[Illustration: Fig. 20.--Sketch Map to show the physical condition of the Rhenish area in the Miocene epoch.--(After Hibbert.)] (_b._) _Physical History._--From the wide distribution of stratified deposits of sand and clay at high levels on both banks of the Rhine north of the Moselle, it would appear that an extensive fresh-water basin, which Dr. Hibbert calls "The Basin of Neuwied," occupied a considerable tract on both banks, in the centre of which the present city of Neuwied stands. This basin was bounded towards the south by the slopes of the Hündsruck and Taunus, which at the time here referred to formed a continuous chain of mountains. (Fig. 20.) To the south of this chain lay the Tertiary basin of Mayence, which was connected at an early period--that of the Miocene--with the waters of the ocean, as shown by the fact that the lower strata contain marine shells; these afterwards gave place to fresh-water conditions. The basin of Neuwied was bounded towards the north by a ridge of Devonian strata which extended across the present gorge of the Rhine between Andernach and Linz, and to the north of this barrier lay another more extensive fresh-water basin, that of Cologne. From this it will be seen that the Rhine, as we now find it, had then only an infantile existence; in fact, its waters to the south of the Hündsruck ridge drained away towards the south. But towards the commencement of the Pliocene period the barriers of the Hündsruck and Taunus, as also that of the Linz, were broken through, and the course of the waters was changed; and thus gradually, as the river deepened its bed, the waters were drained off from the great lakes.[2] This rupture of the barriers may have been due, in the first instance, to the terrestrial disturbances accompanying the volcanic eruptions of the Eifel and Siebengebirge, though the erosion of the gorges at Bingen and at Linz to their present depth and dimensions is of course due to prolonged river action. It was about the epoch we have now arrived at--viz., the close of the Miocene--that volcanic action burst forth in the region of the Lower Rhine. It is probable that this action commenced in the district of the Siebengebirge, and afterwards extended into that of the Moselle and the Eifel, the volcanoes of which bear evidence of recent date. Layers of trachytic tuff are interstratified with the deposits of sand, clay, and lignite of the formation known as that of the Brown Coal--of Miocene age--which underlies nearly the whole of the volcanic district on both sides of the Rhine near Bonn,[3] thus showing that volcanic action had already commenced in that part to some extent; but it does not appear from Dr. Hibbert's statement that any such fragments of eruptive rock are to be found in the strata which were deposited over the floor of the Neuwied basin.[4] It will be recollected that the epoch assigned for the earliest volcanic eruptions of Auvergne