re the shells or bones of animals or the stems
and leaves of plants living in former times, and buried by successive
beds of sand or mud spread over them. Much of the land is covered by a
thin surface deposit of clay, sand, or gravel, which is yet loose
material and which shows the mode of formation of sedimentary rocks.

Some rocks have undergone, since their formation, great pressure or
heat and have been much changed. They are called metamorphic rocks.
Some are now made of crystals though at first they were not; in others
the minerals have become arranged {114} in layers closely resembling
the beds of sedimentary rocks; still others, like slate, tend to split
into thin plates.

The earth's surface is continually being changed; the outcropping hard
rock is worn away by wind and rain, and is broken up by frost, by
solution of some minerals, etc. The loose material formed is blown
away or washed away by rain and deposited elsewhere by streams in
gravel bars, sand beds, and mud flats. The streams cut away their
beds, aided by the sand and pebbles washed along. Thus the hills are
being worn down and the valleys deepened and widened, and the
materials of the land are slowly being moved toward the sea, again to
be deposited in beds.

[Illustration: Wave-cut cliff with beach and spit built by waves and
currents]

Along the coast the waves, with the pebbles washed about, are wearing
away the land and spreading out its materials in new beds elsewhere.
The shore is being cut back in some places and built out in others.
Rivers bring down sand and mud and build deltas or bars at their
mouths.

Volcanoes pour out melted rock on the surface, and much fine material
is blown out in eruptions. Swamps are filled {115} by dead vegetable
matter and by sand and mud washed in. These materials form new rocks
and build up the surface. Thus the two processes, the wearing down in
some places and the building up in others, are tending to bring the
surface to a uniform level. Another process, so slow that it can be
observed only through long periods of time, tends to deform the
earth's crust and to make the surface more irregular. In times past,
layers of rock once horizontal have been bent and folded into great
arches and troughs, and large areas of the earth's surface have been
raised high above sea-level.

[Illustration: Rock ledge rounded smooth and scratched by ice]

[Illustration: Sand-dune with wind-rippled surface]