VIII.] CHEMISTRY OF DOLOMITES AND GYPSUMS. 87 



magnesia, would, by tlieir evaporation, yield gypsum and 

 magnesian carbonate, which would appear as portions of a 

 fresh-water formation, like those of Aix and Auvergne. 



The similar decomposition of soluble sulphates by bicarbon- 

 ates of baryta and strontia will explain the formation of heavy 

 spar and celestine, and their frequent association with gypsif- 

 erous rocks. 



As to the native sulphur which is often associated both with 

 epigenic and sedimentary gypsums, it has doubtless in every 

 case been formed, as Breislak long since indicated, by the de- 

 composition of sulphuretted hydrogen. It is well known that 

 alkaline and earthy sulphates are reduced to sulphurets by or- 

 ganic matters, with the aid of heat, or even at ordinary temper- 

 atures, in presence of water. To the decomposition of these 

 sulphurets by water and carbonic acid we are to ascribe not 

 only the sulphuretted hydrogen of solfataras (which, by its 

 oxidation under different conditions, gives rise either to free 

 sulphur or to sulphuric acid and to gypsum by epigenesis), but 

 also the sulphuretted hydrogen which appears in springs and in 

 stagnant waters, where the sulphur produced by the decompo- 

 sition of the gas is often mingled with sedimentary gypsums.* 

 (Bischof, Lehrbuch, II: 1 39 - 1 85.) Bischof has also suggested 

 the decomposition of chloride of magnesium by alkaline or earthy 

 sulphurets as a source of sulphuretted hydrogen and hydrate of 

 magnesia, into which sulphuret of magnesium is readily resolved 

 in the presence of water. (Chemical Geology, I. 16.) If a salt 

 of calcium were present, this reaction could only take place in 

 the absence of carbonic acid, for carbonate of magnesia is incom- 

 patible with chloride of calcium. The direct reduction and 

 decomposition of sulphate of magnesia by organic matter and 

 carbonic acid may, however, yield sulphuretted hydrogen and 

 carbonate of magnesia, and thus, in certain cases, give rise to 

 magnesian sediments. 



In the preceding sections, we have supposed the waters 

 mingling with the solution of sulphate of magnesia to contain 



* On certain modes of decomposition of the sulphates, see Jacquemin, Comp- 

 tes Rendus, June 14, 1858. 



