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Cambro-Silurian, and Carboniferous rocks are the survivals of ancient land masses that were not base-leveled in pre-Cretaceous time. Their arrangement in long anticlinal folds more or less influences the present relief, and, assisted by erosion, produces a type of configuration quite different from that of the mountains of the Cordilleras.

The mountains of the Trans-Pecos Province are composed not only of the older sedimentary rocks found in the Ouachita uplift and the floor of the Coastward Slope plain, but also of. the Cretaceous rocks which make much of the surface of the latter region. Here they are folded and tilted into mountain structure, while sheets and necks of hard eruptive rock, produced in Mesozoic and Cenozoic time, furnish further relief-making elements. The marine Tertiary and Pleistocene formations of the Coastal Plain are missing, but in the flats and basins between the mountains are extensive unconsolidated nonmarine deposits, probably of synchronous age.

Within the Greater Texas region are two mountain systems-the Ouachita system of Arkansas and Indian Territory, and the Trans-Pecos Mountains. These systems are of different structural types, ages, and configuration, and trend approximately at right angles to each other. The Ouachita system is Appalachian in structure and general resemblance, and is thereby related to the mountains of the eastern half of the United States. The Trans-Pecos Mountains are a part of the great Cordilleran systems which dominate the western half of the continent between the Great Plains and the Pacific.

This system extends east and west between the ninety-third and one hundredth meridians, from the Mississippi embayment of the Coastal Plain to the plateau of the Great Plains, through western Arkansas, Indian Territory, and Oklahoma. The system as a whole is a narrow line of old mountains, whose summit nowhere exceeds 3,000 feet. It is composed of three principal groups, of different types of relief and rock composition. These are the Massern Ranges on the east, the Arbuckle Hills in the center, and the Wichita Mountains on the west.

The Massern Ranges were so named by Thomas Nuttall on a map accompanying his book entitled A Journey of Travels into the Arkansas Territory during the Year 1819: Philadelphia, 1820. These consist of elongated ridges of vertically folded clays and sandstones with some limestone, mostly of Carboniferous age. They extend east and west to longitude 95° 30', where they take a southerly direction, ending in a manner as yet not satisfactorily explained at the northern edge of the Grand Prairie and against the eastern end of the Arbuckle Hills.