rapidly flattened, the carrying power of the rivers is decreased, and the wearing power lessened; hence rivers flowing from mountains in this way tend to eat back into the hills by what is called head-stream erosion. The carrying power of the river depends greatly on the slope; on the mountain-side great masses of rubble and big boulders may be carried down, but once the river reaches the plain the small gradient reduces the carrying power of the river, and the stones and rubble are left behind at the foot of the hills. In the Karroo this feature is found everywhere, and it is essentially a characteristic of semi-desert regions, because in humid regions the action of rain and rivers is more continual and gradual.
In mountainous regions the soil, if there is any, or the bare rocks become occasionally so crumbly by prolonged droughts that when the rain falls it is absorbed as if by a sponge. After a while the crumbled material becomes surcharged with moisture and turns into a quicksand. Being on a slope, the quicksand begins to flow, carrying with it large boulders, and the whole mass rolls down the hill like a lava stream, overwhelming any obstacle, such as houses and trees, in its path, and producing an incredible roaring, as the rocks embedded in the semi-liquid mass grind against each other. When the mud-rush comes to the plains it spreads out harmlessly, and after a while the sand drains away or is washed away by subsequent rains, and there is left behind a trail of great blocks of rock, which are known as stone rivers. On the Karroo hills these are familiar objects, and they are made the more prominent as the moisture retained in them causes a luxuriant growth of bush on the sides of the stone rivers. In the coastal country the mud-rushes are often occasioned by bush
