PREFACE. 



In the preparation of this work it has been the purpose of the authors 

 to present an outhne of the sahent features of geology, as now developed, 

 encumbered as little as possible by technicalities and details whose 

 bearings on the general theme are unimportant. In common with 

 most writers of text-books on geology, the authors believe that the sub- 

 ject is best approached by a study of the forces and processes now in 

 operation, and of the results which these forces and processes are now 

 bringing about. Such study necessarily involves a consideration of 

 the principles which govern the activities of geologic agencies. These 

 topics are presented in Volume I, and prepare the way for the study of 

 the history of past ages, which is outlined in Volume II. 



The general plan of the work has been determined by the experi- 

 ence of the authors as instructors. Little emphasis is laid on the com- 

 monly recognized subdivisions of the science, such as dynamic geology, 

 stratigraphic geology, physiographic geology, etc. The treatment pro- 

 ceeds rather from the point of view that the science is a unit, that its 

 one theme is the history of the earth, and that the discussions of dynamic 

 geology, physiographic geology, etc., apart from their historical bear- 

 ing, lose much of their significance and interest. The effort has, there- 

 fore, been to emphasize the historical element, even in the discussion of 

 speeial themes, such as the work of rivers, the work of snow and ice, 

 and the origin and descent of rocks. This does not mean that phases 

 of geology other than historical have been neglected, but it means that 

 an effort has been made to give a historical cast to all phases of the sub- 

 ject, so far as the topics permit. 



Throughout the work the central purpose has been not merely to 

 set forth the present status of knowledge, but to present it in such a 

 way that the student will be introduced to the methods and spirit of 

 the science, led to a sympathetic interest in its progress, and prepared 

 to receive intelligently, and to welcome cordially, its future advances. 

 Where practicable, the text has been so shaped that the student may 



