444 DOMAIN XII. VOLCANIC. 



pitated by a rapid declivity, and suddenly frozen. 

 There are several currents, one above another ; 

 their thickness varying, in the same current, 

 from one foot to twelve. 



"Some of these substances are .compact; 

 others are so porous that they resemble froth, 

 and float on water. In the cavities of some are 

 observed capillary threads perfectly vitrified. 



" As the volcanoes of Lipari have ceased to 

 be active, even before the times of history, these 

 glassy substances must have existed more than 

 3000 years ; and they have not undergone the 

 least alteration. 



" All volcanoes do not produce these vitreous 

 substances : they are extremely rare in the ejec- 

 tions of Etna, as well as in most countries of 

 Europe. 



France. " Faujas only found obsidian in one place in 

 France ; at Chenavari near Rochemaure, in Vi- 

 varais; and there were but three pieces which 

 he collected. It is an amel, perfectly black, with 

 rounded vesicles of about half a line diameter. 

 Iceland. " The volcanoes of Iceland are very prolific in 

 vitreous substances; and what is improperly 

 called Iceland agate, is a volcanic amel, of a fine 

 black, almost free from pores, and susceptible of 

 a perfect polish. 



" Thepiedra de Galinazzo, regarded by Cay- 



