The American Geologist. 
January. 1898 
more varied. It is represented in figure 24 as projected on a 
plane perpendicular to the edge 001 : 010, the bisectrix Wp , 
being perpendicular or at least less inclined to the surface of 
the projection than n^ , and n^ lying approximately in the 
paper. From this it appears that from albite to anorthite there 
is a gradual rotation of the optic plane in a direction opposite 
to the movement of the hands of a watch about a line parallel 
(or nearly parallel) to Wp, and that the whole movement 
amounts to somewhat less than three-fourths of an entire revo- 
lution. 
pir.toOfO 
Fig. 24. — Projection of the principal indices of the triclinic feldspars upon a 
plane perpendicular to 001 : 010. 1, Albito : 2, Oligoclase ; 3, Andesine 
4, Labradorite ; 5, Anorthite. 
The optic plane is represented in figure 25 as projected on 
the face 010. From this it appears that its projection rotates 
in a similar manner, from this point of view, about a line 
nearly parallel to %. The axis of least elasticity (wg) of all 
the plagioclases is situated nearly in the plane perpendicular to 
the edge 001 : 010, while the axis of greatest elasticity, ?ip , is 
nearly parallel to that edge, and in the plane of symmetry.* 
The value of the acute optic angle (2 V), in the various 
feldspars, and their optical signs, are shown in the following 
table: 
*There is no zone of symmetry in the triclinic feldspars; but for con- 
venience of reference the zone perpendicular to the edge 001:010 is 
jailed the zone of symmetry in the discussion of the plagioclases. 
