S08 Dr Daubeny on the Ancient Volcanoes of Auvergne. 
composition or the manner of its formation. At a cascade 
near the village of the Baths of Mont d’Or, I observed some 
very regular basaltic columns just where the escarpment of the 
rock was touched by the falling spray, and nowhere else, — an 
observation which, with some others tending to the same point, 
I communicated to the Geological Society, iii a paper read be- 
fore them last summer. 
The analogy also of the trachytic formation with the por- 
phyry of Sandy Brae in the same country, although it may not 
hold good in ail points, is nevertheless such as to lead to many 
interesting conclusions. Both present a correspondence in the 
crystals of glassy felspar which enter so largely into their com- 
position ; both pass into pitchstone-porphyry, and the trachyte of 
Cantal even contains, like that of Sandy Brae, imbedded mas- 
ses of opal. 
They differ, indeed, inasmuch as the Irish porphyry abounds 
in quartz, — a mineral so rare in the trachyte of France, that M. 
Daubuisson says it is found only in one spot of Cantal ; but it 
should be recollected that the absence of quartz may be con- 
sidered an accidental circumstance, since M. Beudant has dis- 
covered varieties which contain it abundantly in that porphyritic 
formation of Hungary, which, from its general characters and 
relations, he ranks with the trachytes of Auvergne. Whether 
these varieties approach more nearly to the Sandy Brae por- 
phyry, it will be for future travellers to determine * ; but, even 
according to the present state of our knowledge, we cannot 
help regarding the association of clay stone -porphyry, containing 
pitchstones and opal, with basalt, as tending to bring the rocks 
of the Gianf s Causeway a step nearer the volcanic formations of 
Auvergne. 
3. The third or lowest of the formations found at Mont 
d’Or is the Granite, which, concealed throughout the greater 
* Since writing the above, I find, from the valuable work of Dr Boue on the 
Geology of Scotland, recently published, that these varieties do in reality ap- 
proach very near to the porphyry of Drumodoon in the Isle of Arran, (Ferfe 
p. 295.) which h^ justly considers analogous to the rock of Sandy Brae in Ireland, 
Vide p. 384.) I may therefore regard my conjecture as to the resemblance be- 
tween the quartzous trachyte of Hungary and the Irish porphyry as confirmed. 
