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Show 118 IIABlTS OF \VOHMS. CuAP. n. the surface of tl10 grouucl. 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 casting in such places. So again any lw1low beneath a large stone lying on the surface of ·the ground, is soou filled up with their castings. According to fiensen, 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 dng ground. I think that IIensen may have been deceived by the walls of old burrows, lined with black earth, having sunk in or collapsed; for black streaks are thus ]eft, and these are conspicuous when passing through lightcoloured soil, and might be mistaken for completely filled-up burrows. It is certain that old burrows collapse iu 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 -} of <lD 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, CrrAP. II. 'l'IIE COLLAPSE 01~ OLD BURUOWS. 119 the whole grouncl would be fir.-t thickly riddlocl with holes to a depth of about ten incl1o , and in fifty years a hollow unsupported space, ten inches in depth, would b left. The holes left by the decay of succe - sively formed roots of trees and plants mu t likewise collapse in the conrso of time. The burrows of worms run down p I') 'ndicularly or a little obliquely, and where th 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 cau hardly be viscous enough to flow inward. 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; durin7 dry weather it sinks again. For instance, a large flat stone laid on the surface of a field Rank 3·33 mm. whilst tho weather was dry between May 9th and June 13th, and rose 1•91 mm. between September 7th and 19th, much rain having fallen during the latter part of this time. During fro ts and thaws |