212 Walcot Gibson — Palceozoic Rocks of South Africa. 



surface of some of the reefs. The powerful effects of these over- 

 thrusts have been further demonstrated by the presence of shearing 

 on the edges of the igneous dykes which are frequently situated 

 along thrust planes ; and also by the conversion, by dynamic action, 

 of the smaller dykes into hornblendic and chloritic schists.' Minor 

 movements are indicated by a conglomerate band known as the Main 

 Eeef Leader, being so firmly attached to the overlying quartzite that 

 the reef is described as 'frozen ' into it, while the footwall or under- 

 lay consists of a band of soft schist accompanied in many places by 

 a vein of secondary quartz. These phenomena suggest a minor 

 thrust plane coincident with the footwall. That this is so becomes 

 more apparent in the Spes Bona Mine, where on the third level the 

 conglomerate band known as the Main Eeef is brecciated and is 

 separated from the Main Eeef Leader by eight feet of quartzite ; 

 while on the fourth level the two reefs are separated by a wedge-like 

 mass, 40 feet wide, composed ot a rock looking like a hard siliceous 

 mudstone.- 



Major thrusts have not been definitely proved underground, but 

 there is little doubt that a dislocation of this nature separates the 

 Hospital Hill Series from the Witwatersrand (Banket) Beds, as has 

 been frequently described and figured since its existence was pointed 

 out in 1892. Another major thrust of great intensity is indicated by 

 the junction of the Hospital Hill Series with tlie granites and schists 

 north of Johannesburg, where the two formations are so altered 

 by dynamic action along their junction that all trace of a line 

 of demarcation disappears (Moleugraaff, op. cit.. p. 20). The dis- 

 position of the quartzite ridges of the Hospital Hill Series north of 

 Johannesburg and their progressive metamorphisiu as they approach 

 this major tluust, taken in conjunction with their apparent great 

 thickness, 8,000-10,000 feet,^ strongly suggests that the various 

 quartzite beds and intervening shales are due to duplication or even 

 reduplication by overthrusting. Dr. Molengraaff denies the existence 

 of anything more than a local duplication, though lie acknowledges 

 (op. cit., p. 22) that the " orogenic movements from south to north 

 have been very energetic against the granite massif, which plays the 

 part of a buttress." 



Whatever may be the extent of the overthrusting, it is evident 

 that the simple basin theory does not receive the support of recent 

 investigation. The theory was mainly deduced from the generally 

 observed decrease in the inclination of the reefs when followed in 

 the direction of dip. It now appears that a secondary steepening 

 of the dip has an extended occurrence along the Witwatersrand. 

 This is most marked in the Simmer and Jack Mine, where from the 

 3rd to the ith level the dip is 10° 40', from the 6th to the 7th level 

 0° 30', from the 10th to the 12th level 37°, and from the 14th 

 to the 16th 29° 35'.* How vastly important to the future of the 



1 F. H. Hatch: QJ.G.S., vol. liv (1898), p. 85. 



* S. J. Truscott : " The "Witwatersrand Goldtields ; Banket and Mining- Practice," 

 1898, p. 109. 

 5 F. H. Hatch : op. cit., p. 79. * S. J. Truscott: op. cit., p. 13. 



