EXTINCT VOLCANOES OF CENTRAL FRANCE.
(_a._) _General Structure of the Auvergne District._--From a granitic
and gneissose platform situated near the centre of France, and separated
from the western spurs of the Alps by the wide valley of the Rhone,
there rises a group of volcanic mountains surpassing in variety of form
and structure any similar mountain group in Europe, and belonging to an
epoch ranging from the Middle Tertiary down almost to the present day.
This volcanic group of mountains gives rise to several important rivers,
such as the Loire, the Allier, the Soule (a branch of the Loire), the
Creuse, the Dordogne, and the Lot; and in the Plomb du Cantal attains an
elevation of 6130 feet above the sea. Its southern section, that of Mont
Dore, the Cantal, and the Haute Loire, is characterised by magnificent
valleys, traversing plateaux of volcanic lava, and exhibiting the
results of river erosion on a grand scale; while its northern section,
that of the Puy de Dome, presents to us a varied succession of volcanic
crater-cones and domes, with their extruded lava-streams, almost as
fresh and unchanged in form as if they had only yesterday become
extinct. A somewhat similar, but less important, chain of extinct
volcanoes also occurs in the Velay and Vivarais, between the upper
waters of the Loire and the Allier, in the vicinity of the town of Le
Puy.[1] The principal city in this region is Clermont-Ferrand, lying
near the base of the Puy de Dome, and ever memorable as the birthplace
of Blaise Pascal.[2]
[Illustration: Fig. 17.--Generalised Section through the Puy de Dome and
Vale of Clermont, distance about ten miles. The general floor formed of
granite and gneiss (G); D. Domite-lava of the Puy de Dome; Sc. Cones of
ashes and scoriae; L. Lava-sheets; A. Alluvium of the Vale of Clermont
and Lake deposits.]
The physical structure of this region is on the whole very simple. The
fundamental rocks consist of granite and gneiss passing into schist, all
of extreme geological antiquity, forming a vast platform gradually
rising in a southerly direction towards the head waters of the Loire
and the Allier in the Departments of Haute Loire, Lozere, and Ardeche.
On this platform are planted the whole of the volcanic mountains. (See
Fig. 17.)
The granitic plateau is bounded on the east, throughout a distance of
about 50 miles, by the wide and fertile plain of Clermont, watered by
the Allier and its numerous branches descending from the volcanic
mountains, and is about 25 miles in width from east to west in the
parallel of Clermont, but gradually narrowing in a southerly direction,
till at Brioude it becomes an ordinary mountain ravine. The eastern
margin of the plain is formed by another granitic ridge expanding into a
plateau towards the south, and joining in with that already described;
but towards the north and directly east of Clermont it forms a high
ridge traversed by the railway to St. Etienne and Lyons, and descending
towards the east into the valley of the Loire. No more impressive view
is to be obtained of the volcanic region than that from the summit of
this second ridge, on arriving there towards evening from the city of
Lyons. At your feet lies the richly-cultivated plain of Clermont, dotted
with towns, villages, and hamlets, and decorated with pastures,
orchards, vineyards, and numerous trees; while beyond rises the granitic
plateau, breaking off abruptly along the margin of the plain, and deeply
indented by the valleys and gorges along which the streams descend to
join the Allier. But the chief point of interest is the chain of
volcanic crater-cones and dome-shaped eminences which rise from the
plateau, amongst which the Puy de Dome towers supreme. Their individual
forms stand out in clear and sharp relief against the western sky, and
gradually fade away towards the south into the serried masses of Mont
Dore and Cantal, around whose summits the evening mists are gathering.
Except the first view of the Mont Blanc range from the crest of the
Jura, there is no scene perhaps which is calculated to impress itself
more vividly on the memory than that here faintly described.[3]
[Illustration: Fig. 18.--Transverse view of the Puy de Dome and
neighbouring volcanoes from the Puy de Chopine.--(After Scrope.)]
