Page:South African Geology - Schwarz - 1912.djvu/197

 (c) Malips River type: Crystalline schists, in which the alteration has gone on both by contact and pressure metamorphism.

In the west, in Bechuanaland and Prieska, the series is more extensive and the rocks are more ferruginous. It has been called the Griquatown Series, but the term is no longer necessary. Beginning at the southernmost point of the Doornbergen, the rocks are blue-black slates and quartzites; but as one passes northwards to Prieska's Poort the character of the rocks changes; the slates turn to red, brown, and yellow jaspers, and the quartzites become white. Farther north, at the Orange River, again, the rocks become less altered, and blue-black slates once more appear; but north of this again the banded jaspers occur and continue into the Kalahari. The typical mineral of this formation is crocidolite or blue asbestos, which forms in crevices in the blue-black slates, the fibres extending from side to side of the cracks. When the country rock changes to jasper the crocidolite also alters, and changes to a beautiful honey-brown silky fibrous mass made up of quartz fibres coloured with limonite. It is really a pseudomorph of silica after crocidolite, and its technical name is tiger-eye quartz, though commercially it is known as crocidolite. A further alteration may take place when the tiger eye changes to a red variety as all the iron becomes leached out and a simple mass of white fibrous quartz is left. Frequently the crocidolite, instead of crystallizing out as fibres ranged parallel to one another, forms a felted mass of fibres; the stone thus made up is a lustrous blue-black hard mass, taking a very fine polish and being practically a blue jade. Deep-green crystals of grunerite, silicate of iron, also form from the blue-black shales, and differ chemically from