of eruption, as
suggested further on.

[5] Only very closely; for Mr. Scrope considers that the crater-cones of
the chain of the Haute Loire give evidence of a somewhat earlier epoch
of activity than those of the Puy de Dome, as they have undergone a
greater amount of subaerial erosion.

[6] The extent of this river erosion has been clearly brought out by
Scrope, and is admirably illustrated by several of his panoramic views,
such as that in Plate IX. of his work.

[7] Scrope, _loc. cit._, p. 147.

[8] Scrope, _loc. cit._, p. 144.

[9] Scrope gives a view of these remarkable basaltic cliffs in Plate
XII. of his work, from which the above account is taken. At one spot
near the village of Le Gua there is a break in the continuity of the
sheet.

[10] See Scrope, _loc. cit._, p. 181; also Appendix, p. 228. While there
is no _prima facie_ reason for questioning the origin of the Demise
skull, yet from what Lyell states in his _Antiquity of Man_, p. 196, it
will be seen that he found it impossible to identify its position, or to
determine beyond question that its interment was due to natural causes.
But assuming this to be the case, he shows how the individual to whom it
belonged might have been enveloped in volcanic tuff or mud showered down
during the final eruption of the volcano of Demise. MM. Hebert and
Lartet, on visiting the locality, also failed to find _in situ_ any
exact counterpart of the stone now in the museum of Le Puy.

[11] See Daubeny, _Volcanoes_, p. 31.

[12] That is to say, the surfaces of the lava-streams are not at all, or
only slightly, decomposed into soil suitable for the growth of plants,
except in rare instances.

[13] E. G. Hull, "On the Domite Mountains of Central France," _Scien.
Proc. Roy. Dublin Society_, July 1881, p. 145. Dr. Hull determined the
density of the domite of the Puy de Dome to be 2.5, while that of lava
is about 3.0.

CHAPTER VII.

THE VOLCANIC DISTRICT OF THE RHINE VALLEY.

The region bordering the Rhine along both its banks above Bonn, and
extending thence along the valley of the Moselle and into the Eifel, has
been the theatre of active volcanic phenomena down into recent times,
but at the present day the volcanoes are dormant or extinct.

(_a._) _Geological Structure._--The fundamental rocks of this region
belong to the Silurian, Devonian, and Carboniferous systems, consisting
of schists, grits, and limestones, with occasional horizontal beds of
Mioc