72 Dr Boue’s Observations on M, Beudanfs Opinions 
We must add, also, that the absence of pearlstone in old pitch- 
stones is quite natural, for we know that vitreous matter pre- 
sents a similar concretionary structure only in the case of a cer- 
tain slow cooling. Now, may it not be plausibly supposed, that 
the accessory circumstances of the trachytic deposits have been 
very favourable to the production of this particular structure ; 
and that, on the contrary, those which have happened at the 
period of the old pitchstone formation, have been in no degree 
favourable to the formation of this variety of vitreous product, 
as well as to that of the very porous vitreous rocks 
The question, then, is always hanging upon some trifling cir- 
cumstance of no essensial importance ; and I believe I have al- 
ready sufficiently shewn how little weight those pretended Nep- 
tunian proofs have, which are elicited from the comparison of 
two species of vitreous deposites. 
M. Beudant next proceeds to the porphyries* He assures us 
that some of these rocks in the neighbourhood of the old second- 
ary pitchstones have often a great similarity to certain varieties 
of trachyte. In another place, he says that the secondary por- 
phyries present a pretty well marked analogy with the trachytic 
porphyries, and his porphyre molaire^ (p. 199) ; and he adds, 
further, that some masses of porphyry would, from their cha- 
racters, lead one to suppose them of volcanic origin, while others 
again impress the idea of a Neptunian one. Now, I shall agree 
with M. Beudant in saying, that the one and the other must ne- 
cessarily have had the same origin ; and it will only be requisite to 
shew, in such of the porphyries as seem to be of aqueous origin, 
a sufficient number of igneous characters, to be able to attribute 
to these the same origin as to the other class. 
in the first place, these porphyries present scoriaceous or po- 
rous portions, which seem to me to demand a much greater at- 
tention than M. Beudant seems inclined to bestow upon them, 
for these varieties are not always distributed without a certain 
order-: they are to be observed principally in the upper, and 
less frequently in the lower parts, — an observation which I have 
fully verified, contrary to the assumption of M. Beudant, as well 
at Halle, as in the Thuringerwald, the Erzgebirge, Scotland, 
&c. Besides, these porosities have exactly the form of those of 
the igneous deposites, and are in no way to be compared to thos,e 
