334 ME. F. W. PENNY ON THE YREDEFORT GRANITE. [Dec. I914, 



Postscript. 



The margin of the granite a few miles to the north-west of the 

 area described above, near the boundary of the farms ' Zandf ontein ' 

 and 'Anna's Rust,' is also of interest in connexion with the 

 relationship of the granite to the Witwatersrand Beds. 



A certain thickness of amygdaloidal diabase is intercalated 

 between the lowest Witwatersrand quartzite and the granite. A 

 well-bedded strip of quartzite occurs along the inside edge of the 

 amygdaloid, in a manner similar to that already described near the 

 south-western boundary of the farm ' Vergenoeg ' ; it terminates 

 against a small fault, on the other side of which its course is 

 continued entirely inside the granite for some distance, its 

 stratification and nearly vertical dip being perfectly maintained. 

 Between the amygdaloid and the granite is a shear-zone, 10 yards 

 or so broad, of vertically banded mica-schist, with inconstant bands 

 and lenticular masses of quartz developed along planes parallel 

 to the foliation. 



[While this paper was going through the press, my attention was drawn to 

 Dr. C. Sandberg's view of the age of the Vredefort granite, Geol. Mag. 

 dec. 5, vol. v (1908) p. 559, with which my own conclusions appear to 

 coincide. F.W.P., Oct 1914.'] 



Fig. 2. — Relationship of quartzite, diabase, and granite near the 

 boundary of the farms l Zandf ontein ' and ' Anna's Bust.'' 



x * x x x x x *~*-*rx * x * * K ^jT* 

 "XyyCXXXXXXx x *xy XX**** 



■x y x x * x Amygdaloid *xxxxx* x ;* 

 xxxx xxxxrxxxx x x xx** ^. 



VXXX XXXX >^X XXXXX^xxxV^ 



xx xX * *x 



Quartzite 1 



-+■ + 



*XXXXx-><:*xy Xx -^ 



x x J^-z^^ ■■■■■■Sli ear-zone 



T -L "T -j- Granite 



+ + + -+■ 



EXPLANATION OF PLATES XLVI & XLVII. 



Plate XLVI. 



Pig. 1 a. Quartz-soda-porphyrite, showing green hornblende, acid plagioclase, 

 and quartz. Ordinary light, x 18 diams. (See p. 329.) 

 1 b. Quartz-soda-porphyrite, showing micropegmatite. Crossed nicols. 

 X 18 diams. (See p. 329.) 



2. Magnetite-actinolite-staurolite rock, showing well-formed crystals of 



staurolite with dark centres. Ordinary light. X 8 diams. (See 

 p. 330.) v 



3. Basic rock with inclusions of granitic material. Ordinary light. 



X 18 diams. (Seep. 331.) 



Plate XLVII. 



•Geological map of portions of the Tweefontein, Vergenoeg, and Zyferfontein 

 districts of the Orange Free State, on the approximate scale of 2375 

 inches to the mile, or 1 : 26,680. 



