194 M. Von Buch on Volcafios and Craters of Elevation. 



regard it as a model of this volcanic form, and nothing is visible 

 in this mountain which indicates a resemblance to a real volcano, 

 or which is similar to a real stream of lava. Not only are the 

 beds of leucitophyre, of which it consists, spread over a great 

 part of the circumference, as we now see it, but they are inclined 

 to the exterior, with angles varying from 20° to 30°, without 

 any diminution of their often considerable magnitude — a state 

 of things completely at variance with the phenomena presented 

 by streams of lava having so high an inclination. But the ele- 

 vation of the whole of the vast mountain, in its full extent, is 

 proved in a still more striking manner by the mode in which the 

 Neapolitan tufFa is disposed round the declivities. The tuffa is 

 a white porous rock, composed chiefly of pumice, and extends 

 over the whole plain between the Apennines and the sea. It is 

 found from Capua to the hills of Sorrento, from Nola to be- 

 yond Naples, and almost always in horizontal strata, reposing 

 immediately one above another, and thus the surface is perfectly 

 level. These white strata approach the Somma without inter- 

 ruption, but when they reach its base they immediately ascend, 

 and follow the inclination of the acclivity, at a high angle. At 

 a certain height, which remains perfectly the same round the 

 mountain, they stop, and then we observe, rising with a high 

 inclination, the black leucitophyre beds of the walls of Somma, 

 which continue to the summit. The boundary of the tuffa 

 round the mountain is rendered distinctly visible from a distance 

 by the little platform which results from this slight difference 

 of inclination between the beds of tuffa and of leucitophyre ; 

 and indeed these relations which are of such high importance for 

 the history of the whole, are beautifully exhibited, and in a man- 

 ner as clear as it is picturesque, in the superb view of the vol- 

 cano, and its neighbourhood, obtained from the town of Naples.* 

 The height to which the tuffa ascends is about 1900 feet above 

 the sea, both on the acclivity on the Somma and Ottajano side, 

 and on that above Pompeii and Torre del Greco. Its limit is 

 the long hill on which the well known house of the hermit is 

 built. The upper part of the Somma, that without tuffa, rises 



• The boundary of the tuffa is well marked in the two cuts on the pre- 

 ceding page. 



