brought to the surface on each acre of land; so that the whole
superficial bed of vegetable mould passes through their bodies in the
course of every few years. From the collapsing of the old burrows the
mould is in constant though slow movement, and the particles composing it
are thus rubbed together. By these means fresh surfaces are continually
exposed to the action of the carbonic acid in the soil, and of the
humus-acids which appear to be still more efficient in the decomposition
of rocks. The generation of the humus-acids is probably hastened during
the digestion of the many half-decayed leaves which worms consume. Thus
the particles of earth, forming the superficial mould, are subjected to
conditions eminently favourable for their decomposition and
disintegration. Moreover, the particles of the softer rocks suffer some
amount of mechanical trituration in the muscular gizzards of worms, in
which small stones serve as mill-stones.
The finely levigated castings, when brought to the surface in a moist
condition, flow during rainy weather down any moderate slope; and the
smaller particles are washed far down even a gently inclined surface.
Castings when dry often crumble into small pellets and these are apt to
roll down any sloping surface. Where the land is quite level and is
covered with herbage, and where the climate is humid so that much dust
cannot be blown away, it appears at first sight impossible that there
should be any appreciable amount of sub-aerial denudation; but
worm-castings are blown, especially whilst moist and viscid, in one
uniform direction by the prevalent winds which are accompanied by rain.
By these several means the superficial mould is prevented from
accumulating to a great thickness; and a thick bed of mould checks in
many ways the disintegration of the underlying rocks and fragments of
rock.
The removal of worm-castings by the above means leads to results which
are far from insignificant. It has been shown that a layer of earth, 0.2
of an inch in thickness, is in many places annually brought to the
surface; and if a small part of this amount flows, or rolls, or is
washed, even for a short distance, down every inclined surface, or is
repeatedly blown in one direction, a great effect will be produced in the
course of ages. It was found by measurements and calculations that on a
surface with a mean inclination of 9 26 , 2.4 cubic inches of earth
which had been ejected by worms crossed, in the course of a year, a
horizontal line one yard in length; so that 240 cubic inches would cross
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