TOPOGRAPHIC FEATURES OF THE DESERT BASINS. 51 
caused the formation of playas and alluvial dams? This region then, 
was perhaps what the Great Basin is now, and salt accumulations 
descended from such a period are by no means impossible. But 
whether or not these speculations have a basis of truth, they are as 
yet without supporting evidence. No direct indication of such a 
moderately arid period has been discovered, and the country is so in- 
hospitable that it has not invited the efforts of speculative geologists. 
For the present the matter must remain open and it would seem 
useless to search here for hypothetical salt accumulations when there 
are other regions, the promise of which rests less on speculation and 
more on fact. 3 
THE COCHISE BASIN. 
In the eastern part of the region just discussed there is one basin of more usual 
type. The Arivaipa-Sulphur Springs Valley, being better watered than its more 
westerly analogues, has had a history more nearly parallel to that of the Great Basin 
valleys, and the central portion of it has been cut off by alluvial dams to form the Co- 
chise Basin. Northward the valley drains to the Gila and southward to the Rio 
Yaqui. The northward divide is the lower and probably the more recent, and there is 
little question that the basin once had free drainage in this direction. The area of 
the basin is approximately 1,250 square miles. It contains a playa of usual character. 
THE LORDSBURG-MEMBRES REGION AND THE CHIHUAHUA BOLSONS. 
In the southwestern corner of New Mexico are two trough valleys 
not essentially dissimilar to the Arizona trough valleys which border 
them on the west, but of much less regular structure. The western 
of these troughs contains the present Lordsburg and San Luis Valleys 
and belongs in many ways with the Cochise Basin and the San 
Simon Valley, in the group last discussed. Once it drained north- 
ward into the Gila and it is now cut off thereform only by a low 
alluvial divide. Internally the valley shows a topography essen- 
tially similar to that of the valleys of the Great Basin. The struc- 
tural trough has two branches, each of which once contained a con- 
siderable stream, the two uniting somewhat south of their mutual 
discharge into the Gila. Stream decay and alluvial dams have cut 
these tributaries into chains of shallow basins and local playas, 
notable among which are the Llano de los Playas, or Playas Valley, 
lying southeast of the Pyramid Mountains, and the Lordsburg Dry 
Lake near the railroad junction of that name. All of these sub- 
sidiary basins are recent and unimportant and their individual areas 
have not been computed. The total area of inclosed drainage in the 
trough is about 2,900 square miles. 
East of this trough is another irregular trough valley now con- 
taining the valley of the Membres River and the Florida Plains. In 
this trough the inclination is reversed and the former drainage was 
southward across the international line. Indeed, there is now 
scarcely any barrier to southward drainage and a very moderate 
