ALKALI ROCKS IN THE TRANSVAAL 747 



mother-magma and considered the Waterberg sandstone as younger 

 than the effusive, but older than the main intrusive, period; then, 

 however, there is no genetic connection between effusion and 

 intrusion. 



With H. Kynaston and E. T. Mellor, 1 we can admit a prolonged 

 activity of the Bushveld magma, while the sandstone series were 

 deposited between the main intrusive and effusive period and the 

 later intrusions. 



That the nepheline syenites are younger than both the norites 

 and the granites will be proved in the following pages. Only 

 detailed geological and petrographical investigations can clear the 

 true succession from the remnants left after the advanced denuda- 

 tion. 



In any case, the following reasons make it undesirable to unite 

 the volcanic series and the sandstone in the Waterberg system. 



Over large surfaces at the base of the sandstone series we find 

 developed a basal conglomerate, which rests unconformably, not 

 only upon the felsites, but also upon the red granites; and in the 

 Zoutpansbergen we also find them resting upon much older forma- 

 tions, such as the old granites. Thus it forms a geological horizon 

 over the whole of Central Transvaal and marks an extensive 

 unconformity. 



As a rule the pebbles of felsites are rare in this conglomerate, 

 although the relative quantity may increase locally and the uncon- 

 formity be locally less well developed. In the Waterberg district 2 

 the conglomerate is principally composed of pebbles of jasper, 

 quartzite with magnetite, white quartzite, schistose quartzite with 

 muscovite, quartz, chert, and felsophyre, which, except the fel- 

 sophyre, belong to the Swaziland system (Barberton series). Con- 

 sequently this unconformity must be maintained as a horizon of 

 separation, and the volcanic series must be separated from the 

 sandstone series of the Waterberg system and can either be included 

 in the igneous complex of the Bushveld or else considered as a 



1 H. Kynaston and E. T. Mellor, The Geology of the Waterberg Tin Fields. Memoir 

 No. 4, Geological Survey of the Transvaal, Pretoria, 1909. 



2 G. A. F. Molengraaff, "Geologische Aufnahme der Sud-Afrikanischen Repub- 

 lik," Jahresbericht iiber das Jahr i8q8, Pretoria, 1900. 



