( 38(5 ) 



gives an illustration of the strong force of crvstallization of some 

 minerals under certain circumstances, which enables them to crystal- 

 lize in large crystals still in a nearly completely soliditied magma. 

 For the development of large crystals, the circumstances must have 

 been most favourable shortly before the end of the crystallization, 

 because the same minerals wei-e often already formed in small crystals 

 before. Further the richness in pneumatolytical and thermal minerals 

 is characteristic for all the rocks of this region ; accordijig to this the 

 loss on ignition is always very considerable. 



Probably undei- certain detinite pressures and temperatuj-es the 

 above described structnres can be formed in magma's which, in conse- 

 quence of their richness in pneumatolytical gases, tire very fluid, 

 whilst the changes in the succession of crystallization are principally 

 determined by the chemical composition of the magma. 



In connection with this, we find that in. rocks i-ich in ncpheline, 

 although they nuiy have widely dilferent structures, ue[»heline is 

 constantly the first mineral to crystallize, whereas in rocks poorer in 

 nepheline this mineral has crystallized after the felspars. This we 

 see as well in rocks in which the felspar encloses the nepheline, as 

 in rocks in which the nepheline encloses the felspar. In lujaurite- 

 porphyries from Wijdhoek (701) the felspars enclose numerous needles 

 of aegirine, whilst the fels[)atoids contain those sometimes, but onlj' 

 in the marginal zone. After the crystallization of the larger felspatoids, 

 a mosaic of small idiomor})hic crystals of felspatoids w^as formed, 

 still later the crystals of molengraaffite and aegirine enclosing the 

 small felspatoids poicilitically. 



These rocks are relatively rich in nepheline ; after a period of 

 crystallization of larger crystals of felspatoids followed the ciystalli- 

 zation of the larger felspars and small needles of aegirine, while 

 the crystals of felspatoids grew on slowly. Then the principal part 

 of the felspatoids crystallized as a mosaic, probably under suddenly 

 changed conditions, and at last the larger perforated crystals of aegirine 

 and molengraaffite were formed. 



In the aegirineamphibolebiotitenephelinesyeniteporphyries which are 

 rich in nepheline and poor in dark minerals, a j)eriod of crystallization 

 of felsi)atoids free from inclusions, and of some felspar was followed 

 by the crystallization of small short felsparlaths and some biotite, 

 while the larger crystals of nepheline grew on simultaneously ; 

 smaller crystals of this latter mineral were not formed, wiiicli is 

 evident from the occurrence of perforated crystals of nepheline 

 englobing the felspars only. The inclusions are often limited to their 

 marginal zone. The amphiboles and pyroxenes have been formed 



