Page:South African Geology - Schwarz - 1912.djvu/205

 DAMARALAND

On the east the whole country is covered with Kalahari sand with occasional patches of surface limestone, but towards the coast the older platform of South Africa appears. It is thinly overlain by Dwyka Conglomerate and the overlying shales with Eurydesma and Conularia. The shales are some 600 ft. thick, and contain a little limestone and micaceous, light-coloured sandstones. Besides the marine fossils mentioned, there are Calamites (Schizoneura?) stems and blocks of Dadoxylon wood turned into stone. The continuity of these beds with those of Cape Colony is doubtful. It seems rather that all the Karroo sediments formed north of the well-marked shore line, which runs through Prieska, were deposited in independent lakes or estuaries. Dolerite dykes and sheets occur in the Damaraland-Karroo beds as they do in Cape Colony.

Near the coast calcareous sands have been found in places which have been doubtfully referred to as Cretaceous: the fossils obtained are rolled casts and their identification is uncertain. It has been supposed that these Cretaceous sandstones received the wash from the diamondiferous country round Kimberley, and that, consequently, the agates and other peculiar stones found in the Vaal River gravels, together with the diamonds, became embedded in the sand as it formed. When the land rose, and these beds became exposed to desert weathering, their friable nature caused them to crumble; the wind, catching the lighter particles, blew them away to form the dunes of the sand belt, but the heavier agates, pebbles, and diamonds were left behind. Large areas of gravel swept clear of the sand by the wind