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 canons, 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 cylindro- 

 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 Opuniia 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 

 tesola 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, Bcer- 

 haaina pterocarpa, Bouteloua aristidoides, Andropogon saccharoides, 

 Wedelia incarnata, Machceranthera tanacetifolia, Triodia mutica, Evol- 

 vulus arizonicus, Allionia gracillima, and Cassia covesii. The bases of 

 boulders and partially shaded ledges of rock are the habitats of Selagi- 

 nella rupincola, Cheilanlhes lindheimeri, and Notholcena 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 canons 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 

 Fouquieria splendens are conspicuously confined to southerly slopes 

 (see plates 6 and 8). Parkinsonia torreyana, which is confined to arro- 



