118 M. Foiirnet's Opinions on the 



lization into mica. In support of this proposition, M. Fournet 

 adduces, 1st, The argillaceous slate of the mountain of Bel- Air, 

 above Tarare, which exhibits in the vicinity of porphyritic 

 masses many modifications of the rock in the form of bronze 

 mica, and fine chloritic mica, containing crystals of felspar : this 

 rock again becomes argillaceous slate when it is beyond the in- 

 fluence of the porphyries. 2d, A similar observation has been 

 made by M. Mitscherlich in the Eifel. 3d, The case in which 

 the argillaceous slate is modified into chlorite, when it has been, 

 as it were, plunged into the liquid, or into the paste which has 

 been crystallized into quartz ; the example which he quotes of 

 this singular change is taken from a gallery in the mine of Saint 

 Bel. " In the fourth case (that of the bridge of Gassie on the 

 road from Chessy to L'Etrat) the fragments of grey argillace- 

 ous slate which are found in contact with the quartz porphyries, 

 after having undergone various alterations, have been definitely 

 changed into beautiful dark green hornblende crystals. All 

 the schisto-crystalline rocks of the country are either hornblendic 

 or micaceous. The two preceding modifications give us the key 

 to their formation by a simple softening, and, adding to this the 

 subsequent facts regarding cementing, they easily explain to us 

 the most part of the other changes which the argillaceous schists 

 have subsequently undergone." These facts are so remarkable, 

 and their connection with the conclusions the author draws are 

 so bold, that we have thought it best expressly to quote from 

 the memoir. 



A fifth mode is that which the author denominates change 

 hij penetration and cementing. Thus he regards arkose, an ag. 

 gregate rock composed of quartz and felspar, as resulting from 

 a sandstone being penetrated by a euritic paste, that is to say, 

 a felspar one. 



The last and the greatest change which the author contends for, 

 is that which tiie argillaceous schist undergoes when subjected to 

 the influence of granites: this first and powerful rock of eruption, 

 bv producing mica with the argillaceous slate, by introducing 

 felspar between its laminae, transformed this slate into gneiss ; 

 and not upon a single point, or even a small space, but through- 

 out an entire mountain. This gneiss, then, according to the apt 

 expression of the autlipr, is nothing more than a felspathised 

 slate {schistejelspathise.). 



