226 



but that it indicates only a certain state, a ca- 

 pillary or fibrous form, under which several sub- 

 stances thrown out by volcanoes are seen. The 

 nature of these substances is as different as the 

 thickness, the tenacity, the flexibility, the paral- 

 lelism, or the direction of their fibres. We may 

 consequently doubt, whether pumice ought to 

 hold any place in a system of oryctognosy ; or 

 whether, like compound rocks, they do not ra- 

 ther belong to the domain of geognosy. I have 

 seen black pumice stones, in which augit and 

 hornblende are easily recognised ; they are less 

 light, of & spongy texture, and rather cellular 

 than fibrous. We might be tempted to think, 

 that these substances owe their origin to basal- 

 tic lavas, I have observed them in the volcano 

 of Pichincha, as well as in the tufa of Pausilippo, 

 near Naples. Other pumice stones, and these 

 the most common, are of a grayish white, or of 

 a blueish gray, with numerous parallel fibres, and 

 containing vitreous feldspar and mica. The 

 greater part of the pumice stones of the iEolian 

 islands^ and those I Collected at the foot of the 

 Volcano of Sotara, near Popayan, belong to this 

 class* They seem to have been originally gra- 

 nitic rocks, as Dolomieu first recognised in his 

 voyage to the islands of Lipari *. Assembled 

 in enormous blocks, they sometimes form whole 



* Dolomieu, Voy. aux Iks de Lipuri, p. 67, Id. Man, surles 

 lies Ponces, p. 89. 



