King also observed that the passage up these towers hardly ever ran in
the same exact line with the underlying burrow, so that a thin
cylindrical object such as a haulm of grass, could not be passed down the
tower into the burrow; and this change of direction probably serves in
some manner as a protection.
Worms do not always eject their castings on the surface of the ground.
When they can find any cavity, as when burrowing in newly turned-up
earth, or between the stems of banked-up plants, they deposit their
castings in such places. So again any hollow beneath a large stone lying
on the surface of the ground, is soon filled up with their castings.
According to Hensen, old burrows are habitually used for this purpose;
but as far as my experience serves, this is not the case, excepting with
those near the surface in recently dug ground. I think that Hensen may
have been deceived by the walls of old burrows, lined with black earth,
having sunk in or collapsed; for black streaks are thus left, and these
are conspicuous when passing through light-coloured soil, and might be
mistaken for completely filled-up burrows.
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 3.33 mm. whilst the weather was
dry between May 9th and June 13th, and rose 1.91 mm, between September
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