M. Von Buch on Volccmos and Craters of Elevation. 205 



Hoffmann, that the rock forming the beds and lava-streams of 

 that volcano is a very fine-granular mixture of augite and labra- 

 dorite — an Etna dolerite. Now, since the Vesuvian leucitophyres 

 are distinguished from this rock only by the addition of leucite, 

 and the diminution in quantity of the labradorite, it is clear that 

 these relations prove a resemblance and a connection of the 

 whole eastern part of the series of volcanic phenomena in Italy, 

 viz. of Etna, Stromboli, Vesuvius and Somma, Rocca Monfina 

 near Sessa, Monte Albano near Rome, and Monte Mario on 

 the Tuscan frontier. Trachyte and trachytic products occur, 

 on the contrary, towards the west. They form the Lipari 

 Islands, and the Ponza Islands lying far out at sea. There is 

 an essential geognostical distinction in the position of these 

 rocks. 



Another important distinction in the rocks composing volcanos 

 presents itself when albite takes the place of felspar. A rock 

 is then formed, which can no longer be named trachyte ; for it 

 is not a mere alteration of the trachyte, but is a very constant 

 compound, which invariably occurs, having entirely different re- 

 lations. In Europe it is rare ; for the Italian islands, as well as 

 the greater part of Mont d'Or and Cantal, the Siebengebirge 

 near Bonn, and also the mountains of Iceland, are composed of 

 rocks in which the real and true felspar cannot be mistaken — 

 therefore of trachyte and of trachytic masses. But this is not 

 the case on the other side of the ocean. According to Rose's 

 investigations, we may assume, with considerable certainty, that 

 not one of the almost innumerable volcanos of the Andes con- 

 sists of trachyte, but that all contain albite in their component 

 masses. So general an assertion may seem extremely bold ; but 

 it loses this appearance when we reflect, that, by means of Hum- 

 boldt''s Travels, we had already obtained information regarding 

 nearly the half of these volcanos and their products, in both 

 hemispheres. We are indebted to Meyer for our knowledge of 

 the volcanos lying to the south, viz. in Bolivia and northern 

 Chili, and which were entirely unknown until his journey. 

 Poeppig has extended this information to the most southern 

 limits of Chili. Since also it would appear from Erman's dis- 

 covery, that the northern volcanos of Schevelutsch in Kam- 



VOL. XXXI. NO. XLII.— OCTOBER 1836. P 



