Abbey, which was destroyed by Henry VIII., were collected from a level expanse of turf, overlying the buried tesselated pavement, through which worm-burrows passed; and these castings contained innumerable particles of tiles and bricks, of concrete and cement, the majority of which had manifestly undergone some or much attrition. There were also many minute flakes of a micaceous slate, the points of which were rounded. If the above supposition, that in all these cases the same minute fragments have passed several times through the gizzards of worms, be rejected, notwithstanding its inherent probability, we must then assume that in all the above cases the many rounded fragments found in the castings had all accidentally undergone much attrition before they were swallowed; and this is highly improbable.

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; * 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.

*This conclusion reminds me of the vast amount of extremely fine chalky mud which is found within the lagoons of many atolls, where the sea is tranquil and waves cannot triturate the blocks of coral. This mud must, as I believe ('The Structure and Distribution of Coral-Reefs,' 2nd edit. 1874, p. 19), be attributed to the innumerable annelids and other animals which burrow into the dead coral, and to the fishes, Holothurians, &c., which browse on the living corals

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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 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 Too of an inch in diameter. Perhaps, then, we may conclude that a grain of an inch in diameter would be "worn as much or more in drifting a mile as "a grain of an inch in being drifted "100 miles. On the same principle a pebble 66 one inch in diameter would be worn re"latively more by being drifted only a few

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"hundred yards."* 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, together with Scotland, the land which is cultivated and is well fitted for these animals, has been estimated at above 32 million acres. The product is 320 million

million tons of earth.

* Anniversary Address: The Quarterly Journal of the Geological Soc.' May 1880, p. 59.

CHAPTER VI.

THE DENUDATION OF THE LAND-continued.

Denudation aided by recently ejected castings flowing down inclined grass-covered surfaces-The amount of earth which annually flows downwards-The effect of tropical rain on worm castings-The finest particles of earth washed completely away from castings-The disintegration of dried castings into pellets, and their rolling down inclined surfacesThe formation of little ledges on hill-sides, in part due to the accumulation of disintegrated castings-Castings blown to leeward over level land--An attempt to estimate the amount thus blown-The degradation of ancient encampments and tumuli-The preservation of the crowns and furrows on land anciently ploughed-The formation and amount of mould over the Chalk formation.

WE are now prepared to consider the more direct part which worms take in the denudation of the land. When reflecting on subaerial denudation, it formerly appeared to me, as it has to others, that a nearly level or very gently inclined surface, covered with turf, could suffer no loss during even a long lapse of time. It may, however, be urged that at long intervals, debacles of rain or