IX.] CHEMISTRY OF NATURAL WATERS. 133 



found that predazzite, in virtue of the hydrate of magnesia 

 which it contains, readily decomposes solutions of gypsum 

 holding dissolved carbonic acid, and gives rise to sulphate 

 of magnesia. 



In the second place, sulphuric-acid waters, like those de- 

 scribed in 47, by their action upon calcareous and magne- 

 sian rocks, or by the intervention of carbonate of soda, may, as 

 already suggested, give rise to neutral sulphated waters of the 

 sixth class. It is evident also that waters impregnated with 

 sulphates of alumina and iron from oxidizing sulphates, as 

 mentioned in 28, may be decomposed in a similar manner, 

 and with like results. 



Neutral sulphated waters generated by . any" of the above 

 processes are evidently subject to admixtures of saline matters 

 from other sources, and may thus become impregnated with 

 chlorides and carbonates. Indeed, it is rare to find waters of 

 the sixth class without some portion of chlorides ; and a tran- 

 sition is thus presented to the waters of the first four classes, 

 in which also portions of sulphates are of frequent occurrence. 

 The presence of sulphates being one of the conditions required 

 for the generation of sulphuretted hydrogen ( 10), we find 

 that the waters of the sixth class are very often sulphurous. 



51. Waters of the sixth class are very frequently met with 

 in the palaeozoic rocks of New York and western Canada, and 

 are probably derived from the gypsum which is found -in great- 

 er or less abundance at various horizons, from the Calciferous 

 sand-rock to the Onondaga salt-group. It is, however, not 

 improbable that the sulphuric-acid waters which abound in this 

 region (48) may, by their neutralization, give rise to similar 

 springs. In the waters of the district under consideration, the 

 sulphate of lime generally predominates over the sulphates of 

 the other bases, and chlorides are frequently present in consid- 

 erable quantities. For numerous analyses of these waters, see 

 Beck, Mineralogy of New York. The results of an examina- 

 tion by me of the Charlotteville spring, remarkable for the 

 amount of sulphuretted hydrogen which it contains, will be 

 found in the Amer. Jour. Sci. (2), VIII. 369. A copious sul- 



