334 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



possible occurrence of acid lava observed by the writer was noted as a 30-foot 

 intercalation in the Cretaceous (?) argillite at the crossing of the Boundary 

 line and Little Sheep creek. This is a white, aphanitic, massive rock showing 

 a fairly distinct banding which in the field was taken for bedding. The rock 

 weathers yellow to brown. Under the microscope the one thin section made 

 from the rock showed no certain proof of the origin of the rock but the general 

 appearance was that of a devitrified, partially spherulitic obsidian. The very 

 small spherules seem to be poorly developed radial aggregates of quartz and 

 feldspar and their matrix is a very fine-grained granophyric intergrowth of the 

 same minerals. No other minerals have been certainly determined in the thin 

 section. The banding may be a flow-structure. 



Since its characters are obscure and largely negative the writer must 

 regard his reference of this rock to the acid obsidians as tentative. 



Tuffs and Agglomerates. — Most of the area occupied in the Boundary belt 

 by the Rossland volcanic group is underlain by massive flows of the latites, 

 andesites, and basalts. A considerable tract, estimated as covering at least 

 fifteen square miles, is, however, underlain by a thick, more or less continuous 

 mass of coarse volcanic agglomerate. This pyroclastic composes the majority 

 of the outcrops between Lake mountain on the east and the top of Sophie 

 mountain on the west, besides extending for several miles along Record moun- 

 tain ridge, northward, from the Boundary line. 



The constituent fragments are angular to subangular, ranging in size 

 from dust-particles to blocks four feet in diameter. The deposit is usually 

 without stratification but consists of a tumultuous, massive aggregation of 

 fragments which were evidently never sorted by water-action. Most of them 

 are composed of augite latite, biotite-augite latite, or, to a less extent, of basalt 

 and augite andesite. Besides these, abundant angular blocks of fossiliferous 

 white crystalline limestone occur in the breccia throughout the whole eastern 

 slope of Sophie mountain, and are likewise conspicuous in the breccia on the 

 eastern side of Little Sheep creek valley. 



A basic agglomerate macroscopically similar to the Sophie mountain type 

 but lacking the limestone blocks, crops out three miles to the westward, between 

 the Boundary line and Santa Rosa creek. As already noted, microscopic 

 examination of the fragments showed them to be chiefly augite andesite, with 

 a notable propoi-tion of blocks of dark coloured biotite-quartz porphyry. 



These breccias bear numerous intercalations of the massive lava flows, 

 thin basic ash-beds and a few, thin beds of black, carbonaceous shale. In a 

 few localities the dip could be taken; in general it is high and ranges from 

 70° to 90°, showing that the whole group has been heavily mountain-built. 



Dunites Cutting the Rossland Volcanics. 



At various points the andesites encircling the Coryell batholith within 

 the ten-mile belt are cut by dikes and irregular masses of dunite, now partly 

 serpentinized. The largest body occurs on Record mountain ridge, one mile 



