Ch. X.] 
DIKES OK SOMMA. 
121 
from an ordinary limestone altered by heat and volcanic va- 
pours. 
Carbonate of lime enters into the composition of so many of 
the simple minerals found in Somma, that M. Mitscherlich, 
with much probability, ascribes their great variety to the action 
of the volcanic heat on subjacent masses of limestone. 
Dikes of Somma, — The dikes seen in the great escarpment 
which Somma presents towards the modern cone of Vesuvius 
are very numerous. They are for the most part vertical, and 
traverse at right angles the beds of lava, scoria?, volcanic 
breccia, and sand, of which the ancient cone is composed. They 
project in relief several inches, or sometimes feet, from the face 
of the cliff, like the dikes of Etna already described (see wood- 
cut No. 19), being, like them, extremely compact, and less 
destructible than the intersected tuffs and porous lavas. In 
height they vary from a few yards to 500 feet, and in breadth 
from one to twelve feet. Many of them cut all the inclined beds 
in the escarpment of Somma from top to bottom, others stop 
short before they ascend above half way, and a few terminate 
at both ends, either in a point or abruptly. In mineral com- 
position they scarcely differ from the lavas of Somma, the rock 
consisting of a base of leucite and augite, through which large 
crystals of augite and some of leucite are scattered *. Exam- 
ples are not rare of one dike cutting through another, and in 
one instance a shift or fault is seen at the point of intersection. 
We observed before f , when speaking of the dikes of the 
modern cone of Vesuvius, that they must have been produced 
by the filling up of open fissures by liquid lava. In some ex- 
amples, however, the rents seem to have been filled laterally. 
* Consult the valuable memoir of M. L. A. Necker, Mem. de la Soc. de Phys. 
et d'Hist. Nat. de Geneve, tome ii. part i., Nov. 1822. 
f Vol. i. chap. xx. 
