WEATHERING 125 
varying conditions. Some easily decay, while others 
resist destruction; but every rock will crumble to some 
extent, no matter what its surface conditions and com- 
position. Besides a difference in chemical durability, 
there is a variation in mechanical strength. Some 
a 
Fig. 57. 
A characteristic arid land view (Crow Heart Butte, Wind River valley, 
Wyoming). Surface free from forest and almost free from vegetation, 
excepting near the rivers. 
rocks are very porous, some’very compact; and other 
conditions being equal, the more porous rock will decay 
faster than the more compact. Water follows the joints 
and crevices, and produces much more effect along these 
planes than elsewhere. 
