. Annual report of the United States Geological Survey to the Secretary of the Interior . of the Coast Prairieupon which they are established. To the first class belong the Sabine,Sulphur, Neches, and Angelina. The second includes a number ofshort streams (creeks and bayous) which in their coastal extent aresluggish and brackish. In that portion of the Coastal Plain south ofthe Colorado there are streams of another category, which have theirorigin in remarkable fissure springs that break out at the foot of theBalcones escarpment. Among these spring rivers may be mentionedthe San Marcos, Comal,
. Annual report of the United States Geological Survey to the Secretary of the Interior . of the Coast Prairieupon which they are established. To the first class belong the Sabine,Sulphur, Neches, and Angelina. The second includes a number ofshort streams (creeks and bayous) which in their coastal extent aresluggish and brackish. In that portion of the Coastal Plain south ofthe Colorado there are streams of another category, which have theirorigin in remarkable fissure springs that break out at the foot of theBalcones escarpment. Among these spring rivers may be mentionedthe San Marcos, Comal, San Antonio, Las Moras, and San Pedro. Resume of the Drainage System. In resume it may be said that the rivers of the mountains are com-plex, composite streams. The rivers of the Coastward Slope plainconsist of four distinct systems of consequent streams which havedeveloped during different epochs of geologic history, each recordingdistinct changes of level, accompanied by a migration of the coast lineback and forth. By provinces it may be said the drainage of the Trans-Pecos Country. HILL.] RESUME OF DRAINAGE. 57 is of a nascent type, being practically nil on the bolson plains. Thisis partially due to lack of slope, but chiefly to meteorologic and struc-tural conditions, the evaporation and absorption being so much inexcess of the rainfall that there is not sufficient run-off to developstreams on these desert plains. The minor drainage of the mountainsis also faintly developed in comparison with that of other regions,owing- to the lack of sufficient rainfall. The through-flowing river ofthis region (the Rio Grande) gathers no local drainage from it. On the Great Plains, also, the drainage is undeveloped, and for thesame reason as in the case of the bolson deserts. Its summit run-offis deficient in quantity and of the flood-sheet type, which tends todestroy rather than to establish surface drainways. The real drainageof this province is underground. Such water as is not evapor
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