D1STEICTS OF VEGETATION. 271 



montories, since the southern limit is nearly coincident 

 throughout with the arctic circle, on which it approaches 

 Beering's Straits — clumps of spruce fir*, the usual outliers 

 of the forest, having been observed on Buckland or Noatak 

 River which falls into Eschscholtz Bay. The fertile 

 alluvial deposits of the well-sheltered valley of the Mac- 

 kenzie interrupt the continental continuity of the Barren 

 Grounds by carrying the woods nearly to the sea shore ; 

 but there seems to be no other material indentation of the 

 barren district ; and even on the Mackenzie the valley is 

 bridged, as it were, by the naked summits of the alpine 

 ridges. 



3rd. The prairie slope forms a third physiognomical 

 district of vegetation, which has the greatest transverse 

 expansion on the Missouri, and, narrowing as it goes north, 

 runs out on the 60th parallel, having, after passing the 

 Saskatchewan, been much indented by the woods which 

 feather the numerous rivers that drain the declivity. 

 These prairies have much analogy with chalk downs in 

 aspect as well as in mineral constitution. 



4th. The Rocky Mountain chain, and the alpine ranges 

 and isolated peaks which rise to the westward of it, may be 

 considered as a fourth district which nourishes some pecu- 

 liar species of plants. 



5th. And the lower woodland country on the Pacific 

 side of the range forms a fifth. 



If we trace any one of these districts northwards, making 

 due allowance for the varying altitude of the country above 

 the sea, we may ascertain the effect of increase of latitude 

 on the vegetation of that meridian ; but, if we compare one 

 district with another, we must keep in view the climato- 

 logical fact of the rise of the isothermal lines in proceeding 

 westward. The course of the forest boundary is one illus- 

 * The species is doubtful. 



