Tlie Geology of the East Rand and Heidelberg. 253 



(1) Numerous conglomerate beds belonging to the Kimberley 



series. Over 100 were cut in Grootvlei borehole No. 2, 

 and there were even more in the rirootvlei-Dagga- 

 fontein joint borehole. 



(2) A band of slates, varying from 200 to 400 feet in thickness, 



at the base of the Kimberley series. 



(3) A bed of amygdaloidal basic lava, from 130 to 3G0 feet thiclc, 



immediately above the Bird conglomerate. I called this 

 " the amygdaloidal diabase " ; it has since been called 

 the " Bird amygdaloid ", and this is a better name. 



(4) A bed of slate immediately under the Van Ryn -Nigel, or 



Main, Reef. 



The most (;omplete section of these Upper Witwatorsrand bod? 

 was afforded by the Grootvlei-Daggafontein joint borehole. It is 

 as follows : — 



Karroo Beds .... 



Dolomite with Black Reef at base 



Kimberley-Elsburg Series. — 

 Quartzites and numerous 

 conglomerate beds 



Kimberley Slates 



Quartzites .... 



Bird Amygdaloid 



Main-Bird Series. — Quartzites 

 with few strings of con- 

 glomerate .... 778 4,880 



Both the Kimberley Slates and the Bird Amygdaloid formed 

 most useful " markers " for calculating the depth at which the Main 

 Reef would be cut. A preliminary estimate could always be made 

 as soon as the base of the Kimberley Slates had been reached, and 

 a still closer estimate was possible as soon as the Bird Amygdaloid 

 had been intersected. 



By means of the data furnished by these borings, together with 

 the results of previous borings on the farms Geduld, Daggafontein 

 94, Rietfontein 182, Vlakfontein 21, and Grootfontein 152, I was 

 able to trace the course under the Dolomite of the sub-outcroj) of 

 the Van Ryn or Main Reef as far as the Nigel mine, and, by drawing 

 contour lines at intervals of 1,000 feet depth, to indicate on a plan ^ 

 the nature of the svncline in which it occurs. (See the section in 

 Fig. 2.) _ " 



The completion of the v/ork of the Geological Survey in the 

 Witwatersrand District and its publication in " Sheet 52, 

 Johannesburg ", fully confirms the correlation of the Nigel with 

 the Van Ryn reef ; and the recent publication by Dr. Rogers, 



^ Corrected for deflection from the vertical. 



" See the plan accompanying my paper on " The Extension of the Witwaters- 

 rand Beds eastward, etc.", loc. cit. 



