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dry and porous stream bed and by evaporation. Hence the water does not persist far enough to form outlets to the sea.

Streams of a third type are frequent in the Central Province and around the interior margin of the Southern Province, which derive their normal run-off from springs draining the substructure of the plateau (structurally impeded drainage). These streams, which are usually vigorous at their head, are often interrupted in their lower courses, presenting irregular alternate sections of dry and watered channels, the water disappearing by absorption in sands, gravel, or fissures and reappearing at other places. They may be called spring rivers, and are of two general types; the first, those which rise in the margins of the Llano Estacado and Edwards Plateau and receive their water from gravity springs; the second, those which originate in great fissure springs that rise by hydrostatic pressure, like those of the Balcones scarp line.

Drainways of a fourth type may be called through-flowing rivers; they derive their water from the snow-covered ranges of the Rocky Mountains of northern New Mexico and Colorado and traverse all the geographic provinces. These streams receive their principal volume from the Cordilleras and in their courses across the Regional Coastward Slope are practically great antecedent canals passing across the Greater Texas region without serious lateral reenforcement. Such streams are the Canadian, the Pecos, and, at certain seasons of the year, the Rio Grande.

Most of the streams normally follow the continental slope toward the sea across the various provinces and are of the kind called consequent streams. Others, which are exceptional, flow at right angles to that of the normal regional slope, following parting valleys. The Pecos west of the Plateau of the Plains is the most conspicuous type of the latter class. The Clear Fork of the Brazos, Hubbard Creek, and Jim Ned Creek, of the Central Province, and certain forks of the Trinity in the East-Central Province are minor examples. The Rio Grande, in portions of its course through the Trans-Pecos Mountains and the Rio Grande Plain, follows great structural troughs.

Still another class of streams consist of headwater ramifications (caletas) of the longitudinal streams which drain the inland-facing scarps. These are called obsequent streams. They are usually short obsequent headwaters, and flow in a direction the reverse of that of the consequent streams, which follow the continental slope. They are found along the north-south scarp lines of the Central and East-Central provinces and along the western breaks of the Llano Estacado.

Some streams are of one of these types throughout, and may be called simple in character. Others present in different portions of