The Determination- of Feldspars. — Spiirr. 381 
the lower half respectively of the vertical line which is the pro- 
jection of the zone of symmetry. In ordinary sections, it is 
impossible to determine which reading is positive and which 
negative, but the two readings are conjugated with respect to 
the center of the projections — that is, appear at equal dis- 
tances from the center. Therefore the projection is sought 
out in which the two readings have the necessary conjugation 
— in the above case the figures point to labradorite Ab., An^. 
If a section is not cut exactly in the zone of symmetry, the 
angles on each side of the albite twinning line will vary, and 
their values will be at equal distances to the right and left of 
the vertical line, the distance varying with the deviation from 
the zone of symmetry. 
Foiique Method — Find a bisectrix in convergent light and 
determine its optical character. To do this first bring the 
section into the position of extinction, when a cross is 
formed. Next, rotate the stage 45°, noticing the direction in 
which the hyperbolae move out of the field, and bring the line 
joining the apices of the curves into the 45° position in diag- 
onally opposite quadrants of the field. In this position deter- 
mine by means of the quartz wedge, teinte sensible, or gypsum 
plate, whether this line is of greater or less elasticity than the 
bisectrix perpendicular to which the section is cut. The quartz 
wedge is ordinarily the most convenient means, and in using 
it remember that when like axes are superposed the colors of 
the section rise, and that when they are crossed the colors fall. 
If the line joining the hyperbolae, which lies in the plane of the 
optic axes, is the direction of greatest elasticity in the section 
(«) then the section is cut perpendicular to a positive bisec- 
trix. If this direction is that of least elasticity (c) the bisec- 
trix is negative. Now bring the trace of the optic axes into 
position parallel with one of the cross-hairs and measure the 
angles by the method stated below. It is often convenient to 
remember that the positive bisectrix always emerges upon the 
brachypinacoid. If the bisectrix is positive, measure the angle 
between it and the trace of the basal plane. This may be re- 
cognized in the section as a crystal boundary or as a cleavage. 
If these are not present the angle may be measured against 
the pericline twinning planes, but in this case it is necessary 
to remember that this twinning plane is usuallx' not exactly 
