be called animals. In some cases these forces acted singly;
in others, all acted together to rend and crumble the unbroken stretch
of rock. Let us glance at some of the methods used by these skilled
soil-makers.

Heat and cold are working partners. You already know that most hot
bodies shrink, or contract, on cooling. The early rocks were hot. As the
outside shell of rock cooled from exposure to air and moisture it
contracted. This shrinkage of the rigid rim of course broke many of the
rocks, and here and there left cracks, or fissures. In these fissures
water collected and froze. As freezing water expands with irresistible
power, the expansion still further broke the rocks to pieces. The
smaller pieces again, in the same way, were acted on by frost and ice
and again crumbled. This process is still a means of soil-formation.

Running water was another giant soil-former. If you would understand its
action, observe some usually sparkling stream just after a washing rain.
The clear waters are discolored by mud washed in from the surrounding
hills. As though disliking their muddy burden, the waters strive to
throw it off. Here, as low banks offer chance, they run out into
shallows and drop some of it. Here, as they pass a quiet pool, they
deposit more. At last they reach the still water at the mouth of the
stream, and there they leave behind the last of their mud load, and
often form of it little three-sided islands called _deltas_. In the same
way mighty rivers like the Amazon, the Mississippi, and the Hudson, when
they are swollen by rain, bear great quantities of soil in their sweep
to the seas. Some of the soil they scatter over the lowlands as they
whirl seaward; the rest they deposit in deltas at their mouths. It is
estimated that the Mississippi carries to the ocean each year enough
soil to cover a square mile of surface to a depth of two hundred and
sixty-eight feet.

[Illustration: FIG. 1. ROCK MARKED BY THE SCRAPING OF A GLACIER OVER IT]

The early brooks and rivers, instead of bearing mud, ran oceanward
either bearing ground stone that they themselves had worn from the rocks
by ceaseless fretting, or bearing stones that other forces had already
dislodged. The large pieces were whirled from side to side and beaten
against one another or against bedrock until they were ground into
smaller and smaller pieces. The rivers distributed this rock soil just
as the later rivers distribute muddy soil. For ages the moving