The Elements of Geology | Page 5

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