KRUGER ALKALINE BODY 351 



break with the sonorous ring characteristic of phonolite (specific gravity, 

 2.606 to 2.719). The third class of rocks is much less important as to 

 volume ; they are always coarse in grain, of gray color, and charged with 

 abundant tabular phenocrysts of microperthite which range from 2 to 5 

 centimeters in length. These phenocrysts as well as the alkaline feldspars 

 of the coarse groundmass are usually twinned, following the Carlsbad 

 law — a characteristic very seldom observed in the malignites or nepheline 

 syenites. 



The nepheline syenites often send strong apophysal offshoots into the 

 malignites, but such tongues are highly irregular and intimately welded 

 with the adjacent basic rock as if the latter were still hot when the nephe- 

 line syenites were intruded. Moreover, there are all stages of transition 

 in a single broad outcrop between typical malignite and more leucocratic 

 rock indistinguishable from the nepheline syenite of the apophyses. Sim- 

 ilarly, even with tolerably good exposures, no sharp contacts could be dis- 

 covered between the coarse, porphyritic syenites and the other phases. 

 The porphyritic rocks almost invariably showed strong and unmistakable 

 flow structure, evidenced in the parallel arrangement of undeformed 

 phenocrysts; these generally lie parallel to the contact walls of the body 

 as a whole. The phasal variety of the Kruger body and the field relations 

 of the different types seem best explained on the hypothesis that the 

 phases are all nearly or quite contemporaneous — the product of rapid 

 magmatic differentiation accompanied by strong movements of the magma. 

 These movements continued into the viscous stage immediately preceding 

 crystallization. 



The average composition of the whole Kruger body is probably that of 

 a malignite . transitional into true nepheline syenite; its specific gravity, 

 about 2.750. 



Metamorphism. — Few of the specimens collected are free from signs of 

 crushing. This has sometimes induced a decided gneissic structure, and 

 almost always the microscope shows fracture and granulation. The abun- 

 dant development of metamorphic melanite and biotite and the occasional 

 production of large poikilitic scapolites indicate some recrystallization 

 through dynamic metamorphism. The abundance of microcline and the 

 generally subordinate character of the orthoclase is another, yet more 

 familiar, relation brought about through the crushing. The mechanical 

 alteration of these rocks is far from being as thorough as in the case of 

 the Osoyoos batholith. This is a principal reason for believing that the 

 alkaline mass was intruded after the Osoyoos granodiorite had been itself 

 well crushed. No other definite field evidence for or against that view 

 has been discovered. However, the magmatic relationships between the 



