14 INTRODUCTION. 



(4) Next, the general plan or laws of progress in the earth and its life. 



(5) Finally, there are the active forces and mechanical agencies which 

 were the means of physical progress, — spreading out and consolidating 

 strata, raising mountains, ejecting lavas, wearing out valleys, bearing the 

 material of the heights to the plains and oceans, enlarging the oceans, 

 destroying life, and performing an efficient part in evolving the earth's 

 structure and features. 



These topics lead to the following subdivisions of the science : — 



I. Physiographic Geology, — a general survey of the earth's surface- 

 features. 



II. Structural Geology, — a description of the rock-materials in the 

 structure of the globe, — that is, of its kinds of rocks, and of their arrange- 

 ment or positions. 



III. Dynamical Geology, — an account of the agencies or forces that 

 have produced geological changes, and of the laws, methods, and results of 

 their action. 



IV. Historical Geology, — an account of the earth's geological his- 

 tory, or the successive events or steps in the making of the rock-strata, and 

 of the continents, seas, mountains, and valleys, in the progress of the 

 earth's living species, and in all changes that have gone forward in the 

 earth's development. 



In the study of the science, a previous knowledge of the methods of change taught in 

 the Dynamical section is desirable in order fully to comprehend Historical geology ; and 

 a knowledge of the actual facts and their succession given in the Historical section is 

 desirable to understand the causes of events and methods of change. There is reason, 

 therefore, for studying Dynamical geology before Historical as well as after it. It is here 

 made to precede. But the last topic under it — that of the formation of mountains — will 

 be best appreciated after the student is familiar with the facts presented in the Historical 

 section. 



