has taken a
southerly direction from the crater of the Laacher See towards Nieder
Mendig and Mayen, for a distance of about six miles. This great stream
is covered throughout half its distance by beds of volcanic ash and
lapilli, but emerges into the air at a distance of about two miles from
the edge of the crater (see Fig. 23), and was formerly extensively
quarried in underground caverns for millstones. Here the rock is a
vesicular trachyte, of a greyish colour, solidified in vertical columns
of hexagonal form, about four feet in diameter, and traversed by
transverse joint planes. These quarries have been worked from the time
of the Roman occupation of the country; and, before the introduction of
iron or steel rollers for grinding corn, millstones were exported to all
parts of Europe and the British Isles from this quarry.[11]

The district around the Laacher See is covered by laminated _ejecta_ of
the old volcano, probably of subaerial origin, through which bosses of
the fundamental slate peer up at intervals, while the surface is
diversified by several truncated cones.

(_g._) _Trass of the Bruehl Valley._--The Bruehl Valley, which unites
with that of the Rhine at the town of that name, and drains the northern
side of the volcanic region, has always been regarded with much interest
by travellers for the presence of a deposit of "trass" with which it is
partially filled. The origin of this valley was pre-volcanic, as it is
hewn out of the slaty rocks of the district. But at a later period it
became filled with volcanic mud (tuffstein), out of which the stream has
made for itself a fresh channel. The source of this mud is considered by
Hibbert[12] to have been the old volcano of the Lummerfeld, which, after
becoming dormant, was filled with water, and thus became a lake. At a
subsequent period, however, a fresh eruption took place near the edge of
the lake, resulting in the remarkable ruptured crater known as the
Kunkskoepfe, which rises about four miles to the north of the Laacher
See. The eruptions of this volcano appear to have displaced the mud of
the Lummerfeld, causing it to flow down into the deep gorge of the
Bruehl, which it completely filled, as stated above.

On walking down the valley one may sometimes see the junction of the
tuff with the slate-rock which enfolds it. The tuff consists of white
felspathic mud, with fragments of slate and lava, reaching a depth in
some places of 150 feet. After it has been