The gold is carried in important quantities in several of the most northern conglomerate d beds of the Witwatersrand. The Witwatersrand gold fields are located on the central plateau of the Transvaal on a well-defined ridge and running nearly east and west, which forms the northern fringe of a basin whose southern edge is the Heidelberg range, about 45 km distant. To the north of the Witwatersrand are large masses of diorite and in the basin are numerous dykes of this material filling cross and lateral fissures and faulting the reefs in numerous places. The gold bearing formation consists of beds of sandstone, quartzite, conglomerates locally known as banket, and frequently shales. The northern part of the basin is strongly tilted and has been uplifted by the intrusive rocks, but at a shallow depth the strata flatten.
Conglomerated beds are the main reef series, which are interstratified in the sandstone and quartzite, and these consist of a mass of water worn quartz pebbles, which are cemented together by quartz sand, argillaceous and talcose matter, and iron oxide, which in depth changes to pyrite. The gold is contained in the cementing material and not in the pebbles. Nearly all the mines on these fields were located in the pyritic zone, as the upper levels where the free milling or oxidized ores occur, and were worked out in most of the mines and the average content seem to remain the same, with the difference that more of the gold seems to be locked up in the pyrite, and it is not easy to determine if with greater depth the portion of pyrite will increase and the free gold diminish.
The deep levels like may Deep, Village Main reef, Champ d’Or Deep, Durban Roodeport and Henry Nourse Deep are at a depth of over 300 m, and Geldenhius deep has irregular changes and the cores from the deep bore-holes which probed the reefs to a depth of 700 m, indicated a large percentage of free gold. It is difficult to enter into a details description of this vast and interesting gold bearing area, but it is important to say explorations appear to have sufficiently demonstrated that the gold mining industry will be maintained for a long time and the main deposits should be at depths of over 600 m. the dykes do not seen to thicken at greater depth and beds are not affected. The dip of the beds at greater depth seems to be 30 degrees in average and therefore over the greatest portion of the Witwatersrand every additional 300 m of vertical depth means a distance of the dip of the reef. Then, if the banket beds have interesting contents of gold, the some reefs at deeper levels will be worked successfully.