108 GEOLOGY 



comes an easy prey to running water, so that the work of the at- 

 mosphere prepares the way for that of other eroding agencies. 



Evaporation and precipitation. Perhaps the most important 

 work of the atmosphere as a geologic agent, lies in its relation to 

 the evaporation, circulation, and distribution of water. Atmos- 

 pheric temperature is the primary factor governing evaporation, 

 an important factor in the circulation of the vapor after it is formed, 

 and controls its condensation and precipitation. 



It is impossible to separate sharply the geologic work of the 

 water of the atmosphere from that of other waters; but so long 

 as moisture is in the atmosphere (including the time of its pre- 

 cipitation) its effects are best considered in connection with the 

 atmosphere. 



The mechanical work of the rain. In falling, the rain washes 

 the atmosphere, taking from it much of the dust, spores, etc., 

 which the winds have lifted from the surface of the dry land. Not 

 only this, but in passing through the atmosphere, the water dis- 

 solves some of its gases, so that when the rain reaches the land, 

 it is no longer pure, and some of the gases it has taken up in its 

 descent enable it to dissolve various mineral matters on which pure 

 water has little effect. 



As it falls on the surface of the land, the rain produces various 

 effects of a mechanical nature. In the first place, it leaves on the 

 surface the solid matter taken from the air. Clayey soils are 

 baked under the influence of the sun, and when in this condition 

 are softened by the rain, and more easily removed by running 

 water. Under the influence of the expansion and contraction 

 caused by wetting and drying, the soils and earths on slopes creep 

 slowly downward. When rain falls on dry sand or dust the co- 

 hesion is at once increased, and shifting by the wind is temporarily 

 stopped. 



After the water has fallen from the air, its further work cannot l>o 

 looked upon as a part of the work of the atmosphere; but the proper 

 conception of the geological work of the atmosphere must recognize 

 that the waters of the land have come through the atmosphere. 



Effects of electricity. Another dynamic effect conditioned 

 by the atmosphere is that produced by lightning. In the aggre- 



