Vol. 54.] WITWATERSEAND AND OTHER DISTRICTS IN S. TRiNSVAAL. 81 



when present, is in intimate association with this mineral. Galena, 

 blende, and copper pyrites occur but rarely. In the surface-rocks, 

 down to about 100 feet, the sulphides of iron are replaced by the 

 oxides of that metal. 



The silicification, or more generally the mineralization, of these 

 conglomerates is, in my opinion, the result of secondary processes 

 of infiltration and crystallization. It is significant in this connexion 

 that when most mineralized the conglomerates are often seamed 

 with veins of white quartz (as, for example, Ferreira Mine and New 

 Eietfontein Mine) ; and there can be little doubt that these veins 

 were formed in connexion with the mineralization of the conglo- 

 merates. There is no reason for ascribing a special method of 

 origin to the gold ; and I hold, therefore, that the gold has been 

 introduced with the pyrites with which it is in intimate association, 

 as a part of the general process of mineralization, and I cannot 

 agree with those authors who describe the conglomerates as deposits 

 in which the gold was either pre-existing (alluvial gold) or was 

 introduced contemporaneously with the deposition of the pebbles 

 and sand. 



Some particulars of the Auriferous Conglomerates or 

 ' Banket Reefs ^ of the Main Reef Series. 



Much detailed information relating to the auriferous con- 

 glomerates, now generally known as the ' Main Reef Series,' has 

 been already published.^ Consequently I shall confine myself here 

 to a few points only. 



The outcrop of these conglomerates or ' Banket Reefs ' follows a 

 fairly regular line, extending east and west of Johannesburg from 

 Witportje at the western extremity to Boksburg at the eastern 

 extremity, a distance of close on 30 miles. In the central section, 

 that is, near Johannesburg, the strike is east and west ; but as Boks- 

 burg is approached the outcrop turns towards the south-east, passing 

 finally under the coal-beds of the Karoo Formation at Boksburg. On 

 emerging some 4 or 5 miles farther east they strike to the north- 

 east, appearing successively on the farms Benoni, Kleinfontein, Vlak- 

 fontein, Modderfontein, and Klipfontein. On the latter farm they 

 •again disappear under the Karoo Formation. Although much com- 

 plicated by faulting on these farms (see tig. 1, p. 82), the different series 

 of conglomerates (Main, Bird, and Kimberley) can be recognized, 

 both by stratigraphical position and petrological character. Turning 

 now to the western section of the Rand, we find on the farm Wit- 

 portje, near Roodepoort, 13 miles west of Johannesburg, the Main 

 Reef Series thrown north a distance of 3 miles by a great fault, 

 known as the ' Witportje break.' The beds resume their normal 



^ Gribson, Quart. Journ. Greol. Soe. vol. xlviii (1892) p. 409 ; Schmeisser, 

 * Ueber Vorkomraen u. Grewinnung der nutzbaren Mineralien in der siidafrikan. 

 Republik/ Berlin, 1894; Hatch & Chalmers, 'The Gold Mines of the Eand,' 

 London, 1895, pp. 22-87; Karl Futterer, 'Afrika in seiner Bedeutung f'iir 

 die Groldproduktion,' Berlin, 1895 ; De Launay, ' Les Mines d'Or du Transvaal 

 Paris, 1896 ; also Trans. Fed. Inst. Min. Eng. vol. xi (1896) p. 378. 



Q. J. G. S. No. 213. G 



