526 
'ELEMIENTS OF GEOLOGY. 
It has also been stated above (p. 206), that when we ex¬ 
amine this same region, it is found to consist largely of tufa- 
ceous strata, of a date anterior to human history or tradi¬ 
tion, which are of such thickness as to constitute hills from 
500 to more than 2000 feet in height. Some of these strata 
contain marine shells which are exclusively of living species, 
others contain a slight mixture, one or two per cent., of species 
not known as living. 
The ancient part of Vesuvius is called Somma, and consists 
of the remains of an older cone which appears to have been 
partly destroyed by explosion. In the great escarpment 
which this remnant of the ancient mountain presents towards 
the modern cone of Vesuvius, there are many dikes which 
are for the most part vertical, and traverse the inclined beds 
of lava and scoriae which were successively superimposed 
during those eruptions by which the old cone was formed. 
They project in relief several inches, or sometimes feet, from 
the face of the cliff, being extremely compact, and less de¬ 
structible than the intersected tuffs and porous lavas. In 
vertical extent 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. 
In mineral composition 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. 
Nothing is more remarkable than the usual parallelism of 
the opposite sides of the dikes, which correspond almost as 
regularly 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 heterogeneous composition, like those 
composing the cone of Somma. In explanation of this phe¬ 
nomenon, M. Necker refers us to Sir W. Hamilton’s account 
of an eruption of Vesuvius in the year 1779, who records the 
following fact: “ The lavas, w^hen they either boiled over 
the crater, or broke out from the conical parts of the vol¬ 
cano, constantly formed channels as regular as if they had 
been cut by art down the steep part of the mountain ; and 
whilst in a state of perfect fusion, continued their course in 
those channels, which were sometimes full to the brim, and 
at other times more or less so, according to the quantity of 
matter in motion. 
“These channels (says the same observer), I have found, 
upon examination after an eruption, to be in general from 
