approaching to the horizontal, the rock of the Gleichenburg, being forced up through the midst of them, imparted the inclination which they are now seen to possess. For my own part I am most disposed to adopt the latter opinion, on the same ground on which I assented to M. Bertrand Roux's ideas with respect to the rock of the Mount Mezen; for it seems probable that if the trachyte had been formed in the first instance, fragments of it ought to appear intermixed with the other materials of the tuff, which I did not discover to be the case. The inclination likewise possessed by the strata of tuff seems to me too considerable to be consistent with the former hypothesis, but accords very well with the latter. The following sketch may give an idea of the disposition of the central trachyte. Where a. & b. are alternating beds of tuff and loam or sand, C is the trachyte, and D a valley of denudation separating the two rocks. EUGANEAN HILLS. On entering Italy by the side of Venice, we have not far to go before we meet with a very extensive and interesting volcanic district. To the south of Padua lie the Euganean hills, an isolated tract of high ground in the midst of a level country, consisting of a trachytic formation, not unlike that of Hungary, which, from its cellular structure in some cases, and its semi-vitreous aspect in others, would at once be taken for a volcanic product. Like the formation too of the latter country, it consists of several kinds of rock, which however are so allied, and so connected by mutual passages, as to shew that they have been all derived from a modification of the same process. The most characteristic variety is a rock of an ash-grey colour and uneven fracture, very like the porphyry of Mont Dor, or the first species of Beudant's trachyte formation, (Monselice). It contains numerous crystals of glassy fel spar, sometimes decomposed, sometimes fresh, and occasional specks of black mica, which is also accumulated in nests, the several parts of which have a slaty structure, like that of mica slate. Crystals of augite are also found under the same circumstances. Associated with this is a rock possessing a splintery fracture, waxy lustre, and vitreous appear→ ance, which may be called an hornstone porphyry. Some varieties are cellular, and contain infiltrations of quartz and chalcedony, like the millstone trachyte of Hungary. Others approach very nearly to the characters of pearlstone, presenting, together with the vitreous aspect of that substance, an approach to a similar concentric arrangement. (Monte Siave). In these cases the crystals of glassy felspar, which distinguish true trachyte, are either absent, or very rarely occur. This formation is associated at Monte Venda with basalt, the relation of which to the trachyte is as obscure as in the parallel case of the Siebengebirge. It is also surrounded, H EUGANEAN HILLS. at Castelletto, by strata of tuff, and of pumiceous conglomerate, in a manner analogous to what I have described as taking place near the Gleichenburg in Styria, but disposed more vertically. In some parts a conglomerate or breccia occurs, (Monte Nuovo) which seems to be principally made up of the hornstone above described, intermixed with a white powdery siliceous substance, which fills up the interstices. The whole of this mass might be imagined, as well from its vitreous appearance, as from the intimate union of its parts, to have been consolidated by fusion, or at least by the action of heat. The trachyte of the Euganean hills rests upon a calcareous rock, which appears to correspond with the chalk of Great Britain. It is called Scaglia, from its slaty structure, being disposed in thin horizontal layers. Its colour is commonly white, now and then with a shade of red, and its compactness usually is quite equal to that of our hardest chalk, though softer varieties are sometimes met with. The points however chiefly to be insisted on, as establishing the identity of the two formations, are, the kidneyshaped masses of flint disposed in beds throughout the Scaglia, as in the chalk of England, and the nature of the petrifactions that occur in it, which, from the list given in the Abbé Maraschini's late work,* appear to consist of ammonites, terebratulites, and various species of the echinus family; viz. the echinoneus, galerites, ananchytes, spatangus, cidaris, nucleolites, and echinus proper, of Lamarck. By comparing this list with the one given in Messrs. Conybeare and Phillips' Geology of England and Wales, p. 73, it will be seen, that the analogy between the two formations is in this respect considerable. I know not whether the redness and brittleness of the flints in a part of the rock which lies near the trachyte, not * Sulle Formazioni delle Rocce del Vicentino. Padova, 1824. p. 122. EUGANEAN HILLS. far from the village of Battaglia, is to be explained in the same manner in which Messrs. Buckland and Conybeare have accounted for a similar change in some of those near the Giant's Causeway, namely, by the influence of the melted matter upon them; but it is at least certain, that this volcanic rock has sometimes produced, upon the surface of the subjacent bed, alterations, which afford additional evidence of its igneous origin. Thus at the village of Schevanoya on the southern slope of the Euganean hills, the trachyte is incumbent on an argil laceous variety (as I presume) of the scaglia, which is naturally so incoherent as to be softened by every shower of rain. As we trace it upwards however, we find it gradually becoming more and more compact, until at last, where it touches the incumbent trachyte, it becomes perfectly hard and splintery in its fracture. Other indications of volcanic action may perhaps be ga thered from the springs of hot water impregnated with sulphuretted hydrogen, which gush out from the rock near the village of Battaglia,* and are still in repute, as they were in the time of the Romans, for their medicinal qualities. Perhaps the fable of Phaeton, who was said to have fallen from heaven, or to have been struck by lightning on the * It was the Fons Aponi mentioned by Lucan, Lib. 7. Euganeo, si vera fides memorantibus, Augur Claudian also celebrates it. + Tzetzes, in his Schol. on Lycoph, says, that some supposed the Lake Avernus to exist among the Euganean Hills, and the circumstances that gave rise to the fable of Phaton, to have happened there. Martial too has these lines: Emula Baianis Altini littora villis, Et Phaethontei conscia sylva rogi; Quæque Antenorio, Dryadum pulcherrima, Fauno Nupsit, ad Euganeos sola puella lacus! VICENTIN. borders of the Po, may refer to some tradition that existed of volcanic phenomena, which may have continued here as they now do in Transylvania, long after the formation of the trachyte. The neighbouring country to the north of Vicenza is interesting to the volcanist, as enabling him to trace the differences that exist between the ignigenous rocks of the very same country, according to their relative degrees of antiquity. It appears that all the formations of that country from the talc slate, which is the fundamental rock, up to the scaglia,* which is an equivalent of our chalk, are accompanied by trap rocks, both in beds and in dykes, having an uniformly compact structure, or cells completely filled with crystalline matter; whereas the tertiary beds that lie above them all alternate with a tuff, consisting of materials, the volcanic nature of which is more plainly attested by the scoriform and vitreous aspect which so often belongs to them. It is impossible to imagine any combination of phenomena more in accordance with the idea, that the compactness of lavas is regulated cæt. par. by the pressure which they have undergone, and that the absence of vacuities in the case of all those formed during the deposition of the older rocks, arose from the mass of water superimposed; since it is seen that The Abbe Maraschini was good enough to shew me a hill, near Recuaro, north of Schio, where according to him the greater part of the formations met with in that neighbourhood are seen united. In his Memoir entitled "Observationi sopra alcuni Localitâ del Vicentino," which was published in the Biblioteca Italiana, and in his late work referred to above, a full enumeration of the series is given; I shall therefore content myself with stating, that on this hill are seen, resting on the talc slate, which appears to be the fundamental rock of the country, 1st, a red sandstone, 2d, an augite rock (dolerite), 3d, another red sandstone with seams of slate coal, and above, three alternations of sandstone and limestone, which the Abbe is inclined to refer, I know not how correctly, to distinct formations analogous to those in England and Germany. |