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Show 86 NEWEll. PLIOCENE PERiOD. LCh. VII. No. 18. A A, Zocolaro. B, Monte di Calanua. C Plain at the head of the Valley of Calanna. ' Lava of 1819 descending the precipice and flowing through the valley. :; Lavas o£1811 and 1819 flowing rotmd the hill of Calanna. The flows of melted matter have been deflected from their course by this projecting mass, just as a tidal current, after setting against a line of sea-cliffs, is often thrown off into a new direction by some rocky headland. Lava-streams, it is well known, become solid externally, even while yet in motion, and their sides may be compared to two rocky walls, which are sometimes inclined at an angle of fortyfive degrees. When such streams descend a considerable slope at the base of a line of precipices, and are turned from their course by a projecting rock, they move right onwards in a new direction, so as to leave a considerable space (as in the Valley of Calanna) between them and the cliffs which may be continuous below the point of deflection. It happened in 1811 and 1819, that the flows of lava overtopped the ridge intervening between the bills of Zocolaro and Calanna, so that they fell in a cascade over a lofty precipice, and began to fill up the valley. (See letter a, diagram No. 18.) The narrow cavity of St. Giacomo will admit of an explanation precisely similar to that already offered for Calanna. Ch. VII.] VAL DEL DOVE. 87 f/al del Bo·ve.-After passing up through the defile, called the ' Rocca di Calanna,, we enter a third valley of truly magnificent dimensions-the Val del Bove-a vast amphi~ theatre four or five miles in diameter, surrounded by nearly vertical precipices, varying from one thousand to above three thousand feet in height, the loftiest being at the upper end, and the height gradually diminishing on both sides. The feature which first strikes the geologist as distinguishing this valley from those before mentioned, is the prodigious multitudes of vertical dikes, which are seen in all directions traversing the volcanic beds. ,.fhe circular form of this great chasm, and the occurrence of these countless dikes, amounting perhaps to several thousands in number, so forcibly recalled to my mind the phenomena of the Atrio del Cavallo, on Vesuvius, that I imagined once more that I had entered a vast crater, on a scale as far exceeding that of Somma, as Etna surpasses Vesuvius in magnitude. But having already been deceived in regard to the crescentshaped precipice of the valley of Calanna, I began attentively to explore the different sides of the great amphitheatre, in order to satisfy myself whether the semicircular wall of the Val del Dove had ever formed the boundary of a crater, and whether the beds bad the same qu!lqua-versal dip which is so beautifully exhibited in the escarpment of Somma. If the supposed analogy between Somma and the Val del Bove should hold true, the tuff.'3 and lavas, at the head of the valley, would dip to the west, those on the north side towards the north, and those on the southern side to the south. But such I did not find to be the inclination of the beds ; they all dip towards the sea, or nearly east, as was before seen to be the case in the Valley of Calanna. There arc undoubtedly exceptions to this general rule, which might deceive a geologist who was strongly prepossessed with a belief that he had discovered the hollow of an ancient crater. It is evident that, wherever lateral cones are intersected in the precipices, a series of tuffs and lavas, very similar to those which |