BOTANICAL GEOGRAPHY. 465 



which forms dense thickets on the banks of the rivers, and 

 odoratum Torr., one of the Mimoseee, 20' in height. The northern forms 

 distributed thus far were (36 N. lat.) : Pinus monophylla, Purshia tridentata, 

 and Populus and Salix on the banks of the rivers. 



The snow-line of the Rocky Mountains was estimated on the Snow-Peak 

 (42 43 N. lat.) at 11,800' (i. e. 1800' above the measured point 10,000'), 

 This mountain, the summit of which, 13,570' high, Fremont ascended, belongs 

 to the accessory chain of the Wind River mountains, but is regarded as the 

 most elevated of the entire system. Above the Coniferous region, the alti- 

 tudinal limits of which were not determined there, it contains a copious 

 alpine vegetation, which, according to the examples brought forward, are 

 principally characterised by Hudsonian forms, just as those of the Alps are 

 by arctic forms. 



Fremont's observations upon the tree-limits of the continent of North 

 America are extremely remarkable ; they show that they are much higher than 

 in corresponding latitudes of Europe. Not only in the Californian Andes 

 were the pine forests found to extend above 8000', but on the east side of the 

 Rocky Mountains, in the district of the so-called Park, in the region of the 

 source of the southern bifurcation of the River Platte and the Arkansas 

 (39 20' N. lat.), Fremont found that even at an elevation of 10,430', " the 

 pine forest continued, and grass was good. We continued our road, occa- 

 sionally through open pines 3 with a very gradual ascent ; and having ascended 

 perhaps 800 feet, we reached the summit of the dividing ridge, which would 

 thus have an estimated height of 11,200' " (p. 314.) Hence the altitude of 

 the tree-limit of the Rocky Mountains in the latitude of Valencia may be 

 assumed as 11,000' ; the most elevated tree-limits of the south of Europe, 

 the isotherms of which are of such very different temperatures, scarcely ascend 

 beyond 7000'. If the influence of the highlands of North America is so 

 great in moderating the vertical diminution of the temperature of the summer, 

 we are justified in anticipating similar phenomena in central Asia. There is 

 especially, one observation which corresponds with this supposition, and it is 

 the only one with which I am acquainted, viz. that relating to the valley of 

 Spiti, in Lesser Thibet, where, according to Jacquemont, at the same altitude, 

 but a more southern latitude (32 N. lat.), dwarf trees alone occur. But it 

 is not only the heat which causes the dense tall forests to ascend to such 

 considerable elevations in North America ; the humidity of the air or of the 

 soil must also be taken into account. In the south of Europe, the tree-limit 

 does not ascend in proportion to the increase of heat, since it is frequently 

 situated at a greater altitude on the south side of the Alps than at any 

 more southern point of the continent. In Thibet, where the highlands even 

 ascend to the level of the tree -limit, the limitation of the growth of trees is 

 not caused by cold, but by dryness. Now, it is a circumstance common 

 to both the mountain-chains of North America, that under the latitudes 



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