MODE XI. GNEISS. 



porphyries of Saussure, $ 150, belong to this 

 class ; but granitic porphyroids are so abundant 

 in all primitive mountains, that it is scarcely- 

 necessary to select examples, being a mere va- 

 riation in the construction of granite or granitin. 



Granitic porphyroid, from Mount Cenis. 



The same, from Cornwall, Wales, Scotland, 

 the Vosges mountains in France, the Alps, 

 &c. &c. 



Saussure, 155, gives some curious observa- 

 tions on the transitions from granite to granitic 

 porphyry. A great portion of Forez is of por- 

 phyry; while the adjacent portion of Auvergne 

 is granitic. 



MODE XI. GNEISS. 



When the materials of granite are disposed in Distinctions, 

 thin layers, or plates, the substance assumes the 

 name of Gneiss ; which consequently consists of 

 quartz, felspar, and mica. In his two first vo- 

 lumes, published before Werner had introduced 

 greater precision into the science, Saussure has 

 sometimes used the term veined granite, to ex- 

 press what is now denominated gneiss. But in 

 his latter volumes, as already explained, his 

 reined granite differs from gneiss, as presenting 



