The use of technical and unusual terms has been reduced to a minimum compatible with a reasonable understanding of the subject by the layman. Each of the relatively few scientific terms is explained where first used in the text, and a glossary of common geological terms has been appended. The matter of illustrations has received very careful attention, and only pictures, maps, and diagrams are used which actually illustrate important features of the text. A special point has been made to introduce only cuts of simple construction comparatively free from technicalities. Nearly every illustration is accompanied by a really explanatory title. A number of the pictures are from the author's collection of photographs, and many of the line-cuts have either been made or considerably modified by the author. Among the numerous sources of illustrations, special mention should be made of the United States Geological Survey, the New York State Museum, the American Museum of Natural History, the University of Chicago Press, and various individuals, full credit being given wherever due. William J. Miller. Northampton, Mass. CONTENTS CHAPTER PAGE I. Introduction 9 II. Weathering and Erosion 22 III. Stream Work 30 IV. The Sea and Its Work 51 V. Glaciers and Their Work 59 VI. The Action of Wind 71 VII. Instability of the Earth's Crust 76 VIII. Volcanoes and Igneous Rocks 99 IX. Waters Within the Earth 113 X. How Mountains Come and Go 130 XI. A Study of Lakes 142 XII. How the Earth May Have Originated 158 XIII. Very Ancient Earth History 164 XIV. Ancient Earth History 179 XV. Medieval Earth History 208 XVI. Modern Earth History 221 XVII. Evolution of Plants 249 XVIII. Geological History of Animals (Excluding Vertebrates) 259 XIX. Geological History of Vertebrate Animals (Including Man) 281 XX. Mineralogy 307 XXI. Economic Geology 342 Glossary of Common Geological Terms 377 LIST OF ILLUSTRATIONS Ledge of Rock with Nearly Vertical Strata _Frontispiece_