326 POMA1N XII. VOLCANIC. 



Etna ; it may even be said that porphyry is the 

 essential base of almost all the lavas of that vol- 

 cano ; that it chiefly characterises the productions 

 of Etna, and distinguishes it from other volcanoes, 

 where in general porphyries are more rare. 



" The size, number, and form of the crystals 

 of felspar, and the colour of their base, will dis- 

 tinguish the varieties of this species ; but I shall 

 not consider as varieties, the accidents of the 

 fractures, which, according to their direction, of- 

 fer inequalities in the form and size of the felspar, 

 especially when the crystals are very much flat- 

 tened, and resemble a piece of money. 



" Felspar is not always solitary in these lavas, 

 it is often accompanied with black schorl, and 

 sometimes chrysolites ; both these substances are 

 equally found in some antique porphyries. 



" The base, or ground of all these porphyritic 

 lavas resembles those simple lavas described in 

 the first species : some, however, are more sub- 

 ject to be inflated, and have a more vitreous grain ; 

 besides the felspar is never altered in its form, or 

 organisation, only sometimes it is a little cracked. 

 It is generally observed that the more the lavas 

 have undergone 3, violent action of fire, the whiter 

 the felspar has become ; an effect which may be 

 produced by exposing green porphyry to the fire, 

 or antique serpentine, in which the base becomes 



