. The earliest
one-celled protozoa were probably succeeded by many-celled animals of
the type of the blastosphere, and these by gastrula-like organisms.
From the gastrula type the higher subdivisions of animal life
probably diverged, as separate branches from a common trunk. Much or
all of this vast differentiation was accomplished before the opening
of the next era; for all the subkingdoms are represented in the
Cambrian except the vertebrates.

=Evidences of pre-Cambrian life.= An indirect evidence of life during
the pre-Cambrian periods is found in the abundant and varied fauna of
the next period; for, if the theory of evolution is correct, the
differentiation of the Cambrian fauna was a long process which might
well have required for its accomplishment a large part of pre-Cambrian
time.

Other indirect evidences are the pre-Cambrian limestones, iron ores,
and graphite deposits, since such minerals and rocks have been formed
in later times by the help of organisms. If the carbonate of lime of
the Algonkian limestones and marbles was extracted from sea water by
organisms, as is done at present by corals, mollusks, and other humble
animals and plants, the life of those ancient seas must have been
abundant. Graphite, a soft black mineral composed of carbon and used
in the manufacture of lead pencils and as a lubricant, occurs widely
in the metamorphic pre-Cambrian rocks. It is known to be produced in
some cases by the metamorphism of coal, which itself is formed of
decomposed vegetal tissues. Seams of graphite may therefore represent
accumulations of vegetal matter such as seaweed. But limestone, iron
ores, and graphite can be produced by chemical processes, and their
presence in the pre-Cambrian makes it only probable, and not certain,
that life existed at that time.

=Pre-Cambrian fossils.= Very rarely has any clear trace of an organism
been found in the most ancient chapters of the geological record, so
many of their leaves have been destroyed and so far have their pages
been defaced. Omitting structures whose organic nature has been
questioned, there are left to mention a tiny seashell of one of the
most lowly types,--a _Discina_ from the pre-Cambrian rocks of the
Colorado Canyon,--and from the pre-Cambrian rocks of Montana trails of
annelid worms and casts of their burrows in ancient beaches, and
fragments of the tests of crustaceans. These diverse forms indicate
that before the Algonkian had closed, life was abundant and had widely
differentiated. We may expect that other forms will be discovered as
the rocks are closely searched.

=Pre-Cambrian geography.= Our knowledge is far too meager to warrant
an attempt to draw the varying outlines of sea and land during the
Archean and Algonkian eras. Pre-Cambrian time probably was longer than
all later geological time down to the present, as we may infer from
the vast thicknesses of its rocks and the unconformities which part
them. We know that during its long periods land masses again and again
rose from the sea, were worn low, and were submerged and covered with
the waste of other lands. But the formations of separated regions
cannot be correlated because of the absence of fossils, and nothing
more can be made out than the detached chapters of local histories,
such as the outline given of the district about Lake Superior.

The pre-Cambrian rocks show no evidence of any forces then at work
upon the earth except the forces which are at work upon it now. The
most ancient sediments known are so like the sediments now being laid
that we may infer that they were formed under conditions essentially
similar to those of the present time. There is no proof that the sands
of the pre-Cambrian sandstones were swept by any more powerful waves
and currents than are offshore sands to-day, or that the muds of the
pre-Cambrian shales settled to the sea floor in less quiet water than
such muds settle in at present. The pre-Cambrian lands were, no doubt,
worn by wind and weather, beaten by rain, and furrowed by streams as
now, and, as now, they fronted the ocean with beaches on which waves
dashed and along which tidal currents ran.

Perhaps the chief difference between the pre-Cambrian and the present
was the absence of life upon the land. So far as we have any
knowledge, no forests covered the mountain sides, no verdure carpeted
the plains, and no animals lived on the ground or in the air. It is
permitted to think of the most ancient lands as deserts of barren rock
and rock waste swept by rains and trenched by powerful streams. We may
therefore suppose that the processes of their destruction went on more
rapidly than at present.




CHAPTER XVI

THE CAMBRIAN


=The Paleozoic era.= The second volume of the geological record,
called the Paleozoic (Greek, _palaios_, ancient; _zoe_, life), has
come down to us far less mutilated and defaced than has the first
volume, which contains the traces of the most ancient life of the
globe. Fossils are far more abundant in the Paleozoic than in the
earlier strata, while the sediments in which they were entombed have
suffered far less from metamorphism and other causes, and have been
less widely buried from view, than the strata of the pre-Cambrian
groups. By means of their fossils we can correlate the formations of
widely separated regions from the beginning of the Paleozoic on, and
can therefore trace some outline of the history of the continents.

Paleozoic time, although shorter than the pre-Cambrian as measured by
the thickness of the strata, must still be reckoned in millions of
years. During this vast reach of time the changes in organisms were
very great. It is according to the successive stages in the advance of
life that the Paleozoic formations are arranged in five systems,--the
_Cambrian_, the _Ordovician_, the _Silurian_, the _Devonian_, and the
_Carboniferous_. On the same basis the first three systems are grouped
together as the older Paleozoic, because they alike are characterized
by the dominance of the invertebrates; while the last two systems are
united in the later Paleozoic, and are characterized, the one by the
dominance of fishes, and the other by the appearance of amphibians and
reptiles.

Each of these systems is world-wide in its distribution, and may be
recognized on any continent by its own peculiar fauna. The names first
given them in Great Britain have therefore come into general use,
while their subdivisions, which often cannot be correlated in
different countries and different regions, are usually given local
names.

The first three systems were named from the fact that their strata are
well displayed in Wales. The Cambrian carries the Roman name of Wales,
and the Ordovician and Silurian the names of tribes of ancient Britons
which inhabited the same country. The Devonian is named from the
English county Devon, where its rocks were early studied. The
Carboniferous was so called from the large amount of coal which it was
found to contain in Great Britain and continental Europe.


The Cambrian

=Distribution of strata.= The Cambrian rocks outcrop in narrow belts
about the pre-Cambrian areas of eastern Canada and the Lake Superior
region, the Adirondacks and the Green Mountains. Strips of Cambrian
formations occupy troughs in the pre-Cambrian rocks of New England and
the maritime provinces of Canada; a long belt borders on the west the
crystalline rocks of the Blue Ridge; and on the opposite side of the
continent the Cambrian reappears in the mountains of the Great Basin
and the Canadian Rockies. In the Mississippi valley it is exposed in
small districts where uplift has permitted the stripping off of
younger rocks. Although the areas of outcrop are small, we may infer
that Cambrian rocks were widely deposited over the continent of North
America.

=Physical geography.= The Cambrian system of North America comprises
three distinct series, the _Lower Cambrian_, the _Middle Cambrian_,
and the _Upper Cambrian_, each of which is characterized by its own
peculiar fauna. In sketching the outlines of the continent as it was
at the beginning of the Paleozoic, it must be remembered that wherever
the Lower Cambrian formations now are found was certainly then sea
bottom, and wherever the Lower Cambrian are wanting, and the next
formations rest directly on pre-Cambrian rocks, was probably then
land.

   [Illustration: Fig. 264. Hypothetical Map of Eastern North
     America at the Beginning of Cambrian Time

   Unshaded areas, probable land]

=Early Cambrian geography.= In this way we know that at the opening of
the Cambrian two long, narrow mediterranean seas stretched from north
to south across the continent. The eastern sea extended from the Gulf
of St. Lawrence down the Champlain-Hudson valley and thence along the
western base of the Blue Ridge south at least to Alabama. The western
sea stretched from the Canadian Rockies over the Great Basin and at
least as far south as the Grand Canyon of the Colorado in Arizona.

Between these mediterraneans lay a great central land which included
the pre-Cambrian U-shaped area of the Laurentian peneplain, and
probably extended southward to the latitude of New Orleans. To the
east lay a land which we may designate as _Appalachia_, whose western
shore line was drawn along the site of the present Blue Ridge, but
whose other limits are quite unknown. The land of Appalachia must have
been large, for it furnished a great amount of waste during the entire
Paleozoic era, and its eastern coast may possibly have lain even
beyond the edge of the present continental shelf. On the western side
of the continent a narrow land occupied the site of the Sierra Nevada
Mountains.

Thus, even at the beginning of the Paleozoic, the continental plateau
of North America had already been left by crustal movements in relief
above the abysses of the great oceans on either side. The
mediterraneans which lay upon it were shallow, as their sediments
prove. They were _epicontinental seas_; that is, they rested _upon_
(Greek, _epi_) the submerged portion of the continental plateau. We
have no proof that the deep ocean ever occupied any part of where
North America now is.

The Middle and Upper Cambrian strata are found together with the Lower
Cambrian over the area of both the eastern and the western
mediterraneans, so that here the sea continued during the entire
period. The sediments throughout are those of shoal water. Coarse
cross-bedded sandstones record the action of strong shifting currents
which spread coarse waste near shore and winnowed it of finer stuff.
Frequent ripple marks on the bedding planes of the strata prove that
the loose sands of the sea floor were near enough to the surface to be
agitated by waves and tidal currents. Sun cracks show that often the
outgoing tide exposed large muddy flats to the drying action of the
sun. The fossils, also, of the strata are of kinds related to those
which now live in shallow waters near the shore.

The sediments which gathered in the mediterranean seas were very
thick, reaching in places the enormous depth of ten thousand feet.
Hence the bottoms of these seas were sinking troughs, ever filling
with waste from the adjacent land as fast as they subsided.

=Late Cambrian geography.= The formations of the Middle and Upper
Cambrian are found resting unconformably on the pre-Cambrian rocks
from New York westward into Minnesota and at various points in the
interior, as in Missouri and in Texas. Hence after earlier Cambrian
time the central land subsided, with much the same effect as if the
Mississippi valley were now to lower gradually, and the Gulf of Mexico
to spread northward until it entered Lake Superior. The Cambrian seas
transgressed the central land and strewed far and wide behind their
advancing beaches the sediments of the later Cambrian upon an eroded
surface of pre-Cambrian rocks.

The succession of the Cambrian formations in North America records
many minor oscillations and varying conditions of physical geography;
yet on the whole it tells of widening seas and lowering lands. Basal
conglomerates and coarse sandstones which must have been laid near
shore are succeeded by shaly sandstones, sandy shales, and shales.
Toward the top of the series heavy beds of limestone, extending from
the Blue Ridge to Missouri, speak of clear water, and either of more
distant shores or of neighboring lands which were worn or sunk so low
that for the most part their waste was carried to the sea in solution.

In brief, the Cambrian was a period of submergence. It began with the
larger part of North America emerged as great land masses. It closed
with most of the interior of the continental plateau covered with a
shallow sea.


The Life of the Cambrian Period

It is now for the first time that we find preserved in the offshore
deposits of the Cambrian seas enough remains of animal life to be
properly called a fauna. Doubtless these remains are only the most
fragmentary representation of the life of the time, for the Cambrian
rocks are very old and have been widely metamorphosed. Yet the five
hundred and more species already discovered embrace all the leading
types of invertebrate life, and are so varied that we must believe
that their lines of descent stretch far back into the pre-Cambrian
past.

=Plants.= No remains of plants have been found in Cambrian strata,
except some doubtful markings, as of seaweed.

=Sponges.= The sponges, the lowest of the multicellular animals, were
represented by several orders. Their fossils are recognized by the
siliceous spicules, which, as in modern sponges, either were scattered
through a mass of horny fibers or were connected in a flinty
framework.

   [Illustration: Fig. 265. Sponge Spicules as seen in Flint under
      the Microscope]

=Coelenterates.= This subkingdom includes two classes of interest to
the geologist,--the _Hydrozoa_, such as the fresh-water hydra and the
jellyfish, and the _corals_. Both classes existed in the Cambrian.

   [Illustration: Fig. 266. Graptolites]

The Hydrozoa were represented not only by jellyfish but also by the
_graptolite_, which takes its name from a fancied resemblance of some
of its forms to a quill pen. It was a composite animal with a horny
framework, the individuals of the colony living in cells strung on one
or both sides along a hollow stem, and communicating by means of a
common flesh in this central tube. Some graptolites were straight, and
some curved or spiral; some were single stemmed, and others consisted
of several radial stems united. Graptolites occur but rarely in the
Upper Cambrian. In the Ordovician and Silurian they are very
plentiful, and at the close of the Silurian they pass out of
existence, never to return.

=Corals= are very rarely found in the Cambrian, and the description of
their primitive types is postponed to later chapters treating of
periods when they became more numerous.

=Echinoderms.= This subkingdom comprises at present such familiar
forms as the crinoid, the starfish, and the sea urchin. The structure
of echinoderms is radiate. Their integument is hardened with plates or
particles of carbonate of lime.

   [Illustration: Fig. 267. Cystoids, one showing Two Rudimentary
      Arms]

Of the free echinoderms, such as the starfish and the sea urchin, the
former has been found in the Cambrian rocks of Europe, but neither
have so far been discovered in the strata of this period in North
America. The stemmed and lower division of the echinoderms was
represented by a primitive type, the _cystoid_, so called from its
saclike form, A small globular or ovate "calyx" of calcareous plates,
with an aperture at the top for the mouth, inclosed the body of the
animal, and was attached to the sea bottom by a short flexible stalk
consisting of disks of carbonate of lime held together by a central
ligament.

=Arthropods.= These segmented animals with "jointed feet," as their
name suggests, may be divided in a general way into water breathers
and air breathers. The first-named and lower division comprises the
class of the _Crustacea_,--arthropods protected by a hard exterior
skeleton, or "crust,"--of which crabs, crayfish, and lobsters are
familiar examples. The higher division, that of the air breathers,
includes the following classes: spiders, scorpions, centipedes, and
insects.

=The trilobite.= The aquatic arthropods, the Crustacea, culminated
before the air breathers; and while none of the latter are found in
the Cambrian, the former were the dominant life of the time in
numbers, in size, and in the variety of their forms. The leading
crustacean type is the _trilobite_, which takes its name from the
three lobes into which its shell is divided longitudinally. There are
also three cross divisions,--the head shield, the tail shield, and
between the two the thorax, consisting of a number of distinct and
unconsolidated segments. The head shield carries a pair of large,
crescentic, compound eyes, like those of the insect. The eye varies
greatly in the number of its lenses, ranging from fourteen in some
species to fifteen thousand in others. Figure 268, C, is a restoration
of the trilobite, and shows the appendages, which are found preserved
only in the rarest cases.

   [Illustration: Fig. 268. Trilobites

   A, a Cambrian species; B, a Devonian species showing eyes;
   C, restoration of an Ordovician species]

During the long ages of the Cambrian the trilobite varied greatly.
Again and again new species and genera appeared, while the older types
became extinct. For this reason and because of their abundance,
trilobites are used in the classification of the Cambrian system. The
Lower Cambrian is characterized by the presence of a trilobitic fauna
in which the genus Olenellus is predominant. This, the _Olenellus
Zone_, is one of the most important platforms in the entire geological
series; for, the world over, it marks the beginning of Paleozoic time,
while all underlying strata are classified as pre-Cambrian. The Middle
Cambrian is marked by the genus Paradoxides, and the Upper Cambrian by
the genus Olenus. Some of the Cambrian trilobites were giants,
measuring as much as two feet long, while others were the smallest of
their kind, a fraction of an inch in length.

Another type of crustacean which lived in the Cambrian and whose order
is still living is illustrated in Figure 269.

   [Illustration: Fig. 269. A Phyllopod]

=Worms.= Trails and burrows of worms have been left on the sea beaches
and mud flats of all geological times from the Algonkian to the
present.

=Brachiopods.= These soft-bodied animals, with bivalve shells and two
interior armlike processes which served for breathing, appeared in the
Algonkian, and had now become very abundant. The two valves of the
brachiopod shell are unequal in size, and in each valve a line drawn
from the beak to the base divides the valve into two equal parts
(Fig. 270). It may thus be told from the pelecypod mollusk, such as the
clam, whose two valves are not far from equal in size, each being
divided into unequal parts by a line dropped from the beak (Fig.272).

   [Illustration: Fig. 270. A Cambrian Articulate Brachiopod, Orthis]

   [Illustration: Fig. 271. Cambrian Inarticulate Brachiopods

   A, Lingulella; B, Discina]

Brachiopods include two orders. In the most primitive order--that of
the _inarticulate_ brachiopods--the two valves are held together only
by muscles of the animal, and the shell is horny or is composed of
phosphate of lime. The _Discina_, which began in the Algonkian, is of
this type, as is also the _Lingulella_ of the Cambrian (Fig. 271). Both
of these genera have lived on during the millions of years of geological
time since their introduction, handing down from generation to
generation with hardly any change to their descendants now living off
our shores the characters impressed upon them at the beginning.

The more highly organized _articulate_ brachiopods have valves of
carbonate of lime more securely joined by a hinge with teeth and
sockets (Fig. 270). In the Cambrian the inarticulates predominate,
though the articulates grow common toward the end of the period.

=Mollusks.= The three chief classes of mollusks--the _pelecypods_
(represented by the oyster and clam of to-day), the _gastropods_
(represented now by snails, conches, and periwinkles), and the
_cephalopods_ (such as the nautilus, cuttlefish, and squids)--were all
represented in the Cambrian, although very sparingly.

   [Illustration: Fig. 272. A Cambrian Pelecypod]

   [Illustration: Fig. 273. Gastropods]

Pteropods, a suborder of the gastropods, appeared in this age. Their
papery shells of carbonate of lime are found in great numbers from
this time on.

   [Illustration: Fig. 274. Cambrian Pteropods]

Cephalopods, the most highly organized of the mollusks, started into
existence, so far as the record shows, toward, the end of the Cambrian,
with the long extinct _Orthoceras_ (_straighthorn_) and the allied
genera of its family. The Orthoceras had a long, straight, and tapering
shell, divided by cross partitions into chambers. The animal lived in
the "body chamber" at the larger end, and walled off the other chambers
from it in succession during the growth of the shell. A central tube,
the _siphuncle_ (_s_, Fig. 275, _B_), passed through from the body
chamber to the closed tip of the cone.

   [Illustration: Fig. 275. Orthoceras

   A, fossil; B, restoration]

The seashells, both brachiopods and mollusks, are in some respects the
most important to the geologist of all fossils. They have been so
numerous, so widely distributed, and so well preserved because of
their durable shells and their station in growing sediments, that
better than any other group of organisms they can be used to correlate
the strata of different regions and to mark by their slow changes the
advance of geological time.

=Climate.= The life of Cambrian times in different countries contains
no suggestion of any marked climatic zones, and as in later periods a
warm climate probably reached to the polar regions.




CHAPTER XVII

THE ORDOVICIAN[2] AND SILURIAN

[2] Often known as the Lower Silurian.


The Ordovician

In North America the Ordovician rocks lie conformably on the Cambrian.
The two periods, therefore, were not parted by any deformation, either
of mountain making or of continental uplift. The general submergence
which marked the Cambrian continued into the succeeding period with
little interruption.

=Subdivisions and distribution of strata.= The Ordovician series, as
they have been made out in New York, are given for reference in the
following table, with the rocks of which they are chiefly composed:

  5 Hudson       .....  shales
  4 Utica        .....  shales
  3 Trenton      .....  limestones
  2 Chazy        .....  limestones
  1 Calciferous  .....  sandy limestones

These marine formations of the Ordovician outcrop about the Cambrian
and pre-Cambrian areas, and, as borings show, extend far and wide over
the interior of the continent beneath more recent strata. The
Ordovician sea stretched from Appalachia across the Mississippi
valley. It seems to have extended to California, although broken
probably by several mountainous islands in the west.

=Physical geography.= The physical history of the period is recorded
in the succession of its formations. The sandstones of the Upper
Cambrian, as we have learned, tell of a transgressing sea which
gradually came to occupy the Mississippi valley and the interior of
North America. The limestones of the early and middle Ordovician show
that now the shore had become remote and the lands had become more
low. The waters now had cleared. Colonies of brachiopods and other
lime-secreting animals occupied the sea bottom, and their debris
mantled it with sheets of limy ooze. The sandy limestones of the
Calciferous record the transition stage from the Cambrian when some
sand was still brought in from shore. The highly fossiliferous
limestones of the Trenton tell of clear water and abundant life. We
need not regard this epicontinental sea as deep. No abysmal deposits
have been found, and the limestones of the period are those which
would be laid in clear, warm water of moderate depth like that of
modern coral seas.

   [Illustration: Fig. 276. Hypothetical Map of the Eastern United
      States in Ordovician Time

   Shaded areas, probable sea; broken lines, approximate shore lines]

The shales of the Utica and Hudson show that the waters of the sea now
became clouded with mud washed in from land. Either the land was
gradually uplifted, or perhaps there had arrived one of those periodic
crises which, as we may imagine, have taken place whenever the
crust of the shrinking earth has slowly given way over its great
depressions, and the ocean has withdrawn its waters into deepening
abysses. The land was thus left relatively higher and bordered with
new coastal plains. The epicontinental sea was shoaled and narrowed,
and muds were washed in from the adjacent lands.

=The Taconic deformation.= The Ordovician was closed by a deformation
whose extent and severity are not yet known. From the St. Lawrence
River to New York Bay, along the northwestern and western border of
New England, lies a belt of Cambrian-Ordovician rocks more than a mile
in total thickness, which accumulated during the long ages of those
periods in a gradually subsiding trough between the Adirondacks and a
pre-Cambrian range lying west of the Connecticut River. But since
their deposition these ancient sediments have been crumpled and
crushed, broken with great faults, and extensively metamorphosed. The
limestones have recrystallized into marbles, among them the famous
marbles of Vermont; the Cambrian sandstones have become quartzites,
and the Hudson shale has been changed to a schist exposed on Manhattan
Island and northward.

In part these changes occurred at the close of the Ordovician, for in
several places beds of Silurian age rest unconformably on the upturned
Ordovician strata; but recent investigations have made it probable
that the crustal movements recurred at later times, and it was perhaps
in the Devonian and at the close of the Carboniferous that the greater
part of the deformation and metamorphism was accomplished. As a result
of these movements,--perhaps several times repeated,--a great mountain
range was upridged, which has been long since leveled by erosion, but
whose roots are now visible in the Taconic Mountains of western New
England.

=The Cincinnati anticline.= Over an oval area in Ohio, Indiana, and
Kentucky, whose longer axis extends from north to south through
Cincinnati, the Ordovician strata rise in a very low, broad swell,
called the Cincinnati anticline. The Silurian and Devonian strata thin
out as they approach this area and seem never to have deposited upon
it. We may regard it, therefore, as an island upwarped from the sea at
the close of the Ordovician or shortly after.

=Petroleum and natural gas.= These valuable illuminants and fuels are
considered here because, although they are found in traces in older
strata, it is in the Ordovician that they occur for the first time in
large quantities. They range throughout later formations down to the
most recent.

   [Illustration: Fig. 277. Diagram Illustrating the Conditions of
      Accumulation of Oil and Gas

   _a_, source; _b_, reservoir; _c_, cover. What would be the result
   of boring to the reservoir rock at _d_? at _d´_? at _d´´_?]

The oil horizons of California and Texas are Tertiary; those of
Colorado, Cretaceous; those of West Virginia, Carboniferous; those of
Pennsylvania, Kentucky, and Canada, Devonian; and the large field of
Ohio and Indiana belongs to the Ordovician and higher systems.

Petroleum and natural gas, wherever found, have probably originated
from the decay of organic matter when buried in sedimentary deposits,
just as at present in swampy places the hydrogen and carbon of
decaying vegetation combine to form marsh gas. The light and heat of
these hydrocarbons we may think of, therefore, as a gift to the
civilized life of our race from the humble organisms, both animal and
vegetable, of the remote past, whose remains were entombed in the
sediments of the Ordovician and later geological ages.

Petroleum is very widely disseminated throughout the stratified rocks.
Certain limestones are visibly greasy with it, and others give off its
characteristic fetid odor when struck with a hammer. Many shales are
bituminous, and some are so highly charged that small flakes may be
lighted like tapers, and several gallons of oil to the ton may be
obtained by distillation.

But oil and gas are found in paying quantities only when certain
conditions meet:

1. A _source_ below, usually a bituminous shale, from whose organic
matter they have been derived by slow change.

2. A _reservoir_ above, in which they have gathered. This is either a
porous sandstone or a porous or creviced limestone.

3. Oil and gas are lighter than water, and are usually under pressure
owing to artesian water. Hence, in order to hold them from escaping to
the surface, the reservoir must have the shape of an _anticline_,
_dome_, or _lens_.

4. It must also have an _impervious cover_, usually a shale. In these
reservoirs gas is under a pressure which is often enormous, reaching
in extreme cases as high as a thousand five hundred pounds to the
square inch. When tapped it rushes out with a deafening roar,
sometimes flinging the heavy drill high in air. In accounting for this
pressure we must remember that the gas has been compressed within the
pores of the reservoir rock by artesian water, and in some cases also
by its own expansive force. It is not uncommon for artesian water to
rise in wells after the exhaustion of gas and oil.


_Life of the Ordovician_

During the ages of the Ordovician, life made great advances. Types
already present branched widely into new genera and species, and new
and higher types appeared.

Sponges continued from the Cambrian. Graptolites now reached their
climax.

   [Illustration: Fig. 278. Stromatopora]

=Stromatopora=--colonies of minute hydrozoans allied to corals--grew
in places on the sea floor, secreting stony masses composed of thin,
close, concentric layers, connected by vertical rods. The Stromatopora
are among the chief limestone builders of the Silurian and Devonian
periods.

=Corals= developed along several distinct lines, like modern corals
they secreted a calcareous framework, in whose outer portions the
polyps lived. In the Ordovician, corals were represented chiefly by
the family of the _Chætetes_, all species of which are long since
extinct. The description of other types of corals will be given under
the Silurian, where they first became abundant.

=Echinoderms.= The cystoid reaches its climax, but there appear now
two higher types of echinoderms,--the crinoid and the starfish. The
_crinoid_, named from its resemblance to the lily, is like the cystoid
in many respects, but has a longer stem and supports a crown of
plumose arms. Stirring the water with these arms, it creates currents
by which particles of food are wafted to its mouth. Crinoids are rare
at the present time, but they grew in the greatest profusion in the
warm Ordovician seas and for long ages thereafter. In many places the
sea floor was beautiful with these graceful, flowerlike forms, as with
fields of long-stemmed lilies. Of the higher, free-moving classes of
the echinoderms, starfish are more numerous than in the Cambrian, and
sea urchins make their appearance in rare archaic forms.

