THE PHYSIOGRAPHY OF MEXICO 67 



between the Atlantic and Pacific, but between the Pacific and 

 the interior basins of the Anahuac Desert province. 1 There is also 

 a drop in elevation of a few thousand feet from south to north in 

 conformity with the slope of the tilted plateau block which con- 

 stitutes the Mexican country. 



The Sierra Madre Occidental is generally regarded as a moun- 

 tainous country, and experienced geographers write of its "ridges," 

 but it is important to note that the roughness of its topography is 

 due to down-cutting rather than upheaval or upbuilding; in other 

 words, the slopes of the region are to be regarded as canyon walls 

 rather than mountain flanks. 



This province has no important drainage to the east save by the 

 Rio Conchos and the Rio San Juan, both tributary to other streams. 

 All of the other streams which rise in the mountains and flow east- 

 ward are of the "suicidal" or "lost" 2 type and disappear soon after 

 reaching the desert below, or flow into undrained lagunas (lakes) , 

 where their water is gradually evaporated. The profound work of 

 dissection and denudation which one sees in progress east of the 

 mountains is not due to these streams, but to other processes which 

 will be described later. 



On the west, normal processes of erosion are under way, and 

 numerous subsequent streams are working headward and pushing 

 back the western scarp of the plateau at a rapid rate. In many 

 places these have captured the streams occupying the longitudinal 

 structural valleys, the latter entering the former at angles varying 

 from 45 to 90 . 3 Some of these western streams, particularly the 

 Aros, the Fuerte, and the Rio Grande de Santiago, have incised 

 their canyons a mile below the top of the plateau, and emphasize the 

 fact that the rugged character of the country is not due to upward 

 departures from the general level, but to profound down-cutting in 

 the plateau. The greatest depression is in the Grand Canyon of 

 the Bolanos, a tributary of the Santiago. The depth of this 

 canyon is accentuated by the fact that it lies at the foot of the 

 maximum crest of the mountains — the Sierra de la Yesca. 4 Experi- 

 enced travelers testify that the scenery to be found in these canyons 



1 E. O. Hovey, Bull. Phil. Geog. Soc, No. 4, p. 247. 3 Aguilera, op. cit. 



2 Hill, Eng. and Min. Jour., LXXXIV, 633. 4 Ordonez, op. cit. 



