. Annual report of the United States Geological Survey to the Secretary of the Interior . of the plain, presenting a steep escarpment tothe north and sloping toward the Rio Grande. PLAINS OF THE TRANS-PECOS PROVINCE. The Trans-Pecos Province, as previously stated, is a region of com-bined plain and mountain, the total area of which is about equally dividedbetween these two forms of relief. (See PI. VI.) The plains differ fromthose of the Regional Coastward Slope in origin, geologic formation,details of relief, and vegetation. The former are chiefly the result ofthe emergence and destructional
. Annual report of the United States Geological Survey to the Secretary of the Interior . of the plain, presenting a steep escarpment tothe north and sloping toward the Rio Grande. PLAINS OF THE TRANS-PECOS PROVINCE. The Trans-Pecos Province, as previously stated, is a region of com-bined plain and mountain, the total area of which is about equally dividedbetween these two forms of relief. (See PI. VI.) The plains differ fromthose of the Regional Coastward Slope in origin, geologic formation,details of relief, and vegetation. The former are chiefly the result ofthe emergence and destructional base-leveling of the Coastal Slope,unaccompanied by structural deformation. The latter are largelystructural valleys originated by the deformation that produced themountains, which have been converted into constructional areas byaccumulation 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 plainsoccur. U. S. GEOLOGICAL SURVEY TWENTY -FIRST ANNUAL REPORT PART VII PL. VI. B. DESERT PLAINS NEAR TERLINGO CREEK, TRANS-PECOS TEXAS. HILL.] STREAMS AND STREAM WAYS. 51 THE DRAINAGE. The extensive region under discussion has a diverse drainage, itsstreams varying in origin, number, length, and volume with the topo-graphic and structural conditions, rainfall, evaporation, slope, andporosity of the surface. Some parts have numerous streamways whichdrain the surface and lead the water to the sea. Others, like the greatbolson deserts of the Trans-Pecos region and the Plateau of the Plains,are practically without surficial drainage, and the surface precipita-tion is disposed of by evaporation and imbibition. There is little surficial obstruction to the direct run-off of therainfall, whereby a constant supply of water to stream ways may beregulated, except such as the forest growth of the Eastern Province,and the absorbent character of the dry soil and rocks in certain localare
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