worm-burrows passed; and these castings contained innumerable particles
of tiles and bricks, of concrete and cement, the majority of which had
manifestly undergone some or much attrition. There were also many minute
flakes of a micaceous slate, the points of which were rounded. If the
above supposition, that in all these cases the same minute fragments have
passed several times through the gizzards of worms, be rejected,
notwithstanding its inherent probability, we must then assume that in all
the above cases the many rounded fragments found in the castings had all
accidentally undergone much attrition before they were swallowed; and
this is highly improbable.
On the other hand it must be stated that fragments of ornamental tiles,
somewhat harder than common tiles or bricks, which had been swallowed
only once by worms kept in confinement, were with the doubtful exception
of one or two of the smallest grains, not at all rounded. Nevertheless
some of them appeared a little worn, though not rounded. Notwithstanding
these cases, if we consider the evidence above given, there can be little
doubt that the fragments, which serve as millstones in the gizzards of
worms, suffer, when of a not very hard texture, some amount of attrition;
and that the smaller particles in the earth, which is habitually
swallowed in such astonishingly large quantities by worms, are ground
together and are thus levigated. If this be the case, the terra
tenuissima, the pate excessivement fine, of which the castings largely
consist, is in part due to the mechanical action of the gizzard; {234}
and this fine matter, as we shall see in the next chapter, is that which
is chiefly washed away from the innumerable castings on every field
during each heavy shower of rain. If the softer stones yield at all, the
harder ones will suffer some slight amount of wear and tear.
The trituration of small particles of stone in the gizzards of worms is
of more importance under a geological point of view than may at first
appear to be the case; for Mr. Sorby has clearly shown that the ordinary
means of disintegration, namely, running water and the waves of the sea,
act with less and less power on fragments of rock the smaller they are.
Hence, as he remarks, even making no allowance for the extra buoying
up of very minute particles by a current of water, depending on surface
cohesion, the effects of wearing on the form of the grains must vary
directly as their diameter or thereabouts. If so, a grain of 1/10 an
inch in diameter would be worn ten times as much as one of an inch in
diameter, and at least a hundred times as much as one of 1/100 an inch in
110