52 BULLETIN 54, U. S. DEPAETMENT OF AGEICULTUEE, 



increase of rainfall would suffice to reestablish the outflow. Stream 

 decay has produced a few local plaj'-as, but none of any importance. 



Southward of the liae the ancient drainage line reaches the Laguna 

 Guzman, which is the sink of the northernmost of the Chiliuahua 

 bolsons. The area of the Membres Valley and its tributaries within 

 the United States is over 5,000 square miles. In Mexico an area of 

 6,800 square miles is or has been tributary to the Laguna Guzman, 

 making a total of about 11,800 square miles for the area of this 

 bolson. 



These bolsons are wide, shallow basins once tributary to the Rio 

 Grande and now cut off therefrom only by low dams of alluvium 

 and dune sand. They are products of the decay of the drainage sys- 

 tem which once served the broad featureless plains between the 

 Rio Grande and the Sierra Madre. All are very recent and unim- 

 portant. The larger ones contain intermittent or permanent lakes 

 fed by the perennial streams which head in the well-watered high- 

 lands of the Sierra Madre. The region of the bolsons extends from 

 the western boundary of Chihuahua southeastward to the edge of 

 the drainage system of the Rio Salado, about haK way across the 

 State of Coahuila, but this region is divided into two parts by the 

 still vigorous drainage system of the Rio Conchos. The north- 

 western portion is the smaller and contains the bolsons of the Laguna ^ 

 Guzman (already discussed), the Laguna de Santa Maria, the Rio 

 Carmen, and the Laguna de Patos, as well as many smaller playas 

 and transient ponds. The more important bolsons of the southern 

 division are those of Laguna Palomas, Laguna de Coyote, Laguna 

 Parras, Laguna Viesca, Laguna de Jaco, and Laguna de Agua Verde. 

 Areas have not been computed in detail. The total area covered 

 by all the bolsons, including the Guzman, is probably not less than 

 125,000 square miles. 



THE ROCKY MOUNTAIN BASINS. 



The crests of broad mountain ranges are frequently regions of 

 poorly determined drainage and wherever the crest of the Rockies 

 is flat and imperfectly defined, advancing desiccation has left small 

 valleys and local depressions partially or entirely without outward 

 drainage. All such basins are more or less recent and nearly all are 

 small. Only two require specific notice. 



THE SAN LUIS BASIN. 



The San Luis Valley or San Luis Park lies in south-central Colorado at the head 

 of the great trough of the Rio Grande. It is separated from the valley of this river 

 by a broad and featureless alluvial plain, crossed by an inconspicuous divide. The 

 present drainage of the valley is not sufficient to overflow this divide, and accumulates 



' Laguna is the Spanish word for lake; rio is that for river. 



