SOLFATARA.

appears to have shewn very satisfactorily, that it results entirely from the sulphuretted hydrogen, and is not sublimed in an uncombined state; for we only find it in those parts of the mountain, which are near enough to the surface to admit of the ready access of atmospheric air. It is sometimes accompanied with sulphuret of arsenic, which has probably also been disengaged, combined with hydrogen, and be the cause of the destructiveness of those exhamay lations, which frequently succeed an eruption. I have been assured, that the paradoxical statement of Breislac's with respect to the pear and olive being proof against the deleterious influence of these gases, when all other kinds of vegetables are destroyed, is not unfounded. As selenium is now ascertained to exist among the products of the crater in the Island of Volcano, it appears probable, that a gas composed of that metal with hydrogen, will, ere long, be detected together with the gases before enumerated.

The influence of these exhalations is by no means confined within the compass of the Solfatara, for it extends to a considerable distance on the hills which bound that volcano towards the north, as is evinced by the whiteness and decomposed condition of the rocks, in consequence of being acted upon during so many ages by sulphureous vapours. It would appear that the antients designated them from this circumstance under the name of the Colles Leucogæi, for Pliny* mentions medicinal waters existing between Puzzuoli and Naples, called Leucogei Fontes, and there is still found a hot spring at a place called Pisciarelli, situated on the slope of these hills.

The Solfatara returns an hollow sound when any part of its surface is struck, and appears not to be made up of one entire rock, but of a number of detached blocks, which hanging as it were by each other, form a sort of vault over the abyss, within which the volcanic operations are going on. Hence there would seem, not to be one aperture larger

*Plin. Hist. Nat. Lib. 31. c. 8.

SOLFATARA.

than the rest, such as we are wont to suppose existing in the centre of every active volcano, but a series of fissures which allow of the escape of elastic fluids, but would prevent the fall of any bulky body.

It has given off however from its south-eastern side a stream of lava, which extends in one unbroken line to the sea, forming the promontory, called the Monte Olibano, on the road between Naples and Puzzuoli. It is remarkable from its want of resemblance to the lavas of most other volcanos, as it consists of a rock which can hardly, I think, be separated by its mineralogical characters from trachyte.

It consists essentially of crystals of glassy felspar, imbedded in a basis of a felspathic nature, having an uneven fracture, and ash-grey colour. It differs in these respects from the rock of the Solfatara itself, which I have found in general to possess a darker colour, and a conchoidal fracture. Both however agree in being composed essentially of felspar, and containing augite only as an occasional ingredient, so that we are obliged to class them equally under the head of trachyte. The stone of Mount Olibano is in general compact, but cells are sometimes present, especially in the upper part of the rock; it is seen to rest immediately on a thin bed of loose fragments of lava, and is covered by a sort of tuff, which, being placed on a part of the hill difficult of access, I left unexamined, but which I conjecture to consist of scoriæ, and sand, that succeeded the ejection of the lava.

The whole rests upon the extensive formation which reaches from Puzzuoli to Cumæ, and appears to be continuous with the rock found in the immediate neighbourhood of Naples. This, which has long been known by the name of Puzzolana, is a formation of volcanic tuff, bearing many analogies to the trass of the Rhine, and the pumiceous conglomerates of Hungary.

Its basis is generally of a straw yellow colour, dull and harsh to the feel, with an earthy fracture, and commonly a

PUZZOLANA.

loose degree of consistence. It contains imbedded fragments of pumice, obsidian, trachyte, and many other varieties of compact as well as cellular lava, the softer kinds often rounded, the harder mostly angular. I did not succeed in detecting any kind of rock except of a volcanic nature, imbedded in it, and as the construction of a new road round the promontory of Gaiola, gave me a good op portunity of examining this point, I do not believe that such specimens are common. Shells have been met with in this rock as is noticed by Sir W. Hamilton, and I have heard of the bones of ruminating animals having been likewise discovered.

This mass of tuff is sometimes separated into beds by strata of loam, pumice, or of a ferruginous sand, and Von Buch notices a bed consisting of fragments of limestone, cemented by calc-sinter, occurring in the midst of it between Naples and Puzzuoli.

