214 On the Antiquity of the Volcanos of Auvergne. | | April, 
which Sir John Herschel and others have regarded as volcanic 
craters. Without discussing the mode of their formation, which 
would detain us too long, it may be enough to say that they 
would seem to be more modern than the amphitheatre of vol- 
canic rocks which encompasses them, though their elevation, 
which places them far beyond the reach of the eroding action 
of rivers, prevents our fixing with any certainty the degree of their 
antiquity. 
Let us therefore pass from these problematical rocks to others of 
far greater antiquity than any that have yet come before us. 
They may be divided into two classes—namely, those of a basaltic 
and of a trachytic character, and of these the latter seem in general 
to lie lowest, having the basaltic superimposed. 
But since the trachyte at the same time rises to the most elevated 
points in the country, as at the Pic de Sancy, near the Baths of Mont 
Dor, where it attains the height of 6,217 feet above the sea, and in 
the neighbouring department of Cantal, where it reaches, at the sum- 
mit of the Plomb de Cantal, that of 6,096, the basalt seems in 
some places to he beneath it. 
Although volcanic, these rocks bear but a remote resemblance 
to the rocks above alluded to; for not only are the materials of 
which they are composed in general more compact, but when 
scoriform, they consist for the most part of pumice, a material not 
met with, it is believed, amongst the more recent class of volcanos. 
Still more distinct, too, is their general structure ; for, mstead 
of constituting streams of lava traceable for the most part to a crater 
as their point of issue, they are spread out into vast sheets, extend- 
ing continuously over wide areas, in some places indeed rising to a 
great elevation, but even then exhibiting no traces of anything which 
bears the shghtest resemblance to a crater. 
Indeed, so contrasted are the general characters of the volcanic 
rocks we are considering, with those in the neighbourhood of Cler- 
mont, that Messrs. Dufrenoy and lie de Beaumont, the French 
geologists alluded to, conceived that their structure may be best ex- 
plained upon the supposition, that they had been first spread almost 
horizontally over the surface of the subjacent gravel, and afterwards 
were upheaved at three different points, the Pic de Sancy being the 
centre of one elevatory movement, Roche Sanadoire of a second, and 
the Puy de la Tache of a third, these representing the highest spots 
in the vicinity of the Baths of Mont Dor, around which the sup- 
posed elevatory movement took place. 
Now the deep valley in which these Baths lie is conceived by 
these same geologists to have been originally formed, not by the 
erosion of water, but by a disruption of the rocks on either side, 
consequent upon the elevation of the range at these three several 
points. 
Ret 
