326 THE CAMBRIAN. Ibull.81. 



in the Black Hills. The Potsdam sandstone in this region is quite well 

 developed, attaining a thickness of 200 feet, and exhibiting, as usual, 

 various lithologic characters. Near the base, the rock is of a reddish 

 flesh color, very compact, composed of an aggregation of quartz peb- 

 bles, varying in size from a minute grain of quartz to masses half an 

 inch in diameter, cemented by siliceous matter. Above, the rock is in 

 thin ferruginous layers, slightly calcareous but mostly siliceous, and 

 with many small particles of mica. These thin layers are also charged 

 with fossils, such as Lingula antiqua, Obolella nana, Theea gregaria, and 

 Arionellus ? oweni. Many of the slabs are covered with fucoidal mark- 

 ings and what appear to be tracks and trails of worms. 1 



CANADIAN EXTENSION. 



Our knowledge of the Cambrian rocks of this region is mainly limited 

 to the vicinity of the line of exploration across the Rocky Mountains in 

 the vicinity of the Canadian Pacific Railway. They occur in western 

 Alberta and eastern British Columbia, as is shown in the plates and 

 sections accompanying Mr. R. G. McConnelPs paper. 2 The geographic 

 distribution and the localities mentioned by Mr. McConnell are repre- 

 sented on the map accompanying the report of the Geological Survey 

 of Canada for 1882. 



The Cambrian section, identified by fossils, includes the upper por- 

 tion of the Bow River group and the lower part of the Castle Mountain 

 group. The Bow River group forms the basal member of the section in 

 the Bow River Valley and Cathedral Mountains. It consists mainly of 

 a series of dark colored argillites, associated with some sandstones, 

 quartzites, and conglomerates. The argillites are usually dark grayish 

 in color, but become greenish and purplish in places ; are very impure, 

 and frequently grade into flaggy sandstones that are often slightly cal- 

 careous. The conglomerates of this series are characterized by pebbles 

 of milky or semitransparent quartz, and by pieces, similar in size and 

 fresh looking, of whitish feldspar, and the matrix contains an abundance 

 of pale mica. These constituents have evidently been derived from 

 some not far distant exposures of coarse granite or gneissic rock. The 

 conglomerates characterize more especially the top of the formation, 

 and occur in thick, massive looking bands, alternating with quartzites 

 and shales. The quartzites, like the conglomerates, are mostly found 

 in the upper part of the formation, and sometimes, as in Cathedral 

 Mountains, replace the latter altogether. The only fossils detected in 

 the Bow River series are specimens of Olenellus gilberti, found about 

 2,000 feet below the top of the formation and at the summit of Vermilion 

 Pass. 



4 Tho Primordial Sandstone of the Rocky Mountains in the Northwestern Territories of the United 

 States. Am. Jou '. Sci., 2d ser., vol. 33, 1862, pp. 71, 72. 



6 Ropovt on the Geological Structure of a portion of the Itocky Mountains, with a section. Geol. 

 Surv. Canada, new ser., vol. 2, 1886-'87, pp. 24D-30D. 



