The beautiful city of Naples is entirely founded on volcanic sub stances. Among these the tufa predominates, which has also contributed not a little to the materials of many buildings. To the north and west it is accumulated in large heaps, and forms spacious hills. A philosophical stranger, on his arrival in this country, when he views these immense masses of a substance which must excite in his mind the idea of fire, cannot but feel astonishment, and enquire with a kind of serious thoughtfulness, what has been their origin. It is known that on this subject naturalists are divided. Some conjecture that the volcanic tufa was generated within the sea when it bathed the foot of the burning mountains; others suppose that the cinders ejected by the fire, have, in a long course of years, been hardened into this species of stone by the filtration of rain water; lastly, others incline to think that the tufa derives its origin from the slimy and fluid substances thrown out by the volcanoes in some of their eruptions.

The diversity of volcanic tufas has, perhaps, been the cause of these different opinions, each of which may possibly be true with respect to different kinds of tufa. Those, however, which are found in the vicinity of Naples are probably the produce of thick erup. tions, as we may conclude from the curious discovery of Sir William Hamilton, who, in digging up, in the tufa which had covered Herculaneum, the head of an antique statue, observed that the perfect impression of the head was visible in the tufa, which cannot be supposed to have happened but by its having enveloped the statue in a liquid or moist state.

To the observation of Sir William let me be permitted to add one of my own, which I made in the grotto of Posilipo. It is well known that this grotto has been excavated within the tufa, and serves as a public road from Naples to Pozzuolo. This tufa, which is of a clear grey, has for its base an earth in part argillaceous, of a slight hardness, which contains vitreous flakes, pieces of feltspars and fragments of yellowish pumice-stone, which by the changes it has undergone has become extremely friable, and almost reducible to powder. This tufa has been in some measure analysed by the excavation made in by art, which furnishes a proof of the nature of its origin. For if any person, in the summer time, enters the grotto about the rising of the sun, since at other times of the day there is

not sufficient light; the solar rays, shining on the entrance which looks towards Naples, will sufficiently illuminate the roof and sides to shew layers or flakes, similar to those which may be observed on the steep sides of mountains, or in perpendicular sections of the earth, in low places, where sediments of various kinds of slime have been formed by the inundations of rivers. It seems, however, impossible to doubt, that this accumulation of tufa, through the midst of which the Romans opened that long and spacious grotto, has been produced by the thick eruptions which have frequently issued from volcanoes, and which, heaping up one upon another, have hardened in time into this tufaceous stone; since both Vesuvius and Etna furnish sufficient examples of such eruptions. And as in many other tufas in the vicinity I have observed a similar constructure, I cannot suppose their origin to have been different.

Coming out of this subterraneous passage, and proceeding towards Solfatara, I observed, on the right-hand side of the road, a ridge of lava, nearly parallel with it, which had every appearance of having been thrown out of the volcano when burning, both because it was - extremely near to it, and had its highest part in that direction. Its thickness exceeded five-and-thirty feet, and it was situated between two layers of tufa, one above and the other below. It formed a high rock, perpendicular to one side of the road. A number of labourers were continually employed in separating pieces of this lava, with pickaxes, or other instruments proper for such work. It is compact, heavy, somewhat vitreous, gives sparks with steel, and appeared to me to have for its base the petrosilex. Incorporated with it are found shoerls and feltspars. The former are shining, of a dark violet-colour, in shape rectangular needles, vitreous, in length from the sixth of a line to two lines: it besides contains a considerable quantity of others which have no regular form. But the feltspars are more conspicuous than the shoerls; both from their larger size and greater number. They are, in general, of a flat rhomboidal form, and consist of an aggregate of small white lamellæ, dully transparent, brilliant, marked with longitudinal streaks parallel to each other, closely adhering together, but easily separated by the hammer, giving sparks with steel more readily than the lava; and, in the full light of day, exhibiting that changing colour which usually accompanies this stone. The largest are ten lines

long and six broad, and the smallest exceed one line. The shoerls are also found in the lava, in the same manner, and are so fixed in it, that they occupy nearly the half of it. It is impossible to extricate them entire. They are distributed within it without any order, and frequently cross and intersect each other at right angles.

In some situations of this lava, which are more than other exposed to the inclemency of the air and seasons, the feltspars are visible on the superficies, by a mixture of emerald and paonazzo, probably occasioned by the action of the atmosphere, as from the same cause some volcanic vitrifactions acquire externally their peculiar colour.

This lava has not equal solidity throughout, being in some places porous, or rather cavernous; and, in some of its varieties, it was remarkable, that it abounded with specular iron. This was found in very thin leaves, for the most part closely connected together. These are extremely friable; and the finger being passed over them, they adhere to it like particles of mica. But their small size, which, in the largest, is scarcely a line, renders it necessary to make use of a lens to examine them properly; by the aid of which we shall find that they are of very different shapes, have the lustre of burnished steel, and that many of them appear to be an aggregate of small thin scales, closely united.

