We are therefore under the necessity of attributing to the Eyfel volcanos, a date historically very antient, though geologically speaking modern, since geological research may be said almost to terminate, where history begins: if we adopt the opinions of Professor Buckland respecting the excavation of the vallies, we must suppose these rocks to have been formed, like some of those in Auvergne, subsequently to the deluge recorded by Moses; or, if limiting ourselves to those views in which all geologists concur, we choose to speak more indefinitely, the date of their eruptions must be pronounced to be posterior to the event which reduced the surface of the globe to its present condition.

It would seem however that these are the only volcanos in this part of Europe to which the same remarks will apply, for the remaining rocks near the Rhine to which we attribute a similar origin, no less than those in Hessia and on the borders of the Thuringerwald, belong evidently to a more remote period. The latter indeed all appear to have been submitted to the same agents that have affected the older formations; their craters, if ever they existed, have been obliterated; the evidences of their destruction are seen in the rolled pebbles at their foot; and they are intersected by deep vallies, which the action of running water could never have occasioned.

Yet it would appear that the Rhine volcanos have been principally formed during the period at which the tertiary beds were deposited, for I know of no instance where they are covered by any thing older than diluvial detritus, and in many cases they are seen to rest on the brown coal and other beds belonging to the plastic clay formation. Such is the case at the Habichwald near Cassel, at the Meisner near Eschwege, in the Westerwald east of Coblentz, and in the Siebengebirge near Bonn.

The latter chain of hills presents in some measure the general features of the whole, and shall therefore be noticed more particularly.

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On the eastern bank of the Rhine, rising abruptly from the borders of the river, are the mountains which have obtained this appellation from the seven peaks which strike the eye at a distance. To the north they seem to sink more gradually into the plain, but on the south they terminate in the "castled crag of Drachenfels," which offers on that side a very precipitous front.

This chain of hills consists partly of basalt, partly of trachyte, and the association of these two rocks without any determinate order being discoverable ought to stagger those, who with Beudant consider the two formations as produced by different processes, and at distinct epochs.

Here at least it seems difficult to determine which of the two be the most recent, or whether indeed they have not been ejected at one and the same time.

That it is in this manner that the seven mountains have been formed, appears probable from an examination of them on the side that overlooks the river. We here may trace for some distance up the hill a micaceous slaty sandstone belonging to the greywacke formation of the plain below, containing thin seams of anthracite. This is covered by nearly horizontal beds of a kind of trass, like that near the Lake of Laach, but containing less pumice. On ascending to a greater height we meet at length with the trachyte, which seems therefore to lie under the other rocks, at the same time that it rises above them.

This trachyte is traversed by vertical fissures, like the lava of Niedermennig, and like it is often cellular, and contains imbedded portions of a mixed rock looking like altered granite, but in which the felspar is changed into kaolin. The rock of Drachenfels consists of a somewhat different variety of trachyte, marked by its large and regular crystals of glassy felspar, and the mountain adjoining it on the west, called the Wolkenburg, differs from either, consisting of a number of minute acicular crystals of hornblende. It seems

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evident, that these rocks have both been forced up through the greywacke, which indeed continues until we reach nearly two-thirds of the entire height of the Wolkenburg. It is covered by the same marl which I have before noticed.

Near Königswinter however we observe a very compact quartzose conglomerate, lying betwixt the greywacke and the trachyte, which Professor Nöggerath has referred to the brown coal formation, seen so extensively on the western side of the Rhine. It is rarely observed so compact as in this instance, and it contains masses of opal in which the fibrous structure of the wood is plainly discernible. Thus the date of the trachyte is at least as recent as that of the oldest tertiary formation.

The bed of trass or trachytic conglomerate in the Seven Mountains is traversed by several dykes of a substance allied to the volcanic formation which it encircles.

One of these is seen on the hills above Königswinter pursuing an oblique direction, and at the same time sending out horizontal branches into the trass. The substance of the dyke is very various in composition, approaching in some cases to trachyte, and in others partaking of the characters of basalt.

Opposite to Bonn, at the northern extremity of the Siebengebirge, are the quarries of Obercassel, which have been described by Professor Nöggerath in the work already so often referred to.

They are composed of basalt, which, though more commonly compact, is occasionally found vesicular, and the vacuities, which are generally oval, appear to be of the same age with the rock, and not to have resulted from decomposition. Some of them are filled with calcareous spar, carbonate of iron, and other minerals, but others are void. The point however most worthy of notice in this rock, is the concentric arrangement of its parts.

The principal quarry, that of Ruckersberg, situated near

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the summit of the hill, displays this structure in the most satisfactory manner. We observe here not a cluster of globular concretions, like those of Bertrich, nor yet a line of prisms parallel to each other as in many parts of the Vivarais, but a succession of concentric coats, which on the northern side of the rock are broken away, but in that which is entire, are seen encircling one another in such a manner as to create an impression, that the whole may have once formed an immense spheroid, composed of a series of laminæ wrapped round a common centre.

That the form of this mass was elliptical, having its longest axis in an horizontal direction, appears from this circumstance, that to the north of this quarry, but exactly parallel to it, another section of the rock is exposed, in which the same concretionary structure presents itself, but the lamina form a portion of a curve turned exactly in the opposite direction to that of the former locality.

Thus let A be the quarry of Ruckersberg, and B the section exposed to the north of it, the following will be the disposition of the strata, from which it is evident that if the rock had not been broken away in the interval, an ellipsis would have been formed.

B

* See Nöggerath's Rheinland Westphalen, vol. 2. p. 250.

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We observe too as we descend, that whenever a section is exposed, the direction of the strata is such as corresponds very well with this idea, the rock being found at a short distance below, dipping towards the same point, but at a slighter angle, and at the bottom becoming at length completely horizontal.

Now this admits of a ready explanation if we suppose the whole mountain to have constituted the same great elliptical mass, since it is evident that agreeably with this structure, the dip of the strata would diminish progressively in proportion to their distance from the supposed centre, so that at length they would appear, when viewed within a limited space, altogether horizontal.

It is necesssary however for this hypothesis to assume, that the upper portion of the mountain has been swept away, since the axis of this immense spheroid is placed not in the present centre of the mountain, but about the site of the quarry of Ruckersburg near its summit. Now as this quarry is about 300 feet above the base of the hill, we must attribute to the latter an original elevation of between 500 and 600 feet.

Besides this division into concentric laminæ, the basalt of Obercassel has likewise a tendency to form columnar concretions, which always range at right angles to the curvature of the strata; hence when the latter are vertically disposed, the prisms are horizontal, and vice versâ; appearing in every intermediate position in accordance with the dip of the laminæ themselves.

The Seven Mountains, although they may appear isolated, are in fact a prolongation of the extensive basaltic formation of the Westerwald, which is connected with another considerable volcanic district, north-east of Frankfort, termed the Vogelsgebirge.

From the latter, the isolated basaltic cones of Frankfort