128 M. Dufrenoy on the Volcanic Formations 



these blocks are not found exclusively on Vesuvius: we have 

 collected them in the tuff of Pausilippo ; and it appears that 

 they exist in the tuft' of the Ponza Islands. 



Trachytes of the Phlegrcean Fields. 



6. The hills of the Phlegraean Fields are composed of pu- 

 mice-tuff*; but in the centre of some of them, there are little 

 elevations of trachyte, round which the beds of tuff" are ar- 

 ranged. From the relative position of the pumice-tuff' and the 

 trachyte in these hills, it appears certain that the latter rock is 

 more modern than the former, and that the relief of the Phle- 

 graean Fields was caused by the appearance of the trachyte. 

 The direction of the beds also leads us to think that these tra- 

 chytes have been produced at the same time that the elevation 

 of the granites of the Alps took place. 



7. Monte Epomeo, in the Island of Ischia, also owes its ele- 

 vation to the trachyte of the second period. 



On Vesuvius, 



8. Vesuvius is composed of two distinct portions, Somma 

 and Vesuvius. These two parts have been produced by causes 

 of a diff'erent order. The Somma forms round Vesuvius a zone 

 of abrupt escarpments, the parallel masses composing which 

 rise on all sides towards the centre : it is the result of a general 

 upraising, which has elevated circularly its component paral- 

 lel masses, previously horizontal. Vesuvius is the product of 

 partial eruptions and upraisings. The position of the cone 

 of Vesuvius, in the centre of the crater of elevation (soulevement) 

 of the Somma, might make us presume that an intimate con- 

 nection exists between the two mountains ; but they belong to 

 periods separated the one from the other by several great phe- 

 nomena, which have occurred in the following order : 



a, Formation of the lavas of the Somma in horizontal 

 masses. 



6, Deposition under the sea of beds of pumice-tuff" in ho- 

 rizontal beds. 



c, Soulevement or upraising of the Somma at the epoch of 



the formation of the Phlegraean Fields, 



d, Formation of the cone of Vesuvius in the year 7i). 



