one variety of which seems to approach to clinkstone porphyry.

This trachyte rests upon a bed of clay, sometimes red and ferruginous, at others blue, in which are imbedded several species of Arca, Murex, Turbo and Trochus, enumerated by Brocchi *, and found to be identical with those now living in the Mediterranean. Thus the date of the trachyte cannot be anterior to that of the newest members of the tertiary class of rocks.

As we proceed from thence in an easterly direction round the coast of Ischia, we meet with evidences of volcanic operations belonging to a still more recent date.

At Castiglione the ground is covered with loose fragments of pumice and obsidian, which appear to be derived from one or other of two neighbouring craters, namely Monte Rotaro and Montagnone. Still further we cross the stream of lava, called Arso, which issued from the side of Monte Rotaro in the year 1302, remarkable from the large crystals of glassy felspar which are imbedded in it. Its surface is still undecomposed and consequently barren, moss alone growing upon it, and that only in a few parts, a proof of the number of ages required for bringing some lavast into a state fit for cultivation. This current may be readily traced up the mountain to the point from whence it issued, which is marked by the existence of a crater originating apparently from the eruption itself.

The castle of Ischia itself stands on a projecting mass of lava, which appears to have made a part of a current that may be traced to the neighbouring heights of Campignano, where it constitutes a sort of ridge resting upon the pumiceous conglomerate. Its high antiquity is evident from the changes that must have since taken place in the figure of the island; for not only has this promontory been separated from the island by some subsequent convulsion, but we immediately perceive that a stream of lava at the present time

* Conch. Subapp. p. 354.

+ This lava I find very sparingly soluble in muriatic acid. I dissolved little more than ten per cent. by repeated digestion in that menstruum, whereas nearly half the weight was taken up in the case of the lavas of Vesuvius.

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would pursue a very different direction, and, instead of reaching the promontory, would fill up the valleys and indentations in the coast which the present current overlooks.

Thus Ischia appears to have been subjected to volcanic action of as many different periods as the neighbourhood of Naples itself, its pumiceous conglomerate corresponding with the Puzzolana, its trachytes to the rock of the Solfatara, and the lava of the Capo d'Arso to those of Vesuvius. Even the ancients were fully aware of its volcanic nature, attributing it to the giant Typhæus being confined under the mountain ; and Strabo relates that a colony sent over by Hiero, tyrant of Syracuse, was so alarmed by the frequent earthquakes, that they deserted the island *.

Not less was the consternation excited by the eruption which gave rise to the lava-stream of Arso already noticed. Thus Villani in his · Florentine History' (lib. viii. c. 53) observes, that in the year 1302 a tremendous conflagration broke out from this crater, so that through the whole extent of the island much of the country was consumed and laid waste, and even many of the people and of the cattle in it were destroyed. Multitudes also, to escape from the danger, fled to Procida, Capri and the mainland, and remained there during the continuance of the internal commotions, which lasted more than two months.

At present the only indications of volcanic action are those afforded by the hot springs so common throughout the island.

In the year 1834 I visited almost every one of them, principally with the view of ascertaining whether, like others that had fallen within my notice elsewhere, they evolved nitrogen gas. The following were the particulars which I noted at the time respecting them :

Near the town of Ischia are thermal springs with a temperature, the one of 122.5° Fahr., the other of 102°. They have a saline taste, and contain a few white confervæ, but no gas rises up through them.

Near Castiglione are stufes or clefts in the earth evolving steam, but no saline matter is present.

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Near Casamicciola is the hot spring of Gurgitello, having a temperature of 142°, with a mawkish and peculiar, but not a saline taste. Near it is the Aqua di Cappone, where I collected from the Baths an efflorescence, which appeared to consist of sulphate of soda and a little common salt. There are likewise some stufes proceeding manifestly from the same source as the spring of Gurgitello. Under Mount Thabor are the stufes of Cayuco, issuing from the trachytic rock above noticed.

Near the sea-shore at Lacco are the warm springs of Santa Restituta, the temperature of which is 115°, and a little above are the stufes of San Lorenzo.

