Page:South African Geology - Schwarz - 1912.djvu/50

 (1185 fathoms have been recorded), and the outlet at the Bosporus is not deep. As a matter of fact, the surface of the Caspian Sea, which is just such a lake without outlet, and lies in a region of small rainfall, is 86 ft. below sea level. A similar thing might happen to Lake Baikal which, though it lies 1500 ft. above sea level, is 4748 ft. deep where the Trans-Siberian railway crosses it.

Oceans. — The rivers that discharge their water into the oceans account for their existence, for the water cannot flow elsewhere, as the ocean basins are the lower portions of the earth's crust. There has been a theory that the water of the oceans is due to extrusion of original water vapour from the interior of the globe; but this depended on the supposition that volcanoes discharged vast quantities of water vapour during their explosions. Lately Brun has investigated the exhalations of volcanoes and found them to contain no trace of moisture, so that this theory falls to the ground.

The rivers carry with them a large amount of matter in suspension; the fine solid particles of mud subside very slowly in fresh water and the current of the river is sufficient to keep most of this from settling, so that it is discharged into the ocean. The mud settles quickly in salt water, but the incessant motion of the sea is sufficient to keep it moving, and thus this fine material is carried far out from the shore. The greatest distance mud can be carried from the shore is 200 ml. The heavier sand, which for the most part is dragged along the bottom of the river, is also discharged into the ocean, and is swept up and down the coast by the currents; but as a rule it is not carried beyond a few miles. These two classes of rock waste, the sands and the muds, form