rather an old date,* or or a short German tract of Von Buch's,+ and that of the reputed volcanos of Hungary, we should possess absolutely no authentic account, since one author represented the whole country as of aqueous origin,‡ whilst another described the very craters from whence the lava was ejected.

Our information with regard to the volcanic rocks of the Vicentin, of Sicily, and even of the country round Naples, was at that time still more imperfect, and although I am far from wishing to conceal that since the period to which I allude,§ many additions have been made to our knowledge of these and other volcanic districts, yet I believe it must be at the same time admitted, that even up to the present date the information communicated, at least in the English language, has been too scanty to enable us to determine the relation of these rocks to the trap formations of this and other parts of Europe; whilst in elucidating the nature of existing volcanos, the only direct fruit of the Huttonian

• Montlosier sur les Volcans d'Auvergne, 1802.

+ Mineralogische Briefe aus Auvergne, in the 2d volume of his Geognostiche Beobachtungen, Berlin. 1809.

Esmarck Kurze Bescreibung Einer Reise durch Ungarn, 1799.

Fichtel Mineralogische Bemerkungen von den Karpathen, Wien, 1791.

§ I allude to the winter of 1816-17, which I spent at Edinburgh. In 1819 I visited Auvergne, and published a short account of my observations in Jameson's Journal, vol. 3 and 4. In the same year the first notice of Beudant's researches in Hungary appeared in Daubuisson's Traité de Geognosie. They were three years afterwards more fully detailed in his own work, from which I have drawn a great part of my account of the structure of the volcanic rocks of that country. In 1820 Professor Buckland examined the Vicentin, and satisfied himself with respect to the analogy which the strata in that country seen alternating with volcanic products bear to the beds above the chalk in England. He also visited Auvergne and part of Hungary. Mr. Bakewell likewise has noticed Auvergne in his "Travels in the Tarentaise, London, 1823." The foreign contributions to the knowledge of volcanos will be seen by reference to my general list of works on the subject. It is to be regretted that the death of Professor Playfair so soon after his return from Italy should have prevented the public from being benefited by his researches in that country.

controversy was the work of Sir G. Mackenzie on the rocks of Iceland.

I have therefore been since induced to devote such a portion of my time as could be spared from other occupations, to an examination of various volcanic districts, and as the facts collected in the course of several journies on the Continent, undertaken chiefly with that object, however inadequate they may be to supply the deficiency complained of, promise at least to contribute something to the existing mass of information, I propose to embody them in the following sketch of the general structure of volcanos in various parts of the world.

The most obvious and practical division of volcanos appears to be that into active and extinct, the former class comprehending those which at any period since the existence of authentic records have been in a state of eruption; the latter such, as, though incontestably of the same nature, have never been remembered to exhibit signs o 'activity.

By this definition we exclude from the immediate consideration all rocks, which, whether of igneous origin or not, are so constituted as to evince that they have been formed in a manner different from those of existing volcanos, and confine ourselves to such as consist in part at least of products, which not only are known to result from fire, but seem never to have occurred without its intervention.

Thus independently of the circumstances connected with the figure of the mountain, the direction of its strata, and the existence of a crater, by which a volcano is usually characterized, there are certain circumstances in the aspect of the individual masses which appear to afford decided indications of a similar origin.

When for instance we observe a mountain constituted of materials possessing even in part a vitreous aspect and fracture, together with a cellular structure, especially if these cells are elongated in the same direction, if they are in general unoccupied by crystalline matter, and have a glazed

internal appearance, we need not hesitate in pronouncing the whole mass as volcanic, although all vestiges of a crater may be lost, and the form possess no analogy to that which belongs in general to mountains of the same class.

.. Now if we examine the rocks, which, from their possess ing the above mechanical structure, and from other circumstances, we are led to consider as incontestably volcanic, it will be found, wherever their mineralogical characters are not wholly obliterated, that they appear to belong to one of two substances distinguished by the nature of the simple minerals of which they consist.

