SALT DEPOSITS OF INCLOSED BASINS. 85 



inense dunes, especially along the northern border of the plaja, and have 

 nearly buried a rhyolitic butte beneath snow white drifts, thus acquiring 

 for it the local name of the " White Mountain." 



A stratification of the various salts found in playas in the order of their 

 solubility, as commonly occurs from the slow evapoi-ation of brines, is not 

 usual, for the reason, apparently, that they owe their accumulation to repeated 

 desiccations. In some instances, however, as at Rhode's salt marsh in Ne- 

 vada, the more soluble salts are gathered most abundantly in the central 

 part of the basin. 



A study of the playas of the Far West renders it evident that saline 

 deposits of great extent may result in tlie manner described above, and 

 sustains the suggestion that beds of rock-salt, gypsum, etc., found in various 

 geological formations may have been accumulated in interior basins by the 

 evaporation of ordinary surface waters, and are not in all cases, as fre- 

 quently inferred, the result of the evaporatioii of isolated bodies of sea- 

 water. 



Besides the playas proper, the formation and characteristics of which 

 we have sketched, there are other desert areas in the Far Wes-t that are 

 frequently designated b}' the same name, but which are of a somewhat 

 different nature. These are mud-plains left by the evaporation of large 

 lakes, and composed of ordinary lake sediments. Deserts of this nature 

 are in many instances nearly as desolate as the true playas. Their borders 

 are commonly poorly defined, being more or less covered with shrubs; 

 their surfaces, too, are commonly uneven and irregular. In substance, they 

 are usually composed of tenacious greenish clay of the same character as 

 the sediments now forming in many large lakes. Usually the deserts of 

 this character occupy nearly the entire breadth of an ancient lake-bed, 

 and are overlaid by playas proper in their lowest de^^ressions. In a deep 

 section of such a playa the light-colored saline clays, of which they are 

 almost invariably composed, would be found to pass into the more tena- 

 cious and darker clays beneath. The strata at the base of such a section 

 were deposited in a deep lake of broad extent, while the playa-beds proper 

 are the record of frequent desiccations. 



