Page:South African Geology - Schwarz - 1912.djvu/185

 enormous residual mass of molten matter forced its way upwards, but failed to pierce the Waterberg sediments. Instead, it lifted them and swelled out as an enormous laccolite between them and the underlying rocks; thus was formed the Bushveld laccolite, 250 ml. in breadth. This part of the continent now became dry land, and contributed sediments which went to form the Table Mountain Sandstone and succeeding beds in the south. At the beginning of Karroo times the land was glaciated, and terrestrial boulder clay lies over a large portion of the southern and western edge of the area. In the Transvaal the Karroo lake transgressed the southern portion of the area, and formed a shallow, marshy tract surrounded by dense forests, from which good coal seams were formed. The last event of the history is the fracturing of the plateau by the southern extension of the Great Rift System, which crept northwards in ever-increasing intensity till it expanded in the gigantic rifts constituting the Red Sea and Gulf of Aden in quite recent times. The rift cut off the Transvaal on the west, and along the rift broke out the volcanoes that formed the Lebombo range. On the east side of this are the low-lying plains of Portuguese East Africa, covered with Upper Cretaceous deposits; the Lebombo lavas are of the same age as the Drakensberg lavas. The centre of the Bushveld laccolite was also affected by the fracturing, and after a pouring forth of dust torn from the chimneys, which formed again a repetition of the Cave Sandstone, the lavas followed and spread out as a strip of amygdaloidal rock.