The Geology of the East Rand and Heidelberg. 251 



I added : — 



" It must be remembered that what are termed ' reefs ' are 

 simply the marginal outcrops of large sheets of conglomerate 

 covering many square miles and extending down to the Vaal 

 River and into the Free State. In tracing the extension of the 

 ' reefs ', therefore, we are simply determining the boundaries 

 ■of these sheets. The problem is evidently one which admits 

 of solution and should present no great difficulties to a trained 

 geologist who has the time to devote to it. The main points that 

 should guide the intending investigator are : — 



(1) The slate footwall of the Main Reef series. 



(2) The presence of a series of big Reefs higher up in the order 



of stratification (Bird series). 



(3) The presence of another group of Reefs still higher up in 



the succession (Kimberley series). 



(4) A large series of coarse conglomerates at the top (Elsburg 



series). 



(5) The presence of an alternating series of quartzites and slates 



below the Main Reef series (Hospital Hill formation). 



I have already communicated the results of my investiga- 

 tions on Vlakfontein (i.e. the Van Ryu property) and Modder- 

 fontein, showing that the conditions enumerated above are in 

 most parts fulfilled in this district, namely : the Main Reef with 

 its slate footwall, the slates and quartzites (Hospital Hill forma- 

 tion), underlying the banket formation, and the presence of the 

 Bird Series above. Through Modderfontein I have traced the 

 outcrop of the reefs to Klipfontein. I now find that the Nigel 

 fulfils the same conditions : the reef rests on slate, and, at the 

 proper distance above, are found the Bird and the Kimberley 

 series." 



In 1897 I published these results in the first edition of my 

 Geological Map of the Southern Transvaal, which was reproduced in 

 a paper read before the Geological Society of London in February, 

 1898.^ In this paper I said : 



" East of Johannesburg the Hospital Hill series can be traced 

 to Klipfontein, about 12 miles north-east of Boksburg, where it 

 disappears under the Dolomite. It is next found at Boschmanskop 

 and Vlakfontein, some 10 miles north-west of the Nigel, and is 

 there striking nearly due south and dipping west. It is apparent, 

 therefore, that, while hidden by the Dolomite and the Karroo 

 formations, the strata have swerved roxmd to the south -an 

 important point in connexion with following up the auriferous 

 conglomerates of the Main Reef series. The outcrops of this 

 formation on Uitkyk, Frischgewaagd and Kuilfonteiu closely 



^ Hatch, " A Geological Survey of the Witwatersrand and other districts 

 in the Southern Transvaal " : Quart. Jovrn. Geol. Soc, vol. liv, 1898, p. 73. 



