Determination of the Feldspars. — WineJiell. 
39 
(I) 
(I') 
(2) 
(2') 
Notes. 
Albite 
OHgochise (Ab4Aii,/ . . 
Oligoclase (AbsAn,). • 
Andesine 
Labradorite (AbiAm). 
Anorthite 
A+62° 
A go^ 
A— 82° 
A— 71° 
A-49« 
A— 20° 
+37° 
0«(2) 
— 40« (2) 
-48° (2) 
-40" 
-53° 
+30° 
+25" 
+22^ 
+ 15° 
+ I0« 
-25° 
+28°.(I) 
-25° 
+22° 
+35° 
+27° 
+15° 
(i) Near the 
axis B. 
(2) Near the 
axis A. 
For the optic axis (B) the following table gives the same 
elements. It is constructed in part from the numbers of the 
general epures of Michel- Levy: 
Remarks. 
Albite 
Oligoclase (Ab4An, ).. 
Oligoclase (AbsAn,)-. 
Andesine 
Labradorite (Ab,An,). 
Anorthite 
(I) 
(I') 
(2) 
(2') 
B+63« 
+40° 
+45° 
-32° 
B 90° 
90^(1) 
-35- 
-25° 
B— 82° 
—42° (I) 
-17- 
— 16° 
B— 69^ 
-31° (I) 
— 6" 
0^ 
B-57° 
-23° 
+ 3" 
+ 14° 
B— 62« 
— 12° (2) 
+47° 
-56° 
(i) Near the 
axis A. 
(2) Near the 
axis B. 
(/) The Use of Zonal Sections. 
Michel-Levy has investigated the principal crystallo- 
graphic zones of the feldspars, and has deduced the charac- 
teristic extinction angles in each. These zones are the fol- 
lowing: 
1. Zone perpendicular to 010. 
2. Zone normal to the edge 100:010 situated in 010. 
3. Zone parallel to 100:010. 
4. Zone parallel to 001:010. 
The examination of the zone perpendicular to 010 is al- 
ready described under the Statistical Method, (p. 30). With 
this as a basis Michel-Levy has constructed a series of very 
ingenious and complicated tables, represented graphically in 
the plates (epures) for the various plagioclases (plates I, II, 
III, IV V, VI and VII). These circular plates show not only 
the maximum equal -extinctions of the simple crystal in each 
plagioclase, when cut in a plane perpendicular to 010, but 
also the extinctions of the same when cut in any zone whose 
axis is in 010. The former is shown by the highest number 
seen along the vertical diameter of each plate, and the latter 
