From the facts enumerated in the last chapter, it will appear that, on the borders of the ocean, currents co-operating with tides are most powerful instruments in the destruction and transportation of rocks; and as numerous tributaries discharge their alluvial burden into the channel of one great river, so we find that many great rivers often deliver their earthy contents to one marine current, to be borne by it to a distance, and deposited in some deep receptacles of the ocean. The current not only receives this tribute of sedimentary matter from streams draining the land, but acts also itself on the coast, as does a river on the cliffs which bound a valley. The course of currents on the British shores is ascertained to be as tortuous as that of ordinary rivers. Sometimes they run between sandbanks which consist of matter thrown down at certain points where the velocity of the stream had been retarded; but it very frequently happens, that as in a river one bank is made of alluvial gravel, while the other is composed of some hard rock constantly undermined, so the current, in its bends, strikes here and there upon a coast which then forms one bank, while a shoal under water forms the other. If the coast be composed of solid materials, it yields slowly, or if of great height, it does not lose ground rapidly, since a large quantity of matter must then be removed before the sea can penetrate to any distance. But the openings where rivers enter are generally the points of least resistance, and it is here, therefore, that the ocean makes the widest and longest breaches.