260 REPORT ON THE DENEHOLE EXPLORATION
temperature throughout the year, they are used as cellars for the
famous Mendig beer. The rock may be described as a Nepheline
basalt, and is remarkable for containing crystals of various
minerals, notably hauyne, zircon, and leucite. The hauyne,
which is perhaps the most common enclosure, is easily detected
by its blue colour, and often enables a mere fragment of the rock,
though found in some distant locality, to be readily identified.
The vesicular texture of this old lava led long ago to its employ-
ment as a millstone, the worn surface always maintaining the
roughness needed for triturating grain. The quarries are about
eight miles distant from the picturesque town of Andernach on the
Rhine, the old Roman station of Antonacum, near the spot where
Caesar constructed his wooden bridge across the river. In and
around Andernach the lava is everywhere used as a building stone,
but this application of the material seems to be only local. The
quern, or hand-mill (mola manuaria), formed of this Rhenish rock
was carried by the Romans wherever they settled, and fragments
are found at most Roman stations in this country. It would,
however, be rash to assign any discovery to the Roman period on
the mere evidence of these millstones, for there can be little doubt
that they found their way into England at very various dates.
They were formerly imported in large numbers under the name
of "Dutch Blues," having been sent down the Rhine to Holland,
and thence to this country. Although largely superseded by the
French burr-stones, it is probable that they are still used here to
a limited extent. It seems, therefore, unsafe to base on the
discovery of this fragment of millstone any conclusion as to the
age at which the Essex deneholes were used.
III.—On Chalk Wells.
By F. J. BENNETT, F.G.S., of the Geological Survey of England and Wales.
[Read November 12th, 1887.]
The practice of raising marl or chalk, and of putting it on land as a
top-dressing, must go back to very early times, and may be prehistoric.
In Norfolk, to take a district with which I am familiar, pits were
sunk in the Boulder clay, locally called "marl," from its containing
so much chalk; this was especially beneficial in those parts where
the Boulder clay is masked by a covering of sand. This process has