172 G. M. DAWSON STRUCTURE OF THE SELKIRK RANGE. 



River series of the Rocky Mouutains are paralleled by similar conglomerates 

 which abound in the upper series of the Selkirks. No unconformity has 

 been observed between the upper and the lower masses of strata in either 

 place. 



Though in the Selkirk section the lower of the two great series which have 

 been described resembles the Nisconlith of the interior plateau so closely as 

 to warrant extending the same name to it, the fact that the overlying mem- 

 ber of the section differs considerably from the Adams Lake series of the 

 interior plateau, while on the other side it probably represents not only the 

 whole Castle Mountain group but also the upper part of the Bow River 

 series of the Rocky Mountains, renders necessary the application to it of a 

 provisional distinctive name. It is therefore proposed to refer to this rock- 

 mass as the SeWirl: Series. 



General Relations of the Cambrian. — Regarded as a whole, we find reason 

 to believe that the Selkirk section exhibits a great Cambrian formation 

 which (by analogy with the Rocky Mountains) includes the lower part of 

 the Cambro-Siluriau and reaches down from it to and far beneath a horizon 

 at which the Olenellus or lower Cambrian fauna has been found, with an 

 aggregate thickness of about 40,000 feet. 



The comparatively pure limestones of which the Cambrian of the eastern 

 part of the Rocky Mountains is composed are replaced in the western part 

 of that range by rocks largely clastic in origin. This change in lithological 

 character appears to continue and to become still more marked and to be 

 accompanied by increasing thickness in the Selkirk range. Much of the 

 clastic material is silicious, and the introduction of an increased proportion 

 of such material may be explained by considering it as a result of approach 

 to the shore line of Archean rocks on the west. While the principal devel- 

 opment of contemporaneous volcanic products^ whether in the Paleozoic, 

 Mesozoic or Tertiary, is confined to a region west of the local Archean axis, 

 the writer is inclined to believe that a portion of the remarkable difference 

 found to occur in the western extension of the Cambrian may be due to the 

 inclusion in its rocks, on this side, of volcanic ash deposits or other fine- 

 grained volcanic materials, of which the composition was such as to favor 

 the subsequent production of sericitic or sericite-like schists. 



Speaking generally, the great Cambrian formation of the Rocky Mountain 

 and Selkirk ranges shows many points of resemblance to the Cambrian and 

 so-called "Algonkian " rocks of Utah and Nevada, the resemblance being 

 particularly close in some respects to the series shown in the well-known 

 Wasatch section, in which more or less distinctly micaceous schists are also 

 found. It is, further, not at all unlike the Cambrian of Wales, which, though 

 the organic remains are chiefly confined to some upper beds, has a thickness 

 of 25,000 feet and is believed to exceed this in Shropshire.* The provisional 



*Text Book of Geology; Geikie. 2ad edition, 1885, p. 651. 



