60 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



iiK^ridian. As here defined, the prairie-plains region inchides the 

 "Central Province" of Hill and the strip of prairie that extends 

 southward from Austin to the Rio Grande plain. The Rio Grande 

 plain, as we use the term, is synonymous with the "Lower Rio 

 Grande Country" of Hill (see fig. 21). 



To the east of the East Front Ranges, the western Texas region 

 descends from the high plateaus bordering the Rocky ]\[ountains 

 in northern New Mexico b}" a series of broad plains arranged in the 

 form of a great stair (as described by Hill) to the Gulf of Mexico. 

 The higher part of tins area constitutes ]iart of the plateau region of 

 western United States and is bounded on the south and east hy a 



Fig. L'l— The natural divisions (f:NViR()NMENTAi. complexes) of the (iREATEii Texas Region. 



line running from Del Rio to Austin and from here west and north 

 to Oklahoma. To the eastward the plateaus grade down into the 

 prairie region, whicli forms a l)r()ad band across central Texas and 

 extends to the northward beyond the State. 



The climatic conditions of these regions are well illustrated b}?" 

 the character of the vegetation. The Trans-Pecos region is a ])art 

 of the Proplateau region of southern Arizona and New Mexico, and 

 the conditions are very similar to those in the latter States. The 

 rainfall is mostly below 10 inches, and the evaporation is high, which 

 with the scanty rainfall prevents the occurrence of arboreal vegeta- 



