222 



contained in the obsidian of Quito, Mexico, and 

 Lipari, and which resemble the fibrous plates of 

 the crystallites of our glass houses, on which Sir 

 James Hall, Dr. Thomson, and Mr. Fleuriau de 

 Belle vue, have published some very curious ob- 

 servations % 



The third variety of obsidian of the Peak is the 

 most remarkable of the whole, from it's connec- 

 tion with pumice stones. It is, like the former, 

 of a greenish black, sometimes of a murky gray, 

 but it's very thin plates alternate with layers of 

 pumice stone. Dr. Thomson's fine collection 

 at Naples contained similar examples of lithoid 

 lava of Vesuvius, divided into very distinct plates, 

 only a line thick. The fibres of the pumice stone 

 of the Peak are very seldom parallel to each 

 other, and perpendicular to the strata of obsi- 

 dian ; they are most commonly irregular, as- 

 bestoidal, like fibrous glass-gall ; and instead of 

 being disseminated in the obsidian, like crystal- 

 lites, they are found simply adhering to one of 

 the external surfaces of this substance. During 

 my stay at Madrid, Mr. Hergen showed me se- 



* Bibl. Britann. t. xv, p. 340 ; t. xxvii, p. 147. Edin. 

 Trans. , vol; v, PI. X, No. 3. Journ. de Phys. an 12, floreal, 

 et an 13, prairial. The name of crystallites has been given to 

 the crystallized thin plates included in glass cooling slowly. 

 Dr. Thomson and others indicate by the word verre glastenise', 

 glass which by slow cooling is wholly unvitrified, and has as- 

 sumed the appearance of a fossile substance, or real glass 

 atone. 



