In many places, where streets ran beneath the surface, or where old buildings stood, the mould was only 8 inches in thickness; and Dr.Johnson was surprised that in ploughing the land, the ruins had never been struck by the plough as far as he had heard.He thinks that when the land was first cultivated the old walls were perhaps intentionally pulled down, and that hollow places were filled up.This may have been the case; but if after the desertion of the city the land was left for many centuries uncultivated, worms would have brought up enough fine earth to have covered the ruins completely; that is if they had subsided from having been undermined.The foundations of some of the walls, for instance those of the portion still standing about 20 feet above the ground, and those of the marketplace, lie at the extraordinary depth of 14 feet; but it is highly improbable that the foundations were generally so deep.The mortar employed in the buildings must have been excellent, for it is still in parts extremely hard.Wherever walls of any height have been exposed to view, they are, as Dr.Johnson believes, still perpendicular.The walls with such deep foundations cannot have been undermined by worms, and therefore cannot have subsided, as appears to have occurred at Abinger and Silchester.Hence it is very difficult to account for their being nowcompletely covered with earth; but how much of this covering consists of vegetable mould and how much of rubble I do not know.The market-place, with the foundations at a depth of 14 feet, was covered up, as Dr.Johnson believes, by between 6 and 24 inches of earth.The tops of the broken- down walls of a caldarium or bath, 9 feet in depth, were likewise covered up with nearly 2 feet of earth.The summit of an arch, leading into an ash- pit 7 feet in depth, was covered up with not more than 8 inches of earth.Whenever a building which has not subsided is covered with earth, we must suppose, either that the upper layers of stone have been at some time carried away by man, or that earth has since been washed down during heavy rain, or blown down during storms, from the adjoining land; and this would be especially apt to occur where the land has long been cultivated.In the above cases the adjoining land is somewhat higher than the three specified sites, as far as I can judge by maps and from information given me by Dr.Johnson.If; however, a great pile of broken stones, mortar, plaster, timber and ashes fell over the remains of any building, their disintegration in the course of time, and the sifting action of worms, would ultimately conceal the whole beneath fine earth.
Conclusion.--The cases given in this chapter show that worms have played a considerable part in the burial and concealment of several Roman and other old buildings in England; but no doubt the washing down of soil from the neighbouring higher lands, and the deposition of dust, have together aided largely in the work of concealment.Dust would be apt to accumulate wherever old broken-down walls projected a little above the then existing surface and thus afforded some shelter.The floors of the old rooms, halls and passages have generally sunk, partly from the settling of the ground, but chiefly from having been undermined by worms; and the sinking has commonly been greater in the middle than near the walls.The walls themselves, whenever their foundations do not lie at a great depth, have been penetrated and undermined by worms, and have consequently subsided.The unequal subsidence thus caused, probably explains the great cracks which may be seen in many ancient walls, as well as their inclination from the perpendicular.