740 R. A. F. PENROSE, JR. 
latter being what has made the Transvaal pre-eminent as a gold 
producer. 
Mode oj occurrence of the gold deposits—The Main Reef series, 
which, as just stated, carries most of the gold of the Witwatersrand 
district, contains several gold-bearing conglomerates separated by 
quartzites of a light-gray or greenish-gray color, dense, brittle, and 
of either a vitreous or hard sandy structure. More rarely slaty strata 
occur. On the extreme eastern part of the Rand the conglomerates 
Fic. 3.—Underground photograph in the Ferreira-Deep Mine, Witwatersrand 
District, Transvaal, showing the gold-bearing conglomerate running diagonally across 
the picture. The speckled rock indicates the conglomerate. 
often come close together and are sometimes all within a distance of 
a few feet of each other. To the west they are scattered over a greater 
thickness of strata, sometimes one hundred feet or more, measured 
vertically to the dip. This widening is due chiefly to the widening of 
the interbedded quartzites, though the conglomerates also increase to 
some extent. The conglomerate beds vary in number in different 
places, but certain of them have become especially prominent as gold- 
producers, the chief ones being known locally as the Main Reef and 
the South Reef, while the Main Reef Leader and the South Reef 
