Читать книгу The Romance of Modern Geology. Describing in simple but exact language the making of the earth with some account of prehistoric animal life онлайн

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Off Shetland masses of rock twelve or thirteen tons in weight have been cut out from the cliff seventy feet above the smooth-water level. The sea's battering-rams are the masses of shingle, gravel, and loose blocks of stone which it carries with it; but it has subtler methods in the corrosive action of its salts, for just as it rusts or wears away iron, so its salts and acids must eat their way into many rocks.

But, after all, the coast-line of the world is a small fraction of the whole land surface of the globe; and a smaller fraction of the sea's own wide area. On that area are flung all the records and treasures which the sea has wrested from the land. The rivers, as we have already several times repeated, are the chief carriers of deposits to the sea. By their deltas they may be known. The deltas of the Ganges and Brahmaputra cover an area as large as that of England and Wales. The delta has been bored through to a depth of nearly five hundred feet, and has been found to consist of numerous alternations of fine clays, marls, and sands or sandstones, with occasional layers of gravel. In all this accumulation of sediment there are no traces of marine animals; but land plants and the plants and animals of the river and of the surrounding land have been discovered in quantity. The sea most often destroys land; but it sometimes deposits beaches; and, we might almost say, silts up the land. At Romney Marsh, for example, a tract of eighty square miles which was marsh in Julius Cæsar's time is now dry land, and has become so partly by the natural increase of shingle thrown up by the waves. The coarsest shingle usually accumulates towards the upper part of the beach, and the rest arranges itself generally according to size and weight, that which is finest being nearest to low-water mark.