Other striking examples of the action of worms are found in the ruins of the old Roman town of Silchester, of which Figs. 7 and 8 show the extent to which the basilica has been covered. In Fig. 7, the concrete floor, still covered here and there with tesseræ, is found at three feet below the surface, and on it are piles of charred wood, represented by the black. Fig. 8 does not reach to the natural sub-soil. Worm-castings were observed on the floors of several of the rooms, in one of which the tesselation was unusually perfect. One or
occasionally two open worm-burrows were found beneath all the loose tesseræ. Worms have also penetrated the old walls of the ruins, and were found in them, with traces of the mold which they had carried to them. In almost all the rooms the pavement has sunk considerably, as Mr. Darwin shows in three sections, one of which he gives (Fig. 9). Dust blown by the wind and earth washed from the hills may have partly aided in covering up these buildings, but the chief share of the work is attributable to worms.
Worms also contribute to the disintegration of the rocks and the denudation of the land, by generating humus acids which act on the carbonate by grinding up in their crops the stones they swallow, and