270 COSMOS. 



(equal in hardness to those found in the East), olivine, and 

 augite.* These minerals constitute the main constituents of 

 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 consti- 

 tuents of granite, potash not excepted.f It would not be 

 very surprising, therefore, as is well observed by the distin- 

 guished geognosist, von Dechen, if we were to meet with a 

 fragment of gneiss formed on the walls of a smelting furnace, 

 which was built of argillaceous slate and graywacke. 



After having taken this general view of the three classes of 

 erupted, sedimentary, and metamorphic rocks of the earth's 

 crust, it still remains for us to consider the fourth class, com- 

 prising conglomerates, or rocks of detritus. The very term 

 recalls the destruction which the earth's crust has suifered, 

 and likewise, perhaps, reminds us of the process of cementa- 

 tion, which has connected together, by means of oxide of iron, 

 or of some argillaceous and calcareous substances, the some- 

 times rounded and sometimes angular portions of fragments. 



Hallows Miller, M.A., 1847. Dr. Percy, in a communication with which 

 he has kindly favoured me, says, that the minerals which he has found 

 artificially produced and proved by analysis, are humboldtilite, gehlen- 

 ite, olivine, and magnetic oxide 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. 



* Of minerals purposely produced, we may mention idocrase and 

 garnet (Mitscherlich, in Poggend. Annalen der PhysiTc, bd. xxxii. 

 s. 340) ; ruby (Gaudin, in the Comptes rendus de I'Academie de Science, 

 t. iv. pt. i. p. 999) ; olivine and augite (Mitscherlich and Berthier, in 

 the Annales de Chimie et de Physique, t. xxiv. p. 376). Notwith- 

 standing 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. a. 340, and Rose, Reise nach dem Ural, 

 bd. ii. s. 358 und 363). See also Beudant in the Mem. de I'Acad. des 

 Sciences, t. viii. p. 221, and Becquerel's ingenious experiments in his 

 TraitS de V Electricite, t. i. p. 334, t. iii. p. 218, and t. v. pp. 148 and 185. 



t D'Aubuisson, in the Journal de Physique, t Ixviii. p. 128. 



