CHAPTER II 

 THE GEOLOGIC WORK OF THE ATMOSPHERE 



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

 its history are proper subjects of geologic study. As a part of 

 geology, the study of the atmosphere is restricted, commonly, to 

 its effects on the other parts of the earth. The origin and history 

 of the atmosphere must, however, be considered, in any thorough- 

 going history of the earth. 



In the history of the earth, the atmosphere has played a part 

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

 direct work is partly (i) mechanical and partly (2) chemical. Its 

 indirect effects are even more important, for it furnishes the condi- 

 tions under which (3) the sun produces its temperature effects, and 

 (4) evaporation and precipitation take place. The atmosphere, 

 too, furnishes the necessary conditions for plants and animals, and 

 the important influences that spring from them. 



MECHANICAL WORK 



The mechanical work of the atmosphere is accomplished chiefly 

 through its movements. A feeble breeze moves particles of dust, a 

 wind of moderate velocity blows dry sand, and exceptionally strong 

 winds move small pebbles. 



The principal movement of the wind is horizontal; but every 

 obstacle against which it blows deflects some of the air, and some of 

 it is deflected upward. Furthermore, there are exceptional winds, 

 in which the vertical element predominates. Particles of dust are 

 caught by these upward currents, and carried to great heights. 

 This facilitates their transportation great distances. 



Dust. 1 Transportation of dust by the wind is nearly universal. 

 No house, no room, and scarcely a drawer is so tightly closed but 

 that dust enters, and the movements of dust in the open are much 



1 Udden, Jour, of Geol., Vol. II, pp. 318-331; also Pop. Sci. Mo., Sept., '96. 



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