The height of this tuff, in many places near Naples, is very considerable; the hill of the Camalduli, the loftiest eminence next to Vesuvius in the whole country, is composed of it, and to the west of Naples it forms a sort of wall, so lofty and abrupt, that the former inhabitants of the country apparently found it easier to avail themselves of the soft and friable nature of the stone, and to cut through, than to make a road over it.

This is the origin of the celebrated Grotto of Posilippo, a cavern 363 toises, or 2178 feet in length, 50 feet in height, and 18 in breadth, which strikes every stranger with surprise from the mass of rock cut through, until he reflects at the ease with which a stone of such a description admits of being hollowed out.

This immense mass of Puzzolana forms some considerable hills round Naples, many of which, as the Monte Barbara, Astroni, and others, have very regular craters, but do not appear to have thrown out any currents of lava.

LAGO D'AGNANO-GROTTO del cane.

The Lake Agnano has every appearance of occupying the original site of a crater, as the strata, whose edges are visible in its interior, instead of having a corresponding dip on the opposite sides of the cavity, as is commonly the case with vallies formed by diluvial action, seem on the contrary to slope in all directions away from the crater, just as would happen, if the strata had been thrown up by a force acting from below.

Perhaps the deep fissures, that are found so commonly in the neighbourhood of this and other hills near Naples, may owe their origin, like the Barancos of the Canary Islands,* to the upheaving of the contiguous rocks, rather than to the action of torrents.

The volcanic action here, as in many other places round Naples, seems hardly extinct, for there are exhalations of warm vapour constantly rising near the Lake, which are much esteemed in various complaints.

The celebrated Grotto del Cane, situated on its borders, is constantly giving out volumes of carbonic acid gas, containing in combination much aqueous vapour, which is condensed by the coldness of the external air, thus proving the more exalted temperature of the spot from whence it proceeded.

The mouth of the cavern being somewhat more elevated than its interior, a stratum of carbonic acid goes on constantly accumulating at bottom, but upon rising above the level of its mouth, flows like so much water over the brim. Hence the upper part of the cavern is free from any noxious vapour, but the air of that below is so fully impregnated, that it proves speedily fatal to any animal that is immersed in it, as is shewn to all strangers by the experiment with the dog.

The sensation I experienced on stooping my head for a

See the next Lecture.

LAKE AVernus.

moment to the bottom, resembled that of which we are sometimes sensible on drinking a large glass of soda water in a state of brisk effervescence. The cause in both instances is plainly the same.

The quantity of carbonic acid present in the cavern at various heights, was shewn by immersing in it various combustibles in a state of inflammation. I found that phosphorus would continue lighted at about two feet from the bottom, whilst a sulphur match went out a few inches above, and a wax taper at a still higher level.

It was impossible to fire a pistol at the bottom of the cavern, for though gunpowder may be exploded even in carbonic acid by the application of a heat sufficient to decompose the nitre, and consequently to envelop the mass in an atmosphere of oxygen gas, yet the mere influence of a spark from steel produces too slight an augmentation of temperature for this purpose.

It is probable that the Lake Avernus may likewise have been the crater of a volcano, and afterwards a Solfatara, which will account for the noxious properties attributed to it by the ancients.t

* Forsyth has erroneously stated that though torches of gunpowder lose their inflammability there, yet phosphorus resists the carbonic acid.

+ When we consider the considerable specific gravity of sulphuretted hydrogen gas, it is very reasonable to suppose that the thick woods which in former times surrounded the Lake Avernus would favour materially the accumulation of this noxious vapour. The surface of the lake screened from the access of the winds in every quarter, must have been covered with a thick stratum of unrespirable gas, which would be very slowly dissipated. If car'bonic acid were present, the same thing would take place with this, in even a much greater degree than with the former. But when the woods were cut down, as we are told was done by Agrippa, the air of the lake would become continually intermixed with the surrounding atmosphere, so that, unless a pretty rapid disengagement of gas took place, the noxious qualities would cease. This coincides with the accounts of Strabo, Silius Italicus, and other classical writers.