30 
Tlie AincriciDi Geologist. 
January, 189S 
anorthite shows the black bar associated with )h^. Compare 
also the page following — "Means of discovering sections par- 
allel to oio." 
When the examination has to do with the microlitic feld- 
spars of the second consolidation, it is usually impossible to 
obtain cleavage fragments for the foregoing process. It is 
then necessary to search for favorable sections cut at random 
in a thin section of the rock, when the same distinctions are to 
be observed, or resort may be had to the methods mentioned 
below. Sections parallel to oio do not show the albite twin- 
ning lines. 
In general, when the extinction angles on both ooi and 
OIO are large that fact indicates bytownite or anorthite. When 
both are small the feldspar is either oligoclase or andesine. 
Intermediate extinction angles are seen in albite and labrador- 
ite; while the potash and soda-potash feldspars have extinction 
on ooi practically parallel to the cleavages (except microcline, 
which has extinction at 15° 30'), and on 010 their extinction 
varies from 5° to 9°. 
Following are the extinction angles of the feldspars on the 
base and brachypinacoid: 
Orthoclase . . . 
Microcline. . 
Anorthoclase 
Albite 
Oligoclase . . 
Andesine . . . 
Labradorite 
Bytownite .. . 
Anorthite . . . 
Extinction 
on 
Extinction on 
OOI. 
010. 
qo 
+5° to r 
+i5°3o' 
+5° 30' 
+2- 
+9" 
+4° 
+ i9°30' 
+2° 
-fS" 
-2^30 ' 
—10" 
-5«3o ' 
— 20« 
^15'=' to — 
2S" 
—zb" to — 32«' 
-36''30 
~4i''3o' 
{b) The Statistical Method* 
The method proposed by Michel-Levy, often designated 
the statistical method, is applicable to all cases in which cleav- 
age pieces of sufficient size cannot be obtained, but in which 
still the albite twinning is evident. Since the albite twinning 
forms lamellae parallel to the face 010, whose edges are inclined 
*De I'emploi du microscope polarisant a lumiere parallel pour I'etude 
des plaques minces des roches eruptives. Ann. des Mines, Dec, 1877, 
pp. 392 to 471 (v. p. 451). 
