osed to the conditions of the atmosphere, in
process of being taken to pieces and returned to the sea. This action
goes on everywhere; every drop of rain helps it. It is aided by frost,
or even by the changes of expansion and contraction which occur in the
rocks from variations of heat. The result is that, except where the
slopes are steep, the surface is quickly covered with a layer of
fragments, all of which are in the process of decay, and ready to
afford some food to plants. Even where the rock appears bare, it is
generally covered with lichens, which, adhering to it, obtain a share
of nutriment from the decayed material which they help to hold on the
slope. When they have retained a thin sheet of the _debris_, mosses
and small flowering plants help the work of retaining the detritus.
Soon the strong-rooted bushes and trees win a foothold, and by sending
their rootlets, which are at first small but rapidly enlarge, into the
crevices, they hasten the disruption of the stones.

If the construction of soil goes on upon a steep cliff, the quantity
retained on the slope may be small, but at the base we find a talus,
composed of the fragments not held by the vegetation, which gradually
increases as the cliff wears down, until the original precipice may be
quite obliterated beneath a soil slope. At first this process is
rapid; it becomes gradually slower and slower as the talus mounts up
the cliff and as the cliff loses its steepness, until finally a gentle
slope takes the place of the steep.

From the highest points in any river valley to the sea level the
broken-up rock, which we term soil, is in process of continuous
motion. Everywhere the rain water, flowing over the surface or soaking
through the porous mass, is conveying portions of the material which
is taken into solution in a speedy manner to the sea. Everywhere the
expansion of the soil in freezing, or the movements imposed on it by
the growth of roots, by the overturning of trees, or by the
innumerable borings and burrowings which animals make in the mass, is
through the action of gravitation slowly working down the slope. Every
little disturbance of the grains or fragments of the soil which lifts
them up causes them when they fall to descend a little way farther
toward the sea level. Working toward the streams, the materials of the
soil are in time delivered to those flowing waters, and by them urged
speedily, though in most cases interruptedly, toward the oce