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, temperature 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 contraction, 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 cracks. As these processes are repeated 
from day to day, from year to year, and from genera- 
tion to generation, the surfaces of the rocks crumble. 
The smaller rocks so formed are acted upon by 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 water thus 
