But dust has been seen to fall at a distance of 1030 miles from the shores of Africa. During a stay of three weeks at St. Jago in the Cape Verde Archipelago, the atmosphere was almost always hazy, and extremely fine dust coming from Africa was continually falling. In some of this dust which fell in the open ocean at a distance of between 330 and 380 miles from the African coast, there were many particles of stone, about 1/1000 of an inch square. Nearer to the coast the water has been seen to be so much discoloured by the falling dust, that a sailing vessel left a track behind her. In countries, like the Cape Verde Archipelago, where it seldom rains and there are no frosts, the solid rock nevertheless disintegrates; and in conformity with the views lately advanced by a distinguished Belgian geologist, De Koninck, such disintegration may be attributed in chief part to the action of the carbonic and nitric acids, together with the nitrates and nitrites of ammonia, dissolved in the dew.

In all humid, even moderately humid, countries, worms aid in the work of denudation in several ways. The vegetable mould which covers, as with a mantle, the surface of the land, has all passed many times through their bodies. Mould differs in appearance from the subsoil only in its dark colour, and in the absence of fragments or particles of stone (when such are present in the subsoil), larger than those which can pass through the alimentary canal of a worm. This sifting of the soil is aided, as has already been remarked, by burrowing animals of many kinds, especially by ants. In countries where the summer is long and dry, the mould in protected places must be largely increased by dust blown from other and more exposed places. For instance, the quantity of dust sometimes blown over the plains of La Plata, where there are no solid rocks, is so great, that during the "gran seco," 1827 to 1830, the appearance of the land, which is here unenclosed, was so completely changed that the inhabitants could not recognise the limits of their own estates, and endless lawsuits arose. Immense quantities of dust are likewise blown about in Egypt and in the south of France. In China, as Richthofen maintains, beds appearing like fine sediment, several hundred feet in thickness and extending over an enormous area, owe their origin to dust blown from the high lands of central Asia. {61} In humid countries like Great Britain, as long as the land remains in its natural state clothed with vegetation, the mould in any one place can hardly be much increased by dust; but in its present condition, the fields near high roads, where there is much traffic, must receive a considerable amount of dust, and when fields are harrowed during dry and windy weather, clouds of dust may be seen to be blown away. But in all these cases the surface-soil is merely transported from one place to another. The dust which falls so thickly within our houses consists largely of organic matter, and if spread over the land would in time decay and disappear almost entirely. It appears, however, from recent observations on the snow-fields of the Arctic regions, that some little meteoric dust of extra mundane origin is continually falling.

The dark colour of ordinary mould is obviously due to the presence of decaying organic matter, which, however, is present in but small quantities. The loss of weight which mould suffers when heated to redness seems to be in large part due to water in combination being dispelled. In one sample of fertile mould the amount of organic matter was ascertained to be only 1.76 per cent.; in some artificially prepared soil it was as much as 5.5 per cent., and in the famous black soil of Russia from 5 to even 12 per cent. {62} In leaf-mould formed exclusively by the decay of leaves the amount is much greater, and in peat the carbon alone sometimes amounts to 64 per cent.; but with these latter cases we are not here concerned. The carbon in the soil tends gradually to oxidise and to disappear, except where water accumulates and the climate is cool; {63} so that in the oldest pasture-land there is no great excess of organic matter, notwithstanding the continued decay of the roots and the underground stems of plants, and the occasional addition of manure. The disappearance of the organic matter from mould is probably much aided by its being brought again and again to the surface in the castings of worms.

Worms, on the other hand, add largely to the organic matter in the soil by the astonishing number of half-decayed leaves which they draw into their burrows to a depth of 2 or 3 inches. They do this chiefly for obtaining food, but partly for closing the mouths of their burrows and for lining the upper part. The leaves which they consume are moistened, torn into small shreds, partially digested, and intimately commingled with earth; and it is this process which gives to vegetable mould its uniform dark tint. It is known that various kinds of acids are generated by the decay of vegetable matter; and from the contents of the intestines of worms and from their castings being acid, it seems probable that the process of digestion induces an analogous chemical change in the swallowed, triturated, and half-decayed leaves.

Charles Darwin

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