LECTURE II.

ON THE VOLCANIC COUNTRIES VISITED BY THE AUTHOR IN 1823-24.

Volcanic rocks of Hungary—either trachytic or basaltic— Division of Trachytic rocks, according to Beudant, into five species. Description of the 1st Species, Trachyte proper-Of the 2d, Trachyte Porphyry-Of the 3d, Pearlstone-Of the 4th, Millstone Porphyry-Of the 5th, Trachytic Conglomerate.-Alum rock of Hungary considered— Analogous formations in Great Britain-Older Porphyries of Hungary-Basaltic rocks of Hungary. Account of the Volcanic rocks of Transylvania communicated by Dr. Bouè.

Trachytic Formation near Grætz in Styria.

Volcanos of the North of Italy-Euganean Hills-Vicentine-Comparison of the Basaltic rocks as old as the Scaglia, and posterior to that deposit.

Volcanos of central Italy. - Radicofani — Lagoni of Tuscany.-Campagna di Roma-Monte Albano-Tuff of the Seven Hills-Tiburtine stone and other freshwater deposits. Volcanos of the South of Italy.-Those of Rocca Monfina Of the Ponza Islands-Of M. Volturno-Lacus Amsanctus.

Of Campania.-Vesuvius.—1st. History of the mountain— Condition of its Crater in the time of the RomansStructure of Monte Somma explained-Antiquity of its eruptions-Renewal of its activity in A. D. 79—Erup

tions since that period.-2d. Structure of the mountain— its Lavas-its ejected masses-and gaseous exhalations3d. Alterations produced in the neighbourhood of Naples by Volcanic action-Temple of Puzzuoli-Monte Nuovo. Solfatara-Chemical phænomena-Lava of M. Olibano. Puzzolana of Naples-Lake Agnano-Grotto del CaneLake Averno-Condition of the Phlegrean fields in the time of Homer.

Islands of Procida-and of Ischia-Lava of 1302-Sili ceous sinter and other volcanic products.

Lipari Islands.-Stromboli-its Volcano-its Dykes.Lipari-its Pumice and Obsidians.-Volcano-ils Crater. Rocks in Sicily connected with Volcanic operations.-Blue Clay or Marl formation.-Macaluba-Lago Naftia.Ante-diluvial Volcanic rocks of the Val di Noto-Of the neighbourhood of Etna.-Post-diluvial ditto.—Brydone's statement with respect to the Lavas of Jaci Reale corrected.-Antiquity of the Eruptions of Mount Etna considered.-History of the Volcano-Changes it has produced in the face of the neighbouring country-Port of Ulysses-Formation of the Monti Rossi.

HAVING in my preceding Lecture given a detailed account of the volcanos of France and Germany, I propose in my present to lay before you a briefer description of those which I visited in the years 1823 and 1824, supplying from time to time the deficiencies of personal examination by reference to the best authorities.

As my tour itself comprehended a portion of Hungary and Styria, together with the greater part of Italy, Sicily, and the adjoining islands, we may expect in the course of the details that I shall give, if not to light upon facts relative to this branch of Natural History that have hitherto escaped notice, at least to elucidate certain particulars less fully developed in the structure of the preceding countries, than in those about to be described.

Thus in order to study the Natural History of Trachyte, Hungary is the country that should be chiefly consulted, since from the extended scale on which the rocks belonging to this formation are developed, we are there enabled to follow them through all their modifications with a minuteness not practicable in Auvergne or in Germany.

With the assistance therefore of the elaborate work of M. Beudant, I shall endeavour to lay before you as complete a synopsis as possible of the principal varieties, which the trachytic formation of that country is found to present.

Beudant enumerates five distinct groups of mountains consisting wholly of trachyte, the characters of which are in all nearly the same, although particular parts of the formation may be more developed in one than in the rest.

The first of these groups, situated in the north-western part of Hungary, namely, in the district of Schemnitz and Kremnitz, occupies an elliptical space of about 20 leagues in its greater diameter, and 15 in its smallest.

