TOPOGEAPHIC 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. 



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 GUa 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 GUa. 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 raOroad 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 vaUey 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 



