OF COMMON SALT. 



141 



overcome before the theory can. be finally established. To 

 explain away these difficulties, and to prove that the sea is the 

 true mother of rock salt, is the task that I have set myself to 

 accomplish in this paper. 



I will now notice some of the chief physical characteristics 

 of rock salt. Its chemical composition, it will be hardly 

 necessary to state, is precisely the same as that of ordinary 

 sea salt. 



Eock salt is as unlike sea salt, as we know it under the form 

 of " bay " salt, as any two salts having the same composition 

 could well be. Rock salt, in some places, the Trans- Indus 

 range, and Cordona in Spain, for example, forms solid masses 

 swelling out of the earth for hundreds of feet in cliffs and 

 mountains ; in others, it sinks deep into the bosom of the 

 ground — a hard rock — which has to be mined with pick and 

 blasting-powder. 



It is for the most part translucent, and of a bluish or 

 pinkish color, resembling masses of dimly transparent, 

 flawed glass; yet fine blocks of colorless transparent salt 

 occur in nearly all mines. A rectangular polished block of 

 pure salt, 37 J cubic feet, weighing 2\ tons, was sent from 

 the Mayo mines to the Yienna Exhibition. Newspaper print 

 can be read, through more than six inches of such transparent 

 rock salt. It is as hard or harder than gypsum, and various 

 fancy articles, such as balls, platters, salt-cellars, etc., are 

 turned from it on the lathe. In continental mines it is 

 worked up into statuary having the light effects of alabaster. 

 It forms the pillars, stairs, and horse stalls, used in the rock 

 salt mines. 



If we examine closely, however, we find that in all its 

 other properties — chemical, physical, therapeutic, organo- 

 leptic, &c. — there is no difference between rock salt and 

 ordinary bay salt. Eock salt, like sea salt, crystallises, in 

 the first, or cubic system, as is demonstrated by its clcavago, 



