GEOLOGY OF THE SOUTHERN TKANSVAAL. 459 



ft. in. 



20 Yellowish sandstone. 



t> Yellow and purple laminated clay. 



o Micaceous shale. 



1 4 Shaly coal. 



(> Shaly coal, ferruginous. 



1 2 Good coal. 



4 Finely laminated coal. 



IV. Denudation. 



As in all other parts of the world, there has been here enormous 

 denudation of the more ancient rocks, resulting in the most marked 

 unconformity between the Silurians (?) of the De-Kaap Valley and 

 the Devonians (?) of the Godwaan Plateau. Some points in the later 

 planings-down of this region are well worthy of consideration, both 

 in that {<i) which preceded the deposition of the coal-bearing rocks, 

 and in that (h) to which the present surface-configuration is due. 



(a) Before the High-Veldt Coal-formation. — In treating of the 

 auriferous rocks I have not commented upon their characters as 

 gold-producing rocks ; but in one respect the occurrence or non- 

 occurrence of that metal, or of any other metal or mineral, in them 

 (or in deposits resulting from them) may afford strict geological 

 evidence. Such evidence will be of even more value in connexion 

 with gold, Avhich under ordinary conditions cannot travel far, as its 

 absence from rocks where its presence would naturally have been 

 surmised renders more probable any explanation of its removal by 

 extraordinary or unexpected agencies. 



The tilting of the "NVitwatersrand series began at that period of 

 volcanic activity which gave rise to the outpouring of the dolerite (d) 

 in an extensive sheet that can even now be traced for a distance of 

 at least 300 miles. But this evidently slow upheaval still continued 

 at the same centres, as the trap-rock also was turned up with the 

 beds beneath it, and now forms a fringe to the Klip-Eiver series 

 around the inner part of the Kand basin. Meanwhile the upper 

 series of rocks were deposited, and in some places overlapped the 

 bent-up, faulted, and locally-denuded edges of the Witwatersrand 

 series, as for instance some twenty miles west of Johannesburg. 

 Even then there must have been extensive denudation going on over 

 the areas upheaved, that is to say above the granite bosses south of 

 Pretoria and at Parys in the Orange Free State ; a similar argument 

 to that which follows, although in reference to a still earlier period, 

 in connexion with the upheaval of the beds of the De-Kaap Valley, 

 might also apply. 



It might naturally be expected that large alluvial deposits of 

 gravel, as such or as old conglomerate, would result from the removal 

 o£ the masses of rock that once overlay these granite centres, espe- 

 cially as many of the beds consisted of conglomerates, and were 

 made up, to a great extent, of pebbles of hard quartz and quartzite. 

 But, as a matter of fact, the country is almost destitute of alluvial 

 deposits worthy of the name. There are some few of recent origin, 



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