This book had its origin in the attempt to write an introductory work, dealing principally with American Geology, upon the lines of Sir Archibald Geikie's excellent little " Class-Book." In spite of vigorous efforts at compression, it has expanded to its present size, though the difference from the "Class-Book," in this respect, lies not so much in the quantity of matter as in the larger size of the type and illustrations.

The book is intended to serve as an introduction to the science of Geology, both for students who desire to pursue the subject exhaustively, and also for the much larger class of those who wish merely to obtain an outline of the methods and principal results of the science. To the future specialist it will be of advantage to go over the whole ground in an elementary course, so that he may appreciate the relative significance of the various parts, and their bearing upon one another. This accomplished, he may pursue his chosen branch much more intelligently than if he were to confine his attention exclusively to that branch from the beginning of his studies.

Students, and only too often their instructors, are apt to prefer a text-book upon which they can lean with implicit confidence, and which never leaves them in doubt upon any subject, but is always ready to pronounce a definite and final opinion. They dislike being called upon to weigh evidence and balance probabilities, and to suspend judgment when the testimony is insufficient to justify a decision. This is a habit of mind which should be discouraged; for it deludes the learner into the belief that he knows the subject when he has only acquired some one's opinions and dogmas, and renders further progress exceedingly difficult to him. In no science are there more open questions than in Geology, in none are changes of view more frequent, and in none, consequently, is it more important to emphasize the distinction between fact and inference, between observation and hypothesis. An open-minded hospitality for new facts is essential to intellectual advance.

In preparing this book, I have of course availed myself of material wherever it was to be found, but I wish to acknowledge my special obligations to the text-books of Dana, Le Conte, Geikie, Green, Prestwich, Credner, Kayser, Neumayr, Koken, de Lapparent, and Jukes-Brown. From the last-named writer is taken the arrangement of the Dynamical Agencies, which experience in the class-room has led me to consider as the best.

Princeton, N.J., Jan. 15, 1897.