32S MR. F. W. PEXST OX THE YREDEFORT GEAXTTE. [Dec. 1914* 



15. On the Relationship of the Yeeheeort Geaxite to the 



WlTWATEESEAND SYSTEM. By FREDERICK WlLLOUGHBY 



Pexxy, B.Sc, F.G.S. (Read May 13th, 1914.) 



[Plates XL VI & XL VII.] 



Introduction. 



The Vredefort Granite has always been considered as a member 

 of that ' old granite ' group which throughout the Transvaal and 

 the Orange Free State is found emerging from beneath the Wit- 

 watersrand Beds. Although mineralogical differences could be 

 observed from area to area, this grouping together was a matter 

 of convenience, dictated by their stratigraphical position. 



The first attempt to indicate the extent of the granite outcrop 

 with any pretence of accuracy was made by Dr. F. H. Hatch in 

 his Geological Map of the Southern Transvaal, published in 1897. 

 With regard to the encircling sedimentary rocks, it is clear from 

 the stratigraphical and lithological evidence that they are of 

 Witwatersrand age — the southern rim of the basin of Witwaters- 

 rand rocks, the northern rim of which crops out along the Rand. 



Several geologists who have visited the Vredefort area appear to 

 have been struck by indications pointing to the intrusive nature of 

 the granite into the Witwatersrand Beds. Thus Dr. Molengraaff 

 in 1903, l arguing from the steep dip and overtilting of the sur- 

 rounding strata, the occurrence in them of small enclosures of 

 granite at some distance from the main outcrop, and the evidence 

 of contact-metamorphism in the adjacent beds — particularly the 

 production of corundum — proved it conclusively to his own satis- 

 faction. But, in the following year, he was led to retract his 

 views. 2 on considering evidence collected in the meantime from 

 other parts of the Transvaal as to the non-intrusive relationship of 

 this ' old granite ' group to the Witwatersrand System. To argue, 

 however, in this way from one area to another is to assume con- 

 temporaneity of intrusion as well as stratigraphical similarity, which 

 is too hazardous an assumption. Any definite views held about 

 one area should only suggestively bias the observer when dealing 

 with another. This point has also been previously emphasized by 

 Mr. C. B. Horwood.s 



In the present paper evidence is brought forward to prove clearly 

 the intrusive nature of the Vredefort Granite into the Witwaters- 

 rand Beds, based on an examination of the adjacent sediments 

 and a detailed map of several miles along the contact, made in my 

 spare time during a year's sojourn in the neighbourhood. 



1 Trans. Geol. Soc. S. A. vol. vi (1903) pp. 20-26. 



- Ibid. vol. vii (1904) p. 115. 



3 Geol. Mag. dec. 5, vol. vi (1909) p. 467. 



