288 



CHAP. XLL 



Gypsum, Rock Salt. 



ALTHOUGH Gypsum has been ranked among- the 

 rocks, it ought rather, like Salt, to be considered an 

 occasional mineral occurring among the strata; but, 

 presenting geological features of interest, it demands 

 consideration here, though, for its characters, I may 

 refer to systems of mineralogy. It occurs both among 

 the primary and secondary rocks ; but, while common 

 in the latter, its existence in the former is so rare as to 

 have been doubted by some geologists. 



It is, however, described by Daubuisson, as forming 

 thin strata, alternating with micaceous schist, and with 

 primary limestone, near Aosta ; and by Freiesleben 

 and others, in other parts of the Alps. It has also 

 been found imbedded in graywacke slate by Von Buch 

 and Brongniart, in the Pyrenees, in Switzerland, and 

 in other places ; forming the transition gypsum of 

 geologists. Under both associations, the strata are of 

 small extent ; or the masses are so separated and 

 mixed with clay, or with the surrounding rocks, that 

 they cannot be considered as forming a stratum. 



In the secondary class, it is far more abundant, 

 either in the form of persistent strata or of imbedded 

 masses ; but is chiefly, if not exclusively, associated 

 with the red marl. Thus it is found in various parts 

 of Germany, France, England, Russia, and Spain ; 

 appearing, in all these, as in other instances, to be 

 peculiarly characteristic, like rock salt, of that parti- 

 cular deposit. They who have described two distinct 

 formations, the one superior, and the other inferior to 

 that rock, have probably generalized from local facts. 



The last geological situation in which gypsum 



