64 WARREN N. THAYER 



This province extends from near the International Boundary 

 southward, roughly parallel to the Pacific coast, to the valleys of the 

 Rio Grande de Santiago and the Rio Lerma. It has been suggested 

 that the Chiricahua Range of Arizona may mark its former extent 

 to the northward, and show the transition between it and the closely 

 related province of the Colorado Plateau in the United States. 1 

 Throughout the greater part of its extent it will average ioo miles 

 in width, but in the south it attains a width of 300 miles or more. 



There are many Sierra Madres in Mexico; so many, in fact, as 

 to be confusing to the casual reader of Mexican geography. The 

 Sierra Madre Occidental, however, is the real "mother" Sierra, 

 the progenitor of all the other Sierras in Mexico. If a section be 

 drawn across Mexico from the Atlantic to the Pacific in almost any 

 latitude, it will be found that every feature on the profile of that 

 section, save a narrow strip of coastal plain, is related in some 

 manner to this Sierra (see accompanying profile sketches, Fig. 2). 



The northern boundary has been drawn at the International 

 Boundary, where the Sierra Madre Occidental Mountains end 

 rather abruptly and overlook the vast stretches of desert plateau to 

 the northward. 



The western boundary, from Naco, Arizona, to the mouth of 

 the Rio Yaqui, follows the Rio Sonora and the Rio Moctezuma, 

 which flow in structural valleys at the foot of the mountain scarp. 2 

 From Trinidad, Sonora, to the Rio Grande de Santiago, Tepic, 

 this western line marks the highest escarpment overlooking the 

 desert below, and is coincident with the state boundaries separating 

 Chihuahua, Durango, and Zacatecas on the east from Sonora, 

 Sinaloa, and Tepic on the west. There is no doubt that these 

 boundaries between states are the result of natural selection, because 

 the prominent topographic break makes an indisputable parting 

 between political divisions. This line, which marks the present 

 limit of the eastward retreat of the plateau, was determined by one 

 of the prominent N. 45 W. fault fines which cut the plateau. 3 



1 Hill, Eng. and Mitt. Jour., LXXXIV, 631. 



2 See the Bartholomew map of Mexico, published by the National Geographic 

 Society. 



3 J. D. Villarello, Guides des excursions, 10th Inter. Cong. Geol. Mew, iqo6, No. 18. 



