1916 ] Camp: Amphibians and Reptihs 505 



movement, the last two resulting in a high rate of evaporation. No- 

 where in the United States are lizards so numerous, both in species and 

 individuals, as along the lower Colorado River. 



The Turtle Mountains have never been surveyed. They rise to 

 certainly not more than three thousand feet above the lowest surround- 

 ing depressions, the latter being about nine hundred feet above sea- 

 level. At no points are their summits high enough to support the 

 piiion and golden-oak associations such as occur on nearby desert 

 mountains. Detrital materials washed from the slopes, and lying up 

 against the hill-sides in great fans, nearly bury the lowest passes. In 

 many places beds of brown scoriaceous rock cover the alluvial slopes 

 and mountain sides. Farther out on the plains are stretches of 

 gravelly ash peppered with lapillae. The scoriae are glossy and re- 

 flect much heat on bright days. Igneous rocks make up a large part of 

 the mountain mass and there are a few eroded volcanic plugs, up to 

 four or five hundred feet in diameter. Other formations occurring in 

 some abundance are granites, quartz ledges, and, in the canon bottoms, 

 conglomerates; the latter, judging from their position, are of recent 

 formation. 



The hillsides are uniformly steep. They are covered with loose 

 rocky material and retain but little soil. The canon beds are boulder- 

 strewn and often very narrow. At their mouths they are encroached 

 upon by alluvial fans. These mesa-like benches are covered with 

 scoriae, or with smooth pavements of flat pebbles, and show little soil 

 on the surface. The arroyos dissecting the mesas are uniformly broad 

 and sandy, giving evidence of copious, even though rare, floods. Under- 

 cut caves in the solidified gravels of the wash banks are frequently 

 present at various levels above the wash floor. These holes furnish homes 

 for the rough-scaled lizards and the desert wood rats (Neotoma), the 

 latter industrious rodents being responsible for loosely piled accumu- 

 lations of stones and dead twigs in and about the caves (see pi. 21, 

 fig. 5). 



The typical desert plains vegetation is not supported on the leached 

 sand of the washes, which, probably because it is poor in nitrogen, has 

 a distinct flora of leguminous plants such as palo verde (Cercidium 

 torreyanum) , smoke tree (Dalea spinosa), and ironwood {Olncya 

 tesota). On the gravelly surface of the plains grows that most 

 abundant of all desert plants, the creosote bush or greasewood (Larrea 

 tridentata). "Wash deltas and gentle slopes lying above 2000 feet in 

 elevation usually bear, in addition to the creosote, stands of the long- 



