ON ITS EXTEKIOE. VOLCANOES. 443 



stones (aerolites) ( 632 ), and in artificial scorise (slags) ( 633 ), 

 examined by Sefstrom. 



Obsidian. 



So long ago as the spring and summer of 1799, when 

 I was preparing in Spain for my voyage to the Canaries, 

 the belief of the formation of pumice solely from ob- 

 sidian prevailed generally among the mineralogists of 

 Madrid, Hergen, Don Jose Clavijo, and others. This 

 belief had been based on the study of fine geological 

 collections from the Peak of Teneriffe, as well as on the 

 comparison with phenomena in Hungary, although the 

 latter had then been for the most part represented 

 according to the interpretation resulting from the Nep- 

 tunian views of the Freiberg school. Doubts as to the 

 insufficiency of this theory of the formation of pumice, 

 which were very early awakened in me by my own 

 observations in the Canaries, in the Cordilleras of Quito, 

 and in the range of the Mexican volcanoes ( 634 ), incited 

 me to direct my most earnest attention to two groups 

 of facts: one being the general diversity of the sub- 

 stances enclosed in obsidians and in pumice-stones; 

 and the other their frequency of association or entire 

 separation in well-examined active volcanic frameworks. 

 My journals are filled with data on this subject ; and 

 my determinations of the species of the minerals form- 

 ing part of the rocks have since been assured by the 

 recent and varied examinations of my ever kind and 

 ready friend Gustav Eose. 



In obsidian, as in pumice, glassy felspar is found as 

 well as oligoclase, and often both together. I may 

 adduce, as examples, the Mexican obsidians collected by 



