PHYTOGEOGRAPHY OF THE ARIZONA DESERT 189 



the same forest of giant cactus and palo verde as noted above, 

 with a sprinkling of ironwood, and Franseria as an abundant 

 ground cover. Opuntiafulgida, Echinocereus fendleri and a variety 

 of flat-jointed prickly pears are the principal cacti. The Califor- 

 nia poppy in places enlivens with yellow the sprouting green of 

 the annuals. Finally a belt of the creosote bush, Covillea tridentata, 

 succeeds, and on reaching the playas and Indian farms we get on 

 the fine sedimentary soils Bigelovia harhvegii, Atriplex canescens, 

 and Baccharis sarothroides. The belt of mesquite in the lowest 

 portion is, as usual, sharply differentiated from the Covillea belts 

 of the gentle slopes adjacent. In ascending westward, the latter 

 species at first holds complete sway, then again gradually gives 

 way to Parkinsonia and Franseria, which latter in turn is dis- 

 placed more or less completely by Encelia before the mountains 

 are reached. A large number of other species begin to appear 

 in proportion as the soil becomes more rocky, this number reaching 

 its maximum on the north slopes of the sheltered canon troughs 

 in the mountains, and in the troughs themselves. 



