80 DE. P. H. HATCH GEOLOGY OP THE [Pob. 1 898, 



From the Nigel Mine the beds can be traced to Heidelberg, and, 

 following the trend of the hills of the Znickerboschrand, they run 

 through the farms Boschhoek, Schickfontein, etc. to Stryfontein and 

 Boschkop on the Vaal Eiver, east of Vereeniging, where they are 

 again covered by the Karoo Formation. 



Farther east there are several inlying portions of the Witwaters- 

 rand. Beds, extending as far as Greylingstad, and constituting 

 a synclinal trough, the axis of which runs south-east. The 

 conglomerates are found outcropping on the northern side of the 

 syncline on the farms E-ietfontein, Rietbult, Van Kolders Kop, 

 Witpoort, and Doornhoek ; and on the southern side at Hoodepoort, 

 Driefontein, Hex E,iver, Barnards Kop, Boodevaal, Eietvlei, Witpoort, 

 Daspoort, Tweefontein, Driefontein, and Malans Kraal. The 

 central portion of the trough is much disturbed by igneous intrusions 

 of a basic character. 



An interesting occurrence of these beds is at Yenterskroon, where 

 they lie north of the Hospital Hill Series on the farms Eooderand, 

 Buffelskloof, Kodoesfontein, Buffelshoek, "Witkoppiesfontein, etc. 

 The beds dip south, and apparently underlie the Hospital Hill 

 Series ; but it is evident that tfaey owe their position here to reversed 

 faulting, and not to an inversion of the beds by folding, as described 

 by H. B. Bunkell.^ 



Description of the Beds. 



The Witwatersrand Beds consist of alternating quartzitic sand- 

 stones, grits, and conglomerates, forming a series which has an aggre- 

 gate thickness of from 11,000 to 15,000 feet. The sandstones consist 

 of particles of white quartz, and are cemented to hard compact 

 quartzite by secondarily deposited silica. They are of a dark bluish- 

 grey where unoxidized, but weather at the surface to reddish and 

 yellow tints. Occasionally, on exposed surfaces, current-bedding 

 and ripple-marks are observable. The pebbles of the conglomerates 

 consist almost entirely of a dark-coloured but pellucid quartz, 

 apparently derived from the breaking-down of ancient vein-quartz. 

 The pebbles vary in size from a pea to a hen's egg, though occasionally 

 much bigger pebbles are found. Examination under the microscope 

 shows that the cementing material of the conglomerates consists of 

 a mosaic of minutely granular but distinctly crj^stalline quartz. 

 The rock is so firmly knit together by this secondarily deposited 

 silica that, when broken, the fracture passes irrespectively through 

 pebbles and matrix. In places the newly-deposited quartz has 

 grown on to the pebbles in such a manner as to obliterate their 

 original margin and to produce a rock resembling a homogeneous 

 and glassy variety of vein-quartz. Of other minerals, finely-divided 

 iron pyrites is the most common, being disseminated through the 

 matrix of the rock in fine crystalline particles ; and native gold, 



^ ' Notes on the Venterskrcon Goldfields,' Trans. N. Eng. Inst, Min. & 

 Mech. Eug. vol. xlvi (1896) pp. 58-61. 



