SIMILKAMEEN BATHOLITH Sod 



For many square miles together the great central portion of the batho- 

 lith is composed of this rock — a soda-rich biotite-hornblende granite of 

 an average specific gravity of 2.706. 



At the head of Toude (or Toat) coulee the rock of a large area within 

 the batholith is generally porphyritic and distinctly finer grained than the 

 staple granite, the specific gravity averaging 2.675. The phenocrysts are 

 poikilitic microperthites bearing many inclusions of the other constit- 

 uents. In the specimens so far examined, orthoclase tends to dominate 

 over microperthite. Near the contacts with the normal equigranular 

 rock, oligoclase replaces the alkaline feldspars to a great extent; yet this 

 phase is always poorer in both hornblende and biotite than the normal 

 phase, which is thus slightly the more basic rock. The finer grained 

 phase was seen at several places only a few feet from the coarser ; the con- 

 tact is there sharp, but the absolute relation between the two phases could 

 not be determined. It is highly probable that both are of nearly contem- 

 poraneous origin, the intrusion of the porphyritic phase having followed 

 that of the equigranular rock by a short interval, as if in consequence of 

 massive movements in one slightly heterogeneous, partially cooled magma. 

 The porphyritic phase often shades into the other so imperceptibly that a 

 separation of the two phases on the map is a matter of great difficulty, if 

 not of impossibility. 



The material of the batholith is further varied by rather rare basic 

 segregations. These have the composition of hornblende-biotite diorite, 

 being made up of the minerals of earlier generation in the host. 



Contact basification. — Much more important products of differentia- 

 tion, as shown by microscopic analysis, are illustrated- in a wide zone of 

 contact basification. Here there occur several related types of alkaline 

 or subalkaline syenites. In specimens collected along the contact with 

 the Kruger alkalines, quartz nearly or altogether fails, biotite is absent, 

 and abundant diopsidic augite accompanies the essential hornblende. The 

 feldspars are the same as in the staple rock, with basic oligoclase, Ab 2 An 1? 

 yet more abundant than there. Zircon is added to the list of accessories. 

 These facts and the general habit of the rock relate it both to monzonite 

 and to genuine alkaline syenites. The chemical analysis closely resembles 

 that of the typical rock from Monzoni, except that the soda is strongly 

 dominant over the potash (4.60 per cent of Na 2 to 3.00 per cent of 

 K 2 0). This basic phase may be called an augite-hornblende soda mon- 

 zonite of a specific gravity of 2.800-2.819. It is known to extend at least 

 1,200 yards from the main eastern contact of the batholith. It is an 

 open question as to how far this basification is due to absorption of ma- 



