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Show 122 NEWER PLIOCENE PERIOD. [Ch.X.; No.25. Diltes or l!eins at the Punta del Nasone on Somma. The reader will remember our description of the manner in which the plain of J erocarne, in Calabria, was fissured by the earthquake of 1783 *, so that the Academicians compared it to the cracks in a broken pane of glass. If we suppose the side walls of the ancient crater of Vesuvius to have been cracked in like manner, and the lava to have entered the rents and become consolidated, we can explain the singular form of the veins figured in the accompanying ~ood~cut t. . . Pa·rallelism of their oppostte stdes.-Nothmg IS more re. markable than the parallelism of the opposite sides of the dikes, which usually correspond with as much regularity as the two opposite faces of a wall of masonry. This. characte~ appears at :first the more inexplicable, when we consider how Jagged and uneven are the rents caused by ea_rthquakes in masses of heterocreneous composition like those composing the cone of So~ma; but M. Necker has offered an ingenious and, we think ' satisfactory explanation of the phenomenon. He .re fe.rs us to Sir W. Hamilton's account of an eruption of VesuviUs m the year 1779, who records the following facts. ' The lavas, when they either boiled over the crater, or broke out from the c~nical parts of the volcano, constantly formed channels as regular as if they had been cut by art, down the steep part of "' See vol. i. chap. :xxiv., wood-cut No. 22. t From a drawiug of M. Necker, ibid. Ch.X.] PARALLEL BIDES OF DIKES. 123 the mountain, and, whilst in a state of perfect fusion, continued their course in those channels, which were sometimes full to the brim, and at other times more or less so according to the quantity of matter in motion. ' These channels, upon ~xamination after an eruption, I have found to be in general from two to :five or six feet wide, and seven or eight feet deep. They were often hid from the sight by a quantity of scorire that had formed a crust over them, and the Java, having been conveyed in a covered way for some yards, came out fresh again into an open channeL After an eruption I have walked in some of those subterraneous or covered ga1leries, which were exceedingly curious, the sides, top, and bottom, being worn perfectly smooth and even in most parts, by the violence of the currents of the red-hot lavas, which they had conveyed for many weeks successively.' In another place, in the same memoir, he describes the liquid and red-hot matter as being received (into a regular channel, l'aised upon a sort of wall of scorire and cinders, almost perpen· dicularly, of about the height of eight or ten feet, resembling much an ancient aqueduct*.' Now, if the lava in these instances had not run out from the covered channel, in consequence of the declivity whereon it was placed-if, instead of the space being left empty, the lava had been retained within until it cooled and consolidated, it . would then have constituted a small dike with para~Jel sides. But the walls of a vertical :fissure through which lava has ascended in its way to a volcanic vent, must have been exposed to the same erosion as the four sides of the channels before adverted to. The prolonged and uniform friction of the heavy fluid as it flows upwards cannot fail to wear and smooth down the surfaces on which it rubs, and the intense heat must melt all such masses as project and obstruct the passage of the incandescent fluid. We do not mean to assert that the sides of :fissures caused by earthqunkes are never smooth and parallel, but they are usually uneven, and are often seen to have been so where volcanic * Phil. Trans., vol.lxx, 1780. |