432 Proceedings of the Royal Society of Edinburgh. [Sess. 
1 UuP, twin axis of (1) twin of chalcopyrite on 101 (Fletcher, Phil. Mag., 
vol. xiv., 1882, p. 276), (2) twins of right and left quartz symmetrical to a 
twin plane oblique to the principal axis, e.g. 125, or the plane at right 
angles to [210]; lUuL, twin axis of twins of right (or left) quartz with 
oblique twin plane; 1 Uni, all lines in the “Brazilian” twin of quartz 
except lines of symmetry and lines at right angles to them (§ 10, ii.). 
SUu (in III Uu, GUu): 3 UuP] 3UuL ] 3 Uul, normals to the tetra- 
hedron faces in the interpenetrant twins of sodium chlorate ( G Uu). 
lBu (in II Uk, II Bu, I V Bk, I V Bn, III Bu, VI Uk, VI Bk, VI Bn, G Bu ) : 
1 BuP, the normal to a face of a trigonal prism of the first order in the twin 
of proustite described under 3 BuL (1) (see § 16, ii.); 1 BuL, twin axes of (1) 
twin of chalcopyrite on 111 ; (2) twin of pyrargyrite on rhombohedron 
face ; (3) twin of pyrargyrite on a face of a trigonal prism of the 
first order, described by Haidinger (. Edinb . Journ. Sci., vol. i., 1824, 
