On the other hand it must be stated that fragments of ornamental tiles, somewhat harder than common tiles or bricks, which had been swallowed only once by worms kept in confinement, were with the doubtful exception of one or two of the smallest grains, not at all rounded. Nevertheless some of them appeared a little worn, though not rounded. Notwithstanding these cases, if we consider the evidence above given, there can be little doubt that the fragments, which serve as millstones in the gizzards of worms, suffer, when of a not very hard texture, some amount of attrition; and that the smaller particles in the earth, which is habitually swallowed in such astonishingly large quantities by worms, are ground together and are thus levigated. If this be the case, the “terra tenuissima,”—the “pâte excessivement fine,”—of which the castings largely consist, is in part due to the mechanical action of the gizzard; {234} and this fine matter, as we shall see in the next chapter, is that which is chiefly washed away from the innumerable castings on every field during each heavy shower of rain. If the softer stones yield at all, the harder ones will suffer some slight amount of wear and tear. The trituration of small particles of stone in the gizzards of worms is of more importance under a geological point of view than may at first appear to be the case; for Mr. Sorby has clearly shown that the ordinary means of disintegration, namely, running water and the waves of the sea, act with less and less power on fragments of rock the smaller they are. “Hence,” as he remarks, “even making no allowance for the extra buoying up of very minute particles by a current of water, depending on surface cohesion, the effects of wearing on the form of the grains must vary directly as their diameter or thereabouts. If so, a grain of 1/10 an inch in diameter would be worn ten times as much as one of an inch in diameter, and at least a hundred times as much as one of 1/100 an inch in diameter. Perhaps, then, we may conclude that a grain 1/10 of an inch in diameter would be worn as much or more in drifting a mile as a grain 1/1000 of an inch in being drifted 100 miles. On the same principle a pebble one inch in diameter would be worn relatively more by being drifted only a few hundred yards.” {236} Nor should we forget, in considering the power which worms exert in triturating particles of rock, that there is good evidence that on each acre of land, which is sufficiently damp and not too sandy, gravelly or rocky for worms to inhabit, a weight of more than ten tons of earth annually passes through their bodies and is brought to the surface. The result for a country of the size of Great Britain, within a period not very long in a geological sense, such as a million years, cannot be insignificant; for the ten tons of earth has to be multiplied first by the above number of years, and then by the number of acres fully stocked with worms; and in England,