NORTH AMERICA 



79 



limits of the cold deserts and of the northern coniferous 

 belt. 



Tundra Region. The polar margin of the North 

 American vegetation is characterized as in other con- 

 tinents by the treeless tundra. From west to east of 

 the continent the isotherms are driven gradually further 

 south, and the northern belts of vegetation are correspond- 

 ingly lowered in latitude on the Atlantic side. Thus 

 the northern limit of trees which, on the Mackenzie, 



Fig. 21. Mean Temperature of 

 North America in January reduced 

 to sea-level 



Fig 22. Mean Temperature of 

 North America in July reduced 

 to sea-level. 



runs near the Arctic coast, not far from 70° N., is driven 

 south to about 55° N. in Labrador. This line marks 

 also the southern limit of what is really treeless tundra. 

 In Alaska the tundra occupies a narrow strip of coast 

 along the Bering Sea : on the northern coast it covers 

 the polar slopes of the northernmost branch of the Rocky 

 Mountains: from the Mackenzie River eastward, its 

 southern limit strikes inland across the lake region to 

 reach Hudson Bay at Fort Churchill. It extends, as 



