19 



Mnp lo illustrate the paper on 



the Nbrnencld-ture pf a part of Uie 



NORTH AMERICAN CORDILLERA 



8> REGINALD A. DALY. 



GEOLOGICAL HISTORY. 



The geological history of the North American Cordillera 

 can be clearly expressed with reference to two large 

 geosynclinals ; an eastern or Rocky Mountain geosyn- 

 clinal and a western or Pacific geosynclinal. It can also be 

 shown that previous to the Mesozoic these two geosyn- 

 clinals, as regards their relative periods of deposition and 

 erosion, bore reciprocal relations to each other. 



The Rocky Mountain geosynclinal lies between the 

 Great Plains and the Purcell trench. It embraces sedi- 

 ments from the base of the Belt (pre-Olenellus) terrane 

 up to and including the Mississippian and is composed of a 

 single group of comformable strata varying in composition 

 and texture according to relative proximity to the ancient 

 shore lines which border such basins of sedimentation. 

 The four type sections which illustrate this principle from 

 east to west are the Lewis Galton, Purcell and Summit 

 series which have an average thickness of about 20,000 

 feet. 



36425— 2 1 



