THE PHYSIOGRAPHY OF MEXICO 77 



cuts across the mouth of the Gulf of California to Cape Corrientes, 

 where it closes with the southern boundary. 1 



The eastern boundary has been described in connection with the 

 Sierra Madre Occidental and Anahuac Desert Plateau provinces. 



The southern boundary is really a prolongation of the eastern 

 boundary, which makes a turn to the southwest from the valley of 

 the Rio Grande de Santiago to Cape Corrientes. 



Topography. — Although there is a certain homogeneity of 

 topography which distinguishes this province from its neighbors, 

 there is, nevertheless, a minor diversity of features within its 

 boundaries. The features which force themselves upon the 

 attention of the student of physiography are as follows: (1) a great 

 structural trough, occupied by the Gulf of California, the Salton 

 Sink, and the Coachilla and Imperial valleys; 2 this trough, if not 

 an actual southern extension of the same, is at least analogous to 

 the great downwarp of the Pacific coast which is occupied in suc- 

 cession northward by the San Joaquin, Sacramento, and Willamette 

 valleys and the Puget Sound basin; (2) a destructional coastal 

 plain of varying width bordering the open ocean and the Gulf of 

 California; (3) a foothill region bordering the western foot of the 

 Sierra Madre Occidental and the Arizona Highlands, of which the 

 mountains of the Peninsula of Lower California are a counterpart. 3 



The bottom of the structural trough is below sea-level from 

 Riverside County, California, to Cape Corrientes, with the excep- 

 tion of the delta of the Colorado River, which lies slightly above 

 sea-level and separates the submerged portion, known as the Gulf 

 of California, from the Salton Sink and the Coachilla and Imperial 

 valleys, which are at present land surfaces, though one point is 275 

 feet below sea-level. 



The coastal plain is very narrow where it borders the open ocean 

 and on the east side of the peninsula. On the mainland it assumes 

 a wedge shape, and varies in width from zero in Tepic to 70 miles 

 in the northern part of the province. It is nearly level, with few 

 elevations above 200 feet, and "plainly the bottom of the Gulf of 



1 Ordonez and Aguilera, Inst. Geol. Mex., IV and VI. 



- Bowman, op. cit., p. 235. 



; C. Botsford, Eng. and Min. Jour., LXXXIX, 223. 