   [Illustration: Fig. 279. Crinoid, a Jurassic Species]

   [Illustration: Fig. 280. An Ordovician Starfish]

   [Illustration: Fig. 281. An Ordovician Sea Urchin]

   [Illustration: Fig. 282. Eurypterus]

=Crustaceans.= Trilobites now reach their greatest development and
more than eleven hundred species have been described from the rocks of
this period. It is interesting to note that in many species the
segments of the thorax have now come to be so shaped that they move
freely on one another. Unlike their Cambrian ancestors, many of the
Ordovician trilobites could roll themselves into balls at the approach
of danger. It is in this attitude, taken at the approach of death,
that trilobites are often found in the Ordovician and later rocks. The
gigantic crustaceans called the _eurypterids_ were also present in
this period (Fig. 282).

The arthropods had now seized upon the land. Centipedes and insects of
a low type, the earliest known land animals, have been discovered in
strata of this system.

   [Illustration: Fig. 283. A Bryozoan]

=Bryozoans.= No fossils are more common in the limestones of the time
than the small branching stems and lacelike mats of the
bryozoans,--the skeletons of colonies of a minute animal allied in
structure to the brachiopod.

   [Illustration: Fig. 284. Ordovician Brachiopods]

=Brachiopods.= These multiplied greatly, and in places their shells
formed thick beds of coquina. They still greatly surpassed the
mollusks in numbers.

=Cephalopods.= Among the mollusks we must note the evolution of the
cephalopods. The primitive straight Orthoceras has now become
abundant. But in addition to this ancestral type there appears a
succession of forms more and more curved and closely coiled, as
illustrated in Figure 285. The nautilus, which began its course in
this period, crawls on the bottom of our present seas.

   [Illustration: Fig. 285. A, Cyrtoceras; B, Trochoceras; C, Lituites]

   [Illustration: Fig. 286. Nautilus]

=Vertebrates.= The most important record of the Ordovician is that of
the appearance of a new and higher type, with possibilities of
development lying hidden in its structure that the mollusk and the
insect could never hope to reach. Scales and plates of minute fishes
found in the Ordovician rocks near Canon City, Colorado, show that the
humblest of the vertebrates had already made its appearance. But it is
probable that vertebrates had been on the earth for ages before this
in lowly types, which, being destitute of hard parts, would leave no
record.


The Silurian

The narrowing of the seas and the emergence of the lands which
characterized the closing epoch of the Ordovician in eastern North
America continue into the succeeding period of the Silurian. New
species appear and many old species now become extinct.

=The Appalachian region.= Where the Silurian system is most fully
developed, from New York southward along the Appalachian Mountains, it
comprises four series:

  4 Salina   .....  shales, impure limestones, gypsum, salt
  3 Niagara  .....  chiefly limestones
  2 Clinton  .....  sandstones, shales, with some limestones
  1 Medina   .....  conglomerates, sandstones

The rocks of these series are shallow-water deposits and reach the
total thickness of some five thousand feet. Evidently they were laid
over an area which was on the whole gradually subsiding, although with
various gentle oscillations which are recorded in the different
formations. The coarse sands of the heavy Medina formations record a
period of uplift of the oldland of Appalachia, when erosion went on
rapidly and coarse waste in abundance was brought down from the hills
by swift streams and spread by the waves in wide, sandy flats. As the
lands were worn lower the waste became finer, and during an epoch of
transition--the Clinton--there were deposited various formations of
sandstones, shales, and limestones. The Niagara limestones testify to
a long epoch of repose, when low-lying lands sent little waste down to
the sea.

The gypsum and salt deposits of the Salina show that toward the close
of the Silurian period a slight oscillation brought the sea floor
nearer to the surface, and at the north cut off extensive tracts from
the interior sea. In these wide lagoons, which now and then regained
access to the open sea and obtained new supplies of salt water, beds
of salt and gypsum were deposited as the briny waters became
concentrated by evaporation under a desert climate. Along with these
beds there were also laid shales and impure limestones.

In New York the "salt pans" of the Salina extended over an area one
hundred and fifty miles long from east to west and sixty miles wide,
and similar salt marshes occurred as far west as Cleveland, Ohio, and
Goderich on Lake Huron. At Ithaca, New York, the series is fifteen
hundred feet thick, and is buried beneath an equal thickness of later
strata. It includes two hundred and fifty feet of solid salt, in
several distinct beds, each sealed within the shales of the series.

Would you expect to find ancient beds of rock salt inclosed in beds of
pervious sandstone?

The salt beds of the Salina are of great value. They are reached by
well borings, and their brines are evaporated by solar heat and by
boiling. The rock salt is also mined from deep shafts.

Similar deposits of salt, formed under like conditions, occur in the
rocks of later systems down to the present. The salt beds of Texas are
Permian, those of Kansas are Permian, and those of Louisiana are
Tertiary.

=The Mississippi valley.= The heavy near-shore formations of the
Silurian in the Appalachian region thin out toward the west. The
Medina and the Clinton sandstones are not found west of Ohio, where
the first passes into a shale and the second into a limestone. The
Niagara limestone, however, spreads from the Hudson River to beyond
the Mississippi, a distance of more than a thousand miles. During the
Silurian period the Mississippi valley region was covered with a
quiet, shallow, limestone-making sea, which received little waste from
the low lands which bordered it.

The probable distribution of land and sea in eastern North America and
western Europe is shown in Figure 287. The fauna of the interior
region and of eastern Canada are closely allied with that of western
Europe, and several species are identical. We can hardly account for
this except by a shallow-water connection between the two ancient
epicontinental seas. It was perhaps along the coastal shelves of a
northern land connecting America and Europe by way of Greenland and
Iceland that the migration took place, so that the same species came
to live in Iowa and in Sweden.

    [Illustration: Fig. 287. Hypothetical Map of Parts of North America
       and Europe in Silurian Time.

    Shaded areas, probably seas; broken lines, approximate shore lines]

=The western United States.= So little is found of the rocks of the
system west of the Missouri River that it is quite probable that the
western part of the United States had for the most part emerged from
the sea at the close of the Ordovician and remained land during the
Silurian. At the same time the western land was perhaps connected with
the eastern land of Appalachia across Arkansas and Mississippi; for
toward the south the Silurian sediments indicate an approach to shore.


_Life of the Silurian_

In this brief sketch it is quite impossible to relate the many changes
of species and genera during the Silurian.

=Corals.= Some of the more common types are familiarly known as cup
corals, honeycomb corals, and chain corals. In the _cup corals_ the
most important feature is the development of radiating vertical
partitions, or _septa_, in the cell of the polyp. Some of the cup
corals grew in hemispherical colonies (Fig. 288), while many were
separate individuals (Fig. 289), building a single conical, or
horn-shaped cell, which sometimes reached the extreme size of a foot
in length and two or three inches in diameter.

   [Illustration: Fig. 288. A Compound Cup Coral]

   [Illustration: Fig. 289. A Simple Cup Coral]

   [Illustration: Fig. 290. Honeycomb Corals]

   [Illustration: Fig. 291. A Chain Coral]

   [Illustration: Fig. 292. A Syringopora Coral]

_Honeycomb corals_ consist of masses of small, close-set prismatic
cells, each crossed by horizontal partitions, or _tabulæ_, while the
septa are rudimentary, being represented by faintly projecting ridges
or rows of spines.

_Chain corals_ are also marked by tabulæ. Their cells form elliptical
tubes, touching each other at the edges, and appearing in cross
section like the links of a chain. They became extinct at the end of
the Silurian.

The corals of the _Syringopora_ family are similar in structure to
chain corals, but the tubular columns are connected only in places.

   [Illustration: Fig. 293. A Blastoid: A, side view, showing
      portion of the stem; B, summit of calyx (species
      Carboniferous)]

   [Illustration: Fig. 294. A Silurian Scorpion]

To the echinoderms there is now added the _blastoid_ (bud-shaped). The
blastoid is stemmed and armless, and its globular "head" or "calyx,"
with its five petal-like divisions, resembles a flower bud. The
blastoids became more abundant in the Devonian, culminated in the
Carboniferous, and disappeared at the end of the Paleozoic.

The great eurypterids--some of which were five or six feet in
length--and the cephalopods were still masters of the seas. Fishes
were as yet few and small; trilobites and graptolites had now passed
their prime and had diminished greatly in numbers. Scorpions are found
in this period both in Europe and in America. The limestone-making
seas of the Silurian swarmed with corals, crinoids, and brachiopods.

With the end of the Silurian period the _Age of Invertebrates_ comes
to a close, giving place to the Devonian, the _Age of Fishes_.

   [Illustration: Fig. 295. Block of Limestone showing Interior Casts of
      _Pentamerus oblongus_, a Common Silurian Brachiopod]




CHAPTER XVIII

THE DEVONIAN


In America the Silurian is not separated from the Devonian by any
mountain-making deformation or continental uplift. The one period
passed quietly into the other. Their conformable systems are so
closely related, and the change in their faunas is so gradual, that
geologists are not agreed as to the precise horizon which divides
them.

=Subdivisions and physical geography.= The Devonian is represented in
New York and southward by the following five series. We add the rocks
of which they are chiefly composed.

  5 Chemung      .....  sandstones and sandy shales
  4 Hamilton     .....  shales and sandstones
  3 Corniferous  .....  limestones
  2 Oriskany     .....  sandstones
  1 Helderberg   .....  limestones

The Helderberg is a transition epoch referred by some geologists to
the Silurian. The thin sandstones of the Oriskany mark an epoch when
waves worked over the deposits of former coastal plains. The
limestones of the Corniferous testify to a warm and clear wide sea
which extended from the Hudson to beyond the Mississippi. Corals
throve luxuriantly, and their remains, with those of mollusks and
other lime-secreting animals, built up great beds of limestone. The
bordering continents, as during the later Silurian, must now have been
monotonous lowlands which sent down little of even the finest waste to
the sea.

In the Hamilton the clear seas of the previous epoch became clouded
with mud. The immense deposits of coarse sandstones and sandy shales
of the Chemung, which are found off what was at the time the west
coast of Appalachia, prove an uplift of that ancient continent.

The Chemung series extends from the Catskill Mountains to northeastern
Ohio and south to northeastern Tennessee, covering an area of not less
than a hundred thousand square miles. In eastern New York it attains
three thousand feet in thickness; in Pennsylvania it reaches the
enormous thickness of two miles; but it rapidly thins to the west.
Everywhere the Chemung is made of thin beds of rapidly alternating
coarse and fine sands and clays, with an occasional pebble layer, and
hence is a shallow-water deposit. The fine material has not been
thoroughly winnowed from the coarse by the long action of strong waves
and tides. The sands and clays have undergone little more sorting than
is done by rivers. We must regard the Chemung sandstones as deposits
made at the mouths of swift, turbid rivers in such great amount that
they could be little sorted and distributed by waves.

Over considerable areas the Chemung sandstones bear little or no trace
of the action of the sea. The Catskill Mountains, for example, have as
their summit layers some three thousand feet of coarse red sandstones
of this series, whose structure is that of river deposits, and whose
few fossils are chiefly of fresh-water types. The Chemung is therefore
composed of delta deposits, more or less worked over by the sea. The
bulk of the Chemung equals that of the Sierra Nevada Mountains. To
furnish this immense volume of sediment a great mountain range, or
highland, must have been upheaved where the Appalachian lowland long
had been. To what height the Devonian mountains of Appalachia attained
cannot be told from the volume of the sediments wasted from them, for
they may have risen but little faster than they were worn down by
denudation. We may infer from the character of the waste which they
furnished to the Chemung shores that they did not reach an Alpine
height. The grains of the Chemung sandstones are not those which would
result from mechanical disintegration, as by frost on high mountain
peaks, but are rather those which would be left from the long chemical
decay of siliceous crystalline rocks; for the more soluble minerals
are largely wanting. The red color of much of the deposits points to
the same conclusion. Red residual clays accumulated on the mountain
sides and upland summits, and were washed as ocherous silt to mingle
with the delta sands. The iron-bearing igneous rocks of the oldland
also contributed by their decay iron in solution to the rivers, to be
deposited in films of iron oxide about the quartz grains of the
Chemung sandstones, giving them their reddish tints.


Life of the Devonian

=Plants.= The lands were probably clad with verdure during Silurian
times, if not still earlier; for some rare remains of ferns and other
lowly types of vegetation have been found in the strata of that
system. But it is in the Devonian that we discover for the first time
the remains of extensive and luxuriant forests. This rich flora
reached its climax in the Carboniferous, and it will be more
convenient to describe its varied types in the next chapter.

=Rhizocarps.= In the shales of the Devonian are found microscopic
spores of rhizocarps in such countless numbers that their weight must
be reckoned in hundreds of millions of tons. It would seem that these
aquatic plants culminated in this period, and in widely distant
portions of the earth swampy flats and shallow lagoons were filled
with vegetation of this humble type, either growing from the bottom or
floating free upon the surface. It is to the resinous spores of the
rhizocarps that the petroleum and natural gas from Devonian rocks are
largely due. The decomposition of the spores has made the shales
highly bituminous, and the oil and gas have accumulated in the
reservoirs of overlying porous sandstones.

=Invertebrates.= We must pass over the ever-changing groups of the
invertebrates with the briefest notice. Chain corals became extinct at
the close of the Silurian, but other corals were extremely common in
the Devonian seas. At many places corals formed thin reefs, as at
Louisville, Kentucky, where the hardness of the reef rock is one of
the causes of the Falls of the Ohio.

Sponges, echinoderms, brachiopods, and mollusks were abundant. The
cephalopods take a new departure. So far in all their various forms,
whether straight, as the Orthoceras, or curved, or close-coiled as in
the nautilus, the septum, or partition dividing the chambers, met the
inner shell along a simple line, like that of the rim of a saucer.
There now begins a growth of the septum by which its edges become
sharply corrugated, and the suture, or line of juncture of the septum
and the shell, is thus angled. The group in which this growth of the
septum takes place is called the _Goniatite_ (Greek _gonia_, angle).

   [Illustration: Fig. 296. A Goniatite]

=Vertebrates.= It is with the greatest interest that we turn now to
study the backboned animals of the Devonian; for they are believed to
be the ancestors of the hosts of vertebrates which have since
dominated the earth. Their rudimentary structures foreshadowed what
their descendants were to be, and give some clue to the earliest
vertebrates from which they sprang. Like those whose remains are found
in the lower Paleozoic systems, all of these Devonian vertebrates were
aquatic and go under the general designation of fishes.

The lowest in grade and nearest, perhaps, to the ancestral type of
vertebrates, was the problematic creature, an inch or so long, of
Figure 297. Note the circular mouth not supplied with jaws, the lack
of paired fins, and the symmetric tail fin, with the column of
cartilaginous, ringlike vertebræ running through it to the end. The
animal is probably to be placed with the jawless lampreys and hags,--a
group too low to be included among true fishes.

   [Illustration: Fig. 297. Palæospondylus]

=Ostracoderms.= This archaic group, long since extinct, is also too
lowly to rank among the true fishes, for its members have neither jaws
nor paired fins. These small, fishlike forms were cased in front with
bony plates developed in the skin and covered in the rear with scales.
The vertebræ were not ossified, for no trace of them has been found.

   [Illustration: Fig. 298. An Ostracoderm]

=Devonian fishes.= The _true fishes_ of the Devonian can best be
understood by reference to their descendants now living. Modern fishes
are divided into several groups: _sharks_ and their allies;
_dipnoans_; _ganoids_, such as the sturgeon and gar; and
_teleosts_,--most common fishes, such as the perch and cod.

   [Illustration: Fig. 299. A Paleozoic Shark]

=Sharks.= Of all groups of living fishes the sharks are the oldest and
still retain most fully the embryonic characters of their Paleozoic
ancestors. Such characters are the cartilaginous skeleton, and the
separate gill slits with which the throat wall is pierced and which
are arranged in line like the gill openings of the lamprey. The sharks
of the Silurian and Devonian are known to us chiefly by their teeth
and fin spines, for they were unprotected by scales or plates, and
were devoid of a bony skeleton. Figure 299 is a restoration of an
archaic shark from a somewhat higher horizon. Note the seven gill
slits and the lappetlike paired fins. These fins seem to be remnants
of the continuous fold of skin which, as embryology teaches, passed
from fore to aft down each side of the primitive vertebrate.

Devonian sharks were comparatively small. They had not evolved into
the ferocious monsters which were later to be masters of the seas.

   [Illustration: Fig. 300. A Devonian Dipnoan]

=Dipnoans, or lung fishes.= These are represented to-day by a few
peculiar fishes and are distinguished by some high structures which
ally them with amphibians. An air sac with cellular spaces is
connected with the gullet and serves as a rudimentary lung. It
corresponds with the swim bladder of most modern fishes, and appears
to have had a common origin with it. We may conceive that the
primordial fishes not only had gills used in breathing air dissolved
in water, but also developed a saclike pouch off the gullet. This sac
evolved along two distinct lines. On the line of the ancestry of most
modern fishes its duct was closed and it became the swim bladder used
in flotation and balancing. On another line of descent it was left
open, air was swallowed into it, and it developed into the rudimentary
lung of the dipnoans and into the more perfect lungs of the amphibians
and other air-breathing vertebrates.

One of the ancient dipnoans is illustrated in Figure 300. Some of the
members of this order were, like the ostracoderms, cased in armor, but
their higher rank is shown by their powerful jaws and by other
structures. Some of these armored fishes reached twenty-five feet in
length and six feet across the head. They were the tyrants of the
Devonian seas.

   [Illustration: Fig. 301. A Devonian Fringe-Finned Ganoid]

=Ganoids.= These take their name from their enameled plates or scales
of bone. The few genera now surviving are the descendants of the
tribes which swarmed in the Devonian seas. A restoration of one of a
leading order, the _fringe-finned_ ganoids, is given in Figure 301.
The side fins, which correspond to the limbs of the higher
vertebrates, are quite unlike those of most modern fishes. Their rays,
instead of radiating from a common base, fringe a central lobe which
contains a cartilaginous axis. The teeth of the Devonian ganoids show
a complicated folded structure.

=General characteristics of Devonian fishes.= _The notochord is
persistent._ The notochord is a continuous rod of cartilage, or
gristle, which in the embryological growth of vertebrate animals
supports the spinal nerve cord before the formation of the vertebræ.
In most modern fishes and in all higher vertebrates the notochord is
gradually removed as the bodies of the vertebræ are formed about it;
but in the Devonian fishes it persists through maturity and the
vertebræ remain incomplete.

=The skeleton is cartilaginous.= This also is an embryological
characteristic. In the Devonian fishes the vertebræ, as well as the
other parts of the skeleton, have not ossified, or changed to bone,
but remain in their primitive cartilaginous condition.

   [Illustration: Fig. 302. Vertebræ of Sturgeon in side view _A_;
      and vertical transverse section _B_, showing Notochord _ch_, and
      Neural Canal _m_]

=The tail fin is vertebrated.= The backbone runs through the fin and is
fringed above and below with its vertical rays. In some fishes with
vertebrated tail fins the fin is symmetric (Fig. 300), and this seems
to be the primitive type. In others the tail fin is unsymmetric: the
backbone runs into the upper lobe, leaving the two lobes of unequal
size. In most modern fishes (the _teleosts_) the tail fin is not
vertebrated: the spinal column ends in a broad plate, to which the
diverging fin rays are attached.

But along with these embryonic characters, which were common to all
Devonian fishes, there were other structures in certain groups which
foreshadowed the higher structures of the land vertebrates which were
yet to come: air sacs which were to develop into lungs, and
cartilaginous axes in the side fins which were a prophecy of limbs.
The vertebrates had already advanced far enough to prove the
superiority of their type of structure to all others. Their internal
skeleton afforded the best attachment for muscles and enabled them to
become the largest and most powerful creatures of the time. The
central nervous system, with the predominance given to the ganglia at
the fore end of the nerve cord,--the brain,--already endowed them with
greater energy than the invertebrates; and, still more important,
these structures contained the possibility of development into the
more highly organized land vertebrates which were to rule the earth.

=Teleosts.= The great group of fishes called the teleosts, or those
with complete bony skeletons, to which most modern fishes belong, may
be mentioned here, although in the Devonian they had not yet appeared.
The teleosts are a highly specialized type, adapted most perfectly to
their aquatic environment. Heavy armor has been discarded, and
reliance is placed instead on swiftness. The skeleton is completely
ossified and the notochord removed. The vertebræ have been
economically withdrawn from the tail, and the cartilaginous axis of
the side fins has been found unnecessary. The air sac has become a
swim bladder. In this complete specialization they have long since
lost the possibility of evolving into higher types.

It is interesting to note that the modern teleosts in their
embryological growth pass through the stages which characterized the
maturity of their Devonian ancestors; their skeleton is cartilaginous
and their tail fin vertebrated.




CHAPTER XIX

THE CARBONIFEROUS


The Carboniferous system is so named from the large amount of coal
which it contains. Other systems, from the Devonian on, are coal
bearing also, but none so richly and to so wide an extent. Never
before or since have the peculiar conditions been so favorable for the
formation of extensive coal deposits.

With few exceptions the Carboniferous strata rest on those of the
Devonian without any marked unconformity; the one period passed
quietly into the other, with no great physical disturbances.

The Carboniferous includes three distinct series. The lower is called
the _Mississippian_, from the outcrop of its formations along the
Mississippi River in central and southern Illinois and the adjacent
portions of Iowa and Missouri. The middle series is called the
_Pennsylvanian_ (or Coal Measures), from its wide occurrence over
Pennsylvania. The upper series is named the _Permian_, from the
province of Perm in Russia.

=The Mississippian series.= In the interior the Mississippian is
composed chiefly of limestones, with some shales, which tell of a
clear, warm, epicontinental sea swarming with crinoids, corals, and
shells, and occasionally clouded with silt from the land.

In the eastern region, New York had been added by uplift to the
Appalachian land which now was united to the northern area. From
eastern Pennsylvania southward there were laid in a subsiding trough,
first, thick sandstones (the Pocono sandstone), and later still
heavier shales,--the two together reaching the thickness of four
thousand feet and more. We infer a renewed uplift of Appalachia
similar to that of the later epochs of the Devonian, but as much less
in amount as the volume of sediments is smaller.


The Pennsylvanian Series

The Mississippian was brought to an end by a quiet oscillation which
lifted large areas slightly above the sea, and the Pennsylvanian began
with a movement in the opposite direction. The sea encroached on the
new land, and spread far and wide a great basal conglomerate and
coarse sandstones. On this ancient beach deposit a group of strata
rests which we must now interpret. They consist of alternating shales
and sandstones, with here and there a bed of limestone and an
occasional seam of coal. A stratum of fire clay commonly underlies a
coal seam, and there occur also beds of iron ore. We give a typical
section of a very small portion of the series at a locality in
Pennsylvania. Although some of the minor changes are omitted, the
section shows the rapid alternation of the strata:

  9 Sandstone and shale  .....  25
  8 Limestone            .....  18
  7 Sandstone            .....  10
  6 Coal                 .....  1-6
  5 Shale                .....  0-2
  4 Sandstone            .....  40
  3 Limestone            .....  10
  2 Coal                 .....  5-12
  1 Fire clay            .....   3

This section shows more coal than is usual; on the whole, coal seams
do not take up more than one foot in fifty of the Coal Measures. They
vary also in thickness more than is seen in the section, some
exceptional seams reaching the thickness of fifty feet.

=How coal was made.= 1. Coal is of vegetable origin. Examined under
the microscope even anthracite, or hard coal, is seen to contain
carbonized vegetal tissues. There are also all gradations connecting
the hardest anthracite--through semibituminous coal, bituminous or
soft coal, lignite (an imperfect coal in which sometimes woody fibers
may be seen little changed)--with peat and decaying vegetable tissues.
Coal is compressed and mineralized vegetal matter. Its varieties
depend on the perfection to which the peculiar change called
bituminization has been carried, and also, as shown in the table
below, on the degree to which the volatile substances and water have
escaped, and on the per cent of carbon remaining.

                   Peat           Bituminous
                  Dismal  Lignite   Coal     Anthracite
                  Swamp    Texas    Penn.      Penn.

  Moisture        78.89    14.67     1.30       2.74
  Volatile matter 13.84    37.32    20.87       4.25
  Fixed carbon     6.49    41.07    67.20      81.51
  Ash              0.78     6.69     8.80      10.87

2. The vegetable remains associated with coal are those of land
plants.

3. Coal accumulated in the presence of water; for it is only when thus
protected from the air that vegetal matter is preserved.

4. The vegetation of coal accumulated for the most part where it grew;
it was not generally drifted and deposited by waves and currents.
Commonly the fire clay beneath the seam is penetrated with roots, and
the shale above is packed with leaves of ferns and other plants as
beautifully pressed as in a herbarium. There often is associated with
the seam a fossil forest, with the stumps, which are still standing
where they grew, their spreading roots, and the soil beneath, all
changed to stone. In the Nova Scotia field, out of seventy-six
distinct coal seams, twenty are underlain by old forest grounds.

The presence of fire clay beneath a seam points in the same direction.
Such underclays withstand intense heat and are used in making fire
brick, because their alkalies have been removed by the long-continued
growth of vegetation.

Fuel coal is also too pure to have been accumulated by driftage. In
that case we should expect to find it mixed with mud, while in fact it
often contains no more ash than the vegetal matter would furnish from
which it has been compressed.

   [Illustration: Fig. 303. Fossil Tree Stumps of a Carboniferous
      Forest, Scotland]

These conditions are fairly met in the great swamps of river plains
and deltas and of coastal plains, such as the great Dismal Swamp,
where thousands of generations of forests with their undergrowths
contribute their stems and leaves to form thick beds of peat. A coal
seam is a fossil peat bed.

=Geographical conditions during the Pennsylvanian.= The Carboniferous
peat swamps were of vast extent. A map of the Coal Measures (Fig. 260)
shows that the coal marshes stretched, with various interruptions of
higher ground and straits of open water, from eastern Pennsylvania
into Alabama, Texas, and Kansas. Some individual coal beds may still
be traced over a thousand square miles, despite the erosion which they
have suffered. It taxes the imagination to conceive that the varied
region included within these limits was for hundreds of thousands of
years a marshy plain covered with tropical jungles such as that
pictured in Figure 304.

On the basis that peat loses four fifths of its bulk in changing to
coal, we may reckon the thickness of these ancient peat beds. Coal
seams six and ten feet thick, which are not uncommon, represent peat
beds thirty and fifty feet in thickness, while mammoth coal seams
fifty feet thick have been compressed from peat beds two hundred and
fifty feet deep.

