Page:South African Geology - Schwarz - 1912.djvu/37

 to peneplanation to a base-level determined by the granite of the Augrabies Falls. The second case is where the river is sufficiently powerful to cut through the bar: the softer rock on either side is removed, and the bar emerges as a mountain; the course of the river continues the same and its valley forms a gorge in the mountain. The finest scenery of Cape Colony is due to this latter cause: the great coastal mountains are cleft with stupendous gorges. At Meiring's Poort, through the Zwartberg range in Oudtshoorn, the mountains on either side rise to 7000 ft. and the level of the river is 1500 ft, the walls in many cases being almost perpendicular.

When the hardness of the bar is about equal to the cutting power of the river, the latter forms a lake or vley behind the bar.

If there are two rivers running parallel, and the one runs over soft ground, but the other runs over hard ground, or has a bar in its course, the former will lower its bed more rapidly than the latter. The drainage area of the one river will tend to enlarge more rapidly than is the case with the other river, and it may happen that a tributary of the lower river may eat back so far as to draw off the water from the upper part of the river in the higher level. This frequently happens in nature, and the process is called river capture, the river with its head waters drained into another basin is said to be beheaded.

The greater the slope the more the water tears down it, and the more gravel and shingle will it carry to wear away its bed. On the side of the mountain the torrents wear out their beds very rapidly, but directly the same rivers emerge on to the plains at the foot, the slope is