FORESTS AND TREES OF NORTH AMERICA. 273 



a very well marked, thougli irregular, boundary, and of trees only 

 two or three western species extend east of" it, and as many eastern 

 ones westward. The general elevation of the country on this line is 

 from 3,000 to 4,500 feet, and the sudden change from plains to moun- 

 tains causes a corresponding change in climate, productions, and general 

 features. 



The line 19 — 20 — 21 is a geographical boundary, which, from 

 the great elevation of the dividing ridge, and the different climates of 

 its opposite sides, causes a more marked limitation here than on most 

 of the other western mountains. 



The line 24 — 26 — 25, provisionally bounding the Arizonian re- 

 gion, is founded chiefly on the isothermal lines. The great summer 

 heat of 80° and upwards is connected with the growth of the Cactaceae 

 and other peculiar plants, and the mountains between the Colorado 

 and Gila also have many peculiar forms even on their cool, high 

 summits ; and 26 — 20, bounding the upper Colorado valley, is the 

 geographical boundary of the Wasatch region on the west. 



26 — 27 and 20 — 27 form the geographical boundaries of the Utah 

 basin; and the latter appears also to be near the summer isotherm of 

 75°. The whole of this region is more than 4,500 feet above the sea. 



20 — 31 — 27 forms the northern boundary of the great Columbian 

 plains or Shoshonee region, and is also a well marked limit of the 

 continuous forests northward of it. It coincides also pretty well with 

 the summer temperature of 70° and 35° for winter, or 45° annually. 

 The sudden change in the surface by forests causes these isothermals 

 to be more abrupt than in a country uniformly bare or wooded. This 

 is probably the effect of increased rains, which, of course, affect other 

 products also. For this reason it is believed that the summer temper- 

 ature of about 65° will be found to run along the line of 13 — 4 — 23, 

 instead of being parallel to 70° on 13 — 14 — 22. 



26 — 27 — 31 is the eastern limit of the continuous forests of the 

 Sierra Nevada and Cascade ranges, and a very good natural boundary. 

 27 — 28 is the approsimate limit between W and X, and though in 

 reality very irregular from the rough character of the country, its 

 general direction is sufficiently shown. It limits the range of many 

 trees and other products. 



A boundary probably exists between Upper and Lower California, 

 but cannot yet be determined naturally. 



The broken lines 31 — 29, 21 — 30, 23 — 2, are of course merely con- 

 jectural, but are made to correspond to the general features of the 

 country as well as the direction of the isothermal lines. On the 

 coast at 30° the annual temperature is as high as 43° 5', corresponding 

 to that of tlie coast of Maine ; while what little is known of the inte- 

 rior shows that tliough exceedingly rugged and often inaccessible the 

 Yukon region possesses a good climate and a great variety of natural 

 products, probably comparable with Sweden and Norway. Its winters 

 are milder near the coast than those of Maine, though fifteen degrees 

 farther north. 



These few notes, hastily thrown together,, will, it is hoped, make 

 the design intelligible, and lead others to investigate and increase our 

 very imperfect knowledge of animal and vegetable distribution. The 

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