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Books: The Elements of Geology

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THE ELEMENTS OF GEOLOGY

BY WILLIAM HARMON NORTON

PROFESSOR OF GEOLOGY IN CORNELL COLLEGE

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
paleontological 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





CONTENTS

INTRODUCTION.--THE SCOPE AND AIM OF GEOLOGY

PART I

EXTERNAL GEOLOGICAL AGENCIES

I. THE WORK OF THE WEATHER
II. THE WORK OF GROUND WATER
III. RIVERS AND VALLEYS
IV. RIVER DEPOSITS
V. THE WORK OF GLACIERS
VI. THE WORK OF THE WIND
VII. THE SEA AND ITS SHORES
VIII. OFFSHORE AND DEEP-SEA DEPOSITS

PART II

INTERNAL GEOLOGICAL AGENCIES

IX. MOVEMENTS OF THE EARTH'S CRUST
X. EARTHQUAKES
XI. VOLCANOES
XII. UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN
XIII. METAMORPHISM AND MINERAL VEINS

PART III

HISTORICAL GEOLOGY

XIV. THE GEOLOGICAL RECORD
XV. THE PRE-CAMBRIAN SYSTEMS
XVI. THE CAMBRIAN
XVII. THE ORDOVICIAN AND SILURIAN
XVIII. THE DEVONIAN
XIX. THE CARBONIFEROUS
XX. THE MESOZOIC
XXI. THE TERTIARY
XXII. THE QUATERNARY
INDEX





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 LAMINAE. 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. Two sets of joints,
running at about right angles to each other, together with the
bedding planes, divide the sandstone into quadrangular blocks.

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.

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.

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.

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.