188 



symmetry should be orthorhombic, with a twinning chiefly occurring 

 with respect to a face of the prism {110} and of the pyramid {111}. 



Fig. 148. 

 Garnet,- section parallel to (110). 



Fig. 149. 

 Analcite; section parallel to (100). 



Fig. 150. 

 Fluorspar; section parallel to (100) and (111). 



Fig. 151. 



Perowskite; section parallel 

 to (100). 



Fig. 152. 

 Apophyllite; section parallel to (001). 



Fig- 153- 

 Rutile; section parallel to (001). 



In several of these cases, Brauns and others have tried to demon- 

 strate that the optical anomalies are caused by internal stresses, 



