A Contribution to the Geology of the Southern Transvaal

Abstract

I. I ntroduction . (a) The Southern Transvaal .—The area occupied by the Witwatersrand beds offers many facilities for accurate observation. I have been enabled to collect sufficient evidence to fairly establish the relation between the geology of the district and that of the older known gold-fields, and thus to briefly describe the geology of the Southern Transvaal. The accompanying map (PI. Xu shows an area about 350 miles in length and 50 miles in breadth (or 17,500 square miles in extent). Some points require further elucidation ; but the geological features, as expressed on the map and sections, are generally correct according to my interpretation. For a description of the district here referred to and its eastern border, see Quart. Journ. Geol. Soc. vol. xl. (1884) pp. 658 et seqq ., with map and sections. The granitic rocks are most frequently fine-grained, and generally include little mica, but sometimes much hornblende. Whether pegmatite, syeni.te, or granitd, they are comparatively little exposed in the Southern Transvaal. The granite ( a , on map and sections) is frequently broken through by dykes of diorite, which almost always run nearly north and south. So also are the stratified rocks of all ages broken through in this way ; but the older these are the more numerous are the dykes within them. In the granite, the various minerals have sometimes been re-assorted by the effects of the dykes—layers of quartz and felsite, or of kaolin, flanking the dykes on both sides for considerable distances. The general absence of fossils