OSOBB VALLEY SALT-FIELDS. 707 



125°. They are strong brines, common salt being the chief ingredient. 

 Several other saline springs are found bordering the valley. 



OsoBB Valley Salt-fields. — The lowest depression of Osobb Valley, 

 which lies between the Pah-Ute Range and Augusta Mountains, has an 

 altitude, according to barometric measurement, of 3,360 feet above sea- 

 level, which is by far the lowest point attained in the Grreat Basin within 

 the limits of the Fortieth Parallel Exploration. It is occupied by a Lower 

 Quaternary salt-field, which represents the bottom of a salt lake, which, 

 at the period of its greatest saturation or smallest area, must have occupied 

 a basin 18 or 20 miles in length along the middle of the valley. It is now 

 a solid field of white salt, covered with a thin layer of brine, which, in the 

 extremely dry autumnal months, completely evaporates. The rains of 

 winter and spring, however, accumulate on the surface of the salt, and 

 form a briny lake, covering, two-thirds or three-quarters of the salt-field. 

 In the middle, the deposit of solid salt is 5 feet thick, and toward the west 

 shore it is overlaid by fine saline mud, which is in turn capped by a thin, 

 constantly-renewed, crystalline film of salt, about one-half an inch thick. 

 In the salt mud, which thickens toward the west shore to an unknown depth, 

 are found beautifully-modified octahedrons of salt, nearly pellucid, but 

 containing always some discoloration from minute particles of clay that 

 give the crystals a dark, smoky appearance. An analysis of this salt from 

 the thick deposit yields nearly 97 per cent, of chloride of sodium, the 

 remaining 3 per cent, being an admixture of carbonate and sulphate of 

 soda, with slight impurities. The analysis gave Mr. E,. W. Woodward the 

 following : 



Chloride of sodium . . . .' 96.49 



Sulphate of soda 1.91 



Carbonate of soda 0.96 



Water 0.52 



Insoluble residue 0.12 



100.00 



The insoluble residue of 12 per cent, consists mainly of oxide of 

 iron and a trace of lime. 



