SO^J^ Dr Daubeiij on th€ Ancient Volcanoes of Auvergne. 
and hornbiende, are most frequently found imbedded, whilst it 
is the former variety which contains the finest and most regular 
crystals of glassy felspar. It passes sometimes into pitchstone 
porphyry as at the Vallee d’Enfer, at others into a kind of horn- 
stone porphyry, as on the road to Murat, both near the village 
where the baths are situated. It is frequently coloured red by 
iron, and sometimes incloses flattened balls of clay-ironstone. 
In its fissures and cavities are also found plates of specular iron- 
ore, a substance which we have noticed as occurring among the 
recent volcanoes near Clermont, and particularly at the Euy 
de Dome. 
The trachytic formation has also associated with it beds of 
tuff, containing portions of scoriaceous and vesicular lava, as 
well as of basalt, united generally by a felspar basis. To this 
probably must be referred those singular beds occasionally seen 
interstratified with the trachyte, which consist of an apparently 
homogeneous rock, bearing a resemblance to tripoli, possessing 
a rough earthy feel, and slaty fracture, generally grey, but some- 
times of an ochreous yellow colour, from the intermixture of 
oxide of iron. M. D’Aubuisson, in his late work on Geo- 
gnosy, supposes, that these beds as well as the tuff, owe their 
origin to the disintegration of the trachyte, and the subsequent 
agglutination of the finely divided fragments into an uniform 
mass. He states, that M. Beudant has found, that the Opals 
of Hungary lie in a matrix constituted of this regenerated de- 
scription of rock ; and some observations I have recorded in a 
paper of mine, which was lately read before the Geological 
Society, may contribute to render this fact somewhat more cre- 
dible. To this formation of tuff we may probably also refer 
those fragments of a breccia, containing sulphur and alumstone, 
found in the Gorge d’Enfer, near the village of the Baths, in 
the bed of the River Dordogne, which takes its rise in the 
mountain above. Of this rock M. Cordier has published, in 
the “ Annales des Mines,” a description as well as an analysis, 
from both which he infers that it is analogous to the alumstone of 
Tolfa, like which it yields, on exposure to heat and moisture, 
numerous capillary crystals of alum. It has never been found 
in situ, but it seems probable, that if the middle regions of the 
l^ic de Sancy, above the spot to which it has been brought by 
