OCR Text |
Show 70 NEWER PLTOCENE PERIOD• [Ch. Vl. The annexed diagrams (Nos. 6 and 7) ~eptesent a ~round plan of the l'Ocks as they are exposed to view ~n .a horizontal surface. We think it l1ighly probable that Similar appearances would be seen, if we could examine the floor of the sea in that part of the Mediterranean where the waves have r~cently washed away the new volcanic island, for when a supermcnmbent mass of ejected fragments has been removed by denudation, we may expect to see sections of dikes traversi~g t~ff, or, in other words, sections of the channels of commumcat10n by which the subterranean lavas reached the surface. On the summit of the limestone platform of the Val di Noto, I more than once saw analogous dikes, not only of lava but of volcanic tuff, rising vertically through the horizontal strata, and having no connexion with any igneous masses now apparent on the surface. In regard to the dilces of tuff or peperino, we may suppose them to have been open fissures at the bottom of the sea, into which volcanic sand and scorire were drifted by a current. Tuffs and Peperinos.-In the hill of Novera, between Viz. zini and Militelli, a mass of limestone, horizontally stratified, comes in contact with inclined strata of tuff (see diagram No.8), No.8. A, Limestone. aa, Calcareous breccia with fragments of lava. b, Black tufF. c, TufF. while a mixed calcareous and volcanic breccia, a a, supports the inclined layers of tuff, c. The vertical fissure, b b, is filled with volcanic sand of a different colour. Arr inspection of this section will convince the reader that the limestone must have been greatly dislocated during the time that the submarine eruptions were taking place. At the town of Vizzini, a dike of lava intersects the argilla· ceous strata, ::llld coq.verts ~he111 into siliceous schist, which has Cll. VI.] VOLCANIC TUFFS AND PEPERINOS, 71 been contorted and shivered into an immense number of fragments. We have stated that the beds of limestone, clay, and sand, in the Val di N oto, are often partial! y intermixed with volcanic ejections, such as may have been showered down into the sea during eruptions, or may have been swept by rivers from the land. When the volcanic matter predominates, these compound rocks constitute the peperinos of the Italian mineralogists, some of which are highly calcareous, full of shells, and extremely hard, being capable of a high polish like marble. In some parts of the Val di Noto they are variously mottled with spots of red and yellow, and contain small angular fragments, similar to the lapilli thrown from volcanos. It is recorded that, during the late eruption off the southern coast of Sicily, opposite Sciacca, the sea was in a state of violent ebullition, and £lied, for several weeks continuously, with red or chocolate-coloured mud, consisting of finely-comminuted scorire. During this period, it is clear that the waves and currents that have since had power to sweep away the island, and disperse its materials far and wide over the bed of the sea, must with still greater ease have carried to vast distances the £ne red mud, which was seen boiling up from the bottom, so that it may have entered largely into the composition of modern peperinos. Professor Hoffmann relates that, during the eruption (June. 1831), the surface of the sea was strewed over, at the distance of thirty miles from the new volcano, with so dense a covering of scorire, that the fishermen were obliged to part it with their oars, in order to propel their boats through the water. It is, therefore, quite consistent with analogy, that we should £nd th.e anCient tuffs and peperinos so much more generally dis .. tnbuted than the submarine lavas. In the road which leads from Patagonia to Lago Naftia, and at the distance of about a mile and a half from the former place, there is a small pass where the hills, on both sides, consist of a calcareous grit, intermixed with some grains of volcanic sand. |