the other side was immediate, and as steep as the one we had ascended; but by throwing our legs across it, as would be done on the ridge of a house, and moving ourselves forward by our hands, weat length reached that part of it where it gradually widened itself and formed the summit of the cliff, which we found to have a perfectly flat surface, of the dimensions before stated.

Judging this to be the most conspicuous situation, we here planted the Union, and left a bottle sealed up containing a small account of the origin of the island, and of our having landed upon it, and naming it Sabrina Island.

Within the crater I found the complete skeleton of a guard fish, the bones of which being perfectly burnt, fell to pieces upon attempting to take them up; and by the account of the inhabitants on the coast of St. Michael's, great numbers of fish had been destroyed during the early part of the eruption, as large quantities, probably suffocated or poisoned, were occasionally found drifted into the small inlets or bays.

The island, like other volcanic productions, is composed prinei. pally of porous substances, and generally burnt to complete cinders, with occasional masses of a stone, which I should suppose to be a mixture of iron and lime-stone.

[Lowenorn. Payne's Geog. Extr. Phil. Trans. 1728. 1812.1

CHAP. XVII.

MINERALOGICAL REMARKS ON THE NATURE OF VOLCANIC MATERIALS.

THIS is a subject which still requires the attention of oryctologists. We are scarcely acquainted, says Dr. Thomson in his History of the Royal Society, with the nature of the rocks of which Etna or Vesuvius is composed: indeed, the task is extremely difficult; for these mountains are so surrounded with lava on all sides, that the rock itself, of which the mountain consisted before the volcano commenced, may perhaps be entirely concealed from view. The prodigious extent of

Etna, about a hundred miles in circumference, and the vast number of volcanic hills attached to its sides, at least forty-four in number, render the geognostic examination of the rocks, of which it was originally composed, almost impossible. Sir William Hamilton and several other writers on volcanoes suppose, that, previous to the commencement of the volcano, no mountain whatever existed; and that the whole mountain has been formed by successive eruptions from the crater. But this opinion is both improbable in itself, and destitute of evidence. Werner conceives, that volcanic hills are always composed of green stone, basalt, and the other rocks which constitute the independent coal formation, and the floetztrap. But the observations of Humboldt, on the volcanoes of America, will not admit of such conclusions; unless, indeed, we conceive granite and porphyry to belong to these formations; a supposition not very improbable, if we attend to the late observations of Von Buch and Professor Jameson; the former of whom in Norway, and the latter in Scotland, have found granite among secondary rocks.

Geologists are still deplorably ignorant of every thing relating to volcanic rocks. As a proof of this, it may be stated with truth, that the best treatise on volcanoes, which has hitherto appeared, was written by Bergmann, and published before the year 1780. Even lava itself, or the melted matter which issues from volcanoès, has been frequently confounded with basalt and green-stone. At one time it was the fashion to consider all hills composed of these two last rocks; and, in short, all floetz-trap hills as extinct volcanoes. Thus, Fojas de St. Fond, in his travels through Scotland, finds every where abundance of extinct volcanoes; and Mr. Raspe inserted a paper in the Philosophical Transactions, describing similar hills in Hessia, under the same appellation*. In like manner, the hills of Auvergne, in France, have been considered as extinct volcanoes by the French mineralogists; even D'Aubaisson, who was educated in an opposite school, has come to the same conclusion. Notwithstanding this, there can be little doubt that these hills stand in the same predicament with the floetz-trap hills of Scotland and Germany. These opinions, respecting the volcanic nature of basalt and green-stone, are now pretty generally laid aside. The followers of

* Phil. Trans. 1771. vol. lxi. p. 580.

Dr. Hutton, indeed, contend that they have been melted by heat, but do not suppose that they have ever belonged to a volcanic mountain.

Considerable doubts are entertained by some, whether pumice be a volcanic substance or not; and these doubts are founded on the supposition that pumice, though often observed in the neighbourhood of volcanoes, has never been seen mixed with lava, or actually flowing from a volcano. If the evidence of Tournefort be considered as sufficient, there can be no doubt that pumice is occasioually thrown out of volcanoes; for he describes various examples of it in his Voyage to the Levant. Pumice has been repeatedly observed floating on the surface of the sea, in immense quantity. To give one example, Mr. Dove, the captain of an Indiaman, observed it floating in the Atlantic Ocean, in immense abundance, over a tract of not less than 317 miles in length. The pumice was first observed in south latitude 35° 36′, west longitude 4° 9', and the shoals of it continued for several days. Now it is impossible to account for such appearances on any other supposition than that the pumice has been thrown from the bottom of the sea by some volcanic force; and, if that be admitted, it will follow that pumice, at least some times, though perhaps not always, is volcanic.

The following observations of Spalanzani upon an accurate investigation of the Solfatara †, near Naples, are peculiarly entitled to the attention of the mineralogist.

* Phil. Trans. 1728. vol. xxxv. p. 444.

+ The lofty side of Solfatara, near Naples, seems to exhibit, in a miauter degree, whatever is seen of this horrible kind on the great theatre of nature. This plain, which is about twelve hundred feet long, and a thousand broad, is embosomed in mountains, and has in the middle of it a lake of noisome blackish water, covered with a bitumen that floats upon its surface. In every part of this plain, caverns appear smoking with sulphur, and often emitting flames. The earth, wherever we walk over it, trembles beneath the feet. Noises of flames, and the hissing of waters, are heard at the bottom. The water sometimes spouts up eight or ten feet high. The most noisome fumes, foetid water, and sulphureous vapours, offend the smell. A stone thrown into any of the caverns, is ejected again with considerable violence. These appearances generally prevail when the sea is any ways disturbed; and the whole seems to exhibit marks of an earthquake in miniature.

The beautiful city of Naples is entirely founded on volcanic substances. 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 bills. 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 coutains 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 their origin to have been different.

cannot suppose

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 la mellæ, 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