C. B. Honi-ood 8^ A. Wade— The Old Granites of Africa. 461 



description of the Old Granite in the Klerksdorp District that I quote 

 it in full — 



'■'■ Microcline Granite: — Specimens from Haartebeestfontein (624), 

 Oorbietjesfontein (201), and Rhenosterhoek (585). Macroscopic: 

 a grey granite of medium grain. The lamellar twinned character of 

 the triclinic feldspar is easily recognisable with a pocket lens. 



" Under the microscope the rock shows a typical granitic hypidio- 

 morphic structure. The primary constituents are microcline, plagio- 

 clase, orthoclase, quartz, muscovite, some iron ore, biotite, rutile, 

 zircon, and tourmaline. The secondary constituents are chlorite and 

 muscovite. Feldspar and quartz preponderate so much over the other 

 minerals that the latter only occur in clusters here and there in the 

 rock, or as enclosures in the quartz or feldspar. Iron ore is very 

 sparingly represented by some crystals of magnetite and by some- 

 hydrated iron oxide, which may have been derived from haematite. 

 Mica is chiefly represented by muscovite ; part of the muscovite is 

 altered into chlorite in such a way that each crystal of muscovite 

 is altered into a chlorite crystal with the same crystallographic 

 orientation. The chlorite is distinctly pleochroic and shows ' Hofe ' 

 with very strongly marked pleochroism, and those often contain in 

 their centres minute grains of zircon. 



" The muscovite enclosed in quartz is always (juite undecomposed. 

 Biotite occurs only in small quantities enclosed in quartz or in feldspar. 



"Feldspar occurs chiefly as microcline, which forms, in fact, the 

 bulk of the rock. With the exception of quartz, the microcline 

 was the last mineral to crystallize in the rock, which is proved by 

 the fact that numerous well-planed crystals of orthoclase — mostly 

 Carlsbad twins — and also crystals of plagioclase are enveloped by 

 microcline. The characteristic ' Gitterstructur ' of the twinning is 

 very fine. The microcline was found to be quite fresh and un- 

 decomposed in all the specimens, whereas both orthoclase and 

 plagioclase was less pellucid and partly decomposed, numerous small 

 flakes of secondary muscovite having been formed in them. 



" Quartz fills the gaps left after the crystallization of all the other 

 minerals, and also contains all these as enclosures; zircon, rutile, and 

 tourmaline are found only as enclosures in the quartz. The rock 

 shows signs of having been subjected to great pressure, which is 

 revealed under the microscope by the undulatory extinction of the 

 quartz, and by the numerous bends and cracks in the other minerals ; 

 this last phenomenon is, of course, most clearly shown in the lamellar 

 plagioclase." 



Dr. J. A. Leo Henderson,^ referring to the Yokeskey River 

 granite (in the Johannesburg-Pretoria massif), remarks that "a regular 

 mutual intergrowth between biotite, muscovite, and green pleochroic 

 chlorite was observed parallel to the cleavage traces, and it w'ould 

 appear that, as MolengraaU suggests, the chlorite has been derived 

 from the primary biotite. A good deal of the muscovite is primary, 

 containing tiny zircon inclusions, but much is secondarily derived from 



1 Petrographical and Geological Investigations of certain Transvaal Xorites, Gahbros, 

 Pyroxenites, and other South African Bocks, by Dr. J. A. Leo Henderson, 1899, 

 p. 44, Dulau & Co., Loudon. 



