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has been so dissected into remnants by erosion that the level of the original stratum plain is still recognizable in the summits of the dissected members.

The term " rolling prairies " is well established by popular usage in Texas for plains of undulating or rounded hilly relief, as distinguished from flat plains. Such plains are usually built upon unconsolidated strata of clays or sand.

The plains of the Texas region may be geographically classified into two groups: The plains of the Regional Coastward Slope and those of the Trans-Pecos Province.

The term Regional Coastward Slope is here used for all the non-mountainous portions of the Greater Texas region, including the Great Plains, the Central, East-Central, and Southern provinces.

Before the individual plains of the Regional Coastward Slope are separately discussed its broader features will be considered. As a whole it has a general eastward slope from the foot of the Cordilleras to the coast, from an altitude of 4,000 feet or more to sea level. This slope is primarily caused by a tilt, or the sum of several tilts, which the region has received through uplifts of the Cordilleran region in Tertiary and later time. The slope of the Coastal Plain north of the twenty-ninth parallel is to the east, and in a direction at right angles to the east front line of the Cordilleras north of the thirty-second parallel. Between the twenty-ninth and thirty-third parallels the slope continues east as far as the interior borders of the Eastern and Southern provinces. Within these provinces the direction of the slope changes to the southeast. The first-mentioned direction of the slope pertains to the Great Plains region in general, and the second to the more restricted coast plains proper. South of the thirty-second parallel the gradient of the Regional Coastward Slope is adjusted to the curve of the mountain front in a peculiar manner. Normally it should change in direction sympathetically with the curve, but, instead, it continues east, in a direction no longer at right angles to the mountain front, until the twenty-ninth parallel is reached. At the latter parallel the uniformity of direction of slope is suddenly broken by the structural deformation of the Rio Grande syncline, making a low synclinal basin between the Balcones scarp and the Mexican Cordilleras.

The general average inclination of the Coastward Slope from the mountains to the sea is 8.7 feet per mile. It varies in the different provinces, being approximately 8.6 feet per mile across the Llano Estacado, 9 feet across the western part of the Central Province, 6.7 feet across the eastern part of the Central Province, 16 feet across