ORIGIN OF RAND BANKETS. 36 1 



was only relieved on the south. Does the thrusting and folding 

 in the Transvaal System to the south of the laccolite represent the 

 whole of the relief requisite? It would appear to be totally 

 inadequate. The crumpled Transvaal Beds and the granite must 

 have been thrust bodily southwards and have affected the beds 

 lying between the Johannesburg-Pretoria granite and the next 

 line of granite bosses ; that is to say, the beds lying between the 

 Johannesburg-Pretoria granite and the Heidelberg and Vredefort 

 granite bosses must have been intensely folded. The beds that 

 now occupy that area are the Witwatersrand Beds with a layer of 

 Transvaal Beds riding on top. The normal mapping of the 

 Witwatersrand Beds does not take into account any folding; 

 each successive bed of banket on the Upper Series is taken to 

 represent a normal succession. It is true that great numbers of 

 thrust faults occur everywhere which by themselves indicate that 

 folding has taken place ; but unless we recognise that the Wit- 

 watersrand Beds are themselves folded upon themselves, the 

 technic geology of the country is inexplicable. We will now 

 take the Karroo laccolite to illustrate what does occur when such 

 a laccolite is intruded, and the effects are not yet obscured ])y 

 denudation. 



The Central Karroo area is perfectly plain ; on the south a 

 number of sills of dolerite appear dipping slightly inwards to the 

 north, and on the north a similar series of sills dip in towards 

 the south ; between the two margins is an upland region, the 

 High Karroo, about Sutherland, Fraserburg, Carnarvon, and so 

 on, where the country is riddled v/ith dykes and great expan- 

 sions which in themselves form considerable laccolites, ten to 

 fifteen miles in diameter. In this area the southern limit of the 

 dolerite lies approximately along the main water-shed of the 

 country, and the whole breadth of the laccolite is only exposed 

 on the flat high-veld area. In the east, however, the laccolite is 

 dissected ; the main water-shed lies to the north, and the streams 

 flowing south from it to the sea have deeply cut into the land, so 

 that everywhere one sees stupendous hills capped with dolerite 

 and traversed by connecting dykes. Further east, in the Drakens- 

 berg area, the same dolerite occurs; on the coast the same type of 

 sill occurs, but under the Drakensberg, more in the centre of the 

 laccolite, the great secondary laccolites of the Insizwa, at Mount 

 Aylifl^. Mount Currie and so forth, are found as in the west 

 The tendency to form these subsidiary laccolites seems to be 'a 

 property of the axial region of the main laccolite ; one finds them 

 from the Drakensberg plateau,, through Burghersdorp, at Cra- 

 dock, and so on through Sutherland to Calvinia; with the lacco- 

 litic expansion there is also a less amount of the vast sills that on 

 the exterior margins may be followed for scores of miles, 

 branching, connecting by dykes, and often undulating instead of 

 maintaining a horizontal position in the strata. The eastern 

 dolerite, beginning at Cradock. frequently has a diorite accom- 



