This explains the chamber at the base of the shaft, and the necessity of a cement to hold the walls together, for otherwise the creature's continual comings and goings would result in a landslip.
A matter less easy of explanation is the complete disappearance of the material which originally filled the excavated space.

Where are the twelve cubic inches of earth that represent the average volume of the original contents of the shaft?
There is not a trace of this material outside, nor inside either.

And how, in a soil as dry as a cinder, is the plaster made with which the walls are covered?
Larvae which burrow in wood, such as those of Capricornis and Buprestes, will apparently answer our first question.

They make their way through the substance of a tree-trunk, boring their galleries by the simple method of eating the material in front of them.

Detached by their mandibles, fragment by fragment, the material is digested.