said. The fertile soil is made
of rock meal and decayed leaves and roots. Only recently have ploughs
been invented. But the great forest crops have grown in soil made mellow
by the earthworm's ploughing.

QUIET FORCES THAT DESTROY ROCKS

Wind and water are the blustering active agents we see at work tearing
down rocks and carrying away their particles. They do the most of this
work of levelling the land; but there are quiet forces at work which
might not attract our attention at all, and yet, without their help,
wind and running water would not accomplish half the work for which they
take the credit.

The air contains certain destructive gases which by their chemical
action separate the particles of the hardest rocks, causing them to
crumble. Now the wind blows away these crumbling particles, and the
solid unchanged rock beneath is again exposed to the crumbling agencies.

The changes in temperature between day and night cause rocks to contract
and expand, and these changes put a strain upon the mineral particles
that compose them. Much scaling of rock surfaces is due to these causes.
Building a fire on top of a rock, and then dashing water upon the heated
mass, shatters it in many directions. This process merely intensifies
the effect produced by the mild changes of winter and summer. Water is
present in most rocks, in surprising quantities, often filling the
spaces in porous rocks like sandstones.

When winter brings the temperature down to the freezing point, the water
near the surface of the rock first feels it. Ice forms, and every
particle of water is swollen by the change. A strain is put upon the
mineral particles against which the particles of ice crowd for more
room. Frost is a very powerful agent in the crumbling of rocks, as well
as of stubborn clods of earth. In warm climates, and in desert regions
where there is little moisture in the rocks, this destructive action of
freezing water is not known. In cold countries, and in high altitudes,
where the air is heavy with moisture, its greatest work is done.

Some kinds of rock decay when they become dry, and resist crumbling
better when they absorb a certain amount of moisture. Alternate wetting
and drying is destructive to certain rocks.

One of the unnoticed agents of rock decay is the action of lowly plants.
Mosses grow upon the faces of rocks, thrusting their tiny root processes
into pits they dig deeper by means of acids secreted by the delic