i818.1 ON THE FORMATION OF ROCKS, S2^ 



as at St. Vincent, and the other small volcanic islands 

 of the West Indies. These Cinder eruptions throw 

 cut large quantities of rocks half roasted, that have all 

 the appearance of primitive rocks ; some like granites, 

 otiiers like gneiss, and some hornblend and feld-- 

 spar rocks, neatly crystallized and brilliant, having 

 all the feldspar half vitreous. There is a great similarity, 

 both in structure and appearance, in the roasted rocks, 

 thrown out with the Cinders, in the environs of Roihe^ 

 and those thrown by different eruptions in the West 

 Indies. When such beds of Cinders have lain long 

 exposed to the weather, the greatest part wasli away^ 

 and the remainder become earthy and lose most of 

 the characters of volcanic productions. 



Pumicestone is of volcanic origin, produced by the 

 interference of a medium that is a good conductor of 

 heat, such as water ; it is generally found on islands ; 

 and attends most submarine eruptions. Most of the 

 Pumice of commerce comes from the islands of Lipari; 

 it is likewise abundant in the West India islands ; 

 and generally near the sea. The rapid cooling of the 

 melted glass, before the elastic fluids are disengaged, 

 seems to be necessary to the formation of Pumice- 

 stone. In an extensive field of volcanic productions at 

 the Cape du Gat, in Spain, the Pumice joins to the 

 Pearlstone and Obsidian, and appears to be the out-* 

 side, while the Pearlstone and Obsidian occupy the in- 

 terior, and have l)een subject to a more gradual cool- 

 ing. At the Cape du Grat, vast excavations were made 

 by the Romans, in .scarcii of gold, according to tlic opi^ 



