426 



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



has any fault been laid down without strong evidence of its 

 existence. 



The majority of the smaller faults in the mines cut at a very 

 acute angle across the beds, till in very many instances they become 

 strike-faults having an east-and-west trend. Such faults are ex- 

 tremely common, scarcely a mine being without one or more ex- 

 amples. North-and-south faults are somewhat rarer, the outcrop 

 of the Main Eeef Series being very little shifted along its whole 

 length. 



It has been previously mentioned that west of the Banket Mine 

 there is evidence for a large fault cutting off the Main Eeef Series 

 (see Map, PI. XI.). Now, around Krugersdorp, about six miles 

 north-west of the Banket Mine, several reefs are found having no 

 definite relationship to the Main Reef beds, but resembling those 

 that occur to the south. The dip is very various, a shallow dip at 

 the surface becoming vertical below, and vice versa, and the exist- 

 ence of numerous small faults is shown in the mines. None of 

 these beds, however, can be traced far eastwards, while the Main 

 Reef cannot be traced westwards. Taking all these points into 

 consideration, it seems highly probable that these reefs to the south 

 of Krugersdorp are dislocated portions of those lying south of the 

 Main Reef, and that they are cut off eastwards by a strike -fault 

 which is an extension, or a branch, of that ruiming up the Banket 

 Spruit. The Main Reef Series should, unless the thinning-out 

 shown in the Durban Roodeport and Princess Mines represent an 

 actual dying-out in the beds, be met with to the north-west of 

 Krugersdorp. 



On the map two nearly north-and-south faults are represented as 

 cutting off the Main Reef Series to the east and west. The western 

 fault has just been referred to, and the evidence for the existence of 

 the eastern fault is nearly identical in character. At Boksburg the 

 reefs are found to be turning sharply to the south, and finally to 

 end off against a mass of diorite. In the Chimes Mine, 3 miles 

 N.N.E. of Boksburg, a reef is worked, which, if mineralogical com- 

 position can be relied on as indicating identity, is one of the Main 

 Reef Series. Even if the Boksburg reefs belong to those lying south 

 of the series just mentioned, still the reef in the Chimes and Van 

 Ryn's ground lies too far north to be one of the Main Reef beds, 

 unless brought into its present position by faulting. 



The great east-and-west fault behind Johannesburg can be easily 

 detected by its cutting off one member after another of the quartz- 

 ites and red shale-bands in going from west to east. My reasons 

 for supposing this to be a reversed fault have been already given, as 

 have also those for placing a fault up the Klip Riversberg valley. 



Throughout the Randt the result of the faulting has been to cut 

 up the country into a series of strips striking east and west. How 

 much of the sequence of the beds is concealed, and how much of 

 the apparent thickness is due to repetition, the writer is unable to 

 say. 



Yery few instances occur of igneous rocks intruding into the 



