M. Beudairit on the Piichstone of Saxony, ^71 
or pltchstone, analogous to those of Tribisch, and of many other 
places. 
But, after having spoken of the general position of pitchstone, 
it may not be useless to say a few words on the origin whick 
may be attributed to it, following the probabilities which result 
from our actual recognitions. Some mineralogists consider these 
rocks as of igneous, others as of aqueous origin. This is al- 
ways the great question which divides the Schools, and'^on which 
it is impossible to agree at all times, when the facts which might 
lead to the one or the other opinion are not rigorously contrast- 
ed. We shall endeavour, in this place, to exhibit a short ac- 
count of both, that we may be enabled to judge of their respec- 
tive validity. 
In adopting the opinion of an ^ Igneous origin, the principal 
argument is founded upon the resemblance which exists between 
the pitchstones and certain varieties of claystone with which they 
are associated, and products evidently or very probably of igneous 
origin, It is thus that a very remarkable analogy exists between 
the vitreous pitchstones and certain varieties of the obsidian of 
active volcanoes, and a perfect resemblance between these rocks 
and certain varieties of perlite (perlite retinite), of trachy tic for- 
mation, which are in all probability of igneous origin. In the 
same way, on comparing certain earthy pitchstones, both simple 
and porphyritic, as well as the porphyritic clajstones, with cer- 
tain varieties of earthy perlite (perlite lithoide), of trachytic 
porphyry, and of porphyre molaire, we find that there is a mi- 
neralcgical identity. Now, as the perlites, trachytic porphyries, 
and porphyres molaires, are very proba:bly of igneous origin, 
the same proba!bility holds equally with regard to the rocks 
which resemble them. Such, I believe, are the sole data that 
can be given with regard to the igneous origin of the masses of 
pitchstone. 
The Neptunian origin is founded upon several data, which 
we proceed in like manner to state. It is necessary to admit 
the analogies which we have mentioned as existing between the 
pitchstones and the rocks of igneous Origin, because they are 
real, and because to reject them would be to refuse assent to evi- 
dence ; but, it cannot also be admitted^ that these are the only 
analogies which lead to this opinion, and that all the rest are of 
