Dctcnm/iatiofi of the Feldspars. — IVi/ichcll. 
17 
Fig. 7a. Fig. 8a. 
Albite-Twinned Feldspathic Microlites. 
produces a fine superficial striation visible to the naked eve; 
it is caused by a succession of changes in direction of growth 
of the crystal, each layer being turned from its fellow preced- 
ing by an angle of about 172°. When the twinning axis is a 
normal to 010, this twinning forms the albite type. When it is 
parallel to b it forms the pericline type. In the albite type the 
lamellae are parallel to 010 and produce striations on the sides 
001 and 100. They are not visible in thin sections parallel 
to GIG, but in all others they are apparent in narrow bands 
which polarize and extinguish alternately, on being rotated 
between crossed nicols, the colored bands being parallel to 
the pinacoidal cleavage. The external pericline striations are 
visible on all faces of the crystal. If striations ai)pear on the face 
GIG, they are necessarily of the pericline type. In thin sections, 
if the pericline twinning exists, it is visible in sections cut in 
all directions except parallel to the composition face, and in 
andesine this face is practically parallel to the base goi. In 
the other plagioclases it is in the same zone, but makes an 
angle with the base (fig. i6.) 
Figures 7a and 8a represent each a pair of microlitic twins 
of the albite type, the former having an elongation parallel to 
the axis a and the latter a flattening parallel to gig. 
Figure 9 represents a triclinic feldspar included between 
the principal crystal faces ogi, igg and gig. The lines whicli 
cross each other on the faces ogi and 100 indicate the external 
striations due to the albite and pericline types of polysynthetic 
twinning; those that appear on the face gig represent the ex- 
ternal .striations due to pericline twinning. In the various 
plagioclases the latter make different angles with the basal 
