22 VEGETATION OF A DESERT MOUNTAIN RANGE. 
THE LOWER DESERT SLOPES. 
On leaving the uppermost edge of the bajada and commencing the 
ascent of the mountain over the rather abrupt slopes which lie between 
the larger cafions, a region is entered upon in which the physical con- 
ditions differ from those of the bajada chiefly in the pronounced slope 
exposure to the south, southwest, or southeast, and in the occurrence 
of large masses of rock in situ, with the coarse soil limited to small 
benches, pockets, and fissures. The vegetation of these lowest slopes 
is very similar to that of the Upper Bajadas, and is composed of a 
nearly identical flora. Prosopis, Parkinsonia, and Acacia are repre- 
sented by smaller and less frequent individuals, and both the tylindro- 
puntias and platopuntias occur somewhat less frequently. Carnegiea 
gigantea is even more abundant on the slopes than on the bajadas, 
being represented by smaller individuals, among which relatively few 
have reached the size at which branching begins. For Carnegiea and 
the above-mentioned trees the relatively rapid erosion of the soft gneiss 
and the shifting of the shallow soil are apparently too great to permit 
the attainment of great size or age. Fouquieria, Encelia, and Chrysoma 
laricifolia are even more abundant on the slopes than on the Upper 
Bajada, and Opuntia bigelovii, the most densely spiny of all the cylin- 
dropuntias, is found exclusively on southerly slopes and cliffs, in very 
rocky substratum, at elevations below 3,500 feet (1,067 m.). Olneya 
tesota and Covillea have not been detected on the mountain slopes (see 
plates 4 and 5). 
The summer and winter ephemerals of the bajada are nearly all 
to be found on the Desert Slopes of the mountain, but rarely in such 
abundance as they attain on level ground. Among the most common 
of the ephemerals and root-perennials to be observed in the summer 
are Cladothrox lanuginosa, Pectis papposa, Euphorbia florida, Ber- 
haavia pterocarpa, Bouteloua aristidoides, Andropogon saccharoides, 
Wedelia incarnata, Macheranthera tanacetifolia, Triodia mutica, Evol- 
vulus arizonicus, Allionia gracillima, and Cassia covesti. The bases of 
boulders and partially shaded ledges of rock are the habitats of Selagi- 
nella rupincola, Cheilanthes lindheimeri, and Notholena hookeri. The 
ferns are not common and are conspicuous only during rainy periods, 
but the Selaginella is abundant here and becomes even more so at 
slightly higher elevations, where it frequently clothes the rocky walls 
of steep cafions to such an extent that their usual grayness is converted 
to a vivid green a few hours after a heavy rain (see plate 7). 
The ascent from 3,500 to 4,000 feet (1,067 to 1,220 m.) witnesses 
the first essential changes in the vegetation. At the latter elevation 
nearly all of the typical desert forms may be found, but Opuntia has 
become infrequent and Carnegiea gigantea, Echinocactus wislizeni, and 
Fouguieria splendens are conspicuously confined to southerly slopes 
(see plates 6 and 8). Parkinsonia torreyana, which is confined to arro- 
