OBSERVATIONS ON THE FELDSPARS 205 
I also have a specimen of albite from Auburn, Me., which shows 
pericline parting. The angle between the cleavage and the parting 
is +13°, as nearly as can be measured with a contact goniometer, 
which places it as about Ab,, An;. In this specimen there is peri- 
cline twinning as well as albite twinning and good cleavage parallel 
to joor}, }r1o$, j110$, and jo1o}. The mineral splits easily parallel 
to joro}, and hence there is 
both parting and cleavage 
parallel to this face. The ex- 
tinction angle on the (o10) 
face is about 20. 
Pericline parting was also 
noticed on a specimen of 
oligoclase from Bamle, Nor- 
way. A cover-glass was ce- 
mented to the dull parting 
surface and the angle between 
it and the cleavage was found 
to be +3° 50’, measuring with the contact goniometer. This 
measurement places the feldspar as about Ab,An,, which is 
confirmed by the extinction angle of o° on the (oro) cleavage face. 
For another plagioclase from Labrador, showing well-defined 
pericline twinning and only faint albite twinning, the cleavage and 
the parting are parallel, as nearly as can be judged. Even in this 
case there is lamellar structure due to pericline parting. According 
to Penfield and Sperry‘ the angle of the rhombic section for Ab,An, 
is +o°. This places the feldspar as andesine-labradorite, which is 
confirmed by the extinction angle of —15° on (oro). 
“no 
artis 
pe 
Fic. 5.—Albite-oligoclase, AbsAnx 
PRISMATIC CLEAVAGE IN THE FELDSPARS 
Labradorite from Labrador also occasionally shows dull sur- 
faces due to pericline twinning. These dull surfaces are almost 
parallel to the cleavage, but careful measurement with the reflec- 
tion goniometer proves the angle between the cleavage and the 
parting (using a cover-glass on the latter) to be about —3° 40’ 
t Am. Jour. Sci.,(3) XXXIV, 390 (1887). 
