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County, are masses of solid basalt (see P1. XXII); the Anacacho Hills, extending east and west in southern Kinney County and constituting the most rugose part of the plain, are of still another type, consisting of a monoclinal plateau, or cuesta, sloping southward and presenting a steep scarp to the north. (See geologic profile, fig. 66, p. 260.)
The eastern part of the plain belongs climatically to the humid and subhumid regions; the western part, to the arid. The fertile "black lands" occupy large areas as far southwest as San Antonio; but contlnuous cultivation is limited by increasing aridity west of Bexar County. The plain as a whole is mostly grazing land, and supports one of the greatest stock raising industries of the United States. San Antonio is its commercial center.
In passing westward from comparative humidity to aridity, there is a corresponding decrease in the depth and richness of the residual soils. Soils derived from certain geologic formations which in the Black Prairie and eastern Texas timber regions are fertile, here become progressively more sterile and barren, and are impregnated or coated with a peculiar calcareous encrustation known in the Southwest as "tierra blanca" or "tepetate."
Much of the region is covered by the peculiar flora known as chaparral, consisting of a thorny growth of many species of scrubby acacias-mesquite, guaxillo, and huisatche-between which is an undergrowth of cactus, especially the large Opuntia known as nopal. For this reason the region is sometimes called the "chaparral country." There are also stretches of open prairies covered by nutritious grasses.
It is not within the province of the present paper to deal with the plain in its entirety, but only with a strip along its western margin not exceeding 30 miles wide, a belt underlain by the Cretaceous formations.
BALCONES SCARP LINE.
The Balcones scarp line is the frayed and ragged coastward border of the Edwards Plateau. From the more open and level lower country it appears as a sharp line of timber covered hills, and these are universally called "mountains" by the people of the region. It commences near the northern line of Travis County and continues a little south of west, through Travis, Rays, Comal, Bexar, Medina, Uvalde, and Kinney, to Valverde County, where it meets the Rio Grande. Near Austin its highest summits are about 400 feet above the margin of the lower plain; in Uvalde County, nearly 1,000 feet.
"The "Black Prairie" region is that underlain by the soft Upper Cretaceous chalky limestones and marly clays, where there is considerable rainfall. It comprises the richest agricultural lands of the State. The cities of Austin and Dallas are situated near its western margin; Terrell and Corsicana are near its eastern margin.
A singular fact in the progressive cartography of our country is that notwithstanding the conspicuousness of the Balcones scarp line as a topographic feature and that it was shown upon all the mother maps of the region, such as those made by Bartless, Roemer, and J. De Cordovas (J. H. Colton & Co., New York), it has been omitted from the more recent maps.