422 MR. W. GIBSON ON THE GEOLOGY OF THE GOLD-BEAKING 



The quartzites are identical in thickness, but the shales occupying 

 the first valley are as thick again as those filling the other ravines, 

 owing to the presence of the ribhed black shales, A section across 

 the latter may be seen behind the Landdrost's house. Their dispo- 

 sition is such as to afford most unquestionable evidence that the 

 rocks have been much folded and overthrust. The beds have been 

 thrust forward to such an extent that the axes of the folds have 

 passed the vertical ; and even hand-specimens are marked by minor 

 ripplings, which in situ have the same general southerly dip and 

 east-and-west strike as the beds themselves. The red shales 

 following the black ones to the north are as a rule inclined at a 

 much steeper angle. 



A tunnel on the Houghton Estate (see Map, PL XI.), driven into 

 the escarpment formed by the northernmost band of quartzite, passes 

 first of aU through an exceedingly crushed and altered shale. 

 Pressure has almost entirely disguised its true nature ; it is crossed 

 and recrossed by numerous veins of crystalline quartz, while the 

 original felspathic constituents have become highly talcose, and the 

 banding caused by the different tints of red and green resembles 

 that of serpentine. Traced westward the shale becomes less altered, 

 and can finally be recognized as a hardened red shale identical with 

 that occupying the valleys. In the road-cutting, a little west of the 

 tunnel, the shales are seen to be twisted in and out among the 

 quartzite. They are not, however, so metamorphosed as in the 

 tunnel, and are still recognizable as shales, although intensely 

 hardened and partially changed into schist. Farther in the hiU the 

 tunnel shows the strata curving upwards. This is very markedly 

 the case with a two-foot band of conglomerate. The pebbles in it 

 are small and much shattered. The cementing-material (matrix) 

 is a green chloritic substance of schistose character, and this flows 

 between and around the pebbles, forming a rock resembling in 

 appearance an aug en-gneiss. Gold is present, but is not equally 

 disseminated through the rock. 



The crushed state of the shale, the upward curving of the strata, 

 and the visible features of the conglomerate are all strong evi- 

 dences that a thrust-plane of great importance is being approached ; 

 and before long we find the quartzites of the series resting at once 

 on gneisses and schists, the plane of contact being probably the plane 

 where the maximum differential movement of a powerful thrust 

 was brought about. 



The third valley is largely filled up by a sheet of diorite. This is 

 very fresh-looking and apparently unaltered. It is clearly later in 

 age than the powerful crust-movements to which the quartzites and 

 shales have been subjected. 



The first band of quartzite which has just been mentioned 

 lies about a mile to the north of the Main Reef conglomerate series. 

 Throughout the whole length of its outcrop it retains an average 

 dip of 45° S., never being much higher or lower. The Main Reef 

 Series, on the contrary, is nearly vertical in the centre, while east and 

 west the dip decreases to an average of 15° S. Now the strata 





