t of the Rivers Bruehl and Nette._--The volcanic region of
the Lower Eifel, drained by these two principal streams which flow into
the Rhine, will amply repay exploration by the student of volcanic
phenomena, owing to the variety of forms and conditions under which
these present themselves within a small space. The fundamental rock is
slate or grit of Devonian age, furrowed by numerous valleys, often
richly wooded, and diversified by conical hills of trachyte; or by
crater-cones, formed of basalt or ashes, sometimes ruptured on one side,
and occasionally sending forth streams of lava, as in the cases of the
Perlinkopf, the Bausenberg, and the Engelerkopf. The district attains
its greatest altitude in the High Acht (Der Hohe Acht), an isolated cone
of slate capped by basalt with olivine, and reaching a level of 2434
Rhenish feet.[8]

(_f._) _The Laacher See._--It would be impossible in a work of this kind
to attempt a detailed description of the Eifel volcanoes, often of a
very complex character and obscure physical history, as in the case of
the basin of Rieden, where tufaceous deposits, trachytic and basaltic
lavas and crater-cones, are confusedly intermingled, so that I shall
confine my remarks to the deservedly famous district of the Laacher
See, which I had an opportunity of personally visiting some years
since.[9]

[Illustration: Fig. 23.--Plan and Section of the Laacher See, a lake on
the borders of the Eifel, occupying the crater of an old volcano.--G.
Gravel and volcanic sand forming banks of the lake and rim of old
crater; L. Sheet of trachytic lava with columnar structure; B. Basaltic
dyke; S. Devonian slate, etc.]

The Laacher See is a lake of an oval form, over an English mile in the
shorter diameter, and surrounded by high banks of volcanic sand, gravel,
and scoriae, except on the east side, where cliffs of clay-slate, in a
nearly vertical position, and striking nearly E.W., may be observed. Its
depth from the surface of the water is 214 feet.[10] The ashes of the
encircling banks contain blocks of slate and lava which have been torn
from the sides of the orifice or neck of the volcano and blown into the
air; and there can be no doubt that the ashes and volcanic gravel is the
result of very recent eruptions.

At the east side of the lake we find a stream of scoriaceous lava of a
purple or reddish colour, highly vesicular, and containing crystals of
mica; but the most important lava-stream is that which