THE WORK OF RUNNING WATER. 



113 



is being broken. The process illustrated by the figure involves weath- 

 ering as well as other factors. 



The foregoing are among the comnioner processes of weathering, 

 although they do not exhaust the hst. The naore active and tangible 



m 







m 





'ii 



Ih 



w ,J 



1 



Fig. 95. — Shows the downward creep of soil and slaty rock under the influence 



of gravity. 



processes by which surface rocks are broken up, such as wave wear, 

 river wear and glacier wear, are processes of corrasion. The mechani- 

 cal wear effected by wind-driven sand might be considered either as 

 corrasion or as weathering. It is more likely to be regarded as corrasion 

 if the amount of wear is considerable enough to be obvious. Rock is 

 sometimes decomposed by the chemical action of hot vapors, gases, and 

 waters rising to the surface from considerable depths. This is often 

 seen in ^'olcanic regions. A conspicuous illustration is seen in the 

 canyon of the Yellowstone in the National Park. Decay of this sort 

 is perhaps not properly weatliering, but is not always readily distin- 

 guished from it. 



