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badlands. The most conspicuous mountain group in the area, the Chisos Mountains, is a group of sharp peaks that stand in the center of a dissected lowland and are composed of masses of intrusive igneous rock and remnants of lava flows.

Southeast of the Davis Mountains and east of the Chisos Mountains are the narrow ridge of the Santiago Mountains and the high broken mountain mass of the Sierra del Carmen. These trend southeast and extend beyond the Rio Grande into Mexico. Northward the Santiago Mountains die out near the line of the Southern Pacific Railroad. The Santiago and Carmen Mountains are composed of folded, resistant limestones. East of the folds of these mountains are several domical uplifts (pl. 22). One of these, expressed topographically as the Serrania del Burro, lies wholly in Mexico, with its northern edges reaching up to the Rio Grande. It is a high dissected plateau, for the limestone cover of the dome is complete over its crest. Farther northwest, on the Texas side of the Rio Grande, is the Marathon dome. Here the limestone cover has been stripped from an extensive area on the dome's crest, and a lowland, the Marathon Basin, has been excavated from the nonresistant underlying beds.

-East of the Marathon Basin are escarpments of limestone which form the west edge of an extensive plateau area. The plateau summits descend gently eastward from the flanks of the Marathon region and the Serrania del Burro. The 50- to 75-mile belt between the Marathon region and the Pecos River on the east consists wholly of such plateau country, which has been carved into a maze of canyons and low tablelands. The plateau region is the western edge of the Edwards Plateau section of the Great Plains province, which extends far eastward into central Texas.

Trans-Pecos Texas has an arid or semiarid climate. The average annual rainfall at Marathon and nearby stations is about 17 inches. However, this figure is the average of greatly varying observations of many years, and the amount of rainfall is erratic in both extent and time. Some spots may receive half a dozen rains within a year, whereas others may remain nearly rainless for several years. The yearly rainfall at Fort Stockton, not far north of Marathon, has been as slight as 4 inches and as great as 34 inches, although its average is 15 inches.

One-half or more of the year's rainfall comes during the summer, when most of it is of torrential character. The precipitation during any one of such rains may amount to several inches. Now and then during this time there may be one or more weeks of continuous rain, when the mountains are cloaked in clouds. The entire rainfall of a year may be produced by only a few storms. During the winter some snow falls in the mountains, but as this is the dry part of the year, the amount of such precipitation is not great.

Temperatures at Marathon range from 110° in the summer to below zero in the winter, but ordinarily the variation is not so large. In the summer the diurnal temperature range is as much as 50°. Winds are strongest in the spring, when violent gales, without rain, may persist for a week or more. Violent wind storms of short duration at times accompany the summer thunder-showers.

The Marathon region and surrounding parts of trans-Pecos Texas have a vegetation adapted to the semiarid climate. The smooth plains of the Marathon Basin are grass-grown, but in the low places, where ground water is nearest to the surface, there are expanses of creosote bushes (Covillea) and dense thickets of mesquite (Prosopis juliflora) and catclaw (Acacia greggi). Low rocky ledges in the plains and terraces of limestone gravel that fringe the mountains support clumps of sotol (Basylirion wheeleri), lechuguilla (Agave lecheguilla), and other yuccas. Prickly pear or nopal (Opuntia) and ocotillo (Fouquieria splendens) grow on the low foothills. Higher in the mountains a sparse growth of juniper and pinon spreads over the exposed surfaces and summits and gathers in groves on the northern shaded slopes. Small clusters of live oak and manzanita (Arctostaphylos pungens) grow in protected valleys. Near water holes and stretches of flowing water in the stream channels is a lush growth of reeds and alders, shaded by cottonwood trees. The giant cacti that characterize the Sonoran Desert farther west are lacking in trans-Pecos Texas, but otherwise there is much similarity in the vegetation of the two regions.

The region is most attractive in the spring when numerous small plants come into blossom, covering the hillsides with a mat of brightly colored flowers. After the summer rains also, the brown and barren hills turn green as the vegetation comes to new life. Some of the plants, by reason of desert adaptation, show an immediate and wonderful rejuvenation after these unexpected downpours. The leafy clumps of resurrection plants (Selaginella pringlei ?), matted over many of the limestone surfaces, are dry and brown most of the year, but unfold and turn green within an hour after a rain.

For a useful discussion of the vegetation of this part of Texas see Bray, W. L., The vegetation of the sotol country in Texas: Texas Univ. Bull. 60, 1905. A concise summary is also given in Carter, W. T., and others, op. cit., pp. 7-11.