Vol. 70.] MR. F. W. PEXXY OX THE VREDEFORT GRAXTTE. 329 



Locality. 



The area to be described is situated in the Orange Free State, a 

 few miles south of the Vaal River, on the north-east side of the 

 Vredefort Granite outcrop. It has proved to be a particularly 

 favourable locality for examination, because here for a considerable 

 distance the granite abuts directly against some member of the 

 Witwatersrand Series ; while on the north, on the Transvaal side of 

 the river, basic intrusions intervene to a greater or less extent. 



Although, in a general way, the strike of the sedimentary beds 

 gradually curves round so as to become tangential to the margin of 

 the granite boss, the actual junction is far from being a simple curve. 

 The area mapped is particularly noteworthy for the varying 

 horizons of the Witwatersrand Beds with which the granite comes 

 into contact, owing to the removal of varying amounts of the 

 lower beds hj the granite, and is further complicated by the 

 profuse intrusion of basic rock. 



The generic term diabase has usually been applied to these 

 basic intrusions in the Witwatersrand Beds ; and therefore, as a 

 matter of convenience, it will continue to be used throughout this 

 paper. A microscopic examination, however, shows — at any rate 

 in the area under discussion — that this basic rock is alkaline and 

 intermediate in composition, and should more correctly be termed 

 a soda-porphyrite. It is holocrystalline, and consists prin- 

 cipally of hornblende and felspar. The hornblende occurs in 

 idiomorphic crystals, dark green and very pleochroic : there is no 

 evidence that this hornblende is of secondary origin ; it is too 

 dark for uralite, nor does the shape of its sections suggest that it is 

 derived from augite. Surrounding these crystals is a border of 

 dark blue-green hornblende, in optical continuity with the main 

 mass and of later growth ; it is intermediate in composition, but 

 near arfvedsonite, and also occurs in a second generation as 

 minute blades in the groundmass (PI. XL VI, fig. lrt). The 

 hornblende must have continued crystallizing quite late in the 

 consolidation of the rock. The felspar is a medium oligoclase, 

 usually rather decomposed and then crowded with epidote- grains : 

 it is sometimes surrounded by an edging of orthoclase. Inter- 

 stitial quartz and micropegmatite are common constituents 

 (PI. XL VI, fig. 1 h), and the orthoclase of the latter is in optical 

 continuity with that surrounding the oligoclase-crystals. 



Description. 



Starting from the south-western end of the farm Vergenoeg (see 

 map, PL XLVII), we find the lowest quartzite of the Witwatersrand 

 System, W l a , forming a ridge that abuts against the edge of the 

 granite. This is evidently the section mentioned by Dr. E. Jorissen 1 



1 Trans. Geol. Soc. S. A. vol. vii (1904) pp. 156-57. 



