80 Dr Boue'^s Obsermtims on M. Beudanfs Opinions 
thickness alternating with rocks evidently arenaceous, as at the 
Calton Hill, &c. 
But geologists accustomed, like M. Beudant, to distinguish 
with so much accuracy the immediate products of volcanoes 
from those which are only results of aqueous destruction and 
re-aggregation of igneous matters, will find these alternations 
quite natural ; nor will he be at all astonished to find them ex- 
tending over a great space, although this space will never equal 
the extent of the common subordinate beds of the sandstone 
formation ; and this phenomenon will only furnish him wdth an 
excellent argument for maintaining that these igneous deposits 
have taken place during the formation of the sandstone deposite, 
in the same manner as the conchiferous basaltic tuffa of the 
Veronese, alternating with tertiary beds, shows that basaltic 
eruptions have happened during that period of time. 
Yet M. Beudant finds it extraordinary that these igneous 
matters should have been able to introduce themselves in such a 
way among Neptunian arenaceous rocks. This is not a little 
surprising ; for I may venture to predict, that his able descrip- 
tion of trachytic districts will soon induce geologists in general 
to extend with me the igneous domain. It may even be said, 
that the Wernerian idea of restricting volcanic productions to 
existing volcanoes only, was much more consistent with sound 
logic, than to extend the empire of Pluto to the extinct volca- 
noes and trachytes, and to stop there. In short, the geological 
thermometer must rise to a degree indicated by Nature, and not 
by partial views of natural phenomena. I can scarcely believe 
that M. Beudant should in reality doubt the alternation of ba- 
salts with tertiary deposites, for it is a fact too well determined 
to admit of dispute ; and those who believe it, or who have seen 
it, will see nothing extraordinary in the alternation of older ba- 
salts with sandstones. 
The geognostical position of porphyries has the greatest rela- 
tion with that of trachytes ; they often form bell-like hillocks or 
dbmes^ composed of concentric circular layers ; as, for example, 
near Freyberg, Halle, &c. one might fancy himself in front of 
the Puy of Sarcouy, in Auvergne. In other places, they rise, 
like certain trachytic districts, into conical and acuminated hills, 
which are sometimes as striking, on account of their isolated po- 
