wards in all directions from the axis of the cone
of Vesuvius, are intersected by veins or dikes of compact lava, for the
most part in a vertical position. In 1828 these were seen to be about
seven in number, some of them not less than four or five hundred feet in
height, and thinning out before they reached the uppermost part of the
cone. Being harder than the beds through which they pass, they have
decomposed less rapidly, and therefore stand out in relief. When I
visited Vesuvius, in November, 1828, I was prevented from descending
into the crater by the constant ejections then thrown out; so that I got
sight of three only of the dikes; but Signor Monticelli had previously
had drawings made of the whole, which he showed me. The dikes which I
saw were on that side of the cone which is encircled by Somma. The
eruption before mentioned, of 1828, began in March, and in the November
following the ejected matter had filled up nearly one-third of the deep
abyss formed at the close of the eruption of 1822. In November I found a
single black cone at the bottom of the crater continually throwing out
scoriae, while on the exterior of the cone I observed the lava of 1822,
which had flowed out six years before, not yet cool, and still evolving
much heat and vapor from crevices.

Hoffmann, in 1832, saw on the north side of Vesuvius, near the peak
called Palo, a great many parallel bands of lava, some from six to eight
feet thick, alternating with scoriae and conglomerate. These beds, he
says, were cut through by many dikes, some of them five feet broad. They
resemble those of Somma, the stone being composed of grains of leucite
and augite.[528]

There can be no doubt that the dikes above mentioned have been produced
by the filling up of open fissures with liquid lava; but of the date of
their formation we know nothing farther than that they are all
subsequent to the year 79, and, relatively speaking, that they are more
modern than all the lavas and scoriae which they intersect. A
considerable number of the upper strata are not traversed by them. That
the earthquakes, which almost invariably precede eruptions, occasion
rents in the mass, is well known; and, in 1822, three months before the
lava flowed out, open fissures, evolving hot vapors, were numerous. It
is clear that such rents must be ejected with melted matter when the
column of lava rises, so that the origin of the dikes is easily
explained, as also the great solidity and