OF COMMON SALT. 



14? 



up again, we would expect to find it with rock salt, if the 

 latter were of marine origin. As it happens, we do find it. 

 Gypsum is always present with rock salt. They go together 

 so much as a matter of course, that even the unobservant 

 Afghans have taken notice of the circumstance, and have 

 christened gypsum, " the brother of rock salt." This rela- 

 tion of gypsum to rock salt will have to be gone into, more 

 particularly, when reviewing the objections to the theory of 

 the aqueous origin of rock salt. 



Bock salt and the gypsum associated with it are often 

 stained, by colored clays, a variety of hues, of which, however, 

 pink or some shade of red, is the most common. Common 

 salt, manufactured from the sea, with indifferent care as 

 to cleanliness, is much stained in the same colors. It is 

 remarkable, too, that a pink scum forms on the surface of 

 the salt water at many salt manufactories, staining the salt 

 a pink color ; so that the produce of some salt works may be 

 known by this tinge, even as the produce of some salt mines 

 is known by its red shade. It would be useless here to enter 

 into a discussion as to the nature of this coloring matter ; it 

 is either organic, or an oxide of iron. It may be one or the 

 other, according to circumstances. 



We now come to the crystalline structure of rock salt, and 

 here we touch the edge of the difficulty regarding the aqueous 

 theory of its formation. It is objected to this theory that the 

 homogeneous, transparent, crystal, structure of the rock is 

 unlike any product of the evaporation of salt water as we 

 know it. Such indeed is the case, but we have no large 

 inland salt sea, evaporating, to compare with at the present 

 time, except the Dead Sea, in which the process is not 

 sufficiently advanced for our purpose. There are, however, 

 some inland salt lakes from which we may take a lesson 

 with advantage, Lake Oroomiah, in Persia, and the Elton 

 Lake, in the lowest part of the great Aralo- Caspian plain, 



