REPORT OF THE CHIEF ASTRONOMER 461 



SESSIONAL PAPER No. 25a 



In the Norm classification the rock enters the sodipotassic subrang, 

 toscanose, of the domalkalic rang, toscanase, in the persalane order, britannare. 

 In the older classification it is a biotite granite rich in soda. The specific 

 gravities of three fresh specimens vary from 2-621 to 2-637, averaging 2-631. 



A local varietal phase, bearing olive-green hornblende as a second essential, 

 was found in the contact zone, 400 yards or more in width, alongside the Simil- 

 kameen hornblende-biotite granite; here there may also be some slight enrich- 

 ment in oligoclase at the expense of the microperthite. Neither hornblende nor 

 biotite is abundant. The specific gravity of this phase is 2-644. The cause of 

 ■(he basification must once more be left undecided; it may lie in assimilation, in 

 differentiation, or in both. 



The ordinary basic segregation is notably rare in this batholith. A few, 

 with the composition of biotite-quartz diorite, were seen, but they seldom 

 exceeded a few inches in diameter. 



Younger Phase. — The coarse granite had been intruded, and apparently so 

 far cooled that joints had developed within its mass, when a second eruptive 

 efrort thrust a great wedge of nearly identical magma into the heart of the 

 batholith. This may be called the Younger phase of the Cathedral batholith. It 

 forms a large dike-like mass 3 J miles long and averaging 400 yards in width; 

 its length runs about north 60 degrees west and lies parallel to a system of 

 master joint planes within the Older phase. 



The Younger phase has the same general colour as the coarse granite, but 

 is finer-grained, more regularly porphyritic, and more acid. The microperthite 

 of the older granite is here largely replaced by orthoclase and microcline, both 

 sodiferous; at the same time the plagioclase is more acid, being oligoclase near 

 Ab 8 An x . The accessories are the same as in the coarse granite, but are much 

 rarer. Biotite also is here less abundant. The weight percentages are approx- 

 imately: — 



Quartz 38-8 



Orthoclase and microcline 33-4 



Oligoclase 17-6 



Microperthite 5-8 



Biotite 3-5 



Magnetite -6 



Apatite -3 



100-0 



The Younger phase approaches an aplitic relation to the Older. The con- 

 tacts between the two were seen at several points; they are sharp, yet the two 

 rocks are closely welded together, and it seems probable that the coarser granite 

 was still hot when the younger granite was injected. 



Relation to Similkameen Batholith. — The Cathedral granite is unquestion- 

 ably consanguineous with the Similkameen granodiorite. Apart from their 

 obviously close association both in the field relations and in the geological 



