456 C. B. Horwood ^ A. Wade— The Old Granites of Africa. 



corroded ; pink varieties also occasionally occur, as, for example, near 

 the Tokeskey River on the farms Rietfontein (15) and Driefontein 

 (461)^ in the central portion of the Johannesburg-Pretoria boss; also 

 in the Yredefort granite massif and in some localities of the Vryheid 

 district; Dr. Yoit - mentions a red variety in German South- West 

 Africa, and I have found red and pink varieties in the Gordonia 

 division of Cape Colony near the borders of German South- West 

 Africa. Mr. H. Kynaston^ mentions the occurrence of a coarse pink 

 variety with large porphyritic felspars near the Swaziland border in 

 the Komati Poort neighbourhood ; also pegmatite veins of a pale-pink 

 colour are found in the Old Granites.' 



The Orange Grove granite is a dark-grey rock, characterized by 

 large porphyritic crystals of plagioclase and orthoclase felspar, 

 frequently corroded, measuring up to about 1^ inches in length by 

 about half an inch in width, by rounded, lustrous blebs of quartz 

 a quarter of an inch or more in diameter, and by the abundant 

 presence of small particles of a dark-green biotite scattered throughout 

 the mass, sometimes occurring in the felspar, which imparts a distinctly 

 dark shade to the rock as a whole. 



Dr. G. A. F. Molengraaff in his Annual Report for the year 1898, 

 as State Geologist to the late South Afi'ican Republic, in describing 

 the Old Granite of the Transvaal at the same time drew a vivid 

 comparison between it and the much more recent Red Granite. 



In the light of subsequent investigations some slight alterations 

 must now be made in this description, but for the most part it holds 

 as good to-day as it did then, and it is the best general description 

 which has so far been published, and I therefore reproduce it in full, 

 as follows : — 



Old Granite. Red Granite. 



"The Old Granite is invariably "This granite is, as the name 



of a grey colour, seldom red. implies, usually of a red colour. 



" The Old Granite is on most " The Red Granite is on most 



places a biotite, or two -mica places an amphibole - biotite 



granite ; less frequently an granite and seldom biotite granite, 



amphibole-biotite granite or a Up to the present muscovite has 



muscovite granite, and seldom an never been observed in the Red 



amphibole granite. Granite. 



"The felspar in the Old Granite ' ' The felspar in the Red Granite 



always consists in a great measure is almost exclusively orthoclase. 

 of plagioclase or microcline. Not 



1 The Witwatersrand and Associated Beds, by C. B. Horwood, 1905 (Esson and 

 Perkins, Johannesburg), p. 67. 



2 "A Contribution to the Geology of German South - West Africa," by 

 F. W. Voit: Trans. Geol. Soc. S.A., 1904, vol. vii, pt. ii, p. 84. 



^ "The Geology of the Neighbourhood of Komati Poort," by H. Kynaston: 

 Trans. Geol. Soc. S.A., 1906, vol. ix, p. 21. 



* A pink graphic granite was encountered 50 feet from the surface in sinking 

 a well near the south-west corner of Illovo To^ynship, about 5 miles north of 

 Johannesburg. I have examined this in section under the microscope and find it 

 consists entirely of a beautiful intergrowth of quartz and of water-clear microcline. 

 Presumably the occurrence is that of a pegmatite vein in the Old Granite. 



