Till- olMCIN OF MIM-:i{.\I.S 



265 



place (In- gypsum deposit, as gypsum is trail-formed to tin- anhy- 

 o 1 roiis sulphite as concentration advance.-, being induced by a 

 rai>e nf temperature and ;i concentration of sodium chloride. On 

 top of the calcium sulphate, halite or sodium chloride is deposited, 

 :uid when tiually the mother lirines reach t he point of saturation for 

 the more soluble sulphate and chloride of magnesium) then double 

 salts are precipitated, as at Stassfurt in Prussia, where some thirty 

 species of minerals have separated from a concentrating brine. 

 Hock salt and gypsum or anhydrite are constant companions, their 

 positions indicating their separation from concentrating brines, 

 though deposits of gypsum will occur without the salt, or it is often 

 interbedded with clay and salt, indicating periods of changed con- 

 ditions in the concentration of the mother brine caused by an 

 influx of the sea water or by periodic additions of a dilute solution. 

 Often the concentration has never reached the stage when rock salt 

 is deposited, and in such cases the deposit of calcium sulphate will 

 exist unassociated with the usual stratum of halite ; or again the 

 salt may have been entirely carried away in solution by the ground 

 water, as salt springs are not uncommon. The minerals of saline 

 deposits include borates, carbonates, chlorides, nitrates, and double 

 salts with various amounts of water of crystallization. They are 

 all quite soluble in water, and as concentration of natural solutions 

 is favored only in very dry climates, they are all characteristic of 

 arid regions. 



