The Origin of the Desert Climax and Climate 125 



Larrea-Franseria into a savannah of these with the grasses men- 

 tioned. 



Toward the margins o£ the desert, where there is a rainfall of 

 5 inches, as in the vicinity of Goffs and Yucca, sand similarly 

 increases the availability of the water content and supports a 

 grass cover of Hilaria, Sporobolus cryptandrus flexuosus, Muh- 

 lenbergia porteri, and Aristida purpurea. With a corresponding 

 rainfall in che desert generally, these constituted the chief vege- 

 tation, mixed or alternating with Stipa and Oryzopsis and dotted 

 with taller Larrea and similar shrubs. Such a grassland was 

 doubdess able to maintain at least a sparse population of camels, 

 antelope, and horses, but bison were probably absent. Xeric 

 forms of nearly all the species of rodents at present known for 

 the region must have been present in abundance, and this pre- 

 supposes a corresponding balance in carnivores. 



Still proceeding backward, a further rise in precipitation to 

 from 10 to 12 inches was marked by a prairie denser in texture 

 and richer in species. In terms of the grazing animals, this was 

 even more significant because of the presence of the grama- 

 grasses of the genus BotUeloua. The evidence for this community 

 is drawn from transads found in marginal areas as well as from 

 mountain relicts, which are really transads in climatic terms. 

 Thus, the relict prairie at 4300 feet in the Avawatz Mountains 

 between the Mohave and Death Valley consists of Poa scabrella 

 and Hilaria jamesi, in addition to the more xeric Triodia, Ory- 

 zopsis, Stipa, and Aristida, while at the northern edge on the 

 slopes of the Argus Range occur Hilaria jamesi, Poa scabrella, 

 and Stipa comata. In the district about Cabezon at the outermost 

 edge of the Colorado Desert, a fine example of bunchgrass prairie 

 persisted until it was plowed a few years ago. It was composed 

 of Stipa speciosa, coronata, setigera, and eminens, Poa scabrella, 



