( i86 ) 



fubftances expofed to their fires. Reflect-' 

 ing on the immenfe quantities of vitrifica- 

 tions at Vulcano and Lipari, which are 

 almoft all derived from feltfpars and petro- 

 filex, the idea fuggefted irfelf to mv mind, 

 whether fo great an abundance in thefe two 

 places, and fo great a fcarcity in others, 

 might not be attributed to thefe ftones be- 

 ing here extremely abundant, and very rare 

 elfevvhere. But this fuppofition does not 

 accord v/ith fa£t ; as we have (e^n. in many 

 other volcanic tradts, which I have defcribed, 

 and fhall defcribe when I come to fpeak of 

 the Euganean mountains, that both thefe 

 ftones may be changed into lava without 

 that lava exhibiting the fiighteft appearance 

 of glafs. On the other hand we have 

 fliown, that, befides feitfpar and petrofilex, 

 many pumices have for their bafe the horn- 

 ftone and afbeilus, and many granite, as 

 M. Dolomieu has obferved. I am, there- 

 fore, of opinion that the caufe which has 

 produced them lliould rather be fought in 

 the volcanic fire, which rarely has fufficient 

 a6:ivity to vitrify the ftones and rocks on 



which 



