Page:South African Geology - Schwarz - 1912.djvu/209

 The whole country is covered with superficial deposits either of concretionary limonite or ou klip, or of Kalahari sand with its accompanying calcareous or siliceous tufa. True laterite does not appear to be common. The plateau gravels occur far above the present levels of the rivers, and often contain diamonds, as in the Seta gravels on the Limpopo, which is an interesting occurrence as affording an instance where true ruby and true jade occur together with the diamond. In the Somabula Forest the gravels are even richer in gem stones. The matrix consists of rounded pebbles of quartz, banded jasper, agate, quartzite, silicified wood, granite, and chlorite schist, cemented by ferruginous sandy clay. In this there are found diamond, ruby, sapphire, beryl, chrysoberyl, topaz, garnet, staurolite, andalusite, kyanite, zircon, and tourmaline. True diamond pipes of the Kimberley type also occur on the Bembezi River.

CENTRAL AFRICA

The determination of the strata in Central Africa is at present impossible, owing to want of information. There is always the great granite basis with gneiss, schists, and occasionally limestone folded in or intruded by the granite. Above this is a thick series of coloured sandstones and flags which have been correlated with the Table Mountain Sandstone, but which are more probably Waterberg Sandstone. These are found in Nyasaland (Mafingi System), in German East Africa, and round Lake Tanganyika (Old African Sandstones). In Nyasaland the series is fully 10,000 ft. thick, and in the same country it is followed unconformably by an