PAET I. 



DYNAMICAL GEOLOGY. 



The agencies now in operation, modifying the structure of the sur- 

 face of the earth, may be classed under four heads, viz., atmospheric 

 agencies, aqueous agencies, igneous agencies, and organic agencies. 

 These agencies have operated from the beginning, and are still in 

 operation. We study their operation now, in order that we may un- 

 derstand their effects in previous epochs of the earth's history — i. e., 

 the structure of the earth. 



While all geologists agree that the nature of the agencies which 

 have operated in modifying the earth's surface has remained the same 

 from the beginning, they differ in their views as to the energy of their 

 operation in different periods. Some believe that their energy has been 

 much the same throughout the whole history of the earth, while others 

 believe that many facts in the structure of the earth require much 

 greater operative energy than now exists. We will attempt to show 

 hereafter that neither of these extreme opinions is probably true, but 

 that some of these agencies have been decreasing, while others have 

 been increasing, with the progress of time. It is the constant change 

 of balance between these which determines the development of the 

 earth. 



CHAPTER I. 



ATMOSPHERIC AGENCIES. 



The general effect of atmospheric agencies is the disintegration of 

 rocks and the formation of soils. The atmosphere is composed of nitro- 

 gen and oxygen, with small quantities of watery vapor and of carbonic 

 acid. There are but few rocks which are not gradually disintegrated 

 under the constant chemical action of the atmosphere. The chemical 

 agents of these changes are oxygen, carbonic acid, and watery vapor, 

 the nitrogen being inert. To these must be added, where vegetation 



