68 THE VENTERSDORP SYSTEM 



locality. The unconformity between the Black Beef 

 series and the rocks of the Ventersdorp system seems to 

 be of a similar nature to those within the latter, and lavas 

 very like those of the Pniel series occur near the base of 

 the Transvaal system in Vryburg. Though these facts 

 might be held to justify the inclusion of the Ventersdorp 

 beds in the Transvaal system, the latter as ordinarily 

 denned has a far wider distribution in a uniform state 

 than it would have were the meaning of the term 

 altered. 



Another group of rocks, the Koras series, is for con- 

 venience placed in this system, though its true position 

 is not known. 



A. THE ZOETLIEF SERIES. 1 



The characteristic feature of the oldest member of 

 the Ventersdorp system is the predominance of quartz- 

 porphyries (devitrified rhyolites), but at places there is a 

 small thickness of sedimentary rocks at the base and 

 also above the lavas. 



The acid lavas are often amygdaloidal. They are 

 usually much altered, especially by the replacement of 

 the felspar by silica. They vary in colour from pale 

 grey or pink to dark green and black. The quartz cry- 

 stals are more or less corroded ; crystals of felspar are 

 often very plentiful. In many of the lavas no ferro- 

 magnesian constituent is seen, but in others there are 



1 G. C., x., p. 235 ; xi., p. 14 and 93 ; xii., p. 50 and 164. Also R. H. 

 Bastall, Rep. S. A. A. A. S. for 1905 and 1906, p. 275, " Acid Volcanic 

 Series" ; and Schwarz, Rec. Albany Museum, 1907, vol. ii., p. 34. The 

 " Beervley Volcanic Series" of G. C., iv., p. 86, and of the first edition 

 have been found to be part of this group. 



