434 
Proceedings of the Royal Society of Edinburgh. [Sess. 
(§ 4, xiii. (3)), also yield different twin crystals with different modes of 
twinning. With plane twinning the twin plane is co-directional, while 
with line and point twinning it is contra-directional ; the twin axis is 
always co-directional. 
1 Uh (in II Un, II Bn, II Uh, IV Uh, IV Bk, IV Uu, IV Bn, IV Uh, 
III UK, VI Bk, VI Un, VI Bn, VI Uh, G Un, G Bn, G Uh ): !UhP,lUhL, 
3UhL 3UhI 
and 1 Uhl. Examples occur in the twins of quartz enumerated under 
SUh and those of nepheline described on p. 445. 
3 Uh (in III Uh, GUh): SUhP, the principal axis of quartz in the 
laevo-dextrogyral (3 twin of quartz ; 3 UhL, the principal axis of quartz in 
the dextrogyral (or lmvogyral) twin of quartz; SUhl, the principal axis in 
the “ Brazilian ” twin of quartz (for a discussion of these twins of quartz, 
see Lewis, Crystallography , 1899, pp. 519-523). 
