When water lodges in the crevices of rocks, and
_freezes_, it expands, and bursts the rock, on the same principle as
causes it to break a pitcher in winter. This power is very great, and by
its assistance, large cannon may be burst. Of course the action of frost
is the same on a small scale as when applied to large masses of matter,
and, therefore, we find that when water freezes in the _pores_[M] of
rocks or stones, it separates their particles and causes them to
crumble. The same rule holds true with regard to stiff clay soils. If
they are _ridged_ in autumn, and left with a rough surface exposed to
the frosts of winter, they will become much lighter, and can afterwards
be worked with less difficulty.

[What is the effect of water on certain rocks?

How are some rocks affected by exposure to the atmosphere? Give an
instance of this.]

2. The action of water. Many kinds of rock become so soft on being
soaked with water, that they readily crumble.

3. The chemical changes of the constituents of the rock. Many kinds of
rock are affected by exposure to the atmosphere, in such a manner, that
changes take place in their chemical character, and cause them to fall
to pieces. The red kellis of New Jersey (a species of sandstone), is,
when first quarried, a very hard stone, but on exposure to the
influences of the atmosphere, it becomes so soft that it may be easily
crushed between the thumb and finger.

[What is the similarity between the composition of soils and
the rocks from which they were formed?

What does feldspar rock yield? Talcose slate? Marls?

Does a soil formed entirely from rock contain organic matter?

How is it affected by the growth of plants?]

Other actions, of a less simple kind, exert an influence on the
stubbornness of rocks, and cause them to be resolved into soils.[N] Of
course, the composition of the soil must be similar to that of the rock
from which it was formed; and, consequently, if we know the chemical
character of the rock, we can tell whether the soil formed from it can
be brought under profitable cultivation. Thus feldspar, on being
pulverized, yields potash; talcose slate yields magnesia; marls yield
lime, etc.

The soil formed entirely from rock, contains, of course, no organic
matter.[O] Still it is capable of bearing plants of a certain class, and
when these die, they are deposited in the soil, and thus form its
organic portions, rendering it capable of supporting those plants