NOMB XX. GNEISS, WITH BLUE SIDERITE. 



may be as ancient as the massive granite -, that 

 substance sometimes rising into natural walls, as 

 in Cornwall : or, in the great antiquity of the 

 earth, the veins may have been formed in a 

 softer granitic substance (more compact veins 

 and nodules being observable on a small scale), 

 which afterwards wasted away, and its place 

 was supplied by the clay-slate. 



HYPONOME i. 

 Granite in slate. 



Micronome 1. Slate in granite. 



NOME XX. GNEISS, WITH BLUE SIDE- 

 RITE. 



Near Breuil, Saussure observed a gneiss full 

 of garnets, the surface being incrustated with 

 little crystals of a beautiful steel blue, oblong, 

 irregular, opake, very brilliant, striated in the 

 longest direction, frequently porous in that di- 

 rection, and with difficulty scratched by a knife 

 when the streak is grey. The fracture laminar, 

 equally blue and brilliant; and they are easily 

 fusible under the blow-pipe into a shining black 

 amel, attractable by the magnet, although the 



