59 



Saliferous Deposits. 



No satisfactory hypothesis has yet been advanced to account 

 for the occurrence of rock-salt (muriate of soda) *■ Some have 

 endeavoured to maintain that it was deposited by the ocean like 

 the sedimentary rocks. The strata above the salt contains organic 

 remains, as do also those below, but the salt contains none. The 

 entire absence of marine exuviae from the strata of salt shows that 

 such rocks could not have been deposited in the sea ; and it would 

 be difficult to conceive how such a very soluble substance could 

 crystallize in a large body of water like the sea. Others think 

 that it was produced by heat. Not only are salt rocks always 

 found impregnated with water, but they sometimes inclose sub- 

 stances incompatible with the action of heat. 



Crystals of salts can only be produced from aqueous solvents, 

 and although heat is employed to expedite the process in arti- 

 ficial works, it is not essential for the formation of the crystals, 

 as a crystalline mass formed from solution is effected by polar 

 action alone at common temperature. When salts are crystal- 

 lized, they frequently retain a portion of the solvent not me- 

 chanically mixed with them, but as an essential component, 

 to which their regularity of figure and colour is in some instances 

 referrible. Thus Glauber's salt contains a great quantity of 

 water, and crystallizes in six-sided prisms, transparent and beau- 

 tiful ; expose them to heat, the water of crystallization flies off, 

 the crystals lose their shape, and crumble down into white 

 powder. Crystals of gypsum are of glossy transparency, and of 

 regular figure ; this is due to water : heat them, they crumble 

 into a white powder, well known as plaster of Paris. Such is 

 the aqueous nature of all natural crystals as well as salts : hence 

 we have abundant reason for assuming that crystalline masses 

 have been gradually formed out of aqueous solutions of the 

 substances permeating the rocks. 



It was once supposed that the deposit of salt, or muriate of 

 soda, was confined to a particular series in the order of the se- 

 dimentary rocks ; but the fact is, that salt springs are found to 



* Salt, in the common acceptation of the word, implies culinary salt ; but it 

 denotes a variety of substances, totally distinct from common salt. Thus marble 

 is a salt ; so is pearlash, blue vitriol and plaster of Paris. 



