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valley stretches of "mesquite flats," or level prairie where extensive beds of shale prevail. Country of this character also extends a short distance south of the Callahan Divide, where it is called the Brownwood Country.

The Burnet Country.-Still farther south, adjacent to the drainage basin of the Llano River, the so-called Burnet Country (see Pl. XIV) is found. This is so very rugged that it is considered mountainous by the inhabitants. It is composed of a series of erosion levels cut below the Edwards Plateau, imposed in succession first upon the Carboniferous and then upon the Silurian and Cambrian strata, and finally cut down to a basement plane of ancient granite and schists, upon which the drainage of the Llano and a section of the Colorado is now established. These various levels of the Burnet Country are often extensive features, such as the Backbone Plateau west of Burnet and the Packsaddle Mountains. Notwithstanding its rugosity, this area, which may have been a monadnock-or remnantal hill in a generally planated region-before Cretaceous times, is now a subcircular basin of erosion, below the general level of the Plateau of the Plains and the Grand Prairie, surrounded by overlooking scarps of their strata, which once completely buried it.

The Coastal Plain of Texas includes the area lying within the Eastern and Southern provinces, and consists of three broad types of country-the Coast Prairie, the East-Texas Timber Belt, and the Rio Grande embayment.

This plain is a continuation of the Atlantic Coastal Plain of the Eastern and Gulf States, yet is essentially different from that in many features. At the east line of Texas it has the general characters seen in the other Southern States, consisting of a gently sloping plain, extending from a mountainous background (the Ouachita Mountains) to the Gulf, its eastern border attaining an elevation of about 500 feet in southern Arkansas. As it bends to the southwest its features become gradually modified until they present several notable variations from the Coastal Plain to the east. The most notable differences are the absence of a well-defined interior border between the Ouachita Mountains and the Colorado River, the increasing proportion of prairie, the different adjustment of the fall line of the rivers, and a great embayment which the plain makes up the valley of the Rio Grande, somewhat analogous to that of the Mississippi.

Between the Ouachita Mountains and the Colorado River the Coastal Plain is so continuous with the Regional Coastward Slope, which extends westward to the Rocky Mountains, that its border in this region must be an arbitrarily assumed line practically coinciding with the coastward border of the Black Prairie as far south as the Brazos.