At the same time, the thousands of feet of marine and fresh-water
sediments, with their repeated alternations of limestones, sandstones,
and shales, in which the seams of coal occur, prove a slow subsidence,
with many changes in its rate, with halts when the land was at a
stillstand, and with occasional movements upward.

When subsidence was most rapid and long continued the sea encroached
far and wide upon the lowlands and covered the coal swamps with sands
and muds and limy oozes. When subsidence slackened or ceased the land
gained on the sea. Bays were barred, and lagoons as they gradually
filled with mud became marshes. River deltas pushed forward, burying
with their silts the sunken peat beds of earlier centuries, and at the
surface emerged in broad, swampy flats,--like those of the deltas of
the Mississippi and the Ganges,--which soon were covered with
luxuriant forests. At times a gentle uplift brought to sea level great
coastal plains, which for ages remained mantled with the jungle, their
undeveloped drainage clogged with its debris, and were then again
submerged.

   [Illustration: Fig. 304. Ideal Landscape of the Pennsylvanian
      Epoch]

=Physical geography of the several regions.= _The Acadian region_ lay
on the eastern side of the northern land, where now are New Brunswick
and Nova Scotia, and was an immense river delta. Here river deposits
rich in coal accumulated to a depth of sixteen thousand feet. The area
of this coal field is estimated at about thirty-six thousand square
miles.

_The Appalachian region_ skirts the Appalachian oldland on the west
from the southern boundary of New York to northern Alabama, extending
west into eastern Ohio. The Cincinnati anticline was now a peninsula,
and the broad gulf which had lain between it and Appalachia was
transformed at the beginning of the Pennsylvanian into wide marshy
plains, now sinking beneath the sea and now emerging from it. This
area subsided during the Carboniferous period to a depth of nearly ten
thousand feet.

_The Central region_ lay west of the peninsula of the Cincinnati
anticline, and extended from Indiana west into eastern Nebraska, and
from central Iowa and Illinois southward about the ancient island
in Missouri and Arkansas into Oklahoma and Texas. On the north
the subsidence in this area was comparatively slight, for the
Carboniferous strata scarcely exceed two thousand feet in thickness.
But in Arkansas and Indian Territory the downward movement amounted to
four and five miles, as is proved by shoal water deposits of that
immense thickness.

The coal fields of Indiana, and Illinois are now separated by erosion
from those lying west of the Mississippi River. At the south the
Appalachian land seems still to have stretched away to the west across
Louisiana and Mississippi into Texas, and this westward extension
formed the southern boundary of the coal marshes of the continent.

The three regions just mentioned include the chief Carboniferous coal
fields of North America. Including a field in central Michigan
evidently formed in an inclosed basin (Fig. 260), and one in Rhode
Island, the total area of American coal fields has been reckoned at
not less than two hundred thousand square miles. We can hardly
estimate the value of these great stores of fossil fuel to an
industrial civilization. The forests of the coal swamps accumulated in
their woody tissues the energy which they received from the sun in
light and heat, and it is this solar energy long stored in coal seams
which now forms the world's chief source of power in manufacturing.

=The western area.= On the Great Plains beyond the Missouri River the
Carboniferous strata pass under those of more recent systems. Where
they reappear, as about dissected mountain axes or on stripped
plateaus, they consist wholly of marine deposits and are devoid of
coal. The rich coal fields of the West are of later date.

On the whole the Carboniferous seems to have been a time of subsidence
in the West. Throughout the period a sea covered the Great Basin and
the plateaus of the Colorado River. At the time of the greatest
depression the sites of the central chains of the Rockies were
probably islands, but early in the period they may have been connected
with the broad lands to the south and east. Thousands of feet of
Carboniferous sediments were deposited where the Sierra Nevada
Mountains now stand.

=The Permian.= As the Carboniferous period drew toward its close the
sea gradually withdrew from the eastern part of the continent. Where
the sea lingered in the deepest troughs, and where inclosed basins
were cut off from it, the strata of the Permian were deposited. Such
are found in New Brunswick, in Pennsylvania and West Virginia, in
Texas, and in Kansas. In southwestern Kansas extensive Permian beds of
rock salt and gypsum show that here lay great salt lakes in which
these minerals were precipitated as their brines grew dense and dried
away.

In the southern hemisphere the Permian deposits are so extraordinary
that they deserve a brief notice, although we have so far omitted
mention of the great events which characterized the evolution of other
continents than our own. The Permian fauna-flora of Australia, India,
South Africa, and the southern part of South America are so similar
that the inference is a reasonable one that these widely separated
regions were then connected together, probably as extensions of a
great antarctic continent.

Interbedded with the Permian strata of the first three countries named
are extensive and thick deposits of a peculiar nature which are
clearly ancient ground moraines. Clays and sand, now hardened to firm
rock, are inset with unsorted stones of all sizes, which often are
faceted and scratched. Moreover, these bowlder clays rest on rock
pavements which are polished and scored with glacial markings. Hence
toward the close of the Paleozoic the southern lands of the eastern
hemisphere were invaded by great glaciers or perhaps by ice sheets
like that which now shrouds Greenland.

These Permian ground moraines are not the first traces of the work of
glaciers met with in the geological record. Similar deposits prove
glaciation in Norway succeeding the pre-Cambrian stage of elevation,
and Cambrian glacial drift has recently been found in China.

=The Appalachian deformation.= We have seen that during Paleozoic
times a long, narrow trough of the sea lay off the western coast of
the ancient land of Appalachia, where now are the Appalachian
Mountains. During the long ages of this era the trough gradually
subsided, although with many stillstands and with occasional slight
oscillations upward. Meanwhile the land lying to the east was
gradually uplifted at varying rates and with long pauses. The waste of
the rising land was constantly transferred to the sinking marginal sea
bottom, and on the whole the trough was filled with sediments as
rapidly as it subsided. The sea was thus kept shallow, and at times,
especially toward the close of the era, much of the area was upbuilt
or raised to low, marshy, coastal plains. When the Carboniferous was
ended the waste which had been removed from the land and laid along
its margin in the successive formations of the Paleozoic had reached a
thickness of between thirty and forty thousand feet.

Both by sedimentation and by subsidence the trough had now become a
belt of weakness in the crust of the earth. Here the crust was now
made of layers to the depth of six or seven miles. In comparison with
the massive crystalline rocks of Appalachia on the east, the layered
rock of the trough was weak to resist lateral pressure, as a ream of
sheets of paper is weak when compared with a solid board of the same
thickness. It was weaker also than the region to the west, since there
the sediments were much thinner. Besides, by the long-continued
depression the strata of the trough had been bent from the flat-lying
attitude in which they were laid to one in which they were less able
to resist a horizontal thrust.

There now occurred one of the critical stages in the history of the
planet, when the crust crumples under its own weight and shrinks down
upon a nucleus which is diminishing in volume and no longer able to
support it. Under slow but resistless pressure the strata of the
Appalachian trough were thrust against the rigid land, and slowly,
steadily bent into long folds whose axes ran northeast-southwest
parallel to the ancient coast line. It was on the eastern side next
the buttress of the land that the deformation was the greatest, and
the folds most steep and close. In central Pennsylvania and West
Virginia the folds were for the most part open. South of these states
the folds were more closely appressed, the strata were much broken,
and the great thrust faults were formed which have been described
already. In eastern Pennsylvania seams of bituminous coal were altered
to anthracite, while outside the region of strong deformation, as in
western Pennsylvania, they remained unchanged. An important factor in
the deformation was the massive limestones of the Cambrian-Ordovician.
Because of these thick, resistant beds the rocks were bent into wide
folds and sheared in places with great thrust faults. Had the strata
been weak shales, an equal pressure would have crushed and mashed
them.

Although the great earth folds were slowly raised, and no doubt eroded
in their rising, they formed in all probability a range of lofty
mountains, with a width of from fifty to a hundred and twenty-five
miles, which stretched from New York to central Alabama.

From their bases lowlands extended westward to beyond the Missouri
River. At the same time ranges were upridged out of thick Paleozoic
sediments both in the Bay of Fundy region and in the Indian Territory.
The eastern portion of the North American continent was now well-nigh
complete.

The date of the Appalachian deformation is told in the usual way. The
Carboniferous strata, nearly two miles thick, are all infolded in the
Appalachian ridges, while the next deposits found in this
region--those of the later portion of the first period (the Trias) of
the succeeding era--rest unconformably on the worn edges of the
Appalachian folded strata. The deformation therefore took place about
the close of the Paleozoic. It seems to have begun in the Permian, in,
eastern Pennsylvania,--for here the Permian strata are wanting,--and
to have continued into the Trias, whose earlier formations are absent
over all the area.

With this wide uplift the subsidence of the sea floor which had so
long been general in eastern North America came to an end. Deposition
now gave place to erosion. The sedimentary record of the Paleozoic was
closed, and after an unknown lapse of time, here unrecorded, the
annals of the succeeding era were written under changed conditions.

In western North America the closing stages of the Paleozoic were
marked by important oscillations. The Great Basin, which had long been
a mediterranean sea, was converted into land over western Utah and
eastern Nevada, while the waves of the Pacific rolled across
California and western Nevada.

The absence of tuffs and lavas among the Carboniferous strata of North
America shows that here volcanic action was singularly wanting during
the entire period. Even the Appalachian deformation was not
accompanied by any volcanic outbursts.

   [Illustration: Fig. 305. Carboniferous Ferns]

   [Illustration: Fig. 306. Calamites]


Life of the Carboniferous

=Plants.= The gloomy forests and dense undergrowths of the
Carboniferous jungles would appear unfamiliar to us could we see them
as they grew, and even a botanist would find many of their forms
perplexing and hard to classify. None of our modern trees would meet
the eye. Plants with conspicuous flowers of fragrance and beauty were
yet to come. Even mosses and grasses were still absent.

Tree ferns lifted their crowns of feathery fronds high in air on
trunks of woody tissue; and lowly herbaceous ferns, some belonging to
existing families, carpeted the ground. Many of the fernlike forms,
however, have distinct affinities with the cycads, of which they may
be the ancestors, and some bear seeds and must be classed as
gymnosperms.

Dense thickets, like cane or bamboo brakes, were composed of thick
clumps of _Calamites_, whose slender, jointed stems shot up to a
height of forty feet, and at the joints bore slender branches set
with whorls of leaves. These were close allies of the Equiseta or
"horsetails," of the present; but they bore characteristics of higher
classes in the woody structures of their stems.

There were also vast monotonous forests, composed chiefly of trees
belonging to the lycopods, and whose nearest relatives to-day are the
little club mosses of our eastern woods. Two families of lycopods
deserve special mention,--the Lepidodendrons and the Sigillaria.

   [Illustration: Fig. 307. Lepidodendron]

   [Illustration: Fig. 308. Sigillaria]

The _Lepidodendron_, or "scale tree," was a gigantic club moss fifty
and seventy-five feet high, spreading toward the top into stout
branches, at whose ends were borne cone-shaped spore cases. The
younger parts of the tree were clothed with stiff needle-shaped
leaves, but elsewhere the trunk and branches were marked with
scalelike scars, left by the fallen leaves, and arranged in spiral
rows.

The _Sigillaria_, or "seal tree," was similar to the Lepidodendron,
but its fluted trunk divided into even fewer branches, and was dotted
with vertical rows of leaf scars, like the impressions of a seal.

Both Lepidodendron and Sigillaria were anchored by means of great
cablelike underground stems, which ran to long distances through the
marshy ground. The trunks of both trees had a thick woody rind,
inclosing loose cellular tissue and a pith. Their hollow stumps,
filled with sand and mud, are common in the Coal Measures, and in them
one sometimes finds leaves and stems, land shells, and the bones of
little reptiles of the time which made their home there.

It is important to note that some of these gigantic lycopods, which
are classed with the _cryptogams_, or flowerless plants, had pith and
medullary rays dividing their cylinders into woody wedges. These
characters connect them with the _phanerogams_, or flowering plants.
Like so many of the organisms of the remote past, they were connecting
types from which groups now widely separated have diverged.

Gymnosperms, akin to the cycads, were also present in the
Carboniferous forests. Such were the different species of _Cordaites_,
trees pyramidal in shape, with strap-shaped leaves and nutlike fruit.
Other gymnosperms were related to the yews, and it was by these that
many of the fossil nuts found in the Coal Measures were borne. It is
thought by some that the gymnosperms had their station on the drier
plains and higher lands.

The Carboniferous jungles extended over parts of Europe and of Asia,
as well as eastern North America, and reached from the equator to
within nine degrees of the north pole. Even in these widely separated
regions the genera and species of coal plants are close akin and often
identical.

=Invertebrates.= Among the echinoderms, crinoids are now exceedingly
abundant, sea urchins are more plentiful, and sea cucumbers are found
now for the first time. Trilobites are rapidly declining, and pass
away forever with the close of the period. Eurypterids are common;
stinging scorpions are abundant; and here occur the first-known
spiders.

We have seen that the arthropods were the first of all animals to
conquer the realm of the air, the earliest insects appearing in the
Ordovician. Insects had now become exceedingly abundant, and the
Carboniferous forests swarmed with the ancestral types of dragon
flies,--some with a spread of wing of more than two feet,--May flies,
crickets, and locusts. Cockroaches infested the swamps, and one
hundred and thirty-three species of this ancient order have been
discovered in the Carboniferous of North America. The higher
flower-loving insects are still absent; the reign of the flowering
plants has not yet begun. The Paleozoic insects were generalized types
connecting the present orders. Their fore wings were still membranous
and delicately veined, and used in flying; they had not yet become
thick, and useful only as wing covers, as in many of their
descendants.

   [Illustration: Fig. 309. Carboniferous Brachiopods

   _A_, Productus; _B_, Spirifer, the right-hand figure showing the
   interior with the calcareous spires for the support of the arms]

=Fishes= still held to the Devonian types, with the exception that the
strange ostracoderms now had perished.

=Amphibians.= The vertebrates had now followed the arthropods and the
mollusks upon the land, and developed a higher type adapted to the new
environment. Amphibians--the class to which frogs and salamanders
belong--now appear, with lungs for breathing air and with limbs for
locomotion on the land. Most of the Carboniferous amphibians were shaped
like the salamander, with weak limbs adapted more for crawling than for
carrying the body well above the ground. some legless, degenerate forms
were snakelike in shape.

   [Illustration: Fig. 310. A Carboniferous Dragon Fly

   One tenth natural size]

The earliest amphibians differ from those of to-day in a number of
respects. They were connecting types linking together fishes, from
which they were descended, with reptiles, of which they were the
ancestors. They retained the evidence of their close relationship with
the Devonian fishes in their cold blood, their gills and aquatic habit
during their larval stage, their teeth with dentine infolded like
those of the Devonian ganoids but still more intricately, and their
biconcave vertebræ which never completely ossified. These, the
highest vertebrates of the time, had not yet advanced beyond the
embryonic stage of the more or less cartilaginous skeleton and the
persistent notochord.

   [Illustration: Fig. 311. A Carboniferous Amphibian]

   [Illustration: Fig. 312. Transverse Section of
   Segment of Tooth of Carboniferous Amphibian]

On the other hand, the skull of the Carboniferous amphibians was made
of close-set bony plates, like the skull of the reptile, rather than
like that of the frog, with its open spaces (Figs. 313 and 314).
Unlike modern amphibians, with their slimy skin, the Carboniferous
amphibians wore an armor of bony scales over the ventral surface and
sometimes over the back as well.

   [Illustration: Fig. 313. Skull of a Permian Amphibian from Texas]

   [Illustration: Fig. 314. Skull of a Frog]

It is interesting to notice from the footprints and skeletons of these
earliest-known vertebrates of the land what was the primitive number
of digits. The Carboniferous amphibians had five-toed feet, the
primitive type of foot, from which their descendants of higher orders,
with a smaller number of digits, have diverged.

The Carboniferous was the age of lycopods and amphibians, as the
Devonian had been the age of rhizocarps and fishes.

=Life of the Permian.= The close of the Paleozoic was, as we have
seen, a time of marked physical changes. The upridging of the
Appalachians had begun and a wide continental uplift--proved by the
absence of Permian deposits over large areas where sedimentation had
gone on before--opened new lands for settlement to hordes of
air-breathing animals. Changes of climate compelled extensive
migrations, and the fauna of different regions were thus brought into
conflict. The Permian was a time of pronounced changes in plant and
animal life, and a transitional period between two great eras. The
somber forests of the earlier Carboniferous, with their gigantic club
mosses, were now replaced by forests of cycads, tree ferns, and
conifers. Even in the lower Permian the Lepidodendron and Sigillaria
were very rare, and before the end of the epoch they and the Calamites
also had become extinct. Gradually the antique types of the Paleozoic
fauna died out, and in the Permian rocks are found the last survivors
of the cystoid, the trilobite, and the eurypterid, and of many
long-lived families of brachiopods, mollusks, and other invertebrates.
The venerable Orthoceras and the Goniatite linger on through the epoch
and into the first period of the succeeding era. Forerunners of the
great ammonite family of cephalopod mollusks now appear. The antique
forms of the earlier Carboniferous amphibians continue, but with many
new genera and a marked increase in size.

A long forward step had now been taken in the evolution of the
vertebrates. A new and higher type, the reptiles, had appeared, and in
such numbers and variety are they found in the Permian strata that
their advent may well have occurred in a still earlier epoch. It will
be most convenient to describe the Permian reptiles along with their
descendants of the Mesozoic.




CHAPTER XX

THE MESOZOIC


With the close of the Permian the world of animal and vegetable
life had so changed that the line is drawn here which marks the
end of the old order and the beginning of the new and separates
the Paleozoic from the succeeding era,--the Mesozoic, the Middle
Age of geological history. Although the Mesozoic era is shorter
than the Paleozoic, as measured by the thickness of their strata,
yet its duration must be reckoned in millions of years. Its
predominant life features are the culmination and the beginning of
the decline of reptiles, amphibians, cephalopod mollusks, and
cycads, and the advent of marsupial mammals, birds, teleost
fishes, and angiospermous plants. The leading events of the long
ages of the era we can sketch only in the most summary way.

The Mesozoic comprises three systems,--the _Triassic_, named from
its threefold division in Germany; the _Jurassic_, which is well
displayed in the Jura Mountains; and the _Cretaceous_, which
contains the extensive chalk (Latin, _creta_) deposits of Europe.

In eastern North America the Mesozoic rocks are much less
important than the Paleozoic, for much of this portion of the
continent was land during the Mesozoic era, and the area of the
Mesozoic rocks is small. In western North America, on the other
hand, the strata of the Mesozoic--and of the Cenozoic also--are
widely spread. The Paleozoic rocks are buried quite generally from
view except where the mountain makings and continental uplifts of
the Mesozoic and Cenozoic have allowed profound erosion to bring
them to light, as in deep canyons and about mountain axes. The
record of many of the most important events in the development of
the continent during the Mesozoic and Cenozoic eras is found in
the rocks of our western states.


The Triassic and Jurassic

=Eastern North America.= The sedimentary record interrupted by the
Appalachian deformation was not renewed in eastern North America
until late in the Triassic. Hence during this long interval the
land stood high, the coast was farther out than now, and over our
Atlantic states geological time was recorded chiefly in erosion
forms of hill and plain which have long since vanished. The area
of the later Triassic rocks of this region, which take up again
the geological record, is seen in the map of Figure 260. They lie
on the upturned and eroded edges of the older rocks and occupy
long troughs running for the most part parallel to the Atlantic
coast. Evidently subsidence was in progress where these rocks were
deposited. The eastern border of Appalachia was now depressed. The
oldland was warping, and long belts of country lying parallel to
the shore subsided, forming troughs in which thousands of feet of
sediment now gathered.

These Triassic rocks, which are chiefly sandstones, hold no marine
fossils, and hence were not laid in open arms of the sea. But
their layers are often ripple-marked, and contain many tracks of
reptiles, imprints of raindrops, and some fossil wood, while an
occasional bed of shale is filled with the remains of fishes. We
may conceive, then, of the Connecticut valley and the larger
trough to the southwest as basins gradually sinking at a rate
perhaps no faster than that of the New Jersey coast to-day, and as
gradually aggraded by streams from the neighboring uplands. Their
broad, sandy flats were overflowed by wandering streams, and when
subsidence gained on deposition shallow lakes overspread the
alluvial plains. Perhaps now and then the basins became long,
brackish estuaries, whose low shores were swept by the incoming
tide and were in turn left bare at its retreat to receive the rain
prints of passing showers and the tracks of the troops of reptiles
which inhabited these valleys.

The Triassic rocks are mainly red sandstones,--often feldspathic,
or arkose, with some conglomerates and shales. Considering the
large amount of feldspathic material in these rocks, do you infer
that they were derived from the adjacent crystalline and
metamorphic rocks of the oldland of Appalachia, or from the
sedimentary Paleozoic rocks which had been folded into mountains
during the Appalachian deformation? If from the former, was the
drainage of the northern Appalachian mountain region then, as now,
eastward and southeastward toward the Atlantic? The Triassic
sandstones are voluminous, measuring at least a mile in thickness,
and are largely of coarse waste. What do you infer as to the
height of the lands from which the waste was shed, or the
direction of the oscillation which they were then undergoing? In
the southern basins, as about Richmond, Virginia, are valuable
beds of coal; what was the physical geography of these areas when
the coal was being formed?

   [Illustration: Fig. 315. Section of Triassic Sandstones of the
      Connecticut Valley

   _ss_, sandstones; _ll_, lava sheets; _cc_, crystalline igneous
   and metamorphic rocks]

Interbedded with the Triassic sandstones are contemporaneous lava
beds which were fed from dikes. Volcanic action, which had been
remarkably absent in eastern North America during Paleozoic times,
was well-marked in connection with the warping now in progress.
Thick intrusive sheets have also been driven in among the strata,
as, for example, the sheet of the Palisades of the Hudson,
described on page 269.

The present condition of the Triassic sandstones of the
Connecticut valley is seen in Figure 315. Were the beds laid in
their present attitude? What was the nature of the deformation
which they have suffered? When did the intrusion of lava sheets
take place relative to the deformation? What effect have these
sheets on the present topography, and why? Assuming that the
Triassic deformation went on more rapidly than denudation, what
was its effect on the topography of the time? Are there any of its
results remaining in the topography of to-day? Do the Triassic
areas now stand higher or lower than the surrounding country, and
why? How do the Triassic sandstones and shales compare in hardness
with the igneous and metamorphic rocks about them? The Jurassic
strata are wanting over the Triassic areas and over all of eastern
North America. Was this region land or sea, an area of erosion or
sedimentation, during the Jurassic period? In New Jersey,
Pennsylvania, and farther southwest the lowest strata of the next
period, the Cretaceous, rest on the eroded edges of the earlier
rocks. The surface on which they lie is worn so even that we must
believe that at the opening of the Cretaceous the oldland of
Appalachia, including the Triassic areas, had been baseleveled at
least near the coast. When, therefore, did the deformation of the
Triassic rocks occur?

=Western North America.= Triassic strata infolded in the Sierra
Nevada Mountains carry marine fossils and reach a thickness of
nearly five thousand feet. California was then under water, and
the site of the Sierra was a subsiding trough slowly filling with
waste from the Great Basin land to the east.

Over a long belt which reaches from Wyoming across Colorado into
New Mexico no Triassic sediments are found, nor is there any
evidence that they were ever present; hence this area was high
land suffering erosion during the Triassic. On each side of it, in
eastern Colorado and about the Black Hills, in western Texas, in
Utah, over the site of the Wasatch Mountains, and southward into
Arizona over the plateaus trenched by the Colorado River, are
large areas of Triassic rocks, sandstones chiefly, with some rock
salt and gypsum. Fossils are very rare and none of them marine.
Here, then, lay broad shallow lakes often salt, and warped basins,
in which the waste of the adjacent uplands gathered. To this
system belong the sandstones of the Garden of the Gods in
Colorado, which later earth movements have upturned with the
uplifted mountain flanks.

The Jurassic was marked with varied oscillations and wide changes
in the outline of sea and land.

Jurassic shales of immense thickness--now metamorphosed into
slates--are found infolded into the Sierra Nevada Mountains. Hence
during Jurassic times the Sierra trough continued to subside, and
enormous deposits of mud were washed into it from the land lying
to the east. Contemporaneous lava flows interbedded with the
strata show that volcanic action accompanied the downwarp, and
that molten rock was driven upward through fissures in the crust
and outspread over the sea floor in sheets of lava.

=The Sierra deformation.= Ever since the middle of the Silurian, the
Sierra trough had been sinking, though no doubt with halts and
interruptions, until it contained nearly twenty-five thousand feet
of sediment. At the close of the Jurassic it yielded to lateral
pressure and the vast pile of strata was crumpled and upheaved
into towering mountains. The Mesozoic muds were hardened and
squeezed into slates. The rocks were wrenched and broken, and
underground waters began the work of filling their fissures with
gold-bearing quartz, which was yet to wait millions of years
before the arrival of man to mine it. Immense bodies of molten
rock were intruded into the crust as it suffered deformation, and
these appear in the large areas of granite which the later
denudation of the range has brought to light.

The same movements probably uplifted the rocks of the Coast Range
in a chain of islands. The whole western part of the continent was
raised and its seas and lakes were for the most part drained away.

=The British Isles.= The Triassic strata of the British Isles are
continental, and include breccia beds of cemented talus, deposits
of salt and gypsum, and sandstones whose rounded and polished
grains are those of the wind-blown sands of deserts. In Triassic
times the British Isles were part of a desert extending over much
of northwestern Europe.


The Cretaceous

The third great system of the Mesozoic includes many formations,
marine and continental, which record a long and complicated
history marked by great oscillations of the crust and wide changes
in the outlines of sea and land.

=Early Cretaceous.= In eastern North America the lowest Cretaceous
series comprises fresh-water formations which are traced from
Nantucket across Martha's Vineyard and Long Island, and through
New Jersey southward into Georgia. They rest unconformably on the
Triassic sandstones and the older rocks of the region. The
Atlantic shore line was still farther out than now in the northern
states. Again, as during the Triassic, a warping of the crust
formed a long trough parallel to the coast and to the Appalachian
ridges, but cut off from the sea; and here the continental
deposits of the early Cretaceous were laid.

Along the Gulf of Mexico the same series was deposited under like
conditions over the area known as the Mississippi embayment,
reaching from Georgia northwestward into Tennessee and thence
across into Arkansas and southward into Texas.

