354 ft. A. DALY — THE OKAttAGAtt COMPOSITE BATHOLITH 



terial from the adjacent malignite-syenite series and how far to ordinary 

 spontaneous differentiation along the batholith walls. 



On the contact with the quartzites and schists of mount Chopaka the 

 basification is less pronounced; compared to the staple granite, this phase 

 is poor in quartz and rich in oligoclase-andesine and hornblende. It 

 may be called a hornblende-biotite soda monzonite of a specific gravity of 

 2.712-2.748. 



For a half mile or more northwest of the contact with a large body of 

 schist forming the Horseshoe pendant (figure 4) the batholith exhibits a 

 third basic phase. There is an almost complete disappearance of alkaline 

 feldspars, other characters of the rock remaining essentially like those of 

 the granite. This phase is a hornblende-biotite quartz diorite of a specific 

 gravity of 2.736. Here again there is doubt as to the exact cause of the 

 basification. The Horseshoe pendant is largely amphibolitic in composi- 

 tion, and it is possible that assimilation of material from these schists is 

 partly responsible for the development of the quartz diorite. 



CATHEDRAL GRANITE BATHOLITH 



Character of the material. — The youngest of the batholithic intrusives 

 is petrographically the simplest of all. Its material is singularly homo- 

 geneous, both mineralogically and texturally. The rock is a coarse 

 grained, light, pinkish-gray biotite granite of common macroscopic habit. 

 The essential minerals are microperthite, quartz, oligoclase, Ab 3 An 1? 

 orthoclase, and biotite ; the accessories, apatite and magnetite, with rather 

 rare titanite and zircon. The order of crystallization is normal for gran- 

 ites. Sometimes, and especially along contact walls, the rock is porphy- 

 ritic, with the microperthite developed in large idiomorphic and poiki- 

 litic phenocrysts, which, as described by Calkins, weather out with a reten- 

 tion of the crystal form. 



A determination of weight percentages by the Rosiwal method afforded 



the following result : 



Per cent 



Microperthite 40.3 



Quartz 35.7 



Oligoclase 11.0 



Orthoclase 7.0 



Biotite 5.0 



Magnetite and titanite .7 



Apatite .3 



100.0 



