on the mind of the 'Stygian
cave forlorn,' the entrance to Dante's 'City of Sorrow,' and that
other cave where Aeneas witnessed in cold terror the prophetic fury of
the Sibyl.

This effect of gloom, horror and sublimity is the result of geological
conditions and the action of water, which together have produced many
similar phenomena in the region of the _causses_, but in no other
case, I believe, with such power in composing the picturesque. Imagine
an open plain which in the truly Dark Ages whereof man has had no
experience, but of whose convulsions he has learnt to read a little
from the book whose leaves are the rocks, cracked along a part of its
surface as a drying ball of clay might do, the fissure finishing
abruptly and where it is deepest in front of a mass of rock that
refused to split. This was apparently the beginning of the Gouffre de
Revaillon. Then came another submersion which greatly modified the
appearance of things. There was evidently a deluge here after the land
had dried and cracked, and it must have lasted a very long time for
the waves to have hollowed, smoothed and polished the rocks inside the
caverns and elsewhere as we now see them. Those who have observed with
a little attention a rugged coast will, without being geologists,
recognise the distinctly marine character of the greater number of
these orifices in the calcareous district of the _causses_. The
washing and smoothing action of the sea along the sides of the gorges
which cut up the surface of the country in such an astonishing manner
is not so easy to distinguish. But the reason is obvious. This
limestone rock is by its nature disintegrating wherever it is exposed
to the air and frost, and the foundations of the bastions which
support the _causses_ are being continually sapped by water which
carries away the lime in solution and deposits a part of it elsewhere
in the form of stalactite and stalagmite in the deep galleries where
subterranean rivers often run, and which probably descend to the
lowest part of the formation. Thus by the dislodgment of huge masses
of rock which have rolled down from their original positions, and the
breaking away of the surfaces of others, the most convincing traces of
the sea's action here have nearly disappeared. In the gorge of the
Alzou, however, near Roc-Amadour, about 100 feet above the channel of
the stream, there is a considerable reach of hard rock approaching
marble, the polished and undulating s