Dr. C. Sandberg—The ' Old Granite' of the Transvaal 559 



north, on the farm Rietfontein (Ko. 555), near the contact of the Black- 

 reef and Ventersdorp series, most convincingly convey the impression of 

 being situated on the anticlinal axis of the respective subordinate folds. 

 They are consequently drawn out in the direction of the axes of these 

 folds, and thus stretch, in a curved line parallel to the semicircular 

 periphery of the main granite mass.^ Much further north and west 

 we again find indubitable evidence, and now on a more gigantic 

 scale, of the granite having been folded together with the overlying 

 W-WJl. strata," the Klerksdorp-Johannesburg granite masses being 

 situated on the axes of the Pretoria-Blackreef anticlinal. 



.The Yredefort granite, as well as that of the Klerksdorp and 

 Johannesburg (as also the Barberton and Zoutpansberg) regions (all 

 supposed to be ' old or grey granite '), is, however, a homogeneous 

 rock, hypidiomorphic in grain and massive in structure, showing no 

 signs of crushing or contortion either macroscopically or micro- 

 scopicall5^ It could therefore not have been folded after its consolidation.'^ 

 And as the AV.W.Pt. Beds (at least during their last period of folding) 

 have been folded together simultaneously with the younger deposits 

 covering them, right up to and including the Pretoria Series, it 

 would logically follow that the time of the consolidation, that is, the 

 age, of the Vredefort, Johannesburg, and Klerksdorp granite masses 

 must be fi.xed as post-Pretorian, that is, near to, perhaps even 

 synchronous with, that of the so-called Btishveld or neiv or red granite. 

 Since during the last couple of years the Bushveld area attracted 

 considerable attention economically, our knowledge of the typical 

 rock of the region was consequently considerably extended. .It 

 gradually became evident that all the features which once seemed to 

 constitute characteristic differences between this red granite and the 

 old or grey granite equally pertain to both rocks. There thus seems 

 to exist a distinct consanguinity of the magmas. This and the great 

 probability above demonstrated of these two igneous rocks being of 

 approximately equal age, make it just possible that the old and the 

 new or Bushveld granite are in reality identical, and derivatives from 

 one and the same magma at the same period of ' eruption.' The 

 numerous and powerful syenite dykes striking north and south, and 

 evidently connecting the Johannesburg and even the Vredefort granite 

 masses with the Bushveld igneous rock,* might then, perhaps, be 

 explained as representing an endomorphic modification of this common 

 magma due to the influence thereon of the overlying dolomite. 



1 Ou Dr. Hatch's geological map of the Southern Transvaal the jjhenomeuon of 

 the ahove- mentioned granite fenetres is clearly expressed (Koedoeslaagte, No. 59). 



2 See also A. E,. Sawyer, " New Eand Gold Fields, Orange Eiver Colony" : Trans. 

 Inst. Min. Eng., vol. xxxiii, pt. v, pp. 530-4. 



^ E. "Weinschenk : " Grundziige der Gesteiuskunde," i, pp. 168, 169. Since the 

 assimilation of the Johannesburg- Vredefort-Klerksdorp ' old granite ' with the 

 Zoutpansberg-Swaziland-Ehodesia rock is exclusively based ou lithological groimds 

 only and must therefore be considered as inadequate, I have purposely, although 

 personally inclined to regard these igneous rocks as identical, not included them 

 directly in the summary of my argument. 



* A. L. Hall & W. A. Humphrey, loc. cit., p. 11, pi. iii. 



