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
was that here inferred for those of the Lower Rhine--viz., the close of
the Miocene stage--and from evidence subsequently to be adduced from
other European districts, it will be found that there was a very widely
spread outburst of volcanic action at this epoch.

(_c._) _The Range of the Siebengebirge._--This range of hills--formed of
the older volcanic rocks of the Lower Rhine--rises along the right bank
of this noble river opposite Bonn, where it leaves the narrow gorge
which it traverses all the way from Bingen, and opens out on the broad
plain of Northern Germany. The range consists of a succession of conical
hills sometimes flat-topped--as in the case of Petersberg; and at the
Drachenfels, near the centre of the range it presents to the river a
bold front of precipitous cliffs of trachyte porphyry. The sketch (Fig.
21) here presented was taken by the author in 1857 from the old extinct
volcano of Roderberg, and will convey, perhaps, a better idea of the
character of this picturesque range than a description. The
Siebengebirge, although appearing as an isolated group of hills, is in
reality an offshoot from the range of the Wester