MODE IX. GYPSUM. 437 



a simple rock, like limestone, and sometimes 

 occurs in large masses and beds ; but sometimes 

 in layers, alternating with orsten, clay, or sand- 

 stone. It sometimes contains crystals of quartz, 

 and in rare instances arragonite and boracite. 

 It also sometimes presents native sulphur; and 

 often appears in the neighbourhood of rock salt. 

 It seldom attains a great height, but forms little 

 precipices, which, when of the purer kind, are 

 distinguished by their white lustre. Hollows, 

 like funnels, are sometimes formed in gypsum, 

 which Saussure, 1238, ascribes to the rain water, 

 which, attacking a soft part, remains, and, gra- 

 dually increasing, melts the surrounding circle. 



Gypsum is generally of a whitish yellow, or 

 yellowish brown cast; but it also occurs of an 

 ash grey colour, in which case, as Saussure has 

 observed, it can only be distinguished from lime- 

 stone by the nitrous acid producing no effect. 



One of the most remarkable gypsous hills in Montmartre. 

 Europe is that of Montmartre, near Paris, not 

 only from its producing the plaster best known 

 in commerce, but from its peculiar constructure, 

 and the singular animal remains which have 

 there been discovered. It is thus described by 

 the venerable M. Sage, whose chemical mistakes 

 may be forgiven, in the consideration of the 

 great services he has rendered to mineralogy, 



