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New Mexico, reported to be 13,000 feet high, is the highest summit. The highest mountain of the system in Texas is Guadalupe Peak, near the New Mexican line-9,000 feet, or 5,000 feet above the interior margin of the Coastward Slope plain. Southward the mountains do not attain high altitudes until they cross the Rio Grande in Mexico. Livermore Peak (8,500 feet) and Mount Emory (8,500 feet) are the highest summits south of Guadalupe Peak. Immediately upon crossing into Mexico the ranges again rise to higher altitudes-10,000 feet or more.

The Trans-Pecos Mountains lack continuity and exhibit many irregular and eccentric forms of relief. In general the individual mountains present sharp and rugged outlines. They are usually barren of timber, except a few summits of the Sacramento, Davis, and Chisos mountains that rise above the base of the timber zone, which is about 6,000 feet along the Rio Grande.

The individual mountains may be primarily classified as Sierras, disconnected peaks, and groups of peaks. The various mountain forms, whose lineaments are so clear in the arid atmosphere, have generally been given individual and descriptive names by the former explorers and inhabitants, who were of Spanish speech. Thus we find the various eminences called by such names as cabezas (heads), cornudas (horns), chinos (ghosts), corazones (hearts), sandias (watermelons), sillas (saddles), cuchillas (knife edges), etc. The term "s cuesta" is also entensively used adjectively for the surface of a limited plain, plateau, or mountain which has a distinctly visible slope or tilt-a tilted mesa. Font ranges.-The east front of the Cordilleras, as the parting between the mountains and plains may be called, has a southerly trend from Colorado to the thirty-first parallel. From the latter line it curves southeast through Trans-Pecos Texas and northern Mexico, closely following in direction the general course of the Rio Grande toward the Gulf, which it reaches in Mexico near the northern tropic. This change in direction of Cordilleran trend is due to certain orogenic processes which can not here be set forth, but it has an important bearing on the geography of the west Texas and north Mexican regions.

By far the larger portion of the Greater Texas region is occupied by plains, and, as these are diverse in structure and relations, their character may best be explained by a few preliminary words on the nature and origin of plains in general.

The term "plain," while applied to a region which is dominated by a conspicuous and persistent subhorizontal surface, is not always intended to signify an unbroken monotonous physiographic feature,