Page:South African Geology - Schwarz - 1912.djvu/161

 the blue and yellow ground in the diamond pipes, but the presence of ice-scratched boulders at once tells one what the deposit is. In the south the matrix is harder, and weathers with a peculiar pillow structure which is very characteristic, the rounded blocks standing up like tombstones. It was behind these that the Bushmen were fond of hiding to shoot off their poisoned arrows at the unwary Boer; hence the popular name for the Dwyka Conglomerate is Bushman's Klip. The ice-scratched boulders in the south are quite as plentiful as in the north, and when the matrix has been softened a little by weathering, the surface of the Dwyka Conglomerate is identical with that of the glacial drift of England or north Europe. The Dwyka Conglomerate forms an extremely tough band which does not readily give to expansion and contraction except on the surface; hence springs are very rare in the country covered by it. The Upper Dwyka Shales are largely made up of extremely fine-grained shales which weather in peculiar starch-like fragments. Some of it is carbonaceous, and there is also sufficient pyrites in the rock to make it spontaneously ignite when water gains access to it. The fallen "reef" in the Kimberley mines has thus caught fire. Usually on the natural surface the ignition is slow and the black shale burns to a brilliant white calcined rock which forms a conspicuous white band. The pyrites, in decomposing, yields sulphur, which, combining with lime, forms gypsum, and the iron itself crumbles to powder as yellow or red ochre. Large flakes of gypsum and whole hills of ochre occur on the Upper Dwyka Shales, but they are only superficial deposits. A narrow band of flint or chert usually caps the carbonaceous portion. In the Upper Dwyka Shales there have been found in