REACTION OF THE INTERIOR OF THE EARTH 



the igneous products. Thus I have found angular frag- 

 ments of feldspathic syenite imbedded in the black augitic 

 lava of the volcano of Jorullo, in Mexico. But the masses 

 of dolomite and granular limestone found in the neigh- 

 bourhood of Vesuvius, containing magnificent groups 

 of crystallized minerals (vesuvian and garnets, covered 

 with mionite, nepheline, and sodalite), are not substances 

 which have been erupted from that volcano ; et they belong 

 rather to a very generally distributed formation to beds 

 of tufa, which are older than the elevation of the Somma 

 and of "Vesuvius, and were probably produced by a sub- 

 marine and deeply-seated volcanic action/' ( 221 ) We find 

 among the products of existing volcanoes five metals, iron, 

 copper, lead, arsenic, and selenium, the latter of which was 

 discovered by Stronger in the crater of Volcano. The 

 vapours from the small cones contain chlorides of iron, 

 copper, lead, and ammonia ; specular iron, ( 222 ) and common 

 salt, (the latter often in large quantities) are found in 

 cavities of recent lava currents, and in fissures in the 

 margin of the crater. 



The mineral composition of lava differs according to the 

 nature of the crystalline rock of which the volcano consists, 

 according to the height of the point at which the eruption 

 takes place (whether at the foot of the mountain or near the 

 crater), and according to the degree of heat of the interior. 

 Vitreous volcanic rocks, obsidian, pearl stone, and pumice, 

 are entirely wanting in some volcanoes; in others they 

 proceed from the crater itself, or at least from consi- 

 derable depths beneath it. These important and complicated 

 relations can only be investigated by very exact crystallo- 

 graphical and chemical examination. My Siberian tra- 



