TRUE VOLCANOES. 447 



springs and even in sea-water. The occurrence of olivin in 

 meteoric stones* and in artificial scoriae, as investigated by 

 Seifstrom,t I have already mentioned. 



Obsidian. 



As eai'ly as in the spring and summer of 1799, while I 

 was preparing in Spain for my voyagS to the Canary Isles, 

 there prevailed generally among the mineralogists in Madrid 

 — Hergen, Don Jose Clavijo, and others — the opinion that 

 pumice was entirely derived from obsidian. This opinion 

 had been founded on the study of some line geological collec- 

 tions from the Peak of TeneriiFe, and a comparison of them 

 with the phenomena which Hungary furnishes, although the 

 latter were at that time explained chiefly in accordance with 

 the Neptunian views of the Freiberg school. Doubts of the 

 correctness of this theory of formation, awakened at an early- 

 period in my mind by my observations in the Canary Isles, 

 the Cordilleras of Quito, and in the range of Mexican volca- 

 noes, J impelled me to direct my most earnest attention to 

 two groups of facts : first, the different nature of the inclosures 

 of obsidians and pumice in general ; and, secondly, the fre- 

 quency of the association or entire separation of them in well 

 investigated active volcanic structures. My journals are filled* 

 with notices on this subject, and the specific definition of the 

 imbedded minerals has been ascertained by the most varied 

 and most recent investigations of my ever-ready and obliging 

 friend, Gustav Rose. 



Both glassy feldspar and oligoclase occur in obsidian as 

 well as in pumice, and frequently both of them together. As 

 examples may be cited — the Mexican obsidians of the Cerro 

 de las Navajas, on the eastern slope of the Jacal, collected by 

 me — those of Chico, with many crystals of mica — those of 

 Zimapan, to the S.S.W. of the capital of Mexico, mixed with 

 small distinct crystals of quartz, and the pumice of the Rio 

 Mayo (on the naountain road from Fopayan to Pasto), as 

 well as those of the extinct volcano of Sorata. near Popayan. 

 The subterranean pumice quarries near Lactacunga§ contain 

 a large quantity of mica, oligoclase, and (which is very rare 

 in pumice and obsidian) hornblende also*; the latter, how- 

 ever, is also found in the pumice of the volcano of Arequipa, 



* Cosmos, vol. i., p. 131, and vol. iv., p. 225. 



t Ibid., vol. i., p. 267, note *. 



i Humboldt, Personal Narrative, vol. i., p. 113 (Bohn's edition). 



§ See above, p. 322. 



