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The southward continuation of these folds was planed off and buried beneath the Cretaceous rocks of the Grand Prairie of Texas.

The Arbuckle Ranges extend from the ninety-sixth to the ninety-eighth meridian, in a series of low limestone ridges and granitic hills which strike in a direction north of west. These are old mountains composed of vertically folded limestone strata with a granitic base, exposed toward their eastern end, in the vicinity of Tishomingo. They have been so degraded that they have lost that magnitude which is usually associated with mountains.

The Wichita Range is the western end of the Ouachita system and forms a rugged sierra between longitudes 98° 30' and 100° . The highest peak, Mount Sheridan, rises 2,800 feet-about 1,300 feet above the surrounding plains, which are composed of old granitic and volcanic rocks projecting through flanks of Silurian limestone.

The mountains of this system in general are old, and represent the remnants of once more lofty and extensive ranges which have undergone degradation since early Mesozoic time.

The Trans-Pecos Province is a peculiar combination of mountains and stretches of plateau, plain, and bolsons surrounding, bordering, and lying between mountain ranges. The plains will be discussed later under a separate head.

The Trans-Pecos Mountains of Texas and New Mexico lie between the Pecos and the Rio Grande south of latitude 35° 30'. They represent the eastern front ranges of that portion of the North American Cordilleras lying between the southern end of the Rocky Mountains in northern New Mexico and the northern end of the eastern sierras of Mexico. These mountains are called by some the Continental Divide, but erroneously, for the Cordilleran region has no single dividing ridge in this latitude, but is a canoe-shaped area, bordered toward the Pacific and the Great Plains by broken crests, between which are large basin plains and low, disconnected mountain ridges.

By origin these mountains are of three distinct types, as follows:

• 1. True mountains of deformation, composed of structural folds or tilted fault blocks of sedimentary rocks, the mountain forms corresponding in trend to the strike of the structure.
• 2. Plateau mountains, consisting of high subhorizontal plateaus void of serious deformation, occurring either as summits or as shoulders attending higher relief features.
• 3. Mountains of igneous material, of three subtypes-old igneous vents (such as dikes and necks), craters, and summits of circumdenudation capped by sheets of ejecta.

The highest altitudes attained are in the mountains of the eastern front ranges. Sierra Blanca, of the Sacramento Range in southern