102 
BARE MINEEAR SPECIMENS. 
a long time the Spanish mineralogists considered them as 
furnishing undoubted proofs, that pumice-stcne owes its 
origin to obsidian, in some degree deprived of colour, and 
swelled by volcanic fire. I was formerly of this opinion, 
which, however, must be understood to refer to one variety 
only of pumice. I even thought, with many other geologists, 
that obsidian, so far from being vitrified lava, belonged to 
rocks that were not volcanic ; and that the fire, forcing its 
way through the basalts, the green-stone rocks, the phono- 
lites, and the porphyries with bases of pitchstone and obsi- 
dian, the lavas and pumice-stone were no other than these 
same nodes altered by the action of the volcanoes. The 
deprivation of colour and extraordinary swelling which the 
greater part of the obsidians undergo in a forge-fire, their 
transition into pitch-stone, and their position in regions verv 
distant from burning volcanoes, appeiir to be phenomena 
very difficult to reconcile, when we consider the obsidians 
as volcanic glass. A more profound study of nature, hqiv 
journeys, and observations made on the productions of 
burning volcanoes, have led me to renounce those ideas. 
It appears to me at present extremely probable, that obsi- 
dians, and porphyries with bases of obsidian, arc vitrified 
masses, the cooling of which has been too rapid to change 
them into lithoid lava. I consider even the pearlstone as 
an unvitrified obsidian: for among the minerals in the 
King s cabinet at Berlin there are volcanic glasses from 
Lipari, in which we see striated crystalites, of a pearl-gray 
colour, and of an earthy appearance, forming gradual"' ap- 
proaches to a granular lithoid lava, like the "pearlstone of 
Cinapecuaro, in Mexico. The oblong bubbles observed in 
the obsidians of every continent are incontestiblc proofs of 
their ancient state of igneous fluidity ; and Dr. Thompson 
possesses .specimens from Lipari, which are very instructive 
in this point of view, because fragments of red porphyry or 
porphyry lavas, which do not entirely fill up the cavities of 
the obsidian, are found enveloped in them. We might say, 
that these fragments bad not time to enter into complete 
solution in the liquified mass. They contain vitreous feld- 
spar, and augite, and are the same' as the celebrated co- 
lumnar porphyries of the island of Panaria, which, without 
having been part of a current of lava, seem raised up in the 