(_b._) _The Vale of Clermont._--The plain upon which we look down was
once the floor of an extensive lake, for it is composed of various
strata of sand, clay, marl, and limestone, containing various genera and
species of fresh-water shells. These strata are of great thickness,
perhaps a thousand feet in some places; and along with such shells as
_Paludina_, _Planorbis_, and _Limnaea_ are also found remains of various
other animals, such as fish, serpents, batrachians, crocodiles,
ruminants, and those of huge pachyderms, as _Rhinoceros_, _Dinotherium_,
and _Caenotherium_. This great lake, occupying a hollow in the old
granitic platform of Central France, must have been in existence for an
extensive period, which MM. Pomel, Aymard, and Lyell all unite in
referring to that of the Lower Miocene. But what is to us of special
interest is the fact that, in the deposits of this lake of the Haute
Loire, with the exception of the very latest, there is no intermixture
of volcanic products such as might have been expected to occur if the
neighbouring volcanoes had been in activity during its existence. Hence
it may be supposed that, as Scrope suggested, the waters of the lake
were drained off owing to the disturbance in the levels of the country
caused by the first explosions of the Auvergne volcanoes.[4] If this be
so, then we possess a key by which to determine the period of the first
formation of volcanoes in Central France; for, as the animal remains
enclosed in the lacustrine deposits of the Vale of Clermont belong to
the early Miocene stage, and the earliest traces of contemporaneous
volcanic _ejecta_ are found only in the uppermost deposits, we may
conclude that the first outburst of volcanic action occurred towards the
close of the Miocene period--a period remarkable for similar exhibitions
of internal igneous action in other parts of the world.
(_c._) _Successive Stages of Volcanic Action in Auvergne._--The volcanic
region here described, which has an area of about one hundred square
miles, does not appear to have been at one and the same period of time
the theatre of volcanic action over its whole extent. On the contrary,
this action appears to have commenced at the southern border of the
region in the Cantal, and travelling northwards, to have broken out in
the Mont Dore region; finally terminating its outward manifestations
among the craters and domes of the Puy de Dome. In a similar manner the
volcanic eruptions of the Haute Loire and Ardeche, lying to the
eastward, and separated from those of the Cantal by the granitoid ridge
of the Montagnes de Margeride, belong to two successive periods
referable very closely to those of the Mont Dore and the Puy de Dome
groups.[5] The evidence in support of this view is very clear and
conclusive; for, while the volcanic craters formed of ash, lapilli, and
scoriae, together with the rounded domes of trachytic rock of which the
Puy de Dome group is composed, preserve the form and surface indications
of recently extinguished volcanoes, those which we may assume to have
been piled up in the region of Mont Dore and Cantal have been entirely
swept away by prolonged rain and river action, and the sites of the
ancient craters and cones of eruption are only to be determined by
tracing the great sheets of lava up the sides of the valleys to their
sources, generally situated at the culminating points of their
respective groups. Other points of evidence of the great antiquity of
the latter groups might be adduced from the extent of the erosion which
has taken place in the sheets of lava having their sources in the vents
of the Plomb du Cantal and of Mont Dore, owing to which, magnificent
valleys, many miles in length and hundreds of feet in depth, have been
cut out of these sheets of lava and their supporting rocks, whether
granitic or lacustrine, and the materials carried away by the streams
which flow along their beds. These points will be better understood when
I come to give an account of the several groups; and in doing so I will
commence with that of the Cantal.[6]
(_d._) _The Volcanoes of the Cantal._--The original crater-cones of
this group have entirely disappeared throughout the long ages which have
elapsed since the lava-streams issued forth from their internal
reservoirs. The general figure of this group of volcanic mountains is
that of a depressed cone, whose sides slope away in all directions from
the central heights, which are deeply eroded by streams rising near the
apex and flowing downwards in all directions towards the circumference
of the mountain, where they enter the Lot, the Dordogne, and the Allier.
The orifice of eruption was situated at the Plomb du Cantal, formed of
solid masses of trachyte, which, owing, as Mr. Scrope supposes, to a
high degree of fluidity, were able to extend to great distances in
extensive sheets, and were afterwards covered by repeated and
widely-spread flows of basalt; so that the trachyte towards the margin
of the volcanic area becomes less conspicuous than the basalt by which
it is more or less concealed from view, or overlapped. Extensive beds of
tuff and breccia accompany the trachytic masses.
Magnificent sections of the rocks are laid open to view along the sides
of the valleys, which are steep and rock-bound. Except towards the
south-west, about Aurillac, where lacustrine strata overlie the granite,
the platform from which rises the volcanic dome is composed of granitic
or gneissose rocks. Accompanying the lava-streams are great beds of
volcanic agglomerate, which Mr. Scrope considers to have been formed
contemporaneously with the lava which they envelop, and to be due to
torrents of water tumultuously descending the sides of the volcano at
periods of eruption, and bearing down immense volumes of its fragmental
_ejecta_ in company with its lava-streams.[7] Nowhere throughout this
region do beds of trachyte and basalt alternate with one another; in all
cases the basalt is the newer of the two varieties of rock, and this is
generally the case throughout the region here described.
