348 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



The mode (Rosiwal method) is approximately: — 



Quartz 25-9 



Andesine 28-1 



Orthoclase and microcline 19-2 



Biotite 134 



Hornblende 12-3 



Magnetite -6 



Apatite -3 



Titanite -2 



Zircon trace 



100-0 



In the Norm classification the rock enters the sodipotasisic subrang, 

 harzose, of the alkalicalcic rang, tonalase, in the dosalane order, austrare; 

 although the ratio of potash molecules to soda molecules is very close to the 

 limit separating harzose from tonalose. According to the older classification 

 the rock is a basic granodiorite. 



The average specific gravity of four fresh specimens of the rock is 2-749. 



The dominant granodioritic type often passes gradually into a more acid 

 biotite granite or hornblende granite in which the feldspar is chiefly orthoclase. 

 sometimes microperthitic. These types sometimes form streaks in the main 

 body but are chiefly developed in the numerous apophyses. They furnish a 

 transition to the very abundant aplitic dikes, also apophysal from the batholith. 

 Many of the larger apophyses illustrate the differentiation of two quite different 

 rocks in the same fissure. The middle part of each of these dikes is composed 

 of granodiorite or hornblende-biotite granite, while along each wall, a zone of 

 splite, gradually passing into the more basic rock of the middle zone, is devel- 

 oped. The feldspars of the aplite are orthoclase and microperthite, with acces- 

 sory oligoclase-albite. Quartz and a little biotite are the remaining essentials. 

 The aplite zones make up one-quarter to one-half of these apophyses. Other 

 dikes are composed entirely of the aplite. Its composition and specific gravity 

 (2-592 for one fresh specimen) closely resemble those of the younger Sheppard 

 granite (spec. grav. 2-600 — 2-617). 



Differentiation in Place. — At the batholithic contact where it crosses the 

 railway branch between Trail and Rossland, there is a large body of relatively 

 acid granite which is regarded as genetically connected with the granodiorite. 

 The body measures about 400 yards in width. The contacts are hidden and it 

 is uncertain whether this more acid rock represents a contact-phase of the 

 batholith or a slightly later intrusion. The structure of the smaller body is, 

 so far as known, throughout porphyritic and the^balance of probability is in 

 favour of its being a late differentiate of the batholith and intruded into the 

 zone of contact of the granodiorite and the volcanics. The microscope shows 

 that the porphyritic rock is an alkaline biotite-hornblende granite. The pheno- 

 crysts are orthoclase (sometimes microperthitic), oligoclase, biotite, and horn- 

 blende. The ground-mass is typical gi'anophyre. A little magnetite and 

 apatite are accessory. 



