MAY, 1904. GEOLOGY OF WESTERN MEXICO—FARRINGTON. 199 
In the absence of detailed. and long-continued observations 
regarding the amount of rainfall, however, such opinions must be 
-accepted with hesitation, since less obvious causes may have pro- 
duced the results noted. 3. Abstraction of the waters by westward- 
flowing streams. That to this cause may be chiefly assigned the 
diminution of the waters to the eastward seems probable. The 
westward streams are torrential in character, and hence are capable 
of cutting backward rapidly. Studies by Meek, as yet unpublished, 
- of the fish fauna of the Rio Tunal, show that its fauna is also that of 
the Rio Grande. Hence the Rio Tunal has probably cut back into 
streams that once flowed eastward. While no further data on this 
point are available, it is highly probable that studies of the adjoin- 
ing rivers would show similar results, and prove that the divide 
of the Western Sierra Madre as a whole is gradually moving 
eastward. 
Passing westward from Durango, the country appears of a 
somewhat rolling character, sparsely coated with soil, and semi-arid 
as to climate. Cacti, mesquite, and scrub oaks constitute the chief 
natural vegetation. Where the soil is deep, however, corn can be 
successfully grown. The Rio Chico, here a narrow and easily ford- 
able stream, was crossed some hours after leaving Durango. Its 
valley is several hundred feet deep, with steep and terraced sides. 
Three distinct terraces were noted, but of their origin nothing can 
be stated. 
The rocks for a long distance westward were plainly eruptive 
in character, generally porphyritic, and appearing to be dacites and 
rhyolites. The topography is characterized by a series of mono- 
clines dipping to the east, with abrupt faces to the west. Thus the 
plateau rises westward by series of low steps. 
The aridity of the country continued until toward the close of the 
second day’s travel, when pines began to appear. These soon thick- 
ened into extensive forests such as are shown in Plate LV. During 
the journey of three following days, this pine forest continued almost 
without interruption. Variations in the size and height of the 
trees occurred, and there were occasional rocky areas where trees 
could not grow, but the general character of the region was forested. 
Some flat areas were also treeless, on account, probably, of their sub- 
mergence during the rainy season. The species of pine constituting 
the forests was, in general, a long-leaved sort. The larger trees were 
about two feet in diameter at the butt and perhaps fifty to sixty feet 
high. They rarely exceeded this size, and averaged smaller. As 
usual in pine forests, there was an entire absence of underbrush. 
