116 OUTLINE OF PLANT GEOGRAPHY 



Nowhere do the deciduous trees play an important role. Except 

 for thickets of aspens the deciduous trees are confined to the 

 canyons and river banks of the lower elevations. Birches, wil- 

 lows and cotton-woods may be found in such localities, and 

 several species of oaks occur in Colorado, but are never trees 

 of large size, and usually merely shrubs, forming part of the 

 "chaparral" or scrub on the dry hillside- 



North of the high mountains of Colorado is an elevated plateau 

 which is traversed by the Union Pacific railway in Wyoming. At 

 the continental divide, about 8,000 feet elevation, the country is 

 an undulating dry prairie or steppe with almost no trees, and 

 in general scanty vegetation. 



The Yellowstone Park is approached over much the same 

 type of country, and is also a relatively level area with no high 

 mountains in the immediate vicinity. The hills are covered 

 with a forest of lodge-pole pine, but the trees are small. In 

 the early summer there are a good many attractive flowers 

 growing in the woods and on the grassy slopes. One of the most 

 abundant is a large yellow adder-tongue (Erythrom'um grandi- 

 florum). Other characteristic flowers are some fine larkspurs 

 and fringed gentians, and a small orange fritillary {Fritillaria 

 pudica). 



The barren soil of the peculiar thermal formations is mostly 

 bare, but it is interesting to note that some species, usually found 

 on the seashore, or salt marshes (Salicoimia herbacca, Rumex mari- 

 time!), have established themselves on the hot alkaline geyser for- 

 mations. 



The main range of the Rockies, in northwest Montana and 

 Canada, is extremely rugged, broken up into separate mountain 

 masses with steep slopes and sharp peaks. Owing to the latitude 

 there is much more snow than in the southern Rockies, and glaciers 

 are common, especially in the Canadian mountains, and the snow 

 line is much lower. 



The main range traverses Glacier Park in northwest Montana, 

 and affords a most interesting study in plant distribution. 



The prairie flora to the east has already been referred to, and an 

 analysis of its constituents shows a mixture of eastern and western 

 species, the former rather predominating. 



The eastern slopes of the mountains are exposed to the extremely 



