[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. The arrangement of the particles in layers parallel with the margins of the beins, the regular recurrence of analogous layers on the opposite sides of the veins (on their different walls), and, finally, the elongated cellular 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 existing 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.*
[footnote] *Constantin von Veust, 'Ueber die Porphyrgebilde', 1835, s. 89-96; also his 'Belenchtung der Werner'schen Gangtheorie', 1840, s. 6; and C. von Wissenbach, 'Abbildungen merkwurdiger Gangverhaltnisse', 1836, fig. 12. The ribbon-like structure of the veins is, however, no more to be regarded of general occurrence than the periodic order of the different members of these masses.
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.*
[footnote] *Mitscherlich, 'Ueber die kunstliche Darstellung der Mineralien', in the 'Abhandl. der Akademie der Wiss. zu Berlin', 1822-3, s. 25-41.
In all these operations, the same affinities manifest themselves which determine chemical combinations both in our laboratories and in the interior of the earth. The most considerable part of p 268 the simple minerals which characterize the more generally diffused Plutonic and erupted rocks, as well as those on which they have exercised a metamorphic action, have been produced in a crystalline state, and with perfect identify, in artificial mineral products. We must, however, distinguish here between the scoriae accidentally formed, and those which have been designedly produced by chemists. To the former belong feldspar, mica, augite, olivine, hornblende, crystallized oxyd of iron, magnetic iron in octahedral crystals, and metallis titanium;* to the latter, garnets, idocrase, rubies (equal in hardness to those found in the East), olivine, and augite.**
[footnote] *In scoriae crystals of feldspar have been discovered by Heine in the refuse of a furnace for copper fusing, near Sangerhausen, and analyzed by Kersten (Poggend., 'Annalen', bd. xxxiii., s. 337); crystals of augite in scoriae at Sahle (Mitscherlich, in the 'Abhandl. der Akad. zu Berlin', 1822-23, s. 40); of oliving by Seifstrom (Leonhard, 'Basalt-Gebilde', bd. ii., s. 495); of mica in old scoriae of Schloss Garpenberg (Mitscherlich, in Leonhard, op. cit., s. 506); of magnetic iron in the scoriae of Chatillon sur Seine (Leonhard, s. 441); and of micaceous iron in potter's clay (Mitscherlich, in Leohnard, op. cit., s. 234). [See Ebelmer's papers in 'Ann. de Chimie et de Physique', 1847; also 'Report on the Crystalline Slags', by John Percy, M.D., F.R.S., and William Hallows Miller, M.A., 1847. Dr. Percy, in a communication with which he has kindly favored me, says that the minerals which he has found artificially produced and proved by analysis are Humboldtilite, gehlenite, olivine, and magnetic oxyd of iron, in octahedral crystals. He suggests that the circumstance of the production of gehlenite at a high temperature in an iron furnace may possibly be made available by geologists in explaining the formation of the rocks in which the natural mineral occurs, as in Fassathal in the Tyrol.] — Tr.
[footnote] **Of minerals purposely produced, we may mention idocrase and garnet (Mitscherlich, in Poggend., 'Annalen der Physik', bd. xxxii., s. 340); ruby (Gaudin, in the 'Comptes Rendus de l'Academie de Science', t. iv., Part i., p. 999); olivine and augite (Mitscherlich and Berthier, in the 'Annales de Chimie et de Physique', t. xxiv., p. 376). Notwithstanding the greatest possible similarity in crystalline form, and perfect identity in chemical composition, existing, according to Gustav Rose, between augite and hornblende, hornblende has never been found accompanying augite in scoriae, nor have chemists ever succeeded in artificially producing either hornblende or feldspar (Mitscherlich in Poggend., 'Annalen', bd. xxxiii., s. 340, and Rose, 'Reise nach dem Ural', bd. ii., s. 358 und 363). See also, Beaudant, in the 'Mem. de l'Acad. des Sciences', t. viii., p. 221, and Becquerel's ingenious experiments in his 'Trait de l'Electricite,' t. i., p. 334; t. iii., p. 218; and t. v., p. 148 and 185.
These minerals constitute the main constituents of granite, gneiss, and mica schist, of basalt, dolerite, and many porphyries. The artificial production of feldspar and mica is of most especial geognostic importance with reference to the theory of the formation of gneiss by the metamorphic agency of argillaceous schist, which contains all the constituents of granite, p 269 potash not excepted.*
[footnote] *D'Aubuisson, in the 'Journal de Physique', t. lxviii., p. 128.