CHAPTER III 

 THE GEOLOGICAL WORK OF THE ATMOSPHERE 



Since the atmosphere is a part of the earth, its activities and 

 its history are proper subjects of geologic study. This view is in 

 no way vitiated by the fact that the special study of the atmosphere 

 constitutes the science of Meteorology. The atmosphere is one of 

 the three great formations of the earth, and as a factor in geology, 

 it takes its place beside the hydrosphere and the lithosphere. The 

 study of the atmosphere, so far as taken up in geology, is restricted, 

 commonly, to the effects of the atmosphere on the other parts of 

 the earth; but the origin and history of the atmosphere are surely 

 proper subjects of inquiry, in any thorough-going history of the 

 earth. 



The atmosphere has played a part in the history of the earth 

 comparable to that of water, though its record is less clear. Un- 

 substantial as the atmosphere seems when contrasted with the 

 liquid and solid portions of the earth, its extreme mobility and its 

 chemical activity make it an agent of importance in geological 

 history. It plays and has played a direct part as (1) a mechanical 

 and (2) a chemical agent, and it serves and has long served an 

 indirect function in furnishing the conditions under which (3) the 

 sun produces its temperature effects, and (4) evaporation and piv- 

 cipitation take place. The atmosphere, too, furnishes the necessary 

 conditions for land plants and animals, and the important in- 

 fluences that spring from them. 



The atmosphere is not only an agent of decomposition, as 

 already noted in connection with the decay of igneous rock, but 

 it is also one of the great transporting agents of the earth, and 

 helps to carry away the fine material produced by the chemical 

 activity of its elements. 



