REPORT OF THE CHIEF ASTRONOMER 303 



SESSIONAL PAPER No. 25a 



of the same aplitic phase. At certain points numerous blocks and shreddy frag- 

 ments of quartzite and schist were observed in the granite. These xenoliths, 

 especially the schists, have undergone much metamorphism, with the generation 

 of abundant andalusite in stout prisms, broad leaves of muscovite, and biotite in 

 aggregates which mottle the rock in striking fashion. Again large amounts of 

 micropertbite are disseminated through the altered schist, as if introduced from 

 the magma. 



The northern arm of the body has the form of a huge irregular dike or sill 

 which follows the strike of the invaded schists. The exposures do not favour 

 the decision as to whether or not the mass here follows planes of bedding or 

 schistosity. The other and larger arm of the mass is clearly in cross-cutting 

 relations. The width of this band is doubtless the greater because of the excava- 

 tion of the deep canyon of Lost creek. If erosion should remove a few thousand 

 feet more of the sedimentary cover at the head of the creek, the Lost creek body 

 and the summit stocks would doubtless be found to form one continuous 

 batholithic mass. 



A small intrusion of the Lost creek granite occurs-on the divide between 

 Sheep and Lost creeks and 1-5 miles east of Salmon river. It cuts schists and 

 limestone probably of Carboniferous age, and the youngest bedrock forma- 

 tions with which this whole group of granites, including the Bayonne batholith 

 and its satellites, is known to make contact. The date of these intrusions will 

 be further discussed in a following summary. 



BUNKER HILL STOCK. 



Within the ten-mile belt an igneous body which appears to be the most wes- 

 terly satellite of the Bayonne batholith is a stock covering about eighteen square 

 miles and lying almost wholly on the western side of the Salmon river. This 

 stock is composed of a medium to rather coarse, alkaline biotite-granite (specific 

 gravity, 2-610) which, in all essential respects, is identical with the granite 

 forming the small summit stocks and the Lost Creek body. The Bunker Hill 

 mine (now shut down) is situated in the metamorphic aureole of this stock and 

 it may, for convenience be referred to as the Bunker Hill granite stock. 



Being generally more weathered, this granite has a more reddish tint than 

 the Lost creek and Summit granites. The stronger weathering effect may be 

 partly due to the fact that the Bunker Hill granite has been much more strained 

 and crushed than the more easterly bodies. A distinct schistosity has been 

 thus produced at many points in the stock. The gneissic structure is most 

 pronounced near the southeastern contact, at the confluence of Lost creek and 

 Salmon river. For a distance of 500 feet or more from the contact the granite 

 is specially basic and consists essentially of quartz, biotite, plagioclase 

 (labradorite Ab x An^ to basic oligoclase, Ab 2 AnJ, with very subordinate ortho- 

 clase, and abundant muscovite foils. The plates of the white mica lie in the 

 planes of schistosity and are of metamorphic origin. This basic phase recalls 

 the muscovite-bearing quartz diorite which forms the many apophyses in the 

 shatter-zone about the summit stocks. 



