76 Tlie American Geologist. Feb. i89o 
The mountains of the Trans-Fecos country. 
Entirely distinct in every characteristic of age and structure 
are the mountains constituting the southern margin of the 
Texas region. These can be briefly defined as occupying the 
country west of the Pecos in Texas and New Mexico, and their 
continuation into the republic of Mexico, their northern 
margin approximately coinciding with the Rio Grande, east of 
the mouth of the Pecos. No attempt has been made to study 
and classify them, and to-day almost nothing is published con- 
cerning the details of their structure, history and relations to 
each other. So far as known they consist of numerous peaks, 
mesas and ridges — isolated or in groups — to the individuals of 
which over thirty distinct names have been applied in Texas 
alone. Some of these, as has been recorded, may possess 
granitic axes. Others are highly disturbed sediments of all 
ages from the Silurian to Post-Laramie, sometimes capped 
with lava, as seen in the Eagle mountains. Concerning their 
minute stratigraphy little has been published of a reliable 
nature. In crossing the region several times, however, I have 
been able to see that the whole system has been elevated 
through the characteristic topography and rocks of the Grand 
and Black Prairie regions, i. e., the lower and upper Cretaceous 
respectively. This is readily seen by transecting them from 
east to west, along the line of the Southern Pacific railway. 
The first intimation of the mountain disturbance west of the 
Pecos is the increased dip in the eastward inclination of the 
rocks of all the Black and Grand Prairie region or the upper 
and lower Cretaceous S3'stems respectively. This eastward 
dip gradually increases as we go westward, and mountain 
peaks soon succeed the rugged edges of the plains, and the 
eastwardly dipping Cretaceous limestones are seen to rest on 
the almost vertical rocks of presumably paleozoic schists. 
Continuing westward to the apparent heart of the region, near 
Eagle Springs, the Cretaceous rocks become highly metamor- 
phosed, and are crumpled in various directions, and overlain by 
great eruptive sheets?. Still continuing westward in the vicin- 
ity of El Paso there is an appearance of faulting on an exten- 
sive scale, as seen along the south side of the Rio Grande for 
a hundred miles below that city. The main questions of 
mountain structure and relation, however, as before stated, 
still remain unsolved. Between the individuals or 
