LECTURES ON VOLCANOS.

LECTURE I.

ON THE VOLCANOS OF FRANCE AND GERMANY.

Introductory remarks.-Volcanos when to be considered active-when extinct-Characters by which the latter may be known—Classification of Volcanic products according to their Mineralogical constitution.

Extinct Volcanos of France-In Auvergne two classesPost-diluvial and Ante-diluvial-the former class considered their antiquity.-Problematical Rock of the Puy de Dome, &c.—Mode of its formation discussed.Second Class of Volcanic Rocks-1st, near Clermont, where they consist of Tuffs alternating with Freshwater Limestones.-2d, at the Mont Dor-its Basaltic and Trachytic Formation considered-3d, in Cantal.-Volcanic Rocks of the Puy en Velay-of the Vivarais— their age.--Volcanic Rocks in other parts of France briefly considered.

Volcanic Rocks of Germany-those near the Rhine-1st class, Post-diluvial.-Volcanos of the Eyfel-Crater of Laach-Lava of Niedermennig, Gerolstein, Mosenburg, Bertrich.-Trass of the Rhine considered-Whether the Post-diluvial Volcanos were in action since the existence

A

of Historical Records.-2d Class, Ante-diluvial considered. Of the Siebengebirge-Of the Westerwald— Of the Vogelsgebirge-Basalts near Eisenach, Budingen, Hanau, Frankfort.- Volcanic Rocks of the Brisgau, near Constance and in Wirtemburg, in Silesia, Bohemia, Moravia, &c. briefly considered.

It has often been a subject of dispute amongst Geologists, whether the processes, to which the earth is supposed to owe its actual condition, were the same with any that are taking place at present, differing only in magnitude, extent, and duration; or whether they must be explained by assuming a totally distinct system of causes, which, since the commencement of the present order of things, have ceased to exist.

The latter is the opinion expressed by Dr. Kidd in the close of his Geological Essay; and it is favoured more particularly by the appearances presented by the rolled masses met with every where at the bottom of vallies, which are now attributed by almost universal consent, to a body of water differing both in its cause and mode of action from our present rivers.

But this remark, however applicable it may be to the other forces that are now in operation, does not seem to extend either to earthquakes or volcanos, from both which agents effects have resulted even within the narrow limits of our own observation, which, although inferior in point of magnitude to some of those produced at former periods, seem nevertheless analogous in kind. It is therefore rather remarkable, that the progress of other departments of geology in this country should have tended so little to advance our knowledge of these particular subjects, and that the inquiries instituted with regard to the changes which the earth has undergone, should not have produced more frequently an appeal (to a class of phænomena so capable of illustrating them.

This is the more surprising when we consider that until lately the very questions that engrossed a more than ordinary share of attention, were precisely those which bore the nearest relation to the phænomena of volcanos; such, for instance, as the discussions relative to the origin of rocks of the trap family.

It is true that the resemblance existing between the pro ducts of actual volcanos, and the last mentioned class of formations, was often urged in proof of the general opera. tion of heat; but the discussions to which this hypothesis gave rise, do not appear to have occasioned the same minute inquiry into the structure of volcanic districts, which they were the principal means of exciting with regard to whin stone and basalt.

This neglect may perhaps be ascribed in the first instance to the circumstances of the times, which were such during the period at which these questions were most warmly agitated, as rendered the parts of Europe wherein active volcanos occur but little accessible; and at a later period perhaps the same effect may have been produced by the preponderance of the arguments in favour of the igneous theory, which might seem to render it superfluous to hunt for proofs in distant countries, while facts were every day accumulating from sources nearer home.

Those however who have been withheld by the latter consideration from attending to these phænomena, ought to recollect, that it cannot be reckoned sufficient to have established the bare position that trap rocks are produced by heat, since many subordinate inquiries still remain with regard to the particular structure and relations of these rocks, which call for a more minute examination, and render an appeal to the phænomena of existing volcanos still of importance.

Nor will it be viewed as an unprofitable undertaking, if we are able to shew by an extensive induction of particulars, that the differences between the products of antient and modern volcanos in all these respects are such, as we

might deduce à priori from considering the condition of the earth's surface at the several periods at which they were in action.

I recollect so long ago as the year 1816, when I was pursuing my studies at Edinburgh, being led by something like this train of thought, to meditate the excursions that I have since accomplished, in the hope of supplying in some measure this gap in our geological knowledge. It is true, that at the time I made this resolution, I was far from viewing the question as I do at present, or from being persuaded, as I now am, that volcanic and trap rocks are, for the most part at least, analogous formations, calculated mutually to reflect light upon each other; on the contrary, I was then rather a convert to the views of Professor Jameson, whose opinions on all subjects connected with geology, were received among his pupils with that respect, to which his acknowledged accuracy and extent of practical information, justly entitled them.

Still, with all my deference for the Professor's judgment, I never rose from the enquiry without a conviction that something was yet wanting to compleat the chain of his proofs, and that in order to determine whether trap rocks were really of igneous origin or not, the most effectual method would be to compare them in all their details with products universally acknowledged to be volcanic.

I felt that for this purpose a mere examination of hand specimens was not sufficient, the very spots themselves should be visited, and the circumstances of geological position, as well as the nature of the rocks associated, carefully compared with what we see in the trap districts, which have excited so much attention and dispute.

I could not help wondering that this inquiry, intimately connected as it is with the basaltic question, should never have been taken up by any of the zealous supporters of either system, that the volcanos of Auvergne for instance, should be known to us chiefly through a French work of