80 THE BLACK REEF SERIES 



conformity below it, but this unconformity is of less 

 importance than those at the base of the Ventersdorp 

 and Waterberg systems in Cape Colony. 



It gets its name from a conglomerate which occurs 

 in it in the Transvaal, 1 where it was first definitely sepa- 

 rated from the overlying Dolomite formation. 



In Cape Colony 2 the series consists of usually light- 

 coloured quartzites, grits, gritty shales, conglomerates 

 and limestones, with lavas and green volcanic ash-beds 

 near Vryburg. Where it rests directly on granite, as is 

 the case in the Mashowing Valley at and below Motiton, 

 there are several feet of rock made of granite debris at 

 the' base. There is a rather gradual decrease in the 

 amount of felspar present from the lowest layers up- 

 wards, and it is at places difficult or impossible to see 

 where the old granite surface ends, for the granite is 

 weathered as well as the arkose. Where the series rests 

 on the Pniel beds at Takoon the lowest beds are quart- 

 zitic grits with little felspar. In some localities, such 

 as the neighbourhood of Vryburg, the quartzites and 

 flagstones have a darker colour than usual, owing to 

 their containing much dark-coloured material derived 

 from the Pniel lavas. The quartzites are often current 

 bedded, and both they and the flagstones, thin-bedded 

 somewhat argillaceous rocks, frequently have very well- 

 developed ripple marks on their surfaces, a feature 



1 Penning, Q. J. G. S., xlvi., p. 456. The name Black Reef series is 

 most inappropriate as applied to the group of beds commonly known by 

 it, as indeed are most group names first derived from some local peculi- 

 arity and extended to contemporaneously formed rocks over a wide area. 



2 For details of the Black Reef series in Cape Colony see Holmes, T. G. 

 S. S. A., vii., 130. G. C., x., 245 ; xi., 19, 105 ; xii., 59, 174 ; xiii., 83. 



