Detcrnii nation of the Feldspars. — Wincliell. 41 
circles in black which show the planes of principal elasticity. 
The fine dotted lines unite poles of the same extinction angle. 
The curves composed of crosses separate the negative from 
the positive extinctions, and coincide with the curves of 0° 
extinction. 
The various planes of an inclined zonal axis contained in 
the face 010 would find their poles arranged on an arc of a 
great circle which would cross the plate along a meridian ex- 
tending between the poles 010. The numbers that are seen 
along this meridian denote the extinction angles for the dif- 
ferent planes of such zone. The greater the inclination of the 
zonal axis the greater the separation of its meridian from the 
horizontal diameter of the epure. 
These epures represent the projection and the optical char- 
acters of simple crystals or lamellae in the conventional posi- 
tion. In the case of polysynthetic albite twinning the extinc- 
tion of every alternate lamella is of a negative sign, if read ac- 
cording to Schuster's rule, but such negative reading would 
not be that expressed on these epures. This change of sign is 
owing to the rotation of the albite made 180° about an axis 
perdendicular to 010. Again in the case of the Carlsbad twin- 
ning in which there is a rotation of 180° about a line parallel 
to the vertical crystallographic axis, it is evident that two in- 
dividuals have contrary signs for the same reason. In the 
case of both Carlsbad and albite twinning in the same com- 
pound individual, only one albite lamella and its homologues 
occupy the conventional position, although several may extin- 
guish at the same point. 
The zone normal to the edge 100:010 situated in 010 has 
all its poles in the horizontal diameter of the epures already 
referred to. The maximum extinction is always in the vicin- 
ity of 010, and it increases regularly from a .point near the 
edge 100:010, as below. This zone is very characteristic for 
the acid feldspars if the study is not carried to sections beyond 
an inclination of 50° with the section perpendicular to the 
prism. It afifords a distinction between albite and andesine, 
and also separates the oligoclases. 
It appears, therefore, that the increments are continuous 
from albite to anorthite, and this zone would be most charac- 
