422 THE OLDEST VOLCANIC ROCKS 



The three south-western formations mentioned above, 

 Ibiquas, French Hoek, and Cango series, are alike in in- 

 cluding conglomerates in which granite and slaty rocks 

 are conspicuous, but there is not yet enough evidence 

 to correlate the three of them. One of the chief fea- 

 tures of the Cango series is the great development of 

 dolomitic limestone, which probably represents a marine 

 deposit. 



Both the Wilgenhout Drift and Kheis series include 

 limestones, and also contemporaneous volcanic rocks of 

 acid and basic composition, which are found again in the 

 Kraaipan series. These lavas and tuffs are the earliest 

 volcanic rocks known in the Colony. After the forma- 

 tion of these three groups of rocks they were greatly 

 folded and faulted. 



At the present time the known distribution of the 

 Wilgenhout Drift and Kraaipan series is very limited, 

 but they doubtless formerly covered a very wide area. 

 They probably formed great ranges in the north of the 

 Colony, but their destruction was brought about during 

 a long period preceding the deposition of the rocks of 

 the Vaal River or Ventersdorp system. During this 

 period of denudation the Witwatersrand system was 

 deposited and again partly denuded away in the Trans- 

 vaal. Whether that system is now, or was formerly, 

 represented in Cape Colony is unknown ; it has not 

 been identified here. Its existence in the Transvaal 

 adds very greatly to the already enormous length of 

 time required for the successive deposition, folding, and 

 denudation of the unfossiliferous rock groups of Cape 

 Colony. 



