I may here append a modern instance of the sinking of a pavement, communicated to me in 1871 by Mr.Ramsay, Director of the Geological Survey of England.A passage without a roof, 7 feet in length by 3 feet 2 inches in width, led from his house into the garden, and was paved with slabs of Portland stone.Several of these slabs were 16 inches square, others larger, and some a little smaller.This pavement had subsided about 3 inches along the middle of the passage, and two inches on each side, as could be seen by the lines of cement by which the slabs had been originally joined to the walls.The pavement had thus become slightly concave along the middle; but there was no subsidence at the end close to the house.Mr.Ramsay could not account for this sinking, until he observed that castings of black mould were frequently ejected along the lines of junction between the slabs; and these castings were regularly swept away.The several lines of junction, including those with the lateral walls, were altogether 39 feet 2 inches in length.The pavement did not present the appearance of ever having been renewed, and the house was believed to have been built about eighty-seven years ago.Considering all these circumstances, Mr.Ramsay does not doubt that the earth brought up by the worms since the pavement was first laid down, or rather since the decay of the mortar allowed the worms to burrow through it, and therefore within a much shorter time than the eighty-seven years, has sufficed to cause the sinking of the pavement to the above amount, except close to the house, where the ground beneath would have been kept nearly dry.
Beaulieu Abbey, Hampshire.--This abbey was destroyed by Henry VIII., and there now remains only a portion of the southern aisle- wall.It is believed that the king had most of the stones carried away for building a castle; and it is certain that they have been removed.The positions of thenave and transepts were ascertained not long ago by the foundations having been found; and the place is now marked by stones let into the ground.Where the abbey formerly stood, there now extends a smooth grass-covered surface, which resembles in all respects the rest of the field.The guardian, a very old man, said the surface had never been levelled in his time.In the year 1853, the Duke of Buccleuch had three holes dug in the turf within a few yards of one another, at the western end of the nave; and the old tesselated pavement of the abbey was thus discovered.These holes were afterwards surrounded by brickwork, and protected by trap- doors, so that the pavement might be readily inspected and preserved.When my son William examined the place on January 5, 1872, he found that the pavement in the three holes lay at depths of 6.75, 10 and 11.5 inches beneath the surrounding turf- covered surface.The old guardian asserted that he was often forced to remove worm-castings from the pavement; and that he had done so about six months before.My son collected all from one of the holes, the area of which was 5.32 square feet, and they weighed 7.97 ounces.Assuming that this amount had accumulated in six months, the accumulation during a year on a square yard would be 1.68 pounds, which, though a large amount, is very small compared with what, as we have seen, is often ejected on fields and commons.When I visited the abbey on June 22, 1877, the old man said that he had cleared out the holes about a month before, but a good many castings had since been ejected.I suspect that he imagined that he swept the pavements oftener than he really did, for the conditions were in several respects very unfavourable for the accumulation of even a moderate amount of castings.The tiles are rather large, viz., about 5.5 inches square, and the mortar between them was in most places sound, so that the worms were able to bring up earth from below only at certain points.The tiles rested on a bed of concrete, and the castings in consequence consisted in large part (viz., in the proportion of 19 to 33) of particles of mortar, grains of sand, little fragments of rock, bricks or tile; and such substances could hardly be agreeable, and certainly not nutritious, to worms.
My son dug holes in several places within the former walls of the abbey, at a distance of several yards from the above described brickedsquares.He did not find any tiles, though these are known to occur in some other parts, but he came in one spot to concrete on which tiles had once rested.The fine mould beneath the turf on the sides of the several holes, varied in thickness from only 2 to 2.75 inches, and this rested on a layer from 8.75 to above 11 inches in thickness, consisting of fragments of mortar and stone- rubbish with the interstices compactly filled up with black mould.In the surrounding field, at a distance of 20 yards from the abbey, the fine vegetable mould was 11 inches thick.
We may conclude from these facts that when the abbey was destroyed and the stones removed, a layer of rubbish was left over the whole surface, and that as soon as the worms were able to penetrate the decayed concrete and the joints between the tiles, they slowly filled up the interstices in the overlying rubbish with their castings, which were afterwards accumulated to a thickness of nearly three inches over the whole surface.If we add to this latter amount the mould between the fragments of stones, some five or six inches of mould must have been brought up from beneath the concrete or tiles.The concrete or tiles will consequently have subsided to nearly this amount.The bases of the columns of the aisles are now buried beneath mould and turf.It is not probable that they can have been undermined by worms, for their foundations would no doubt have been laid at a considerable depth.If they have not subsided, the stones of which the columns were constructed must have been removed from beneath the former level of the floor.