S30 Dr Bone’s Comparison of the Volcanic Roclcs (f Auvergne 
pitchstone, as in the Valley des Gardes in the Cantal, along with 
two other basalt veins, traverse trachyte or volcanic porphyry. 
(Vid. La Coste’s Lettres sur les Volcans (VAuvergne, p. 255,) 
Not having the specimens I collected at this moment beside me, 
I cannot affirm that the rocks are identical with the remarkable 
pitchstone veins which you discovered on the eastern extremity 
of the Isle of Lamlash, although I am convinced that they are 
nearly of the same nature. All the other minerals in the Can- 
tal group which have been referred to the pitchstone species, 
appear to me to be varieties of semi-opal. Some of them are 
found in irregular beds in the calcareous fresh- water formation, 
and these appear to pass into Menilite, while others occur in 
large masses in volcanic tuffa. These latter appear to be singu- 
lar siliceous volcanic productions, that bear some relation to the 
volcanic rocks of Hungary. At St Pierre Eynat, in the Mezen 
group, semi-opal occurs in concretions, in what appeared to me, 
during my cursory visit, to be a fresh-water formation. In 
the Mont d’Or group is a resinous-like mineral, which has 
been considered as a variety of pitchstone, and it is so arran- 
ged in the Royal Museum of France, under the name Retinitc 
porphyritique et subretinite, I am quite satisfied of the resem- 
blance of these to your pitch stone-porphyry, but they are by 
no means identical with it. W e must, I think, look for illus- 
trations in Germany and other countries ; for the single fact just 
mentioned, although ad hoc^ is not sufficient to decide the que- 
stion about the pitchstone. 
7. The next rock I shall mention is the Trachyte^ or volca- 
nic porphyry. Under this name are comprehended many diffe- 
rent porphyritic rocks, having the general character of contain- 
ing crystals of glassy felspar. It is almost impossible to give 
a definition of such a rock ; and how could it be otherwise with 
a volcanic production, which is formed from all the different 
members of the granite formation The porphyry rock of 
Drumadoon Point in Arran bears some resemblance to tra- 
chyte : in some parts of the Croix Morand, in the Mont d’Or 
group, I found specimens which recalled to my mind the green 
and more compact parts of the trap-porphyry of Rue Varey 
Point, also in Arran. But still the structure and position of 
the Arran porphyries, on a general view, are so different, that 
