CATHEDRAL GRANITE BATHOLITH 355 



It is seen by inspection of the weight percentage tables that this granite 

 carries more silica than does the granite of the Similkameen batholith, 

 which in its turn is more acid than the granodiorites. 



The great bulk of the batholith is thus composed of biotite soda granite 

 (specific gravity, 2.631). 



A local varietal phase, bearing dark green hornblende as a second essen- 

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

 the Similkameen hornblende-biotite granite ; here there may also be some 

 slight enrichment 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 the basification must once more be left un- 

 decided; 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. 



Young er phase. — The coarse granite had been intruded, and appar- 

 ently so far cooled that joints had developed within its mass, when a sec- 

 ond eruptive effort 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!/2 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 color 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 sodif erous ; at the same time the plagioclase is more 

 acid, being oligoclase near AbgAn^ The accessories are the same as in 

 the coarse granite, but are much rarer. Biotite, too, is here less abundant. 

 The weight percentages are approximately : 



Per cent 



Quartz 38.8 



Orthoclase and microcline 33.4 



Oligoclase 17.G 



Microperthite 5.8 



Biotite 3.5 



Magnetite 6 



Apatite 3 



100.0 



The Younger phase approaches an aplitic relation to the older. The 

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



