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Show 200 OLDER PLIOCENE PERIOD. [Ch, XIV, '.fhe other organic remains of the brown coal arc principally fishes; they arc found in a bituminous s~ale, called pnper-~oa~, from beinO' divisible into extremely tlnn leaves. The 1nd1~ viduals ar: extr~mely numerous, but they appear to belong to about five species, which M. Agassiz informs me are aU extinct, and hitherto peculiar to the brown coal. They belong to the fresh-water genera Leuciscus, Aspius, and Perea. The remains of frogs also, of an extinct species, have been discovered in the paper coal, and a perfect series may be seen in the museum at Bonn, from the most imperfect state of the tadpole to that of the full-grown animal. With these a sala~ mander, scarcely distinguishable from the recent species, has been found. All the distinguishable remains of plants in the lignite and associated beds are said to belong to dicotyledonous trees and shrubs, bearing a close resemblance to those now existing in the country. The same is declared to be the case with the remains found in the trachytic tuffs and in the trass; but the absolute identification of species on which some geologists have insisted must be received with great caution. As trachytic tuff has been observed at several places inter .. stratified with the clay beds of the brown coal formation, and containing the same impressions of plants, there can be no doubt that the oldest eruptions began when :the fresh-water deposits were still in progress, and when the geographical features of the country must have been extremely different from those which it has now assumed. We have stated that the volcanic t::jections of the Roder berg repose upon a bed of gravel. This gravel forms part of an ancient alluvium which is quite distinct in character from that now found in the plains of the valley of the Rhine. It consists chiefly of quartz pebbles, and is found at considerable elevations both on the graywacke and brown coal beds. It forms indeed a general capping to the latter, varying from ten to thirty-five feet in thickness, and was probably an alluvium formed at that period when the ancient lake, in which the Ch. XIV.] AGE OF EIFEL VOLCANOS. 201 brown coal strata. were deposited, was drained ; for the disappearance of that great body of fresh water may naturally be supposed to have taken place when the country was undergoing great changes in its physical geography. Beds and large veins of quartz are found in the Hundsruck, Taunus, and Eifel, the nearest mountain-chains which border this part of the Rhine, and their degradation may have supplied the quartz found in this gravel called Kiesel gerolle by the Germans. It has been supposed by some writers that the latest volcanic eruptions of the Eifel and Rhine coincided in epoch with the deposition of the Loess before described (chap. xi.). Such an association, if established, would give a comparatively recent date to the most modern igneous eruptions; but I looked in vain for any clear indications of such a connexion, and all the sections which I saw appeared to indicate the posteriority of the Loess. The integrity of the volcanic cones is, for reasons before explained, a character to which we attach no value. We have, therefore, in this region, graywacke covered by brown coal, and some volcanic formations so blended with the latter as to prove the igneous eruptions to have been contemporaneous. Yet when we endeavour to assign a chronological position to any one part of the series by reference to organic remains, we discover that the evidence is vague and inconclusive. I have as yet been unable to obtain satisfactory proof that any one species of fossil animal or plant has been found in the brown coal, or superimposed formations which was com. mon to a tertiary group of known date in any other part of Europe; whereas the reader will bear in mind that the relative age of different tertiary formations, of which we have before spoken, was usually determined by reference to a comparison of several hundred, often more than a thousand, species of testacea *. • A memoir has lately been communicated to the Geological Sotiety of London, by Mr. Horner, on the geology of this district. For fuller dt>tails consult Noeggerath's Rheinland Westphalen, and the works of Vou Dechen, Oyen~ hausen, Von Buch, Steininger, Van der Wyck, Scropc, Daubeny, Leonhanl, and Hibbel't. |