188 
In this work, weathering is the great preparer. 
ELEMENTARY GEOLOGY 
It is 
of the first importance; for if rocks did not decay, 
not only would there be no soils formed, but the rain- 
born streams, flowing over bare rock, would not carry 
large quantities of sediment as now. 
IMPORTANT AGENTS OF WEATHERING 
MECHANICAL. 
CHEMICAL. 
Mechanical action  in- 
~ Direct effect of the air 
Alternation from dry to 
wet condition. 
“AIR. ‘ ; ; nee 
cluded under erosion. in promoting oxidation. 
Direct blow of rain. Solution. 
Freezing in_ crevices. : ; 
WATER. 8 Chemical change; oxi- 
dation. 
CHANGES IN 
Change from warm to 
cold. 
Warming of percolat- 
ing water. 
Bringing soil to the air 
(earthworms, ants, prairie 
dogs, and other burrowing 
animals). 
pe hbase Frost (combined with 
TURE. 
water). 
Action of roots on rock ‘ee 
‘ Solution in sap. 
and in soil. 
PLANTs. Indirectly by furnish- 
Overturning of trees. ing substances to perco- 
lating water. 
Pulverizing soil by bur- 
rowing through it and by Slight chemical action 
taking it into their stom- | in stomach (earthworms). 
achs (earthworms). 
ANIMALS. Indirectly, after death, 
by furnishing substances 
to percolating water 
which increase its chemi- 
cal power. 
