C. B. Honvood <^ A. Wade— The Old Granifes of Africa. 467 



the granite the shaly and slaty rocks are charged with contact minerals. 

 Of these he describes two chief types — [a) apparently porphyritic, 

 more or less schistose rocks, composed mainly of corundum and 

 biotite in fairly large crystalline grains, all interlarded by a kind 

 of network of quartz ; in some varieties phenocrysts of corundum 

 are very conspicuous on the surface of the rock, having been so left 

 by reason of their strong resistance to weathering ; [h) highly 

 ferruginous and magnetic banded slates, being identical with the 

 well-known Hospital Hill Slates, but for their being charged with 

 spherulites or bunches of actinolite. And further, he points out 

 that the steep dip and overtilting of the beds surrounding the 

 granite boss is not due to one main axis of dislocation at right- 

 angles to which lateral pressure was exerted, but the axis of 

 overtilting gradually changes in direction all round the granite 

 boss, keeping tangental to it, so that evidently the pressure 

 has been exerted in every instance from its centre towards the 

 periphery. Since all the series of beds from the granite up to and 

 including the Pretoria Series has been more or less overtilted, and 

 since sandstones of the Karroo formation near Vredefort rest in 

 a perfectly horizontal and undisturbed position on the granite, the 

 intrusion must have taken place after the deposition of the Pretoria 

 Series and before the deposition of the rocks of the Karroo System. 

 Dr. Hatch,' in discussing this paper, stated that he thought it 

 extremely probable that Dr. Molengraaff was correct in his view 

 with regard to the overtilting of the Witwatersrand Beds. 



Although Dr. Molengraaff had treated this occurrence in such 

 a masterly way, and his conclusions were based on such sound 

 premises, yet because one occurrence of the Old Granite was after- 

 wards shown to be non-intrusive in the Lower Witwatersrand Beds 

 in the Heidelberg district,^ instead of realizing that his previous 

 work in the Vredefort district had proved that the Old Granite was 

 intrusive in places, even if not so everywhere, and therefore did 

 not all belong to one and the same pei'iod of intrusion, he in August, 

 1904, says: ^ "the recent studies of Messrs. Dorffel and Corstorphine 

 have led me to doubt the correctness of my opinion that the Old 

 Granite is intrusive in the Lower Witwatersrand Beds (Hospital 

 Hill Series), and I consider this question open to revision in the 

 case of the granite of the Vredefort boss also." 



In his Geology of the Transvaal (1904) he states that the whole of 

 the South African Primary System represents one single series, of 

 which the rocks are greatly modified in their structure by contact- 

 metamorphism produced by granitic intrusions. He then states that 

 in the districts where the South African Primary System has been 

 well developed, as in the Barberton district and in the Witwatersrand, 

 it is preferable to distinguish a lower series confined to the vicinity 

 of the old intrusive granite, consisting of crystalline schists, and an 

 upper series composed of rocks of clastic origin. He adds a footnote 



1 Trans. Geol. Soc. S.A., vol. vi, pt. iii, p. 30. 



- "The Geological Relation of the Old Granite to the Witwatersrand Series," 

 by G. S. Corstorphine: Trans. Geol. Soc. S.A., vol. vii, pt. i, pp. 9-12. 

 3 Trans. Geol. Soc. S.A., 1904, vol. vii, pt. ii, p. 115. 



