850 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 

 Conglomerate Formations. 



Because of the extremely rare occurrence of water-laid deposits in the- 

 Rossland mountains, the discovery of fossiliferous horizon-markers is, through- 

 out the mountain group, a field problem of special difficulty. For that reason- 

 certain small patches of conglomerate with sandy and shaly interbeds, which 

 may yield useful fossils, have the particular interest of the geologist who 

 attempts to understand the structural tangle of this region. Four small and 

 quite detached areas of the conglomerate appear within the Boundary belt; 

 these will be described in order from east to west. 



Conglomerate at Lalce Mountain.. — One mile southwest of Lake Mountain- 

 summit, a patch of the conglomerate covering about a third of a square mile 

 has been mapped by McConnell and re-traversed by the writer. The rock is there 

 chiefly a coarse, massive conglomerate, dipping at an average angle of 20° to 

 the northeast and showing an apparent thickness of about 800 feet. The mass 

 is truncated by an erosion-surface, so that 300 feet is a minimum thickness at 

 the locality. At no point was the conglomerate found in actual contact with 

 the Rossland volcanics which surround it. There are two possibilities as to the 

 relation between the two formations; the conglomerate may overlie the vol- 

 canics, as postulated by McConnell, or, secondly, it may represent a pre-volcanic 

 conglomerate forming a knob which was first buried under the lavas and since 

 uncovered by their denudation. The choice between these alternatives is 

 not ensured by any known fact. The comparatively low dips suggest that the 

 first view is the correct one. At the same time, there are no lava-fragments 

 among the pebbles of the conglomerate, which are composed of gray and 

 greenish-gray quartzite, silicious grit, vein quartz, phyllite, and slate. A few 

 badly altered pebbles of a rock like granite are also present. 



Practically all of the material observed in the pebbles could have been 

 derived from the Paleozoic and pre-Cambrian terranes now exposed in the 

 Selkirk range, twenty-five miles to the eastward; in the absence of any other 

 known source, that place of origin appears probable. The pebbles are of all 

 sizes, up to the diameter of one foot. They are of rounded, subangular, and 

 angular shapes. In places the deposit approximates a true breccia in appear- 

 ance. The imperfect rounding, and, in addition, the generally tumultuous 

 aggregation of the pebbles suggest rapid deposition, as if by a rapid mountain 

 stream. Small irregular lenses of quartz-sandstone and grit form the only 

 breaks in the pebbly mass. Similar arenaceous material composes the cement 

 of the conglomerate, which is also quite highly ferruginous. One dike of basic 

 andesite or latite (character not determined) and a large (mapped) apophysis 

 of the Sheppard granite cut the conglomerate. 



Conglomerate at Sophie Mountain. — A second body of coarse conglomerate, 

 covering a square mile or more, crowns the summit of Sophie mountain at the 

 International line. In structure, size of pebbles, and composition this rock 

 resembles the conglomerate at Lake mountain very closely, but here there are 



