Page:South African Geology - Schwarz - 1912.djvu/198

 crocidolite by the absence of soda. Near the top of the lower part of the series, and just below the lavas, there is a band of glacial conglomerate. It is precisely similar to the Dwyka Conglomerate in structure, though the matrix has been altered to a flinty red jasper, and the included boulders, when they can be made to break away from the matrix, show unmistakable glacial striations and facets.

Above the lower sub-series there is a great development of lavas, breccias, fine-grained tuffs, and a few intercalated layers of brilliant red jaspers. These are the Ongeluk lavas. The lavas are compact, blue-green rocks in which no crystalline structure can be seen, and are probably melaphyres altered from unstable basalts.

Above the lavas, on the west, there is a series of brown, red, and black magnetic cherty beds with a thin bed of limestone and some phyllites. These are classed as the Upper Pretoria Beds.

In the flat surface of the Kaap plateau there are long ridges of a breccia formed of blocks of Pretoria jaspers and magnetite quartzites called the Blink-Klip breccia. The history of these is peculiar. Before the Pretoria Beds were cleared away from off the limestone by denudation the water sank in, and, reaching the limestone beneath, hollowed out tunnels just below the jaspers. In course of time the tunnel became too weak to bear the load, and the jaspers fell into the tunnel of the underground stream. A similar thing happens when morainic material falls through into the channel of a sub-glacial stream, owing to the ice on top becoming too weak to sustain the weight of stones.

Veins of ore occur in this formation. The best known are the silver-lead ores of Willows Silver Mine and the