NEIGHBOURHOOD OF ROME.

This rock, it may be observed, is not produced by the waters of the Tiber at the present moment, but we have ocular proof of the manner of its formation in the Lago di Solfatara, near Tivoli, where the same process is constantly going on. The water is there impregnated with sulphuretted hydrogen, and carbonic acid gases, which rise up in bubbles through it, and to the presence of the latter substance it owes the property of dissolving calcareous earth, which is again deposited round the lake in the form of travertino, in proportion as the carbonic acid escapes.

The same process appears to have taken place formerly on a much more extensive scale in the neighbourhood of Rome, and, if we are disposed to theorize, we may attribute it to a general disengagement of these two gases at a period more nearly approaching to that at which the volcanos of the neighbouring country were in activity. For we shall see as we proceed, that the extrication of elastic fluids, of which sulphuretted hydrogen and carbonic acid are the most common, takes place round the site of a volcano for a long period after the cessation of its more violent action, so that there is no absurdity in supposing that the same operations, which, when in their greatest intensity, produced the materials of the volcanic tuff itself, may, during the periods of their partial intermittence, have given rise to these gaseous exhalations, which imparted, to water impregnated with them, the property of dissolving calcareous matter; nor shall we want analogies to support us, if we assume, that a feeble remnant of the same action may even at this distance of time continue, and manifest itself in the sulphureous exhalations near Tivoli.

It would appear, that these indications (if they may be so considered) of languid volcanic action, were more extensively distributed about the neighbourhood in earlier periods than at the present. Thus Varro makes mention of warm baths near the temple of Janus, whence the spot obtained

NEIGHBOURHOOD Of rome.

the name of Lautolæ " à Lavando," a spot on the Esquiline Hill was called Puticulæ, from the sulphureous smell which it emitted; and the wood consecrated to the Goddess Mephitis renders it probable that a noxious gas arose from that place. All these have now ceased, and nothing remains but the Lago de Solfatara to remind us of their existence.

It is remarkable that no kind of animal is seen near this water, a circumstance which can only be attributed to the noxious qualities of the sulphuretted hydrogen, for the Lago de Tartaro near, so well known for its calcareous incrustations, contains abundance of molluscæ. Shells are also rare in the ancient travertine near Rome.

The existence of masses of this latter substance, on the very summits of the Seven Hills proves, that at the period of its formation, the site of Rome must have been covered with water to the depth of at least 140 feet.

From the character of the shells sometimes contained in the Travertine, which Brocchi has ascertained to belong to existing species, we may conclude that the water, which deposited it, was not impregnated with salt, and are consequently enabled to fix the date of the volcanic tuff which accompanies these Neptunian deposits, as corresponding with that of the latest freshwater formation.

Brocchi has further shewn, that the beds above noticed all rest upon a formation containing oysters and other marine shells, which is seen underlying the rest at the Monte Mario, and in the excavations made at the foot of the Capitoline Hill.

On the road between Rome and Naples, the first indications of volcanic action, after passing the Pontine Marshes, occur a little to the south-west of Mola de Gaeta, near the River Garigliano, the antient Liris.* We there find our

See Breislac Campanie, vol. 1. p. 86.

ROCCA MONFINA.

selves between two chains of hills, that to our right the Mount Massico, composed of the Appennine limestone, the other on our left, consisting entirely of volcanic materials. The town of Sessa itself stands on an eminence composed of tuff, which seems to be of comparatively recent date, since, like that of Herculaneum, it covers the remains of an antient city, for on digging the foundation of an house some time ago, there were discovered, many feet beneath the surface, a chamber with antique frescos, and the remains of an amphitheatre. Yet so far from any account existing of such a town having been destroyed, we have no tradition even of an eruption having taken place from any volcano near it since the memory of man. Not far from the same place, the bed of a rivulet, which has washed away the tuff, exposes two subjacent streams of lava, one of which closely resembles in composition that with which the Appian way was paved. Several other masses of lava, having the appearance of Coulées, occur either near Sessa or the neighbouring village of Casale, all of which seem referable to the volcanic mountain of Rocca Monfina, and one indeed from its freshness would seem to have proceeded from it at no very distant epoch.

