Ch. X.] MINOR CONES OF CAMPANIA. 125 



The same phenomenon, observes M. Necker, may readily be 

 exhibited on a smaller scale, if we detach a piece of liquid lava 

 from a moving current. The fragment cools instantly, and we 

 find the surface covered with a vitreous coat, while the interior, 

 although extremely fine grained, has a more stony appearance. 



It must, however, be observed, that although the lateral por- 

 tions of the dikes are finer grained than the central, yet the 

 vitreous parting layer before alluded to is extremely rare. This 

 may, perhaps, be accounted for, as the above-mentioned author 

 suggests, by the great heat which the walls of a fissure may 

 acquire before the fluid mass begins to consolidate, in which 

 case the lava, even at the sides, would cool very slowly. Some 

 fissures, also, may be filled from above ; and in this case the 

 refrigeration at the sides would be more rapid than when the 

 melted matter flowed upwards from the volcanic foci, in an 

 intensely-heated state. 



The rock composing the dikes of Somma is far more com- 

 pact than that of ordinary lava, for the column of melted matter 

 in a fissure greatly exceeds an ordinary stream of lava in weight, 

 and the great pressure checks the expansion of those gases 

 which give rise to vesicles in lava. 



There is a tendency in almost all the Vesuvian dikes to 

 divide into horizontal prisms, which are at right angles to the 

 cooling surfaces *, a phenomenon in accordance with the for- 

 mation of vertical columns in horizontal beds of lava. 



Minor cones of the Phlecjrcean Fields. In the volcanic dis- 

 trict of Naples there are a great number of conical hills with 

 craters on their summits, which have evidently been produced 

 by one or more explosions, like that which threw up the Monte 

 Nuovo in 1538. They are composed of trachytic tuff, which 

 is loose and incoherent, both in the hills and, to a certain depth, 

 in the plains around their base, but which is indurated below. 

 It is suggested by Mr. Scrope, that this difference may be 

 owing to the circumstance of the volcanic vents having burst 

 out in a shallow sea, as was the case with Monte Nuovo, where 

 there is a similar foundation of hard tuff, under a covering of 

 * See wood-cut No. 25. 



