el district and on the left bank of the Rhine--as,
for example, in the case of the Roderberg--may be regarded as sufficient
evidence that this range is of comparatively high antiquity. It seems to
bear the same relation to the more modern craters of the Eifel and
Moselle that the Mont Dore and Cantal volcanoes do to those of the Puy
de Dome. In both cases, denudation carried on throughout perhaps the
Pliocene and Post-Pliocene periods down to the present day has had the
effect of demolishing the original craters; so that what we now observe
as forming these ranges are the consolidated columns of original molten
matter which filled the throats of the old volcanoes, or the sheets of
lava which were extruded from them, but are now probably much reduced in
size and extent.

Having thus given a description of the older volcanic range on the right
bank of the Rhine, we shall cross the river in search of some details
regarding the more recent group of Rhenish volcanoes, commencing with
that of the Roderberg, a remarkable hill a few miles south of Bonn, from
which the view of the Seven Mountains was taken.

[Illustration: Fig. 22.--Section of the extinct crater of the Roderberg
on the bank of the Rhine, above Bonn.--(Original.)]

(_d._) _The Roderberg._--This crater, which was visited by the author in
1857, is about one-fourth of a mile in diameter, and is in the form of a
cup with gentle slopes on all sides. In its centre is a farmhouse
surrounded by corn-fields. The general section through the hill is
represented above (Fig. 22).

The flanks on the north side are composed of loose quartzose gravel
(gerolle), a remnant of the deposits formed around the margin of the
"Basin of Neuwied" described above (p. 114). This gravel is found
covering the terraces of the brown coal formation several hundred feet
above the Rhine. Besides quartz-pebbles, the deposit contains others of
slate, grit, and volcanic rock. On reaching the edge of the crater we
find the gravel covered over by black and purple scoria or slag the
superposition of the scoria on the gravel being visible in several
places, showing that the former is of more recent origin. On the
opposite side of the crater, overlooking the Rhine, we find the cliff of
Rolandsec composed of hard vesicular lava, rudely prismatic, and
extending from the summit of the hill to its base, about 250 feet below.
This is the most northerly of the group of the Eifel volcanoes.

(_e._) _Distric