and Hanau on the one side, and of Cassel and Eisenach on the other, seem to be ramifications. Different as these several chains may be to each other in various particulars, they have the following characters in common, by which, if I mistake not, they are distinguished from the basalts, which occur associated with secondary strata. Although frequently as compact as the latter, they are always, when viewed on the great scale, found to be accompanied more or less with cellular products, the cavities of which do not appear to have ever been filled with infiltrations of crystalline matter, which is the case with the amygdaloids accompanying secondary basalt. These cellular rocks frequently occur on the highest parts of the chain, as near Rennerod in the Westerwald, where they rest on compact basalt, as though the latter had been the effect of the pressure exerted by the superincumbent stratum. In other places the highest portions of the chain consist of compact basalt, whilst beds of cellular lava occur mantling round it, as if after the formation of the central mass, ejections of cellular matters had succeeded, which ranged themselves round its sides. Such is the case in the Vogelsgebirge. The volcanos of the Westerwald are also identified with those of the Siebengebirge in consisting partly of trachyte, and likewise in being accompanied with strata of trass, which seem to fill up the bottom of the vallies. Concerning the manner in which the latter has been formed, I shall only remark at present that the existence in it of slag and pumice, and the rare occurrence of fragments of compact lava, like that of the mountains encircling it, seem to shew that it is derived in a greater degree from ejections of pumiceous matters than from the detritus of the surrounding country. Occasional knolls of basalt lie scattered over the whole space between the Westerwald and Vogelsgebirge, as at Limburg, Wetzlar, &c. which some may consider as the relics of a continuous stratum once covering the country, NEAR EISENACH. whilst others may regard them as produced by distinct eruptions of volcanic matter. That the latter is the true state of the case appears, I think, from comparing them with the conical basaltic hills. near Eisenach, where circumstances have enabled us to ascertain clearly the relative position of the volcanic mass to the contiguous stratum. As the latter appear to me highly interesting, not only in themselves, but also as affording a key to the structure of the whole country we are considering, I shall proceed at once to describe them, without noticing the rocks intermediate. These knolls of basalt occur in the second or variegated sandstone formation, and appear on the surface perfectly unconnected one with the other. As they afford a valuable material for the roads, so much of them has in many cases been removed, that the rock which originally rose above the surface of the sandstone, is now worked considerably below the level of that rock. In tracing it downwards the mass is generally found to enlarge, so that its shape appears to resemble that of a wedge. The quarry in which the relation of the basalt to the contiguous rock is best displayed is the Pflasterkaute near Eisenach on the road to Frankfort.* * For a knowledge of this locality, and in some measure for the power of examining the geological phænomena therein displayed, I hold myself indebted to a very intelligent road-surveyor, M. Sartorius of Eisenach, who began many years ago taking advantage of his situation, to expose the rock in such a manner as might enable geologists in time to determine, whether the basalt merely lay upon the sandstone, as the Wernerians would suppose, or had been forced up through the midst of it from a considerable depth. His first account of this and other similar spots, was published in 1802, in a small pamphlet entitled “Die Basalte in der Gegend von Eisenach," but as every year's consumption of stone renders the excavations deeper. the fact must at present be exhibited in a more satisfactory manner than it was when he wrote his first pamphlet, as it becomes more and more difficult to explain the position of the basalt by any irregularity of surface in the subjacent sandstone, and less likely that any termination will be found to the dyke as we penetrate downwards. Sartorius has since published several other Geological tracts, which will be specified in the Appendix. NEAR EISEN лCH. In this instance the excavations are carried to such a depth, that we are enabled distinctly to see the basalt more than 50 feet below the surface of the sandstone. The line of junction is also well displayed, and we observe the sandstone changed from an horizontal to a vertical position, split in all directions, and rendered harder and whiter where the basalt touches it. The latter is in some places compact, and in others cellular, the cells partly empty, and partly filled with calcareous spar, quartz