THE WESTERN MOUNTAINS 



99 



Of the three main ranges, the Coast and the Cascade 

 chains may be grouped together as representing a moister 

 system ; while the Rocky Mountains, separated from 

 these by a series of broad plateaus, form a much drier 

 eistern wing. Only at one point in the Columbia- 

 Frazer district, thanks to the gap in the western barrier 



Fig. 26. The prairie passing into a brush of summer-green bushes 

 and small birches — Wheat zone, Western Canada. 



through which the river Frazer flows, does the inland 

 chain share some of the moisture and luxuriance of the 

 coast-ranges. 



The forests of Pacific North America are almost ex- 

 clusively composed of conifers. Why this should be so 

 is not satisfactorily explained yet. It is quite certain 

 that many of the varied climates and soils, as they 



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