an.

There is another element in the movement of the soils which, though
less appreciable, is still of great importance. The agents of decay
which produce and remove the detritus, the chemical changes of the bed
rock, and the mechanical action which roots apply to them, along with
the solutional processes, are constantly lowering the surface of the
mass. In this way we can often prove that a soil continuously
existing has worked downward through many thousand feet of strata. In
this process of downgoing the country on which the layer rests may
have greatly changed its form, but the deposit, under favourable
conditions, may continue to retain some trace of the materials which
it derived from beds which have long since disappeared, their position
having been far up in the spaces now occupied by the air. Where the
slopes are steep and streams abound, we rarely find detritus which
belonged in rock more than a hundred feet above the present surface of
the soil. Where, however, as on those isolated table-lands or buttes
which abound in certain portions of the Mississippi Valley, as well as
in many other countries, we find a patch of soil lying on a nearly
level surface, which for geologic ages has not felt the effect of
streams, we may discover, commingled in the _debris_, the harder
wreckage derived from the decay of a thousand feet or more of vanished
strata.

When we consider the effect of organic life on the processes which go
on in the soil, we first note the large fact that the development of
all land vegetation depends upon the existence of this detritus--in a
word, on the slow movement of the decaying rocky matter from the point
where it is disrupted to its field of rest in the depths of the sea.
The plants take their food from the portion of this rocky waste which
is brought into solution by the waters which penetrate the mass. On
the plants the animals feed, and so this vast assemblage of organisms
is maintained. Not only does the land life maintain itself on the
soil, and give much to the sea, but it serves in various ways to
protect this detrital coating from too rapid destruction, and to
improve its quality. To see the nature of this work we should visit a
region where primeval forests still lie upon the slopes of a hilly
region. In the body of such a wood we find next the surface a coating
of decayed vegetable matter, made up of the falling leaves, bark,
branches, and trunks which are constantly descending to