Ill 



INTRODUCTION TO THE GEOLOGY OF THE 

 CORDILLERA. 



BY 



Reginald A. Daly. 



GENERAL TOPOGRAPHY. 



The North American Cordillera, extending from Bering 

 Sea to the intersection with the Antillean mountain system, 

 has a length of 7,000 kilometres (4,350 miles), an average 

 breadth of about 900 kilometres (560 miles), and an area 

 more than two-thirds that of all Canada and nearly two- 

 thirds that of Europe. This gigantic feature of the earth is 

 a tectonic unit, originating in stresses specially exerted 

 from the Pacific basin. The Cordillera as a whole has, 

 therefore, been fitly called the Pacific Mountain system 

 of North America. 



The members of Excursion Ci. will cross the system 

 where it is comparatively narrow; nevertheless, a straight- 

 line measurement of its width is here about 700 kilo- 

 metres (435 miles). Along the somewhat tortuous route 

 of the Canadian Pacific railway, the distance from the 

 eastern foot of the mountains to the city of Victoria is 1 ,050 

 kilometres (650 miles). For purposes of geological 

 description and of orientation in the field, it is necessary to 

 review the general subdivision of the Pacific Mountain 

 system at the railway section. 



Among the conceivable criteria for subdivision, the 

 purely topographic principle used by G. M. Dawson seems 

 to be the only practical one. In the first place we may 

 distinguish a belt characterized by plateau forms and 

 thereby contrasted with the rest of the Cordillera in the 

 Dominion of Canada. This may be called the Belt of 

 Interior Plateaus. It lies on the eastern side of the Coast 

 range, which is of alpine habit. Elsewhere the subdivision 

 of the mountain chain follows the lines of the master valleys. 



The greatest of the intermont depressions is that 

 extending from Flathead lake in Montana to the Yukon 

 boundary, a distance of 1 ,600 kilometres (990 miles) . It is a 

 relatively narrow but actually imposing trough, successively 

 drained by head-waters of most of the great rivers of the 



