S 06 Dr Daiibeny on the Ancient Volcamioe^ of Awvergne. 
fissures in the trachyte below it, and not that they were the 
rents through which the basalt was originally forced into the 
spot which it now occupies. 
Respecting the nature and origin of the traohytic formation, 
many questions arise, which it will be found very difficult to solve. 
That, it has been produced, or at least affected by the agency of 
heat, appears evident from the scoriae and pumice with which it 
is found associated, as well as from its analogy with the undis- 
puted lavas of the Lipari Islands. But the nature of the ma- 
terials from which it has been formed, and the manner by 
which the action of fire has reduced these materials into the 
state in which we now find them, is no less obscure than the 
explanation of the process which has produced an analogous 
rock found among the recent volcanoes near Clermont, namely, 
the Domite of the Puy de Dome. Notwithstanding certain 
differences in appearance existing between these two rocks, the 
general resemblance is such, that we can hardly hesitate to re- 
fer both to the same formation, and admit that the trachyte is 
to. the ancient lavas what the domite is to the modern. Both 
indeed appear to be products of the same mud volcanoes, which 
have had such extensive operation in the New World, and in 
the Old have produced the lavas of the Lipari Islands, and a 
few of those in Italy. 
At the same time, it must be confessed, that certain varieties 
of trachyte bear the most striking resemblance to some of the 
porphyries in Scotland, which are by no means generally ad- 
mitted to be volcanic. 
If the clinkstone of Mont d’Or should, as M. Daubuisson sup- 
poses, be a modification of trachyte, I know not how we are to 
help extending the same inference to -that of North Berwick 
and Trapraine Law in East Lothian, nor can two rocks brought 
from distant parts of the globe resemble each other more close- 
ly than do certain varieties of the trachyte of Mont d’Or, and 
the clay stone-porphyry of Drumodoon in the Isle of Arran. 
It is thus that we are brought to acknowledge the insufficiency 
of our present data to determine many of the great questions 
of geology, and that a little more extensive inforhiation makes 
us doubt the truth of those opinions which we had previously 
iixiagined to be the best established. Before, however, I quit 
