126 ELEMENTARY GEOLOGY 
Not only are there these differences, but the amount 
of weathering varies with the climate and other causes. 
It seems certain that exposed rocks in cold regions 
decay with greater rapidity than those which are situ- 
Fig. 58. 
A talus slope in a river valley at the base of 
a high rock cliff. 
ated in the warm coun- 
tries of the globe. This 
is due to the action 
of frost; but on the 
other hand, rocks de- 
cay to a greater depth 
in warm climates than 
in cold. The reason 
for this is, that in trop- 
ical lands, densely cov- 
ered with forests, the 
water is warmer, and 
hence possesses higher 
chemical powers, 
which are also in- 
creased by the abun- 
dance of organic acids 
and alkalies furnished 
from the forest beds. 
On the other hand, in an arid region, since there is 
little water to enter the ground, there is naturally little 
change in the rock; and vegetation is so scanty that 
the effects of this agent are also lessened. 
