ARTIFICIAL PRODUCTION OP SIMPLE MINERALS. 257 



formed artificially are the most important simple minerals 

 which characterise very widely distributed eruptive plu- 

 tonic and volcanic rocks, as well as nietamorphic rocks 

 altered by them ; and these have been produced in a crys- 

 talline state,, and with complete identity. We must dis- 

 tinguish, however, between minerals formed accidentally in 

 the scoriae, and those produced by chemical operations 

 purposely devised : to the first class belong feldspar, mica, 

 augite, olivine, blende, crystallized oxide of iron (specular 

 iron), octahedral magnetic oxide of iron, and metallic tita- 

 nium ; ( 293 ) to the second, garnet, idocrase, ruby (as hard 

 as the oriental ruby), olivine, and augite. ( 294 ) These mi- 

 nerals form the principal constituents of granite, gneiss, and 

 mica slate, of basalt, dolerite, and many porphyries. The 

 artificial production of feldspar and mica is of singular 

 geological importance, in reference to the theory of the 

 nietamorphic conversion of argillaceous schist into quartz. 

 We have in the schist all the elements of granite, without 

 even excepting potash. ( 295 ) It would not be very sur- 

 prising, therefore, as our ingenious geologist von Deche;* 

 has justly remarked, if on some occasion we were to find a 

 fragment of gneiss formed on the inside wall of a furnace 

 built of argillaceous schist and greywacke. 



Having passed in review the three great classes of erupted, 

 sedimentary, and metamorphic rocks, we have still to notice 

 the fourth class, comprising conglomerates, or rocks formed 

 of detritus. The terms which we employ to designate this 

 class recal the revolutions which the crust of the earth has 

 undergone, as well as the cementing action which has con- 

 solidated, by means of the oxide of iron, or argillaceous or 

 calcareous pastes, the sometimes rounded, and soiretimea 



