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; {75} 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.
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