30 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
At levels slightly lower than the above, such as a locality examined 
near Hazen, Nevada, many other small shrubs and annuals gain in num- 
ber, such as Sarcobatus vermiculatus, Artemisia spinosa, Chrysothamnus, 
Thamnus montana, Tetradymia glabrata, and Lycium andersonu. An 
Ephedra is also prominent, while Lepidium fremonti is abundant in many 
places. With these is also to befound Opuntia pusilla, which forms 
small mounds or tufts of dead branches and gathers débris blown by the 
wind, from which the living stems barely emerge (plate 26). One or 
perhaps two species of prickly-pear are also encountered. 
The absence of succulents and of plants with storage structures is. 
quite noticeable at elevations above 3,000 feet and in the northern part 
of the region under discussion, although the conditions of precipitation 
would render such capacity highly advantageous. The low winter and 
night temperatures, however, probably render such structures impossible 
because of the liability to freezing. As one descends the valleys and 
canyons leading to the drainage of the Colorado River in southern 
Nevada quite a variety of such forms are encountered, among which are 
numerous opuntias and a large Echinocactus, with the tree Yucca coming 
in still lower down. 
A characteristic area is that around Las Vegas, Nevada, a bolson of 
great extent (plates 27 and 28), the surface of which includes great spaces 
covered with a loose alkaline soil which is hardened to a thin, fragile crust 
on the surface, while there also occur soft, slippery patches of moist 
alkali bearing AJdlenroljea. Small arrested dunes bearing low mesquite 
shrubs are numerous, but in no place were free, moving dunes encountered. 
In general the vegetation consists chiefly of spinose types, or of forms 
either with heavily coated leaves or with very restricted blades (plate 
28). The geological studies of the great basins of this region lead to 
well-founded conclusions as to major oscillations in climate in which 
long periods of aridity and humidity have alternated. It is concluded 
that one climax of aridity was reached about three centuries ago and 
that the precipitation is now slowly increasing, at a rate not appreciable 
by direct methods of measurement, however. (See page 104.) 
THE MOHAVE DESERT. 
Ascending from the San Bernardino Valley northward through the 
long climb of Cajon Pass, the railroad at last emerges from the dense 
growth of chaparral and comes out upon the elevated plain of the Mohave 
Desert. About 4 miles north of the summit begin to occur small groves 
of the strange tree for which the western part of the Mohave Desert is 
most widely known, the tree-yucca (Yucca arborescens). Within a few 
miles the desert becomes almost a forest of yucca and juniper (Juniperus 
californica), the former reaching a height, ordinarily, of 12 to 15 feet, 
though occasionally exceeding 25 feet and attaining a diameter of nearly 
