DESERT REGIONS OF NORTH AMERICA. Sh 
Parosela spinosa and another species of the same genus, and the composite 
green-leaved Peucephyllum schottit. 
West of Indio the railroad passes through a strip of mesquite dunes. 
Most of the sand here lies in hummocks, each produced by a mesquite 
tree (Prosopis), about which, and finally through the branches of which, 
the sand has drifted until only the ends of the branches project and the 
hummock presents the appearance of being covered with a growth of 
brambles. 
The total precipitation in this region is probably not more than 3 
inches annually, and as this comes uncertainly and irregularly one finds 
but few forms with pronounced storage organs, although on the mountain 
slopes several species of Opuntia find lodgment, and a barrel-cactus 
(Echinocactus cylindraceus) grows in places where it must depend chiefly 
upon drainage from the slopes above, which receive a greater amount, 
or where the total-of a large area is collected in underground conduits. 
It would be difficult to give accurate records of temperature, although as 
high as 128° F. is reported from certain places in this basin in midsum- 
mer. Ice may be formed in the winter, however. The mountains to 
the west of the Colorado Desert are subject to sudden storms, in which the 
rainfall is very heavy for short periods. Thus at Campo a precipitation 
of 16.10 inches was measured within a few hours in one day. The “‘Sono- 
ras,’’ or storms, which give this precipitation are usually confined to the 
main mountain ridges in their effects, but occasionally one drifts to the 
eastward far enough to cast some water upon the Colorado Desert; and 
in fact most of its total precipitation may come in this manner. 
The surface of the basin consists for the most part of loam, which in 
places is gravelly and in other places very sandy, while elsewhere frag- 
ments of rock cover the entire surface, particularly on the western slopes. 
Almost all of the loam is more or less saline and alkaline, the proportion 
of salts~present being the principal factor determining the character of 
the vegetation on any area, with the physical characters of secondary 
importance. 
The leached gravelly slopes bear Covillea and a variety of woody 
perennials, among which are to be included Fouquieria splendens, Olneya 
tesota, Ephedra, and Parkinsonia torreyana (plate 37), while along the 
beds of the dry streamways Parosela spinosa is abundant and also the 
composite bush with bur-like fruits, Gaertneria dumosa. The salt-bushes 
Atriplex canescens and A. polycarpa inhabit saline areas, the latter show- 
ing a wide range of capacity in being able to live not only in slightly saline 
but highly alkaline soils. 
The more highly alkaline areas bear Allenroljea and Sueda (plate 38), 
chiefly, while in such places the small shallow drainage channels, not 
wider than a hand’s breadth and less than an inch in depth, become 
sufficiently leached to permit the growth of a half dozen small annuals. 
