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find that the principle moil abounding in 

 thefe fix ftones is the filex ; that the alu- 

 mine is but in a moderate or fmall propor- 

 tion ; as are likewife the lime and iron 

 when they enter into the compofition of 

 thefe ftones. It is therefore evident that 

 thefe fix ftones are of the fame fpecies ; 

 nor can the infufibility of feveral pitch- 

 ftones, which are not volcanic, diftingulfh 

 them as eflentially different from the volca- 

 nic, fince we find in the genus of petrofi- 

 lices fome which fufe in the furnace while 

 others remain refradory. It is, befides, 

 not invariably true that volcanic pitch- 

 ftones are eafily fufible. Thofe of Monte 

 Muffato, of which I fhall prefently treat, 

 require a continuance of feveral days in the 

 furnace before they are completely fufed. 

 Some pitch-ftone lavas of the iflands of 

 Ponza, which are of a pale white colour, 

 and of a fmooth and fliining fradlure, after 

 continuing feveral hours in the furnace, 

 only become of a deep red colour, and fu- 

 fion does not take place till after thirty 

 hours. It is remarkable that almoft alLvol- 



canic 