In the Southwest the subsidence continued until the transgressing
sea covered most of Mexico and Texas and extended a gulf northward
into Kansas. In its warm and quiet waters limestones accumulated
to a depth of from one thousand to five thousand feet in Texas,
and of more than ten thousand feet in Mexico. Meanwhile the
lowlands, where the Great Plains are now, received continental
deposits; coal swamps stretched from western Montana into British
Columbia.

=The Middle Cretaceous.= This was a land epoch. The early Cretaceous
sea retired from Texas and Mexico, for its sediments are overlain
unconformably by formations of the Upper Cretaceous. So long was the
time gap between the two series that no species found in the one
occurs in the other.

=The Upper Cretaceous.= There now began one of the most remarkable
events in all geological history,--the great Cretaceous subsidence.
Its earlier warpings were recorded in continental deposits,--wide
sheets of sandstone, shale, and some coal,--which were spread from
Texas to British Columbia. These continental deposits are overlain by
a succession of marine formations whose vast area is shown on the map,
Figure 260. We may infer that as the depression of the continent
continued the sea came in far and wide over the coast lands and the
plains worn low during the previous epochs. Upper Cretaceous
formations show that south of New England the waters of the Atlantic
somewhat overlapped the crystalline rocks of the Piedmont Belt and
spread their waste over the submerged coastal plain. The Gulf of
Mexico again covered the Mississippi embayment, reaching as far north
as southern Illinois, and extended over Texas. A mediterranean sea now
stretched from the Gulf to the arctic regions and from central Iowa to
the eastern shore of the Great Basin land at about the longitude of
Salt Lake City, the Colorado Mountains rising from it in a chain of
islands. Along with minor oscillations there were laid in the interior
sea various formations of sandstones, shales, and limestones, and from
Kansas to South Dakota beds of white chalk show that the clear, warm
waters swarmed at times with foraminiferal life whose disintegrating
microscopic shells accumulated in this rare deposit.

   [Illustration: Fig. 316. Hypothetical Map of Upper Cretaceous
      Epicontinental Seas

   Shaded areas, probable seas; broken lines, approximate shore
   lines]

   [Illustration: Fig. 317. Foraminifera from Cretaceous Chalk,
      Iowa]

At this epoch a wide sea, interrupted by various islands, stretched
across Eurasia from Wales and western Spain to China, and spread
southward over much of the Sahara. To the west its waters were clear
and on its floor the crumbled remains of foraminifers gathered in
heavy accumulations of calcareous ooze,--the white chalk of France and
England. Sea urchins were also abundant, and sponges contributed their
spicules to form nodules of flint.

=The Laramie.= The closing stage of the Cretaceous was marked in
North America by a slow uplift of the land. As the interior sea
gradually withdrew, the warping basins of its floor were filled with
waste from the rising lands about them, and over this wide area
there were spread continental deposits in fresh-water lakes like the
Great Lakes of the present, in brackish estuaries, and in river
plains, while occasional oscillations now and again let in the sea.
There were vast marshes in which there accumulated the larger part
of the valuable coal seams of the West. The Laramie is the
coal-bearing series of the West, as the Pennsylvanian is of the
eastern part of our country.

=The Rocky Mountain deformation.= At the close of the Cretaceous we
enter upon an epoch of mountain-making far more extensive than any which
the continent had witnessed. The long belt lying west of the ancient
axes of the Colorado Islands and east of the Great Basin land had been
an area of deposition for many ages, and in its subsiding troughs
Paleozoic and Mesozoic sediments had gathered to the depth of many
thousand feet. And now from Mexico well-nigh to the Arctic Ocean this
belt yielded to lateral pressure. The Cretaceous limestones of Mexico
were folded into lofty mountains. A massive range was upfolded where the
Wasatch Mountains now are, and various ranges of the Rockies in Colorado
and other states were upridged. However slowly these deformations were
effected they were no doubt accompanied by world-shaking earthquakes,
and it is known that volcanic eruptions took place on a magnificent
scale. Outflows of lava occurred along the Wasatch, the laccoliths of
the Henry Mountains (p. 271) were formed, while the great masses of
igneous rock which constitute the cores of the Spanish Peaks (p. 271)
and other western mountains were thrust up amid the strata. The high
plateaus from which many of these ranges rise had not yet been uplifted,
and the bases of the mountains probably stood near the level of the sea.

North America was now well-nigh completed. The mediterranean seas
which so often had occupied the heart of the land were done away
with, and the continent stretched unbroken from the foot of the
Sierras on the west to the Fall Line of the Atlantic coastal plain
on the east.

=The Mesozoic peneplain.= The immense thickness of the Mesozoic
formations conveys to our minds some idea of the vast length of time
involved in the slow progress of its successive ages. The same
lesson is taught as plainly by the amount of denudation which the
lands suffered during the era.

The beginning of the Mesozoic saw a system of lofty mountain ranges
stretching from New York into central Alabama. The end of this long
era found here a wide peneplain crossed by sluggish wandering rivers
and overlooked by detached hills as yet unreduced to the general
level. The Mesozoic era was long enough for the Appalachian
Mountains, upridged at its beginning, to have been weathered and
worn away and carried grain by grain to the sea. The same plain
extended over southern New England. The Taconic range, uplifted
partially at least at the close of the Ordovician, and the block
mountains of the Triassic, together with the pre-Cambrian mountains
of ancient Appalachia, had now all been worn to a common level with
the Allegheny ranges. The Mesozoic peneplain has been upwarped by
later crustal movements and has suffered profound erosion, but the
remnants of it which remain on the upland of southern New England
and the even summits of the Allegheny ridges suffice to prove that
it once existed. The age of the Mesozoic peneplain is determined
from the fact that the lower Tertiary sediments were deposited on
its even surface when at the close of the era the peneplain was
depressed along its edges beneath the sea.


Life of the Mesozoic

=Plant life of the Triassic and Jurassic.= The Carboniferous forests
of lepidodendrons and sigillarids had now vanished from the earth.
The uplands were clothed with conifers, like the Araucarian pines
of South America and Australia. Dense forests of tree ferns throve
in moist regions, and canebrakes of horsetails of modern type, but
with stems reaching four inches in thickness, bordered the lagoons
and marshes. Cycads were exceedingly abundant. These gymnosperms,
related to the pines and spruces in structure and fruiting, but
palmlike in their foliage, and uncoiling their long leaves after
the manner of ferns, culminated in the Jurassic. From the view
point of the botanist the Mesozoic is the Age of Cycads, and after
this era they gradually decline to the small number of species now
existing in tropical latitudes.

   [Illustration: Fig. 318. A Living Cycad of Australia]

   [Illustration: Fig. 319. Stem of a Mesozoic Cycad]

=Plant life of the Cretaceous.= In the Lower Cretaceous the woodlands
continued of much the same type as during the Jurassic. The
forerunners now appeared of the modern dicotyls (plants with two seed
leaves), and in the Middle Cretaceous the monocotyledonous group of
palms came in. Palms are so like cycads that we may regard them as the
descendants of some cycad type.

In the _Upper Cretaceous_, cycads become rare. The highest types of
flowering plants gain a complete ascendency, and forests of modern
aspect cover the continent from the Gulf of Mexico to the Arctic
Ocean. Among the kinds of forest trees whose remains are found in
the continental deposits of the Cretaceous are the magnolia, the
myrtle, the laurel, the fig, the tulip tree, the chestnut, the
oak, beech, elm, poplar, willow, birch, and maple. Forests of
Eucalyptus grew along the coast of New England, and palms on the
Pacific shores of British Columbia. Sequoias of many varieties
ranged far into northern Canada. In northern Greenland there were
luxuriant forests of magnolias, figs, and cycads; and a similar
flora has been disinterred from the Cretaceous rocks of Alaska and
Spitzbergen. Evidently the lands within the Arctic Circle enjoyed
a warm and genial climate, as they had done during the Paleozoic.
Greenland had the temperature of Cuba and southern Florida, and
the time was yet far distant when it was to be wrapped in glacier
ice.

   [Illustration: Fig. 320. A Jurassic Long-Tailed Crustacean]

=Invertebrates.= During the long succession of the ages of the
Mesozoic, with their vast geographical changes, there were many
and great changes in organisms. Species were replaced again and
again by others better fitted to the changing environment. During
the Lower Cretaceous alone there were no less than six successive
changes in the faunas which inhabited the limestone-making sea
which then covered Texas. We shall disregard these changes for the
most part in describing the life of the era, and shall confine our
view to some of the most important advances made in the leading
types.

Stromatopora have disappeared. Protozoans and sponges are
exceedingly abundant, and all contribute to the making of Mesozoic
strata. Corals have assumed a more modern type. Sea urchins have
become plentiful; crinoids abound until the Cretaceous, where they
begin their decline to their present humble station.

   [Illustration: Fig. 321. A Fossil Crab]

   [Illustration: Fig. 322. Cretaceous Mollusks

   _A_, Ostrea (oyster); _B_, Exogyra; _C_. Gryphæa]

Trilobites and eurypterids are gone. Ten-footed crustaceans abound of
the primitive long-tailed type (represented by the lobster and the
crayfish), and in the Jurassic there appears the modern short-tailed
type represented by the crabs. The latter type is higher in
organization and now far more common. In its embryological development
it passes through the long-tailed stage; connecting links in the
Mesozoic also indicate that the younger type is the offshoot of the
older.

Insects evolve along diverse lines, giving rise to beetles, ants,
bees, and flies.

Brachiopods have dwindled greatly in the number of their species,
while mollusks have correspondingly increased. The great oyster family
dates from here.

Cephalopods are now to have their day. The archaic Orthoceras lingers
on into the Triassic and becomes extinct, but a remarkable development
is now at hand for the more highly organized descendants of this
ancient line. We have noticed that in the Devonian the sutures of some
of the chambered shells become angled, evolving the Goniatite type.
The sutures now become lobed and _corrugated_ in _Ceratites_. The process
was carried still farther, and the sutures were elaborately frilled in
the great order of the Ammonites. It was in the Jurassic that the
Ammonites reached their height. No fossils are more abundant or
characteristic of their age. Great banks of their shells formed beds
of limestone in warm seas the world over.

   [Illustration: Fig. 323. Ceratites]

   [Illustration: Fig. 324. An Ammonite

   A portion of the shell is removed to show frilling of suture]

   [Illustration: Fig. 325. Slab of Rock covered with
      Ammonites,--a Bit of a Mesozoic Sea Bottom]

   [Illustration: Fig. 326. Representative Species of Different
      Families of Ammonites]

The ammonite stem branched into a most luxuriant variety of forms.
The typical form was closely coiled like a nautilus. In others the
coil was more or less open, or even erected into a spiral. Some
were hook-shaped, and there were members of the order in which the
shell was straight, and yet retained all the internal structures
of its kind. At the end of the Mesozoic the entire tribe of
ammonites became extinct.

The Belemnite (Greek, _belemnon_, a dart) is a distinctly higher
type of cephalopod which appeared in the Triassic, became numerous
and varied in the Jurassic and Cretaceous, and died out early in
the Tertiary. Like the squids and cuttlefish, of which it was the
prototype, it had an internal calcareous shell. This consisted of
a chambered and siphuncled cone, whose point was sheathed in a
long solid guard somewhat like a dart. The animal carried an ink
sac, and no doubt used it as that of the modern cuttlefish is
used,--to darken the water and make easy an escape from foes.
Belemnites have sometimes been sketched with fossil sepia, or
india ink, from their own ink sacs. In the belemnites and their
descendants, the squids and cuttlefish, the cephalopods made the
radical change from external to the internal shell. They abandoned
the defensive system of warfare and boldly took up the offensive.
No doubt, like their descendants, the belemnites were exceedingly
active and voracious creatures.

   [Illustration: Fig. 327. Internal Shell of Belemnite]

=Fishes and amphibians.= In the Triassic and Jurassic, little
progress was made among the fishes, and the ganoid was still the
leading type. In the Cretaceous the teleosts, or bony fishes, made
their appearance, while ganoids declined toward their present
subordinate place.

The amphibians culminated in the Triassic, some being formidable
creatures as large as alligators. They were still of the primitive
Paleozoic types. Their pygmy descendants of more modern types are
not found until later, salamanders appearing first in the
Cretaceous, and frogs at the beginning of the Cenozoic.

No remains of amphibians have been discovered in the Jurassic. Do
you infer from this that there were none in existence at that
time?


Reptiles of the Mesozoic

The great order of Reptiles made its advent in the Permian, culminated
in the Triassic and Jurassic, and began to decline in the Cretaceous.
The advance from the amphibian to the reptile was a long forward step
in the evolution of the vertebrates. In the reptile the vertebrate
skeleton now became completely ossified. Gills were abandoned and
breathing was by lungs alone. The development of the individual from
the egg to maturity was uninterrupted by any metamorphosis, such as
that of the frog when it passes from the tadpole stage. Yet in
advancing from the amphibian to the reptile the evolution of the
vertebrate 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.

The reptiles of the Permian, the earliest known, were much like
lizards in form of body. Constituting a transition type between the
amphibians on the one hand, and both the higher reptiles and the
mammals on the other, they retained the archaic biconcave vertebra of
the fish and in some cases the persistent notochord, while some of
them, the theromorphs, possessed characters allying them with mammals.
In these the skull was remarkably similar to that of the carnivores,
or flesh-eating mammals, and the teeth, unlike the teeth of any later
reptiles, were divisible into incisors, canines, and molars, as are
the teeth of mammals (Fig. 328).

   [Illustration: Fig. 328. Skull of a Permian Theromorph]

At the opening of the Mesozoic era reptiles were the most highly
organized and powerful of any animals on the earth. New ranges of
continental extent were opened to them, food was abundant, the climate
was congenial, and they now branched into very many diverse types
which occupied and ruled all fields,--the land, the air, and the sea.
The Mesozoic was the Age of Reptiles.

=The ancestry of surviving reptilian types.= We will consider first
the evolution of the few reptilian types which have survived to the
present.

Crocodiles, the highest of existing reptiles, are a very ancient
order, dating back to the lower Jurassic, and traceable to earlier
ancestral, generalized forms, from which sprang several other orders
also.

Turtles and tortoises are not found until the early Jurassic, when
they already possessed the peculiar characteristics which set them off
so sharply from other reptiles. They seem to have lived at first in
shallow water and in swamps, and it is not until after the end of the
Mesozoic that some of the order became adapted to life on the land.

The largest of all known turtles, _Archelon_, whose home was the great
interior Cretaceous sea, was fully a dozen feet in length and must
have weighed at least two tons. The skull alone is a yard long.

Lizards and snakes do not appear until after the close of the
Mesozoic, although their ancestral lines may be followed back into the
Cretaceous.

We will now describe some of the highly specialized orders peculiar to
the Mesozoic.

=Land reptiles.= The _dinosaurs_ (terrible reptiles) are an extremely
varied order which were masters of the land from the late Trias until
the close of the Mesozoic era. Some were far larger than elephants,
some were as small as cats; some walked on all fours, some were
bipedal; some fed on the luxuriant tropical foliage, and others on the
flesh of weaker reptiles. They may be classed in three divisions,--the
_flesh-eating dinosaurs_, the _reptile-footed dinosaurs_, and the
_beaked dinosaurs_,--the latter two divisions being herbivorous.

The _flesh-eating dinosaurs_ are the oldest known division of the
order, and their characteristics are shown in Figure 329. As a class,
reptiles are egg layers (_oviparous_); but some of the flesh-eating
dinosaurs are known to have been _viviparous_, i.e. to have brought
forth their young alive. This group was the longest-lived of any of
the three, beginning in the Trias and continuing to the close of the
Mesozoic era.

   [Illustration: Fig. 329. Ceratosaurus]

Contrast the small fore limbs, used only for grasping, with the
powerful hind limbs on which the animal stalked about. Some of the
species of this group seem to have been able to progress by
leaping in kangaroo fashion. Notice the sharp claws, the ponderous
tail, and the skull set at right angles with the spinal column.
The limb bones are hollow. The ceratosaurs reached a length of
some fifteen feet, and were not uncommon in Colorado and the
western lands in Jurassic times.

   [Illustration: Fig. 330. Diplodocus]

The _reptile-footed dinosaurs_ (Sauropoda) include some of the
biggest brutes which ever trod the ground. One of the largest,
whose remains are found entombed in the Jurassic rocks of Wyoming
and Colorado, is shown in Figure 330.

Note the five digits on the hind feet, the quadrupedal gait, the
enormous stretch of neck and tail, the small head aligned with the
vertebral column. Diplodocus was fully sixty-five feet long and
must have weighed about twenty tons. The thigh bones of the
Sauropoda are the largest bones which ever grew. That of a genus
allied to the Diplodocus measures six feet and eight inches, and
the total length of the animal must have been not far from eighty
feet, the largest land animal known.

The Sauropoda became extinct when their haunts along the rivers
and lakes of the western plains of Jurassic times were invaded by
the Cretaceous interior sea.

The _beaked dinosaurs_ (Predentata) were distinguished by a beak
sheathed with horn carried in front of the tooth-set jaw, and
used, we may imagine, in stripping the leaves and twigs of trees
and shrubs. We may notice only two of the most interesting types.

   [Illustration: Fig. 331. Stegosaurus]

_Stegosaurus_ (plated reptile) takes its name from the double row of
bony plates arranged along its back. The powerful tail was armed
with long spines, and the thick skin was defended with irregular
bits of bone implanted in it. The brain of the stegosaur was
smaller than that of any land vertebrate, while in the sacrum the
nerve canal was enlarged to ten times the capacity of the brain
cavity of the skull. Despite their feeble wits, this well-armored
family lived on through millions of years which intervened between
their appearance, at the opening of the Jurassic, and the close of
the Cretaceous, when they became extinct.

A less stupid brute than the stegosaur was _Triceratops_, the
dinosaur of the three horns,--one horn carried on the nose, and a
massive pair set over the eyes (Fig. 332). Note the enormous wedge-shaped
skull, with its sharp beak, and the hood behind resembling a
fireman's helmet. Triceratops was fully twenty-five feet long, and
of twice the bulk of an elephant. The family appeared in the Upper
Cretaceous and became extinct at its close. Their bones are found
buried in the fresh-water deposits of the time from Colorado to
Montana and eastward to the Dakotas.

   [Illustration: Fig. 332. Restoration of Triceratops

   By courtesy of the American Museum of Natural History]

=Marine reptiles.= In the ocean, reptiles occupied the place now
held by the aquatic mammals, such as whales and dolphins, and
their form and structure were similarly modified to suit their
environment. In the Ichthyosaurus (fish reptile), for example, the
body was fishlike in form, with short neck and large, pointed head
(Fig. 333).

   [Illustration: Fig. 333. Ichthyosaurus]

A powerful tail, whose flukes were set vertical, and the lower one
of which was vertebrated, served as propeller, while a large
dorsal fin was developed as a cutwater. The primitive biconcave
vertebræ of the fish and of the early land vertebrates were
retained, and the limbs degenerated into short paddles. The skin
of the ichthyosaur was smooth like that of a whale, and its food
was largely fish and cephalopods, as the fossil contents of its
stomach prove.

These sea monsters disported along the Pacific shore over northern
California in Triassic times, and the bones of immense members of
the family occur in the Jurassic strata of Wyoming. Like whales
and seals, the ichthyosaurs were descended from land vertebrates
which had become adapted to a marine habitat.

   [Illustration: Fig. 334. Plesiosaurus]

_Plesiosaurs_ were another order which ranged throughout the
Mesozoic. Descended from small amphibious animals, they later
included great marine reptiles, characterized in the typical genus
by long neck, snakelike head, and immense paddles. They swam in
the Cretaceous interior sea of western North America.

   [Illustration: Fig. 335. Restoration of a Mosasaur]

_Mosasaurs_ belong to the same order as do snakes and lizards, and
are an offshoot of the same ancestral line of land reptiles. These
snakelike creatures--which measured as much as forty-five feet in
length--abounded in the Cretaceous seas. They had large conical
teeth, and their limbs had become stout paddles.

The lower jaw of the mosasaur was jointed; the quadrate bone,
which in all reptiles connects the bone of the lower jaw with the
skull, was movable, and as in snakes the lower jaw could be used
in thrusting prey down the throat. The family became extinct at
the end of the Mesozoic, and left no descendants. One may imitate
the movement of the lower jaw of the mosasaur by extending the
arms, clasping the hands, and bending the elbows.

=Flying reptiles.= The atmosphere, which had hitherto been tenanted
only by insects, was first conquered by the vertebrates in the
Mesozoic. _Pterosaurs_, winged reptiles, whose whole organism was
adapted for flight through the air, appeared in the Jurassic and
passed off the stage of existence before the end of the
Cretaceous. The bones were hollow, as are those of birds. The
sternum, or breastbone, was given a keel for the attachment of the
wing muscles. The fifth finger, prodigiously lengthened, was
turned backward to support a membrane which was attached to the
body and extended to the base of the tail. The other fingers were
free, and armed with sharp and delicate claws, as shown in Figures
336 and 337.

   [Illustration: Fig. 336. Restoration of a Pterosaur]

   [Illustration: Fig. 337. Skeletons of Pterosaur Ornithostoma,
      _A_, and of the Condor, _B_

   After Lucas]

These "dragons of the air" varied greatly in size; some were as
small as sparrows, while others surpassed in stretch of wing the
largest birds of the present day. They may be divided into two
groups. The earliest group comprises genera with jaws set with
teeth, and with long tails sometimes provided with a rudderlike
expansion at the end. In their successors of the later group the
tail had become short, and in some of the genera the teeth had
disappeared. Among the latest of the flying reptiles was
_Ornithostoma_ (bird beak), the largest creature which ever flew,
and whose remains are imbedded in the offshore deposits of the
Cretaceous sea which held sway over our western plains.
Ornithostoma's spread of wings was twenty feet. Its bones were a
marvel of lightness, the entire skeleton, even in its petrified
condition, not weighing more than five or six pounds. The sharp
beak, a yard long, was toothless and bird-like, as its name
suggests.

   [Illustration: Fig. 338. Archæopteryx]

=Birds.= The earliest known birds are found in the Jurassic, and
during the remainder of the Mesozoic they contended with the
flying reptiles for the empire of the air. The first feathered
creatures were very different from the birds of to-day. Their
characteristics prove them an offshoot of the dinosaur line of
reptiles. _Archæopteryx_ (_ancient bird_) (Fig. 338) exhibits a
strange mingling of bird and reptile. Like birds, it was fledged
with perfect feathers, at least on wings and tail, but it retained
the teeth of the reptile, and its long tail was vertebrated,
a pair of feathers springing from each joint. Throughout the
Jurassic and Cretaceous the remains of birds are far less common
than those of flying reptiles, and strata representing hundreds of
thousands of years intervene between Archæopteryx and the next
birds of which we know, whose skeletons occur in the Cretaceous
beds of western Kansas.

=Mammals.= So far as the entries upon the geological record show,
mammals made their advent in a very humble way during the Trias.
These earliest of vertebrates which suckle their young were no
bigger than young kittens, and their strong affinities with the
theromorphs suggest that their ancestors are to be found among
some generalized types of that order of reptiles.

   [Illustration: Fig. 339. Jawbone of a Jurassic Mammal]

During the long ages of the Mesozoic, mammals continued small and
few, and were completely dominated by the reptiles. Their remains
are exceedingly rare, and consist of minute scattered teeth,--with
an occasional detached jaw,--which prove them to have been flesh
or insect eaters. In the same way their affinities are seen to be
with the lowest of mammals,--the _monotremes_ and _marsupials_.
The monotremes,--such as the duckbill mole and the spiny ant-eater
of Australia, reproduce by means of eggs resembling those of
reptiles; the marsupials, such as the opossum and the kangaroo,
bring forth their young alive, but in a very immature condition,
and carry them for some time after birth in the marsupium, a pouch
on the ventral side of the body.




CHAPTER XXI

THE TERTIARY


=The Cenozoic era.= The last stages of the Cretaceous are marked by a
decadence of the reptiles. By the end of that period the reptilian
forms characteristic of the time had become extinct one after another,
leaving to represent the class only the types of reptiles which
continue to modern times. The day of the ammonite and the belemnite
also now drew to a close, and only a few of these cephalopods were
left to survive the period. It is therefore at the close of the
Cretaceous that the line is drawn which marks the end of the Middle
Age of geology and the beginning of the Cenozoic era, the era of
modern life,--the Age of Mammals.

In place of the giant reptiles, mammals now become masters of the
land, appearing first in generalized types which, during the long ages
of the era, gradually evolve to higher forms, more specialized and
ever more closely resembling the mammals of the present. In the
atmosphere the flying dragons of the Mesozoic give place to birds and
bats. In the sea, whales, sharks, and teleost fishes of modern types
rule in the stead of huge swimming reptiles. The lower vertebrates,
the invertebrates of land and sea, and the plants of field and forest
take on a modern aspect, and differ little more from those of to-day
than the plants and animals of different countries now differ from one
another. From the beginning of the Cenozoic era until now there is a
steadily increasing number of species of animals and plants which have
continued to exist to the present time.

The Cenozoic era comprises two divisions,--the _Tertiary_ period and
the _Quaternary_ period.

In the early days of geology the formations of the entire geological
record, so far as it was then known, were divided into three
groups,--the _Primary_, the _Secondary_ (now known as the Mesozoic),
and the _Tertiary_, When the third group was subdivided into two
systems, the term Tertiary was retained for the first system of the
two, while the term _Quaternary_ was used to designate the second.

=Divisions of the Tertiary.= The formations of the Tertiary are
grouped in three divisions,--the _Pliocene_ (more recent), the
_Miocene_ (less recent), and the _Eocene_ (the dawn of the recent).
Each of these epochs is long and complex. Their various subdivisions
are distinguished each by its own peculiar organisms, and the changes
of physical geography recorded in their strata. In the rapid view
which we are compelled to take we can note only a few of the most
conspicuous events of the period.

=Physical geography of the Tertiary in eastern North America.= The
Tertiary rocks of eastern North America are marine deposits and occupy
the coastal lowlands of the Atlantic and Gulf states (Fig. 260). In
New England, Tertiary beds occur on the island of Martha's Vineyard,
but not on the mainland; hence the shore line here stood somewhat
farther out than now. From New Jersey southward the earliest Tertiary
sands and clays, still unconsolidated, leave only a narrow strip of
the edge of the Cretaceous between them and the Triassic and
crystalline rocks of the Piedmont oldland; hence the Atlantic shore
here stood farther in than now, and at the beginning of the period the
present coastal plain was continental delta. A broad belt of Tertiary
sea-laid limestones, sandstones, and shales surrounds the Gulf of
Mexico and extends northward up the Mississippi embayment to the mouth
of the Ohio River; hence the Gulf was then larger than at present, and
its waters reached in a broad bay far up the Mississippi valley.

