3d. Masses of black porous lava covered by the prismatic basalt, which again serves as a support for masses of porous lava extending upwards for 200 feet towards the conical portion of the hill of Montpezat, from which it has probably descended, although all vestiges of a crater are obliterated on its summit.* I shall not detain you by enumerating the other postdiluvial volcanic products which I noticed in the Vivarais, as my examination was of a very hasty nature, and limited to ascertaining their general relations. There is indeed so much of a common character in the post-diluvial lavas of that country, that a detailed description would be of more interest with reference to the topography of that part of France, than to the natural history of volcanos in general. Compared however with the products of Etna and Vesuvius, they present one important difference, namely, in their more compact and basaltic appearance; connected with which is the general occurrence of a columnar arrangement, not derived like that met with in the latter localities, from the contraction, and consequent splitting of the mass, but arising probably out of a tendency to form a series of globular concretions, which the pressure exerted by the parts upon each other has reduced to prisms more or less regular according to the circumstances of the case.† The great similarity that exists between the volcanic formations of Auvergne and of the Vivarais, in age as well as in character, is sufficiently apparent from the facts already stated. In both countries we have proofs of volcanic eruptions which, though not noticed by history or tradition, must have been posterior to the formation of the vallies, and in both instances we observe a more extensive formation of trachyte and of tuff, indicating the operation of the same causes at a period which, though antecedent to the latter See Faujas St. Fond, Volcans du Vivarais. + See this more fully explained in my fourth Lecture. epoch, were either subsequent to, or coincident with, that of the deposition of the latest class of rock formations. In either case the general tenor of the phænomena leads us alike to the conclusion, that the period, at which these latter mentioned eruptions must have occurred, was that, at which the great mass of the ocean had retired, having left however in the hollows those lakes, to which we attribute the formation of the calcareous and gypseous deposits containing freshwater shells, seen alike in the vallies of Cantal, of the Limagne, and of the Puy. These eruptions seem more generally to have given rise to a formation of volcanic tuff or breccia, but associated with the latter are those trachytic lavas, which seem referable to the same or to a somewhat more recent epoch. The trachyte of Cantal, for example, seems to be contemporaneous with the tuff which alternates with the freshwater limestone at Salers, but its greater compactness, and the absence of cellular products, lead us to imagine it more ancient than the analogous formation which I have described as existing in the Mont Dor. This corresponds well with what M. Bertrand Roux has inferred with respect to the age of the trachyte and porphyry slate of the Mezen, which he concludes to be more modern than the greater part of the tuff that surrounds it, from the occurrence of fragments of these rocks only in its upper strata. Evidences of volcanic agency are not exclusively confined to the districts of France we have been considering, they occur likewise still farther south among the Cevennes, and near the shores of the Mediterranean in the neighbourhood of Marseilles, and of Montpellier. Of these the rock which has been most noticed by geologlists is that of Beaulieu, near Aix in Provence, described by Saussure, Faujas St. Fond, and still more lately by Menard de Groye.* It is stated by the latter as about 1200 * See Journ de Phys. vol. 82, 83. fathoms in length, six or seven hundred in breadth, and rising to about two hundred fathoms above the level of the sea. It is composed of basalt which, as we trace it downwards, is seen to pass into a very crystalline greenstone, and is covered by an amygdaloidal wacke, the cells of which are empty near the surface, probably from the decay of the rock, but in the interior are filled with calcareous matter. The latter is sometimes so diffused through the substance of the rock, that it forms with it a kind of breccia, and even swells out into nests or geodes of considerable size, imbedded in the midst of the tuff. Shells are also contained in this formation, and serve to connect it still more closely with the limestone covering it, the recent origin of which may be inferred from the existence in it of bones of ruminating animals. The limestone is compact, and passes into a siliceous kind of rock, probably a chert, which is called by Haüy quartz agate calcifere. Upon the whole I suspect that the basalt of Beaulieu, like the rocks of Auvergne and the Vivarais, belongs to the epoch at which the tertiary class of rocks were formed, though M. Menard de Groyé's account is not such as to be completely decisive. Neither is there any thing in the structure of the rock of Beaulieu, so far as we can judge from his description, which can be viewed as establishing its volcanic origin more fully than might be done in the case of almost every other trap rock, for the analogies he has pointed out between the products of existing volcanoes and those of which he has given us an account, would hold good equally in every other case, and the evidence of igneous injection derived from the presence of dykes is in this instance wanting. A league to the south-east of Agde, a town placed near the sea at a distance of about twelve miles west of Cette in Languedoc, is the hill of St. Loup, which belongs to the most recent order of volcanos.* * See Journal des Mines, No. 141, p. 231. Its crater, which is considerable, has given rise to two currents of lava, on one of which the town of Agde is erected, whilst the other, having taken the direction of the sea, has formed a neck of land called Cape Agde, and a little island at a short distance from the shore. M. Marcel de Serres has also described two other rocks to which he assigns the same origin and date; the one that of St. Thibery about four miles north of Agde, the other that of Montferrier near Montpellier. The latter I visited some years ago, and found to consist entirely of compact trap, so that I should be disposed to view it as considerably more ancient. On the Volcanos of Germany. After this general description of the Volcanos of France, I shall proceed to lay before you a short sketch of those which occur distributed over various parts of Germany. Although no active volcanos are found in any part of that extensive country, and the recognition of those which are extinct dates only from the last century, yet those who have visited the spots themselves will feel no more doubt as to their having once existed, than an American who had witnessed the burning mountains of his own hemisphere, but had never heard of those in Europe, would entertain with respect to the real nature of Vesuvius, if landed at its foot when it chanced to be in a tranquil state. This remark applies to no case more completely than to that of the rocks which occur in a district commonly known by the name of the Eyfel, situated between the Rhine and the present frontier of the Netherlands.*. * This account of the Rhine volcanos is principally drawn (where the reverse is not stated) from observations made by myself during a tour in that country in the Summer of 1825. This country is bounded on the south-east by the Moselle, on the north-east by the Rhine, on the west by the Ardennes and the other mountains round Spa and Malmedi, and on the south by the level country about Cologne. The fundamental rock which comes to view is clay-slate, associated with greywacké, and with a saccharoid magnesian limestone containing trilobites and other petrifactions, which stamp it as belonging to the transition series. These rocks in a few places support horizontal beds of what appears to be the second or variegated sandstone formation. Scattered however over the greater part of the district alluded to, are a number of little conical eminences, often with craters, the bottoms of which are usually sunk much below the present level, and have thereby in many cases received the drainage of the surrounding country, thus forming a series of lakes, known by the name of "Maars, which are remarkably distinguished from those elsewhere seen by their circular form, and by the absence of any apparent outlet for their waters. Steininger, a geologist of Treves, who has published the most circumstantial account of this district that has yet appeared, distinguishes these craters into three classes. The first includes those properly speaking known by the name of "Maars,"-volcanos, which have ejected nothing but loose fragments of rock with sand and balls of scoriform lava. In this class are: 1. The Lake of Laach. 4. Two at Gillenfield. 5. One at Bettenfield. 6. One at Dochweiler. 7. One at Walsdorf. 8. One at Masburck. No. 6 and 7, however, have fallen in. *See, for an enumeration of his works, the Appendix. |