OF VOLCANOS. 227 



at different epochs, in which the different parts of the exa- 

 mination are all truly comparable with each other. We 

 learn from it that the margins of craters are a phenomenon 

 of far more permanent character than had been previously 

 inferred from passing observations, and this not only where 

 (as in the Peak of Teneriffe, and in all the volcanos of the 

 chain of the Andes,) they are visibly composed of trachyte, 

 but also elsewhere. According to my last determinations, 

 the north-west edge of Vesuvius has, perhaps, not altered 

 at all since the time of Saussure, an interval of 49 years ; 

 and the south-eastern side, on the side towards Bosche Tre 

 Case, which, in 1794, had become 400 French (426 English) 

 feet lower, has since then hardly altered 10 toises (60 French 

 or 64 English feet). 



If the public journals, in describing great eruptions, often 

 state the shape of Vesuvius to have undergone an entire 

 change, and if these assertions appear to be confirmed by 

 picturesque views sketched at Naples, the cause of the error 

 consists in the outlines of the margin of the crater having 

 been confounded with those of the cones of eruption acci- 

 dentally formed in the middle of the crater on its floor or 

 bottom which has been upheaved by vapours. Such a cone 

 of eruption, consisting of loosely heaped-up rapilli and 

 scoriae, had in the course of the years 1816-1818 gradually 

 risen so as to be seen above the south-eastern margin of the 

 crater; and the eruption of the month of February 1822 

 augmented it so much, that it even became from 100 to 110 

 (about 107 to 117 English) feet higher than the north- 

 western margin of the crater (the Rocca del Palo), This 



