162 Br. F. H. Hatch — Geological History of South Africa. 



sea had become suflficiently shallow to allow of the accumulation of 

 shingle and gravel. There is evidence in the Southern Transvaal 

 that the land from which the sediments were mainly derived lay 

 to the west, the sea to the east, for the lower Witwatersrand Beds, 

 which consist solely of mudstones and fine sandstones in the east, 

 gradually develop conglomerates with a decreasing amount of shale 

 towards the west. 



The northern shore-line of the Witwatersrand sea probably did 

 not extend north of the 25th parallel of latitude, for in the Northern 

 Transvaal we find the Waterberg Sandstone resting directly on the 

 granites and gneisses of the Swaziland System, while in Southern 

 lihodesia, north of Bulawayo, the oldest deposits resting on these 

 rocks are the Sijarira sandstones,^ which immediately underlie the 

 IMatobola Coal-measures, in which fossils indicating a Permo- 

 Carboniferous age have been found. 



After the close of the AVitwatersrand period, which was brought 

 about by the final elevation of these beds above sea-level, they in 

 their turn became exposed to the disintegrating forces of denudation, 

 the resultant debris being transported and spread out by the heavy 

 flood waters of torrential rivers, since we find the Ventersdorp 

 period ushered in by the accumulation of coarse conglomerates and 

 boulder beds, in which occur fragments of such characteristic beds 

 of the preceding formation as Hospital Hill Slate and the auriferous 

 conglomerates of the Upper Witwatersrand Series. This denudation 

 produced the second break in the succession, marked as Unconformity 

 No. II in the scheme given on page OS. The Ventersdorp period 

 is particularly marked by volcanic manifestations, vast thicknesses 

 of basic and acid lavas, volcanic breccia and tuffs having been 

 accumulated, probably on a land surface. 



Before the next submergence, that of the Potchefstroom period, 

 the vast piles of volcanic accumulations, together with the boulder 

 beds and coarse conglomerates of the Ventersdorp System, were 

 long exposed to denudation, as there is a break (Unconformity 

 No. Ill) between this and the succeeding system, the lowest 

 member of which, the Black Reef Series, is found lying uncon- 

 formably on every older formation down to the granite and schists 

 of the Swaziland System. At some time before the deposition of 

 the Black Reef and Dolomite Series, the Witwatersrand Beds had 

 been subjected to a folding movement, as the latter are found bent 

 into gentle synclines and anticlines, on the denuded remnants of 

 which the former lie undisturbed.- The Potchefstroom System 

 consists of three members: a small development of sandstone at 

 the bottom with a basal conglomerate, dolomitic limestone and 

 shales in the middle, and a great thickness of shales and sandstones 

 on the top. This succession indicates a comparatively rapid sub- 

 mergence, continued until clear water conditions were reached, 

 in which the accumulation of calcareous sediments became possible. 



1 A. J. C. Molyneux: Q.J.G.S., vol. lix (1903), p. 283. 



- F. H. Hatch," " The Exteusion of the Witwatersraml Beds eastward under the 

 Dolomite," etc. : loc. cit., p. 68. 



