globe of solid rock,--the lithosphere,--surrounded by two mobile
envelopes: the envelope of air,--THE ATMOSPHERE, and the envelope
of water,--THE HYDROSPHERE. Under the action of solar energy these
envelopes are in constant motion. Water from the hydrosphere is
continually rising in vapor into the atmosphere, the air of the
atmosphere penetrates the hydrosphere,--for its gases are
dissolved in all waters,--and both air and water enter and work
upon the solid earth. By their action upon the lithosphere they
have produced a third envelope,--the mantle of rock waste.

This envelope also is in movement, not indeed as a whole, but
particle by particle. The causes which set its particles in
motion, and the different forms which the mantle comes to assume,
we will now proceed to study.

MOVEMENTS OF THE MANTLE OF ROCK WASTE

At the sandstone ledges which we first visited we saw not only
that the rocks were crumbling away, but also that grains and
fragments of them were creeping down the slopes of the valley to
the stream and were carried by it onward toward the sea. This
process is going on everywhere. Slowly it may be, and with many
interruptions, but surely, the waste of the land moves downward to
the sea. We may divide its course into two parts,--the path to the
stream, which we will now consider, and its carriage onward by the
stream, which we will defer to a later chapter.

GRAVITY. The chief agent concerned in the movement of waste is
gravity. Each particle of waste feels the unceasing downward pull
of the earth's mass and follows it when free to do so. All