150 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 75 



we find evidence of the former existence of several troughs in which 

 distinct series of sediments were deposited with the marine Hfe of 

 their time embedded in them. 



There is also evidence of longitudinal undulations of the bottom 

 of the troughs that influenced the de]:»th and character of the sedi- 

 ments to such an extent as to now cause a most disconcerting dis- 

 appearance and reappearance of formations with their characteristic 

 faunas. 



It is clear that at the beginning of Proterozoic time the Cordilleran 

 Geosyncline was broad, and relatively shallow, with extended land 

 areas on both its eastern and western sides. This is indicated by the 

 immense amount of fine sand and silts that were brought into the 

 trough and distributed by gentle currents and wave action. There are 

 neither great conglomerates indicating lands of high relief, nor coarse, 

 cross-bedded sandstones suggesting a sea with strong tidal currents 

 and waves attacking the coast line. The Beltian limestones, of which 

 great thicknesses occur in Montana ' and possibly in Alberta, are rel- 

 atively shallow water deposits similar to those of the great inland 

 lakes of Tertiary time, east- of the Rocky Mountains. 



Many thousand feet of arenaceous, calcareous and siliceous de]:)Osits 

 accumulated as the geosyncline gradually deepened because of the 

 downward pressure of the load of sediments it was receiving. There 

 may have been longitudinal troughs within the geosyncline as in 

 Paleozoic time, but at present the study of the areal geology is not 

 sufficiently advanced to outline more than one of them. 



PROTEROZOIC DEPOSITS IN THE CORDILLERAN GEOSYNCLINE 

 At the close of Proterozoic time the greatest thickness of deposits 

 was in the area that is now the Purcell and Selkirk Mountains 

 of British Columbia. Daly gives the following sections of the 

 formations : ' 



Feet Meters 



Ross quartzite (in part) 2,500 762 



Nakimu limestone 350 -|- 107 + 



Cougar formation 10,800 + 3,292 + 



Laurie formation 15,000 + 4,572 -|- 



Illecillewaet quartzite 1,500 -|- 457 -f- 



Moose metargillite 2,150 655 



Limestone (marble) 170 52 



Basal quartzite 280 + 85 -(- 



32,750 9,982 



' Bull. Geol. Soc. America, Vol. 17, 1906, p. 7. Bull. LI. S. Geol. Surv., 

 No. 384, 1909, p. 41. 



''Geol. Surv. Canada, Transcontinental Excursion No. C i. Guide Book 

 No. 8, pt. II, 1913, p. 183. 



