ate
tips. You have seen shaded green patches of lichens, like little rugs,
of different shapes, spread on the surface of rocks. But you cannot see
so well the work these growths are doing in etching away the surface,
and feeding upon the decaying mineral substance.

Mosses and lichens do a mighty work, with the help of water, in
reducing rocks to their original elements, and thus forming soil. No
plants but lichens and mosses can grow on the bare faces of rocks. As
their root-like processes lengthen and go deeper into the rock face,
particles are pried off, and the under-substance is attacked. Higher
plants then find a footing. Have you not seen little trees growing on a
patch of moss which gets its food from the air and the rock to which it
clings? The spongy moss cushion soaks up the rain and holds it against
the rock face. A streak of iron in the rock may cause the water to
follow and rust it out, leaving a distinct crevice. Now the roots of any
plant that happens to be growing on the moss may find a foot-hold in the
crack. Streaks of lime in a rock readily absorb water, which gradually
dissolves and absorbs its particles, inviting the roots to enter these
new passages and feed upon the disintegrating minerals. Dead leaves
decay, and the acids the trickling water absorbs from them are
especially active in disintegrating lime rocks.

From such small beginnings has resulted the shattering of great rock
masses by the growth of plants upon them. Tree roots that grow in rock
crevices exert a power that is irresistible. The roots of smaller plants
do the same great work in a quieter way.

When a hurricane or a flood tears down the mountain-side, sweeping
everything before it, trees, torn out by the roots, drag great masses
of rock and soil into the air, and fling them down the slope. Wind and
water thus finish the destruction which the humble mosses and lichens
began. What seemed an impregnable fortress of granite has crumbled into
fragments. Its particles are reduced to dust, or are on the way to this
condition. The plant food locked up in granite boulders becomes
available to hungry roots. Forests, grain-fields, and meadows cover the
work of destructive agencies with a mantle of green.

HOW ROCKS ARE MADE

The granite shaft is made out of the original substance of the earth's
crust. Its minerals are the elements out of which all of the rock masses
of the earth are formed, no matter how different they look fr