These thicknesses must, however, be corrected, for the triturated castings, after being well shaken down and pressed, did not make nearly so compact a mass as vegetable mould, though each separate particle was very compact. Yet mould is far from being compact, as is shown by the number of air-bubbles which rise up when the surface is flooded with water. It is moreover penetrated by many fine roots. To ascertain approximately by how much ordinary vegetable mould would be increased in bulk by being broken up into small particles and then dried, a thin oblong block of somewhat argillaceous mould (with the turf pared off) was measured before being broken up, was well dried and again measured. The drying caused it to shrink by 1/7 of its original bulk, judging from exterior measurements alone. It was then triturated and partly reduced to powder, in the same manner as the castings had been treated, and its bulk now exceeded (notwithstanding shrinkage from drying) by 1/16 that of the original block of damp mould. Therefore the above calculated thickness of the layer, formed by the castings from the Terrace, after being damped and spread over a square yard, would have to be reduced by 1/16; and this will reduce the layer to 0.09 of an inch, so that a layer 0.9 inch in thickness would be formed in the course of ten years. On the same principle the castings from the Common would make in the course of a single year a layer 0.1429 inch, or in the course of 10 years 1.429 inch, in thickness. We may say in round numbers that the thickness in the former case would amount to nearly 1 inch, and in the second case to nearly 1½ inch in 10 years. In order to compare these results with those deduced from the rates at which small objects left on the surfaces of grass-fields become buried (as described in the early part of this chapter), we will give the following summary:— SUMMARY OF THE THICKNESS OF THE MOULD ACCUMULATED OVER OBJECTS LEFT STREWED ON THE SURFACE, IN THE COURSE OF TEN YEARS. The accumulation of mould during 14¾ years on the surface of a dry, sandy, grass-field near Maer Hall, amounted to 2.2 inches in 10 years. The accumulation during 21½ years on a swampy field near Maer Hall, amounted to nearly 1.9 inch in 10 years. The accumulation during 7 years on a very swampy field near Maer Hall amounted to 2.1 inches in 10 years. The accumulation during 29 years, on good, argillaceous pasture-land over the Chalk at Down, amounted to 2.2 inches in 10 years. The accumulation during 30 years on the side of a valley over the Chalk at Down, the soil being argillaceous, very poor, and only just converted into pasture (so that it was for some years unfavourable for worms), amounted to 0.83 inch in 10 years.