OCR Text |
Show 236 DISINTEGRATION CHAP. V. 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 CHAP. v. AND DENUDATION. 237 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.* 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 con- "' For La Plata, sec my 'Journal of Researches,' during the voyage of tho Beagle, 1845, p. 133. Elie de Beaumont bas given (' Les;ons de Gcolog. pratique,' tom. l. 1845, p. 183) an excellent account of the enormo~1s quantity of du:;t which is transported in some countries. I cannot but think tbat Mr. Proctor has somewhat exaggerated ('Pleasant Ways in ~cicnco,' 1879, p. 379) the agency of dust in a humid country like Great Britain. James Geikie has given (' Prehi$toric EurO})C,' 1880, p. 165) a full abstract of Richthof n's views, which, however, he disputes. |