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the Rio Grande Plain. On the right is constantly visible a line of low, circular, flat topped hills, the Balcones scarp line, which represents the jagged southeastward front of a higher region which has been called the Edwards Plateau. The Rio Grande Plain, the Edwards Plateau, and the Balcones scarp line are the chief geographic features of the region. Broadly considered, they are a lowland plain inclining gently southeastward to the Gulf of Mexico, an upland plain rising gradually toward the northwest, and a rugged zone of separation which includes a quick ascent from plain to plain.

The Atlantic and Gulf coasts of the United States from the Hudson to the Rio Grande are margined by a broad lowland called the Coastal Plain. The portion of it lying farthest to the southwest is called the Rio Grande Plain. One of the more important geographic divisions of the interior of the continent is the Great Plains. Its most southerly division is the Edwards Plateau. Farther north the Coastal Plain and the Great Plains lie far apart, the Mississippi Valley and the Appalachian belt and other geographic provinces being included between them; but southward they converge, finally meeting in southern Texas, so that the Rio Grande Plain and the Edwards Plateau lie side by side.

In shape this plain is an irregular quadrilateral with angles turned toward the four cardinal points. On the northwest, as just stated, the Balcones scarp line separates it from the Edwards Plateau. On the southeast it is bounded by the Gulf of Mexico, and on the northeast it is arbitrarily limited by the Colorado River. On the southwest it is limited by the folded mountains of Mexico, which lie beyond the valley of the Rio Grande. It also sends a tongue for many miles up the Rio Grande.

The plains drainage, which follows the general slope from northwest to southeast, includes the Nueces, San Antonio, and Guadalupe rivers and their branches, besides various minor streams which join the Rio Grande and Colorado or enter the Gulf direct.

As compared with the adjacent plateau and mountain regions, its characteristic topographic feature is a low relief, but its surface is broken by occasional undulations and in places by hills of considerable height. Some of these are the low scarp lines of drainage valleys; others, like the Dos Hermanos Hills of Webb County, are buttes capped by limestone; others, like Pilot Knob, in Travis County, are old volcanic necks; Pinto, Las Moras, Turkey, and Elm mountains, in Kinney County, are buttes composed below of sedimentary rocks and above of caps of igneous rock; Sulphur Peak and Fort Inge, in Uvalde "

The indentation of this plain up the Rio Grande has been called the "Rio Grande Embayment" by Hill: Bull. Geol. Soc. America, Vol. III, 1891, p. 93.

Hill, ibid., p. 90.