r egarding the Rocks of the Red Sandstone Formation. 69 
“ Every where in the trachytic deposits,” says this learned 
author, the perlites are accompanied with porous and slaggy 
rocks of different varieties. Now, it is very remarkable, that 
there exists no trace of these in all the deposits of pitchstone 
with which we are acquainted. This constant absence of pro- 
ducts, which are precisely those from which the hypothesis of 
an igneous origin might derive the greatest weight, is one of the 
strongest arguments in favour of the Neptunian hypothesis. On 
the other hand, when lithoid varieties have a pretty marked ana- 
logy with the lithoid perlites, if we consider with attention the 
relations of the several varieties through which these rocks pass, 
we cannot help agreeing that a perfect identity does not occur. 
W e do not find in the pitchstone that immense quantity of small 
vitreo-lithoid giolbules, which are seen at every step in the rocks 
which are considered as analogous to them ; nor do we observe 
these lithoid pitchstones modifying themselves successively, un- 
til they assume a cellular structure, with irregular cavities, as 
we see every where in the perlites.” 
It cannot certainly be said, that the pitchstones of the red 
sandstone formation are associated with pumice or with great 
scoriaceous masses, at least in the localities yet known ; still the 
absence of these circumstances is not absolute. 'The pitchstones 
are here and there porous, at Rue Varey in Arran, for example; 
at Corygills (see my Essai sur FEcosse, p. 307), and even in 
some points of the Saxon Trebischthal, they are even some- 
times modified so much as to assume that particular cellular 
structure, (a cellules dechiquetees) as is the case in the lov/er part 
of the pitchstone of the Corygills {Essai, p. 315) ; and at Pla- 
nitz the pitchstones are associated with a small set of very po- 
rous rocks. Besides, the lithoid pitchstones with small vitreo- 
lithoid globules are to be observed at Corygills, and are identi- 
cal with those of the pearlstone, while similar varieties occur al- 
so in the Valley of Trebischthal. 
It is true that all these circumstances are of much less fre- 
quent occurrence in the secondary than in the trachytic pitch- 
stones ; but it appears to me that their occurrence in both is 
enough to prevent our considering this rarity in the secondary 
rocks as one of the strongest proofs in favour of the Neptunian 
idea ; for were ’we to adopt this mode of proceeding, we might 
