76 BE. F. H. HATCH GEOLOGY OF THE [Feb. 1S98, 



granitic complex north, of the Witwatersrand is a microcline-granite, 

 of which he describes a type from the Haif\Yay House on the 

 Johannesburg-Pretoria road. He also mentions tonalite or plagio- 

 clase-granite as of frequent occurrence. 



Sectious made from specimens of granite collected by me on the 

 Vaal River near Yenterskroon show that the rock is a holocrystalline 

 aggregate of quartz, orthoclase, and plagioclase, with a little brown 

 and green mica. The felspar is much clouded through kaolinization, 

 and the quartz is full of inclusions. 



Crystalline schists are of rare occurrence in the Southern Trans- 

 vaal; but Cohen ^ mentions sericite-schist from Grobler's Parm near 

 the Halfway House, and Molengraaff ^ refers to actinolite-schist at 

 Yredefort in the Orange Fiee State. A. Schenck ^ also speaks of 

 metamorphic schists at Grobler's Parm, Xromdraai, and Sterkfon- 

 tein, and correlates them with his Swasi Schists of the De Kaap 

 and Zoutpansberg. 



Associated with the granitic rocks are eruptive masses of basic 

 rocks (norite, gabbro, dolerite), while the whole complex is pierced 

 in many places by dykes and veins of pegmatite, granophyre, micro- 

 granite, and felsite. No doubt there were several periods of eruption, 

 and a portion of the rocks, especially the dykes, are possibly of much 

 later age than the original Archaean formation. 



The gabbros of the Zwartkoppie hills, north of the Magaliesberg, 

 are well described by Dahms."* He divides them into {a) gabbros 

 rich in plagioclase and (b) gabbros rich ia pyroxene. A section ' 

 of gabbro from the north side of the Magaliesberg range (8 miles 

 north of Rustenburg) shows it to be a beautiful specimen of hyper- 

 sthene-gabbro or norite, consisting of a holocrystalline aggregate of 

 unclouded plagioclase and fresh highly-pleochroic hypersthene. 



A section of a specimen from Uitkyk, in the Heidelberg district, 

 discloses a quartz-felsite, composed of rounded crystals of quartz 

 embedded in a cryptocrystalline (felsitic) groundmass. 



III. The Ca.pe System. 



The Cape System is capable of division into five distinct groups 

 or series of beds. 



At the bottom lies a series of quartzites and ferruginous shales, 

 locally known as the Hospital Hill Series. IS^ext follow the sand- 

 stones and conglomerates (partly auriferous) of the Witwatersrand. 

 Lying unconformably above the latter, and resting on a sheet of 

 basic volcanic, rock (' Klipriversberg amygdaloid '), is a small bed of 

 quartzite and conglomerate known as the Black Reef. Above this 

 is a thickly -bedded dolomite, followed by a series of alternating 

 quartzites, shales, and volcanic flows for which the name Magaliesberg 

 and Gatsrand Series is well adapted. 



^ Dahms, Neues Jahrb. Beilage-Bd. vii (1891) p. 116. 



2 Neues Jahrb. Beilage-Bd. ix (1895) p. 194. 



3 Zeitschr. d. Deutsch. geol. G-esellsch. vol. xH (1889) pp. 578 & 579. 

 * Op. supra cit. p. 90. 



^ In the possession of the British Museum (Nat. Hist.). 



