1904] MAC DOUGAL—DELTA AND DESERT VEGETATION 51 
Many parts of the delta and of the adjoining districts in the deserts 
of Sonora and Baja California show traces of recent earthquakes 
and of volcanic action, a tract 2 by 10°" being now occupied by a 
number of active mud volcanoes. 
3.—View to southward on floodplain of Rio Colorado below mouth of Hardy’s 
t of Cressa truxillensis and Distichlis spicata; 
Fic . 
branch; R: ange Hill in distance; carpet 
Prosopis scattered over plain, which also shows great quantities of driftwood. 
The Cocopa Mountains rise directly from the delta to a height 
of over 1300™, and their granite slopes support an island of desert 
vegetation of the types induced by low humidity and precipitation. 
DESERTS. 
The arid region east of the delta, extending southw ard from the 
Gila River, consists principally of long gentle slopes or sandy gravelly 
plains rising gradually toward the interior, and broken here and there 
