790 A TREATISE ON METAMORPHISM. 



katamorphism. I believe that much of the chlorine emitted from volca- 

 noes is not thus derived, but is an original constituent of the deep-seated 

 magmas, and it follows that before the crystallization of the magmas a 

 portion of the chlorine escapes. Consequently the quantity of chlorine in 

 the crystallized igneous rocks is less than originally existed in the magmas. 

 Probably the original chlorine minerals and the emissions in connection 

 with volcanism are sufficient to account for the chlorine of the chlorides. 



Sodium chloride formed in the soil is taken into solution and is 

 transported to the sea or to lakes with no outlets, which are prevented 

 from expansion by evaporation. As the process continues salt accumulates 

 to the point of saturation, when further evaporation leads to chemical 

 precipitation, precisely as with niter. 



Sodium chloride ranks in solubility next to niter, and is abundantly 

 precipitated before the latter compound is. Deposits of rock salt are 

 forming in various desert regions at the present time by the evaporation 

 of the waters of inclosed seas, such as the Dead Sea, Great Salt Lake, etc. 

 Extensive deposits formed under similar conditions during past geological 

 periods have been buried under later formations, and these are the rock- 

 salt beds. Illustrations of such deposits are the thick rock-salt formations 

 of Poland, of central Germany, and of Louisiana. From the foregoing it 

 is plain that rock-salt deposits are strictly products of the zone of kata- 

 morphism, and mainly of the belt of weathering. 



The alterations of rock salt are those of recrystallization and solution. 

 In some cases the material has become coarsely crystalline, as in the case of 

 the Polish deposits. In order that a rock-salt formation shall be preserved 

 it is necessary that it be buried under relatively impervious formations, 

 in order that the underground circulation shall not be so rapid as to 

 dissolve the material and bring it to the surface. That solution of many 

 ancient deposits is taking place is shown by the briny waters which are 

 found at many localities. Where such ground waters issue at the surface 

 we have salt springs. Such salt springs were known in America in early 

 days as "salt licks," because the animals frequented these places in order 

 to lick the salt which was deposited. As illustrations of deposits which are 

 being dissolved and which are artificially abstracted from the earth in v the 

 form of brine, those of the Saginaw district of Michigan and the Syracuse 

 district of New York may be mentioned. 



