437 
1911-12.] The Geometry of Twin Crystals. 
4<Bk (in IV Bk, CBu ): 4B kPI, (a) the principal axis of chalcopyrite in 
twins on the basal plane ; ( b ) the crystallographic axes in the interpenetrant 
twins of eulytine and the diamond. 
4Bk 4BkPI 
2 Uu (in II Uu, III Uh ) : 2 UuPI, the lines of symmetry in the 
Kt Brazilian ” twins of quartz. 
4 Uu (in 1 V Uu) : 4 UuPI . 
6Uu (in VI Uu) : 6 UuPI. The principal axis of the twins of nepheline 
referred to in § 19, vi. (see figures QUu, QUuPI, p. 444). 
2 Bu (in II Bu, VI Bk) : 2 BuPI, vertical axis in the “ symmetric” twins of 
hemimorphite on the basal plane. 
2 BtuPI 
4 Bu (in IV Bu) : 4 BuPI. 
6Bu (in VI Bu) : 6 BuPI. 
2Uh (in II Uh, IV Bk, IV Uh, VI Uh, G Uu, C Uh): 2UhPI, the crystallo- 
graphic axes of the interpenetrant twins of sodium chlorate. 
4Z7A (in IV Uh, C Uh) : 4 UhPI. 
6 Uh (in VI Uh): GUhPI. 
