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more closely related to Mexico and New Mexico than it is to the rest of Texas. It is a region of rugged sierras, of high plateaus and broad cuestas, and of gently sloping intermontane plains. The mountains have no timber except in sheltered valleys and on the higher summits.

In the clear air of the desert the mountain masses loom with sharp outlines and clear detail from a distance of many miles, and the plains that surround them are deceptively foreshortened. More than half of the region is a lowland. These intermontane areas are either bolsons (structural depressions filled by mountain waste) or destructional plains that slope upward as pediments toward the mountain masses from which they have been carved.

Ephemeral streams, which are dry gravel beds most of the year, discharge from the mountains and flow across the plains. Some of these drain into bolsons with no outlet to the sea, such as the Salt Basin, in the northwestern part of trans-Pecos Texas (fig. 1). Most of the drainage channels, however, lead to the two master streams of the area, the Rio Grande and its major tributary, the Pecos River. Their waters flow to the Gulf of Mexico. The Rio Grande is noteworthy more for its persistence through long stretches of desert land than for its breadth or volume. In its southeastward course across the area the river traverses a succession of desert basins and passes from one to the next through separating mountain barriers in which it has cut narrow and imposing canyons.

The mountains, plains, and plateaus of trans-Pecos Texas have been formed by interaction between various crustal movements of post-Mesozoic age and by the forces of erosion working upon the disturbed crust. The forms thus produced are of varying character, and the area may be divided into several geomorphic and structural provinces.

-North of the Texas & Pacific Railway the mountain areas are broad and in part plateaulike, with one side presenting a steep escarpment and the other forming a gentle back slope descending from the crest. Between them are intermontane plains 5 to 15 miles across, whose margins rise as bajada slopes toward the mountains. These mountains are composed of rocks which have been very little folded but which have been broken in the later part of Cenozoic time into numerous fault blocks (pl. 22). Movement along the faults has served to outline the form of the mountain areas, and this form has been modified but slightly by subsequent erosion. The intermontane plains are mostly depressed areas filled by waste carried down from the mountains.

The mountains and desert plains of this part of trans-Pecos Texas resemble those in the adjacent part of central New Mexico, to the north, near the Rio Grande. They are also similar to those in the typical basin and range country farther west, and this part of transPecos Texas is included in the Basin and Range province.

-South of the Texas & Pacific Railway block mountains of basin and range type are well developed in only a few areas. The mountains and plains are not caused directly by the uplift or depression of blocks of the earth's crust, but mostly by the differential erosion of bedrocks of varied character. The sedimentary rocks and the lava flows have been tilted, flexed, and in places strongly folded by crustal movements older than the block faulting of the basin and range province. In many places there are also masses of intrusive igneous rock. The nonresistant rocks of this region have been worn down into valleys and plains, and such harder rocks as limestones, thick lava flows, and igneous intrusions have been left as ridges, plateaus, and peaks. This area is the northern edge of a region of rugged highlands, whose greatest extent is in Mexico, south of the Rio Grande. It is here termed the "Mexican Highlands province."

The western part of the Mexican Highlands province in trans-Pecos Texas, comprising the Quitman and Eagle Mountains (fig. 1), consists of narrow parallel ridges and mountain chains of resistant, steeply tilted limestone and sandstone, between which are longitudinal lowlands carved from less resistant strata. In places the lowlands are covered to a moderate depth by later Cenozoic lake beds and alluvial deposits, but on the whole they seem to have been formed by erosion rather than by downfaulting or downwarping of the earth's crust. Similar parallel mountain ranges and intermontane lowlands are present southwest of the Quitman and Eagle Mountains, on the Mexican side of the Rio Grande, where they extend from the vicinity of El Paso southeastward past the great bend of the river and on into the interior of the State of Chihuahua (pl. 22, fig. 1). The two mountain ranges in Texas and the similar ranges to the southwest and south of them form the north end of the Sierra Madre Oriental of Mexico.

The eastern part of the Mexican Highlands province, comprising the eastern border ranges, is of greater diversity. Toward the north are the Davis Mountains (fig. 1), a high plateau broken up by canyons and in its more eroded parts separated into mesas, ridges, and isolated peaks. The Davis Mountains are carved from flat-lying or gently flexed lava flows. South of the Davis Mountains are various irregular mountain groups, some of them consisting of sharp peaks, and others of plateaulike blocks or narrow ridges. Between the mountains are lowland areas, some of which are smooth, gently sloping plains, whereas others have been greatly dissected and in part form picturesque