176 



FORMATION OF ROCK-SALT. 



beds of gypsum, there was a thin stratum of carbonaceous matter, 

 which soiled the fingers like coal smut ; this is the only instance of 

 carbonaceous matter found in gypsum that I am acquainted with. 



Transparent, colourless rock-salt consists of muriate of soda, near- 

 ly in the highest state of purity ; or, according to Sir. H. Davy, of 

 chlorine and sodium. It has so little water of crystallization, that it 

 scarcely decrepitates when thrown on burning coals, in which it dif- 

 fers from salt prepared artificially by evaporation. Specimens of 

 rock-salt brought from the Polish mines, are less disposed to deli- 

 quesce, than those from Cheshire. The deep red colour, very com- 

 mon to rock-salt, is derived from the oxide of iron. Rock-salt in 

 detached masses frequently lies imbedded in clay or marl ; the clay 

 is often much impregnated with salt, which is extracted from it by so- 

 lution in water. The almost constant occurrence of sulphate of lime 

 (gypsum) with rock-salt, is also a fact of considerable intererest. It 

 is curious to observe the two most powerful acids, the sulphuric and 

 muriatic, so nearly associated in the same place. This fact, in a 

 more advanced state of science, may elucidate the chemical changes 

 which have effected the formation of these minerals. 



The most natural hypothesis respecting the formation of rock-salt, 

 at least in some situations, is that before stated, which attributes it to 

 the gradual evaporation of lakes and pools of salt water, which re- 

 mained, when the ocean retired from the present continents. This 

 mineral, by slow evaporation, would be separated from the impure 

 salts that exist in sea water ; and as these salts are more deliquescent 

 than rock-salt, they might be washed away, before the beds of rock- 

 salt were covered with earthy strata. 



The occurrence of anhydrous gypsum with rock-salt, which is 

 also anhydrous, would, however, indicate the action of heat, in the 

 formation of these minerals, for it is scarcely possible to conceive 

 any mode of aqueous deposition, that could form anhydrous gypsum: 

 but common gypsum might be fused by heat, and its water of crys- 

 tallization expelled ; it would then be converted into anhydrous gyp- 

 sum. From the observations of M. Carpentier at Bex, it appears, 

 that the great beds of gypsum associated with rock-salt, when they 

 are laid open to the atmosphere, are always found to be anhydrous, 

 but they soon absorb water, and are converted into common gypsum. 

 The saliferous gypsum in other parts of the Alps, is also anhydrous; 

 and if it should appear that the beds of gypsum associated with rock^ 

 salt in other countries are anhydrous, where they have not been ex- 

 posed to the action of moisture, it would add much probability to the 

 opinion, that the consolidation of rock-salt and' gypsum had been ef- 

 fected by heat. 



Before concluding the account of the red marl and sandstone for- 

 mation, it may be proper to state, that foreign geologists contend for 

 the existence of a red sandstone over coal, which is laid conformably 



