ROCKS. 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. The arrangement of the par- 

 ticles in layers parallel with the margins of the veins, the regu- 

 lar recurrence of analogous layers on the opposite sides of the 

 veins (on their different walls), and, finally, the elongated cel- 

 lular cavities in the middle, frequently afford direct evidence 

 of the Plutonic process of sublimation in metalliferous veins. 

 As the traversing rocks must be of more recent origin than 

 the traversed, we learn from the relations of stratification ex- 

 isting between the porphyry and the argentiferous ores in the 

 Saxon mines (the richest and most important in Germany), 

 that these formations are at any rate more recent than the 

 vegetable remains found in carboniferous strata and in the red 

 sandstone.* 



All the facts connected with our geological hypotheses on 

 the formation of the earth's crust and the metamorphism of 

 rocks have been unexpectedly elucidated by the ingenious 

 idea which led to a comparison of the slags or scoriae of our 

 smelting furnaces with natural minerals, and to the attempt 

 of reproducing the latter from their elements.! In all these 

 operations, the same affinities manifest themselves which de- 

 termine chemical combinations both in our laboratories and 

 in the interior of the earth. The most considerable part of 



Lussac, in the Annates de Chimie, t. xxii., p. 41.5, and Mitscherlich, in 

 Poggeud., Annalen, bd. xv., s. 630. Moreover, crystals of olivine have 

 been formed (probably by sublimation) in the cavities of the obsidian 

 of Cerro del Jacal, whicla I brought from Mexico (Gustav Rose, in 

 Poggend., Annalen, bd. x., s. 323). Hence olivine occurs in basalt, 

 lava, obsidian, artificial scoria?, in meteoric stones, in the syenite of Elf- 

 dale, and (as hyalosiderite) in the wacke of the Kaiserstahl. 



* Constantin von Beust, Ueber die Porphyrgebilde, 1835, s. 89-96 ; 

 also his Beleuchtung der Werner^ schen Gangtheorie, 1840, s. 6 ; and C 

 von Wissenbach, Ahhildungen merkwurdiger Gangverhdltnisse, 1836, fig. 

 12. The ribbon-like structure of the veins is, how^ever, no more to bo 

 regarded of general occurrence than the periodic order of the differen* 

 members of these masses. 



t Mitscherlich. Ueber die kunstliche DarsteUnng der Mineralien, m 

 the Abhandl. der Akademie der Wiss. zu Berlin, 1822-3, s. 2.5-41 