(_e._) _Volcanoes of Mont Dore._--This mountain lies to the north of
that of Cantal, and somewhat resembles it in general structure and
configuration. Like Cantal, it is destitute of any distinct crater; all
that is left of the central focus of eruption being the solidified
matter which filled the throat of the original volcano, and which forms
a rocky mass of lava, rising in its highest point, the Pic de Saucy, to
an elevation (as given by Ramond) of 6258 feet above the level of the
sea, thus exceeding that of the Plomb du Cantal by 128 feet. Its figure
will be best understood by supposing seven or eight rocky summits
grouped together within a circle of about a mile in diameter, from
whence, as from the apex of an irregular and flattened cone, all the
sides slope more or less rapidly downwards, until their inclination is
gradually lost in the plain around. This dome-shaped mass has been
deeply eroded on opposite sides by the valleys of the Dordogne and
Chambon; while it is further furrowed by numerous minor streams.[8]
The great beds of volcanic rock, disposed as above stated, consist of
prodigious layers of scoriae, pumice-stones, and detritus, alternating
with beds of trachyte and basalt, which often descend in uninterrupted
currents till they reach the granite platform, and then spread
themselves for miles around. The sheets of basalt are found to stretch
to greater distances than those of trachyte, and have flowed as far as
15 or 20 miles from their orifices of eruption; while in some cases, on
the east and north sides, they have extended as far as 25 or 30 miles
from the central height. On the other hand, a radius of about ten miles
from the centre would probably include all the streams of trachyte;--so
much greater has been the viscosity of the basalt over the latter rock.
Some portions of these great sheets of lava, cut off by river valleys or
eroded areas from the main mass of which they once formed a part, are
found forming isolated terraces and plateaux either on the granitic
platform, or resting on the fresh-water strata of the valley of the
Allier, while in a northern direction they overspread a large portion of
the granitic plateau from which rise the Puy de Dome and associated
volcanic mountains. Still more remarkable are the cases in which these
lava-streams have descended into the old river channels which drained
the granitic plateau. Thus the current which took its origin in the Puy
Gros descended into the valley of the Dordogne, while another stream
invaded the gorge of Champeix on the eastern side.
The more ancient lava-streams just described are invaded by currents and
surmounted by cones of eruption of more recent date, similar to those of
the Puy de Dome group lying to the northward. Such cones and currents,
amongst which are the Puy de Tartaret and that of Montenard, present
exactly the same characters as those of this group, to which we shall
return further on.
(_f._) _Volcanoes of the Haute Loire and Ardeche._--Separated by the
valley of the Allier and the granitic ridge of La Margeride from the
volcanic regions of Cantal and Mont Dore is another volcanic region of
great extent, which reaches its highest elevation in the central points
of Mont Mezen, attaining an elevation (according to Cordier) of 5820
feet, and formed of "clinkstone." The volcanic products of Mezen have
been erupted from one central orifice of vast size, and consist mainly
of extensive sheets of "clinkstone," a variety of trachytic lava, which
have taken courses mainly towards the north-west and south-east. These
great sheets, one of which appears to have covered a space more than 26
miles in length with an average breadth of 6 miles, thus overspreading
an estimated area of 156 square miles, has been deeply eroded by streams
draining into the Loire, along whose banks the rocks tower in lofty
cliffs; while it has also suffered enormous denudation, by which
outlying fragments are disconnected from the main mass, and form
flat-topped hills and plateaux as far distant as Roche en Reigner and
Beauzac, at the extreme distance (as stated above) of 26 miles from the
source of eruption.
But even more remarkable than the above are the vast basaltic sheets
which stretch away for a distance of 30 miles by Privas almost to the
banks of the Rhone, opposite Montlimart. These have their origin amongst
the clinkstone heights of Mont Mezen, and taking their course along the
granitic plateau in a south-easterly direction, ultimately pass over on
to the Jurassic and Cretaceous formations composing the plateau of the
Coiron, which break off in vertical cliffs from 300 to 400 feet in
height, surmounting the slopes that rise from the banks of the Ardeche
and Escourtais rivers near Villeneuve de Bere. This is probably one of
the most extensive sheets of basalt with which we are acquainted in the
European area, and it is only a remnant of a vastly greater original
sheet.[9]
[Illustration: Fig. 19.--Mont Demise, near Le Puy, seen from the S.E.
