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 Phlegrcean 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. 
