27^ M, Beudant on the Pitchstone of Saxony. 
a contrary nature. Thus, it is a fact, that nowhere do we find 
in the masses of pitchstones, either porous substances or slags, 
which exist every where in the currents of obsidian poured out 
by modern volcanoes, and in the masses of perlite which belong 
to the trachytic formation. Again, while the earthy varieties of 
pitchstone, as well as the felspar porphyries with which they occur, 
have a pretty marked analogy witli the earthy perlite, trachytic 
porphyries, and porphyres molaires, it must be remarked, that 
an equally perfect identity is not found when we examine at the 
^ame time, on all sides, the series of varieties through which all 
these rocks pass. In fact, we do not see in the pitchstone the 
immense quantity of small vitreous globules, radiated from the 
centre to the circumference, which is every where found in the 
perlites, trachytic porphyries, and porphyres molaires, and of 
which the quantity is so great, that it might in many cases be 
said that they are rather globular rocks than vitreous and por- 
phyritic ones. Nor do we find these varieties of pitchstones suc- 
cessively modified till they assume a cellular structure, with 
hollow cellules, as we find in the rocks with which they are 
compared. 
Thus the comparison of the product, which holds good when 
some isolated specimens are taken, will not do so when all the 
varieties are collected for more scrupulous examination. We 
find, then, that the varieties, which are of the greatest import- 
ance m leading to the opinion of an aqueous origin, are precisely 
those which are wanting. It appears to me, therefore, that, on 
weighing the preceding statements, in order to discover their 
comparative validity, we shall find a little more weight in those 
which have been brought forward in favour of the neptunian 
origin, since they are founded precisely upon the absence of 
products, which wmild give the greatest probability to the hy- 
pothesis of an igneous origin. But I shall not even admit this 
probability, and I shall consider the data which might be pre- 
mised in favour of either opinion, as mutually compensated, so 
that, on limiting ourselves to them alone, the geological relations 
of the rocks in question must rema,in undecided. 
But, after having resmnded the preceding statements, there 
remain other data which are not liable to be counterbalanced. 
These are in favour of the neptunian origin; they are founded 
