368 PRINCIPLES OF STRATIGRAPHY 



a drainless basin it is evident that the salt will be deposited on 

 evaporation of the water, and that such deposits are limited in 

 thickness only by the depth of the basin and the continuation of 

 erosion of the tributary area. Salts deposited in this manner will, 

 of course, be free from marine organic remains, though terrestrial 

 organisms or brine animals may be enclosed. The area tributary to 

 this basin will show extensive erosion during the period of forma- 

 tion of the salt deposits, while the salt beds themselves rest on the 

 formation elsewhere eroded, and will be covered by a formation 

 which elsewhere rests on the eroded surface of the formation below 

 the salts, or on an earlier one. 



Besides the segregation of the salts in desert regions through 

 streams, due credit must be given to the wind. Where rock sur- 

 faces are covered with an efflorescence of salt, the dry wind may 

 sweep it away and redeposit the salt particles mixed with clay or 

 dust. In rare cases large particles of salt may be picked up by the 

 wind and dropped elsewhere, as in the case of the remarkable salt 

 hail which fell near the Lucendro bridge of the Gotthard road on 

 August 30, 1870, at II A. M. (Kengott, quoted by Walther-60. ) 

 Saline clays may be deposited at any altitude by the wind, while 

 playa surfaces may be dusted over with a layer of salt particles, 

 which adhere to the surface rendered damp by the presence in the 

 clay of hygroscopic salts. 



Shallow salt lakes or playas may be dried by a covering of 

 desert sands, which will sponge up the mother liquor and so leave 

 only the pure salt behind. On reaching the surface of the covering 

 sand dunes through capillary action, the efflorescing salts may be 

 scattered far and wide by the winds. Desert salts are probably, as 

 a rule, free from the potash salts. 



These mother liquor salts crystallize, as a rule, only at very 

 high or very low temperatures. Thus kieserite (MgS04.H20) 

 crystallizes out in the laboratory only on evaporation of the solution 

 at temperatures above 100° C., while a solution of carnallite 

 (KMgCl3.6H20) first loses its water at a temperature of 120° C. 

 This same substance, however, has been found to crystallize out 

 at a low winter temperature, such as can readily be produced in 

 continental areas, or where snow is mixed with the brine. 



Sources of Calcium Sulphate. 



Gypsum, like salt, is deposited from sea water direct, or from 

 concentrated lake waters or in playas. It may be said to have the 



