ROCKS. 201 



are thus formed beneath our eyes by volcan.c forces, whera 

 the contiguous rock has akeady attained a certain degree of 

 sohdification, show us how, in a similar manner, mineral and 

 metallic veins may have been every where formed in the more 

 a ncient periods of the world, where the solid but thimier 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 mo^allic 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 aftbrd direct evidence 

 cf 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. t In all these 

 operations, the same affinities manife-st themselves which de- 

 termine chemical combinations both in our laboratories and 

 in the interior of the earth. The mo.st considerable part of 



Lussac, iu the Annales de Ckimie, t. xxii., p. 415, and Mitscherlicb, iu 

 Poggend., 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, wliich I brought from Mexico (Gustav Rose, iu 

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

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

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



* Constantiu von Beust, Ueber die Porphyrgebilde, 1835, s. 89-9G ; 

 also his Beleuchtiing der Werner' schen GangtheoHe, 18-10, s. 6 ; and C. 

 vouWissenbach, Abbildungen merkwurdiger Gangverhdltnisse, 1836, fig. 

 12. The ribbon-like structure of the veins is, however, no more to ba 

 regai'ded of general occurrence than the periodic order of the differeu^ 

 members of these masses. 



t ?»Iitscherlich, Ueber die kunstUche Darstellung der Mineralien, io 

 the Abhandl. der Akadcmie der Wiss. zu Berlin, 18^-3, s. 25-41 



