THE GEOLOGICAL HISTORY OF THE COLONY 423 



The Ventersdorp system has two unconformities 

 within it in Cape Colony. The lowest member is the 

 Zoetlief series, mainly lavas of acid composition which 

 occur at various places in Hopetown, Griqualand West 

 and Bechuanaland ; this is followed unconformably in 

 the T'Kuip hills by the Kuip beds, lavas and sedimentary 

 deposits, arkose, cherts, limestones and flagstones. The 

 Kuip beds are overlain unconformably in the T'Kuip 

 hills by the Pniel series, the most widely distributed 

 member of the Ventersdorp system. The Pniel series 

 includes both volcanic rocks and sedimentary beds de- 

 rived locally from the waste of older formations, but the 

 most characteristic rocks in it are the diabase lavas. A 

 remarkable feature is the varying thickness and oc- 

 casional absence over wide areas of the sedimentary 

 beds at the base of the group. This series of rocks is 

 met with in many places from Morokwen and Mafeking 

 southwards to Hopstown, and it also covers wide areas 

 in the Transvaal, but it has not been definitely re- 

 cognised in Gordonia. The Ventersdorp system records 

 a long-continued period of volcanic activity, and perhaps 

 the unconformities within the group were closely con- 

 nected in their origin with the vulcanicity. 



The rocks of the Ventersdorp system underwent a 

 considerable amount of denudation before the basal 

 member of the Transvaal system was deposited in Cape 

 Colony, but it is certain that this unconformity is not of 

 the same order of importance as the one at the base of 

 the Ventersdorp system ; it is rather to be compared with 

 the unconformities within the latter. In Bechuana- 

 land lavas like the Pniels are interbedded with the 



