296 FORREST SHREVE 



ingly represented at the highest altitudes in the Santa Catalinas. 

 The rugged topography of the Pinaleno range and the numerous 

 constant streams bring about a sharper contrast between the 

 vegetation of canons and slopes at lower and middle altitudes 

 than is the case in the Santa Catalinas. The evergreen scrub 

 of lower altitudes is carried to high elevations on the steep south 

 slopes and the mesophytic vegetation is carried to low elevations 

 along the constant streams of the larger canons. This sort of 

 interdigitation of the lowland and mountain vegetation is con- 

 spicuous in all desert mountains, but its amplitude is greater in 

 the Pinaleno Mountains than in any of the smaller neighboring 

 ranges. 



The narrow belt of -desert which lies between the Gila River 

 and the eastern base of the Pinaleno Mountains is characterized 

 mainly by Covillea tridentata, Atriplex polycarpa and Lycium 

 gracilipes. The commonest representatives of the cacti are 

 Opuntia stanlyi, a low form with club-shaped joints, and 0. 

 engelmannii, an erect platy opuntia with very large joints. The 

 simple vegetation of the" desert floor becomes somewhat richer 

 on entering the interbajadal valleys, where Prosopis, Atriplex 

 canescens and Isocoma wrightii are the most abundant plants. 

 Opuntia spinosior, 0. engelmannii and 0. macrocentra are the 

 only common representatives of the cacti in these valleys and 

 on their gravelly sides. In habitats of this character at eleva- 

 tions only 800 feet lower in other parts of southern Arizona 

 there is a strong representation of such striking cacti as Carnegiea 

 and Opuntia bigelovii and a series of desert shrubs and perennials 

 confined to lower elevations. 



The tops of the largest bajadas lie between 4000 and 5000 feet, 

 sloping upward at a gradient of slightly less than 500 feet to 

 the mile. The extremely coarse material of which they are 

 built is revealed by their gravel soil and numerous rounded 

 boulders of 1 to 4 feet in diameter. The southern crests of the 

 bajadas are thickly clothed with the common species of Opuntia 

 and with Fouquieria, while the tops are very scantily covered 

 with Prosopis and Yucca radiosa, and with a very open summer 

 carpet of Hilaria cenchroides, Eragrostis lugens, Aristida scheidiana. 



