[footnote] *Hoffman, 'Reise', s. 30 und 37.
We still daily find the same iron glance formed by sublimation from the
vapors and the walls of the fissures of open veins on the margin of the
crater, and in the fresh lava currents of the volcanoes of Stromboli,
Vesuvius, and AEtna.*
[footnote] *On the chemical process in the formation of specular iron, see
Gay Lussac, in the 'Annales de Chimie', t. xxii., p. 415, and Mitscherlich,
in Poggend., 'Annalen', bd. xv., s. 630. Moreover, crystals of olivine have
been formed (probaby by sublimation) in the cavities of the obsidian of
Cerro del Jacal, which I brought from Mexico (Gustav Rose, in Poggend.,
'Annalen', bd. x., s. 323). Hence olivine occurs in basalt, lava, obsidian,
artificial scoriae in meteoric stones, in the syenite of Elfdale, and (as
hyalosiderite) in the wacke of the Kaiserstuhl.
The veins that
p 267
are thus formed beneath our eyes by volcanic forces, where the contiguous
rock has already attained a certain degree of solidification, show us how,
in a similar manner, mineral and metallic veins may have been every where
formed in the more ancient periods of the world, where the solid but thinner
crust of our planet, shaken by earthquakes, and rent and fissured by the
change of volume to which it was subjected in cooling, may have presented
many communications with the interior, and many passages for the escape of
vapors impregnated with earthy and metallic substances.
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