234 COSMOS. 



Rocks which are merely broken through by the volcanic ac- 

 tion are often inclosed in the igneous products. Thus I have 

 ibund angular fragments of feldspathic syenite imbedded in the 

 black augitic lava of the volcano of .ToruUo, in Mexico ; but 

 the masses of dolomite and granular limestone, which contain 

 magnificent clusters of crystalline fossils (vesuvian and garnets, 

 covered with mejonite, nepheline, and sodalite), are not the 

 ejected products of Vesuvius, these belonging rather to very 

 generally distributed formations, viz., strata of tufa, which are 

 more ancient than the elevation of the Somma and of Vesu 

 vius, and are probably the products of a deep-seated and con 

 cealed submarine volcanic action.* We find five metals among 

 the products of existing volcanoes, iron, copper, lead, arsenic, 

 and selenium, discovered by Stromeyer in the crater of Volca- 

 no.! The vapors that rise from the fiwtarolles cause the sub- 

 limation of the chlorids of iron, copper, lead, and ammonium ; 

 iron glancel and chlorid of sodium (the latter often in large 

 quantities) fill the cavities of recent lava streams and the fis- 

 sures of the margin of the crater. 



The mineral composition of lava differs according to the na- 

 ture of the crystalline rock of which the volcano is formed, the 

 height of the point where the eruption occurs, whether at the 

 foot of the mountain or in the neighborhood of the crater, and 

 the condition of temperature of the interior. Vitreous volcanic 

 formations, obsidian, pearl-stone, and pumice, are entirely want- 

 ing in some volcanoes, while in the case of otliers they only 

 proceed from the crater, or, at any rate, from very considera- 

 ble heights. These important and involved relations can only 

 be explained by very accurate crystallographic and chemical 

 investigations. My fellow-traveler in Siberia, Gustav Rose, 

 and subsequently Hermann Abich, have already been able, 

 by their fortunate and ingenious researches, to throw much 

 light on the structural relations of the various kinds of vol- 

 canic rocks. 



* Leop. vou Buch, in Poggend., Annalen, bd. xxxvii., s. 179. 



t [The litde island of Volcano is separated from Lipari by a narrow 

 channel. It appears to have exhibited strong signs of volcanic activ- 

 ity long before \he Christian era, and still emits gaseous exhalations. 

 Stromeyer detected the presence of selenium in a mixture of sal ammo- 

 niac and sulphur. Another product, supposed to be peculiar to this 

 volcano, is boracic acid, wliich lines the sides of the cavities in beauti- 

 ful white silky crystals. Daubeney, op. cit., p. 257.] — Tr. 



t Regarding the chemical origin of iron glance in volcanic masses, see 

 Mitscherlich, in Poggend., Annalen, bd. xv., s. 630 ; and on the libera 

 tion of hydrochloric acid in the crater, see Gay-Lussac, in the Annal^ 

 ic Chimique el de Physique, t. xxii., p. 423. 



