460 C. B. Honcood ^ A . Wade— The Old Granites of Africa. 



The apparent contradiction disappears if we take into consideration 

 that we have here to deal with a stream of acid — probably very 

 viscous — lava of enormous extent and thickness. This explains how 

 the intra-liquid portion could cool only very slowly, and thus acquire 

 the characteristic structure of plutonic rocks. We must assume that, 

 after the decomposition of the Magaliesberg Beds in the area which 

 is now the Bushveld, there followed a period of volcanic activity on 

 a very large scale. . . . The bed of the Pienaars River on the farms 

 Baviaanspoort (No. 470), Leeuwfontein (IS"©. 320), Zeekoegat 

 (No. 287), and Roodeplaat (No. 314) offers excellent opportunities 

 for the study of the earlier results of the above-mentioned eruptive 

 activity, followed by the outflow of a colossal stream of granite lava, 

 which in its central and lower portions consolidated to a ' granite ' 

 with micropegmatitic structure, the portion nearer the surface, 

 however, furnished rocks belonging to the quartz - porphyry and 

 felsite-porphyry group." Later ^ he speaks of the intrusion of a 

 magma, rich in soda, which might have been of the nature of 

 a laccolite, intruded between the Waterberg and Pretoria Series. 

 However, whether portion of an enormous sheet or of a laccolite, the 

 origin of the granite, in either case, is the same. 



This explanation of the origin of the Red Granite is not incompatible 

 with modern ideas. Sir Archibald Geikie states- that granite may 

 occur " connected with true volcanic rocks, and forming, perhaps, 

 the lower portion of masses which flowed out at the surface as lavas. 

 In the Tertiary volcanic region of the west of Scotland, masses of 

 granite and granophyie have pierced the sheets of sub-aerial basalts, 

 and must have risen near to, if they did not actuallj' reach, the 

 surface. They prove that granite is not necessarily, though usually 

 an abysmal rock ". 



Returning now to the Old Granite, Mr. H. Kynaston^ has described 

 the granite at the old Half-way House between Pretoria and 

 Johannesburg as a somewhat coarse rock of a pale pinkish - grey 

 colour, with occasional large flesh-coloured felspars showing more 

 conspicuously among the other constituents, and says that under the 

 microscope it is seen to consist mainly of a somewhat coarse aggregate 

 of microcline, plagioclase, and quartz, with a few small flakes of white 

 mica. He states that the proportion of ferro-magnesian constituent is 

 small, and consists of biotite, partially chloritized, and a very small 

 quantity of green hornblende ; also that apatite is accessory, and that 

 a few small grains of epidote appear as a secondary constituent. 



In this same granite boss, near the dynamite factory on the farm 

 Modderfontein, I have found a reddish-grey granite, in which small 

 particles of violet-coloured fluorspar were easily discernible in the 

 hand specimen. 



Dr. G. A. F. Molengraaff * has given such a very full and careful 



1 Geology of the Transvaal, 1904, pp. 57-60. 



' Text Book of Geology, by Sir A. Geikie, 1903, vol. i, p. 208. 



^ ' ' The Marajinal Phenomena and Geological Relations of the Granite North of 

 Johannesburg," by H. Kj-naston: Trans. Geol. Soc. S.A., 1907, vol. x, p. 55. 



* "Note on the Geology of a Portion of the Klerksdorp District, etc.," by 

 Dr. G. A. F. Molengraaff : Trans. Geol. Soc. S.A., 1905, vol. viii, pp. 24, 25. 



