Metamorphic changes of Rucks, ^-c. 115 



signaled severally according to the notions entertained of their 

 mode of formation, also present themselves in the systems of 

 elevation ; not indeed as having contributed with the preceding 

 rocks to produce them, for they cannot be ascribed to this cause, 

 but, if we may so speak, as having availed themselves of the 

 great fissures which were produced by the eruption of these 

 rocks in the crust of the earth, and so insinuating themselves 

 into these numerous clefts, they have there formed metallife- 

 rous veins, which assist in the most striking manner, by the 

 difference of their nature and direction, in determining the dis- 

 tinctness of the several epochs, and the independence of the va- 

 rious elevations. For, in the first elevations, the axis of which 

 runs from north-east to south-west, we find veins of cupreous 

 minerals running in this direction. In the second system, which 

 runs nearly from north-west to south-east, it is chiefly veins of 

 quartz, of barytes, of fluor-spar, and galena, which are found 

 parallel to this direction. It is then apparent, as stated by M. 

 Fournet, that there are the most striking relations on the one 

 hand between the systems of elevation, and the nature of the 

 rocks which compose the uplifted strata, and especially the up- 

 raising rocks themselves ; and on the other, between the direc- 

 tions of the fissures into which the several minerals, crystalline, 

 earthy, and metallic, are as it were injected or secreted, and 

 finally in the very nature of these minerals themselves. 



Another consequence which M. Fournet deduces from his 

 researches, and which appears to us exceedingly striking, should 

 future observations and a more minute examination of the rocks 

 confirm the view, is the connection he imagines exists between 

 the silica, which is the negative element in these rocks, the dif- 

 ferent degrees of their fusibility, and the epoch of their appear- 

 ance. Thus, according to him, the rocks which have first ap- 

 peared in upraising and dislocating the soil, are the common 

 granites ; rocks which, being the most infusible on account of 

 the silica, the negative element which they contain in excess, 

 required for their melting the most direct application to the 

 source of heat. To these rocks succeed, and in the exact or- 

 der of the diminution of their silica, and the augmentation of 

 their fusibility, the porphyritic granites or granites contain- 

 ing large crystals of felspar, and tjuartzose porphyries, then 



H 2 



