32 NORTH AMERICAN FAUNA. [no. 27. 



tained by consulting this table and by assuming that the Liard 

 opened at Fort Liard a few days earlier, since the disruption of the 

 Liard ice is almost invariably the immediate cause of the opening 

 of the Mackenzie below their junction. 



An account of the progress of the seasons and attendant phe- 

 nomena at Fort Simpson, as observed from October, 1903, to June, 

 1904. appears in the discussion of the Mackenzie Valley. (See p. 37.) 



THE MACKENZIE RIVER VALLEY. 



The Mackenzie (taken in a restricted sense as comprehending only 

 that part of the river known under this name) has a course of over 

 900 miles from Great Slave Lake to the Arctic Sea. As its imme- 

 diate valley is more fully described elsewhere (see p. 100), the present 

 account may be confined to a few general statements and brief de- 

 scriptions of the country bordering it on either side. It averages 

 over a mile in width and is usually deep, with a current of from 2 

 to miles an hour. Its general course is to the northwest. It is bor- 

 dered mainly by sandy or gravelly beaches and occupies a narrow, 

 comparatively shallow valley, through which it flows in a succession 

 of gentle curves. Many low islands, usually well wooded, occur 

 throughout its course. Its rocks are chiefly Devonian. 



Issuing from Great Slave Lake, the Mackenzie first follows a 

 general westerly course for nearly 300 miles. The tributaries which 

 it receives in this stretch, with the exception of the Liard, already 

 described, are of minor importance. The Horn Mountains, a long 

 ridge less than a thousand feet in height, lie in an easterly and west- 

 erly direction at some distance north of the middle of the stretch. 

 To the southward of the river occur other lower ranges, the principal 

 one being Trout Mountain. These mountains are very imperfectly 

 known. A large part of the country bordering this part of the 

 .Mackenzie is of a swampy nature, and it is all well wooded. Nearly 

 all the species of trees of the great subarctic forest are represented. 

 (See p. 10.) 



A short distance north of latitude 62° the Mackenzie strikes a spur 

 of the Rocky Mountain system, the Nahanni Mountains, is deflected 

 toward the north, and for some distance flows close to their bases. 

 At the point where the Mackenzie first approaches them the nearest 

 peaks are from 2,000 to 2,500 feet in height and are sparsely wooded 

 to their summits. Farther back they rise much higher, and above 

 an altitude of 2,500 feet are treeless. In early summer these moun- 

 tains are capped with snow, but this disappears entirely beneath the 

 almost continuous sunlight of midsummer. The North Nahanni 

 River, occupying a deep, narrow valley, issues from the mountains at 

 this point, and joins the Mackenzie by several shallow mouths. 

 (See PI. XII, lig. 2.) Its course has never been explored. 



