Before visiting Etna, Lyell had been told by Dr. Buckland that in his opinion the Val del Bove was the most interesting part of Etna, accordingly he specially and minutely examined that part of the mountain. This vast valley is situated on the eastern flanks of the mountain, and it commences near the base of the cone, stretching for nearly five miles downwards. It is a large oval basin formed in the side of the mountain, and surrounded by vast precipices, some of which at the head of the valley are between three and four thousand feet in height. The surface is covered with lava of various 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 scoriæ 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.