Along the Atlantic coast the Mesozoic peneplain may be traced
shoreward to where it disappears from view beneath an unconformable
cover of early Tertiary marine strata. The beginning of the Tertiary
was therefore marked by a subsidence. The wide erosion surface which
at the close of the Mesozoic lay near sea level where the Appalachian
Mountains and their neighboring plateaus and uplands now stand was
lowered gently along its seaward edge beneath the Tertiary Atlantic to
receive a cover of its sediments.

As the period progressed slight oscillations occurred from time to
time. Strips of coastal plain were added to the land, and as early as
the close of the Miocene the shore lines of the Atlantic and Gulf
states had reached well-nigh their present place. Louisiana and
Florida were the last areas to emerge wholly from the sea,--Florida
being formed by a broad transverse upwarp of the continental delta at
the opening of the Miocene, forming first an island, which afterwards
was joined to the mainland.

=The Pacific coast.= Tertiary deposits with marine fossils occur along
the western foothills of the Sierra Nevadas, and are crumpled among
the mountain masses of the Coast Ranges; it is hence inferred that the
Great Valley of California was then a border sea, separated from the
ocean by a chain of mountainous islands which were upridged into the
Coast Ranges at a still later time. Tertiary marine strata are spread
over the lower Columbia valley and that of Puget Sound, showing that
the Pacific came in broadly there.

=The interior of the western United States.= The closing stages of the
Mesozoic were marked, as we have seen, by the upheaval of the Rocky
Mountains and other western ranges. The bases of the mountains are now
skirted by widespread Tertiary deposits, which form the highest strata
of the lofty plateaus from the level of whose summits the mountains
rise. Like the recent alluvium of the Great Valley of California (p.
101), these deposits imply low-lying lands when they were laid, and
therefore at that time the mountains rose from near sea level. But the
height at which the Tertiary strata now stand--five thousand feet
above the sea at Denver, and twice that height in the plateaus of
southern Utah--proves that the plateaus of which the Tertiary strata
form a part have been uplifted during the Cenozoic. During their
uplift, warping formed extensive basins both east and west of the
Rockies, and in these basins stream-swept and lake-laid waste gathered
to depths of hundreds and thousands of feet, as it is accumulating at
present in the Great Valley of California and on the river plains of
Turkestan (p. 103). The Tertiary river deposits of the High Plains
have already been described (p. 100). How widespread are these ancient
river plains and beds of fresh-water lakes may be seen in the map of
Figure 260.

   [Illustration: Fig. 340. View in the Bad Lands of South Dakota]

=The Bad Lands.= In several of the western states large areas of
Tertiary fresh-water deposits have been dissected to a maze of hills
whose steep sides are cut with innumerable ravines. The deposits of
these ancient river plains and lake beds are little cemented and
because of the dryness of the climate are unprotected by vegetation;
hence they are easily carved by the wet-weather rills of scanty and
infrequent rains. These waterless, rugged surfaces were named by the
early French explorers the _Bad Lands_ because they were found so
difficult to traverse. The strata of the Bad Lands contain vast
numbers of the remains of the animals of Tertiary times, and the large
amount of barren surface exposed to view makes search for fossils easy
and fruitful. These desolate tracts are therefore frequently visited
by scientific collecting expeditions.

=Mountain making in the Tertiary.= The Tertiary period included epochs
when the earth's crust was singularly unquiet. From time to time on
all the continents subterranean forces gathered head, and the crust
was bent and broken and upridged in lofty mountains.

The Sierra Nevada range was formed, as we have seen, by strata
crumpling at the end of the Jurassic. But since that remote time the
upfolded mountains had been worn to plains and hilly uplands, the
remnants of whose uplifted erosion surfaces may now be traced along
the western mountain slopes. Beginning late in the Tertiary, the
region was again affected by mountain-making movements. A series of
displacements along a profound fault on the eastern side tilted the
enormous earth block of the Sierras to the west, lifting its eastern
edge to form the lofty crest and giving to the range a steep eastern
front and a gentle descent toward the Pacific.

The Coast Ranges also have had a complex history with many
vicissitudes. The earliest foldings of their strata belong to the
close of the Jurassic, but it was not until the end of the Miocene
that the line of mountainous islands and the heavy sediments which had
been deposited on their submerged flanks were crushed into a
continuous mountain chain. Thick Pliocene beds upon their sides prove
that they were depressed to near sea level during the later Tertiary.
At the close of the Pliocene the Coast Ranges rose along with the
upheaval of the Sierra, and their gradual uplift has continued to the
present time.

The numerous north-south ranges of the Great Basin and the Mount Saint
Elias range of Alaska were also uptilted during the Tertiary.

During the Tertiary period many of the loftiest mountains of the
earth--the Alps, the Apennines, the Pyrenees, the Atlas, the Caucasus,
and the Himalayas--received the uplift to which they owe most of their
colossal bulk and height, as portions of the Tertiary sea beds now
found high upon their flanks attest. In the Himalayas, Tertiary marine
limestones occur sixteen thousand five hundred feet above sea level.

=Volcanic activity in the tertiary.= The vast deformations of the
Tertiary were accompanied on a corresponding scale by outpourings of
lava, the outburst of volcanoes, and the intrusion of molten masses
within the crust. In the Sierra Nevadas the Miocene river gravels of
the valleys of the western slope, with their placer deposits of gold,
were buried beneath streams of lava and beds of tuff (Fig. 258).
Volcanoes broke forth along the Rocky Mountains and on the plateaus of
Utah, New Mexico, and Arizona.

Mount Shasta and the immense volcanic piles of the Cascades date from
this period. The mountain basin of the Yellowstone Park was filled to
a depth of several thousand feet with tuffs and lavas, the oldest
dating as far back as the beginning of the Tertiary. Crandall
volcano (Fig. 263) was reared in the Miocene and the latest eruptions
of the Park are far more recent.

   [Illustration: Fig. 341. Lava Plateau with Lava Domes in the
      Distance]

=The Columbia and Snake River lavas.= Still more important is the
plateau of lava, more than two hundred thousand square miles in area,
extending from the Yellowstone Park to the Cascade Mountains, which
has been built from Miocene times to the present.

Over this plateau, which occupies large portions of Idaho, Washington,
and Oregon, and extends into northern California and Nevada, the
country rock is basaltic lava. 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 floods of molten
rock spread up the mountain valleys for a score of miles and more, the
intervening spurs rising above the lava like long peninsulas, while
here and there an isolated peak was left to tower above the inundation
like an island off a submerged shore.

The rivers which drain the plateau--the Snake, the Columbia, and their
tributaries--have deeply trenched it, yet their canyons, which reach the
depth of several thousand feet, have not been worn to the base of the
lava except near the margin and where they cut the summits of mountains
drowned beneath the flood. Here and there the plateau has been deformed.
It has been upbent into great folds, and broken into immense blocks of
bedded lava, forming mountain ranges, which run parallel with the
Pacific coast line. On the edges of these tilted blocks the thickness of
the lava is seen to be fully five thousand feet. The plateau has been
built, like that of Iceland (p. 242), of innumerable overlapping sheets
of lava. On the canyon walls they weather back in horizontal terraces
and long talus slopes. One may distinguish each successive flow by its
dense central portion, often jointed with large vertical columns, and
the upper portion with its mass of confused irregular columns and
scoriaceous surface. The average thickness of the flows seems to be
about seventy-five feet.

The plateau was long in building. Between the layers are found in
places old soil beds and forest grounds and the sediments of lakes.
Hence the interval between the flows in any locality was sometimes
long enough for clays to gather in the lakes which filled depressions
in the surface. Again and again the surface of the black basalt was
reddened by oxidation and decayed to soil, and forests had time to
grow upon it before the succeeding inundation sealed the sediments and
soils away beneath a sheet of stone. Near the edges of the lava plain,
rivers from the surrounding mountains spread sheets of sand and gravel
on the surface of one flow after another. These pervious sands,
interbedded with the lava, become the aquifers of artesian wells.

In places the lavas rest on extensive lake deposits, one thousand feet
deep, and Miocene in age as their fossils prove. It is to the middle
Tertiary, then, that the earliest flows and the largest bulk of the
great inundation belong. So ancient are the latest floods in the
Columbia 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. Their
surfaces are so fresh and undecayed that here the effusive eruptions
may well have continued to within the period of human history. Low
lava domes like those of Iceland mark where last the basalt outwelled
and spread far and wide before it chilled (Fig. 341). In places small
mounds of scoria show that the eruptions were accompanied to a slight
degree by explosions of steam. So fluid was this superheated lava that
recent flows have been traced for more than fifty miles.

The rocks underlying the Columbia lavas, where exposed to view, are
seen to be cut by numerous great dikes of dense basalt, which mark the
fissures through which the molten rock rose to the surface.

The Tertiary included times of widespread and intense volcanic action
in other continents as well as in North America. In Europe,
Vesuvius (p. 231) and Etna began their career as submarine volcanoes in
connection with earth movements which finally lifted Pliocene deposits
in Sicily to their present height,--four thousand feet above the sea.
Volcanoes broke forth in central France and southern Germany, in
Hungary and the Carpathians. Innumerable fissures opened in the crust
from the north of Ireland and the western islands of Scotland to the
Faroes, Iceland, and even to arctic Greenland; and here great plateaus
were built of flows of basalt similar to that of the Columbia River.
In India, at the opening of the Tertiary, there had been an outwelling
of basalt, flooding to a depth of thousands of feet two hundred
thousand square miles of the northwestern part of the peninsula (Fig.
342), and similar inundations of lava occurred where are now the
table-lands of Abyssinia. From the middle Tertiary on, Asia Minor,
Arabia, and Persia were the scenes of volcanic action. In Palestine
the rise of the uplands of Judea at the close of the Eocene, and the
downfaulting of the Jordan valley (p. 221) were followed by volcanic
outbursts. In comparison with the middle Tertiary, the present is a
time of volcanic inactivity and repose.

   [Illustration: Fig. 342. Map showing the Lava Sheet
      (shaded area) of Western India]

=Erosion of Tertiary mountains and plateaus.= The mountains and
plateaus built at various times during the Tertiary and at its
commencement have been profoundly carved by erosive agents. The Sierra
Nevada Mountains have been dissected on the western slope by such
canyons as those of King's River and the Yosemite. Six miles of strata
have been denuded from parts of the Wasatch Mountains since their rise
at the beginning of the era. From the Colorado plateaus, whose uplift
dates from the same time, there have been stripped off ten thousand
feet of strata over thousands of square miles, and the colossal canyon
of the Colorado has been cut after this great denudation had been
mostly accomplished.

On the eastern side of the continent, as we have seen, a broad
peneplain had been developed by the close of the Cretaceous. The
remnants of this old erosion surface are now found upwarped to various
heights in different portions of its area. In southern New England it
now stands fifteen hundred feet above the sea in western
Massachusetts, declining thence southward and eastward to sea level at
the coast. In southwestern Virginia it has been lifted to four
thousand feet above the sea. Manifestly this upwarp occurred since the
peneplain was formed; it is later than the Mesozoic, and the vast
dissection which the peneplain has suffered since its uplift must
belong to the successive cycles of Cenozoic time.

Revived by the uplift, the streams of the area trenched it as deeply
as its elevation permitted, and reaching grade, opened up wide valleys
and new peneplains in the softer rocks. The Connecticut valley is
Tertiary in age, and in the weak Triassic sandstones has been widened
in places to fifteen miles. Dating from the same time are the valleys
of the Hudson, the Susquehanna, the Delaware, the Potomac, and the
Shenandoah.

In Pennsylvania and the states lying to the south the Mesozoic
peneplain lies along the summits of the mountain ridges. On the
surface of this ancient plain, Tertiary erosion etched out the
beautifully regular pattern of the Allegheny mountain ridges and their
intervening valleys. The weaker strata of the long, regular folds were
eroded into longitudinal valleys, while the hard Paleozoic sandstones,
such as the Medina (p. 335) and the Pocono (p. 350), were left in
relief as bold mountain walls whose even crests rise to the common
level of the ancient plain. From Virginia far into Alabama the great
Appalachian valley was opened to a width in places of fifty miles and
more, along a belt of intensely folded and faulted strata where once
was the heart of the Appalachian Mountains. In Figure 70, the summit of
the Cumberland plateau (ab) marks the level of the Mesozoic peneplain,
while the lower erosion levels are Tertiary and Quaternary in age.

   [Illustration: Fig. 343. Diagram of the Allegheny Mountains,
      Pennsylvania

   From Davis' Elementary Physical Geography]


Life of the Tertiary Period

=Vegetation and climate.= The highest plants in structure, the
_dicotyls_ (such as our deciduous forest trees) and the _monocotyls_
(represented by the palms), were introduced during the Cretaceous. The
vegetable kingdom reached its culmination before the animal kingdom,
and if the dividing line between the Mesozoic and the Cenozoic were
drawn according to the progress of plant life, the Cretaceous instead
of the Tertiary would be made the opening period of the modern era.

The plants of the Tertiary belonged, for the most part, to genera
now living; but their distribution was very different from that of
the flora of to-day. In the earlier Tertiary, palms flourished over
northern Europe, and in the northwestern United States grew the
magnolia and laurel, along with the walnut, oak, and elm. Even in
northern Greenland and in Spitzbergen there were lakes covered with
water lilies and surrounded by forests of maples, poplars, limes, the
cypress of our southern states, and noble sequoias similar to the
"big trees" and redwoods of California. A warm climate like that of
the Mesozoic, therefore, prevailed over North America and Europe,
extending far toward the pole. In the later Tertiary the climate
gradually became cooler. Palms disappeared from Europe, and everywhere
the aspect of forests and open lands became more like that of to-day.
Grasses became abundant, furnishing a new food for herbivorous
animals.

=Animal life of the Tertiary.= Little needs to be said of the Tertiary
invertebrates, so nearly were they like the invertebrates of the
present. Even in the Eocene, about five per cent of marine shells were
of species still living, and in the Pliocene the proportion had risen
to more than one half.

Fishes were of modern types. Teleosts were now abundant. The ocean
teemed with sharks, some of them being voracious monsters seventy-five
feet and even more in length, with a gape of jaw of six feet, as
estimated by the size of their enormous sharp-edged teeth.

Snakes are found for the first time in the early Tertiary. These
limbless reptiles, evolved by degeneration from lizardlike ancestors,
appeared in nonpoisonous types scarcely to be distinguished from those
of the present day.

=Mammals of the early Tertiary.= The fossils of continental deposits
of the earliest Eocene show that a marked advance had now been made in
the evolution of the Mammalia. The higher mammals had appeared, and
henceforth the lower mammals--the monotremes and the marsupials--are
reduced to a subordinate place.

   [Illustration: Fig. 344. Phenacodus]

These first true mammals were archaic and generalized in structure.
Their feet were of the primitive type, with five toes of about equal
length. They were also _plantigrades_,--that is, they touched the
ground with the sole of the entire foot from toe to heel. No foot had
yet become adapted to swift running by a decrease in the number of
digits and by lifting the heel and sole so that only the toes touch
the ground,--a tread called _digitigrade_. Nor was there yet any foot
like that of the cats, with sharp retractile claws adapted to seizing
and tearing the prey. The forearm and the lower leg each had still
two separate bones (ulna and radius, fibula and tibia), neither pair
having been replaced with a single strong bone, as in the leg of the
horse. The teeth also were primitive in type and of full number. The
complex heavy grinders of the horse and elephant, the sharp cutting
teeth of the carnivores, and the cropping teeth of the grass eaters
were all still to come.

Phenacodus is a characteristic genus of the early Eocene, whose
species varied in size from that of a bulldog to that of an animal a
little larger than a sheep. Its feet were primitive, and their five
toes bore nails intermediate in form between a claw and a hoof. The
archaic type of teeth indicates that the animal was omnivorous in
diet. A cast of the brain cavity shows that, like its associates of
the time, its brain was extremely small and nearly smooth, having
little more than traces of convolutions.

The long ages of the Eocene and the following epochs of the Tertiary
were times of comparatively rapid evolution among the Mammalia.
The earliest forms evolved along diverging lines toward the various
specialized types of hoofed mammals, rodents, carnivores,
proboscidians, the primates, and the other mammalian orders as we know
them now. We must describe the Tertiary mammals very briefly, tracing
the lines of descent of only a few of the more familiar mammals of the
present.

=The horse.= The pedigree of the horse runs back into the early Eocene
through many genera and species to a five-toed,[3] short-legged ancestor
little bigger than a cat. Its descendants gradually increased in stature
and became better and better adapted to swift running to escape their
foes. The leg became longer, and only the tip of the toes struck the
ground. The middle toe (digit number three), originally the longest of
the five, steadily enlarged, while the remaining digits dwindled and
disappeared. The inner digit, corresponding to the great toe and thumb,
was the first to go. Next number five, the little finger, was also
dropped. By the end of the Eocene a three-toed genus of the horse
family had appeared, as large as a sheep. The hoof of digit number
three now supported most of the weight, but the slender hoofs of
digits two and four were still serviceable. In the Miocene the stature
of the ancestors of the horse increased to that of a pony. The feet
were still three-toed, but the side hoofs were now mere dewclaws and
scarcely touched the ground. The evolution of the family was completed
in the Pliocene. The middle toe was enlarged still more, the side toes
were dropped, and the palm and foot bones which supported them were
reduced to splints.

   [3] Or, more accurately, with four perfect toes and a
       rudimentary fifth corresponding to the thumb.

   [Illustration: Fig. 345. Development of Forefoot (A), the
      Forearm (B), the Molar (C), of the Horse Family]

While these changes were in progress the radius and ulna of the fore
limb became consolidated to a single bone; and in the hind limb the
fibula dwindled to a splint, while the tibia was correspondingly
enlarged. The molars, also gradually lengthened, and became more and
more complex on their grinding surface; the neck became longer; the
brain steadily increased in size and its convolutions became more
abundant. The evolution of the horse has made for greater fleetness
and intelligence.

=The rhinoceros and tapir.= These animals, which are grouped with the
horse among the _odd-toed_ (perissodactyl) mammals, are now verging
toward extinction. In the rhinoceros, evolution seems to have taken
the opposite course from that of the horse. As the animal increased in
size it became more clumsy, its limbs became shorter and more massive,
and, perhaps because of its great weight, the number of digits were
not reduced below the number three. Like other large herbivores, the
rhinoceros, too slow to escape its enemies by flight, learned to
withstand them. It developed as its means of defense a nasal horn.

Peculiar offshoots of the line appeared at various times in the
Tertiary. A rhinoceros, semiaquatic in habits, with curved tusks,
resembling in aspect the hippopotamus, lived along the water courses
of the plains east of the Rockies, and its bones are now found by the
thousands in the Miocene of Kansas. Another developed along a line
parallel to that of the horse, and herds of these light-limbed and
swift-footed running rhinoceroses ranged the Great Plains from the
Dakotas southward.

The tapirs are an ancient family which has changed but little since it
separated from the other perissodactyl stocks in the early Tertiary.
At present, tapirs are found only in South America and southern
Asia,--a remarkable distribution which we could not explain were it
not that the geological record shows that during Tertiary times tapirs
ranged throughout the northern hemisphere, making their way to South
America late in that period. During the Pleistocene they became
extinct over all the intervening lands between the widely separated
regions where now they live. The geographic distribution of animals,
as well as their relationships and origins, can be understood only
through a study of their geological history.

   [Illustration: Fig. 346. A Tertiary Mastodon]

   [Illustration: Fig. 347. Head of Dinothere]

=The proboscidians.= This unique order of hoofed mammals, of which the
elephant is the sole survivor, has been traced back to the close of
the Eocene. In the middle and later Tertiary it was represented by
huge creatures so nearly akin to the mastodons of the Pleistocene that
they are often included in that genus. The Tertiary _Mastodon_ was
furnished with a long, flexible proboscis, and armed with two pairs of
long, straight ivory tusks, the pair of the lower jaw being smaller.

The _Dinothere_ was a curious offshoot of the line, which developed in
the Miocene in Europe. In this immense proboscidian, whose skull was
three feet long, the upper pair of tusks had disappeared, and those of
the lower jaw were bent down with a backward curve in walrus fashion.

   [Illustration: Fig. 348. Crown of Mastodon Tooth]

In the true _elephants_, which do not appear until near the close of
the Tertiary, the lower jaw loses its tusks and the grinding teeth
become exceedingly complex in structure. The grinding teeth of the
mastodon had long roots and low crowns crossed by four or five peaked
enameled ridges. In the teeth of the true elephants the crown has
become deep, and the ridges of enamel have changed to numerous
upright, platelike folds, their interspaces filled with cement. The
two genera--Mastodon and Elephant--are connected by species whose
teeth are intermediate in pattern. The proboscidians culminated in the
Pliocene, when some of the giant elephants reached a height of
fourteen feet.

   [Illustration: Fig. 349. Tooth of an Extinct Elephant, the Mammoth]

   [Illustration: Fig. 350. Evolution of the Artiodactyl Foot,
      Illustrated by Existing Families

   _A_, pig; _B_, roebuck; _C_, sheep; _D_, camel]

=The artiodactyls= comprise the hoofed Mammalia which have an even
number of toes, such as cattle, sheep, and swine. Like the
perissodactyls, they are descended from the primitive five-toed
plantigrade mammals of the lowest Eocene. In their evolution, digit
number one was first dropped, and the middle pair became larger and
more massive, while the side digits, numbers two and five, became
shorter, weaker, and less serviceable. The _four-toed artiodactyls_
culminated in the Tertiary; at present they are represented only by
the hippopotamus and the hog. Along the main line of the evolution of
the artiodactyls the side toes, digits two and five, disappeared,
leaving as proof that they once existed the corresponding bones of
palm and sole as splints. The _two-toed artiodactyls_, such as the
camels, deer, cattle, and sheep, are now the leading types of the
herbivores.

_Swine and peccaries_ are two branches of a common stock, the first
developing in the Old World and the second in the New. In the Miocene
a noticeable offshoot of the line was a gigantic piglike brute, a root
eater, with a skull a yard in length, whose remains are now found in
Colorado and South Dakota.

=Camels and llamas.= The line of camels and llamas developed in North
America, where the successive changes from an early Eocene ancestor,
no larger than a rabbit, are traced step by step to the present forms,
as clearly as is the evolution of the horse. In the late Miocene some
of the ancestral forms migrated to the Old World by way of a land
connection where Bering Strait now is, and there gave rise to the
camels and dromedaries. Others migrated into South America, which had
now been connected with our own continent, and these developed into
the llamas and guanacos, while those of the race which remained in
North America became extinct during the Pleistocene.

Some peculiar branches of the camel stem appeared in North America. In
the Pliocene arose a llama with the long neck and limbs of a giraffe,
whose food was cropped from the leaves and branches of trees. Far more
generalized in structure was the _Oreodon_, an animal related to the
camels, but with distinct affinities also with other lines, such as
those of the hog and deer. These curious creatures were much like the
peccary in appearance, except for their long tails. In the middle
Eocene they roamed in vast herds from Oregon to Kansas and Nebraska.

=The ruminants.= This division of the artiodactyls includes antelopes,
deer, oxen, bison, sheep, and goats,--all of which belong to a common
stock which took its rise in Europe in the upper Eocene from ancestral
forms akin to those of the camels. In the Miocene the evolution of the
two-toed artiodactyl foot was well-nigh completed. Bonelike growths
appeared on the head, and the two groups of the ruminants became
specialized,--the deer with bony antlers, shed and renewed each year,
and the ruminants with hollow horns, whose two bony knobs upon the
skull are covered with permanent, pointed, horny sheaths.

The ruminants evolved in the Old World, and it was not until the later
Miocene that the ancestors of the antelope and of some deer found
their way to North America. Mountain sheep and goats, the bison and
most of the deer, did not arrive until after the close of the
Tertiary, and sheep and oxen were introduced by man.

The hoofed mammals of the Tertiary included many offshoots from the
main lines which we have traced. Among them were a number of genera of
clumsy, ponderous brutes, some almost elephantine in their bulk.

=The carnivores.= The ancestral lines of the families of the flesh
eaters--such as the cats (lions, tigers, etc.), the bears, the hyenas,
and the dogs (including wolves and foxes)--converge in the creodonts
of the early Eocene,--an order so generalized that it had affinities
not only with the carnivores but also with the insect eaters, the
marsupials, and the hoofed mammals as well. From these primitive flesh
eaters, with small and simple brains, numerous small teeth, and
plantigrade tread, the different families of the carnivores of the
present have slowly evolved.

=Dogs and bears.= The dog family diverged from the creodonts late in
the Eocene, and divided into two branches, one of which evolved the
wolves and the other the foxes. An offshoot gave rise to the family
of the bears, and so closely do these two families, now wide apart,
approach as we trace them back in Tertiary times that the Amphicyon,
a genus doglike in its teeth and bearlike in other structures, is
referred by some to the dog and by others to the bear family. The
well-known plantigrade tread of bears is a primitive characteristic
which has survived from their creodont ancestry.

=Cats.= The family of the cats, the most highly specialized of all the
carnivores, divided in the Tertiary into two main branches. One, the
saber-tooth tigers (Fig. 351), which takes its name from their long,
saberlike, sharp-edged upper canine teeth, evolved a succession of
genera and species, among them some of the most destructive beasts of
prey which ever scourged the earth. They were masters of the entire
northern hemisphere during the middle Tertiary, but in Europe during
the Pliocene they declined, from unknown causes, and gave place to the
other branch of cats,--which includes the lions, tigers, and leopards.
In the Americas the saber-tooth tigers long survived the epoch.

   [Illustration: Fig. 351. Saber-Tooth Tiger]

=Marine mammals.= The carnivorous mammals of the sea--whales, seals,
walruses, etc.--seem to have been derived from some of the creodonts
of the early Tertiary by adaptation to aquatic life. Whales evolved
from some land ancestry at a very early date in the Tertiary; in the
marine deposits of the Eocene are found the bones of the Zeuglodon, a
whalelike creature seventy feet in length.

=Primates.= This order, which includes lemurs, monkeys, apes, and man,
seems to have sprung from a creodont or insectivorous ancestry in
the lower Eocene. Lemur-like types, with small, smooth brains, were
abundant in the United States in the early Tertiary, but no primates
have been found here in the middle Tertiary and later strata. In
Europe true monkeys were introduced in the Miocene, and were abundant
until the close of the Tertiary, when they were driven from the
continent by the increasing cold.

