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For the purpose of this discussion, Texas is readily separable into three divisions:

• 1. The Gulf Slope, . . . . . . Cenozoic.
• 2. The Central Basin, .. Paleozoic.
• 3. The Western Mountain System,

The area covered by the Gulf Slope includes all the region east and south of the western and northern boundary of the Grand Prairie plateau, which stretches southward from the Red River to the Colorado, and thence westward to the Rio Grande. In area this comprises fully one-half of the State and by far the most thickly settled portion.

The Central Basin includes all that portion of the State west and north of the Grand Prairie, extending to the Gaudalupe Mountains on the west.

The Western Mountain System covers the remainder of Trans-Pecos Texas.

The Gulf Slope is in a certain degree a continuation of the topographic and geologic features of the States east of us which border upon the Gulf, but in some ways its differences are as pronounced as its resemblances. Thus, with the exception of a little marshy ground in the southeastern corner there is none along the entire coast. Differences in amount and character of rainfall and of temperature have also resulted in the production of a somewhat different topography, especially toward the Rio Grande, and the soils of certain formations are of far greater fertility than those derived from rocks of similar age in the other States, owing to peculiar conditions of formation.

The different sediments which now appear covering the surface of this area were laid down by the waters of a great sea, which in its present restricted basin we call the Gulf of Mexico.

Beginning at the coast in low and almost level prairies the ascent is gradual towards the interior, in many places not exceeding one foot per mile for the first fifty miles. Through this comparatively level plain, which comprises the exposure of the strata embraced under the general name of "Coast Clays," the streams move sluggishly in tortuous channels, and for the most part through an open prairie country, the only timber being along such water courses and in scattered motts or islands. As we pass inland this is succeeded by other belts which, having been longer subjected to erosion, show a surface more and more undulating as we recede from the Gulf. The ascent is also more rapid, and some elevations of as much as seven hundred feet are found, as at Ghent