304 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



This granite also has thermally metamorphosed its country rock, in this 

 case the Pend D'Oreille schists. The metamorphic aureole is nowhere as wide 

 as those about the summit stocks or the Bayonne batholith; it is thus probable 

 that the contact surface of this stock dips under the invaded rocks at higher 

 angles than those characteristic of the contacts in the eastern bodies. The 

 Bunker Hill aureole has not been systematically studied with the microscope. 

 Thin sections of two specimens collected, one at the southwestern contact, the 

 other at a point about 1,000 feet from the contact, both showed the abundant 

 generation of andalusite prisms in the characteristic micaceous hornfels. At 

 Bunker Hill mine the andalusite schist is enormously crumpled and is cut by 

 veins of gold-bearing quaitz. On one of the veins the mine shaft has been 

 sunk for free-milling ore. 



Salmon Eiver JVIoxzonite. 



Halfway between Sheep creek and Lost creek, and a mile east of the Salmon 

 river, the Pend D'Oreille schist is cut by a small stock of plntonic rock, which, 

 in chemical and mineralogical composition, is unique among the known intru- 

 sives of the Selkirks within the ten-mile belt. The stock has the subcircular 

 ground plan of a typical granitic boss, measuring 700 yards in diameter. The 

 rock is relatively prone to disintegration and it has weathered freely into huge 

 houldery masses, whose forms have been produced by exfoliation iand con- 

 centric weathering on joint blocks. By the energetic intrusion the schists round 

 about have been crumpled, hardened, .and converted into hornfelsy, massive 

 rock. This contact aureole is a few hundred feet in width; it has not been 

 studied microscopically. 



The igneous rock is dark greenish-gray and rather coarse-grained. It is 

 massive and quite uncrushed. Wdth the unaided eye, augite, biotite, and feld- 

 spar can be readily identified as the essential constituents. The first named 

 mineral forms highly idiomorphic, stout prisms of varying lengths up to that 

 of 7 mm. or '8 mm. The biotite occurs in lustrous black, often idiomorphic 

 foils which may be 2 mm. or more in diameter but average about 0-6 mm. 

 Between these femic essentials the feldspar forms a kind of mesostasis, numer- 

 ous individuals ajpproaching 5 mm. in diameter. Many of the larger crystals 

 schillerize in vivid sky-blue colours which are specially brilliant when the rock 

 is wetted. 



Under the microscope the augite shows the cleavages, the very pale green 

 almost colourless tint, double refraction, and extinction angles of a diopside. 

 One crystal in a thin section showed a narrow interrupted mantle of green 

 hornblende about the pyroxene. The biotite is sensibly uniaxial and has power- 

 ful absorption. The feldspar belongs to the alkaline and soda-lime groups, 

 which are represented in nearly equal proportions. The larger, schillerizing 

 individuals have the optical properties of soda-orthoclase and microperthite. 

 The same crystal often has the homogeneous structure of soda-orthoclase in 

 one part and the familiar microperthitic intergrowth irregularly developed 