=Advance of the mammalia during the tertiary.= During the several
millions of years comprised in Tertiary time the mammals evolved from
the lowly, simple types which tenanted the earth at the beginning of
the period, into the many kinds of highly specialized mammals of the
Pleistocene and the present, each with the various structures of the
body adapted to its own peculiar mode of life. The swift feet of the
horse, the horns of cattle and the antlers of the deer, the lion's
claws and teeth, the long incisors of the beaver, the proboscis of the
elephant, were all developed in Tertiary times. In especial the brain
of the Tertiary mammals constantly grew larger relatively to the
size of body, and the higher portion of the brain--the cerebral
lobes--increased in size in comparison with the cerebellum. Some
of the hoofed mammals now have a brain eight or ten times the size
of that of their early Tertiary predecessors of equal bulk. Nor
can we doubt that along with the increasing size of brain went a
corresponding increase in the keenness of the senses, in activity
and vigor, and in intelligence.




CHAPTER XXII

THE QUATERNARY


The last period of geological history, the Quaternary, may be said to
have begun when all, or nearly all, living species of mollusks and
most of the existing mammals had appeared.

It is divided into two great epochs. The first, the _Pleistocene_ or
_Glacial epoch_, is marked off from the Tertiary by the occupation of
the northern parts of North America and Europe by vast ice sheets; the
second, the _Recent epoch_, began with the disappearance of the ice
sheets from these continents, and merges into the present time.


The Pleistocene Epoch

We now come to an episode of unusual interest, so different was it
from most of the preceding epochs and from the present, and so largely
has it influenced the conditions of man's life.

The records of the Glacial epoch are so plain and full that
we are compelled to believe what otherwise would seem almost
incredible,--that following the mild climate of the Tertiary came a
succession of ages when ice fields, like that of Greenland, shrouded
the northern parts of North America and Europe and extended far into
temperate latitudes.

=The drift.= Our studies of glaciers have prepared us to decipher and
interpret the history of the Glacial epoch, as it is recorded in the
surface deposits known as the drift. Over most of Canada and the
northern states this familiar formation is exposed to view in nearly
all cuttings which pass below the surface soil. The drift includes two
distinct classes of deposits,--the unstratified drift laid down by
glacier ice, and the stratified drift spread by glacier waters.

The materials of the drift are in any given place in part unlike the
rock on which it rests. They cannot be derived from the underlying
rock by weathering, but have been brought from elsewhere. Thus where a
region is underlain by sedimentary rocks, as is the drift-covered area
from the Hudson River to the Missouri, the drift contains not only
fragments of limestone, sandstone, and shale of local derivation, but
also pebbles of many igneous and metamorphic rocks, such as granites,
gneisses, schists, dike rocks, quartzites, and the quartz of mineral
veins, whose nearest source is the Archean area of Canada and the
states of our northern border. The drift received its name when it was
supposed that the formation had been drifted by floods and icebergs
from outside sources,--a theory long since abandoned.

   [Illustration: Fig. 352. Stratified Drift overlaying
      Unstratified Drift, Massachusetts]

The distribution also of the drift points clearly to its peculiar
origin. Within the limits of the glaciated area it covers the country
without regard to the relief, mantling with its debris not only
lowlands and valleys but also highlands and mountain slopes.

The boundary of the drift is equally independent of the relief of
the land, crossing hills and plains impartially, unlike water-laid
deposits, whose margins, unless subsequently deformed, are horizontal.
The boundary of the drift is strikingly lobate also, bending outward
in broad, convex curves, where there are no natural barriers in the
topography of the country to set it such a limit. Under these
conditions such a lobate margin cannot belong to deposits of rivers,
lakes, or ocean, but is precisely that which would mark the edge of a
continental glacier which deployed in broad tongues of ice.

=The rock surface underlying the drift.= Over much of its area the
drift rests on firm, fresh rock, showing that both the preglacial
mantle of residual waste and the partially decomposed and broken rock
beneath it have been swept away. The underlying rock, especially if
massive, hard, and of a fine grain, has often been ground down to a
smooth surface and rubbed to a polish as perfect as that seen on the
rock beside an Alpine glacier where the ice has recently melted back.
Frequently it has been worn to the smooth, rounded hummocks known as
roches moutonnées, and even rocky hills have been thus smoothed to
flowing outlines like roches moutonnées on a gigantic scale. The rock
pavement beneath the drift is also marked by long, straight, parallel
scorings, varying in size from deep grooves to fine striae as delicate
as the hair lines cut by an engraver's needle. Where the rock is soft
or closely jointed it is often shattered to a depth of several feet
beneath the drift, while stony clay has been thrust in among the
fragments into which the rock is broken.

In the presence of these glaciated surfaces we cannot doubt that the
area of the drift has been overridden by vast sheets of ice which, in
their steady flow, rasped and scored the rock bed beneath by means of
the stones with which their basal layers were inset, and in places
plucked and shattered it.

=Till.= The unstratified portion of the drift consists chiefly of
sheets of dense, stony clay called till, which clearly are the ground
moraines of ancient continental glaciers. Till is an unsorted mixture
of materials of all sizes, from fine clay and sand, gravel, pebbles,
and cobblestones, to large bowlders. The stones of the till are of
many kinds, some having been plucked from the bed rock of the locality
where they are found, and others having been brought from outside and
often distant places. Land ice is the only agent known which can
spread unstratified material in such extensive sheets.

The _fine material_ of the till comes from two different sources. In
part it is derived from old residual clays, which in the making had
been leached of the lime and other soluble ingredients of the rock
from which they weathered. In part it consists of sound rock ground
fine; a drop of acid on fresh, clayey till often proves by brisk
effervescence that the till contains much undecayed limestone flour.
The ice sheet, therefore, both scraped up the mantle of long-weathered
waste which covered the country before its coming, and also ground
heavily upon the sound rock underneath, and crushed and wore to rock
flour the fragments which it carried.

The color of unweathered till depends on that of the materials of
which it is composed. Where red sandstones have contributed largely to
its making, as over the Triassic sandstones of the eastern states and
the Algonkian sandstones about Lake Superior, the drift is reddish.
When derived in part from coaly shales, as over many outcrops of the
Pennsylvanian, it may when moist be almost black. Fresh till is
normally a dull gray or bluish, so largely is it made up of the
grindings of unoxidized rocks of these common colors.

Except where composed chiefly of sand or coarser stuff, unweathered
till is often exceedingly dense. Can you suggest by what means it has
been thus compacted? Did the ice fields of the Glacial epoch bear
heavy surface moraines like the medial and lateral moraines of valley
glaciers? Where was the greater part of the load of these ice fields
carried, judging from what you know of the glaciers of Greenland?

=Bowlders of the drift.= The pebbles and bowlders of the drift are in
part stream gravels, bowlders of weathering, and other coarse rock
waste picked up from the surface of the country by the advancing ice,
and in part are fragments plucked from ledges of sound rock after the
mantle of waste had been removed. Many of the stones of the till are
dressed as only glacier ice can do; their sharp edges have been
blunted and their sides faceted and scored.

We may easily find all stages of this process represented among the
pebbles of the till. Some are little worn, even on their edges; some
are planed and scored on one side only; while some in their long
journey have been ground down to many facets and have lost much of
their original bulk. Evidently the ice played fast and loose with a
stone carried in its basal layers, now holding it fast and rubbing it
against the rock beneath, now loosening its grasp and allowing the
stone to turn.

Bowlders of the drift are sometimes found on higher ground than their
parent ledges. Thus bowlders have been left on the sides of Mount
Katahdin, Maine, which were plucked from limestone ledges twelve miles
distant and three thousand feet lower than their resting place. In
other cases stones have been carried over mountain ranges, as in
Vermont, where pebbles of Burlington red sandstone were dragged over
the Green Mountains, three thousand feet in height, and left in the
Connecticut valley sixty miles away. No other geological agent than
glacier ice could do this work.

The bowlders of the drift are often large. Bowlders ten and twenty
feet in diameter are not uncommon, and some are known whose diameter
exceeds fifty feet. As a rule the average size of bowlders decreases
with increasing distance from their sources. Why?

=Till plains.= The surface of the drift, where left in its initial
state, also displays clear proof of its glacial origin. Over large
areas it is spread in level plains of till, perhaps bowlder-dotted,
similar to the plains of stony clay left in Spitzbergen by the recent
retreat of some of the glaciers of that island. In places the
unstratified drift is heaped in hills of various kinds, which we will
now describe.

   [Illustration: Fig. 354. Map of a portion of a Drumlin Area near
      Oswego, New York]

=Drumlins.= Drumlins are smooth, rounded hills composed of till,
elliptical in base, and having their longer axes parallel to the
movement of the ice as shown by glacial scorings. They crowd certain
districts in central New York and in southern Wisconsin, where they
may be counted by the thousands. Among the numerous drumlins about
Boston is historic Bunker Hill.

Drumlins are made of ground moraine. They were accumulated and given
shape beneath the overriding ice, much as are sand bars in a river, or
in some instances were carved, like roches moutonnées, by an ice sheet
out of the till left by an earlier ice invasion.

=Terminal moraines.= The glaciated area is crossed by belts of
thickened drift, often a mile or two, and sometimes even ten miles
and more, in breadth, which lie transverse to the movement of the ice
and clearly are the terminal moraines of ancient ice sheets, marking
either the limit of their farthest advance or pauses in their general
retreat.

   [Illustration: Fig. 355. Terminal Moraine, Staten Island]

The surface of these moraines is a jumble of elevations and
depressions, which vary from low, gentle swells and shallow sags to
sharp hills, a hundred feet or so in height, and deep, steep-sided
hollows. Such tumultuous hills and hummocks, set with depressions of
all shapes, which usually are without outlet and are often occupied by
marshes, ponds, and lakes, surely cannot be the work of running water.
The hills are heaps of drift, lodged beneath the ice edge or piled
along its front. The basins were left among the tangle of morainic
knolls and ridges (Fig. 105) as the margin of the ice moved back and
forth. Some bowl-shaped basins were made by the melting of a mass of
ice left behind by the retreating glacier and buried in its debris.

   [Illustration: Fig. 356. Esker, New York]

=The stratified drift.= Like modern glaciers the ice sheets of the
Pleistocene were ever being converted into water about their margins.
Their limits on the land were the lines where their onward flow was
just balanced by melting and evaporation. On the surface of the ice
along the marginal zone, rivulets no doubt flowed in summer, and found
their way through crevasses to the interior of the glacier or to
the ground. Subglacial streams, like those of the Malaspina glacier,
issued from tunnels in the ice, and water ran along the melting ice
front as it is seen to do about the glacier tongues of Greenland. All
these glacier waters flowed away down the chief drainage channels in
swollen rivers loaded with glacial waste.

It is not unexpected therefore that there are found, over all the
country where the melting ice retreated, deposits made of the same
materials as the till, but sorted and stratified by running water.
Some of these were deposited behind the ice front in ice-walled
channels, some at the edge of the glaciers by issuing streams, and
others were spread to long distances in front of the ice edge by
glacial waters as they flowed away.

_Eskers_ are narrow, winding ridges of stratified sand and gravel
whose general course lies parallel with the movement of the glacier.
These ridges, though evidently laid by running water, do not follow
lines of continuous descent, but may be found to cross river valleys
and ascend their sides. Hence the streams by which eskers were laid
did not flow unconfined upon the surface of the ground. We may infer
that eskers were deposited in the tunnels and ice-walled gorges of
glacial streams before they issued from the ice front.

   [Illustration: Fig. 357. Kames, New York]

_Kames_ are sand and gravel knolls, associated for the most part
with terminal moraines, and heaped by glacial waters along the
margin of the ice.

   [Illustration: Fig. 358. Diagram Illustrating the Formation of
      Kame Terraces

   _i_, glacier ice; _t_, _t_, terraces]

_Kame terraces_ are hummocky embankments of stratified drift sometimes
found in rugged regions along the sides of valleys. In these valleys
long tongues of glacier ice lay slowly melting. Glacial waters took
their way between the edges of the glaciers and the hillside, and here
deposited sand and gravel in rude terraces.

_Outwash plains_ are plains of sand and gravel which frequently border
terminal moraines on their outward face, and were spread evidently by
outwash from the melting ice. Outwash plains are sometimes pitted by
bowl-shaped basins where ice blocks were left buried in the sand by
the retreating glacier.

_Valley trains_ are deposits of stratified drift with which river
valleys have been aggraded. Valleys leading outward from the ice front
were flooded by glacial waters and were filled often to great depths
with trains of stream-swept drift. Since the disappearance of the ice
these glacial flood plains have been dissected by the shrunken rivers
of recent times and left on either side the valley in high terraces.
Valley trains head in morainic plains, and their material grows finer
down valley and coarser toward their sources. Their gradient is
commonly greater than that of the present rivers.

=The extent of the drift.= The extent of the drift of North America
and its southern limits are best seen in Figure 359. Its area is
reckoned at about four million square miles. The ice fields which once
covered so much of our continent were all together ten times as large
as the inland ice of Greenland, and about equal to the enormous ice
cap which now covers the antartic regions.

The ice field of Europe was much smaller, measuring about seven
hundred and seventy thousand square miles.

=Centers of dispersion.= The direction of the movement of the ice is
recorded plainly in the scorings of the rock surface, in the shapes of
glaciated hills, in the axes of drumlins and eskers, and in trains of
bowlders, when the ledges from which they were plucked can be
discovered. In these ways it has been proved that in North America
there were three centers where ice gathered to the greatest depth,
and from which it flowed in all directions outward. There were thus
three vast ice fields,--one the _Cordilleran_, which lay upon the
Cordilleras of British America; one the _Keewatin_, which flowed
out from the province of Keewatin, west of Hudson Bay; and one the
_Labrador_ ice field, whose center of dispersion was on the highlands
of the peninsula of Labrador. As shown in Figure 359, the western ice
field extended but a short way beyond the eastern foothills of the
Rocky Mountains, where perhaps it met the far-traveled ice from the
great central field. The Keewatin and the Labrador ice fields flowed
farthest toward the south, and in the Mississippi valley the one
reached the mouth of the Missouri and the other nearly to the mouth of
the Ohio. In Minnesota and Wisconsin and northward they merged in one
vast field.

   [Illustration: Fig. 359. Hypothetical Map of the Pleistocene Ice Sheets
      of North America

   From Salisbury's _Glacial Geology of New Jersey_]

The thickness of the ice was so great that it buried the highest
mountains of eastern North America, as is proved by the transported
bowlders which have been found upon their summits. If the land then
stood at its present height above sea level, and if the average slope
of the ice were no more than ten feet to the mile,--a slope so gentle
that the eye could not detect it and less than half the slope of the
interior of the inland ice of Greenland,--the ice plateaus about
Hudson Bay must have reached a thickness of at least ten thousand
feet.

In Europe the Scandinavian plateau was the chief center of dispersion.
At the time of greatest glaciation a continuous field of ice extended
from the Ural Mountains to the Atlantic, where, off the coasts of
Norway and the British Isles, it met the sea in an unbroken ice wall.
On the south it reached to southern England, Belgium, and central
Germany, and deployed on the eastern plains in wide lobes over Poland
and central Russia (Fig. 360).

   [Illustration: Fig. 360. Hypothetical Map of the Pleistocene
      Ice Sheet of Europe]

At the same time the Alps supported giant glaciers many times the size
of the surviving glaciers of to-day, and a piedmont glacier covered
the plains of northern Switzerland.

=The thickness of the drift.= The drift is far from uniform in
thickness. It is comparatively thin and scanty over the Laurentian
highlands and the rugged regions of New England, while from southern
New York and Ontario westward over the Mississippi valley, and on the
great western plains of Canada, it exceeds an average of one hundred
feet over wide areas, and in places has five and six times that
thickness. It was to this marginal belt that the ice sheets brought
their loads, while northwards, nearer the centers of dispersion,
erosion was excessive and deposition slight.

=Successive ice invasions and their drift sheets.= Recent studies of
the drift prove that it does not consist of one indivisible formation,
but includes a number of distinct drift sheets, each with its own
peculiar features. The Pleistocene epoch consisted, therefore, of
several glacial stages,--during each of which the ice advanced far
southward,--together with the intervening interglacial stages when,
under a milder climate, the ice melted back toward its sources or
wholly disappeared.

   [Illustration: Fig. 361. Diagram illustrating Criteria by which
      Different Drift Sheets are distinguished]

The evidences of such interglacial stages, and the means by which the
different drift sheets are told apart, are illustrated in Figure 361.
Here the country from N to S is wholly covered by drift, but the drift
from N to _m_ is so unlike that from _m_ to S that we may believe it
the product of a distinct ice invasion and deposited during another
and far later glacial stage. The former drift is very young, for its
drainage is as yet immature, and there are many lakes and marshes
upon its surface; the latter is far older, for its surface has been
thoroughly dissected by its streams. The former is but slightly
weathered, while the latter is so old that it is deeply reddened by
oxidation and is leached of its soluble ingredients such as lime.
The younger drift is bordered by a distinct terminal moraine, while
the margin of the older drift is not thus marked. Moreover, the two
drift sheets are somewhat unlike in composition, and the different
proportion of pebbles of the various kinds of rocks which they contain
shows that their respective glaciers followed different tracks and
gathered their loads from different regions. Again, in places beneath
the younger drift there is found the buried land surface of an older
drift with old soils, forest grounds, and vegetable deposits,
containing the remains of animals and plants, which tell of the
climate of the interglacial stage in which they lived.

By such differences as these the following drift sheets have been made
out in America, and similar subdivisions have been recognized in
Europe.

    5 The Wisconsin formation
    4 The Iowan formation
    3 The Illinoian formation
    2 The Kansan formation
    1 The pre-Kansan or Jerseyan formation

In New Jersey and Pennsylvania the edge of a deeply weathered and
eroded drift sheet, the Jerseyan, extends beyond the limits of a much
younger overlying drift. It may be the equivalent of a deep-buried
basal drift sheet found in the Mississippi valley beneath the Kansan
and parted from it by peat, old soil, and gravel beds.

The two succeeding stages mark the greatest snowfall of the Glacial
epoch. In Kansan times the Keewatin ice field slowly grew southward
until it reached fifteen hundred miles from its center of dispersion
and extended from the Arctic Ocean to northeastern Kansas. In the
Illinoian stage the Labrador ice field stretched from Hudson Straits
nearly to the Ohio River in Illinois. In the Iowan and the Wisconsin,
the closing stages of the Glacial epoch, the readvancing ice fields
fell far short of their former limits in the Mississippi valley, but
in the eastern states the Labrador ice field during Wisconsin times
overrode for the most part all earlier deposits, and, covering New
England, probably met the ocean in a continuous wall of ice which set
its bergs afloat from Massachusetts to northern Labrador.

We select for detailed description the Kansan and the Wisconsin
formations as representatives, the one of the older and the other of
the younger drift sheets.

   [Illustration: Fig. 362. Photograph of Relief Map of the United
      States at the Time of the Wisconsin Ice Invasion

   By the courtesy of E. E. Howell, Washington, D.C.]

=The Kansan formation.= The Kansan drift consists for the most part of
a sheet of clayey till carrying smaller bowlders than the later drift.
Few traces of drumlins, kames, or terminal moraines are found upon the
Kansan drift, and where thick enough to mask the preexisting surface,
it seems to have been spread originally in level plains of till.

The initial Kansan plain has been worn by running water until there
are now left only isolated patches and the narrow strips and crests of
the divides, which still rise to the ancient level. The valleys of the
larger streams have been opened wide. Their well-developed tributaries
have carved nearly the entire plain to valley slopes (Figs. 50 B, and
59). The lakes and marshes which once marked the infancy of the region
have long since been effaced. The drift is also deeply weathered. The
till, originally blue in color, has been yellowed by oxidation to
a depth of ten and twenty feet and even more, and its surface is
sometimes rusted to terra-cotta red. To a somewhat less depth it has
been leached of its lime and other soluble ingredients. In the
weathered zone its pebbles, especially where the till is loose in
texture, are sometimes so rotted that granites may be crumbled with
the fingers. The Kansan drift is therefore old.

   [Illustration: Fig. 363. Plain of Wisconsin Drift, Iowa]

=The Wisconsin formation.= The Wisconsin drift sheet is but little
weathered and eroded, and therefore is extremely young. Oxidation has
effected it but slightly, and lime and other soluble plant foods
remain undissolved even at the grass roots. Its river systems are
still in their infancy (Fig. 50, A). Swamps and peat bogs are abundant
on its undrained surface, and to this drift sheet belong the lake
lands of our northern states and of the Laurentian peneplain of
Canada.

The lake basins of the Wisconsin drift are of several different
classes. Many are shallow sags in the ground moraine. Still more
numerous are the lakes set in hollows among the hills of the terminal
moraines; such as the thousands of lakelets of eastern Massachusetts.
Indeed, the terminal moraines of the Wisconsin drift may often be
roughly traced on maps by means of belts of lakes and ponds. Some
lakes are due to the blockade of ancient valleys by morainic débris,
and this class includes many of the lakes of the Adirondacks, the
mountain regions of New England, and the Laurentian area. Still other
lakes rest in rock basins scooped out by glaciers. In many cases lakes
are due to more than one cause, as where preglacial valleys have both
been basined by the ice and blockaded by its moraines. The Finger
lakes of New York, for example, occupy such glacial troughs.

Massive _terminal moraines_, which mark the farthest limits to which
the Wisconsin ice advanced, have been traced from Cape Cod and
the islands south of New England, across the Appalachians and the
Mississippi valley, through the Dakotas, and far to the north over the
plains of British America. Where the ice halted for a time in its
general retreat, it left _recessional moraines_, as this variety of
the terminal moraine is called. The moraines of the Wisconsin drift
lie upon the country like great festoons, each series of concentric
loops marking the utmost advance of broad lobes of the ice margin and
the various pauses in their recession.

Behind the terminal moraines lie wide till plains, in places studded
thickly with drumlins, or ridged with an occasional esker. Great
outwash plains of sand and gravel lie in front of the moraine belts,
and long valley trains of coarse gravels tell of the swift and
powerful rivers of the time.

=The loess of the Mississippi valley.= A yellow earth, quite like
the loess of China, is laid broadly as a surface deposit over
the Mississippi valley from eastern Nebraska to Ohio outside the
boundaries of the Iowan and the Wisconsin drift. Much of the loess was
deposited in Iowan times. It is younger than the earlier drift sheets,
for it overlies their weathered and eroded surfaces. It thickens to
the Iowan drift border, but is not found upon that drift. It is older
than the Wisconsin, for in many places it passes underneath the
Wisconsin terminal moraines. In part the loess seems to have been
washed from glacial waste and spread in sluggish glacial waters, and
in part to have been distributed by the wind from plains of aggrading
glacial streams.

   [Illustration: Fig. 364. Bank of Loess, Iowa]

=The effects of the ice invasions on rivers.= The repeated ice
invasions of the Pleistocene profoundly disarranged the drainage
systems of our northern states. In some regions the ancient valleys
were completely filled with drift. On the withdrawal of the ice the
streams were compelled to find their way, as best they could, over a
fresh land surface, where we now find them flowing on the drift in
young, narrow channels. But hundreds of feet below the ground the
well driller and the prospector for coal and oil discover deep,
wide, buried valleys cut in rock,--the channels of preglacial and
interglacial streams. In places the ancient valleys were filled with
drift to a depth of a hundred feet, and sometimes even to a depth of
four hundred and five hundred feet. In such valleys, rivers now flow
high above their ancient beds of rock on floors of valley drift. Many
of the valleys of our present rivers are but patchworks of preglacial,
interglacial, and postglacial courses (Fig. 366). Here the river winds
along an ancient valley with gently sloping sides and a wide alluvial
floor perhaps a mile or so in width, and there it enters a young,
rock-walled gorge, whose rocky bed may be crossed by ledges over which
the river plunges in waterfalls and rapids.

   [Illustration: Fig. 365. Preglacial Drainage, Upper Ohio Valley

   After Chamberlain and Leverett]

   [Illustration: Fig. 366. A Patchwork Valley

   _a_ and _a´_, ancient courses still occupied by the river;
   _b_, postglacial gorge; _c_, ancient course now filled with drift]

In such cases it is possible that the river was pushed to one side
of its former valley by a lobe of ice, and compelled to cut a new
channel in the adjacent uplands. A section of the valley may have been
blockaded with morainic waste, and the lake formed behind the barrier
may have found outlet over the country to one side of the ancient
drift-filled valley. In some instances it would seem that during the
waning of the ice sheets, glacial streams, while confined within walls
of stagnant ice, cut down through the ice and incised their channels
on the underlying country, in some cases being let down on old river
courses, and in other cases excavating gorges in adjacent uplands.

=Pleistocene lakes.= Temporary lakes were formed wherever the ice
front dammed the natural drainage of the region. Some, held in the
minor valleys crossed by ice lobes, were small, and no doubt many were
too short-lived to leave lasting records. Others, long held against
the northward sloping country by the retreating ice edge, left in
their beaches their clayey beds, and their outlet channels permanent
evidences of their area and depth. Some of these glacial lakes are
thus known to have been larger than any present lake.

Lake Agassiz, named in honor of the author of the theory of
continental glaciation, is supposed to have been held by the united
front of the Keewatin and the Labrador ice fields as they finally
retreated down the valley of the Red River of the North and the
drainage basin of Lake Winnipeg. From first to last Lake Agassiz
covered a hundred and ten thousand square miles in Manitoba and the
adjacent parts of Minnesota and North Dakota,--an area larger than all
the Great Lakes combined. It discharged its waters across the divide
which held it on the south, and thus excavated the valley of the
Minnesota River. The lake bed--a plain of till--was spread smooth and
level as a floor with lacustrine silts. Since Lake Agassiz vanished
with the melting back of the ice beyond the outlet by the Nelson River
into Hudson Bay, there has gathered on its floor a deep humus, rich in
the nitrogenous elements so needful for the growth of plants, and it
is to this soil that the region owes its well-known fertility.

=The Great Lakes.= The basins of the Great Lakes are broad preglacial
river valleys, warped by movements of the crust still in progress,
enlarged by the erosive action of lobes of the continental ice sheets,
and blockaded by their drift. The complicated glacial and postglacial
history of the lakes is recorded in old strand lines which have been
traced at various heights about them, showing their areas and the
levels at which their waters stood at different times.

With the retreat of the lobate Wisconsin ice sheet toward the north
and east, the southern and western ends of the basins of the Great
Lakes were uncovered first; and here, between the receding ice front
and the slopes of land which faced it, lakes gathered which increased
constantly in size.

