CHAPTER II. 



THE ATMOSPHERE AS A GEOLOGICAL AGENT. 



While it is convenient to regard the lithosphere as the earth proper, 

 and the atmosphere as its envelope, the latter is as truly a part of the 

 planet as the former, and its activities and its history are as truly 

 subjects of geological study as the formation of the rocks. This view 

 is in no way vitiated by the fact that the special study of the atmos- 

 phere is set apart under the name Meteorology, for in the same way 

 the special study of rocks is set apart under the name Petrology, that 

 of ancient life under the name Paleontology, and that of other phases 

 of the subject under other names. The atmosphere is one of the three 

 great formations of the earth, and as a geological factor takes its place 

 beside the hydrosphere and the lithosphere. It has played a part 

 in the history of the earth comparable to that of the water, though 

 its mass is less and its record more elusive. Unsubstantial as the 

 atmosphere seems when contrasted with the liquid and solid portions 

 of the earth, its extreme mobility and its chemical activity compensate 

 for its lightness and tenuity, and give it a function of the first order of 

 importance. 



The atmosphere plays a direct part as (1) a mechanical and (2) a 

 chemical agent, and at the same time serves an indirect function in 

 furnishing favorable conditions under which (3) solar radiation pro- 

 duces temperature effects, and (4) evaporation gives origin to precipita- 

 tion and stream effects, and furnishes the necessary conditions for land 

 plants and animals, and the important influences that spring from 

 them. 



This chapter is devoted to the work of the atmosphere in these and 

 some less notable phases. The consideration of the origin and history 

 of the atmosphere will receive attention later. 



A. THE ATMOSPHERE AS A DIRECT AGENCY. 



/. Mechanical Work. 



The mechanical work of the atmosphere is accomplished chiefly 

 through its movement. A feeble breeze is competent to move particles 



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