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Show 120 NEWER PLIOCENJ~ PERlOD. [Ch. X. . d in contemplating the gigantic whole, must appear the pwtoe rk of extraordinary causes, but w1 I en t h e separate portions of which it is made up are carefully stu~ied, ~hey are seen to have been forme d successi.v e ly , and the dimensiOns of ea.c h pa.r t, cons.i dered s•m g 1y , are soon recooo- nized to be compara. tively In- s1. gm. ficant, an< 1 1. t appears no longer extravagant to hken them to the recorded effects of ordinary causes. D-;wn:e rence m• tzF Ze comrv nosition o'Jr Somma and Vesuvius. A s no t ra d1't 'w na 1 accounts have been handed down to us of t I1 e erupt .J ons of the ancient Vesuvius, from the times of the ear1 t.e st G ree 1{ colon1'sts , the volcano must have been dormant for many centuries, perhaps for thousands of years, previous to the great eruption in the reign of Titus. But we sha~l afte:wards show that there are sufficient grounds for presummg th1s mountain, and the other igneous products of Campania, to have been produced during the Newer Pliocene peri~d. We stated in the first volume *, that the ancient and modern cones of Vesuvius were each a counterpart of the other in structure; we may now remark that the principal point of difference consists in the greater abundance in the older cone of frao·ments of stratified rocks ejected during eruptions. We may :asily conceive that the first explosions would act with the greatest violence, rending and shattering whateve~ solid masses obstructed the escape of lava and the accompanymg gases, so that great heaps of ejected pieces of sedimentary rock would naturally occur in the tufaceous breccias formed by the earliest eruptions. But when a passage had once been opened and an habitual vent established, the materials thrown out would con· sist of liquid lava, which would take the form of sand and scori~, or of angular fragments of such solid lavas as may have choked up the vent. Among the angular fragments of solid rock which abound in the tufaceous breccias of Somma, none are more common than a saccharoid dolomite, supposed to have been derived * Chap. xx. Cb.X.] DIKES OF SOMMA. 121 from an ordinary limestone altered by heat and volcanic vapours. Carbonate of lime enters into the composition of so many of the simple minerals found in Somma, that M. Mitscberlich, with much probability, ascribes their great variety to the action of the volcanic heat on subjacent masses of limestone. Dikes of Somma.-The dikes· seen in the great escarpment which Somma presents towards the modern cone of V csnvius are very numerous. They are for the most part vertical, and traverse at right angles the beds of lava, scorire, volcanic breccia, and sand, of which the ancient cone is composed. They project in relief several inches, or sometimes feet, from the face of the cliff, like the dikes of Etna already described (see woodcut No. 19), being, like them, extremely compact, and less destructible than the intersected tufFs and porous lavas. In height they vary from a few yards to 500 feet, and in breadth from one to twelve feet. Many of them cut all the inclined beds in the escarpment of Somma from top to bottom, others stop short before they ascend above half way, and a few terminate at both ends, either in a point or abruptly. In mineral composition they scarcely differ from the lavas of Somma, the rock consisting of a base of leucite and augite, through which large crystals of augite and some of leucite are scattered *. Examples are not rare of one dike cutting through another, and in one instance a shift or fault is seen "at the point of intersection. We observed before t, when speaking of the dikes of the modern cone of Vesuvius, that they must have been produced by the filling up of open fissures by liquid lava. In some examples, however, the rents seem to have been filled laterally. * Consult the valuable memoir of M. L.A. Necker, Mem. de la Soc. de Phys. et d'Hist. Nat. de Geneve1 tome ii. part i., Nov. 182~. t Vol. i. chap. xx. |