Rocca Monfina seems to retain the vestiges of the great original crater from which these volcanic masses proceeded. In many parts indeed its sides have fallen in, but enough yet remains to enable the eye of the traveller to fill up the outline. The now detached hills, which appear to have resulted from the destruction of the walls of the crater, must have enclosed a circumference of no less than nine miles, but it is probable that the actual section is much below its former elevation, and that its height was at first considerably greater.

Within the space occupied by the original crater, two other volcanic cones have since been thrown up, each provided with its crater; the magnitude of one of them may be

PONZA ISLANDS.

judged of by the fact, that on the summit of the cone is a plain near a mile in circumference, bounded by two lofty eminences, which are the remains of it.

It appears therefore that the latest eruptions of this volcano have taken place since the country was inhabited by man, but it will hardly be considered probable that the whole of the tuff, which extends from the River Garigliano, beyond Capua, and connects itself with that of the neighbourhood of Naples, is in the same predicament with that of Sessa, which is of such recent date. It is not likely indeed that the Rocca Monfina was the source from whence the greater part of the tuff was derived, since its position on the summits of detached hills, as well as at their base, seems to shew that it has been produced antecedently to the general revolution to which we attribute the formation of the vallies.

It will not however be necessary to inquire further into its origin at present, as it is clearly the same with that near Naples, which will be afterwards more fully considered.

A few miles west of Mola de Gaeta lie the Ponza Islands, four of which appear to be entirely volcanic, the fifth (Jannone) in part Neptunian. No tradition exists of their having been in activity, nor was Dolomieu able to discover any traces of a crater. The most recent description of them is that communicated by Mr. Poulett Scrope, to the Geological Society, of which I extract the following notice from the Annals of Philosophy for July, 1824.

The whole of these islands, with the exception of Jannone, are composed of rocks, of the trachytic series, fine sections of which are presented along the coast. The Isle of Ponza is long and very narrow, and is eroded by the sea into deep concavities. Harder masses left along its shores show that it once was broader, and protruding ledges mark its former connexion with Quannone and La Gabbia. Prismatic trachyte, variously coloured and disposed, forms

MOUNT VULTUR.

the ossature of the island. It is constantly accompanied by, and alternates with, a semi-vitreous trachytic conglomerate, formed of minute pulverulent matter enclosing fragments of trachyte. The prismátic trachyte seems to have been forcibly injected through the conglomerate, and wherever it touches the latter, its earthy base is converted into a pitchstone porphyry; sometimes it becomes a pearlstone, at others it incloses a true obsidian. These rocks are connected with a siliceous trachyte, resembling in appearance the siliceous buhrstone of Paris.

Resting on the semi-vitreous trachyte, and forming the base of the Montagna della Guardia, is a rock 300 feet thick, which the author distinguishes mineralogically from common trachyte, and proposes to call greystone. In Jannone the trachyte overlies a limestone, which Brocchi describes as transition limestone; at the point of contact the latter becomes a dolomite.

On looking over Mr. Scrope's specimens, I was struck with the resemblance between many of them and the constituents of the Euganean Hills already described.

In the province of Basilicata, near the town of Melfi, rises in the midst of the chain of the Appennines a large isolated hill, the Mount Vultur, which Horace has celebrated as the scene of some of his early poetical adventures.*

This mountain, both from its conical figure, and the rocks composing it, is easily recognised as volcanic. On its slope, and scattered over various parts of its surface, are no less than twelve protuberances, thrown up by the action of

+ This account is extracted partly from a memoir of Brocchi in the Biblioteca Italiana, partly from Romanelli Topografia del regno di Napoli, vol. 2. p. 232, and partly from the Calendario di Napoli, for 1824.