(After Scrope.)--1. Building standing on old breccia, rocks of the Col;
2. Road to Brioude; 3. Croix de la Paille; 4. Orgue d'Expailly (basalt);
5. Spot where human bones were found.]
(_g._) _Newer Volcanoes of the Haute Loire (the Velay and
Vivarais)._--Subsequently to the formation of the lava-streams above
described, and probably after the lapse of a lengthened period, the
region of the Haute Loire and Ardeche became the scene of a fresh
outburst of volcanic action, during which the surface of the older
lavas, or of the fundamental granite, was covered by numerous
crater-cones and lava-streams strewn along the banks of the Allier and
of the Loire for many miles. These cones and craters are not quite so
fresh as those of the Mont Dome group; those of the Haute Loire being
slightly earlier in point of time, and, as Daubeny shows, belonging to a
different system. So numerous are these more recent cones and craters
that Scrope counted more than 150 of them, and probably omitted many.
The volcanic phenomena now described have a special interest as bearing
on the question whether man was an inhabitant of this region at the time
of these later eruptions. The question seems to be answered in the
affirmative by the discovery of a human skull and several bones in the
volcanic breccia of Mont Demise, in company with remains of the elephant
(_E. primigenius_), rhinoceros (_R. tichorhinus_), stag, and other large
mammifers. The discovery of these remains was made in the year 1844, and
the circumstances were fully investigated and reported upon by M.
Aymard, and afterwards by Mr. Poulett Scrope, upon whose mind no
possible doubt of the fact remained. From what we now know of the
occurrence of human remains and works of art in other parts of France
and Europe, no surprise need be felt at the occurrence of human remains
in company with some extinct mammalia in these volcanic tuffs, which
belong to the Post-Pliocene or superficial alluvia antecedent to the
historic period.[10]
(_h._) _Mont Dome Chain._--We now come to the consideration of the most
recent of all the volcanic mountain groups of the region of Central
France, that of the Puy de Dome, lying to the north of Mont Dore and
Cantal. We have seen that there is almost conclusive evidence that man
was a witness to the later volcanic outbursts of the Vivarais, and as
these craters seem to be of somewhat earlier date than those of the Puy
de Dome group, we cannot doubt that they were in active eruption when
human beings inhabited the country, and not improbably within what is
known as the _Historic Period_. No mention, however, is made either by
Caesar, Pliny, or other Roman writers of the existence of active
volcanoes in this region. Caesar, who was a close observer, and who
carried the Roman arms into Auvergne, makes no mention of such; nor yet
does the elder Pliny, who enumerated the known burning mountains of his
day all over the Roman Empire. It is not till we come down to the fifth
century of our era that we find any notices which might lead us to infer
the existence of volcanic action in Central France. This is the
well-known letter written by Sidonius Apollonarius, bishop of Auvergne,
to Alcinus Avitus, bishop of Vienne, in which the former refers to
certain terrific terrestrial manifestations which had occurred in the
diocese of the latter. But, as Dr. Daubeny observes, this is no evidence
of volcanic action in Auvergne, where Sidonius himself resided; the
terrestrial disturbances above referred to may have been earthquake
shocks of extreme severity.[11]
But although we have no reliably historical record of volcanic action
amongst the mountains of the Mont Dome group, the fact that these are,
comparatively, extremely recent will be evident to an observer visiting
this district, and this conclusion is based on three principal grounds:
first, because of the well-preserved forms of the original craters,
though generally composed of very loose material, such as ashes,
lapilli, and slag; secondly, because of the freshness of the
lava-streams over whose rugged surfaces even a scanty herbage has in
some places scarcely found a footing;[12] and thirdly, because the lava
from the crater-cones has invaded channels previously occupied by the
earlier lavas, or those which had been eroded since the overflow of the
great basaltic sheets of Mont Dore. Still, as in the case of the valleys
of Lake Aidot, of Channonat, and of Royat, these streams are
sufficiently ancient to have given time for the existing rivers to have
worn out in them channels of some depth, but bearing no comparison to
the great valleys which had been eroded out of the more ancient lavas,
such as those of the Coiron, of the Ardeche, and of the Dordogne and
Chambon in the district of Mont Dore.
