42 



climato'logical reasons. 



The rapid erosion of the Colorado river and its two confluents 

 the Gi'and and Green have left precipitous walls about 2000 feet 

 high on the west and north and east, and on the south high dome like 

 uplifts and groat box canons. The flcor of the basin is from 4000 

 to 5000 feet elevation, the western crest 8000 feet, the north 12000 

 feet, the east 10000 f<-et, while the south exclusive of the canons Is 

 5000 to 6000 feet. The great barriers efftciLually cut off moisture 

 from almost any region and the rainfall is little over 4 inches per 

 annum, while the relative humidity is often 10 per cent. There are 

 a few summer showers and there is some snow in the winter or an 

 occasional shower. The annual temperature runs from about 49 de- 

 grees at the north to nearly 60 degrees at the south. There is rare- 

 ly a foot of snow at any one time at the north and none at any 

 time at the south. There are heavy spring frosts at the north, and 

 no permanent winter snow cover. The soil from the center north- 

 ward and westward is a loose and highly alkaline gray clay. Fn ith 

 of the center it is all a light and deep red sand, carrying much alknli. 

 The low rainfall and humidity makes it seem like an absolute desert. 

 It is the nearest to it of any region in Ithe west except the salt aren 

 forty miles wide west of Great Salt Lake. This region is traversed 

 by the Grand, Green, Duchesne, Fremont, San Rafael, Dolores rn'l 

 San Juan rivers, all of which canon more or less below the floor, 

 Wherever the waters can be taken out upon the land the soil be- 

 comes very fertile after about three years' leaching of the alkali. 

 The floor of the region is covered by the Lower Temi)erate flora; 

 the deep canons from the head of the Colorado down by the Troiii- 

 cal. The high walls have the Middle Temperate. The Astragaline 

 flora is unique. On the clay plains and particularly along little 

 draws A. asclepiadoides stands erect mostly singly or in twos fro'^i 

 a deep ta]) root, with its large shiny leaves, and having almost exactly 

 the habit of Asclepias crytoceras. but more erect, which grows in 

 the saino region. A. amiuillarius has much the same habit farther 

 south. The coarse and tufted A. Pattcrsoni grows much the same b't 

 prefers bottoms. A. sabulosus a close relative of Pattersoni grows 

 where alkali seeps out of stiff slopes. A. Haydenianus grows aro'nn 

 irrigation ditches and along trails as if an immigrant but appears 

 more at home on the edges of oak brush higher up. On gentle 

 slopes and where there is a little sand mixed in A. confertiflorus grows 

 In small tufts, and where there is more sand A. argillosus grows 

 along with it with much the same habit. Along the ridges where 

 there is more or less loose sandstone rock A. Preussii grows singly 

 or nearly so. On the ridges themselves the variety Eastwoodfe 

 grows with tufted habit and low. Tn the crevices of flat or gently 

 sloping sandstones masses A. desperatus is at home. . In the loose 

 sand along the bottoms and in gulches A. pubentissimus grows 

 singly and flat on the ground as a winter annual. Tn the canons 

 in sand A. Moencoppensis is local and rare. On open sand stretches 

 and in drifting sand A. pictus is scattered here and there. The 

 densely tufted A. Episcopus prefers sand with rock close below, and A. 

 Woodruffi prefers sand dunes, with its innumerable wiry stems and 

 masses of purple bloom. A. montanus sometimes grows from crev- 

 ices of hot rocks. A. Coltoni abounds on gravelly mesas among the 

 pinons and sometimes on the rocks. A. detritalis grows in gravelly 

 draws near Theodore. A. lutosus is found in tight crevices on bare 

 rocks near Dragon. A. junceus grov/s on gravelly mesas as does A. 

 Wingatensis. A. sesquiflorus grows in mats in the wide crevicp • 

 of hot sandstones at Kanab. Brandegei and straturensis grow :n 



