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increasing altitude and older and more fully developed drainage systems, becomes more undulating, and finally, toward its interior margin, consists of low, rolling hills, which increase in rugosity toward the Balcones scarp line. These hills are capped by the gravel wash from Edwards Plateau. West of a line from San Antonio to Laredo a low synclinal trough (the Rio Grande embayment), threaded by the Rio Grande, sets in and the slope changes from directly coastward toward the river, forming the north side of a trough between the Balcones and Mexican Sierras, which inclose it on the north and south, respectively. The slopes in this district have a normal inclination of less than 8 feet to the mile. The Texas side of the Rio Grande embayment consists of a low broken plain abutting against the sharply defined interior Balcones escarpment of the Plateau of the Plains. Its surface includes long stretches of level country, presenting a few interesting features of minor relief, several solitary volcanic hills, hills of circumdenudation, and a low monoclinal mountain group known as the Anacacho Hills.

Some plains, as in the vicinity of Spofford and Del Rio, are of constructional origin and present the aspect of gravel-covered flats analogous to the Llano Estacado; they have been produced by the distribution of arid material by storm-wash deposition.

Rising out of the interior or northern margin of this plain, in Uvalde and Kinney counties, are a number of low, dome-shaped volcanic or laccolithic necks or stocks and monadnocks of horizontal strata, capped by sills of igneous rock. The Anacacho Hills, in Kinney County, form an exceptional feature of relief-a long cuesta or monoclinal mountain rising out of the plain, presenting a steep escarpment to the north and sloping toward the Rio Grande.

The Trans-Pecos Province, as previously stated, is a region of combined plain and mountain, the total area of which is about equally divided between these two forms of relief. (See Pl. VI.) The plains differ from those of the Regional Coastward Slope in origin, geologic formation, details of relief, and vegetation. The former are chiefly the result of the emergence and destructional base-leveling of the Coastal Slope, unaccompanied by structural deformation. The latter are largely structural valleys originated by the deformation that produced the mountains, which have been converted into constructional areas by accumulation of debris of the surrounding highland.

The plains of the Trans-Pecos Province are of two principal types, plateau plains and bolson plains, though, exceptionally, lava plains occur.