The 2nd, a smaller group, south of the preceding one, forms the mountains of Dregeley near Gran on the Danube.

The 3rd, is the mountain group known by the name of Matra, situated in the heart of Hungary, east of the former. The 4th, a chain which commences at Tokai, and extends north to the heights of Eperies, in length 25 or 30 leagues, and in breadth about 5 or 6.

. The 5th, that of Vihorlet, east of the foregoing group, which is connected with the trachytic mountains of Marmorosch on the borders of Transylvania.

Not only do these several groups appear unconnected with each other, but it is Beudant's opinion, that almost each particular mountain has been separately formed, for their escarpments rarely correspond, as is the case with plateaus comprised of basalt, so that it is impossible to view them as the detached portions of one general bed cut away by the operation of subsequent causes.

Now in the formation distinguished by Beudant underthe generic term of trachyte, that geologist has noticed five kinds of rock, which, although possessing a common origin, present many important differences.

These five varieties he has designated under the names of Trachyte properly so called, Trachytic Porphyry, Pearlstone, Millstone Porphyry, and Trachytic Conglomerate.

Trachyte, properly so called, is characterized by its porphyritic structure, by the scorified and cellular aspect which it has such a tendency to assume, by its harsh feel, and by the presence of crystals of glassy felspar, generally cracked, and sometimes passing into pumice. Besides these, which may be regarded as essential to its composition, crystals of mica and hornblende are often present, and all these minerals are either confusedly united without any apparent cement, or by the intervention of a paste of a felspathic nature, sometimes compact, and sometimes cellular, This paste is generally light coloured, though different shades of red and brown are sometimes communicated to it by the presence of iron, and there is one variety in which the paste is perfectly black and semivitreous, intermediate in its characters between pitchstone and basalt, but distinguished from either rock by melting into a white enamel. Augite is sometimes present, and grains of titaniferous iron are often discoverable, but olivine rarely, if ever, occurs, and therefore appears to be the only mineral which has any claim to be considered as peculiar to basalt.

The 2nd species, called by Beudant Trachytic Porphyry, is distinguished from the preceding by the general absence of scorified substances. Neither hornblende, augite, nor titaniferous iron enter into its composition, but quartz and chalcedony, which are wanting in the former, are commonly present in this species. In its general aspect it bears a much nearer resemblance to the older formations than trachyte properly so called.

This description however applies only to the characters of the larger portion of the mass, for Mons. Beudant is com

pelled, in order to include all the varieties, to establish two subspecies, the one with, the other without quartz, and in both of these he notices a variety possessing a vesicular structure. The subspecies indeed, which is without quartz, even passes into pumice. Many varieties of Trachytic Porphyry contain a number of very small globules, which seem to consist of melted felspar, having often in their centre, a little crystal either of quartz or of mica. The assemblage of these globules, leaving minute cells between them, sometimes gives to the rock a scoriform aspect. The chalcedony often occurs in small geodes, and sometimes intimately mixed with the paste in which the crystals are imbedded.

Trachytic porphyry also appears to pass by imperceptible gradations into the next species, pearlstone, which is characterized by the vitreous aspect generally belonging to its component parts. It is evident, that this definition includes pitchstone and obsidian, but these are of rare occurrence in Hungary, the great mass of this formation being composed of the mineral called pearlstone, some varieties of which pass into pumice.

In its simplest form, this rock presents an assemblage of globules, varying from the size of a nut to that of a grain of sand, which have usually a pearly lustre, and scaly aspect, and are set, as it were, one upon the other, without any substance intervening.

From this, the most characteristic variety, the rock passes through a number of gradations, in which its peculiarities are more or less distinctly marked. In some varieties the globules are destitute of lustre, and exhibit at the same time sundry alterations in their size, structure, and mode of aggregation, till at length they entirely disappear, and the whole mass puts on a stony appearance, which retains none of the characters of pearlstone. On the other hand the globules, becoming less distinct, either resolve themselves into a paste resembling enamel, very fragile, in which separate portions approaching to a spherical form are indistinctly visible, or into a more vitreous and more homogeneous mass,