IX.] CHEMISTRY OF NATURAL WATERS. 117 



37. The waters of the first class contain, besides chloride 

 of sodium and a little chloride of potassium, large quantities of 

 the chlorides of calcium and magnesium, amounting together, 

 in several cases, to more than one half the solid contents of the 

 water. Sulphates are either absent, or occur only in small 

 quantities, and the same is true of earthy carbonates. Salts of 

 baryta and strontia are sometimes present, while the propor- 

 tions of bromides and iodides, though variable, are often con- 

 siderable. 



In the large amount of magnesian chloride which they con- 

 tain, these waters resemble the bittern or mother-liquor which 

 remains after the greater part of the chloride of sodium has 

 been removed from sea-water by evaporation. The bitterns 

 from modern seas, however, differ in the constant presence of 

 sulphates, and in containing, when sufficiently concentrated, 

 only traces of lime. The reason of this, as already pointed out 

 in 22, is to be found in the fact that in the waters of the 

 present ocean the sulphates are much more than equivalent to 

 the lime, so that this base separates during evaporation as 

 gypsum.* But as shown in 23 and 24, the waters of the 

 ancient seas, which held in the form of chloride of calcium 

 the greater part of the lime since deposited as carbonate, must 

 have yielded by evaporation bitterns containing a large pro- 

 portion of chloride of calcium. Such is the nature of the brines 

 whose analyses are given above, and such we suppose to have 

 been their origin. The complete absence of sulphates from 

 many of these waters points to the separation of large quantities 

 of earthy sulphates in the Cambrian strata from which these 

 saline springs issue ; and the presence in many of the dolo- 

 mitic beds of the Calciferous sand-rock of small masses of gyp- 

 sum abundantly disseminated is an evidence of the elimination 

 of the sulphates by evaporation. The frequent occurrence of 

 crystalline masses of sulphate of strontian in the Chazy and 

 Black River limestones of this region is also to be noted as 

 another means by which the sulphates were separated from the 

 waters of the palaeozoic seas. From the proportions of chloride 

 * See further on this point, Bischof, Chem. Geology, I. 413. 



