REPORT OF THE CHIEF ASTRONOMER 449 



SESSIONAL PAPER No. 25a 



There is a question as to how far this list of varieties actually represents 

 the original magmatic variation within the body. The evidence is good that 

 the augite and hornblende and a part of the biotite, along with the feldspars and 

 nephelite, crystallized from the magma. It is not certain in the case of melanite 

 which, in the Ontario malignite, as described by Lawson, appears to be a primary 

 essential.* Microscopic study shows that much of the melanite in the Kruger 

 rocks is of magmatic origin, but that perhaps much more of it has replaced 

 the pyroxene during dynamic metamorphism. In such cases the pyroxene, 

 where still in part remaining, is very ragged, with granular aggregates of the 

 garnet occupying the irregular embayments in the attacked mineral. A further 

 stage consists in the complete replacement of the augite by the melanite 

 aggregates which are shot through with metamorphic biotite. These peculiar 

 reactions between the pyroxene and the other components of the rock are wide- 

 spread in both syenite and malignite. 



All the phases so far studied in this natural museum of alkaline types can 

 be grouped in three classes — granular malignites, granular nephelite syenites, 

 and coarsely porphyritic alkaline syenites. The malignitic varieties are always 

 basic in look, dark greenish-gray in colour, and medium to coarse in grain 

 (specific gravity, 2-757 to 2-967). The nephelite syenites are rather light 

 bluish-grey in tint, medium- to fine-grained, and 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 colour, and charged with abundant tabular phenocrysts of microperthite 

 which range from 2 to 5 centimetres 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 

 nephelite syenites. (Plate 39, A) 



The nephelite 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 nephelite 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 nephelite syenite of the apophyses. Similarly, even with tolerably good 

 exposures, no sharp contacts could be discovered 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 move- 

 ments continued into the viscous stage immediately preceding crystallization. 

 (Plate 39, A). 



Three specimens representing as many principal types were submitted to 

 Professor Dittrich for analysis. 



*A. C. Lawson, Bulletin, Dept. of Geology, University of California, vol. 1, 190. 

 25a— vol. ii— 29 



