Page:South African Geology - Schwarz - 1912.djvu/196

 by solution, become filled with secondary minerals, of which galena is the commonest, and a vein so formed is technically called a gash vein, though the term is frequently misapplied. Galena occurs in Prieska and in the Transvaal, and frequently carries silver. At Malmani in the west, and at Pilgrim's Rest in the east of the Transvaal, the gash veins are filled in with brecciated quartz, with limestone cement, or sometimes with pure quartz; the veins are either horizontal or vertical, and contain gold and copper.

The Pretoria Series. — The Pretoria Series, as typically developed round Pretoria, consists of a great band of shales and quartzites overlying the dolomite. There are three quartzites, known, from below upwards, as the Tirneball, Daspoort, and Megaliesberg Quartzites, averaging some thousand feet in thickness, though the Megaliesberg is usually the thickest and the Daspoort the thinnest of the three. These are separated by shales, which vary from normal blue shales and flagstones to actual haematite shales like the lower Witwatersrand shales, with which they were originally classed. The quartzites are usually sandstones hardened by secondary silica; but these again become ferruginous.

All round Pretoria the series has been invaded by innumerable dykes of dolerite, altered by time to diabase, and there are also some contemporaneous igneous sheets. The metamorphism produced by the diabase is extraordinary. Three types are recognized:

(a) The Longsight type: True contact rocks with biotite, staurolite, andalusite, and cordierite crystallized in the slates.

(b) The Groothoek type: Hornstones, entirely recrystallized, with biotite and cordierite forming a cement.