86 CHEMISTRY OF DOLOMITES AND GYPSUMS. [VIII. 



being the less soluble, is first deposited in the form of gyp- 

 sum, while the magnesian carbonate is only separated after fur- 

 ther evaporation ; when, provided the supply of bicarbonate of 

 lime still continues, the two carbonates may fall down in a state 

 of intermixture. In this way sediments will be formed con- 

 taining the elements of dolomite or of magnesite. 



The solution of magnesian bicarbonate remaining after the 

 deposition of gypsum from the solution possesses, as we have 

 seen, the power of decomposing chloride of calcium, and, when 

 deprived of a portion of its carbonic acid by evaporation, reacts 

 in a similar manner with a solution of sulphate of lime. 

 In this way, an influx of sea-water into the basin from 

 which gypsum, and perhaps a portion of magnesian carbonate, 

 has already been deposited, would give rise to a precipitate of 

 carbonate of lime, like the tufaceous limestones whose occur- 

 rence with gypsum and dolomites has been already noticed. 

 In basins which, like the salt-lagoons of Bessarabia on the 

 shores of the Black Sea, receive occasional additions of sea- 

 water, and deposit every summer large amounts of salt (Bischof, 

 Lehrbuch, II. 1717), the influx of waters containing bicarbonate 

 of lime would give rise to the formation of beds of gypsum, 

 alternating with dolomites or magnesian marls and rock-salt. 



We have already referred to the analyses of certain rivers in 

 which the sulphates are more abundant than the chlorides. 

 Thus, in the Rhine, near Bonn, according to Bischof, we have 

 for 100,000 parts of the water, 17.08 of solid matters, of which 

 1.23 are sulphate of lime, and 1.81 sulphate of magnesia, with 

 only 1.45 of chloride and 8.37 of carbonate of lime ; in the 

 Danube, near Tienna, the predominance of sulphates is still 

 more marked. The waters of the Arve, in the month of Feb- 

 ruary, gave to Tingry, for 100,000 parts, 24.5 of solid matt.-rs, 

 of which 6.5 were sulphate of lime, 6.2 sulphate of iii;i t u'H"si;i, 

 and 8.3 carbonate of lime, with only 1.5 of chlorides. Now, 

 as in river-waters there is always present an excess of carbonic 

 acid, and as bicarbonate of lime and sulphate of magnesia in 

 solution are mutually decomposed, these waters, which are to be 

 regarded as solutions of sulphate of lime and bicarbonate of 



