1904] MAC DOUGAL— DELTA AND DESERT VEGETATION 



61 



It is to be seen that the region discussed in the foregoing paper 

 includes a subtropical delta of irregular outline and long extensions, 

 which includes within its outermost boundaries a great alluvial plain 

 subject to floods, bank erosion, and shifting of the soil, and also to 

 the action of salt tidal waters; a mountain range of granite and vol- 

 canic rocks over iooo" 1 in height; a small area of active mud volcanoes; 

 a depressed region, presumably an old sea-floor, the bottom of which 

 is more than ioo m below the level of the sea; and the mud flats near 

 the actual mouth of the river. This delta is directly in contact with 

 the gravel and sand desert mesas of Sonora and Baja California. 



The portion of the delta subject to the direct action of the floods 

 and tides is everywhere slightly alkaline and varies but little in the 

 general constituency of the soil, supporting a luxuriant growth of 

 vegetation, the 'more important elements of which grow in pure for- 

 mations of the greatest possible density. The larger woody plants of 

 this region have their bases submerged in water at a low temperature 

 during the summer season, during which time the crowns are exposed 

 to low relative humidity at temperatures that may be as much as 

 70 F. higher than the roots. In consequence of this condition many 

 species in the low lands have xerophilous foliage. In addition, a 

 comparatively high concentration of soil salts must be endured during 

 the stages of low water. 



An analysis of the flora of the region shows that many of the 

 species of the delta extend down along the coast especially in the 



