THE ROCK-SALT FAMILY. 789 



dration occurs, and thus gypsum passes into anhydrite, with a decrease of 

 volume of 38 per cent. Anhydrite is then a result of the dehydration of 

 gypsum, often in the zone of anamorphism. But it is to be remembered that 

 after a deposit of gypsum has been changed to anhydrite in the lower zone, 

 in order that it shall reach the surface by denudation it must pass through 

 the upper zone, and under these conditions hydration occurs to a greater 

 or less extent, and it is thus transformed to gypsum in part or in whole. 

 At various localities the beds of calcium sulphate are in part composed of 

 gypsum and in part of anhydrite. Doubtless in places where the deposits 

 are deep the change toward anhydrite is taking place, and in other places 

 near the surface the change toward gypsum is taking place. 



CHLORIDE ORDER. 



ROCK-SALT FAMILY. 



Rock salt, impure sodium chloride,' or simply salt, is the only abundant 

 chloride. But potassium chloride, calcium chloride, and magnesium chlo- 

 ride are found as impure compounds. The sodium is derived mainly 

 from the silicates. It is liberated from these compounds in the zone 

 of katamorphism, mainly in the belt of weathering, by the process of 

 carbonation. The chlorine is more difficult to account for. Of the min- 

 erals in the original rOcks, only sodalite, wernerite, and apatite contain 

 chlorine. But sodalite is abundant in the soda rocks, and the last two 

 minerals are very widespread. Therefore these minerals are an impor- 

 tant source of chlorine. It is well known that hydrochloric acid and 

 other chlorine compounds are emitted in large quantities in connection 

 with volcanic action. In this connection it should be noted that at 

 periods of regional volcanism the quantity of chlorine emitted is vastly 

 greater than during periods of local volcanism, like the present." It has 

 been supposed that emissions of chlorine during volcanism are evidence 

 that sea waters from the surface, or the salts of underground solutions, 

 have penetrated to the roots of volcanoes, and thus contributed chlorine 

 to magmas. Doubtless this is true in some instances, but it can not 

 be assumed from occasional cases of this kind that the chlorine very 

 generally emitted in volcanic regions is thus derived from the zone of 



"Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11, 1898, 

 pp. 496-497. 



