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the Grand Prairie, 2.5 feet across the Black Prairie and East-Texas Timber Belt, and 1.3 feet per mile across the Coast Prairie.

The chief relief feature of all the plains of the Coastward Slope is their general inclination toward the sea, in conformity with the tilt or dip of the underlying strata, away from the Cordilleran front, caused by the regional upward movements of the Cordilleran area as a whole, as mentioned elsewhere in this paper.

The relief of the individual plains is due to differences in origin, age, adjustment of erosion to the different geologic formations, climatic conditions (such as wind, humidity, precipitation, and evaporation), and gradient of the regional slope. Some plains, like the Coast Prairie, are so nearly flat and unbroken that undulations or elevations can hardly be detected; others, like some belts of the Central Province and the margin of the Edwards Plateau, are deeply and extensively dissected into high hills of uniform elevation separated by valleys, so that only small remnants of the former surface are here and there preserved; still others, like the eastern and southern extension of the Black Prairie, are eroded into low, rounded hills called rolling prairie.

The plains of the Regional Coastward Slope are of four general types, forming wide belts extending approximately north and south. These are the Great Plains proper, the Central Prairies, the plains of the East-Central Province (the Black and Grand prairies), the Atlantic Timber Belt, and the Coast Prairie. They may be classified by relationship into three major groups: The Great Plains, the prairie plains, and the Coastal Plain.

The Great Plains are a wide north-south belt of sublevel highland extending east from the Rocky Mountains to the prairie plains. The Coastal Plain, including the Coast Prairie and East-Texas Timber Belt, forms a wide stretch of lowland extending west from the Gulf. The prairie plains consist of the plains of the Central Province and the Grand and Black prairies of the East-Central Province. They occupy an intermediate position between the Coastal Plain and the Great Plains proper, and they differ from these in many respects, as will be shown later.

The most conspicuous relief features of the plains of the Coast-ward Slope are the Plateau of the Plains, the bordering Breaks of the Plains, the Valleys of the Plains, the Callahan Divide, the Balcones fault line, the Anacacho Hills, the Llano Hills, the Shumard Knobs, and the White Rock, Grand Prairie, Baird, and Seymour scarps, which will be described in the specific accounts of the plains to which they are related.

The Great Plains Province within the area of our map includes a portion of the middle and all of the southern part of the Great Plains