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Geology. — "On peculiar sieve .structures in igneous rocks, rich in 

 alcalles". By Dr. H. A. Brouwer. (Communicated by Prof. 

 G. A. F. Molengraaff). 



(Communicated in the meeting of September 30, 1911). 



The following poicilitic structures, occurring in nepheline-syenitic 

 rocks from the Pilandsbergen (Transvaal), are very instructive with 

 regard to the insight wdiich they allow into the order of succession 

 of the crystallizations in magma's which are rich in alcalies. 



1. Intergrowtlis of dark nmierals loith felspars 

 and felspatoids. 



A tardy crystallisatiun of the alcalip)roxenes and alcaliamphiboles 

 is known in alcaligranites, alcalisyenites and nephelinesyenites. 

 Sieve structures in which the crystals of the dark minerals are 

 ramiticated between the colourless ones are found e.g. in the paisa- 

 nites and the alcaligraniteporphyries from Zinder (Sokota) ^). This 

 structure is very common in rocks from the Pilandsbergen and is 

 beautifully developed in aegirineamphibolebiotiteneplielinesyenite- 

 porphyries from Wijdhoek (701), in which principally the alcali- 

 amphibole is ramificated in skeletons of equal optical orientation 

 between the elements of the groundmass : nepheline, sodalite, felspar, 

 biotite, fluorspar, iron compounds and traces of apatite "). jMacros- 

 copically those crystals are visible as round spots of a few m.M. in 

 diameter contrasting with a grey, medium- to linegrained "ground- 

 mass". The larger crystals of biotite and aegirine, which are very 

 subordinate, show the same poicilitic structure. The biotite also 

 occurs in small crystals in the groundmass, whilst the pyroxene, 

 amphibole, and larger crystals of felspatoids are only crystallized 

 after the colourless minerals. 



Tiie same sieve structures enclosing the colourless minerals, are 

 found in crystals of uiolengraaftite and astrophyllile. 



In lavenitebearing lujauriteporphyries from Wijdhoek (701), the 

 larger crystals of molengraaffite and aegirine are speckled with small 

 crystals of nepheline ^), whilst the felsparcrystals are completely free 

 from inclusions. 



1) A. Lacroix. "Sur les microgranites alcalins du territoire dc Zinder."' C. R. 

 Ac. des Sciences, t. CXL, 1905 p. 22.. 



-) See Pi. IV, fig. 1 and 2 in H. A. Brouwer. "Oorsprong en samenstelling der 

 Transvaalsche nepheliensyenieten." The Hague. Mouton and Co. 1910. 



1) Seejbid. fig. 9, p. 158. 



