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Show 234 DISINTEGRATION CnAr. V. drainage must be lowered ~ of a foot annually, or 1 foot in 4566 years. Consequently, taking the best estimate of the mean height of the North American continent, viz. 7 48 feet, and looking to the future, the whole of the great Mississippi basin will be washed away, and " brought down to the sea(' level in less than 4,500,000 years, if no '' elevation of the land takes place." Some rivers carry down much more sediment relatively to their size, and some much less than the Mississippi. Disintegrated matter: is carried away by the wind as well as by running water. During volcanic outbursts much rock is triturated and is thus widely dispersed; and in all arid countries the wind plays an important part in the removal of such matter. Wind-driven sand also wears down the hardest rocks. I have shown * that during four months of the year a large quantity of dust is blown from the north-western shores of Africa, and falls on the Atlantic over a • "An account of the fine dust which often fi:l.lls on V csscls in the Atlantic Ocean," Proc. Geolog. Soc. of London, June 4th 1845. ' 0IIAP. V AND DENUDATION. 235 space of 1600 miles in latitude, and for a distance of from 300 to 600 miles from the coast. 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 fron1 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 nfoo 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, |
