TRUE VOLCANOES. 479 



for ornamental purposes,* a granulated mixture of hyper- 

 sthene arid labradorite, which Berzelius has described as sye- 

 nite, likewise contains olivine, 6 as does also (though more 

 rarely) the phonolite of the Pic de Griou, in the Cantal. 6 

 While, according to Stromeyer, nickel is a very constant ac- 

 companiment of olivine, Kumler has on the other hand 

 discovered arsenic in it, 7 a metal which has been found in the 

 most recent times widely diffused in so many mineral springs, 

 and even in sea-water. The occurrence of olivine in meteoric 

 stones 8 and in artificial scoria^ as investigated by Sefstrom," 

 I have already mentioned. 



OBSIDIAN. 



As early as in the spring and summer of 1799, while I was 

 preparing in Spain for my voyage 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 fine geological 

 collections from the Peak of Teneriffe, and a comparison of 

 them with the phenomena which Hungary furnishes, although 

 the latter were at that time explained chiefly in accordance 

 with the jSTeptunian 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 

 volcanoes, 10 impelled me to direct my most earnest attention 



[* It is there cut into vases, sometimes of a considerable size, and 

 other ornamental objects. From the high polish it takes, and the 

 contrast of its colours, it is one of the most beautiful stones in 

 existence. Tr.] 



5 Berzeliufe, Sechster Jaltresbemcht, 1827, p. 392; Gustav Rose, in 

 Poggend. Ann. vol. xxxiv, 1835, p. 14 (Cosmos, vol. i, p. 464). 



6 Jenzsch, Pkonolithe, 1856, p. 37, and Seuft, in his important work, 

 Classification der Felsarten, 1857, p. 187. According to Scacchi olivine 

 occurs also, along with mica and augite, in the lime-blocks of the 

 Somma. I call these remarkable masses erupted blocks, not lavas, for 

 the Somroa appears never to have ejected the latter. 



' Poggend. Annul. Bd. xlix, 1840, s. 591, and Bd. Ixxxiv, s. 302; 

 Daubrde in the Annales des Mines, 4me Serie, t. xix, 1851, p. 669. 



8 Cosmos, vol. i, p. 119, and vol. iv, p. 595. 



9 Ibid. vol. i, p. 269, note*. 



10 Humboldt, Personal Naraiive, vol. i, p. 113 (Bohn's Edition). 



