156 Dr. F. M. Stapff — Sand-grains in Micaceous Gneiss. 



some other cases there has been an insignificant displacement, and 

 this is the case with the quartz-grain here figured ; but these 

 quartz-grains never present such a brilliant, polymorphous and 

 polychromatic variegated mosaic as the small particles of quartz 

 which enter into the constitution of the micaceous gneiss. Their 

 polarization colours are somewhat dull, and fade away con- 

 centrically from the interior outwards, just as in the case of quartz 

 crystals in porphyries. 



Tig. 3. — Quartz-grain in micaceous gneiss, No. 130 (north) St. Gothard 

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



The microlithic (and other) inclusions in the quartz-grains are 

 larger but neither so frequent nor so varied as those in the mag- 



