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region. The middle division, which is locally known as the North Plains, lies between the Arkansas and the Canadian rivers. The southern division is the relief feature which we have described as the Southern Plateau of the Great Plains.

The Southern Plateau of the Plains (the combined Llano Estacado and Edwards Plateau) is the most extensive relief feature of the non-mountainous portion of Texas. It represents the continuation of the Great Plains proper south of the Canadian Valley, and is a vast sub-quadrangular, treeless table-land, about 60,000 square miles in area, slightly tilted toward the sea and surrounded on all sides by escarpments of erosion. (See Pl. III.) At one time it was a continuous plain which extended from the front of the mountain eastward far across the Central Province. Denudation of its margins has restricted the area to its present dimensions. It is surrounded by the escarpments of the deeply eroded Pecos Valley, separating it from the mountains on the west, the drainage groove of the Canadian on the north, and an escarpment of headwater recession on the east. Its southern margin is abruptly terminated by the Balcones fault escarpment, coastward of which the level of the country has dropped several hundred feet. In this manner the Plateau of the Plains has become a subquadrangular plateau surrounded by low escarpments, which are locally known as the Breaks of the Plains.

The prairie plains, which include the vast Central Province of southern Kansas, Oklahoma, Indian Territory, and Texas and the East-Central Province of Texas, consist primarily of destructional plains resulting from the wearing away of the formations of the Coastward Slope from higher to lower beds. The general type of relief is that of a greatly denuded prairie region in which the surfaces have been established upon many different planes of stratified rocks, some of which are very rugged, although extensive stretches of level prairie predominate. These stretches occur mostly in subparallel north-south belts of country, accompanied by scarp lines, isolated circular buttes and mesas, and deeply serrated cut plains. This relief is due to erosive sculpture resulting from the establishment of the natural drainage upon successively lower and lower stratum plains in the geologic series from the Plains (Tertiary) formation on the west to the Algonkian at the southeast, inclusive. In a broad way this relief may be looked upon as a great and deeply cut intaglio, in which the various surfaces, composed of layers of six groups of subhorizontal strata of different age, thickness, hardness, and color, have been successively exposed by erosion.