Page:South African Geology - Schwarz - 1912.djvu/190

 syncline on the south side of the basin, on the north of the granite bosses at Heidelberg and Parys, in the Orange Free State. There are, however, evidences of great thrusting and faulting in the northern area, which suggest that the stratigraphy is by no means so simple as would at first sight appear. Along these thrust faults there are dykes of diabase which also follow along the beds and may possibly be contemporaneous igneous sheets. Some of the dykes are quartz-porphyries belonging to the granite series. Farther east in the Klerksdorp area the Witwatersrand Beds again are found on the inside of the Archaean inlier, between it and the Parys granite. This is significant, and taken in conjunction with the repeated recurrence of similar conglomerates throughout the thickness of the Witwatersrand Beds, it suggests that the Witwatersrand bankets are one and the same series folded upon themselves.

The gold in the bankets has formed the subject of a long controversy. It has been thought to be secondary in origin, that is, has come up along the conglomerates while they were still loosely aggregated, in solution from some deep-seated source of supply. The simplest theory, however, is that the banket is ordinary river gravel which had gold deposited in it as placer gold; then, when the rocks were tilted and compressed, water circulating through the rock averaged the whole by abstracting from the rich patches and depositing the gold in the poorer ones.

The banket on the surface, when it was first discovered by Struben in 1880, was thought to be merely surface gravel; the free-milling ore ran to an ounce to the ton, but this richness was soon dropped as the reef was sunk on, and when the sulphide ore was reached the