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, 



JANUARY 



JULY 



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 



