Dr. C. Sandberg — The ' Old Granite' of the Transvaal. 557 



rocks of the Zwartkop,^ which lithologically and in their mutual 

 succession (only the order being reversed) are identical with Lower 

 W.W.R. Beds, to this same Archaean system. 



From the above it is clear that («) no valid argument to prove 

 the actual existence of pre-Lower "W.W.R. Beds has yet been put 

 forward; and {h) that even had Dr. Hatch's and Mr. Jorissen's 

 conclusions been the logical outcome of their investigations, they must 

 still be deemed unacceptable, being founded, as they are, upon the 

 absolutely inadmissible basis of Dr. Corstorphine's conclusions. But 

 if it is inadmissible to concede the non-intrusiveness of a granite 

 mass from the fact that no injection nor any metamorphic action could 

 be observed along one line of contact between that igneous rock and 

 the overlying sedimentary strata, exposed over some 200 yards, 

 it only needs the proof of the existence of one single spot, be it 

 ever so small, where indubitable contact-metamorphism has taken 

 place, to at once prove this igneous rock to be younger than the 

 transformed sedimentary one. It is clear that the sericite- schists, 

 separating over large areas in different regions the Lower W.W.B,. 

 quartzites from the underlying granite, are intimately connected with 

 magmatic action, being the result either of magmatic segregation or 

 of magmatic transformation of sedimentary rocks. And where now 

 Molengraaff has been able to establish the presence in these sericite- 

 schists of conglomerates (composed of quartz-pebbles), lying in 

 absolute conformable position with and below the Orange Grove 

 quartzites, there seems to be no more opening for doubting their 

 sedimentary origin. 



The conclusions arrived at from the study of these sericite -schists- 

 is materially strengthened by evidence from the Yredefort Massif. 

 Certain strata of the Lower W.W.R. Series are here conspicuous by 

 their peculiar and extensive development of corundum, which can 

 only be attributed to the action on the overlying sedimentary strata of 

 the yet unconsolidated magma. 



But if, as evidence now already available seems to prove, the ' old 

 granite ' is post-W.W.R., where then have we to fix its approximate 

 geological age ? It is here where the great importance of the 

 question lies. It is evident that the crystalline rock has participated 

 in the erogenic movement, and that it has been folded together with 

 the overlying W.W.R. Beds.- This fact finds its expression in the 

 tangential direction to the granite mass periphery of the axes of the 

 folds of the surrounding sedimentary strata, a phenomenon which is 

 magnificently developed round the Vredefort granite mass. It is also 

 evident from an examination of the contact region between this granite 

 mass and the Lower W. W.R. Series, and of the small granite masses 

 which come peeping through the overlying W.W.R. Beds on the farms 

 Brakfontein (673), Koedoeslaagte (59), and Aasvogelrand (see Figure, 

 p. 558). These granite protuberances, as well as the one situated further 



1 A. L. Hall & "W. A. Humphrey, "The Blackreef Series and the underlying 

 formation in the neighbourhood of Kromdrasi and Zwartkop, north of Krugersdorp " : 

 Trans. Geol. Soc. S. Africa, 1906, vol. ix, pp. 10-15. 



- C. Sandberg, " Notes on the Structural Geology of South Africa" : Trans. Inst. 

 Min. Eng., vol. xxxiii, pt. v, pp. 540-57. 



