74 WARREX X. THAYER 



laguna and diverted its waters to the Gulf of Mexico. But the 

 channel of the stream was far to the south and west of the present 

 course of the Rio Grande, to which it was gradually crowded by 

 encroaching volcanic flows. 1 It has never recovered itself, and in 

 the vicinity of El Paso wanders about with an abandon that per- 

 plexes those who are charged with the duty of maintaining a definite 

 line for the International Boundary. - 



The desert plains of this province are of two types — degraded 

 and constructional. The former are the result of denudation of 

 greater plateau heights; the latter represent valleys or basins filled 

 by detritus derived from the degradation of neighboring areas. 



The mountains show several types of structure, though gener- 

 ally they are tilted fault blocks. Mountains in which an igneous 

 core has been thrust up through the sedimentaries are not uncom- 

 mon. Many are of the ''mesa'' type, the volcanic capping pro- 

 tecting the soft limestones beneath from erosion. A typical 

 "lost'' mountain is to be found in the Santa Eulalia, situated a few 

 miles southeast of Chihuahua City. It is a mass of folded Coman- 

 chean limestone mantled by superficial eruptive material, tuffs, 

 and cinders, and at present rises-about 1.500 feet above the desert 

 Level. Scattered over the surrounding desert lie piles of debris 

 derived from the denudation of the original mountain, which stood 

 several thousand feet above its present height. 



Physiographic history. — The history of this province coincides 

 with that of the Sierra Madre Occidental until the beginning of the 

 second erosion cycle. In brief, its known history begins with the 

 withdrawal of the waters of the Gulf of Mexico and the emergence 

 of the land mass at the end of the Cretaceous period. Then 

 followed deformation along general north-south lines, with accom- 

 panying or subsequent elevation to a height sufficient to be sus- 

 ceptible of active erosion. This elevation, which probably occurred 

 during the early Eocene. 3 ushered in the first erosion cycle, which 

 ended with the base-leveling of the country to the condition of the 

 Cordilleran Peneplain. 



1 Richardson, El Paso Folio, U.S. Geol. Survey. 



- Mrs. A. S. Burleson, Natl. Geog. Mag.. XXIV, 3S1. 



5 Ordonez and Aguilera. Inst. Geol. Mac.. IV. 



