THE PHYSIOGRAPHY OF MEXICO 71 



The eastern boundary follows the western slope of the Pecos 

 valley to the southern edge of the Stockton Plateau. 1 Here it turns 

 eastward along the Balcones scarp to the junction of the Pecos and 

 Rio Grande. Here it turns again and continues in a southerly 

 direction almost to Vera Cruz. In this part of its course it has been 

 drawn on the 500-foot contour of the Bartholemew map, which is 

 practically coincident with the western limit of sediments younger 

 than Late Cretaceous as mapped by Willis. In the field, it should 

 coincide with the line separating the undeformed sediments of the 

 Gulf Coastal Plain from the folded rocks of the plateau. 



The western boundary runs southward from the great bend of 

 the Colorado River at the foot of the Grand Wash and Cottonwood 

 Cliffs, 2 which represent the eastward-retreating scarp of the old 

 plateau, 3 to the Gila River. From the Gila to Naco, Arizona, it 

 follows the Rio San Pedro to the International Boundary. From 

 this point southward the line has already been described in con- 

 nection with the Sierra Madre Occidental province. 



The southern boundary is drawn close to the edge of the volcanic 

 area from Nevado de Toluca to Jalapa. It separates two easily 

 distinguishable topographic divisions. North of it, denudation 

 dominates the topography; south of it, great volcanic piles raise 

 themselves above the general level. The Mexican National Rail- 

 road from Vera Cruz to Mexico City follows the same line for a 

 considerable distance, and it is, I think, a safe assumption that the 

 railroad survey took these topographic features into consideration. 



Topography. — It is not possible to give a single descriptive 

 statement of the topography of this province. Broadly speaking, 

 it is a wide expanse of territory in which great islands of isolated 

 mountain blocks rise above a vast sea of desert. This description 

 may be applied with all faithfulness to the province through its 

 entire north-south extent, but an important exception must be 

 made with reference to the eastern and southern borders. In 

 these places it is actually mountainous. 



Beginning with the Sierra del Carmen and the Sierra del Burro, 

 inconspicuous ranges of 5,000 feet altitude or less, just across the 



1 1. Bowman, Forest Physiography, PL I. 



2 Ibid. 3 Hill, Eng. and Min. Jour., LXXII, 563. 



