a B. Horivood 8^ A. Wade— The Old Granites of Africa. 549 



is intrusive in the Swaziland Series by after-protrusions due to the 

 fracturing and crumpling of the old granitic crust produced by secular 

 cooling. These intrusions have occurred at various intervals extending 

 over vast geological time. For example, Dr. Molengraafi ' has shown 

 that the Old Granite bo«s of Vredefort is probably more recent than 

 the Pretoria Formation. 



(3) The increasing prevalence of granite and gneiss as one travels 

 northwards towards Central Africa has ali'eady been noted. To the 

 south and south-east the sediments are well developed. The elevation 

 of the more northern portion and the progressive subsidence of that to 

 south and south-east (and consequent depositions and building-up of 

 the sediments of these latter areas) may probably be due to the same 

 causes as those recently suggested by Dr. Adams- to explain the 

 progressive iiplifting of the Laureutian Highlands of Eastern Canada 

 and sinking of the great plains to the south, namely, the uprising of 

 the granite-magma from the depths accompanied by its withdrawal 

 from below the subsiding areas. 



(4) The gneisses of this formation are of igneous origin, and are 

 probably granites altered chiefly by regional metamorphism. Possibly, 

 however, some of the later gneisses, intimately associated with the 

 crystalline schists of the Swaziland Series, are paragneisses, that is to 

 say, are of sedimentary origin. 



(5) The sediments of the Swaziland Series are derived from this Old 

 Fundamental Formation, and possibly also in part from ocean-borne 

 detritus. 



(6) The Witwatersrand System has been derived from the Swaziland 

 Series and the Old Granite which is intrusive in it, but probably 

 to only a very small extent, if any, from the further removed 

 Fundamental Granite-Gneiss Formation. 



The Granite of Orange Grove. 



By Arthur Wade, B.Sc. (Loud.), F.G.S., A.R.C.S, 



(PLATE XXVIL) 



The rock consists essentially of quartz, albite, microcline, orthoclase, 

 biotite, and sphene, with apatite as a prominent accessory mineral ; 

 epidote, chlorite, sericite, and fiuor are more or less common minerals 

 of secondary origin. It is a biotite-granite, somewhat rich in soda. 

 Under the microscope the rock is seen to be holocrystalline, hypidio- 

 morphic, somewhat porphyritic, and coarse-grained. A tendency to 

 graphic and micrographic structures are sometimes to be noted. 

 Felspars, either orthoclase or plagioclase, are the most abundant 

 minerals, the latter type being most frequent. The orthoclase is 

 practically confined to an early generation of large porphyritic crystals. 

 Microcline is very abundant in some sections and curiously rax-e in 

 others. 



1 " Remarks on the Vredefort Mountain-Land," by G. A. F. Molengraaff : Trans. 

 Geol. Soc. S.A., vol. vi, pt. ii, pp. 20-6. 



- Loc. cit., p. 145. In Great Britain the sequence from the very oldest to the most 

 recent rocks as one travels from the north-west of Scotland to the south-east shores 

 of England is .very marked. 



