THR 



EXPERIMENTAL FORMATION OF IGNEOUS R0(. 



3. . 



and as closely as rhyolites and granites, though the number of analyses 

 of sedimentary and foliated rocks hitherto made is too few to demon- 

 strate correspondence with all the varieties of igneous rocks which 

 might be selected. This deficiency may be remedied hereafter. But 

 if the views enunciated are sufficiently supported by facts, further 

 details will become available, and repeat the general principles which 

 we have attempted to exemplify. 



It remains to be shown that rocks and minerals can be produced 

 artificially. 



Experimental Formation of Volcanic Rocks. Messieurs Fouque" 

 and Levy by a series of ingenious experiments have succeeded in 

 forming volcanic rocks artificially, not indeed by transforming sedi- 

 mentary deposits, but by heating together constituent minerals of the 

 rocks. Thus andesite and andesite porphyry are obtained by com- 

 bining four parts of oligoclase with one part of augite, and heating 

 them together for three days in a platinum crucible half embedded 

 in the furnace. In this process oxide of iron may be produced at the 

 expense of the augite. When a little lime is added microliths of 

 labradorite are formed, and augite forms more abundantly round these 

 than round the oligoclase. In a similar way augite-andesite has been 

 obtained by mixing ten parts of oligoclase and one part of amphibole, 

 when the amphibole is changed into pyroxene. Basalts have been 

 made artificially by a double heating of a black glass, having a composi- 

 tion which corresponds to 6 of olivine, 2 of augite, and 6 of labradorite. 

 This is raised to a white heat, maintained for forty-eight hours, above 

 the melting point of pyroxene and labradorite, when peridote is formed 

 in a brown glass. Subsequently the mass is exposed for forty-eight 

 hours to a red heat, when microliths of labradorite, augite, iron oxide, 

 and picotite are formed. 



By combining nepheline and augite in the proportion of 3 to i "3, 

 nephelinite results after two days' heating ; but when the augite is in 

 the proportion of i to 10, it gives rise to octohedrons of spinelle and 

 dodecahedrons of melanite garnet. In the same way 9 parts of leucite 

 are combined with i part of augite and heated for three days, when 

 crystals of leucite were found to be surrounded by augite and oxide 

 of iron. In this production of leucitite double fusion is necessary, in 

 consequence of the different fusibility of leucite and pyroxene. The 

 formation of Iherzolite is more difficult and less complete. And from 

 negative results, Fouque and Le"vy infer that the rocks containing 

 quartz, orthoclase, black mica, and amphibole, have not been formed by 

 the agency of heat alone. For the methods of artificial formation of 

 the minerals which enter into the composition of igneous rocks, we 

 must refer to " Synthese des Mine"raux et des Roches " by these 

 authors. 1 



1 The three preceding chapters are the matter of Royal Institution lectures, 

 delivered March 15, 25, and April i, 1882. 



