42 
climatological reasons. 
The rapid erosion of the Colorado river and its two confluents 
the Grand 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 great box canons, The floor of the basin is from 4000 
to 5000 feet elevation, the western crest 8000 feet, the north 12000 
feet, the east 10000 feet, while the south exclusive of the canons is 
5000 to 6000 feet. The great barriers effectually 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. South 
of the center it is all a light and deep red sand, carrying much alkali. 
The low rainfall and humidity makes it seem like an absolute desert. 
It is the nearest to it of any region in [the west except the salt area 
forty miles wide west of Great Salt Lake. This region is traversed 
by the Grand, Green, Duchesne, Fremont, San Rafael, Dolores and 
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 Temperate flora; 
the deep canons from the head of the Colorado down by the Tropi- 
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 from 
a deep tap root, with its large shiny leaves, and having almost exactly 
. the habit of Asclepias crytoceras, but. more erect, which grows in 
the same region. A. ampullarius has much the same: habit farther 
south. The coarse and tufted A. Pattersoni grows much the same but 
prefers bottoms. A. sabulosus a close relative of Pattersoni grows 
where alkali seeps out of stiff slopes. A. Haydenianus grows around 
irrigation ditches and along trails as if an immigrant «but appears 
mcre 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 Eastwood 
grows with tufted habit and low.: In the crevices of flat or gently 
sloping ‘sandstones masses A. desperatus is at home. . In the loose 
sand along the bottoms and in gulehes A. pubentissimus grows 
singly and flat on the ground as a winter annual. In 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 grows on gravelly mesas as. does A. 
Wingatensis. A. sesquiflorus grows in mats in the wide. ‚erevices . " 
of hot sandstones at Kanab.  Brandegei and straturensis grow. ¿in iem 
1 E, u 
