sun has been shining. Examine them in the same way early the next
morning. You will find that the rocks are heated by the sun just as
the plate was heated when put into the oven, and when the sun goes
down the rocks cool again. This causes tension in the rocks and
little cracks and checks appear in them just as in the heated plate,
only more slowly. This checking may also be brought about by a cool
shower falling on the sun heated rocks just as the cool water cracked
the warm glass. Many rocks if examined closely will be found to be
composed of several materials. These materials do not expand and
contract alike when heated and cooled and the tendency for them to
check is greater even than that of the plate. This is the case with
most rocks.

[Illustration: FIG. 19.--COMPARING SOILS.]

[Illustration: FIG. 20.--WATER TEST OF SOILS.
Bottle _A_ contains sand and water, bottle _B_ clay and water. The
sand settles quickly, the clay very slowly.]

_Work of Rain._--Rain falling on the rocks may dissolve a part of them
just as it dissolved the rock salt; or, working into the small cracks
made by the sun, may wash out loosened particles; or, during cold
weather it may freeze in the cracks and by its expansion chip off
small pieces; or, getting into large cracks and freezing, may split
the rock just as freezing water splits a water pitcher or the water
pipes.

_Work of Moving Water._--Visit some neighboring beach or the banks of
some rapid stream. See how the waves are rolling the sand and pebbles
up and down the beach, grinding them together, rounding their corners