Chap. IX] 



DESERTS 



637 



The single work dealing with the oecological conditions of the North 

 American desert to the west of the Rocky Mountains is Coville's account 

 of his travels in South California, in the hot, dry ' Death Valley.' 



The North American desert is composed of several belts of different 

 altitudes, of which only the lowest, the Lower Sonoran region, was investi- 

 gated by Coville. It is characterized by two common shrubs, which are 

 confined to it, the zygophyllaceous creosote bush (Larrea tridentata) and 



Fig. 373. North American desert. The Colorado desert of South California. 

 Washingtonia filifera, Wendl. 



the composite Franseria dumosa, and it extends over the portion of Cali- 

 fornia to the west of the Sierra Nevada as well as over parts of Nevada, 

 Utah (Fig. 374), and Arizona (Figs. 37.5-377, 379>- 



This tract is occupied chiefly by extensive ' mesas Y the soil of which is 



1 Mesa = table. 



