black, while the felspar whitens; it then acquires the property of strongly acting on the magnet.

"Most porphyritic lavas are susceptible of a fine polish, which always increases the strength of their colour; they then acquire as much brightness and beauty as natural porphyries, and may be substituted for them; only porphyries of a purple, and green bases, are not found among them, because those two colours become black in a less degree of heat than that of volcanoes.'

The most common porphyritic lava of Etna is of a greyish black with white spots, the base resembling basalt. But the work of Dolomieu having been published before mineralogy had acquired great precision, it is to be feared that he has often confounded the lavas with the original rocks.

In one of his porphyritic lavas he observed crystals of specular iron; and as he also observed this metal in the same state in the dross of Monte Rosso, he concludes that it is formed by sublimation*.

* Etna, 379.

Analysis.

NOME II. VESICULAR LAVA.

This is the most general and undoubted product of volcanic fires. The vesicles are sometimes of an oblong form, but often spherical, especially in those with a base of siderite, which, even in vitrification, does not assume the fibrous form common to other substances.

From the lava which contains leucite, Vauquelin derived silex 53, argil 18, lime 2, oxyd of iron 6, potash about 17. The leucite itself contained very little iron, but presented the same ingredients as the lava, with 20 of potash.

Vesicular lava is the most common and characteristic production of volcanoes, among which Etna has been chiefly celebrated for more than two thousand years. The torrents of liquid fire, vaguely mentioned through a long series of learned and illiterate ages, consisted of inflamed vesicular lava. Many were the attempts to explore the source of this phenomenon, the sumSummit of mit of a mountain so interesting to curiosity and even to science. But the best account is that of Spallanzani, at once a natural philosopher and a mineralogist, and who has sprinkled his description with some learned anecdotes of the his

Etna.

tory of this celebrated mountain. Its length and minuteness will only render it the more acceptable to the intelligent reader, especially as they may serve to diversify the dry brevity of some parts of this domain. It may also be considered as a counterpart to the description of the summit of Mont Blanc, by Saussure, which is given in a former division of this work.

"Three hours before day I, with my companions, left the Grotta delle Capre, which had afforded us a welcome asylum; though our bed was not of the softest, as it consisted only of a few oak leaves scattered over the floor of lava. I continued my journey towards the summit of Etna; and the clearness of the sky induced me to hope that it would continue the same during the approaching day, that I might enjoy the extensive and sublime prospect from the top of this lofty mountain, which is usually involved in clouds. I soon left the middle region and entered the upper one, which is entirely destitute of vegetation, except a few bushes very thinly scattered. The light of several torches, which were carried before us, enabled me to observe the nature of the ground over which we passed, and to ascertain, from such experiments as I was able to make, that our road lay over lavas either perfectly the same with, or analogous to,

those in which the Grotta delle Capre is hollowed.

"We had arrived at within about four miles of the borders of the great crater, when the dawn of day began to disperse the darkness of night. Faint gleams of a whitish light were succeeded by the ruddy hues of Aurora; and soon after the sun rose above the horizon, turbid at first and dimmed by mists, but his rays insensibly became more clear and resplendent. These gradations of the rising day are no where to be viewed with such precision and delight as from the lofty height we had reached, which was not far from the most elevated point of Etna. Here likewise I began to perceive the effects of the eruption of Etna, which took place in July 1787, and which has been so accurately described by Drosses. the Chevalier Gioeni*. These were visible in a coating of black scoriæ, at first thin, but which became gradually thicker as I approached the summit of the mountain, till it composed a stratum of several palms in thickness. Over these scoriæ I was obliged to proceed, not without considerable difficulty and fatigue, as my leg at

"His account of this eruption was printed at Catania, in 1787. There is likewise a French translation at the end of the Catalogue Raisonné of M. Dolomieu." An English translation of this singular account is afterwards here given.

every step sank deep into it. The figure of these scoriæ, the smallest of which are about a line, or somewhat less, in diameter, is very irregular. Externally they have the appearance of scoriæ of iron; and when broken, are found full of small cavities, which are almost all spherical, or nearly of that figure. They are therefore light and friable, two qualities which are almost always inseparable from scoriæ. This great number of cavities is an evident proof of the quantity and vigorous action of the elastic fluids, which in this eruption, imprisoned in the liquid matter within the crater, dilated it on every side, seeking to extricate themselves; and forced it, in scoriaceous particles, to various heights and distances, according to the respective weights of those particles. The most attentive eye cannot discover in them the smallest shorl; either because these stones have been perfectly fused, and with the lava passed into homogenous consistence, or because they never existed in it. Some linear felspars are however found, which by their splendour, semitransparency, and solidity, show that they have suffered no injury from the fire. When these scoriæ are pulverized, they become extremely black; but retain the dryness and scabrous contexture which they had when entire. They abound in iron, and in con