ene sandstone and shale with lignite, resting on the upturned edges
of the older rocks. Scattered over the greater part of the district here
referred to are a number of conical eminences, often with craters, the
bottoms of which are usually sunk much below the present level of the
country, and thus receiving the surface drainage, have been converted
into little lakes called "maars," differing from ordinary lakes by their
circular form and the absence of any _apparent_ outlet for their
waters.[1]

But before entering into details, it may be desirable to present the
reader with a short outline of the physical history of the region (which
has been ably done by Dr. Hibbert in his treatise, to which I have
already referred), so as to enable him better to understand the
succession of physical events in its volcanic history.

[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 Huendsruck 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 Huendsruck ridge drained away towards the south. But towards the
commencement of the Pliocene period the barriers of the Huendsruck 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