Geology of North em Mexico. — /////. 1-^5 
of their debris ) and extending like a smooth floor from one to 
the other are the plains or basins. 
There is also much igneous material, but this is a secondary 
feature, to be mentioned later, and most cases is in the l)eds of al- 
most recent lakes, above which the mountains l)ut lately ( in geologic 
times ) projected as islands. This is the true basin structure so 
ably described in fractional portions of its extent in Utah and 
Nevada, b}- Gilbert. Dutton. Russell. Powell and McGee. In cen- 
tral New Mexico the true Rocky Mountain system ends aljruptly 
against this region, just south of Santa Fe. All mountains north 
of that point are in the basin region. 
The western escarpment of the Llano Estacado and the Pecos 
river and its continued course in the Rio Grande form approxi- 
mately the eastern border of the basin region in Texas and New 
Mexico, while its southern ])order extends southward through 
Comanche and eastern Chihuahua in Mexico to the state of 
Durango. 
The principal mountain blocks of this region are the following: 
In southeastern New ^lexico, the Sierra Oscura, the San Andreas, 
the Juccarillo, Sierra Cal)ella. Sierra Florida, Sierra de la Hacheta 
and many unnamed l)locks; in Trans-Pecos. Texas, the Orgim 
Franklin chain, the hueras. the Van Home, Carrisos and the Las 
Chisos, Davis mountains, the Chenatis: in northern part of Mexico, 
the Juarez mountains ( near city of Juarez ). Sierra del Carmen. 
Sierra San Yincente, Sierra del Burro, Las Arboles, Las Graces, 
Sierra Encantado. Sierra Carrizalyo. Sierra del Santa Rosa, Sierra 
de Guajes, Sierra Lampazos and many others: the eastern conl lu- 
nation of the region into Mexico. 
Intimately and closely associatetl with these l)asins, is the west- 
ward embayment of the Rio Grande from the coast. This em- 
I)ayment is marked l)y the southern escarpment of the Kdwards 
division of the Grand Prairie, on the north from San Antonio to 
Del Rio, and by the Santa Rosa and allied mountains in Coahuila, 
and is a great topographic depression, up which nil the late Cre- 
taceous and Quaternary formations deflect. It is covered more 
or less to the coast liy a detrital deposit of an age probably related 
to the entirel}' enclosed basins. The northern ( Texan ) boundary 
of this l)asin is a fault wall — the great Austin-New Rnumfels 
fault of my previous papers. 
The southern boundary, in Mexico, is composed of typical 
