Determination of the Feldspars. — IVinrhe/l. 
19 
Fig. 12.— Quadrillage of Microcline, 
face 010. In microcline this plane, while nearly perpendicular 
to 001, and quite perpendicular to 010, has a trace on 010 
which forms an angle of — 80° to + ioo° with the edge 
Fig. 13. Fig. 14. 
Albite Twins of Labradorite. 
ooi : GIG (fig. i6). Its position is between the faces lOO and 
20I. The cross-hatching of microcline is represented by fig. 
12. That of anorthoclase is less distinct, being extremely fine 
and badly defined. 
01 
>^ 
^ 
Old 
110 
110 
A- 


110 
110 
-k^ 
010 
^-< 
'" 
y- 
2 01 
sJ 
Fig. 15, — Pericline Twin? of Albite. 
With the plagioclases, properly, so called, the albite and 
periclinic types of twinning play an important role (figs. 13, 
14, 15). They are adopted as characteristic and permanent 
standards from which are measured other optic phenomena. 
