i 9 o4] MAC DOUGAL— DELTA AND DESER7 VEGETATION 



5 1 



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 io km being now occupied by a 

 number of active mud volcanoes. 



Fig. 3. — View to southward on floodplain of Rio Colorado below mouth of Hardy's 

 branch; Range Hill in distance; carpet of Cressa truxillensis and Distichlis spicata; 

 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 southward from the 

 Gila River, consists principally of long gentle slopes or sandy gravelly 

 plains rising gradually toward the interior, and broken here and there 



