120 ELEMENTARY GEOLOGY 
- The Soils. — Residual Soil. The chemical and me- 
chanical agents of destruction go hand in hand; and 
Fie. 52. 
A sandstone rock, showing outer dark rim of decay, 
surrounding the fresh interior. . Here calcite has 
been dissolved from the outer part, and iron rust 
has been deposited, staining the rim a dark red, 
while the color of the natural rock is nearly white. 
in time, the result 
is the production 
of a soil-covering 
for the rock; pro- 
vided that the 
winds and rains 
have not been 
able to wash it 
off as fast as 
formed. This 
class of soil, the 
most widespread 
in the world, is 
called a residual 
soul (Fig. 53), be- 
cause it is made 
up of the residue 
of rock decay. It 
does not repre- 
sent the entire 
product of the 
rock disintegra- 
tion, because considerable has been removed in solution, 
either by the action of plants or of percolating waters. 
The residual soil often accumulates to a depth of a 
