( 57 ) 



common centre, and their bafes at tlie cir- 

 cumference. We find fome very large, 

 pieces of this kind, formed by the aggrega- 

 tion of thefe pyramids. 



The third fpecies is compofed of thin 

 and (hining plates, fomewhat elaftic, tranCr 

 parent, very foft, and forms the indeter- 

 minate cryftallization of fulphate of lime 

 called fpecular ftone; but this fpecies is 

 rare, and its cryftals are always very fmalL 

 In thefe cafes the determinate and primitive 

 eryftallization of this neutral earthy fait is 

 always wanting. 



It is therefore evident that this variety of 

 colours, fuch as yellow, red, or violet, ex- 

 hibited by the decompofed lavas, is a con* 

 fequence of the iron pre-exifting in them ; 

 which being, if not decompofed, at leafl 

 greatly altered, by the fulphureous acids, is 

 varioufly modified, and aflumes this diver- 

 fity of hues. The fame caufe operates, in 

 like manner, on the fulphates of lime, form- 

 ed by the combination of the fulphuric acid 



with 



