416 



MR. W. GIBSON ON THE GEOLOGY OF THE GOLD -BEARING 



as to the west of the Salisbury Mine ; and the soft surface-rocks 

 are also replaced below by hard beds having a schistose character. 



In the May Deep Levels Mine five hard and compact conglome- 

 rates are admirably shown. These dip at 45°, and associated with 

 them are thick beds of crystalline quartz, having the same dip. 

 These are probably segregation-veins. 



Pig. 3. — Section across tJie Cinderella Gold-mine and Bolcshurg 

 Colliery, 



Cinderella 



Gold 

 W.N.W. mine 



Boksburg 



Colliery. 



[Length of section = about 1|- mile 



a. Coal-bearing beds. 



b. Quartzites. 



c. Volcanic Ash, 



d. Conglomerates. 



e. Diorite. 

 /. Fault. 



The distance over which the Main Eeef Series can thus be defi- 

 nitely traced, from the Banket Mine to the Simmer and Jack Mine, 

 is about 2b miles. Both east and west of the Salisbury Mine, as 

 we have seen, only two conglomerates are found at the surface. 

 Yery varying dips are met with, but on the whole the dip decreases 

 both east and west, and the beds gradually assume a southerly 

 trend. Finally, it appears to be invariably the case along the whole 

 line of strike of the Main Eeef Series that the rocks, when traced 

 downwards, assume more and more a schistose character. 



3. The Gold-bearing Conglomerates south of Johannesburg. 



On 'deep-level' properties it is always found that the reef is 

 struck at a much shallower depth than would be calculated from 

 surface indications, and that more reefs are met with below than 

 appear at the surface. These facts are generally considered to 

 show that the conglomerate-beds form a basin, and are decreasing 

 in dip towards its centre. But there is scarcely a mine on the 

 Kandt that does not reveal abundant evidence of reversed faulting. 

 In the case of the two diamond drill-holes placed on the Vogelstruis 

 property (see Map, PI. XI.) there can scarcely be a doubt that the 

 apparent decrease in the dip of the reefs is due to this cause. In 

 one of the drill-holes a sheet of diorite, apparently in a line of 

 fault, was passed through immediately after the reef was struck ; 

 while the shaft north of the second drill-hole showed a reversed 

 fault, with the upthrow portion of the beds flat, while the reef 

 still farther north had a dip of nearly 70°. Such facts are de- 



