350 R. A. DALY — THE OKAttAGAN COMPOSITE BATHOLtTH 



the varietal rock types carry essential feldspars of high alkalinity — micro- 

 perthite, microcline, soda-orthoclase, and orthoclase. xSTepheline, biotite, 

 brown-green hornblende, a pyroxene of the aegerine-augite series, and 

 melanite complete the general list of essentials. Titanite, titaniferous 

 magnetite or ilmenite, rutile, apatite, and acid andesine, Ab s An 3 (the 

 last entirely absent in most of the rock phases), form the staple acces- 

 sories, though any one or more of the colored silicates may be only ac- 

 cessory in certain phases. Muscovite, hydronepheline, kaolin, calcite, 

 epidote, and chlorite are secondary, but on account of the notable fresh- 

 ness of the rocks are believed to be due to crush-metamorphism more than 

 to weathering. 



According to the relative proportions of the essential minerals, at least 

 ten different varieties of alkaline rock have been found in the body. 

 These are — 



Augite-nepheline malignite, Hornblende-nepheline syenite, 



Augite-biotite-nepheline malignite, Biotite-melanite-nepheline syenite, 



Augite-biotite-melanite malignite, Augite-biotite-nepheline syenite, 



Hornblende-augite malignite, Porphyritic augite syenite, 



Augite-nepheline syenite, Porphyritic alkaline biotite syenite. 



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

 sents 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 nepheline, crystallized from the magma. It is not cer- 

 tain 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 ha3 replaced the pyroxene during dynamic 

 metamorphism. In such cases the pyroxene, where still in part remain- 

 ing, 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 reac- 

 tions 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 nepheline 

 syenites, and coarsely porphyritic alkaline syenites. The malignitic varie- 

 ties are always basic in look, dark greenish-gray in color, and medium to 

 coarse in grain (specific gravity, 2.757 to 2.967). The nepheline sye- 

 nites are rather light bluish-gray in tint, medium to fine grained, and 



♦Bulletin, Pept. of Geology, University of California, vol. i, 190. 



