2 GUIDEBOOK OF THE WESTERN UNITED STATES 
more abundant, and many special trees and plants are prevalent, 
notably the mesquite; forests diminish in density, and far to the 
west trees occur only in the bottom lands. Agriculture here depends 
largely on irrigation, and the raising of cattle, sheep, and goats is 
the dominant industry. The principal underlying rocks are shale, 
soft sandstone, and chalk, which do not make strong relief but produce 
hills and ridges of moderate height separated by wide valleys, which 
along the larger streams are bordered by bottom lands. Northwest 
of San Antonio the Coastal Plain gives place rather abruptly to the 
Edwards Plateau, owing to the rapid rise of hard limestones; from 
San Antonio to Del Rio this feature lies north of the railroad but 
is visible at many places. 
For many miles west from Del Rio the railroad is on the plateau, 
which is floored by hard limestone and deeply trenched by the 
drainageways, notably by the canyons of the Devils River, the Rio 
Grande, and the Pecos River. In this district, where semiarid con- 
ditions prevail, vegetation is sparse and trees are mostly confined to 
valley bottoms except where the limestone supports a growth of 
juniper or live oak. The soil is thin, but it sustains grass and shrubs 
which afford good pasturage for many goats, sheep, and cattle. 
Owing to the gradual general rise of the strata to the west the land — 
increases in elevation, and much of the plateau in south-central 
Texas is 2,000 feet above sea level in its eastern part and 3,000 feet 
in its western part. Near Sanderson this rise develops into the great 
dome of the Marathon uplift. The central part of this uplift is 
truncated, revealing a large area of closely folded Paleozoic rocks, 
making sharp ridges of the Appalachian type. The Edwards Plateau 
ends on the east side of this uplift. To the west is the Davis Moun- 
tain region, a wide province of volcanic rocks, characterized by rugged 
peaks and irregularly disposed ridges in great variety, which rise to 
elevations considerably more than 6,000 feet. 
The voleanic rocks continue far west of Marfa, but near that place 
begins the Basin and Range province, which extends thence across 
New Mexico, Arizona, and southern California. In this province 
there is a prevalence of long ridges separated by wide plains or bolsons 
floored by sand and gravel. They present a succession of strata or of 
voleanic flows, mostly tilted or flexed and faulted. Many of the great 
mountain faces stand along lines of uplift. At intervals there are 
large masses of intrusive rocks, which have been forced up in a molten 
condition and are now so hard that they are conspicuous topographic 
features. 
The climate of this region is arid or semiarid. The Rio Grande 
flows between ridges in New Mexico, but at and below El Paso it 
either crosses the axes of the ridges in canyons or passes around their 
ends, The Gila and Colorado Rivers have similar relations in south- 
