Br Daubeny on the Ancient Vokanoes erf Auvergne. 309 
part of that range by the volcanic rocks superimposed, crops 
out near the western extremity of the valley in which the Baths 
are situated, at the village of Bourbeuli. It is small-granu- 
lar, consisting of a white disintegrated felspar, white quartz 
and black mica ; sometimes it is compact, but at others it has 
passed into the state of kaolin. As there is nothing remark- 
able in its appearance, I should hardly have thought it neces- 
sary to notice it, except from the circumstance that the trachyte 
superimposed has been supposed to be derived from it ; and if 
I have succeeded in rendering it probable that the rock of the 
Puy de Dome, Puy de Chopine, &c. mentioned in my former 
letter, are granite, in various states of alteration, the same will 
necessarily follow with regard to the trachyte. The singular 
difference, indeed, between the composition of granite and 
trachyte, the abundance of quartz in the one, the almost total 
absence of it in the other, may appear to some hardly recon- 
cilable with this opinion ; but as, I have remarked in my for- 
mer letter, the silica of the quartz may, in the new compound 
resulting from the fusion of the granite, have gone towards the 
formation of the felspar, — a mineral which, according to the best 
analyses, contains above 60 per cent, of the earth in question ; 
and if we consider that the mica, which occurs in much smal- 
ler proportion in the trachyte than it does in granite, falls short 
of the felspar in the quantity of silex which it contains almost 
as much as the quartz exceeds it, — our difficulty seems in a great 
measure removed, as the absolute quantity of silica in either 
rock does not perhaps materially differ. 
In describing the rocks found at Mont d’Or, I have said 
almost all that appears necessary respecting the trachyte of 
Auvergne, for that of Cantal is distinguished chiefly by its more 
compact form, and by the rarer occurrence in it of scorified 
matter. The highest hills in Cantal are mostly capped with 
porphyry-slate or phonolite^ a substance found also at Mont 
d’Or, capping the trachyte at the rocks Sanadoire and Thui- 
liere. The clinkstone in both instances is considered by Dau- 
buisson as a modification of the trachyte,^ but to me it did not 
appear to pass into that rock ; nor does this conclusion seem 
warranted by its appearance, remarkably distinguished as it is 
even at a distance, by its more harsh and rugged outline, de- 
