ISO COSMOS. 



to two groups of facts ; first, the different nature of the en- 

 closures of obsidians and pumice in general, and secondly, 

 the frequency 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 defi- 

 nition 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 felspar 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 mountain-road from Popayan 

 to Pasto) as well as those of the extinct volcano of Sorata, 

 near Popayan. The subterranean pumice quarries near 

 Liactagunga 11 contain a large quantity of mica, oligoclase, 

 and (which is very rare in pumice and obsidian), hornblende 

 also ; the latter, however, is also found in the pumice of the 

 volcano of Arequipa. Common felspar (orthoclase) never 

 occurs in pumice along with saiiidine, nor is augite ever 

 present. The Somma, not the cone of Vesuvius itself, con- 

 tains pumice, enclosing earthy masses of carbonate of lirne. 

 It is by this remarkable variety of a calcareous pumice that 

 Pompeii was overwhelmed. 1 * Obsidians are rare in genuine 

 lava-like streams ; they belong almost solely to the Peak of 

 Teneriffe, Ltpari, and Volcano. 



Passing now to the association of obsidian and pumice in 

 one and the same volcano, the following facts appear. 

 Pichincha possesses large pumice-fields and no obsidian. 

 Chimborazo, like Etna, whose trachytes, however, have a 



11 See above, p. 342. 



12 Scacchi, Oaservazioni criticJte sulla manicra come fu scpellito Tantica, 

 Pompei, 1843, p. 10, in opposition to the theory proposed by Carmine 

 Lippi, and afterwards shared by Tondi, Tenore, Pilla, and Dufrenoy, 

 that Pompeii and Herculaneum were not overwhelmed by rapilli 

 and ashes direct from the Somma, but that they were conveyed 

 there by water. Iloth, Monogr. cks Vesuvs. 1857, 5. 458, sea 

 above, p. 429. 



