38 WYNNE: TRANS-INDUS SALT REGION, KOHAT DISTRICT. 



up to the tertiary, as in Europe (among the eocene rocks of the Pyrenees,, 

 and in Germany), as well as here, or above the eocene, as in Persia.* 



From the way in which the subject is treated by Lyell, Jukes, Page, 

 and other authors, it would appear that modern writers on geology 

 incline to the belief that rock-salt has been accumulated by the evapo- 

 ration of salt-water or the sea, under favourable circumstances, some of 

 which may be taken to mean such as would enable apparently natural 

 consequences of the evaporation to become reversed — the deposition of 

 sulphate of lime (gypsum), which would ordinarily be thrown down first, 

 being delayed till precipitation of the- salt was accomplished — because 

 usually when the two rocks are present (as in this district) the gypsum 

 seen overlies the salt. 



The unascertained influence of high temperature in producing this 

 result is alluded to by Page, who says that from the connexion of many 

 of these deposits with axes of elevation, it is more than probable igneous 

 action had to do with their formation. It is known that salt (chloride 

 of sodium) is very slightly more soluble in hot water than in cold, but 

 heat has a strong effect on the solubility of gypsum, which forms an 

 exception in being so much less soluble in hot than in cold water, that 

 bv heating a cold aqueous solution containing the mineral the latter 

 will become insoluble and be precipitated. Thus, heat which would not 

 retard the solubility of salt, nor affect it much in either way, may pos- 

 sibly have come into operation to hasten the deposition of the gypsum 

 and so influence its distribution. 



The great difficulty as to superposition of this latter rock might 

 be lessened if it could be supposed that mechanical destruction and 

 recomposition of previously existing gypsum overlaid by salt had taken 

 place. Most cases of such recomposition in stratified deposits tend to 

 transfer the newer rock on removal to situations beneath the materials 

 derived from the older, this being later acted upon. Instances of the 



* Loftus, in Quar. Jour., Geol. Soc, Lond., Vol. XI, p. 334, &c. Blanford, ibid, 

 Vol. XXIX, p. 501. 

 ( 142 ) 



