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The last paper read was, by C. Darwin, Esq. F.G.S. 'On the Formation of Mould.* The author commenced by remarking on the two most striking characters, by which the superficial layer of vegetable mould is distinguished. These are, its nearly homogeneous nature, although overlying different kinds of subsoil, and the uniform fineness of its particles. This may be well observed in any gravelly country, where, although in a ploughed field, a large proportion of the soil consists of small stones, yet in old pasture land not a single pebble will be found within some inches of the surface. The author's attention was called to this subject, by Mr. Wedgwood of Maer Hall, in Staffordshire, who showed him several fields, some of which a few years before had been covered with lime, and others with burnt marl and cinders. These substances in every case were now buried to the depth of some inches beneath the turf, as was ascertained by a careful examination of the several fields; and Mr. Darwin stated, that the appearance in all cases was as if the fragments had, as the farmers believe, worked themselves down. But it did not appear to him at all possible, that either the powdered lime or the fragments of burnt marl and the pebbles, could sink through compact earth to some inches beneath the surface. Nor is it probable that the decay of the grass, although adding to the surface of some of the constituent parts of the mould, should separate in so short a time the fine, from the coarse earth, and accumulate the former on those objects, which had so lately been strewed on the surface.

Mr. Darwin had also observed near towns, in apparently unploughed fields, pieces of pottery and bones some inches below the surface, so on the mountains of Chili, he had been perplexed by marine elevated shells, covered by earth, in situations where rain could not have washed it on them. The explanation which occurred to Mr. Wedgwood of these phenomena, Mr. Darwin does not doubt to be the correct one: namely, that the whole is due to the digestive process, by which the common earth-worm is supported. On carefully examining between the blades of grass in the fields where the observations had been made, the author found that there was scarcely a space of two inches square, without a little heap of the cylindrical castings of worms.

It is well known, that worms in their excavations swallow earthy matter, and having separated the serviceable portion object, at the mouth of their burrows, the remainder in little intestine shaped heaps; hence the fine particles are brought to the surface, and the cinders, burnt marl, or powdered lime, would by degrees be undermined, and eventually become covered by what was previously the underlying earth. In a field on which cinders had been spread only half a year before, Mr. Darwin actually saw the castings of the worms heaped on the smaller fragments.

On the above hypothesis, the great advantage of old pasture land, which farmers are always averse to break up, is explained; for the worms must require a considerable length of time to prepare a thick stratum of mould, by thoroughly mingling the original constituent parts of the soil, as well as the manures added by man. The author observes, that the digestive process of animals is a geological power of greater extent than might at first be imagined. To recent coral formations, the quantity of stone converted into the most impalpable mud, by the excavations of boring shells, and of nereidous animals, must be very great. Numerous large fish (of the genus Sparus) likewise subject by browsing on the living branches of coral. Mr. Darwin believes, that large portions of the chalk of Europe has been produced from coral, by the digestive action of marine animals the, same manner as mould has been prepared by the same process on disintegrated rock.