eds of miles and see no
rocks. One might think that in time Nature's work would be finished. But
before the mountains in one place have crumbled and been washed away,
she raises up new ones somewhere else so that the tearing-down work
begins again.

[Illustration: _H. W. Fairbanks_
Little by little the great rocks break in pieces and crumble finally to
form soil.]

Let us, in imagination, sit down by the side of a rock, prepared to stay
there many years, that we may learn just how Nature makes the soil. It
will be a long, long time before we can see any change in the rock. Each
bright day the sun warms the cold rock and makes it expand a very
little. At night the rock grows cold and shrinks. In this way minute
crevices are finally formed between the grains of the different minerals
that make up the rock.

When it rains, water creeps into the tiny crevices. The water carries
with it a little carbonic acid which the raindrops took from the air.
This substance aids in dissolving some of the rock materials. If the
nights are very cold, the water in the crevices freezes and opens them a
little wider, for ice, as you know, takes up a little more room than it
did when it was water.

Plants also aid in breaking the rock. Often seeds are dropped by the
wind, and the rootlets of some of these seeds, when they sprout, may
find a crevice large enough and deep enough for them to push their way
into the rock. In these crevices they find a little food and slowly grow
larger and stronger. By and by some of the roots are strong enough to
push apart large pieces of rock.

If the rock which we are studying is granite, we shall after a time be
able to pick out the different minerals of which it is composed. We can
tell the grains of quartz, because they look glassy and remain very
hard. Other grains, which we call _feldspar_, soften and change into
clay, which makes the water muddy as it runs over the rocks. We see also
little scales of yellow mica, sometimes called "fool's gold," and a few
grains of iron. There are tiny quantities of other things which we shall
not be able to see, for the rainwater dissolves them and carries them
away.

As the rock slowly crumbles to sand and clay, the bacteria begin to make
their home in it. Hardy plants, that are not particular about what they
grow in, get a foothold, and when they die their stems and leaves decay
and mix with the rock particles until at last this material begins to
look