AND ASSOCIATED ROCKS OF THE SOUTHERN TRANSVAAL. 413 



the Salisbury Mine, on the outcrop of the Main E-eef. There is in 

 this mine, as in others, a great difference between the rocks at the 

 upper and lower levels. No one, probably, examining the beds at 

 the surface alone, would think that the red-stained, crumbling, soft 

 pebble-beds and shales looking like a series of ordinary sedimentary 

 deposits above, become below bluish conglomerates, schistose rock, 

 and hard quartzites. Surface-outcrops in the Transvaal, therefore, 

 are not to be relied upon as showing the true nature of the rocks 

 as they appear below. 



The sequence of the beds on the lower level is shown in the 

 section on p. 411 (fig. 2). 



The quartzite (a) is intensely hard and reveals planes of move- 

 ment. It carries a large proportion (sometimes as much as 10 

 per cent.) of crystals of iron pyrites. Much of the silica has segre- 

 gated out in the form of quartz-veins, many of which reach a 

 thickness of 2 feet. 



The conglomerate-bands or reefs (h-b^) are here Jive in number, 

 and appear from the strike to be continuations of those in the Salis- 

 bury Mine. As far, however, as can be judged from a section of such 

 limited height (5 feet) as a ' driv^e,' the bands are always distinctly 

 wedge-shaped, the thin end of the wedge being downwards. The 

 pebbles are shattered, and the cementing-material is in a schistose 

 condition. On the surface of the ground only two reefs are seen. 

 This difference can be accounted for in two ways : either the bands 

 (h~h^) are lenticles of conglomerate, which die out above and below, 

 and are replaced by others at different horizons ; or they are dupli- 

 cations of one or more bands by overfaulting. Looking at the 

 metamorphosed nature of the surrounding rock, the latter view seems 

 the more likely. 



It should be noticed that it is one of the commonest results 

 of mining operations in this district, as they are continued in 

 depth, to cut through more reefs on the lower than are to be found 

 on the upper levels, or at the surface. This is so in the ' Meyer and 

 Charlton,' the ' Village Main Reef,' and the ' May Deep Levels ' 

 mines. Indeed the writer is unaware of a single instance in which 

 this is not the case. 



With the exception of band (\), which is nearly twice the thick- 

 ness of any of the others, the bands are all of the same size, are 

 identical in composition, and the beds between them are quite 

 similar to each other. It is, however, impossible to say exactly to 

 what extent the same beds have been repeated. Even one band may 

 have been repeated several times. The conglomerate-band (\) is 

 divided in the centre by a curved 3-inch layer of milk-white quartz. 

 It looks, from the amount of displacement in the cementing-material 

 of the conglomerate, and from the crushed nature of the pebbles, as 

 if the conglomerate-bed had snapped and been thrust forward along 

 a divisional plane marked by the thin quartz-layer. 



The igneous rock (c) is probably a decomposed diorite. It is 

 not crushed, and thus belongs to a later period than that of the 

 disturbance which has affected the quartzites and conglomerates. 



The schistose band (d) shows distinctly that very great pressure 

 Q. J. G. S. No. 191. 2 G 



