220 CLIMATOLOGY. 



months being comparatively milder, and the three summer 

 ones less warm. 



Table I. shows that as we advance into the interior, 

 the heat of the summer is kept up, or even augments, 

 notwithstanding the elevation above the sea attained by 

 ascending the successive stages of the St. Lawrence basin. 



The nature of the rock formations has a considerable 

 influence on the climate of a district. In the primitive 

 country, such as has been described in a preceding page 

 as abounding in lakes and swamps, the climate is extreme, 

 the winters being not only longer, but also more severe, 

 the dissolution of the ice in such districts absorbing much 

 heat. A marked difference occurs when we pass from the 

 " intermediate primitive range " to the prairie districts 

 lying to the westward ; for, notwithstanding the greater 

 elevation of the latter, the winters are milder, the snow 

 less deep and less durable, the rivers break up earlier, 

 and the sap flows sooner in the trees. A corresponding 

 difference in the vegetation occurs ; the prairie plants have 

 much less of an arctic aspect than those of the primitive 

 districts. Professor Agassiz, in his work on Lake Supe- 

 rior, has instituted a very interesting comparison between 

 the vegetation of that basin and the lower and middle sub- 

 alpine zones of the higher tracts of the Jura, proving 

 their very great similarity. With the prairie districts, the 

 analogy of the Jura is very much less strong. Many of 

 the plants which give the peculiar character to the prairies 

 south of the Missouri, range northwards to the branches 

 of the Peace River, a main affluent of the Mackenzie; and 

 several prairie plants enter the silurian w r ooded tracts 

 which lie to the westward of the " intermediate primitive 

 range," though they have not been discovered in the more 

 eastern parts of the continent. 



