Dctcnnination of tJic Feldspars. — JVinchc/l. 
31 
to each other alternately outward and inward, a thin section 
cutting these laniallje at right angles to 010 would cut thein 
also at right angles, and the extinction angles on opposite 
sides of any twinning line would be equal, but of contrary 
signs since, according to the law of albite twinning, one 
lamalla is turned 180° from the conventional position about a 
line perpendicular to 010. In case the section be not cut in 
a plane perpendicular to 010 the extinction on one side of the 
twinning line is greater than on the other. The method con- 
sists in finding in some feldspar grain the maximum equal ex- 
tinctions on opposite sides of an albite twinning line. All 
sections that have equal extinctions on adjacent sides of a 
twinning line in the same species are cut in the zone perpen- 
dicular to the faces 010; but only one of these affords the 
maximum equal extinction. The position of the plane which 
affords the maximum extinctions in the zone perpendicular 
to 010, is different for the different species, and the maxima 
also differ for the different species. When this maximum has 
been found it serves for the index to the species according to 
the following tabulation. The table, drawn principally from 
Maxi- 
Compo- 
mum 
Position of the Plane with respect 
sition. 
Extinc- 
tion. 
to the bisectrices. 
Orthoclase .... 
Or. 
0'' 
Monoclinic. 
Microcline 
Or. 
-fi9« 
Inclined (2=;'' N. and 2^° K.). 
Anorthoclase . . 
Ab^Or. 
Albite 
Ab 
- i6« 
Perpendicular to ;/p in the ol)tuse 
angle. 
Oligcjclase 
AbaAn, 
+4" 
Perpendicular to Wp in the acute 
angle. 
Dligoclase 
Ab.An, 
+ ^4" 
Inclined 30'^ on Wm and 2q'^ from 
001, downward toward the front, 
i. e. S. 
Inclined 4" E. and 4° N. from Wp 
Andesine 
Ab^Ans 
+ 16° 
in the acute angle. 
Labradorite . . . 
Ab.Au, 
+27- 
Inclined 10'' E. and 15" N. from 
Labrador- 
;7p in the acute angle. 
Bytownite . . . 
AbaAn^ 
+38° 
Perijendicular to 201, i. e. inclined 
18° E and 22" N. from ;/pin the 
acute angle. 
Anorthite 
An 
+ 52 '^« 
Inclined 180'' N. from the optic 
axis B, and 30° E. from »p in 
the acute angle. 
