122 
NEWER PLIOCENE PERIOD. 
No. 25. 
1 
/ 
y\- 
x\ 
\A 
Dikes or veins at the Punto del Nasone on Somma. 
The reader will remember our description of the manner in 
which the plain of Jerocarne, in Calabria, was fissured by the 
earthquake of 1783 *, so that the Academicians compared it to 
the cracks in a broken pane of glass. If we suppose the side 
walls of the ancient crater of Vesuvius to have been cracked in 
like manner, and the lava to have entered the rents and become 
consolidated, we can explain the singular form of the veins 
figured in the accompanying wood-cut -f-. 
Parallelism of their opposite sides. — Nothing is more re^ 
markable than the parallelism of the opposite sides of the dikes, 
which usually correspond with as much regularity as the two 
opposite faces of a wall of masonry. This character appears at 
first the more inexplicable, when we consider how jagged and 
uneven are the rents caused by earthquakes in masses of hete- 
rogeneous composition like those composing the cone of 
Somma ; but M. Necker has offered an ingenious and, we 
think, satisfactory explanation of the phenomenon. He refers 
us to Sir W. Hamilton's account of an eruption of Vesuvius in 
the year 1779, who records the following facts. ' The lavas, 
when they either boiled over the crater, or broke out from the 
conical parts of the volcano, constantly formed channels as 
regular as if they had been cut by art, down the steep part of 
* See vol. i. chap, xxiv., wood-cut No. 22. 
f From a drawing of M. Necker, ibid. 