(_i._) _Dome-shaped Volcanic Hills._--I have previously (page 15)
referred to the two classes of volcanic eminences to be found in the
chain of the Puy de Dome; one indicated by the name itself, formed of a
variety of trachytic lava called "domite," and of the form of a dome;
the other, composed of fragmental matter piled up in the form of a
crater or cup, often ruptured on one side by a stream of lava which has
burst through the side, owing to its superior density. Of the former
class the Puy de Dome and the Grand Sarcoui (see Fig. 18) are the most
striking examples out of the five enumerated by Scrope, while there is a
large number, altogether sixty-one, belonging to the latter class. These
domes and crater-cones, as already stated, rise from a platform of
granite, either directly or from one formed of the lava-sheets of the
Mont Dore region, which in turn overlies the granitic platform. Of the
nearly perfect craters there are the Petit Puy de Dome, lying partially
against the northern flank of the greater eminence; the Puy de Cone,
remarkable for the symmetry of its conical form, rising to a height of
900 feet from the plain; and the Puys de Chaumont and Thiolet lying to
the north of the Puy de Dome. Of those to the south of this mount, two
out of the three craters of the Puy de Barme and the Puy de Vichatel are
perfect; but most of the crater-cones south of the Puy de Dome are
breached. Some of the lava streams by which these craters were broken
down flowed for long distances. That the lava followed the showers of
ashes and lapilli forming the walls of the craters is rendered very
evident in the case of the Puy de la Vache, whose lava-stream coalescing
with those from the Puy de la Solas and Puy Noir, deluged the
surrounding tracts and flowed down the Channonat Valley as far as La
Roche Blanc in the Vale of Clermont. In the interior of the upper part
of the crater still remaining may be seen the level (so to speak) to
which the molten lava rose before it burst its barrier. This level is
marked by a projecting platform of reddish or yellow material, rich in
specular iron, apparently part of the frothy scum which formed on the
surface of the lava and adhered to the side of the basin at the moment
of its being emptied.
Space does not permit a fuller description of this remarkable assemblage
of extinct volcanoes, and the reader must be referred for further
details to the work of Mr. Scrope. I shall content myself with some
further reference to the central figure in this grand chain, the Puy de
Dome itself.
_Ascent of the Puy de Dome._--On ascending by the winding path up the
steep side of the mount, and on reaching the somewhat flattened summit,
the first objects which strike the eye are the massive foundations of
the Roman temple of Mercury; they are hewn out of solid grey lava,
altogether different from the rock of the Puy de Dome itself, which must
have been obtained from one of the lava-sheets of the Mont Dore group.
To have carried these large blocks to their present resting-place must
have cost no little labour and effort. The temple is supposed to have
been surmounted by a colossal statue of the winged deity, visible from
all parts of the surrounding country which was dedicated to his honour,
and the foundations were only discovered a few years ago when excavating
for the foundation of the observatory, which stands a little further on
under the charge of Professor Janssen. On proceeding to the northern
crest of the platform a wonderful view of the extinct craters and
domes--about forty in number, and terminating in the Puy de Beauny, the
most northerly member of the chain--is presented to the spectator. To
the right is the Vale of Clermont and the rich valley of the Allier
merging into the great plain of Central France. On the south side of the
platform a no less remarkable spectacle meets the eye. The chain of Puys
and broken craters stretches away southwards for a distance of nearly
ten miles, while the horizon is bounded in that direction by the lofty
masses of the Mont Dore, Cantal, and Le Puy ranges. Nor does it require
much effort of the imagination to restore the character of the region
when these now dormant volcanoes were in full activity, projecting
showers of ashes and stones high into the air amidst flames of fire and
vast clouds of incandescent gas and steam.
The material of which the Puy de Dome is formed consists of a light
grey, nearly white, soft felsitic lava, containing crystals of mica,
hornblende, and specular iron-ore. It is highly vesicular, and was
probably extruded in a pasty condition from a throat piercing the
granitic plateau and the overlying sheet of ancient lava of Mont Dore.
