RELATION OF INDIVIDUAL CRYSTALS 



147 



III. When- the t winning axis is normal to a dome, as is well 

 illustrated l>y the cross-shaped twins of staurolite, Fig. 297 6, in 

 which the twinning axis is normal to the dome (032). 



a b 



FIG. 297. Staurolite Twins from Georgia. 



In the didigonal equatorial type all faces may be twinning faces, 

 except the three pinacoids, the normals to which are the axes of 



FI.I. -J'.ts. --Twins of 

 Aragonite, Twin- 

 ning Plane lid. 



Fiu. 2!)9. Triplets of Arunonite from 

 Bastanes, France. 



symmetry. In the sphenoidal and digonal holoaxial types, these 

 normals are st ill digonal axes of symmetry, and only the above three 

 types are possible. In the digonal polar type supplementary 



