NO. 4 PRE-DEVONIAN SEDIMENTATION I49 



tinues northwest to the Blaeberry River. Southeast of Field it passes 

 eastward to the boundary line represented on the map. 



CORDILLERAN GEOSYNCLINE 



The Cordilleran Geosyncline has been characterized by Schuchert 

 as the oldest, longest, and widest continuous seaway known. During 

 the Palaeozoic it extended from the Arctic Ocean southward through 

 what is now the mountainous region of western North America into 

 northwestern Mexico, a distance of 3,000 miles (4,827 km.). In 

 Canada the width of this seaway was in most places a few hundred 

 miles, while in the United States it was many hundreds of miles wide. 

 The eastern shore of this vast geosyncline (seaway) and its marine 

 extensions was the Canadian shield and its southern prolongation, 

 Siouia. On the west it was bounded by Cascadia.' 



Tn this paper we are chiefly concerned with the portion of the 

 geosyncline that is now embraced in the area between the 49th and 

 54th parallels, a distance of about 350 miles (563.2 km.). The width 

 of the geosyncline in pre-Cambrian time is unknown, but judging 

 from the presence of Proterozoic deposit^ of Beltian time far to the 

 west of the Cambrian outcrops, it was probably 300 or 400 miles 

 (482.7 or 643.6 km.). In early Paleozoic time it may have been in 

 places 200 miles (321.8 km.) or more, but as yet we do not know 

 conclusively what pre-Devonian formations are present in the area 

 west of the " Rocky Mountain Trench " ; of the later Paleozoic, 

 limestones of Devonian and Carboniferous age have been recognized 

 which were presumably deposited in bays and along the shores of 

 the old Selkirkian land area. On a line (from / through M on map, 

 pi. 25) extending from the Rocky Mountain front at Devils Gap, 

 northeast of Banff, Alberta, west-southwest through Banff and over 

 the Continental Divide to the pre-Cambrian Proterozoic terraine on 

 the west side of the Columbia River valley, the geosyncline may have 

 been 200 to 250 miles (321.8 to 402.3 km.) broad; at present, after 

 narrowing of the area by compression, folding, and thrusting of the 

 pre-Devonian Paleozoic formations, the width occupied by them is 

 about 160 miles (257.4 km.). On a parallel line that crosses the strike 

 of the Continental Divide and the Cordilleran Trough 100 miles 

 (160.9 km-) north, the area over which the pre-Devonian formations 

 now occur is about 180 miles (289.6 km.) in width. 



To what extent during pre-Devonian time the bottom of the Cor- 

 dilleran Geosyncline was wrinkled and thus made into a complex 

 geosyncline with minor troughs has not been fully determined, but 



* Bull. Geol. Soc. America, Vol. 34, 1923, p. 184. 



