52 BULLETIN 54, U. S. DEPARTMENT OF AGRICULTURE. 
increase of rainfall would suffice to reestablish the outflow. Stream 
decay has produced a few local playas, but none of any importance. 
Southward of the line the ancient drainage line reaches the Laguna 
Guzman, which is the sink of the northernmost of the Chihuahua 
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, ahaneee ‘basis once tributary to the Rio 
Grande and now cut off therefrom only by low dams of alluvium 
and dunesand. 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 half 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 eae 
125,000 square niles 
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 
1 Laguna is the Spanish word for lake; rio is that for river. 
