NOME VIII. D. GNEISS. 



247 



" Rem. This is extremely rare, as the felspar is 

 by far the most subject to decay." 



NOME VIII. D. GNEISS. 



In this substance, as in granite, the felspar 

 and the mica are chiefly affected. Karsten gives 

 irhe following examples : 



r 



HYPONOME I. 



," 95. Coarse fibrous gneiss, with slightly de- 

 composed felspar, but further decomposed mica ; 

 from Swisserland. 



" 96. Gneiss with entirely decomposed felspar; 

 from the Isaac, near Freyberg. 



" 97. Gneiss entirely decomposed, which is 

 scarcely any longer distinguishable, except where 

 the quartz still retains its appropriate structure ; 

 with an adhering compound of brown blende, 

 martial pyrites, and some galena; from Frey- 

 berg." 



The last is properly a vein-stone ; and rocks 

 are generally decomposed when in contact with 

 metallic ores. 



