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