32 Th^ American Geologist. Janimry, isss 
the epures of Michel-Levy,* also expresses the position of the 
plane with respect to the bisectrices. The maximum extinc- 
tion angles here given are found, for each feldspar, on the ver- 
tical diameters in the plates I — VII. 
In the application of this method it is not necessary to ex- 
amine all sections of feldspars at random, but by certain guides 
those in the zonal position can be selected, (i) It is the zone 
of symmetry of the albite twinning, and the alternate lamellae 
extinguish at the same angle. If account be taken of the 
optical sign of the direction of extinction (-|- to the right of 
the twinning line and — to the left) the positions of other 
planes, inclined to this zone may be identified by reference to 
the epures of Michel- Levy (plates I — VII). f (2) The sections 
of this zone, being perpendicular to the face of association of 
both the albite and the Carlsbad twinning, the albite twinning' 
lines ought to be extremely fine and straight. Further the 
feldspathic microlites, however small, cross the thin section 
perpendicularly. They seem to be elongated parallel to these 
lines; their outlines are clear and their colors of double refrac- 
tion are those that comport with the total thickness of the 
prac[ue for the orientation in each case. (3) Sections that are 
perpendicular to 010 have not only equal extinction angles, 
but they may be identified by the fact that the two lamellae 
on opposite sides of the twinning line have, on rotation be- 
tween crossed nicols eight positions of equal luminosity, viz: 
four at 45° from the spider lines, one in each quadrant and 
four at the points of agreement with the spider lines. In these 
positions the lamellae appear to belong to the same crystal, 
being separated only by a very fine dark line. This test is ex- 
tremely delicate and with the least obliquity to the axis of 
the zone the equal luminosity does not appear. 
This method, notwithstanding its tediousness, is one of 
the most serviceable as well as the most reliable, owing to the 
readiness with which sections perpendicular to the plane 010 
can be recognized, and to the characteristic differences in the 
maxima of the various feldspars. The chief obstacles that 
interfere with its use are (i) the possible existence of two or 
^Determination des feldspaths dans les plaques minces, first fascicule, 
1894. 
t For explanation of these plates see p. 40. 
