454 ME. AV. H. PENNING ON THE 



the waste of the underlying Silurian beds." The conglomerates, and 

 in a less degree the sandstones, are auriferous. Subsequent obser- 

 vations convinced me that the inference was correct, and that the 

 Lj'denburg rocks referred to Liter on are of the same age as those, 

 2U0 miles distant, now to be briefly described. 



As in the De-Kaap Valley and Northern Swaziland, so between 

 Pretoria and Johannesburg, the rocks have been partly tilted by, 

 and partly faulted against, a mass of granite, which, by its more 

 rapid disintegration, here also forms the lower ground, surrounded 

 by the tilted and broken stratified deposits. This upheaval was 

 more recent than that of the De Kaap, as in the latter region the 

 Silurian (?) rocks only were disturbed, whilst the Devonian (?) still 

 occupied very nearly their original horizontal position. 



(a) The Witwatersrand Series. — These rocks have been described 

 as " chiefl.y sandstones, sometimes micaceous, with occasional shales, 

 cherts, and quartzites, especially towards the base of the formation, 

 which abuts against the mass of granite already mentioned. The 

 intrusion of this granite has tilted the rocks to a high angle .... 

 further south they gradually assume a less inclination, and eventu- 

 ally arc seen in a nearly horizontal position " *. Here they pass in 

 under a very large and quite conformable sheet of dolerite, the out- 

 crop of which (from beneath an upper series of strata) has recently 

 been traced south-eastward to bej'ond Heidelberg, the beds rising from 

 beneath it, in due order, all round its eastern extremity, and again 

 along its southern margin. This establishes as a fact, what was 

 previously only a surmise, that they form a practically continuous 

 basin, the southern extension of which passes in under the Coal- 

 formation to an indefinite distance in the Orange Free State. Prom 

 several sections across this basin, and many observed dips all over 

 the area, I have worked out the curve which these beds must follow 

 in their downward course ; and, as the results fairly tally at dis- 

 tances of some miles apart, they may be taken as approximating to 

 the truth. I find that the surface of the granite along the central 

 line of the basin, if assuming as regular a curve as the outcrops 

 indicate, would lie at a depth of about 18,000 feet below the 

 ground ; and, as this has an average elevation of about 5000 feet, 

 the granite along that line is no less than 13,000 feet below the 

 level of the sea. 



Of this fact the continuity of the beds all round the eastern end 

 of the depression, with mere local fractures and without much 

 change in thickness, affords satisfactory evidence. The regularity 

 of the lessening dip implies general absence of folding ; and, even 

 if the beds be folded, there must still be room for their great thick- 

 ness, unless (as is unlikely) they have thinned out towards what is 

 now the middle of the depression. The width of outcrop of the 

 rocks of this series near Johannesburg is five miles, and their dip 

 decreases from almost 90° through G0°, 45", and 20° to 10° (where 

 they pass under the dolerite), with an average of about 40°, which 



* Journ. Soc, Arts, Tol. xxxyi. (1888) p. 435. 



