THE CRYSTALS OF THE SEA. 133 



become saturated with brine. This is the case with the Michi- 

 gan Salt Group and the Onondaga salines of New York, as 

 also those on the Kanawha and Ohio rivers. In some cases 

 the home rocks possess sufficient porosity to retain the brine. 

 In other localities the solid salt exists, but it is so mixed with 

 clay as to require redissolving and purification. This as I 

 said, is the case in Cheshire. In some countries, as in Poland 

 and Austria, great mines are excavated in immense salt forma- 

 tions. In Michigan the rock salt possesses great purity, but 

 it lies so deep that the expense of sinking and maintaining 

 shafts has so far, led to the expedient of dissolving the salt in 

 its place, and then pumping out the saturated brine. At 

 Marine City, water pumped from the St. Clair river is forced 

 down the bore-hole, where it dissolves the salt, and is then 

 forced out by the same process into great tanks, where the 

 brine settles, and then in other tanks undergoes evaporation 

 by means of heat from steam pipes immersed in the brine. 

 The precipitated salt is raked out by automatic rakes, allowed 

 to drain, then dried and barreled. At Syracuse and in the 

 Saginaw valley, the brine is pumped from the wells and set- 

 tled and evaporated. Formerly much evaporation was done 

 in kettles over a fire. More recently, pans and steam have 

 been employed. A large amount of salt is produced, especially 

 at Syracuse, by spontaneous evaporation in shallow vats ex- 

 posed to the sun and air, and covered in rainy weather, by 

 light roofs moved on rollers. 



The natural brines of Saginaw and other regions contain 

 impurities. In the process of evaporation those least soluble 

 are first precipitated out, and then the other substances in the 

 inverse order of their solubility. Thus the brine, which is 

 limpid and sparkling on its escape from the earth, after ex- 

 posure to the air forms, by peroxidation of the iron (see Talk 

 XXII) a red deposit which is insoluble, and falls to the bot- 

 tom of the settling vat generally hastened by some coagulable 

 substance. When transferred to the kettles and heated, 

 gypsum is the first deposit, and this adheres firmly to the sur- 

 face. Next, common salt crystallizes out, which forms on the 



