CHAPTER III 

 THE WORK OF THE ATMOSPHERE 



THE erosive effects of the atmosphere upon the surface 

 of the land in general are less important than those of running 

 water, because water is much heavier (over 800 times), and its 

 work is concentrated for the most part along definite lines. 

 Yet the atmosphere is an important geological agent because 

 of its extreme mobility, and because two of its constituents, 

 carbon dioxide and oxygen, by uniting chemically with many 

 rocks, change their character and cause them to decay. The 

 atmosphere is also indirectly of great importance. This is evi- 

 dent when it is remembered that without an atmosphere there 

 would be no life upon the lands, no precipitation of rain or 

 snow, and, in consequence of this, no work by streams and 

 glaciers. 



The work of the atmosphere may be considered under two 

 main headings : (1) the work it accomplishes by mechanical 

 means, and (2) that accomplished by chemical means. 



MECHANICAL WORK 



The mechanical work of the atmosphere is performed largely 

 by the wind, and consists chiefly in transporting, wearing, 

 and depositing rock materials. The atmosphere also in- 

 fluences the changes in rocks that are produced by variations 

 in temperature. 



TRANSPORTATION 



Transportation by the wind is likely to be important wher- 

 ever dry surfaces of fine material are exposed to strong winds. 

 These conditions are fulfilled best over large areas in 



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