The Geology of the East Rand and Heidelberg. 255 



Director of the Survey,^ of the results of his work in the Heidelberg 

 District, shows the general correctness of the lines drawn by me for 

 the outcrops of the Witwatersraud Beds in that area. But his 

 detailed work, which is reproduced in the beautiful little map 

 accompanying the paper, marks a great advance in the knowledge 

 of the tectonic structure of the district and throws a flood of light 

 on complexities that had completely baffled previous observers. 



His most illuminating discovery is the existence, in the southern 

 part of the district, of a great east-and-west fault. This disturbance, 

 named by Dr. Rogers the " Sugarbush Fault ", has been traced for 

 30 miles, from Rietbult 322 in the east, where it emerges from beneath 

 the Karroo rocks, westward to Goedverwachting 306, where it is 

 again covered by Karroo beds. It has a throw which increases from 

 4,000 feet in the west, where it is first seen in the valley of the 

 Zuikerboschrand Eiver to 14,000 feet on Rietfontein 72 in the east, 

 where it lets down the Ventersdorp lavas to the level of the lowest 

 beds of the Witwatersraud System. 



The outcrop of the Ventersdorp beds on the south, or down- 

 throw side of the fault, is from 4 to 5 miles wide, and the volcanic 

 beds are followed in downward succession by Upper and Lower 

 Witwatersraud with the gold-bearing conglomerates of Malanskraal 

 and Tweefontein at their junction. A section drawn in a south- 

 easterly direction through the town of Heidelberg shows a complete 

 duplication of the Witwatersraud succession. Dr. Rogers' map does 

 not extend far enough to the south-east to take in the exposures of 

 Witwatersraud beds at Hexriver and at the Heidelberg-Roodepoort 

 mine, which have been the scene of much prospecting and some 

 mining, but it seems likely that fault lines of a similar nature, and 

 probably parallel to that described by him, will have to be drawn 

 in order to explain the present position of the beds. The extension 

 of his Avork in this direction will be awaited with considerable interest. 



Another important observation of Dr. Rogers is that showing 

 the existence of passage beds between the Kimberley-Elsburg beds 

 at the top of the Witwatersraud System and the volcanic series at 

 the base of the Ventersdorp System, thus demonstrating that there 

 are no grounds for the assumption of an unconformity at this horizon . 

 "' The observed facts," he says, " indicate that volcanic activity 

 began whilst the small pebble conglomerate and grit taken as the 

 the top of the Kimberley-Elsburg series of Heidelberg, was being 

 formed and that for some time afterwards quartz grit was furnished 

 to areas where volcanic tufi formed the bulk of the sediment 

 deposited. The arenaceous deposits were outweighed by the 

 volcanic ; there is no evidence of a sudden and complete cessation 

 of the supply of sand, but it came in at too slow a rate to maintain 



^ Rogers, " The Geolog}' of the neighbourhood of Heidelberg " : Trans. 

 GeoJ. Soc. S.A., vol. xxiv, 1921, p. 17. It is to be noted that Dr. Rogers will 

 have none of Mr. Bleloch's correlation of the conglomerate overlying the 

 Kimberley Slates with the Van Ryn reef. See p. 47, et seq. 



