THE 
AMERICAN GEOLOGIST 
Vol. VIII. SEPTP^MBER, 1891. No. 
PRELIMINARY NOTES ON THE TOPOGRAPHY AND 
GEOLOGY OF NORTHERN MEXICO AND SOUTH- 
WEST TEXAS, AND NEW MEXICO. 
By RoBT T. Hii.L, AuBtiii, Texas. 
The topographic and geologic features of northern Mexico, and 
the Trans-Pecos region of Texas and New Mexico have Ijeen for 
several years a subject of profound interest to the writer, who, 
notwithstanding much stud}', involving thousands of miles of 
travel, still feels that he can contribute only a few data concern- 
ing this vast region, and that the -main facts and details of its 
structure are still unraveled, especially those relating to oro- 
graphic and igneous geology, and he presents the accompanying 
description of a small, but typical portion of the area, with the 
hope that it may be of some assistance to those who are more 
competent to discuss as a whole the grander orographic features of 
our continent. 
I have previously shown the salient topographic features of the 
region to consist of: 
1. A series of present and ancient coast plains, consisting of strata of 
Trinity and later age, which covers the eastern half of the state, and 
collectively form what I will call the coastward incline: This embraces 
the coast prairies, the Washington prairies, the Eo-Lignitic or Forest 
region, the black prairie, the Grand prairie, and the two Cross-Timbers. 
The Llano Estacado in some respects may be classified generally with 
this region, but for the present, I prefer to treat it separately. 
2. The central denuded region, including the great rock sheet of the 
Paleozoic and early Mesozoic ( Red beds ) mostly dipping westward, 
which lie unconformably beneath the group of the coastward incline, 
