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