Page:South African Geology - Schwarz - 1912.djvu/195

 Black Reef Series. It is a magnesian limestone, rather than a dolomite, which is as much as 5000 ft. thick in places. It has received many local names, such as Malmani Dolomite, Ngami Dolomite, and Otawi Dolomite, but the only one recognized now is that given to it by Stow, Campbell Rand Dolomite, and this term is restricted to the western outcrops. It weathers with a peculiar wrinkled brown coat, due to the insolubility of the manganese contained in the rock, which is left behind when the lime and magnesia pass out in solution; this brown coat gives it the local name of Olifant's Klip, Elephant's rock. Intercalated in the dolomite there are frequent beds of chert, often peculiarly aggregated as if of organic origin, but so far no recognizable fossils have been obtained. The chert bands become ferruginous towards the top. There are many peculiar features, due to the easy solubility of dolomite, such as underground rivers. The joints down which the water travels to supply the rivers beneath often become widened and funnel-shaped at the surface; thence arises what is known as a swallow hole, or wondergat, as it is called locally, that is, a circular pool of water in the limestone. In the Kaap plateau, which is composed of horizontal beds of limestone of great thickness, this tendency of all the water to disappear underground leads to the surface being waterless; such a country is called a karst land, from the name given to it in the typical locality on the northeast of the Adriatic in Bosnia and Herzegovina. The underground rivers often emerge as fine springs, as at Kuruman, and these are called dolinen. Great masses of calcareous tufa are deposited at the foot of the escarpment of the Kaap plateau by the escape of the dolinen and the evaporation of the water. The joints, widened