The lake which thus came to occupy the western end of the Lake
Superior basin discharged over the divide at Duluth down the St. Croix
River, as an old outlet channel proves; that which held the southern
end of the basin of Lake Michigan sent its overflow across the divide
at Chicago via the Illinois River to the Mississippi; the lake which
covered the lowlands about the western end of Lake Erie discharged its
waters at Fort Wayne into the Wabash River.

The ice still blocked the Mohawk and St. Lawrence valleys on the east,
while on the west it had retreated far to the north. The lakes become
confluent in wide expanses of water, whose depths and margins, as
shown by their old lake beaches, varied at different times with the
position of the confining ice and with warpings of the land. These
vast water bodies, which at one or more periods were greater than all
the Great Lakes combined, discharged at various times across the
divide at Chicago, near Syracuse, New York, down the Mohawk valley,
and by a channel from Georgian Bay into the Ottawa River. Last of all
the present outlet by the St. Lawrence was established.

The beaches of the glacial lakes just mentioned are now far from
horizontal. That of the lake which occupied the Ontario basin has an
elevation of three hundred and sixty-two feet above tide at the west
and of six hundred and seventy-five feet at the northeast, proving
here a differential movement of the land since glacial times amounting
to more than three hundred feet. The beaches which mark the successive
heights of these glacial lakes are not parallel; hence the warping
began before the Glacial epoch closed. We have already seen that the
canting of the region is still in progress.

=The Champlain subsidence.= As the Glacial epoch approached its end,
and the Labrador ice field melted back for the last time to near its
source, the land on which the ice had lain in eastern North America
was so depressed that the sea now spread far and wide up the St.
Lawrence valley. It joined with Lake Ontario, and extending down the
Champlain and Hudson valleys, made an island of New England and the
maritime provinces of Canada.

The proofs of this subsidence are found in old sea beaches and
sea-laid clays resting on Wisconsin till. At Montreal such terraces
are found six hundred and twenty feet above sea level, and along Lake
Champlain--where the skeleton of a whale was once found among them--at
from five hundred to four hundred feet. The heavy delta which the
Mohawk River built at its mouth in this arm of the sea now stands
something more than three hundred feet above sea level. The clays of
the Champlain subsidence pass under water near the mouth of the
Hudson, and in northern New Jersey they occur two hundred feet below
tide. In these elevations we have measures of the warping of the
region since glacial times.

=The western United States in glacial times.= The western United
States was not covered during the Pleistocene by any general ice
sheet, but all the high ranges were capped with permanent snow and
nourished valley glaciers, often many times the size of the existing
glaciers of the Alps. In almost every valley of the Sierras and the
Rockies the records of these vanished ice streams may be found in
cirques, glacial troughs, roches moutonnées, and morainic deposits.

It was during the Glacial epoch that Lakes Bonneville and Lahontan
were established in the Great Basin, whose climate must then have been
much more moist than now.

   [Illustration: Fig. 367. A Valley in the Driftless Area]

=The driftless area.= In the upper Mississippi valley there is an
area of about ten thousand square miles in southwestern Wisconsin
and the adjacent parts of Iowa and Minnesota, which escaped the ice
invasions. The rocks are covered with residual clays, the product of
long preglacial weathering. The region is an ancient peneplain,
uplifted and dissected in late Tertiary times, with mature valleys
whose gentle gradients are unbroken by waterfalls and rapids. Thus the
driftless area is in strong contrast with the immature drift topography
about it, where lakes and waterfalls are common. It is a bit of
preglacial landscape, showing the condition of the entire region before
the Glacial epoch.

The driftless area lay to one side of the main track of both the
Keewatin and the Labrador ice fields, and at the north it was
protected by the upland south of Lake Superior, which weakened and
retarded the movement of the ice.

South of the driftless area the Mississippi valley was invaded at
different times by ice sheets from the west,--the Kansan and the
Iowan,--and again by the Illinoian ice sheet from the east. Again and
again the Mississippi River was pushed to one side or the other of its
path. The ancient channel which it held along the Illinoian ice front
has been traced through southeastern Iowa for many miles.

   [Illustration: Fig. 368. Cross Section of a Valley in Eastern Iowa

   _a_, country rock; _b_, Kansan till; _c_, loess; _t_, terrace
   of reddish sands and decayed pebbles above reach of present
   stream; _s_, stream; _fp_, flood plain of _s_. What is the age
   of rock-cut valley and of the alluvium which partially fills
   it, compared with that of the Kansan till? with that of the
   loess? Give the complete history recorded in the section.]

=Benefits of glaciation.= Like the driftless area, the preglacial
surface over which the ice advanced seems to have been well dissected
after the late Tertiary uplifts, and to have been carved in many
places to steep valley slopes and rugged hills. The retreating ice
sheets, which left smooth plains and gently rolling country over the
wide belt where glacial deposition exceeded glacial erosion, have made
travel and transportation easier than they otherwise would have been.

The preglacial subsoils were residual clays and sands, composed of the
insoluble elements of the country rock of the locality, with some
minglings of its soluble parts still undissolved. The glacial subsoils
are made of rocks of many kinds, still undecayed and largely ground to
powder. They thus contain an inexhaustible store of the mineral foods
of plants, and in a form made easily ready for plant use.

On the preglacial hillsides the humus layer must have been
comparatively thin, while the broad glacial plains have gathered deep
black soils, rich in carbon and nitrogen taken from the atmosphere.
To these soils and subsoils a large part of the wealth and prosperity
of the glaciated regions of our country must be attributed.

The ice invasions have also added very largely to the water power of
the country. The rivers which in preglacial times were flowing over
graded courses for the most part, were pushed from their old valleys
and set to flow on higher levels, where they have developed waterfalls
and rapids. This power will probably be fully utilized long before the
coal beds of the country are exhausted, and will become one of the
chief sources of the national wealth.

=The Recent epoch.= The deposits laid since glacial times graduate
into those now forming along the ocean shores, on lake beds, and in
river valleys. Slow and comparatively slight changes, such as the
warpings of the region of the Great Lakes, have brought about the
geographical conditions of the present. The physical history of the
Recent epoch needs here no special mention.


The Life of the Quaternary

During the entire Quaternary, invertebrates and plants suffered little
change in species,--so slowly are these ancient and comparatively
simple organisms modified. The Mammalia, on the other hand, have
changed much since the beginning of Quaternary time: the various
species of the present have been evolved, and some lines have become
extinct. These highly organized vertebrates are evidently less stable
than are lower types of animals, and respond more rapidly to changes
in the environment.

=Pleistocene mammals.= In the Pleistocene the Mammalia reached their
culmination both in size and in variety of forms, and were superior
in both these respects to the mammals of to-day. In Pleistocene times
in North America there were several species of bison,--one whose
widespreading horns were ten feet from tip to tip,--a gigantic moose
elk, a giant rodent (Castoroides) five feet long, several species of
musk oxen, several species of horses,--more akin, however, to zebras
than to the modern horse,--a huge lion, several saber-tooth tigers,
immense edentates of several genera, and largest of all the mastodon
and mammoth.

   [Illustration: Fig. 369. Megatherium]

   [Illustration: Fig. 370. Glyptodon]

The largest of the edentates was the Megatherium, a. clumsy ground
sloth bigger than a rhinoceros. The bones of the Megatherium are
extraordinarily massive,--the thigh bone being thrice as thick as
that of an elephant,--and the animal seems to have been well able to
get its living by overthrowing trees and stripping off their leaves.
The Glyptodon was a mailed edentate, eight feet long, resembling the
little armadillo. These edentates survived from Tertiary times, and in
the warmer stages of the Pleistocene ranged north as far as Ohio and
Oregon.

The great proboscidians of the Glacial epoch were about the size of
modern elephants, and somewhat smaller than their ancestral species in
the Pliocene. The _Mastodon_ ranged over all North America south of
Hudson Bay, but had become extinct in the Old World at the end of the
Tertiary. The elephants were represented by the _Mammoth_, which
roamed in immense herds from our middle states to Alaska, and from
Arctic Asia to the Mediterranean and Atlantic.

It is an oft-told story how about a century ago, near the Lena River
in Siberia, there was found the body of a mammoth which had been
safely preserved in ice for thousands of years, how the flesh was
eaten by dogs and bears, and how the eyes and hoofs and portions of
the hide were taken with the skeleton to St. Petersburg. Since then
several other carcasses of the mammoth, similarly preserved in ice,
have been found in the same region,--one as recently as 1901. We know
from these remains that the animal was clothed in a coat of long,
coarse hair, with thick brown fur beneath.

   [Illustration: Fig. 371. Skull of Musk Ox, from Pleistocene
      Deposits, Iowa]

=The distribution of animals and plants.= The distribution of species
in the Glacial epoch was far different from that of the present. In
the glacial stages arctic species ranged south into what are now
temperate latitudes. The walrus throve along the shores of Virginia
and the musk ox grazed in Iowa and Kentucky. In Europe the reindeer
and arctic fox reached the Pyrenees. During the Champlain depression
arctic shells lived along the shore of the arm of the sea which
covered the St. Lawrence valley. In interglacial times of milder
climate the arctic fauna-flora retreated, and their places were taken
by plants and animals from the south. Peccaries, now found in Texas,
ranged into Michigan and New York, while great sloths from South
America reached the middle states. Interglacial beds at Toronto,
Canada, contain remains of forests of maple, elm, and papaw, with
mollusks now living in the Mississippi basin.

What changes in the forests of your region would be brought about, and
in what way, if the climate should very gradually grow colder? What
changes if it should grow warmer?

On the Alps and the highest summits of the White Mountains of New
England are found colonies of arctic species of plants and insects.
How did they come to be thus separated from their home beyond the
arctic circle by a thousand miles and more of temperate climate
impossible to cross?

=Man.= Along with the remains of the characteristic animals of the
time which are now extinct there have been found in deposits of the
Glacial epoch in the Old World relics of Pleistocene _Man_, his bones,
and articles of his manufacture. In Europe, where they have best been
studied, human relics occur chiefly in peat bogs, in loess, in caverns
where man made his home, and in high river terraces sometimes eighty
and a hundred feet above the present flood plains of the streams.

In order to understand the development of early man, we should know
that prehistoric peoples are ranked according to the materials of
which their tools were made and the skill shown in their manufacture.
There are thus four well-marked stages of human culture preceding the
written annals of history:

    4 The Iron stage.
    3 The Bronze stage.
    2 The Neolithic (recent stone) stage.
    1 The Paleolithic (ancient stone) stage.

In the Neolithic stage the use of the metals had not yet been learned,
but tools of stone were carefully shaped and polished. To this stage
the North American Indian belonged at the time of the discovery of the
continent. In the Paleolithic stage, stone implements were chipped to
rude shapes and left unpolished. This, the lowest state of human
culture, has been outgrown by nearly every savage tribe now on earth.
A still earlier stage may once have existed, when man had not learned
so much as to shape his weapons to his needs, but used chance pebbles
and rock splinters in their natural forms; of such a stage, however,
we have no evidence.

   [Illustration: Fig. 372. Paleolithic Implement from Great Britain]

=Paleolithic man in Europe.= It was to the Paleolithic stage that the
earliest men belonged whose relics are found in Europe. They had
learned to knock off two-edged flakes from flint pebbles, and to work
them into simple weapons. The great discovery had been made that fire
could be kindled and made use of, as the charcoal and the stones
discolored by heat of their ancient hearths attest. Caves and shelters
beneath overhanging cliffs were their homes or camping places.
Paleolithic man was a savage of the lowest type, who lived by hunting
the wild beasts of the time.

Skeletons found in certain caves in Belgium and France represent
perhaps the earliest race yet found in Europe. These short,
broad-shouldered men, muscular, with bent knees and stooping gait,
low-browed and small of brain, were of little intelligence and yet
truly human.

The remains of Pleistocene man are naturally found either in caverns,
where they escaped destruction by the ice sheets, or in deposits
outside the glaciated area. In both cases it is extremely difficult,
or quite impossible, to assign the remains to definite glacial or
interglacial times. Their relative age is best told by the fauna with
which they are associated. Thus the oldest relics of man are found
with the animals of the late Tertiary or early Quaternary, such as a
species of hippopotamus and an elephant more ancient than the mammoth.
Later in age are the remains found along with the mammoth, cave bear
and cave hyena, and other animals of glacial time which are now
extinct; while more recent still are those associated with the
reindeer, which in the last ice invasion roamed widely with the
mammoth over central Europe.

   [Illustration: Fig. 373. Paleolithic Sketch on Ivory of the Mammoth]

=The caves of southern France.= These contain the fullest records of
the race, much like the Eskimos in bodily frame, which lived in
western Europe at the time of the mammoth and the reindeer. The floors
of these caves are covered with a layer of bone fragments, the remains
of many meals, and here are found also various articles of handicraft.
In this way we know that the savages who made these caves their homes
fished with harpoons of bone, and hunted with spears and darts tipped
with flint and horn. The larger bones are split for the extraction of
the marrow. Among such fragments no split human bones are found; this
people, therefore, were not cannibals. Bone needles imply the art of
sewing, and therefore the use of clothing, made no doubt of skins;
while various ornaments, such as necklaces of shells, show how ancient
is the love of personal adornment. Pottery was not yet invented. There
is no sign of agriculture. No animals had yet been domesticated; not
even man's earliest friend, the dog. Certain implements, perhaps used
as the insignia of office, suggest a rude tribal organization and the
beginnings of the state. The remains of funeral feasts in front of
caverns used as tombs point to a religion and the belief in a life
beyond the grave. In the caverns of southern France are found also the
beginnings of the arts of painting and of sculpture. With surprising
skill these Paleolithic men sketched on bits of ivory the mammoth with
his long hair and huge curved tusks, frescoed their cavern walls with
pictures of the bison and other animals, and carved reindeer on their
dagger heads.

   [Illustration: Fig. 374. Restoration of Head of Pithecanthropus
      erectus]

=Early man on other continents.= Paleolithic flints curiously like
those of western Europe are found also in many regions of the Old
World,--in India, Egypt, and Asia Minor,--beneath the earliest
vestiges of the civilization of those ancient seats, and sometimes
associated with the fauna of the Glacial epoch.

In Java there were found in 1891, in strata early Quaternary or late
Pliocene in age, parts of a skeleton of lower grade, if not of greater
antiquity, than any human remains now known. _Pithecanthropus erectus_,
as the creature has been named, walked erect, as its thigh bone shows,
but the skull and teeth indicate a close affinity with the ape.

In North America there have been reported many finds of human relics
in valley trains, loess, old river gravels buried beneath lava flows,
and other deposits of supposed glacial age; but in the opinion of some
geologists sufficient proof of the existence of man in America in
glacial times has not as yet been found.

These finds in North America have been discredited for various
reasons. Some were not made by scientific men accustomed to the
closest scrutiny of every detail. Some were reported after a number of
years, when the circumstances might not be accurately remembered;
while in a number of instances it seems possible that the relics might
have been worked into glacial deposits by natural causes from the
surface.

Man, we may believe, witnessed the great ice fields of Europe, if not
of America, and perhaps appeared on earth under the genial climate
of preglacial times. Nothing has yet been found of the line of man's
supposed descent from the primates of the early Tertiary, with the
possible exception of the Java remains just mentioned. The structures
of man's body show that he is not descended from any of the existing
genera of apes. And although he may not have been exempt from the law
of evolution,--that method of creation which has made all life on
earth akin,--yet his appearance was an event which in importance
ranks with the advent of life upon the planet, and marks a new
manifestation of creative energy upon a higher plane. There now
appeared intelligence, reason, a moral nature, and a capacity for
self-directed progress such as had never been before on earth.

=The Recent epoch.= The Glacial epoch ends with the melting of the
ice sheets of North America and Europe, and the replacement of the
Pleistocene mammalian fauna by present species. How gradually the one
epoch shades into the other is seen in the fact that the glaciers
which still linger in Norway and Alaska are the lineal descendants or
the renewed appearances of the ice fields of glacial times.

Our science cannot foretell whether all traces of the Great Ice Age
are to disappear, and the earth is to enjoy again the genial climate
of the Tertiary, or whether the present is an interglacial epoch and
the northern lands are comparatively soon again to be wrapped in ice.

=Neolithic man.= The wild Paleolithic men vanished from Europe with
the wild beasts which they hunted, and their place was taken by
tribes, perhaps from Asia, of a higher culture. 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 the beds of lakes near shore where they
sometimes built their dwellings upon piles.

The successive stages in European culture are well displayed in the
peat bogs of Denmark. The lowest layers contain the polished _stone_
implements of Neolithic man, along with remains of the _Scotch fir_.
Above are _oak_ trunks with implements of _bronze_, while the higher
layers hold _iron_ weapons and the remains of a _beech_ forest.

Neolithic man in Europe had learned to make pottery, to spin and weave
linen, to hew timbers and build boats, and to grow wheat and barley.
The dog, horse, ox, sheep, goat, and hog had been domesticated, and,
as these species are not known to have existed before in Europe, it is
a fair inference that they were brought by man from another continent
of the Old World. Neolithic man knew nothing of the art of extracting
the metals from their ores, nor had he a written language.

The Neolithic stage of culture passes by insensible gradations into
that of the age of bronze, and thus into the Recent epoch.

In the Recent epoch the progress of man in language, in social
organization, in the arts of life, in morals and religion, has left
ample records which are for other sciences than ours to read; here,
therefore, geology gives place to archæology and history.

Our brief study of the outlines of geology has given us, it is hoped,
some great and lasting good. To conceive a past so different from the
present has stimulated the imagination, and to follow the inferences
by which the conclusions of our science have been reached has
exercised one of the noblest faculties of the mind,--the reason. We
have learned to look on nature in new ways: every landscape, every
pebble now has a meaning and tells something of its origin and
history, while plants and animals have a closer interest since we have
traced the long lines of their descent. The narrow horizons of human
life have been broken through, and we have caught glimpses of that
immeasurable reach of time in which nebulae and suns and planets run
their courses. Moreover, we have learned something of that orderly and
world-embracing progress by which the once uninhabitable globe has
come to be man's well-appointed home, and life appearing in the
lowliest forms has steadily developed higher and still higher types.
Seeing this process enter human history and lift our race continually
to loftier levels, we find reason to believe that the onward, upward
movement of the geological past is the manifestation of the same wise
Power which makes for righteousness and good and that this unceasing
purpose will still lead on to nobler ends.




INDEX


  Aa, lava, 241
  Acadian coal field, 354
  Accretion hypothesis, 304
  Acidic rocks, 249
  Adelsberg grotto, 47
  Adirondacks, 309, 316
  Africa, 357
  Agassiz, Lake, 67, 111, 435
  Agates, 251
  Alabama, 317, 360
  Alaska, 85, 138, 140, 378
  Aletsch glacier, 121
  Algæ, 51, 52
  Algonkian era, 306, 310
  Allegheny Mountains, 90, 224, 326, 403
  Alluvial cones, 98
  Alluvium, 62
  Alps, 118, 121, 141, 210, 211, 212, 223, 229, 349, 427, 443
  Amazon River, 175
  Ammonites, 294, 367, 380, 382
  Amphibians, 364, 383
  Amphicyon, 413
  Amygdules, 250
  Andes, 236, 279
  Angle of repose, 25
  Antarctic continent, 294
  Antecedent streams, 209
  Antelope, 413
  Anthracite, 281
  Anticlinal folds, 203, 209
  Ants, 20
  Apennine Mountains, 399
  Appalachia, 317, 351, 358
  Appalachian coal field, 356
  Appalachian deformation, 358
  Appalachian Mountains, 211, 214, 218, 292
  Aquifer, 44
  Aragonite, 296
  Archæopteryx, 393
  Archean era, 305
  Arenaceous rocks, 9
  Argillaceous rocks, 9
  Arizona, 32, 76, 140, 151, 164, 220, 229, 249, 257, 371, 390
  Arkansas, 337, 356, 373
  Arkose, 186, 282, 370
  Artesian wells, 44
  Arthropods, 322
  Artiodactyls, 411
  Assiniboine, Mount, 34
  Atlas Mountains, 399
  Atmosphere, 304, 305
  Atolls, 191, 193
  Augite, 274
  Austin, Tex., 71
  Australia, 190, 357
  Avalanches, 26

  Bad Lands, 397, 398
  Baltic Sea, 170, 171, 199
  Barite, 287
  Barrier Reefs, 191, 192
  Basal conglomerate, 173, 184
  Basalt, 249
  Baselevel, 80, 83
  Basic rocks, 249
  Basin deposits, 103
  Bay bars, 164
  Beaches, 162, 164
  Bears, 413
  Bedding planes, 5
  Belemnites, 382
  Belt Mountains, 309
  Bergschrund, 121, 135, 137
  Bermudas, 148
  Birds, 392
  Bison, 413
  Bitter Root Mountains, 272
  Black Hills, 309, 371
  Blastoids, 339
  Blastosphere, 311
  Block mountains, 222, 226
  Blowholes, 159
  Blue Ridge, 309, 316
  Bomb, volcanic, 256
  Bonneville, Lake, 107, 488
  Bosses, 270
  Bowlders, erratic, 420
      of weathering, 28
  Brachiopods, 328, 383, 343, 364, 380
  Brazil, 18, 286
  Breccia, 218, 255, 264
  British Columbia, 373, 378
  Bronze stage, 443, 448
  Bryozoans, 333
  Bunker Hill, 422

  Calamites, 361, 367
  Calcareous rocks, 9
  Calciferous series, 327
  Calcite, 290
  Caldera, 239
  California, 24, 99, 186, 152, 158, 169, 170, 197, 224, 256, 262, 287,
     357, 360, 371, 400
  Great Valley of, 101, 199, 372, 396
  Cambrian period, 315
      glaciation in, 358
      life of, 319
  Camels, 412
  Canada, 28, 86, 67, 69, 90, 182, 198, 200, 218, 218, 267, 307, 309,
     316, 336, 364, 367, 482, 487
  Cape Breton Island, 198
  Cape Cod, 162
  Carbonated springs, 261
  Carbonates, formation of, 12
  Carboniferous period, 350
      life of, 301
  Carnivores, 418
  Cascade Mountains, 00, 400
  Cats, 418
  Catskill Mountains, 342
  Caucasus Mountains, 399
  Caverns, 46, 241
  Cenozoic era, 394
  Centipedes, 388
  Cephalopods, 324, 388, 389, 344, 367, 380
  Ceratites, 380
  Ceratosaurus, 385
  Chain coral, 389, 843
  Chalcopyrite, 287
  Chalk, 9, 374, 375
  Chalybeate springs, 62
  Champlain subsidence, 487
  Charleston earthquake, 288
  Chazy series, 327
  Chelan, Lake, 141
  Chemung series, 341, 342
  Chesapeake Bay, 169, 170, 197
  Chicago, 146, 198, 486
  Chile, 286
  China, 28, 161
  Christmas Island, 194, 248
  Cincinnati anticline, 329, 366
  Cirques, 136
  Clinton series, 336
  Coal, 362, 370, 375
  Coal Measures, 351
  Coast Range, 101, 372, 399
  Coastal plain, Atlantic, 188
  Coelenterates, 320
  Coke, 271
  Colorado, 18, 29, 88, 87, 158, 288, 266, 271, 334
  Colorado plateaus, 357, 403
  Colorado River, 80, 76, 140, 154, 228, 307, 318, 317
  Columbia lavas, 400
  Columnar structure, 268
  Concretions, 49
  Cones, alluvial, 98
      volcanic, 267
  Conglomerate, 9, 178
  Congo River, 175
  Conifers, 377
  Connecticut, 370
      valley, 408
  Contemporaneous lava sheets, 248, 268
  Continental delta, 176, 183
  Continental shelf, 183
  Continents, 188
  Contours, 60
  Copper, 287, 310
  Coquina, 177
  Coral reefs, 188
  Corals, ancient, 321, 332, 338, 379
  Cordaites, 363
  Cordilleran ice field, 426
  Corniferous series, 341
  Coves, 161
  Crabs, 379
  Crandall volcano, 268, 400
  Crater Lake, 259
  Creodonts, 418
  Cretaceous period, 372
  Crinoids, 382, 303, 379
  Crocodiles, 384
  Cross bedding, 65, 182
  Crustacea, 322, 382, 368, 379
  Crustal movements, 195
  Cumberland plateau, 90
  Cup corals, 388
  Cycads, 377, 378
  Cycle of erosion, 84, 185, 292
  Cystoids, 321, 382, 367

  Dalmatia, 170
  Darwin's theory of coral reefs, 191
  Dead Sea, 221, 279
  Death Gulch, 264
  Deep-sea deposits, 187
  Deer, 413
  Deflation, 152
  Deformation, 279
  Delaware River, 197, 403
  Deltas, 108, 111, 197
      of Ganges, 109
      of Indus, 110
      of Mississippi, 109, 197
  Denudation, 57
  Denver, 398
  Desert, 15, 55
  Devitrification, 257
  Devonian period, 316, 341
  Dicotyls, 377, 404
  Digitigrade, 406
  Dikes, 244, 265
  Dinosaurs, 385
  Dinothere, 410
  Diorite, 274
  Dip, 202
  Dip fault, 225
  Diplodocus, 286
  Dipnoans, 346
  Discina, 324
  Dismal Swamp, 106
  Dogs, 413
  Dragon flies, 364
  Drift, 18, 113, 416
      bowlders of, 420
      englacial, 125
      extent of, 425
      pebbles of, 114, 420
      stratified, 423
      thickness of, 429
  Driftless area, 438
  Drowned valleys, 197
  Drumlins, 421
  Duluth, 436
  Dunes, 147
  Dust falls, 145

  Earth, age of, 292, 298, 302
      interior of, 276
  Earthquakes, 224, 233
      causes of, 233, 237
  Charleston, 233
      distribution of, 236
      geological effects of, 234
      India, 236
      Japan, 237
      New Madrid, 236
  Earthworms, 20, 21
  Echinoderms, 321, 332, 333, 343, 363
  Edentates, 441
  Egypt, 98
  Electric Peak, 269
  Elephants, 410
  Elevation, effects of, 85
      movements of, 197
  Eocene epoch, 395
  Epicontinental seas, 318
  Erratics, 133, 420
  Eskers, 424
  Etna, 248, 402
  Europe, Pleistocene ice sheet of, 427
  Eurypterids, 333, 339, 363, 367
  Evolution, 300, 447