This iron acts on the magnetic needle, at the distance of two lines. Like many other irons exposed to the air, it has acquired polarity; attracting the needle on one side, and repelling it on the other.

When we extract these thin scales of iron from the lava, and examine them with the lens, there frequently appear, intermingled with them, various fragments of microscopic transparent prisms, which I at first thought to be shoerls, or feltspars; but which afterwards I rather conceived to be zeolites, as they exhibited the appearance of radii diverging from a centre: but their extreme minuteness rendered it impossible accurately to ascertain their species.

Proceeding along the road to Solfatara, we find on the left hand a natural ridge of rock, formed of a very light lava, the base of which is horn-stone, of the colour of blue baked brick, of a coarse earthy grain, which attaches slightly to the tongue, and gives an ar

gillaceous scent, on wetting it, or even merely moistening it with the breath.

It is very probable that this lava has been decomposed, and that the decomposition has penetrated to the feltspars with which it abounds, as they are become very friable, though they in general still vetain their natural brilliancy.

Having made these cursory observations, I proceeded to Solfatara: nor did I satisfy myself with one visit only, but repeated it several days; being extremely desirous carefully to examine, and gain every information relative to a place so celebrated.

From reading the notes of M. Dietrich to Ferber's Travels in Italy, I had been induced to imagine that Solfatara was a mountain isolated on every sidet; but the truth is, it is connected with the other neighbouring mountains, with which it forms an uninterrupted chain of considerable extent.

It would be but of little utility for me to describe at length the form, extent, and circuit of this Phlegrean field; the various qualities of the hot vapours which exhale from it; or the hollow noise which is heard on striking the ground in various parts of it; not that these circumstances were not carefully examined by me, or that I think them unworthy of my narrative, but because it appears to me unnecessary to enlarge on them, as they have been already repeatedly described by a great number of travellers. It will, in my opinion, be more agreeable to the naturalist to proceed to a minute examination of the principal productions of this yet unextinguished volcano, as they have hitherto been, for the most part, either unobserved or passed over in silence.

In the obscurity and uncertainty in which we find ourselves, relative to the causes productive of subterraneous conflagrations, the spontaneous inflammation of sulphures of iron (or pyrites) has been considered as one of the most probable. The well-known experiment of Lemery, by which a similar conflagration is produced by

'In many lavas, the scent of clay is perceived, on moistening them with the breath, or by other means: whenever, therefore, I may hereafter mention the argillaceous scent of lava, I always understand it to have been subjected to this humectation, thoguh I omit to mention it, to avoid prolixity.'

+ La Solfatare représente encore aujourd'hui une montagne assez élevée et isolée de tous côtés.—Lettres sur la Mineralogie, &c. de l'Italie, &c.

mixing filings of iron with powdered sulphur properly moistened has given great support to this opinion. But sulphures of iron, in volcanic countries, are less frequent than has been supposed. This has been clearly proved by the accurate observations of mineralogists who have written on them. And though Sir William Hamilton expressly affirms that both Etna and Vesuvius abound with them, it is now well known that he mistook the shoerls for sulphures of iron (or pyrites), from want of mineralogical knowledge. In fact, Signior Dolomieu, in his Catalogo Ragionato de' Prodotti dell' Etna, mentions only one single piece of lava as containing sulpure of iron; and the Chevalier Gieoni, in his Litologia Vesuviana, has never noticed any such production. In Vulcano and Stromboli, two islands which are in a state of actual conflagration, I could trace no vestiges of such sulphures. As the same kind of substance, therefore, is found diffused in several parts of Solfatara, I think it well deserves that we should carefully consider it, and the bodies with which it is found united.

I. The stones which I here undertake to describe are principally found in the interior sides of Solfatara. The first I shall mention exhibits, both externally and internally, a number of shining par. ticles, which, when examined by the lens, appear to be small aggre gates of sulphure of iron, some crystallized in cubes, others in globes, and others in irregular figures. When the flame of the blow-pipe is applied to them, they begin to lose their yellow colour, which quickly, in consequence of their destruction,, entirely disappears; when an odour slightly sulphureous is emitted.

This substance is a lava, the base of which is horn-stone; in part decomposed, light, friable, granulous, and of a cinereous colour.

II. The small sulphures of iron in this second lava are less numerous, but in their qualities very analogous to that already described, except that they are less decomposed and less friable.

III. The appearances exhibited by this lava are two. The external part is extremely white, and so decomposed that the slightest blow reduces it to powder; we likewise find in it some of the external characters of ordinary clay. It tenaciously adheres to the inside of the lip, is soft to the touch, and becomes still more so

♦ Both these mountains abound with pyrites.—Campi Phlegræi.