70 WARREN N. THAYER 



At some date subsequent to the completion of the first cycle 

 extensive orogenic movements changed the whole structural trend 

 from north-south to about N. 45 W. 1 The profound faulting 

 which accompanied these movements blocked out the entire country 

 into its present shape, determined most of the prominent structural 

 features, and opened avenues for the intrusion of vast igneous 

 bodies which have been responsible for the primary mineralization 

 of the province. 



THE ANAHUAC DESERT PLATEAU 



Definition and boundaries. — Many names have been applied 

 to this portion of the old Mexican Plateau, such as the Mesa Central, 

 Chihuahu'a Desert, and Anahuac Plateau. Hill has suggested the 

 name Anahuac and it will be used in this paper, for it seems emi- 

 nently appropriate to fasten upon this decayed remnant of a former 

 widespread feature the name of the aboriginal race which inhabited 

 it, and which centuries ago went into decadence along with it. 

 The terms "desert" and "plateau" are included, for they are 

 descriptive of the province. Furthermore, these terms serve to 

 correlate this province with those of the United States which 

 Fenneman has included under the name of desert plateaus. 2 



The Anahuac Desert Plateau is a degraded area of scattered 

 block mountains and intervening "bolson" desert. It extends 

 from the plains of Texas and New Mexico and Arizona on the north, 3 

 southward through the Mexican states of Chihuahua, Coahuila, 

 Nuevo Leon, Tamaulipas, San Luis Potosi, Queretaro, and Hildalgo 

 to the Valle de Mexico and the base of Xinantecatl (Nevado de 

 Toluca) . 4 The portion lying north of the International Boundary is 

 well known under the names of Trans-Pecos Highlands and Arizona 

 Highlands. 



The northern boundary line runs eastward from the great 

 bend of the Colorado River, along the edge of the Colorado Plateau, 

 through Silver City, New Mexico, then turns northeastward and 

 passes the southern ends of the San Juan and Sangre de Cristo 

 mountains. 



1 Hill, Science, N.S., XXV, 710. 2 Fenneman, op. cil. 



i G. B. Richardson, El Paso Folio, U.S. Geol. Survey. 



4 Ordonez el al., Trans. A.I.M.E., XXXII, 259. 



