The work is intended primarily for mature students, and is designed to furnish the basis for a year’s work in the later part of the college course. By judicious selection of material to be presented and omitted, the volumes will be found useful for briefer courses, and by the use of the numerous references to the fuller discussions of special treatises, they may be made the basis for more extended courses than are commonly given in undergraduate work. The attempt has also been made to make the volumes readable, in the belief that many persons not in colleges or universities will be interested in following a connected account of the earth’s history, and of the means by which that history is recorded and read. Antecedent elementary courses in geology will not be necessary to the use of these volumes, though such courses may be helpful. The arrangement of themes adopted is such as to bring to the fore processes with which all students are immediately in contact, and which are available for study at all seats of learning. The commoner geologic agents, such as the atmosphere and running water, have been elaborated somewhat more fully than is customary, and the common rather than the exceptional phases of the work of these agents have been emphasized, both because of their greater importance and their universal availability. The text has been so shaped as to suggest field work in connection with these topics especially, since work of this sort is everywhere possible. After the preliminary outline, which is intended to give some idea of the scope of the science, and of its salient features, and to show the relations of the special subjects which follow, the order of treatment is such as to pass from the commoner and more readily apprehended portions of the subject to those which are less readily accessible and more obscure. Following the same general conception, the treatment of the topics is somewhat graded, the earlier chapters being developed with greater simplicity and fullness, while the later are somewhat more condensed. Many acknowledgments are made in the text and foot-notes, but it is impossible to adequately acknowledge all the sources which have been drawn upon, since the whole body of literature has been laid under contribution. The authors especially acknowledge the generous assistance of Professor J. P. Iddings in connection with the chapter on The Origin and Descent of Rocks; of Dr. F. R. Moulton, Professor C. S. Slichter, Professor L. M. Hoskins, Mr. A. C. Lunn, and Mr. W. H. Emmons in connection with mathematical problems; of Professor C. R. Barnes in connection with the geologic functions of life; and of Professor Julius Stieglitz in connection with chemical subjects.