Canadian Rocky Mountains. 297 



explains its dry climate. Meeting the western slopes 

 of the Eocky Mountains, the air is still sufficiently moist to 

 afford the relatively abundant precipitation of that region ; 

 but on passing still further eastward, across the summit 

 elevations, the conditions are unfavourable to further rain- 

 fall. Superimposed, however, on these main features, is a 

 tendency to greater rainfall toward the north, which is 

 specially noticeable — whether from a lessened elevation in 

 the mountain barriers to the west, or other causes — in com- 

 paring the conditions in different parts of the Columbia- 

 Kootanie Yalley. The total amount of precipitation is 

 evidently least in that part of this valley near the forty- 

 ninth parallel, which is known as the Tobacco Plains. 

 Much of the surface is there open, covered with bunch- 

 grass and dotted with open groves of yellow pine (Pinus 

 ponderosa), interspersed with the western larch (JLarix occi- 

 dentalism and Douglas fir (Pseudotsuga Douglasii), while the 

 herbaceous plants are of a drought-loving character. North- 

 ward in the valley these gradually disappear, the yellow 

 pine and western larch cease abruptly at the head of the 

 Upper Columbia Lake, and the black pine (Pinus Mur- 

 rayana) and Engelmann's Spruce (Picea Engelmanni) form 

 the chief part of the forest, which becomes relatively dense. 

 Such small efforts at cultivation as have been made, prove 

 that irrigation is necessary for the successful growth of 

 crops in all the southern part of this valley. 



In the lower parts of the eastern foot-hills and the 

 larger valleys in the eastern part of the range, the dry 

 conditions of the Columbia-Kootanie Yalley are again to 

 some extent repeated ; and even within the range, rather 

 extensive patches of dry prairie and slopes clothed with 

 bunch-grass are found in the mouths of the depressions 

 leading to the passes. The open, prairie character of the 

 southern foot-hills has already been alluded to. 



Neither the western larch nor the yellow pine recur on 

 the eastern slopes of the mountains, and the Douglas fir, 

 though abundant in the foot-hills, does not extend within 

 the mountains beyond the larger valleys. 



The tree most characteristic of the valleys of the western 

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