the earth.
Ordinarily, this layer is a foot or more in thickness; at the top it
is almost altogether composed of vegetable matter; at the bottom it
verges into the true soil. An important effect of this decayed
vegetation is to restrain the movement of the surface water. Even in
the heaviest rains, provided the mass be not frozen, the water is
taken into it and delivered in the manner of springs to the larger
streams. We can better note the measure of this effect by observing
the difference in the ground covered by this primeval forest and that
which we find near by which has been converted into tilled fields.
With the same degree of rapidity in the flow, the distinct stream
channels on the tilled ground are likely to be from twenty to a
hundred times in length what they are on the forest bed. The result is
that while the brook which drains the forested area maintains a
tolerably constant flow of clean water, the other from the tilled
ground courses only in times of heavy rain, and then is heavily
charged with mud. In the virgin conditions of the soil the downwear is
very slow; in its artificial state this wearing goes on so rapidly
that the sloping fields are likely to be worn to below the soil level
in a few score years.

Not only does the natural coating of vegetation, such as our forests
impose upon the country, protect the soil from washing away, but the
roots of the larger plants are continually at work in various ways to
increase the fertility and depth of the stratum. In the form of
slender fibrils these underground branches enter the joints and bed
planes of the rock, and there growing they disrupt the materials,
giving them a larger surface on which decay may operate. These bits,
at first of considerable size, are in turn broken up by the same
action. Where the underlying rocks afford nutritious materials, the
branches of our tap-rooted trees sometimes find their way ten feet or
more below the base of the true soil. Not only do they thus break up
the stones, but the nutrition which they obtain in the depths is
brought up and deposited in the parts above the ground, as well as in
the roots which lie in the true soil, so that when the tree dies it
becomes available for other plants. Thus in the forest condition of a
country the amount of rock material contributed to the deposit in
general so far exceeds that which is taken away to the rivers by the
underground water as to insure the deepening of the soil bed to