PRESIDENTIAL ADDRESS. — SECTION B. 



33 



A-ast bodies of granite, probably mountain building, and cer- 

 tainly a prolonged period of denudation intervened between 

 the two but with the exception of the \\'itwatersrand forma- 

 tion, which consists of detrital sediments, the interval is ap- 

 parently unrepresented by rocks in South Africa. The Ven- 

 tersdorp form.ation and its possible correlative, the Koras 

 series near the Orange River in Gordonia and Kenhardt 

 occur over an area more than 70,000 square miles in extent' 

 though the continuity of the group is broken by denudation 

 and concealment under newer rocks, and probably was never 

 ■complete over the whole area. The total thickness of the 

 volcanic rocks and sediments of the A'entersdorp series is con- 

 ^'45r^^Jf' ^^'^ '^ varies greatly in different districts. 



_ Ihe iransvaal system has near its base in Wvburg an in- 

 significant thickness of volcanic rocks closely resembling the 

 more basic of the Ventersdorp lavas, and a" much morS im- 

 portant group of lavas of intermediate composition on a hi^ier 

 horizon, the Ongeluk lavas of Griqualand West and Bechuana- 

 iand, and the probably corresponding lavas in the Pretoria 



rork.' hJ A ?TT-''^- ^^ ''. P""'"'^^^ ^^^t ^h^se volcanic 

 rocks had a distribution as wide as that of the A'entersdorp 



i3.\ 3.S . ^ 



There is a considerable gap between the latest known rocks 

 of the Transvaal system and the earliest Matsap beds W'ith- 

 ■out prejudice to the doubtful correlation questions affecting 

 the \\aterberg and Alatsap series, it will be sufficient for th? 

 present purpose to point out that lavas exist in both those 

 formations; m Bechuanaland the total thickness of volcanic 

 rocks IS less than 1,000 feet, but in the Middelburg district 

 (Transvaalj the Waterberg volcanic rocks are nearly 8,000 feet 



Throughout the period represented bv the Cape formation 

 (assuming for the moment that the Alatsap and Wateibe?" 



or'eate llf'Tif'T °' ^'^ ^''''' ^'^°""^^^" series) and he 

 greater part of the Karroo system, that is from Silurian to 

 early Jurassic times, there were apparently no volcanic out 

 bursts m South Africa; but in the' Jurassic period wWch^n 

 Europe and .\orth America was remarkable in being free from 

 volcanic activity, there came what may be regarded as the 

 fZTfl °t'" ^h^^--ded South AfriLn volcanic outbirts 

 From the Transkei to beyond Nyasa there are large areas of 

 basic lavas which are associated with and follow the vountes 

 Karroo deposits. In Xatal they reach 5.000 feet in thXess 

 These rocks, called the Drakensberg gr'oup in the south are 

 remnants of their former selves, for they have lost much bv 

 denudation. The area affected by volca'i.ic aciv ty at tht 

 penod was probably very much larger than that covered by 

 the surviving lavas, for there is good reason to connect with 

 rUt' ''' mtrusions that are so characteristic of the Karroo 



region. 



The latest phrase of volcanic activity in South Africa left its 

 traces in the pipes and dykes filled with melilite-basalt kim 

 berhte, various other breccias, and lamprophvres of Tate Cr^ 



