It is certain that old burrows collapse in the course of time; for
as we shall see in the next chapter, the fine earth voided by
worms, if spread out uniformly, would form in many places in the
course of a year a layer 0.2 of an inch in thickness; so that at
any rate this large amount is not deposited within the old unused
burrows. If the burrows did not collapse, the whole ground would
be first thickly riddled with holes to a depth of about ten inches,
and in fifty years a hollow unsupported space, ten inches in depth,
would be left. The holes left by the decay of successively formed
roots of trees and plants must likewise collapse in the course of
time.
The burrows of worms run down perpendicularly or a little
obliquely, and where the soil is at all argillaceous, there is no
difficulty in believing that the walls would slowly flow or slide
inwards during very wet weather. When, however, the soil is sandy
or mingled with many small stones, it can hardly be viscous enough
to flow inwards during even the wettest weather; but another agency
may here come into play. After much rain the ground swells, and as
it cannot expand laterally, the surface rises; during dry weather
it sinks again. For instance, a large flat stone laid on the
surface of a field sank 3.
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