52 DRY-FARMING 



produce soil from rocks are of two distinct classes : 

 physical and chemical. The physical agencies of soil 

 production merely cause a pulverization of the 

 rock ; the chemical agencies, on the other hand, so 

 thoroughly change the essential nature of the soil 

 particles that they are no longer like the rock from 

 which they were formed. 



Of the physical agencies, temperalure changes are 

 first in order of time, and perhaps of first importance. 

 As the heat of the day increases, the rock expands, 

 and as the cold night approaches, contracts. This 

 alternate^ expansion and conti-action, in time, cracks 

 the surfaces of the rocks. Into the tiny crevices 

 thus formed water enters from the falling snow or 

 rain. When winter comes, the water in these cracks 

 freezes to ice, and in so doing expands and widens 

 each of the civacks. As these ]:)rocesses are repeated 

 from da}' to day, from year to year, and from genera- 

 tion to generati(jn, the surfaces of the rocks crumble. 

 The smaller rocks so formed are acted upon b}^ the 

 same agencies, in the same manner, and thus the 

 process of pulverization goes on. 



It is clear, then, that the second great agency of 

 soil formation, which always acts in conjunction with 

 temperature changes, is freezing water. The rock 

 particles formed in this manner are often washed 

 down into the mountain valleys, there caught by 

 great rivers, ground into finer dust, and at length 

 deposited in the lower valleys. Moving tcater thus 



