sjlbkbt.] PHYSICAL GEOGRAPHY OF THE REGION. 267 



In western Utah the winter is 80 F. ( U° C.) cooler than the summer: 

 in Yellowstone Park, 50 P. (27 0.); at Leadville, 43 c P. (34° O.). 



The principal zones of vegetation are known as the desert, the piuon, 

 the pine, the balsam fir, the spruce, and the subalpine. s In the desert 

 zone there are QO trees, except that the cottonwood occasionally fol- 

 lows tin 1 lines of streams. Hushes, of which the sa,u'e is the most 

 important, have an open growth, usually offering no impediment to the 

 progress of pedestrian or horseman. With these aregrasses, invaria- 

 bly growing in discrete hunches so as not to constitute a torf. The 



prevailing color of earth, grass, and bushes is ashen. Near springs 



and perennial streams grow other grasses and bushes with more cblo- 

 rophyl, so that bright green is to the desert wanderer the welcome sign 

 of water. 



The pinon zone is characterized by two species of evergreens, the 

 nut pine (Pinus cihilis) and the ••cedar" (Juniperus occidental™ mono- 

 spn-nxi). The pine is ordinarily from 20 to 30 feet (6 to'.) in.) high, and 

 the cedar from L5 to 25 feel (1 to 7 in.). 



The pine /one is characterized by tin' yellow pine (Pinu8 ponderosa)) 



a beautiful tree. 70 to 100 feet (20 to 30 in.) high, the groves of which 

 stand in open order, wit hout underbrush and without low branches. 

 This tree is rarely associated with others, but w aterwavs t raversin^ its 



zone sometimes nourish a few individuals of various deciduous species, 



including the maple, the ash. and the box elder. 



The characteristic tree of the balsam fir zone is the Douglas fir 

 (Pseudotsuga douglasii). With it are associated the Rocky .Mountain 



pine (Pinus flexilis macrocarpa) and the aspen (Populus tremuloides). 



The Douglas fir is a commanding tree, the rival of the yellow pine. 

 The spruce zone is characterized by the Engelmann's spruce (Pitea 



engelmanni) and the foxtail pine {Pinus aristata). The spruce, which 

 ordinarily predominates, is a beautiful tapering cone, its lower branches 

 resting upon the ground. 



In the subalpine zone Engelmann's spruce and the foxtail pine 

 become gnarled and procumbent. 



In the western region the precipitation of the winter is ordinarily 



greater than that of the summer, and on the mountains and uplands it 

 takes the form of snow. In the Spring and early summer this snow is 

 melted and streams are nourished which tlow to the lowlands, where 



the temperature is favorable to agriculture. Thus, despite the aridity 



of the lowlands, man is enabled to cultivate a limited portion of the 

 laud. The land thus cultivated yields a much greater return than can 

 be obtained in the humid districts farther east. The great power of 



solar rays transmitted through a clear at mosphere of low humidity, 

 combined with the rapid evaporation of moisture from the leaves of 

 plants, gives a wonderful stimulus to vegetation, and, where the water 

 for irrigation is abundant and skilfully applied, the yield is large. 



