WEATHERING 117 
do they pulverize the soil, but, by bringing earth to the 
surface, they render it more open to wasting agencies. 
The processes of rock destruction so far considered 
may be called mechanical, in distinction from another 
Fre. 51. 
Forest tree overturned by storm wind. Quantities of soil dragged to the air 
entangled in the roots. 
set that by chemical means codperates with these to 
reduce the rock. 
Chemical Agents. — Percolating Water. When rain 
falls upon the land, part of it enters the ground, and 
even passes into the rock itself. Pure rain water has 
little effect, but it ceases to be pure on beginning to 
