778 H. A. BROUWER 



If ultra-acid rocks have differentiated from the granitic magma, 

 the residual magma will be enriched in A1 2 3 , and alkalies with 

 regard to Si0 2 and its composition will more or less agree with 

 that of the nepheline syenites. This kind of differentiation may 

 have taken place on a much larger scale at a greater depth, which 

 has not been exposed by denudation. It is interesting to find an 

 occurrence of nepheline syenites on Rietfontein [451] and Spitskop 

 [463], close behind the zone of ultra-acid rocks. 



The coarse textures, the rather indefinite order of crystallization, 

 the numerous poikilitic structures, 1 the abundance of fluorine and 

 rare minerals with a highly complex constitution, which are char- 

 acteristic for many of the nepheline syenites in the Transvaal, 

 make it probable that these rocks crystallized from a residual 

 magma in which the volatile constituents were concentrated and 

 which may have crystallized at rather low temperatures. Some 

 of the nepheline syenites are certainly younger and may be con- 

 siderably younger than the sandstones and conglomerates of the 

 Waterberg system ; the time at which the different intrusions have 

 risen to the present level and the time at which they have con- 

 solidated may have varied between wide limits. 



The age of an intrusive rock is determined by the time of its 

 consolidation, and it is very probable that the alkali magmas 

 remained fluid during a very long period of igneous activity. 

 When these magmas which are rich in volatile substances shall 

 crystallize will greatly depend upon the eventual loss of these 

 substances, which may have been the immediate cause of crystal- 

 lization quite as much as of any actual cooling. 2 



Only some characteristic features of the various igneous rocks 

 have been dealt with; as has been stated above, it does not seem 

 improbable that the nepheline syenites and allied rocks have 

 originated from the same sources as the granites and the norites 

 of the Bushveld. To the petrologist there are many very interest- 

 ing problems with regard to the origin and age of the different 

 rock types which would repay further research. 



1 H. A. Brouwer, ' ' On Peculiar Sieve Structures in Igneous Rocks Rich in Alka- 

 lies," Proc. Kon. Akad. Amsterdam, November, 191 1. 



2 A. Harker, The Natural History of Igneous Rocks, 1909, p. 186. 