  Faceted pebbles, 113, 114, 420
  Falls of the Ohio, 343
  Fan folds, 205
  Fault scarps, 219
  Faults, 217
  Faunas, 299
  Feldspar, 9, 10, 42
  Ferns, 361
  Finger lakes, 432
  Fire clay, 353
  Fishes, 334, 339, 345, 364, 405
  Fissure eruptions, 242
  Fissure springs, 44
  Fjords, 139, 142
  Flint, 18, 375
  Flood plains, 85, 93
  Floods, 54
  Floras, 299
  Florida, 46, 163, 177, 178, 188, 396
  Flow lines, 252
  Fluorite, 287
  Folded mountains, 210
  Folds, 201, 208
  Foliation, 283
  Foraminifera, 187, 374:
  Forests, Carboniferous, 354, 361
      Cretaceous, 377, 378
      Devonian, 343
      Tertiary, 404
  Fort Wayne, 436
  Fossils, 177, 296
  Fractures, 215
  Fragmental rocks, 8
  France, 167, 171
      cave men of, 445
  Fringing reefs, 190
  Frogs, 383
  Frost, 15
  Fundy, Bay of, 182

  Gabbro, 274
  Ganges, 58, 109, 197
  Ganoids, 347
  Garnet, 281
  Gases, volcanic, 244
  Gastropods, 324
  Gastrula, 311
  Geneva, Lake, 71
  Geodes, 49
  Geological time, divisions of, 295
  Geology, definition of, 1, 3
      departments of, 4
  Georgia, 18, 373
  Geysers, 52, 260
  Glacial epoch, 142, 416
  Glaciers, 113
      abrasion by, 133
      Alpine, 118
      compared with rivers, 137, 142
      crevasses of, 123
      deposition by, 138
      Greenland, 116
      lower limit of, 129
      melting of, 126
      mode of formation, 118
      moraines, 124
      motion of, 120, 122, 134
      piedmont, 131, 141
      plucking by, 133
      tables, 130
      transportation by, 132
      troughs, 137
      wells, 129
      young and mature, 129
  Glauconite, 176
  Globigerina ooze, 187
  Glyptodon, 441
  Gneiss, 283
  Goats, 413
  Gold, 287, 372
  Goniatite, 344, 367
  Graded slopes, 25
  Granite, 9, 274
  Graphite, 312
  Graptolites, 320, 339
  Gravitation, 22
  Great Basin, 357, 360, 374, 376
  Great Lakes, 198, 436
  Great Plains, 82
  Great Salt Lake, 107
  Greenland, 115, 126, 378
  Green Mountains, 309, 316, 420
  Green sand, 176
  Ground water, 39
  Ground water surface, 40
  Gryphæa, 379
  Gymnosperms, 363, 377
  Gypsum, 12, 335, 357, 371

  Hade, 217
  Hamilton series, 341
  Hanging valley, 1389
  Hanging wall, 217
  Hartz Mountains, 214
  Hawaiian volcanoes, 238, 248, 258, 279
  Heat and cold, 13
  Helderberg series, 341
  Hematite, 310
  Henry Mountains, 271, 376
  High Plains, 100, 398
  Hillers Mountain, 271
  Himalaya Mountains, 122, 209, 210, 399
  Historical geology, 4, 291
  Honeycomb corals, 339
  Hood, Mount, 260, 262
  Hooks, 165
  Hornblende, 274
  Hornblende schist, 284
  Hudson Bay, 90, 170
  Hudson River, 197, 417
  Hudson series, 327, 329
  Humus acids, 10
  Humus layer, 19
  Huronian systems, 308
  Hwang-ho River, 151
  Hydrosphere, 22
  Hydrozoa, 320

  Icebergs, 116, 148
  Iceland, 242, 258
  Ichthyosaurus, 389
  Idaho, 34, 400
  Igneous rocks, 9, 249, 250, 251, 273
  Illinoian formation, 429
  Illinois, 54, 146, 356, 374
  India, 28, 102, 147, 235, 357, 402
  Indian Territory, 356
  Indiana, 48, 104
  Indo-Gangetic plain, 101
  Indus River, 101, 110
  Insects, 333, 364, 380
  Interior of earth, 276
  Internal geological agencies, 195
  Intrusive masses, 270
  Intrusive rocks, 273
  Intrusive sheets, 268
  Inverness earthquake, 236
  Iowa, 29, 69, 73, 80, 86, 336, 356, 374, 431, 433, 439, 442
  Iowan formation, 429
  Iron ores, 13, 53, 279, 310
  Islands, coral, 188
      wave cut, 159, 161

  Japan, 223, 224, 237
  Joints, 5, 31, 216
  Jordan valley, 279
  Jura Mountains, 141, 212
  Jurassic period, 369

  Kame terraces, 424
  Kames, 424
  Kansan formation, 429
  Kansas, 41, 50, 100, 336, 357, 373, 374, 429
  Kaolin, 12
  Karst, 47
  Katahdin, Mount, 420
  Keewatin ice field, 425
  Kentucky, 45, 46, 343, 442
  Keweenawan system, 308, 310
  Kilauea, 239
  Kings River Canyon, 403
  Krakatoa, 245

  Labrador, 198
  Labrador ice field, 426
  Laccolith, 271
  Lagoon, 165, 167
  Lahontan, Lake, 107, 438
  Lake Chelan, 141
  Lake dwellings, 448
  Lake Geneva, 71
  Lake Superior region, 284, 308, 310
  Lakes, 70, 222, 432
      basins, 97, 110, 127, 139, 141, 164, 165, 167, 191, 221, 222, 235,
         259, 423, 432, 435
      deposits, 104
      glacial, 127, 139, 141, 423, 432, 435
      Pleistocene, 435
      salt, 106
  Laminæ, 5
  Landslides, 26, 234
  Lapilli, 255
  Laramie series, 375
  Laurentian peneplain, 84, 308, 432
  Lava, 238, 241
  Lava domes, 243, 400
  Lepidodendron, 362, 367
  Lichens, 16
  Lignite, 271
  Limestone, 7, 177, 178, 190
  Limonite, 13
  Lingulella, 324
  Lithosphere, 21
  Lizards, 384
  Llamas, 412
  Loess, 150, 433
  Long Island, 373
  Louisiana, 336, 396
  Lower Silurian period, 327
  Luray Cavern, 48
  Lycopods, 362

  Magnetite, 279, 310
  Maine, 169, 420
  Malaspina glacier, 181
  Maldive Archipelago, 198
  Mammals, 393, 406, 440
  Mammoth, 442
  Mammoth Cave, 46
  Mammoth Hot Springs, 52
  Man, 414, 443
  Mantle of waste, 17
  Marble, 284, 329
  Marengo Cavern, 48
  Marl, 104
  Marsupials, 393, 406
  Martha's Vineyard, 161, 373, 395
  Maryland, 56, 270
  Massachusetts, 106, 162, 257, 309, 408, 417, 429
  Mastodon, 410, 441, 442
  Matterhorn, 34
  Maturity of land forms, 80
  Mauna Loa, 239
  Meanders, 96
  Medina series, 335, 403
  Megatherium, 441
  Mendota, Lake, 71
  Mesa, 31, 32, 153
  Mesozoic era, 369
  Mesozoic peneplain, 376, 403
  Metamorphism, 281
  Mexico, 373, 375
  Mica, 9
  Mica schist, 284
  Michigan, 104, 356, 443
  Michigan, Lake, 149, 198
  Mineral veins, 49, 286
  Minnesota, 97, 426
  Miocene series, 395
  Mississippi, 337
  Mississippi embayment, 373, 374, 395
  Mississippi River, 56, 57, 82, 94, 96, 109
  Mississippian series, 350
  Missouri, 18, 236
  Missouri River, 55, 97
  Mobile Bay, 197
  Mohawk valley, 436, 437
  Molluscous shell deposits, 177
  Mollusks, 324
  Monadnock,83
  Monkeys, 414
  Monoclinal fold, 204
  Monocotyls, 377, 404
  Monotremes, 393, 406
  Montana, 71, 309, 313, 373
  Montreal, 268, 437
  Monuments, 33
  Moraines, 124
  Mosasaurs, 390
  Mountain sheep, 413
  Mountains, age of, 229
      life history of, 212, 215
      origin of, 90, 210, 222
      sculpture of, 33, 137
  Movements of crust, 195
  Muir glacier, 122, 129

  Nantucket, 373
  Naples, 201
  Narragansett Bay, 197
  Natural bridges, 46
  Natural gas, 330
  Natural levees, 93
  Nautilus, 334
  Nebraska, 50, 82, 100, 255, 356
  Nebular hypothesis, 304
  Neolithic man, 443, 448
  Nevada, 104, 107, 222, 288, 289, 360, 400
  Névé, 120
  New Brunswick, 198
  New England, 88, 373, 376, 378, 395, 403, 429, 432, 437
  Newfoundland, 198
  New Jersey, 148, 166, 168, 176, 196, 268, 269, 309, 310, 373, 437
  New Madrid earthquake, 236
  New Mexico, 31, 371, 399
  New York, 60, 90, 309, 327, 329, 335, 336, 360, 421, 422, 423, 424,
     432, 448
  Niagara Falls, 60, 199
  Niagara series, 335
  Nile, 93, 109, 197
  Normal fault, 217
  North Carolina, 106
  North Dakota, 67
  North Sea, 170
  Notochord, 347
  Nova Scotia, 198
  Nunatak, 116, 132

  Ohio, 82, 198, 329, 335, 441
  Ohio River, 55, 82
  Oil, 330
  Olenellus zone, 328
  Olivine, 274
  Oolitic limestone, 178
  Ooze, deep-sea, 131
  Ordovician period, 316, 327
      life of, 331
  Oregon, 222, 262, 400
  Oreodon, 412
  Ores, 287, 290
  Organisms, work of, 16
  Oriskany series, 341
  Ornithostoma, 392
  Orthoceras, 325, 367, 380
  Oscillations, 196
      a cause of, 273
      effect on drainage, 85
  Ostracoderms, 344
  Ottawa River, 90
  Outcrop, 2
  Outliers, 31
  Outwash plains, 425
  Oxidation, 13
  Oyster, 379, 380

  Pahoehoe lava,241
  Palæospondylus, 344
  Paleolithic man, 444
  Paleozoic era, 315
  Palisades of Hudson, 268
  Palms, 377
  Pamir, 15
  Peat, 94, 104
  Peccaries, 412
  Pelecypods, 324
  Pelée, Mt., 246
  Peneplain, 83
      dissected, 86
      Laurentian, 89, 308, 402
      Mesozoic, 376, 403
  Pennsylvania, 35, 211, 257, 357, 359, 403
  Pennsylvanian series, 350, 351
  Perissodactyl, 408
  Perlitic structure, 252
  Permian series, 350, 357, 360, 366
  Petrifaction, 296
  Petroleum, 330, 343
  Phenacodus, 406
  Phyllite, 283
  Phyllopod, 323
  Piedmont Belt, 87, 214, 309, 374
  Piedmont plains, 99
  Pikes Peak, 18
  _Pithecanthropus erectus_, 446
  Placers, 287
  Plains of marine abrasion, 172
  Planation, 81
  Plantigrade, 406
  Platte River, 82
  Playa, 103
  Playa lakes, 104
  Pleistocene epoch, 416
  Plesiosaurus, 389, 390
  Pliocene epoch, 395
  Plucking, 133
  Po River, 58, 197
  Pocono sandstone, 350, 404
  Porosity of rocks, 40
  Porphyritic structure, 252
  Potholes, 59
  Potomac River, 58, 66, 403
  Predentata, 386
  Pre-Kansan formation, 429
  Primates, 414
  Prince Edward Island, 198
  Proboscidians, 410, 441, 442
  Pteropods, 325
  Pterosaurs, 391
  Puget Sound, 396
  Pumice, 250
  Pyrite, 13

  Quarry water, 15
  Quartz, 6, 9
  Quartz schist, 284
  Quaternary period, 395, 416
  Quebec, 28

  Rain, erosion, 23
  Rain prints, 181
  Recent epoch, 416, 440, 447
  Reconcentration of ores, 289
  Record, the geological, 291
  Red clay, 187
  Red River of the North, 67
  Red Sea, 221
  Red snow, 115
  Reefs, coral, 188
  Regional intrusions, 272
  Reptiles, 367, 383
  Rhinoceros, 408
  Rhizocarp, 343
  Rhode Island, 356
  Rhone glacier, 123
  Rhyolite, 240
  Richmond, Va., 370
  Rift valleys, 221
  Ripple marks, 180
  Rivers, 54
      bars, 65
      braided channels, 94
      deltas, 108
      deposition, 62
      discharge, 55
      erosion, 59
      estuaries, 85
      flood plains, 93
      floods, 54
      graded, 74
      gradients, 82
      load of, 56
      mature, 72, 80, 97, 98
      meanders, 96
      plains, 99
      profile of, 73
      revived, 85
      run-off, 54
      structure of deposits, 102
      terraces, 96
      transportation, 56, 64
      waterfalls, 78
      young, 67
  Roches moutonnées, 134, 418
  Rock bench, 156
  Hock salt, 12, 357, 371
  Rocky Mountains, 375, 399, 437
  Ruminants, 412

  Saber-tooth tiger, 413
  Saguenay River, 90, 201
  Sahara, 15, 146, 150
  St. Elias Range, 399
  St. Peter sandstone, 150
  Salamanders, 383
  Salina series, 335
  Salt, common, 106, 335
  Salt lakes, 106
  San Francisco Bay, 197
  Sand, beach, 163
      of deserts, 149
      reefs, 165, 167
      storms, 145
  Sandstone, 6, 7, 186
  Sarcoui, 258
  Sauropoda, 386
  Schist, 283
  Schladebach, 277
  Scoria, 250, 255
  Scorpions, 339, 340, 363
  Scotland, 170, 220, 402
  Sea, 155
      erosion, 156
      deposition, 174
      transportation, 162
  Sea arch, 159
  Sea cave, 158
  Sea cliff, 156, 157
  Sea cucumber, 363
  Seals, 414
  Sea stacks, 169
  Sea urchin, 332, 379
  Seaweed, 176
  Sedimentary rocks, 8, 9
  Selkirk Mountains, 218
  Septa, 338
  Sequoia, 378
  Shale, 8, 9
  Sharks, 345, 405
  Shasta, Mount, 262, 400
  Sheep, 413
  Shenandoah valley, 403
  Shores of elevation, 167
  Shores of depression, 169
  Siderite, 63
  Sierra Nevada Mountains, 24, 90, 99, 224, 229, 272, 287, 318, 357,
     371, 372, 396, 398, 390, 402, 437
  Sigillaria, 362, 367
  Silica, 6, 178
  Silurian period, 316, 334
      life of, 338
  Sink hole, 46
  Slate, 207, 282
  Slaty cleavage, 207
  Slickensides, 217
  Snake River lavas, 400, 401
  Snakes, 384, 405
  Soil, 19
  Solfatara, 260
  Solution, 11
  Soufriére, 246
  South America, 357
  South Carolina, 233
  South Dakota, 276, 374, 397
  Spanish Peaks, 271, 376
  Spherulites, 252
  Spiders, 363
  Spitzbergen, 378
  Sponges, 320, 379
  Springs, 41
      thermal, 50
  Stalactite, 48
  Stalagmite, 48
  Starfishes, 332
  Staubbach, 140
  Stegosaurus, 387
  Stoss side, 134
  Stratification, 5, 64, 180
  Striæ, glacial, 114, 133, 418
  Strike, 203
  Strike fault, 225
  Stromatopora, 331, 379
  Stromboli, 244
  Subsidence, 85, 183, 197
  Sun cracks, 180
  Superior, Lake, 257
  Superposition, law of, 293
  Susquehanna River, 403
  Sutlej River, 209
  Sweden, 199
  Swine, 412
  Switzerland, 28, 427
  Syenite, 274
  Synclinal fold, 204
  Syracuse, N.Y., 436
  Syringopora, 339

  Tabulæ, 339
  Taconic deformation, 329
  Taconic Mountains, 376
  Talc, 284
  Talc schist, 284
  Talus, 23
  Tapir, 409
  Teleost fishes, 349, 382, 405
  Tennessee, 90, 373
  Terminal moraines, 126, 422, 432
  Terraces, 86, 96
  Tertiary period, 395
  Texas, 15, 69, 71, 166, 336, 356, 357, 371, 373, 374, 378
  Theromorphs, 383
  Throw, 217
  Thrust faults, 217
  Till, 418
  Till plains, 420
  Toronto, 443
  Trachyte, 249, 258
  Travertine, 52
  Trenton series, 327
  Triassic period, 369
  Triceratops, 387
  Trilobites, 322, 332, 339, 363, 367
  Tuff, 255
  Turkestan, 103
  Turtles, 384

  Unconformity, 227
  Undertow, 174
  Utah, 107, 271, 360, 371, 396, 399
  Utica series, 327

  V-Valleys, 74
  Valley drift, 128
  Valley trains, 425
  Valleys, 66
  Vermont, 309, 329, 420
  Vernagt glacier, 129
  Vertebrates, 334, 349
  Vesuvius, 247, 259, 402
  Virginia, 48, 84, 106, 370, 403, 442
  Volcanic ashes, 244, 255
      cones, 257
      necks, 267
      rocks, 249
  Volcanoes, 238
      causes of, 278
      decadent, 260
      submarine, 248
      tertiary, 399

  Walrus, 414
  Warped valleys, 101
  Warping, 198
  Wasatch Mountains, 375
  Washington, 18, 91, 150, 400
  Waterfalls, 59, 78
  Waves, 156
  Weathering, 5
      chemical, 10
      differential, 29
      mechanical, 13
  Wells, 41
      artesian, 44
  West Virginia, 79, 357, 359
  White Mountains, 443
  Wind, 144
      deposition, 147
      erosion, 151
      pebbles carved by, 152
      transportation, 145
  Wisconsin, 15, 18, 70, 71, 90, 94, 422, 426
  Wisconsin formation, 429, 431
  Wyoming, 50, 98, 371

  Yahtse River, 131
  Yellow Sea, 151, 170
  Yellowstone canyon, 74
  Yellowstone National Park, 50, 51, 52, 260, 261, 263, 269, 400
  Yosemite, 403

  Zeuglodon, 414
  Zone of cementation, 49, 180
  Zone of solution, 45
  Zones of flow and of fracture, 207


       *       *       *       *       *




ANNOUNCEMENTS




=================================================================

TEXT BOOKS ON SCIENCE

FOR HIGHER SCHOOLS AND COLLEGES

                                                          _List   _Mailing
                                                          price_  price_

  Bergen's Elements of Botany. (Revised Edition)           $1.30     $1.45
  Bergen's Foundations of Botany                            1.50      1.70
  Blaisdell's Life and Health                                .90      1.00
  Blaisdell's Practical Physiology                          1.10      1.20
  Brown's Physiology for the Laboratory                      .75       .85
  Byrd's Laboratory Manual in Astronomy                     1.25      1.35
  Davis' Elementary Meteorology                             2.50      2.70
  Davis' Elementary Physical Geography                      1.25      1.40
  Davis' Physical Geography                                 1.25      1.40
  Dennis and Whittelsey's Qualitative Analysis              1.00      1.10
  Dolbear's First Principles of Natural Philosophy          1.00      1.10
  Evans' Introductory Course in Quantitative Chemical
    Analysis                                                 .50       .55
  Frost's Scheiner's Astronomical Spectroscopy              4.75      5.00
  Gage's Principles of Physics                              1.30      1.45
  Gage's Elements of Physics. (Revised Edition)             1.12      1.20
  Gage's Physical Experiments                                .35       .45
  Gage's Physical Laboratory Manual and Notebook             .35       .45
  Gage's Introduction to Physical Science                   1.00      1.10
  Gage's Introduction to Physical Science. (Revised)        1.00      1.10
  Hastings and Beach's General Physics                      2.75      2.95
  Higgins' Lessons in Physics                                .90      1.00
  Lincoln's Hygienic Physiology                              .80       .90
  Meier's Herbarium and Plant Description. With directions
    for collecting, pressing, and mounting specimens         .60       .70
  Moore's Laboratory Directions for Beginners in
    Bacteriology                                            1.00      1.05
  Nichols, Smith, and Turton's Manual of Experimental
    Physics.                                                1.00       .90
  Pratt's Invertebrate Zoölogy.                             1.25      1.35
  Sabine's Laboratory Course in Physical Measurements       1.25      1.35
  Sellers' Elementary Treatise on Qualitative Chemical
    Analysis                                                 .75       .80
  Snyder and Palmer's One Thousand Problems in Physics       .50       .55
  Stone's Experimental Physics                              1.00      1.10
  Thorp's Inorganic Chemical Preparations                   1.50      1.60
  Upton's Star Atlas                                        2.00      2.15
  Ward's Practical Exercises in Elementary Meteorology      1.12      1.25
  Wentworth and Hill's Text-Book of Physics                 1.15      1.25
  Wentworth and Hill's Laboratory Exercises in Elementary
    Physics                                                  .25       .27
  White's Elementary Chemistry                              1.00      1.10
  Williams' Chemical Experiments                             .60       .50
  Williams' Chemical Exercises                               .35       .30
  Williams' Elements of Chemistry                           1.10      1.20
  Williams' Introduction to Chemical Science                 .80       .90
  Williams' Laboratory Manual of Inorganic Chemistry         .35       .30
  Williams' Laboratory Manual of General Chemistry           .25       .30
  Young's Elements of Astronomy                             1.60      1.75
  Young's General Astronomy                                 2.75      3.00
  Young's Lessons in Astronomy. (Revised Edition)           1.25      1.40
  Young's Manual of Astronomy                               2.25      2.45


=================================================================

GINN & COMPANY PUBLISHERS




Davis' Elementary Physical Geography

=================================================================

By WILLIAM MORRIS DAVIS

_Author of Davis' "Physical Geography" and Professor of Geology in
Harvard University_

12mo. Cloth. viii + 401 pages + 6 color charts + 16 pages of maps.
Illustrated. List price, $1.25.

The "Elementary Physical Geography" is a new book by Professor W. M.
Davis, the first authority in the United States in the field of physical
geography. It retains the characteristic features of the author's
earlier work, "Physical Geography," but is much less difficult and is
admirably adapted for use with younger pupils.

The plan of this volume, like that of its predecessor, is to give the
problems of physical geography a rational treatment. The object of this
method is not simply to explain physiographic facts, but to increase the
appreciation of the facts themselves by associating them with their
causes and their consequences. This relation is not presented merely
as an afterthought in a detached chapter at the end of the book; it
accompanies the presentation of the facts themselves.

The chapter on the Atmosphere has been considerably expanded, and an
entirely new chapter has been added on the Distribution of Plants,
Animals, and Man, considered from a physiographic standpoint. The
questions at the end of each chapter, prepared by an experienced
high-school teacher, will be found most useful in class-room work with
this book.

Especially notable are the illustrations, which include nearly two
hundred splendid woodcuts, five pages of charts in color, and nineteen
full-page half-tone plates from rare photographs.

       *       *       *       *       *

The Nation, July 3, 1902: Taken all in all, this seems the most
satisfactory elementary text-book in physical geography yet published.
Certainly in its treatment of the land it has not been surpassed unless,
perhaps, by the author's larger work ["Physical Geography"].

=================================================================

GINN & COMPANY Publishers




=================================================================

GEOGRAPHIC INFLUENCES IN AMERICAN HISTORY

By ALBERT PERRY BRIGHAM

Professor of Geology in Colgate University. 12mo. Cloth. 366 pages.
Illustrated. List price, $1.25; mailing price, $1.40

       *       *       *       *       *

In this new book Professor Brigham has presented vividly and clearly
those physiographic features of America which have been important
in guiding the unfolding of our industrial and national life. The
arrangement is mainly geographical. Among the themes receiving special
treatment are: The Eastern Gateway of the United States, the Appalachian
Barrier, the Great Lakes and American Commerce, the Civil War, and
Mines and Mountain Life. Closing chapters deal with the unity and
diversity of American life and with physiography as affecting American
destiny.

The book will be found particularly interesting and valuable to
students and teachers of geography and history, but it will also
appeal to the general reader. The very large number of rare and
attractive photographs and the numerous maps are of importance
in vivifying and explaining the text.

=================================================================

GINN & COMPANY PUBLISHERS




=================================================================

The best example in this country of the kind of books that geography
needs.--_The Nation_

THE LAKES OF NORTH AMERICA

By ISRAEL C. RUSSELL

Professor of Geology in the University of Michigan

8vo. Cloth, xi + 125 pages. Illustrated. List price, $1.50;
mailing price, $1.65

Recent advances have made physical geography almost a new science.
Professor Russell here presents a single phase, but one of great
importance. The work is based upon his original investigations, and
is at once thoroughly scientific in substance and enjoyably popular
in style.

As a text-book or reading book for students in geography and geology
the "Lakes of North America" possesses a unique value. Its interest
to the general reader is enhanced by its illustrations and its happy
descriptions of lake scenery.


GLACIERS OF NORTH AMERICA

By ISRAEL C. RUSSELL, Professor of Geology in the University of Michigan
Author of "Lakes of North America"

8vo. Cloth. x + 210 pages., Illustrated. List price, $1.75;
mailing price, $1.90


Recent explorations have shown that North America contains thousands of
glaciers, some of which are not only vastly larger than any in Europe,
but belong to types of ice bodies not there represented. In the study
of the glaciers of North America, and especially of those in Alaska,
Professor Russell has taken an active part; and this book not only
presents the results of his own explorations but also a condensed and
accurate statement of the present status of glacial investigations. Its
popular character and numerous illustrations will make it of interest to
the general reader.

       *       *       *       *       *

_DEPARTMENT OF SPECIAL PUBLICATION_

=================================================================

GINN & COMPANY PUBLISHERS





       *       *       *       *       *


Transcriber's Notes


This transcription was derived from the 1905 publication obtained from
The Internet Archive. As the Index of the original 1905 book is missing
entries for U and V, the Index from the 1921 version was used to add the
missing sections.

One error was noted in preparing this revision (page 493 under Hudson
River should have been 417). Several hyphenated vs. unhyphenated forms
were standardized to the most prevalent. Minor corrections were made
where periods, commas, etc. were missing. A number of paragraphs which
were split by images were rejoined. In some cases, the text was moved to
the preceding or following page. As ALL CAPS was employed for the
Chapter Titles in the original book, the small caps subchapter headings
and Figure captions were not converted to all caps. The oe ligature in
Coelenterates was converted to "oe".








End of Project Gutenberg's The Elements of Geology, by William Harmon Norton

*** END OF THE PROJECT GUTENBERG EBOOK 40404 ***