358 proceedings: geological society 



son, one that holds the surface water, with its mechanical sediments, and 

 the underground water, with its dissolved solids. This closed basin is, 

 on the whole, being aggraded but is suffering stream erosion (1) on the 

 upper parts of the slopes, and (2) near the playa. The high-level ero- 

 sion results in part from the normal development of the gradational cycle, 

 the large canyons having been cut deeper than the smaller ones, and all 

 of them having been progressingly lowered. The basin held an ancient 

 lake of 120 square miles with 30 miles of beach ridge. Inside the ancient 

 strand is a playa covering 51 square miles. Northeast of the playa is a 

 dune area produced by southwest storm winds. Search for ancient 

 shore features in any bolson in this region should begin on the south- 

 west side of the playa, where such features are not covered by wind 

 deposits. 



A completely closed bolson has two surfaces: a debris surface and an 

 underground-water surface. The base-levels of the two generally coin- 

 cide. The debris base-level is gradually raised by debris from the 

 mountains. Since the water supply is much larger than the debris 

 supply the water base-level is kept up to the debris base-level, and were 

 it not for evaporation the water-level would be raised indefinitely. As 

 long as the water-level does not rise above the playa the debris surface is 

 a self-developed unit, but when it rises above the playa, the submerged 

 part of the debris surface is placed under new conditions and the part 

 remaining above water adjusts itself to the lake-level, which is the new 

 base-level accidentally imposed upon it. In Sulphur Springs Valley 

 the debris surface has not yet readjusted itself since the disappearance of 

 the Pleistocene lake, as is shown by the erosion near the playa. 



In a completely closed bolson the soluble minerals are concentrated in 

 the central area of evaporation, and as the valley is aggraded they are 

 carried upward and reconcentrated at the surface. The very shallow 

 waters in the area of evaporation are generally more highly mineralized 

 than the deeper waters and the waters below the slopes. The waters. 

 below the slopes adjacent to igneous mountains contain less mineral 

 matter than those below the slopes adjacent to mountains of sedimen- 

 tary rock. Also the mineral matter is of a different kind, the soluble 

 material in the soil derived from the igneous mountains being largely 

 sodium carbonate. 



The fill of the Gila Valley has been eroded and this erosion has 

 extended to Sulphur Springs Valley, capturing several of its arroyos. 

 Hence this bolson, with its delicate gradational adjustments, is an ephe- 

 meral feature. 



Certain deposits in Sulphur Springs and adjacent valleys that indicate 

 deposition in lakes or seas lie beneath the stream deposits and represent 

 an older submergence than do the beaches. 



The magmatic sulfide ore body at Elkhorn, Montana: Adolph Knopf. 

 The Golden Curry Mine at Elkhorn, Montana, has produced 50,000 tons 

 of iron ore carrying $4 a ton in gold, which has been used mainly as a 

 flux for the lead smelters. One of the sources of this iron ore was a lens 

 of sulfide ore occurring in quartz monzonite 250 feet from the contact 



