94
qui en se repliant autour d’eux, compose ce qu’on appelle
leur cratère./ All three lakes exhibit striking similar features;
but at fi rst it is necessary to distinguish two important issues
there. The abyss which render them lakes, and which is a
depth that one had long thought was not measurable, and the
widened part above the water level, which, being bent around
the water, creates the so-called crater. [...]
Il faut ici surtout faire bien attention à trois choses : 1°
que ces lacs ne présentent aucun indice de ces matières vio-
lemment torréfi ées, telles qu’on les trouve dans les autres
cratères de nos volcans; 2° qu’ils n’ont fourni aucun courant
de lave; 3° (et ce caractère aurait dû être, je crois, placé le
premier), que si on doit les regarder comme d’anciennes
bouches de volcans, du moins le cratère, et surtout l’abîme
ou la voragine, sont infi niment plus évasés, ont infi niment
plus de circonférence, que n’en ont tous nos autres volcans;
ce qui, en leur supposant plus d’action, contraste singulière-
ment avec le peu d’effet qu’ils ont produit. Or, ces trois car-
actères bien extraordinaires, et pourtant bien constatés, me
font croire que leur théorie ne doit pas être tout à fait la
même que celle des volcans ordinaires. / Here we need to to
pay particular attention to three issues, namely: 1° these
lakes do not present any indication for the violently roasted
materials as those found in other craters of our volcanoes;
2°they did not supply any current of lava; 3° (and, I believe,
this characteristic should have been placed in the fi rst place),
if one must consider them as old mouths of volcanoes, at
least the crater, and especially the abyss or the voragine, are
infi nitely widened, have infi nitely more circumference that
all our other volcanoes; thus, if we suppose that they had
more action, these peculiar lakes singularly contrast with the
little effect that they produced. However, these three extraor-
dinary characters, yet well recognized, make me believe that
their theory should not be quite the same as that of ordinary
volcanoes. [...]
Voici donc ce que je pense sur l’origine de ces lacs : je
suis sûr qu’ils n’appartiennent nullement à la classe des vol-
cans ordinaires, que leur volcanisation n’a été qu’incomplète;
qu’enfi n l’éruption qui a découvert ces abîmes, en com-
posant autour d’eux les vases qui leur servent comme de
rempart et d’abri, n’a point été une éruption torréfi ante,
mais une explosion pulvérulente, causée par l’action de l’air,
ou de l’eau condensée en vapeurs dans ces vastes souter-
rains; et alors je pense qu’un tel effet a pu avoir lieu, lorsque
le vrai volcan, une fois éteint, son cratère sera tombé dans la
voragine, et en aura tellement bouché l’orifi ce, que le reste
de la force volcanique emprisonnée dans ces cavernes, aura
fait un violent et dernier effort, dans le premier endroit où
elle aura pu trouver une issue./Hence this is how I envisage
the origin of these lakes: I am sure that they do not belong to
the category of ordinary volcanoes, that their ‘volcanisation’
was only incomplete; that fi nally the eruption which unveiled
these abysses, by creating around them the mud and silt
which are used as rampart and shelter, was not a torrefying
eruption, but a pulverulent explosion, caused by the action of
the air or by condensate out of vapors in these vast under-
ground passages; and then I think that such an effect could
take place when the true volcano is extinct and its crater falls
into the voragine, and will have so much tapped the opening,
that the rest of the volcanic strength imprisoned into these
caves, will have made a violent and last effort, in the very
place where it will have been able to fi nd an exit.” (Montlosier
1788 , p.159–163).
As depicted here, Montlosier founds his theory only on
geomorphological criteria, the absence of volcanic products
usually found in the region: “ torrefi ed ” scoriae and “ cur-
rents ” of lava, as well as on chronicles relating the eruptive
activity of Etna, which mention the periodic collapse of its
crater. On this basis, he proposed a new eruptive mechanism
that included three stages:- Eruption of a ‘common’ volcano,
- Collapse of this volcano in its chimney leading to its
obturation, - Accumulation of pressure and explosion without molten
magma: a “pulverulent explosion” creating one immense
crater.
This model presents however a weakness, as the debris of
this unique and gigantic explosion are neither visible, nor
they have been looked for.
Here steps in Pierre-Jean-Baptiste Legrand d’Aussy, a
brilliant spirit, Parisian and academician, whose three vol-
umes of “Voyage en Auvergne” ( 1788 , 1794–1795) were a
best-seller back then. At the time of his visit in Auvergne,
he naturally met Montlosier who exchanged his ideas with
him. While the Earl may have shown a thorough knowledge
of the fi eld, Legrand d’Aussy, thanks to his acute sense of
observation, has quickly made his own opinion. In fact,
Montlosier did not appreciate this character, who neverthe-
less acted as a stimulus on him to the point that he decided
to write his essay “Essai sur la théorie des volcans
d’Auvergne” in 1788.
In front of Pavin, which he described accurately, Legrand
d’Aussy alo reported, as Montlosier did beforehand, the
absence of volcanic products: “ Les cratères ordinaires sont
des issues qu’un volcan s’ouvre vers sa calotte pour
l’écoulement ou pour l’éjection des matières qu’il pousse
hors de son foyer. Celles de ces substances qui sortent liqué-
fi ées ou fondues s’épanchent au dehors, sous la forme de
fl euves, et ils produisent d’immenses traînées, que souvent
on peut suivre, depuis le foyer d’où elles se sont élancées
jusqu’au terme où elles cessent. Pavin n’a rien de semblable.
A la vérité, des laves s’étendent sur ses rebords, et même au-
delà; mais loin d’avoir comme les autres des coulées de sor-
tie, il reçoit, au contraire, des coulées étrangères./TheP. Boivin and S. Jouhannel