crystals, and zeolite. The central portion of the mass is always freest from these hollows, and at the surface is generally a kind of tuff made up of fragments of the volcanic rock cemented by clay or sand. The following may serve to give an idea of the relative position of the several rocks displayed in the quarry. Vertical Section of the Pflasterkaute near Eisenach. a a aaaa Basalt. bb Sandstone altered. ccc Sandstone unaltered. The Stoffelskuppe and the Kupfergrube, both which lie at no great distance, present similar phænomena, with this additional circumstance, that the masses of sandstone, which occur entangled in the midst of the volcanic rock, are sometimes prismatic. This last fact is however best displayed near the town of Budingen, north of Hanau, in the Wette rau. NEAR EISEN ACH. The prevailing rock in this country is the new red sandstone, but on the slope of the hill south of the town occurs a little eminence called the Wildenstein, consisting of columnar basalt, which seems altogether isolated, being encompassed on all sides by sandstone. Though we are not enabled, as in the case of the Pflasterkaute, to observe the relation of the trap to the surrounding stratum, yet it can hardly be doubted that it has been forced up in the same manner, and has thus carried with it the portions of sandstone that occur in the very midst of the basalt. These portions appear to have been curiously altered by the heat to which they were subjected; they are hardened almost to the degree of flinty slate, rendered white and splintery, and in most instances form clusters of little prisms, possessing even greater regularity of form than those of the basalt which encircles them. It is curious to mark the resemblance between the prisms here alluded to, and those produced artificially in several parts of Derbyshire and Yorkshire,* where the soft friable sandstone of the country is rendered serviceable for road-making by exposure to heat, which hardens and causes it to split into small columnar concretions. Sartorius has described a number of other basaltic eminences in the neighbourhood of Eisenach, all of which he supposes to have been thrown up in a similar manner, and many indeed of which are proved to be so constituted. He likewise endeavours to shew + that a certain connexion exists between many of these distinct cones, and that they are in some cases grouped round some common centre, forming a system of basaltic eminences, which possess a certain correspondence in position, and, as it should seem, in origin. In the case to which he appeals, the central point (the Dietrichberg) is a large overlying mass of basalt, the direc As at Rotherham. + See Sartorius, Geognost, Beobacht. Eisenach, 1821. p. 100. NEAR EISENACH. tion of which appears to be conformable with that of the subjacent rock, but in the smaller eminences of the same formation which are distributed round it, the basalt always is placed obliquely, and its inclination is found to vary according to its position with reference to the central mass. Sartorius from thence concludes that all these detached cones have been thrown up from a common point, and that the focus of the volcanic action lay immediately underneath the Dietrichberg, at a depth which he thinks might be calculated, by considering the distance of any one of these masses from the centre of the system, coupled with the degree of its inclination.* My principal motive for noticing this statement, which I had no opportunity of verifying, is to lead to an examination of other similarly constituted basaltic districts, with a view to ascertain whether any such arrangement or connexion can be perceived amongst them. At present all that I have stated must be understood to rest on the authority of Sartorious, to whose little publications I may refer for the most detailed account of these basalts, the igneous origin of which he has the merit of having maintained, even at the time when the authority of Werner was at its height. But the rock perhaps which exhibits the greatest combination of phænomena calculated to shake any preconceived opinion with respect to the aqueous origin of these basalts, is the Blaue Kuppe, near Eschwege, a town also in Hessia, but about twenty miles north-east of the above localities.+ He however admits the uncertainty of these calculations, for the depth at which the volcanic force resided, estimated by the obliquity of one of these masses, is 125,248 feet, or upwards of four Geog. miles, whilst by another it would be only 79,104 feet. It is evident indeed that the inclination of the basalt is influenced by too many causes to afford any correct data. + It is curious that Daubuisson in his "Account of the Basalts of Saxony," never alludes to this mountain, although he seems to have particularly examined the Meisner, which lies no more than half a dozen miles off from it, and to which he appeals as affording evidence of the aqueous origin of trap. |