REPORT OF TEE CHIEF ASTRONOMER 



457 



SESSIONAL PAPER No. 25a 



granite, the specific gravity averaging 2-675. The phenocrysts are poikilitic 

 microperthites bearing many inclusions of the other constituents. 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 contact is there sharp, but the absolute relation between 

 the two phases could not be determined. It is highly probable that both are of 

 nearly contemporaneous 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 segrega- 

 tions. These have the composition of hornblende-biotite diorite, being made up 

 of the minerals of earlier generation in the host. 



Basic Phase at Contact. — Much more important products of differentiation, 

 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 x , yet more abundant than there. Zircon 

 is added to the list of accessories. 



A specimen (No. 1107) of the basified shell showing this mineralogical 

 composition was collected at a point two miles north of the Boundary line and 

 about 200 yards from the contact with the Kruger alkaline body. It was 

 analyzed by Professor Dittrich, with the- following result: 



Analysis of basic contact-phase, Similkameen batholith. 



SiO, 5406 



Ti0 2 -80 



A1 2 3 18-75 



Fe-A 4-64 



FeO 3-10 



MnO tr. 



MgO 2.75 



CaO 7-35 



Na 2 4-60 • 



K>0 3-00 



H a O at 110 C C -10 



H,0 above 110°C -41 



P 2 K -55 



C0 2 -11 



100-22 



Sp.gr 2-819 



Mol. 

 901 

 010 

 183 

 029 

 043 



069 

 131 

 074 

 032 



004 



