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Show 68 NEWER PLIOCENE PERIOD. [Ch. VI. l 1 T 1ese mar s are someti·m es gyp:::~. e 0u~::: ., and belonO;>' to a. great '11 .c t' wh1arg1 aceous 1orma wn 'ch stretches ove.r a considerable part of Sicily, and contains sulphur and salt m great abundanc.e. The strata of this group have been in some pl~ces contorteclm the most extraordinary manner, their convolutwns often resembling those seen in the most disturbed districts of primary clay slate. But we wish, at present, to direct the reader's exclusive at-tention to strata decidedly referrible to the ~newer Pliocene era, and we have yet to mention the igneous rocks associated with the sedimentary formations already alluded to. Volcanic Rocks of the Val di Noto.-The volcanic rocks occasionally associated with the limestones, sands, and marls already described, constitute a very prominent feature throughout the Val di Noto. Great confusion might have been expected to prevail, where lava and ejected sand and scorire are intermixed with the marine strata, and, accordingly, we find it often impossible to recognize the exact part of the series to which the beds thus interfered with belong. Sometimes there are proofs of the posterior origin of the lava, and sometimes of the newer date of the stratified rock, for we find dikes of lava intersecting both the marl and limestone, while, in other places, calcareous beds repose upon lava, and are unaltered at the point of contact. Thus the shelly limestone of Capo Santa Croce rests in horizontal strata upon a mass of lava, which had evidently been long exposed to the action of the waves, so that the surface has been worn perfectly smooth. The limestone is unchanged at its junction with the igneous rock, and incloses within it pebbles of the lava*. The volcanic formations of the Val di Noto usually consist of the most ordinary variety of basalt with or without olivine. The rock is sometimes compact, often very vesicular. '!'he some which he obtained from the same localities, but I have met with no zoolo· gists who could name the species. "'This locality is described by Pl'ofcssor Hoffmann, Archiv fur Mineralogic, &c. Brrlin, 1831. Ch. VI.] VOLCANIC ROCKS-VAL DI NOTO. 69 vesicles are occasionally empty, both in dikes and currents, and are in some localities filled with calcareous spar, arragonite, and zeolites. The structure is, in some places, spheroidal, in others, though rarely, columnar. I found dikes of amygdaloid, wacke, and prismatic basalt, intersecting the limestone at the bottom of the hollow, called Gozzo degli Martiri, below Melilli. Dilces.-Dikes of vesicular and amygdaloidal lava are also seen traversing peperino, west of Palagonia, near a mill by the road side. No.6. No.7. ~ -~ ·.,., 'Do, ' 0 b: 0 ••• I I. • D ~:: D·· 0 ., •' a (), >,· o' '• llorizontal section of Dikes near Patagonia. a, Lava. b, Peperiuo, consisting of volcanic sand, mixecl with fragments of lava and of limestone. In this case we may suppose the peperino to have resulted from showers of volcanic sand and scm·ire, together with fragments of limestone thrown out by a submarine explosion, similar to that which lately gave rise to the volcanic island off Sciacca. When the mass was, to a certain degree, consolidated, it may have been rent open, so that the lava ascended through fissures, the walls of which were perfectly even and parallel. After the melted matter that filled the rent had cooled down, it must have been fractured and shifted horizontally by a lateral movement. In the second figure, No. 7, the lava has more the appearance of a vein which forced its way through the peperino, availing itself, perhaps, of a slight passage opened by rents caused by earthqual<es. Some of the pores of the lava, in these dikes, are empty,' while others are filled with carbonate of lime. |