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Show 198 OLDER PLIOCENE PERIOD· [Ch. XIV. the t~faceous alluvium of the Rhine volcanos called t·rass, which has covered large areas, and choked up some valleys now partially re-excavated. This trass is, lik.e the loess, ~nstratified. The base is composed almost entirely of pumice, in which are included fragments of basalt and other lavas, pieces of burnt shale, slate, and sandstone, and numerous trunks and branches of trees. If an eruption, attended by a copious evolution of gases, should now happen in one of the lake basins, we might suppose the water to remain for weeks in a state of violent ebullition, until it became of the consistency of mud, just as the sea became charged with red mud round the new island of Sciacca, in the Mediterranean, in the year 1831. If a breach should then be made in the side of the cone, the flood would sweep away great heaps of ejected fragments of shale and sandstone, which would be borne down into the adjoining valleys. Forests would be torn up by such a flood, which would explain the occurrence of the numerous trunks of trees dispersed irregularly through the trass. Crater of the Roderberg.-One of the most interesting volcanos on the left bank of the Rhine is called the Roderberg. It forms a circular crater nearly a quarter of a mile in diameter, and one hundred feet deep, now covered with fields of corn. The high] y inclined graywacke strata rise even to the rim of one side of the crater, but they are overspread by quartzose gravel, and this again is covered by volcanic scm·ire and tufaceous sand. The opposite wall of the crater is a scoriaceous rock, like that at the summit of Vesuvius. It is quite evident that the eruption in this case burst through the graywacke and alluvium which immediately overlies it; and I observed some of the quartz pebbles mixed with scorire on the flanks of the mountain, so p1aced as if they had been cast up into the air, and had fallen again with the volcanic ashes. On the opposite, or right bank of the Rhine, are the SiebengebirgeJ a group of mountains wherein analogous phenomena are exhibited. There also trachytic lavas have :flowed out and Cb. XIV.] AGE OF EIFEL VOLCANOS, .199 covered the graywacke; and basaltic currents of a somewhat later date have followed. There is, however, such a connexion between these rocks that a suite might be procured from the Siebengebirge, showing an insensible gradation from highly crystalline trachyte into compact basalt, with the accompanying passage of the hornblende ih the former, into augite in the latter. Age of the volcanic rocks of the Eifel uncertain.-Besides the ancient inclined graywacke, we have in the immediate vicinity of the valley of the Rhine, a nearly horizontal tertiary formation, called brown coal, from the association with it of beds of lignite worked for coal. The great mass of the igneous rocks are seen to be newer than this formation; and thus we obtain a relative date of much local importance for the volcanos of the whole region. This brown coal consists of beds of sand and sandstone, with nodules of clay-ironstone, and siliceous conglomerate. Beds of lignite of various thickness are interstratified with the clays and sands, and often irregularly diffused through them. This deposit was classed with the plastic clay at a time when every group of tertiary strata was referred to the age of some one of the subdivisions of the Paris basin ' but as no shells, either marine, fresh-water, or land have yet been found imbedded, it is not easy to decide the age of the formation. Near Marienforst, in the vicinity of Bonn, large blocks are found on the surface of a white opaque quartz rock, containing numerous casts of fresh-water shells which appear to belong to Planorbis rotundatus and Limnea longiscatus, two well-known Eocene species*; but this rock is not in situ, and may possibly have been a local deposit in some small lake, fed by a spring holding silica in solution. Yet, as there are beds of the brown coal at Marienforst, and this formation contains in other places subordinate beds of silex it seems to me most probable that the quartzose blocks alluded. to were derived from some member of that tertiary group. * M. Deshayes, to whom I showed the specimens, said he felt as confident of the above identifications as mere casts would warrant. |