458 C. B. Horivood i<^ A. Wade— The Old Granites of Africa. 



accumulations of the ampbibole, 

 whereby local rocks originate 

 which may be designated horn- 

 blende rocks. Of much moie 

 importance, however, is the segre- 

 gation of the most basic materials 

 — the iron ores from norite — at 

 the base of the Eed Granite, which 

 have conduced to the origin of 

 important ore masses consisting of 

 chrom-iron and magnetite. The 

 norite and gabbro themselves, of 

 Avhich, among others, the range 

 known as the Zwartkoppies con- 

 sists, may not improbably again 

 be considered as basic segregations 

 from the Red Granite magma on a 

 ^ large scale, but up to the present 



the data are insufficient to enable 

 one to decide with certainty on 

 the matter. 

 " The Old Granite is intimately "In the Eed Granite region 



connected with the schists. It schists have never yet been met 



not infrequently even possesses with." 



a more or less banded structure, 



and passing then over to gneissic 



or amphibolitic rocks ; while in 



many places bands of schists are 



interbedded. In sucb cases the 



schists consist of amphibolite, 



especially actinolite schists, chlor- 

 ite schist, sericitic schist, staurolite 



schist, andalusite schist, etc." 



Prom a more recent work we now know that muscovite does 

 occur in the Red Granite.^ Fi'om a specimen of Red Granite which 

 I obtained from Balmoral I have had a section cut which contains 

 microcline ; the occurrence of this mineral in the Red Granite is, 

 however, extremely rare. Granophyric structure is sometimes present 

 in the Old Granite.- Also passages of the Old Granite into quartz- 

 diorite and quartz-porphyrj' sometimes occur, and Mr. H. Kynaston^ 

 has called attention to their occurrence in places along the southern 

 margin of the Johannesburg-Pretoria boss. 



In view of our present more advanced knowledge I would suggest 

 the following comparison between the Old Granite and the Newer Red 

 Granite : — 



1 The Witxvatersrand and Associated Beds, by C. B. Horwood, 1905, pp. 68, 69. 



^ Loc. cit., pp. 65, 68 ; also vide " The Geolosjy of the Neighbourhood of Komati 

 Poort", by H. Kynaston: Trans. Geol. Soc. S.A., 1906, vol. ix, p. 21. 



^ " The Marginal Phenomena and Geological Relations of the Granite North of 

 Johannesburg," by H. Kynaston: Trans. Geol. Soc. S.A., 1907, vol. x, pp. 56-8. 



