INTRODUCTION. 7 



9. In using the present in order to reveal the past, we assume that 

 the forces in the world are essentially the same through all time ; 

 for these forces are based on the very nature of matter, and could 

 not have changed. The ocean has always had its waves, and those 

 waves have ever acted in the same manner. Kunning water on 

 the land has ever had the same power of wear and transportation 

 and mathematical value to its force. The laws of chemistry, heat, 

 electricity, and mechanics have been the same through ^ime. 

 The plan of living structures has been fundamentally one, for the 

 whole series belongs to one system, as much almost as the parts 

 of an animal to the one body ; and the relations of life to light 

 and heat, and to the atmosphere, have ever been the same as now. 



The laws of the existing world, if perfectly known, are, consequently, a key 

 to the past history. But this perfect knowledge implies a complete comprehen- 

 sion of nature in all her departments, — the departments of chemistry, physics, 

 mechanics, physical geography, and each of the natural sciences. Thus fur- 

 nished, we may scan the rocks with reference to the past ages, and feel confident 

 that the truth will declare itself to the truth-loving mind. 



As this extensive range of learning is not within the grasp of a single person, 

 special departments have been carried forward by different individuals, each in 

 his own line of research ; for Geology as it stands is the combined result of 

 the labors of many workers. But the system is now so far perfected that the 

 ordinary mind may readily understand the great principles of the science, and 

 comprehend the unity of plan in the earth's genesis. 



SUBDIVISIONS OF GEOLOGY. 



10. (1.) Like a plant or animal, the earth has its systematic external 

 form and features, which should be reviewed. 



(2.) Next, there are the constituents of the structure to be considered: 

 — first, their nature ; secondly, their general arrangement. 



(3.) Next, the successive stages in the formation of the structure, 

 and the concurrent steps in the progress of life, through past 

 time. 



(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 structures and features. 



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



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

 face-features. 



