134 WALKS AND TALKS. 



bottom and is raked from the kettle, drained and placed in 

 bins. The water remaining is called "bitterns," in conse- 

 quence of other bitter and nauseous substances still remaining. 

 Should the evaporation be further continued, Epsom salts 

 (magnesium sulphate) would be thrown down in needle-shaped 

 crystals. Finally, the chlorides of calcium and potassium 

 possess such affinity for water, that they could only be sepa- 

 rated completely by bringing the residue to a red heat. 



This order of precipitation possesses much geological in- 

 t:rest. In some salt formations, as that of the Salina group, 

 the same order of succession has been noted. At the bottom 

 we find red ferruginous clays. Above are gypseous clays and 

 often, beds of pure gypsum. Next occurs brine or salt. 

 Above all are found limestones still retaining the needle-shaped 

 cavities from which the crystals of Epsom salt have been dis- 

 solved, or in which crystals of some other substance have been 

 deposited as pseudomorphs minerals having the crystalline 

 forms which characterize other minerals. This succession 

 observed in nature is a confirmation of the theory of origin of 

 salt formations by evaporation of gulfs and bays. It is evi- 

 dent, however, that such order of deposit can not generally be 

 observed as one single circuit ; because irregular irruptions of 

 sea-water, alternating with floods from land and periods of dry 

 weather, must break up any continuous succession from begin- 

 ning to end of the history of a salt basin, and must lead to 

 many repetitions of strata of the various kinds. This, how- 

 ever, is the fact universally observed, that all salt formations 

 are characterized by the presence of all or nearly all the sub- 

 stances found existing in sea-water. Gypsum, especially, is 

 always associated with brine and salt ; and that is the reason 

 the two have to be discussed together. Other substances, 

 found equally in sea-water and natural brines, are magnesia, 

 potash, bromine, and iodine. 



