310 Dr Daubeny wi the Ancient Volcanoes of Auvergne, 
rived from the very indestructible nature of the materials 
which compose it. Hence the hills in Cantal are usually 
covered towards their summits, owing to the presence of this 
rockj with massive fragments of stone, of a greyish colour 
and great hardness, whereas the trachyte underneath decom- 
poses in a more rapid and uniform manner. Nor can there be 
a greater contrast than between the luxuriance of some of the 
valleys, as that of Teyzac, in which the substratum is of trachyte, 
and the extreme barrenness of the higher parts, which are 
composed of plionolite. The porphyry-slate, too, found on the 
summit of the Plomb de Cantal, the highest mountain in that 
district, bears a much closer analogy to basalt than to trachyte ; 
and the circumstance of the clinkstone occupying the same re- 
lative position in Cantal which the basalt does at Mont d’Or, 
would lead us to infer that it is more allied to the latter forma- 
tion than to trachyte. We have already alluded to the tuff, 
which, at Mont d’Or, is found associated with trachyte. A simi^ 
lar rock occurs in much greater abundance throughout Cantal ; 
it is there remarkably distinguished by the grotesque appear- 
ances which it assumes, presenting to the eye a range of mural 
precipices, broken into a number of fantastic shapes, — a circum- 
stance very characteristic of rocks of this description, both here 
and in the neighbourhood of the Puy en Velay. 
It is only necessary to refer the reader to M. Faujas St 
Fond’s description of the Rock of St Michael, near the latter 
town, which stands isolated in the midst of the valley, and from 
the small proportion which its diameter beat’s to its height, more 
resembles some work of art than a production, or rather a 
relic of nature. This and a projecting rock, almost equally ex- 
traordinary in its appearance, which is found near the same 
spot, and goes by the name of ‘‘ Le Rocher Rouge,” from the 
red colour which its surface presents from decomposition, M. 
Faujas seems to suppose to have been projected by some con- 
vulsion of nature from the bowels of the earth ; and he even 
refers in the latter instance to an appearance of dislocation in 
the granite which encircles it, as giving countenance to this opi- 
nion. 
This circinnstance ought to render us cautious in deciding on 
the reality of those alterations on the position of strata which 
