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Show 344 SECTION OF VESUVIUS AND SOMMA. No.l3. Suppowl ltctlon qf Vtluviu• and Somma. . f the ancient cone of Vesuvius. a. Monte Somma, or the remall~ks o . ction encircling the base of the recent b. The Pedamentina, a terrace: 1 e proJe ' , cone of Vesuvius, on the south Slde. c A trio del Cavallo* • · C t 1 ft by eruption of 1822. fd.. e. ra er e . 1828 at the bottom of the great crater. Small cone throw:n up m ' Dikes intersectmg Somma. . ~·~· Dikes intersecting the recent cone of Vesuvms. IZ.II. . . volcano of no great height) ; and that the ex- spea~mg, /sthae ear 79 caused it not merely to disgorge the plosiOns o . y 1 • l had loner been choked up, but contents of Its crater, Wf11Ch1 l.t· s~lf. so that the wall of bl reat part o t e cone . ew up a g . f the Pedamentina were never S d the r1dcre or terrace o ' . omma, a·n f clr:?a ter of erup t1' 0n, b· ut are the rehcs of a the· mda rgmd ot a ated cone. It WI'1 1 be seen in the diagram rmne an . ru nc e of Vesuvius become horizontal that the slantmg beds of tl(le co)n l the base of the new cone in the Atrio del Cavallo at c ' w 1teref Somrna . for when the meets the preci· pi· t ous cs carpmen o d · 18'G l~ its descend-h' · t as happene m Tw ' ~ava flows d?wn to t IS po; it then runs in another direction mg course xs arrested, an. . d t1 base of the cone. along this small valley, ctrclmg brou~ l.e ds collec.t at the Sand and scorire, also, blown y t e WI~ ~orrents· so that b f the cone and are then swept away y d ' I the ase o 'h fl. ttish plain as represente . n there is always ere a. a . . ' ) must be composed of same manner the smal~ mt~riOr hco~e (~ I plain ; for while this sloping beds, terminatmg m a onzon a * . . nd mules there when they So called from travellers leaving their horses a prel1are to ascend the cone on foot, VESUVIAN LAVAS, 345 monticule was gradually gaining height by successive ejections of lava and scorire, in 18~8, it was always surrounded by a flat pool of semi-fluid lava, into which scorire and sand were thrown. The escarpment of Somma exhibits a structure precisely similar to that of the cone of Vesuvius, but the beds are intersected by a much greater number of dikes. The formation of this older cone does not belong to the historical era, and we must not, therefore, enlarge upon it in this place; but we shall have occasion presently to revert to the subject, when we speak of a favorite doctrine of some modern geologists, concerning " craters of elevation" (Erhebung's Cratere), whereby, in defiance of analogy, the origin of the identical disposition of the strata and dikes in Vesuvius and Somma has been referred to a mode of operation totally' dissimilar. Vesuvian Lavas.-The modern lavas of Vesuvius are characterized by a large proportion of augite (or pyroxene). When they are composed of this mineral and felspar, they may be said to differ in no way in composition from many of the ancient volcanic rocks of Scotland. They are often porphyritic, containing disseminated crystals of augite, leucite, or some other mineral, imbedded in a more earthy base. These porphyritic lavas are often extremely compact, especially in the dikes both of Vesuvius and. Somma, which, in hardness and specific gravity, are by no means inferior to ordinary veins of trap, and, like them, often preserve a remarkable parallelism in their two opposite faces for considerable distances. In regard to the structure of the Vesuvian lavas on a great scale, there are no sections of sufficient depth to enable us to draw fair comparisons between them and the products of extinct volcanos. At the fortress near 'l"'orre del Greco a section is exposed, fifteen feet in height, of a current which ran into the sea; and it evinces, especially in the lower part, a decided tendency to divide into rude columns. A still more striking example may be seen to the West of Torre del Annunziata, near Forte Scassato, where the mass is laid open by the sea to the depth of twenty feet. In both these cases, however, the r~ck may rather be said to be divided into numerous perpendiCular fiss~res, than to be prismatic, although the same |