dates, and several minor
cones, notably those of 1852, are within its boundaries. The abrupt
precipices reveal the presence of a large number of vertical dikes,
radiating from a point within the valley, some of them, according to Von
Waltershausen, being of ancient greenstone. Other dikes of more modern
doleritic lava radiate from the present crater. From the slope of the
beds in the Val del Bove, Lyell and Von Waltershausen have independently
inferred that there was once a second great centre of eruption in the
Val del Bove between the Sierra Giannicola, and Zoccolaro (_vide_ the
Figure on p. 117). The axis of eruption passing through this point Lyell
calls the _Axis of Trifoglietto_; while he distinguishes the present
centre of eruption as the _Axis of Mongibello_. These centres probably
existed simultaneously, but were unequal as regards eruptive violence;
the crater of Mongibello was the more active of the two, and eventually
overwhelmed the crater of Trifoglietto with its products, by which means
the whole mountain became a fairly symmetrical cone, having the crater
of Mongibello at its apex (_vide_ the Figures on pp. 119 and 121).
Subsequently the Val del Bove was formed, probably by some paroxysmal
explosion, caused by pent-up gases escaping from fissures. Possibly also
subsidence may have occurred.

[Illustration: Profile of Etna]

We must then in the first place think of Etna as a submarine volcano
of the Newer Pliocene age; when it reached the surface it increased
rapidly in bulk by pouring out scoriae and lava from its two centres
of eruption--the centre of Mongibello, and the centre of
Trifoglietto,--general upheaval of the surrounding district followed,
and ultimately the crater of Trifoglietto was obliterated by the
discharges from the crater of Mongibello. Afterwards the Val del Bove
was blown out by sudden eruptive force from beneath, and the mountain
assumed its present aspect. Then the historical eruptions commenced,
and of these we have given an account in the preceding chapter.

The most obvious method of obtaining some idea as to the age of Etna, is
to ascertain the thickness of matter added during the historical period
to the sides of the mountain, and to compare this with the thickness of
the beds of ancient lava and scoriae exposed at the abrupt precipices of
the Val del Bove. There is reason for believing, however, that none of
the ancient lavas equalled in volume the lava streams of 1809 and