The 1st, which, from the harsh and earthy feel it often possesses, has been denominated trachyte, is essentially composed of crystals of glassy felspar, often cracked, which are imbedded in a basis generally considered as being itself a modification of compact felspar. To this are sometimes superadded crystals of hornblende, mica, iron pyrites, specular iron, and more rarely augite, and magnetic or titaniferous iron ore.

The 2d substance resulting from volcanic operations, appears to be some modification of basalt, consisting essentially of augite, felspar, and titaniferous, or magnetic iron ore, generally accompanied with olivine, and occasionally with hornblende. In many cases indeed the ingredients are too intimately mixed to allow of our ascertaining their nature, except perhaps by the ingenious mechanical method of Monsieur Cordier; but it is always easy to distinguish this kind of volcanic product from trachyte, which even when it has the colour of basalt, melts before the blowpipe into a white enamel, whilst the latter retains its original colour after being fused.

As the only essential ingredient therefore of trachyte appears to be felspar, whereas basalt always contains augite, I shall, in speaking of volcanic products, use the liberty of

See Cordier sur les substances Minerales dites en Masse. Journ. de Phys. Vol. 82-83.

employing occasionally the term felspathic lava as synonymous to the former, and that of augitic lava to the latter... In order however not to prejudge the question with reference to trap rocks, I intend to confine myself to the consideration of such trachytes, and such basalts, as possess, at least in part, that mechanical structure which I have laid down as characteristic of indisputable lavas; and hence, as it will appear in the sequel, my attention will be limited to those formations which were produced at a comparatively recent epoch in the history of our planet.

Whatever conclusions indeed we may be disposed to draw with respect to the origin of certain of the older rocks, as for instance of the basalts and porphyries of the county of Antrim, it is clear that they ought to be distinguished from those forming the subject matter of these Lectures, which, from their close resemblance to the products of existing volcanos, are at once recognized as analogous formations; since the nature of the inference in these two cases differs not in degree, but in kind, the one being a deduction of reason-the other an immediate result of observation.

On the Volcanos of France.

The volcanic country to which my attention was originally directed, in the hope of obtaining some additional insight into the nature and origin of basalt, was that which occupies a considerable tract in the centre of France, known under the general term of Auvergne. This, and some other parts of the same country, also of a volcanic nature, were visited by me in the year 1819, and the result of my observations was in part inserted in the Edinburgh Journal of Science for the ensuing year. In the course of the summer of 1823, I also spent a few days in the same country, and

cleared up two or three points which in my former visit I had left undecided.

In my letters to Professor Jameson above referred to, I distinguished these rocks into two classes, the one formed before, the latter since the vallies were excavated; and in order to keep constantly in view this important difference, proposed to apply the term post-diluvial to the one, and ante-diluvial to the other. The adoption of such a nomenclature must not be supposed to imply the expression of any opinion on my part with respect to the much agitated question as to the identity of the particular deluge recorded in the Mosaic history, with the cause to which the excavations of the vallies and the formation of beds of gravel are to be referred.* All that is intended to be conveyed is a statement as to the relative antiquity of the rocks referred to these respective classes; and the terms ante-diluvial and post-diluvial seem preferable to ancient and modern, because the latter imply only their relation to each other; the former also their connection to the rocks with which they are associated.

Now the post-diluvial volcanic products in Auvergne are distinguished, as we shall see, from the remaining class in their external characters, as well as their position. They are more cellular, have in general a harsher feel, and more of a vitreous aspect, their surface presenting a series of minute elevations and depressions, and the scanty portion of soil which covers them affording but little pasturage, and that generally of the worst description.

The mountains referred to this division constitute a chain which rises considerably above the elevated granitic platform on which they rest, and extend at intervals over a space of above eight leagues from north to south; from whence the rocks which compose them may be often traced a considerable way into the valleys contiguous. Above

See the note on that subject attached to my paper entitled "Sketch of the Geology of Sicily." Edinburgh Philos. Journal for October 1825 sub fine.