ROCKY MOUNTAIN PLATEAU. 



361 



being elevated by the deposition of their moisture and 

 continually increasing density as they descend ; but the 

 capacity of air for moisture is well known to be de- 

 pendent upon its temperature within certain limits ; hence 

 the westerly Pacific winds become more warm and more 

 dry as they descend the Eastern Eocky Mountain slope, 

 until they meet the moist winds from the Gulf of Mexico, 

 passing up the Valley of the Mississippi, towards and 

 through the region of the great Canadian Lakes, and over 

 the low height of land separating the waters flowing into 

 Lake Winnipeg from the Mississippi Valley.* 



In the latitude of the Valley of the Saskatchewan, how- 

 ever, the moist south-west winds from the Pacific find a de- 

 pression in the Eocky Mountain range and a low con- 

 tracted plateau ; they lose less humidity than winds passing 

 over the higher and broader ranges to the south, and 

 meet with a prevailing north-easterly wind as they begin 

 to descend their eastern flanks ; their temperature is 

 consequently lessened instead of being elevated, and their 

 capacity for moisture diminished ; hence, precipitation in 

 the form of rain and hail takes place in descending the 

 slope towards Lake Winnipeg. 



There is no doubt that the south-west Pacific winds, 

 passing through the depression in the Eocky Mountains 

 near the 49th parallel, and over the narrow plateau on 

 which they rest, without losing the whole of their mois- 

 ture, give humidity to the large portion of Eupert's Land 

 they traverse. 



The great plateau on which the Eocky Mountain ranges 

 rest, has an average elevation of 4000 feet near the 32nd 

 parallel of latitude, the lowest pass in the most easterly 

 range being there 5717 feet above the ocean. Along the 



* Meteorology in its Connexion with Agriculture, by Professor Joseph 

 Henry. 