It has been suggested that such highly felsitic and acid lavas as that
of which the Puy de Dome, the Grand Sarcoui, and Cliersou are composed,
may have had their origin in the granite itself, melted and rendered
viscous by intense heat. Dr. E. Gordon Hull has suggested that the
domite hills (owing to their low specific gravity) may have filled up
pre-existing craters of ashes and scoriae without rupturing them, as in
the case of the heavier basaltic lavas, and then still continuing to be
extruded, may have entirely enveloped them in its mass; so that each
domite hill encloses within its interior a crater formed of ashes,
stones, and scoriae. In the case of the Puy de Dome there is some
evidence that the domite matter rests on a basis of ashes and scoriae,
which may be seen in a few places around the base of the cone. It is
difficult without some such theory as this to explain how a viscous mass
was able to raise mountains some 2000 or 3000 feet above the surrounding
plain.[13]
(_j._) _Sketch of the Volcanic History of Central France._--It now only
remains to give a brief _resume_ of the volcanic history of this region
as it may be gathered from the relations of the rocks and strata to the
volcanic products, and of these latter to each other.
_1st Stage._--It would appear that at the close of the Eocene period
great terrestrial changes occurred. The bed of the sea was converted
into dry land, the strata were flexured and denuded, and a depression
was formed in the granitic floor of Central France, which, in the
succeeding Miocene period, was converted into an extensive lake peopled
by molluscs, fishes, reptiles, and pachyderms of the period.
_2nd Stage._--Towards the close of the Miocene epoch volcanic eruptions
commenced on a grand scale over the granitic platform in the districts
now called Mont Dore, Cantal, and the Vivarais. Vast sheets of trachytic
and basaltic lavas successively invaded the tracts surrounding the
central orifices of eruption, now constituting the more ancient of the
lava-sheets of the Auvergne region, and, invading the waters of the
neighbouring lake, overspread the lacustrine deposits which were being
accumulated therein. These volcanic eruptions probably continued
throughout the Pliocene period, interrupted by occasional intervals of
inactivity, and ultimately altogether ceased.
_3rd Stage._--Towards the close of the Pliocene period terrestrial
movements took place, owing to which the waters of the lake began to
fall away, and the sheets of lava were subjected to great denudation.
This process, probably accelerated by excessive rainfall during the
succeeding Post-Pliocene and Pluvial periods, was continued until plains
and extensive river-valleys were eroded out of the sheets of lava and
their supporting granitic rocks and the adjoining lacustrine strata.
_4th Stage._--A new outburst of volcanic forces marks this stage, during
which the chain of the Puy de Dome was thrown up on the west, and that
of the newer cones of the Vivarais on the south-east of the lacustrine
tract. The waters of the lake were now completely drained away through
the channel of the Allier, and denudation, extending down to the present
day, began over the area now forming the Vale of Clermont and adjoining
districts. The volcanic action ultimately spent its force; and somewhere
about the time of the appearance of man, the mammoth, rhinoceros, stag,
and reindeer on the scene, eruptions entirely ceased, and gradually the
region assumed those conditions of repose by which it is now physically
characterised.
[1] The literature referring to this region is very extensive. Guettard
in 1775, afterwards Faujas, published descriptions of the rocks of the
Vivarais and Velay; and Desmarest's geological map, published in 1779,
is a work of great merit. The district was afterwards described by
Daubeny, Lyell, Von Buch, and others; but by far the most complete work
is that of Scrope, entitled _Volcanoes of Central France_, containing
maps and numerous illustrations, published in 1826, and republished in a
more extended form in 1858; to this I am largely indebted.
[2] A monument to Pascal, erected by the citizens, occupies the centre
of the square in Clermont. It will be remembered that Pascal verified
the conclusions arrived at by Torricelli regarding the pressure of the
atmosphere, by carrying a Torricellian tube to the summit of the Puy de
Dome, and recording how the mercury continually fell during the ascent,
and rose as he descended. This experiment was made in 1645.
[3] In this visit to Auvergne in the summer of 1880, the author was
accompanied by his son, Dr. E. Gordon Hull, and Sir Robert S. Ball. On
reaching the station at the summit of the ridge it seemed as if the
volcanic fires had again been lighted, for the whole sky was aglow with
the rays of the western sun.
[4] On the other hand, certain beds of ash and other volcanic _ejecta_
occur in _the uppermost_ strata of lake deposits of Limagne, so that
these may indicate the commencement of the period 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.