223 



veral specimens in the mineralogical collection of 

 Don Jose Clavijo ; and for a long time the Spa 

 nish mineralogists considered them as undoubt- 

 ed proofs, that pumice stone owes it's origin to 

 obsidian, in some degree deprived of color, and 

 swelled by volcanic fire. I was formerly of this 

 opinion, which must be confined 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 it's way 

 through the basalts, the green stone rocks, the 

 phonolites, and the porphyries with basis of 

 pitchstone and obsidian, the lavas and pumice 

 stone were no other than these same rocks al- 

 tered by the action of the volcanoes. The de- 

 privation of color and extraordinary swelling, 

 which the greater part of the obsidians undergo 

 in a forge fire, their transition into pechstein, and 

 their position in regions very distant from burn- 

 ing volcanoes, appear * to be phenomena very 

 difficult to reconcile, when we consider the ob- 

 sidians as volcanic glass. A more profound 

 study of nature, new journeys, and observations 

 made on the productions of burning volcanoes, 

 have led me to renounce those ideas. 



It appears to me at present extremely pro- 

 bable, that obsidians, and porphyries with basis 

 of obsidian, are vitrified masses, the cooling of 



* Ann. du Mus. d'Hisi. nat. t. iii, p. 398. 



