Dr Daubeny on the Ancient Volcanoes Auvergne, S07 
this subject, I cannot help remarking the analogy which exists 
generally between the rocks of Auvergne and those of the trap 
formation in the North of Ireland. The basalt of the latter 
country can scarcely be distinguished in external characters 
from that of Mont d’Or, or at least differs from it much less 
than one variety of that rock found in the same formation dif- 
fers from another. It is indeed distinguished by the total ab- 
sence of scorified matter; for I conceive it to be beyond ques- 
tion, that the cinders found close to the bed of brown coal, 
which lies in the midst of the basalt immediately above the 
Causeway, are owing to some artificial fire ; and it differs like- 
wise in the greater abundance of dikes cutting through, and 
hardening in their vicinity the rocks on which the basaltic de- 
posit in Ireland is incumbent, and the regular columnar ar- 
rangement which it more frequently affects. But of these dif- 
ferences you will not be disposed to lay much stress on any ex- 
cept the first, and the natural inference from it seems to be, 
that the basalt of the Giant’s Causeway has been formed at the 
bottom of the then existing ocean, whereas in the case of that of 
Mont d’Or, the necessary pressure was supplied by the masses 
of scorified and cellular matter above it,— -a portion of which, as 
I have already stated, may since have been removed by the 
action of the elements, leaving the subjacent basalt partially un- 
covered. The rarer occurrence of dikes and of columnar con- 
cretions in the basalt of the Mont d’Or than in that of An- 
trim, cannot be considered as of much weight towards deter- 
mining the present question, especially as instances are not 
wanting, even in the former country, of columnar basalt, al- 
most as perfect and regular as we meet with elsewhere In- 
deed, I am inclined to think, that the superior symmetry and 
greater frequency of the basaltic columns in the north of Ire- 
land, is attributable rather to the action of water along such an 
extent of coast, in developing in a gradual manner the latent 
structure of the mass, than to any original peculiarity in its 
• In the Vivarais we meet with numerous instances of the most regularly 
columnar basalt, which has evidently flowed from volcanoes, since the excavation 
of the present valleys. See Favjas St Fond sur les Volcans des Vivarais. 