Near the town of Foria on the sea-shore are the hot springs of Citara, one of which has a temperature of 81° Fahr., the other one of 120°. A little further, the road on the sea-shore is extremely hot, having a temperature of 130° one foot below the surface, and emitting much steam.

In a deep ravine or water-course cut through the tuff and extending to the sea is the warm spring of Olmitello (78° of Fahr.), and in the village of 'Testaccio above is a dry stufe, or an evolution of hot air unaccompanied with steam.

In every one of these cases my attention was particularly directed to ascertaining whether gas was evolved from the spring, but in no one instance did this occur. I also tested the character of the saline efflorescences found near the springs, with a view of learning whether ammonia was present in them, but never succeeded in detecting any by the usual tests. I also found that the air emitted from the stufes contained its usual proportion of oxygen.

It may therefore be concluded, that these warm springs and warm vapours merely arise from the rains which fall in the island, sinking down into a rock which still retains some portion of the heat derived from antecedent volcanic operations, but not from any going on at the present time, in this respect therefore differing from the hot springs of most other localities. (See the Chapter on thermal waters.)

Where steam passes through the rock, the fissures in it are often coated with a white siliceous incrustation, which Dr. Thomson, I believe, was the first to notice under the name of fiorite.

Dr. Macculloch* has noticed a similar phænomenon as occurring in the graphic granite of the Isle of Rona, where the surface of the quartz, or chalcedony as it has been otherwise called, obtains from exposure to the weather a glossy enamel, arising apparently from a partial solution of the silex. A similar enamel is to be observed in

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a

Geological Transactions, vol. ii. p. 392.

vesting the sandstones of Jura and of Schehallion, and in the granite of Rockall.

The circumstances of the case are certainly more favourable to chemical action in the vapour baths of Ischia, as the aqueous particles are presented to the silex at a high temperature, and in a minute state of division, at the moment of their deposition from the state of vapour.

I need not insist upon the analogy between the above phænomena and those presented at the Geysers in Iceland hereafter to be described, but I may remark, that similar concretions are noticed as occurring, among the volcanic rocks of the Solfatara near Naples, and of Santa Fiora in Tuscany; by Humboldt at Teneriffe; and by Von Buch in Lancerote. In these cases, the alkali, which in the Iceland Geysers is supposed to assist in dissolving the silex, does not appear to be present. The late experiments by Dr. Turner, on the power which high-pressure steam possesses to dissolve silica, may perhaps throw light upon this phænomenon.

CHAPTER XIV.

LIPARI GROUP OF ISLANDS.

Stromboli-its crater-unintermitting character of its eruptions--tuff and dykes of lava. Lipari-central portion, consists of tuff--acted on by sulphureous vapours.-Southern portion, composed of pumice and obsidian.-Northern portion the same.-Glassy lavas. Volcano-description of its crater-date of the last eruptions. Panaria described.-Basiluzzo-Salina-Felicudi-Alicudi-Ustica.

THE Lipari Islands, between Naples and Sicily, are made up of a class of volcanic formations analogous to some of those we have been just considering.

Like the neighbourhood of Naples too, they afford us the means of comparing the products of active and half-extinguished volcanos, with those which have arisen from the same cause at earlier periods.

The volcanic action here seems to have been developed along two lines, the one nearly parallel to that of the Apennines, beginning with Stromboli, intersecting Panaria, Lipari and Volcano, and exhibiting traces of its prolongation on the coast of Sicily in the fumaroles which are evolved from the rocks of Cape Colara; the other extending from Panaria to Salina, Alicudi and Felicudi, and again visible in the volcanic products that make their appearance at Ustica.

The Map at the end of the volume shows the connexion between these several islands, which it is the purpose of the present chapter to describe.

Stromboli.

The island of Stromboli consists of a single conical mountain, having on its summit a circular crest of rocks broken away on one of its sides, which Hoffman* regarded as a crater

* Poggend. Annal. vol. xxvi. for 1832.