Dr Daubeny on the Volcanoes qf Auvergne. 
impregnated with bitumen, which covers the external surface 
with a glossy varnish, and fills all the crevices in the rock. The 
trap itself is of that kind which is called, by the French geolo- 
gists, Variolite, meaning to designate, by that term, a trap-rock, 
which, from the unequal manner in which it decomposes, ex- 
hibits a number of light spots disseminated through a darker 
ground. 
The volcanic nature of the variolite is established by the oc-^ 
eurrence in it of occasional fragments of vesicular lava, for, like 
so many of the rocks of this description in Auvergne, it fre- 
quently puts on a tulfaceous character. 
The Puy Cronelle, which is about half a mile distant, is 
composed of the same rock with that we have been describing, 
equally penetrated with asphaltum. Its conical form leads us 
to conclude a 'priori that it is of trap, and we find our suspi- 
cions verified, when we come to examine it. The lower por- 
tions of the hill, which consist of fresh-water limestone, the 
same which extends over other parts of the plain, are covered 
with vineyards, which terminate pretty generally where the 
trap commences^ 
I was therefore able, by following this indication, to trace 
pretty exactly the line of junction between the two formations 
round the greater part of the hill. The line was, indeed, as 
might be expected, somewhat irregular, and in many instances 
I observed the limestone sending forth processes vertically into 
the trap-rock above it. But a still more curious thing was 
the occurrence of perfectly isolated fragments of the same lime- 
stone in the midst of the trap which covers it, apparently un- 
altered in appearance by the heat to which, as it would seem, 
it must have been subjected. 
Nor is this fact altogether uncommon. I observed in many 
of the Coulies’'’ near Clermont, as at Graveneire, fragments 
of a rock much resembling granite, completely surrounded by 
the lava ; and M, Lacoste of Clermont, the author of some 
Letters on Auvergne, shewed me, in his collection, two interest- 
ing specimens, in one of which a ball of granite was inclosed in 
a nucleus of basalt, while, in the other, the inclosed mass of 
granite had a regular octahedral form. 
