Yol. 54.] WITWATEESEAND AND OTHER DISTRICTS IN S. TRANSVAAL. 95 



YI. The Age and Geotectonic Eelations oe the Formations oe 

 THE Southern Transvaal. 



It has been shown above that the Karoo formation rests un- 

 conformably on the much older Cape formation. The age of 

 the former s^^stem has recently been discussed in this Journal 

 by Mr. A. C. Seward/ who places it at the close of the Palaeozoic 

 and beginning of the Mesozoic eras, corresponding to the Permo- 

 Carboniferous system of Europe. With regard to the age of the 

 Cape formation, there is very little to help one to a correlation 

 with European systems, owing to the absence of fossils. It is re- 

 markable that strata of such immense thickness, including beds of 

 the most varied composition, such as sandstones, shales, and linae- 

 stones, should show no trace of fossiliferous remains. One reason 

 for this is, no doubt, the fact that the beds show evidence of consi- 

 derable metamorphism since their first deposition ; thus we find the 

 sandstones converted into quartzites, the shales into haematite- and 

 magnetite-schists, the limestones silicified and marmorized, while 

 the accompanying basic igneous rocks are in part changed to 

 hornblendic and chloritic schists. 



Eor the sole indication of the age of these beds we have to look 

 to the Cape. In the Cape Peninsula an occurrence of Devonian 

 fossils has been recorded in the Bokkeveld Shales.^ These shales 

 have no counterpart in the Transvaal, but they lie on the Table 

 Mountain Sandstone, of which the Witwatersrand sandstones and 

 conglomerates are generally held to be the Transvaal facies. But 

 this, although extremely probable, has not been proved, and 

 nothing short of the completion of a geological survey of South 

 Africa is likely to prove it. 



As already pointed out, the Cape System in the Transvaal is 

 separated by an unconformity into an upper and a lower portion. 

 The upper beds include the sandstones and shales of the Gatsrand 

 and Magaliesberg Series and the wide-spread Dolomite Eormation, 

 while the lower beds comprise the Witwatersrand Beds and the 

 Hospital Hill shales and quartzites. Consequently, if we assume 

 that the lower beds are of Devonian age, it is possible that the 

 upper beds may correspond to the Lower Carboniferous of Europe. 



Turning now to the geotectonic relations of the district, we find' 

 that the central portion of the great anticline formed by the 

 Dolomite Formation and the overlying Magaliesberg and Gatsrand 

 Series has been removed by denudation, so as to bring to view 

 the older beds of the Witwatersrand, and with them the igneous 

 complex of Archaean rocks. The presence of the latter can be ex- 

 plained only by the assumption that they have been elevated to their 

 present position by earth-movements of considerable magnitude, or 

 that the beds of the Cape System have been lowered by ' trough- 



1 Vol. liii (1897) p. 334. 



■■* D. Sharpe and J. W. Salter, Trans. Geol. Soc. ser. 2, vol. vii (1856) p. 203, 

 and F. Sandberger, Neues Jahrb. 1852, p. 581. 



