which those minerals which are found at Monte Somma, and the modern part of the mountain distinguished as Vesuvius, are duly specified.

Minerals found about Vesuvius.

E. B.

S.

S.

tions of volcanic matter seem to have been detached from some torrent of lava that lay within the sphere of the volcanic operations; these disrupted masses are called Erratic Lavas, to express at once their supposed origin, and to separate them from such as constitute a part of some existing current.

It appears from this statement, that the greater number of the above minerals are confined to Monte Somma, and that the only ones at present known to be produced by Vesuvius are, augite, hornblende, breislakite, mica, magnetic iron, sodalite, and more rarely leucite, if indeed the latter be not merely ejected.

The gaseous exhalations given off by Vesuvius both in 1834 and in 1845, when I made them the subject of a particular examination*, proved to be muriatic acid and aqueous vapour; but on neither of these occasions could the mountain be regarded as in a tranquil state, when I suspect sulphur would be found to be disengaged in combination either with oxygen or with hydrogen gases.

* The apparatus I employed for collecting those gases which might be absorbable by water, consisted of the head of a large glass alembic, having luted to its larger aperture an iron cylinder, which admitted of being introduced into the earth at the spot from which the vapour issued.

By keeping the upper part of the glass vessel cold by the external application of ice or wet cloths, I succeeded at Vesuvius on two occasions, and likewise at the Solfatara, and at the Lipari Islands, in condensing a quantity of the steam sufficient for ascertaining the nature of the gases with which it was impregnated.

In order to determine the gases not readily absorbable in water which might issue from the orifice of the volcano, I could devise no better plan, than that of a large funnel covering the aperture, to the tube of which was attached a long flexible india-rubber tube. The latter communicated with a glass bottle having two apertures, into the upper one of which the flexible tube was inserted, whilst the lower could be opened or shut at pleasure by means of a stop-cock. The bottle being filled with water, and the funnel inserted into the fumarole, it is evident, that on opening the stop-cock the air of the funnel would be drawn into the bottle, in proportion as the water escaped.

I made trial of this apparatus at the crater of Vesuvius in October 1845, and found that the air admitted into the glass vessel was charged with carbonic acid, as it rendered lime-water immediately turbid; but not being able to meet, amongst the scoriæ and lapilli which were present in the crater, any material of a plastic and adhesive nature, by which the access of external air could be excluded from the funnel when the latter was placed over the fumarole, I regarded it a waste of time to institute any

pro

I had reason to suspect the presence of carbonic acid, but no unexceptionable means of proving it occurred to me. I have stated that muriate of ammonia was also present in the lava of 1834, but none could be detected at that time ceeding from the crater itself.

It has been asserted, that nitrogen gas is also disengaged, but I was unable to devise a mode of determining whether such was the case when I visited the crater,

Nor ought I to omit stating, that Professor Pilla has lately observed flames on three occasions issuing from the crater whilst in an active condition, from which it may be inferred, that hydrogen gas in some of its combinations ranks amongst the aëriform products of this volcano. A full discussion however of a fact so important will be best introduced, when the theory of volcanos comes under our consideration.

Besides these products however which belong to the more active states of the volcano, fatal Moffettes, or exhalations of noxious gases, are given out from crevices in all parts; they are frequently found in the cellars of Portici and Resina after an eruption, and, as already stated, when they rise up through the land, prove speedily destructive to vegetation. They are supposed to consist of carbonic acid, and this is undoubtedly the prevailing ingredient in their composition. more particular examination of the air which had been collected than could be accomplished by a few hasty trials.

The following is a representation of the apparatus :

B

C

A, Funnel; B, Caoutchouc tube; C, Bottle. The advantage of this apparatus is, that the bottle can be filled with the gases, whilst the operator remains at a safe distance from the fumarole. But no dependence can be placed upon the results, unless the funnel be buried under some material capable of excluding the external air, except in cases where the disengagement of gas from the fumarole is going on with considerable rapidity.

CHAPTER XIII.

ISLANDS OF PROCIDA AND ISCHIA.

Procida-its structure.-Ischia-its tuff-trachyte-lava-streams-hot springs and vapour issuing from the rocks-eruptions in historical times.

THE neighbouring islands of Procida and Ischia are likewise composed of volcanic rocks bearing a considerable resemblance to those of the Campi Phlegrei.

The former island seems to consist entirely of tuff separated by beds of cellular lava. In one part where the coast exposes a section of the strata, I observed that they were so contorted as to represent an arch, whilst in the places intermediate, as well as on either side, they appeared horizontal.

This effect, happening as it here does in a volcanic country, seems attributable to the pressure of elastic vapours from below, which may have heaved up the strata round a given area; and it will be readily perceived, that if the force applied had been considerable enough to cause a disruption of the beds, we should then have had them dipping in all directions away from the opening, as happens where a crater is produced.

In the following sketches No. 1 represents the actual section, and No. 2 that which would have resulted had the force been somewhat greater. In either case A represents the beds of tuff, and B those slaggy lava interposed.

The island of Ischia is somewhat more varied in its composition. It is composed for the most part of a rock which seems to consist of very finely comminuted pumice, reagglutinated so as to form a tuff, sometimes resembling the Puzzolana of Naples.

From the very fine state of division however into which it was reduced at the time when it underwent consolidation, a rock has often resulted of so homogeneous a texture as to be considered a variety of felspathic lava; but I am, upon the whole, rather disposed to believe it to be a substance resulting from the reaggregation of fragments of pumice and other analogous products *.

This formation is often separated by beds consisting of loose portions of pumice and obsidian, and with this exception nearly all the island may be said to consist of tuff, which extends even to the summit of Mount Epomeo, its loftiest point, attaining a height of 2605 feet above the sea.

I could discover nothing like a crater on the summit of this hill, but conceive its superior elevation to arise merely from the rock in this locality having resisted decomposition more than it had done elsewhere.

Although the pumiceous conglomerate, as I shall venture to call this rock, is seen in every part of the island, yet at Monte Vico, near the town of Foria, we observe thrust up through the midst of it huge blocks of trachyte, sometimes thirty feet in diameter, consisting of a congeries of crystals of glassy felspar, without any thing intermediate. These blocks are angular and of irregular shape; they seem in some places scattered without any order through the substance of the tuff.

A little beyond the village of Casamicciola is a conical hill, called the Monte Thabor, composed entirely of trachyte,

* Mr. Forbes and M. Dufrenoy both agree with me in regarding the rock of Mount Epomeo as a species of tuff or conglomerate, although Brocchi and other eminent geologists considered it as an earthy variety of trachyte. It certainly bears a considerable resemblance to the rock of the Puy de Dôme, but there are on the other hand varieties of the pumiceous conglomerate of Hungary which possess in quite as great a degree the characters of a trachytic rock; and this consideration, coupled with the frequent presence of scoriform portions different in colour from the matrix, confirms me in the view I have taken of its nature.