72 WARREN N. THAYER 



Rio Grande from the Stockton Plateau, a chain rises above the 

 desert level and increases gradually in height southward until it 

 forms the Sierra Madre Oriental. These mountains increase in 

 height southward until they attain truly magnificent proportions, 

 with peaks reaching 10,000 feet or more in Hidalgo and Queretaro, 

 where they meet eastward-extending spurs of the Sierra Madre 

 Occidental and merge with them. The Gulf slope of these moun- 

 tains is in many places a scarp dividing the plateau from the Gulf 

 Plain. It is probable that this scarp marks the position of a great 

 fault line having at some points a vertical displacement of at least 

 4,200 feet. 1 These mountains are not a continuous north-south 

 barrier, like the western sierra, and the ramifications of the desert 

 "extend through the passes of the limiting chains in many places 

 and establish easy communication with the coastal lowlands." 2 



Travelers across the Trans-Pecos region of Texas and New 

 Mexico are more or less familiar with the topographic forms which 

 will be described as characteristic of the greater part of this province. 

 Prominent among these are the north-south trending ridges, sepa- 

 rated by broad plains — in part constructional and in part plains of 

 degradation — many of the latter being of the basin type called 

 "bolsons," from the Spanish word meaning "purse." A section 

 across the Anahuac Desert Plateau in almost any latitude would 

 show the same topographic features — "numerous isolated moun- 

 tain ranges with intervening pocket valleys or wide expanse of 

 undulating plain." 3 The general desert level has an altitude of 

 from 2,000 to 4,000 feet, while the mountains generally rise above 

 5,000 feet, with an occasional peak ascending to 8,000 feet or more. 



Other prominent features of the topography are the "lost" 

 mountains, lagunas, medanos, and a peculiar formation known as 

 "La Brisca." 



Lost mountains are isolated mountain blocks standing in the 

 desert, and so far removed from the main mass of mountains as 

 to have no apparent connection with them. They are simply out- 

 liers (monadnocks) which have resisted denudation more effectively 

 than the surrounding territory. 



1 F. L. Nason, Econ. Geol., IV, 421. 



2 Aguilera, Inst. Geol. Mex., VI. 3 Hill, Eng. and Min. Jour., LXXXV, 6S1. 



