1911.] NATURAL SCIENCES OF PHILADELPHIA. 25 



increasing slowly with the altitude up the mountains. The air is 

 naturally excessively dry, the moisture content being, according to 

 Gilbert, 1 but 45 per cent, of that necessary for saturation, as against 

 69 per cent, in the region between the Mississippi River and the 

 Appalachian Mountains, and the power of evaporation annually 80 

 inches, as against 22 inches over Lake Michigan. From the lower 

 ranges the snow that falls generally evaporates without melting or 

 melts without the formation of definite streams. The heavier snows 

 of the higher ranges feed scattered springs and the small streams 

 running down the canyons and out a varying distance into the valleys, 

 where, often after becoming heavily charged with alkali, they sink 

 into the parched soil and are lost. Many of the springs at the bases 

 of the ranges are brackish or salt and some are warm. 



The vegetation of this arid region, while generally scant, is more 

 abundant than most would expect; and there is no part even of the 

 valleys in the driest times wholly devoid of plants, excepting some of 

 the playas most heavily charged with alkali, and especially the Great 

 Salt Lake Desert. In these places scattered clumps of the several 

 " greasewoods " occur about the margins. The vegetation of the 

 valleys and slopes as well as of the hills and of much of the mountain 

 sides presents a monotonous uniformity of appearance due to an 

 immense profusion of individuals of but few species. Those most 

 constant and conspicuous are shrubby and suffrutescent plants which 

 occur almost to the exclusion of other forms. No trees are found 

 among them. Grasses grow in tufts, but these die out with the 

 advancing season everywhere excepting in favored recesses and parks 

 of the mountains. Turfing grasses, such as are so conspicuous in 

 parts of the plains region east of the Rockies, do not occur, excepting 

 certain salt forms almost worthless for pasturage and confined to the 

 alkaline meadow lands. As a protection against the intense dryness 

 of the region, the characteristic plants above mentioned have mostly 

 reduced leaves with tough cuticle and often a dense covering of hair. 

 The prevalent color of the vegetation is a wearisome gray or dull olive. 

 Only at long intervals is this monotony of color relieved by the bright 

 green of the richer vegetation of the oases about springs and along 

 streams. 



It is impossible for plants of the higher orders to thrive in the 

 strongly alkaline soil in the lower portions of the valleys. The plants 

 growing here belong for the most part especially to the Chenopodiacese, 



1 Lake Bonneville, pp. 6 and 7, 1890. 



