110 ELEMENTARY GEOLOGY 
lated at its base. In the same way, the stones in many 
buildings that have been exposed to the weather for sey- 
eral centuries have perceptibly decayed and crumbled. 
Fig. 47. 
The crumbling of a granite ledge exposed to the weather in a railway cut. A 
partial soil accumulation with granite bowlders shown in middle of picture. 
Presence of Water. — Let us suppose that we have a 
cliff of rock exposed to the air; it is a solid granite, 
formed of several very hard minerals which unite the 
