232 COSMOS. 



filling large basins, in which they become solidified in super- 

 imposed strata. These few sentences describe the general 

 character of the products of volcanic activity. 



Rocks which are merely broken through by the volcanic 

 action ,are often inclosed in the igneous products. Thus, 

 I have found angular fragments of feldspathic syenite em- 

 bedded in the black augitic lava of the volcano of Jorullo, in 

 Mexico : but the masses of dolomite and granular limestone, 

 which contain magnificent clusters of crystalline fossils (vesu- 

 viaii and garnets, covered with mejonite, nepheline, and soda- 

 lite), are not the ejected products of Vesuvius, these belonging 

 rather to very generally distributed formations, viz., strata of 

 tuffa, which are more ancient than the elevation of the Somma 

 and of Vesuvius, and are probably the products of a deep- 

 seated and concealed submarine volcanic action.* 1 We find 

 five metals amongst the products of existing volcanoes, iron, 

 copper, lead, arsenic, and selenium, discovered by Stromeyer 

 in the crater of Volcano. f The vapours that rise from the 

 fumarolles cause the sublimation of the chlorides of iron, 

 copper, lead, and ammonium ; iron glance,^; and chloride of 

 sodium (the latter often in large quantities) fill the cavities 

 of recent lava streams and the fissures of the margin of the 

 crater. 



The mineral composition of lava differs according to the 

 nature 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 neighbourhood of the crater 

 and the condition of temperature of the interior. Vitreous 

 volcanic formations, obsidian, pearl-stone, and pumice, are 

 entirely Avanting in some volcanoes, whilst in the case of 



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



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

 channel. It appears to have exhibited strong signs of volcanic activity 

 long before the Christian era, and still emits gaseous exhalations. 

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

 moniac" and sulphur. Another product supposed to be peculiar to this 

 volcano is boracic acid, which lines the sides of the cavities in beautiful 

 white silky crystals. Daubeney, op. cit., p. 257.] TV. 



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

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

 of hydrochloric acid in the crater, see Gay Lussac, in the Annales <* 

 Chimiquc et de Physique, t. xxii. p. 423. 



