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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The aspect of its force with which we are, however, most concerned is that which is directed to lowering gradually the surface of the land. In the last chapter we showed how much mineral might be dissolved in the waters of rivers. If we are to include also the amount of mud, sand, and other things classed altogether as silt which a river carries down, the figures become much more imposing. Sir Archibald Geikie says that, taking the Mississippi as a typical river (it is as good an example as would be found, because in its great length it passes through many different kinds of land, soil, and climate), we may assume that the average amount of sediment carried down by a river is one part of sediment to every 1500 parts of water.

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The Grand Cañon of Arizona

The Colorado River at this point is nearly 200 feet wide. The man is seated about 1200 feet above the river's level. This whole cañon up to the top of the mountains in the distance has been worn away by prehistoric current; and the river has gradually cut its bed deeper.