158 Dr, F. M. Stajiff — Sand-grains in Micaceous Gneiss. 



sand-grains and gravel encrusting the corrugated surface of a pebble. 

 The cracks and fissures are sometimes enlarged at the surface of the 

 bay, which allows the surrounding material to invade the grain. 



The felspar-grains, of which one is reproduced on Fig. 4, are in 

 part poly synthetic tivitis, in part single or twofold individuals. It is 

 not easy to determine particular felspar species in view of the 



Fig. 



4. — Felspar-grain in micaceous gneiss, No. 130 (north) St. Gothard 

 Tunnel. Enlarged 16-17 times. Photographed in polarized light. 



extent to which they are decomposed ; but even with the naked eye 

 one can observe that some of the felspar-grains are greenish, dull, 

 lustreless, striped and dusty (kaolin) ; whilst others are white or 

 limpid, fresh and glossy. Commonly they present the outlines of 



