one of
Etna are exposed. The dikes are for the most part vertical, but
sometimes they run in a tortuous course through the tuffs and breccias,
as represented in fig. 50. In the escarpment of Somma, where similar
walls of lava cut through alternating beds of sand and scoriae, a coating
of coal-black rock, approaching in its nature and appearance to
pitchstone, is seen at the contact of the dike with the intersected
beds. I did not observe such parting layers at the junction of the
Etnean dikes which I examined, but they may perhaps be discoverable.

[Illustration: Fig. 50.

Tortuous veins of lava at Punto di Giumento, Etna.]

The geographical position of these dikes is most interesting, as they
are very numerous near the head of the Val del Bove, where the cones of
1811 and 1819 were thrown up, as also in that zone of the mountain where
lateral eruptions are frequent; whereas in the valley of Calanna, which
is below that parallel, and in a region where lateral eruptions are
extremely rare, scarcely any dikes are seen, and none whatever still
lower in the valley of St. Giacomo. This is precisely what we might have
expected, if we consider the vertical fissures now filled with rock to
have been the feeders of lateral cones, or, in other words, the channels
which gave passage to the lava-currents and scoriae that have issued from
vents in the forest zone. In other parts of Etna there may be numerous
dikes at as low a level as the Valley of Calanna, because the line of
lateral eruptions is not everywhere at the same height above the sea;
but in the section above alluded to, there appeared to me an obvious
connection between the frequency of dikes and of lateral eruptions.

Some fissures may have been filled from above, but I did not see any
which, by terminating downwards, gave proof of such an origin. Almost
all the isolated masses in the Val del Bove, such as Capra, Musara, and
others, are traversed by dikes, and may, perhaps, have partly owed their
preservation to that circumstance, if at least the action of occasional
floods has been one of the destroying causes in the Val del Bove; for
there is nothing which affords so much protection to a mass of strata
against the undermining action of running water as a perpendicular dike
of hard rock.

In the accompanying drawing (fig. 51), the flowing of the lavas of 1811
and 1819, between the rocks Finochio, Capra, and Musara, is represented.
The height of the two last-mentio