8 4 



THE ATMOSPHERE 



of the activity of water. Few regions of the earth's surface are 

 altogether rainless, but nearly all of the continents have great 

 desert areas in which atmospheric precipitation is very light. It 

 might seem that in such deserts the work of rock disintegration 



Fig. 27. 



Weathered and exfoliating granite, Sierra Nevada, California. 

 (U. S. G. S.) 



must be practically at a standstill, and that the circulation of ma- 

 terial must be so slow as to be hardly distinguishable from com- 

 plete stagnation. Even in these regions, however, the rain accom- 

 plishes something, and it is aided by other agencies which in moist 

 climates play a much more modest role ; these are the changes of 

 temperature and the wind. 



