448 cosmos. 



Common feldspar (orthoclase) never occurs in pumice along 

 with sanidine, nor is augite ever present. The Somma, not 

 the cone of Vesuvius itself, contains pumice, inclosing earthy- 

 masses of carbonate of lime. It is by this remarkable vari- 

 ety of a calcareous pumice that Pompeii was overwhelmed.* 

 Obsidians are rare in genuine lava-like streams ; they belong 

 almost solely to the Peak of Teneriffe, Lipari, and Volcano. 



Passing now to the association of obsidian and pumice in 

 one and the same volcano, the following facts appear. Pi- 

 chincha possesses large pumice fields, and no obsidian. Chim- 

 borazo, like JEtna, whose trachytes, however, have a totally 

 different composition (containing Labradorite instead of oligo- 

 clase), shows neither obsidian nor pumice ; this same defi- 

 ciency I observed on my ascent of the Tungurahua. The 

 volcano Purace, near Popayan, has a great deal of obsidian 

 mixed in its trachytes, but has never yielded any pumice. 

 The immense plains out of which rise the Ilinissa, Carguai- 

 razo, and Altar are covered with pumice. The subterra- 

 nean pumice quarries near Lactacunga, as well as those of 

 Huichapa, southeast of Queretaro ; and the accumulations 

 of pumice at the Rio Mayo,f those near Tschegem in the 

 Caucasus, $ and near Tollo§ in Chili, at a distance from act- 

 ive volcanic structures, appear to me to belong to the phe- 

 nomena of eruption from the numerous fissures in the level 

 surface of the earth. Another Chilian volcano, that of An- 

 tuco || (of which Poppig has given a description as scientific- 

 ally important as it is agreeably written), produces, like Ve- 

 suvius, ashes, triturated rapilli (sand), but gives out no pum- 

 ice, no vitrified or obsidian-like mineral. Without the pres- 

 ence of either obsidian or glassy feldspar, we sometimes meet 

 with pumice in trachytes of very dissimilar composition, al- 

 though in many cases it is not present. Pumice, as Charles 

 Darwin observes, is entirely wanting in those of the Archi- 



* Scacchi, Osservazioni critiche sulla maniera come fa sepdlita Vantica 

 Pompei, 1 843, p. 10, in opposition to the theory proposed by Carmine 

 Lippi, and afterward 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. Roth, Monogr. des Vestws, 1857, s. 458 ; see above, p. 401. 



+ Nivellement Barometrique, in Humboldt, Observat. Astron., vol. i., 

 p. 305, No. 149. $ See above, p. 324. 



§ For an account of the pumice hill of Tollo, at a distance of two 

 days' journey from the active volcano of Maypu, which has itself never 

 ejected a fragment of such pumice, see Meyen, Reise inn die Erde, th. 

 I'., s. 338 and 358. 



II Poppig, Reise in Chile und Peru, bd. i., s. 42G. 



