1 8 Tlie AlllCriCiDl Gcolonst. January, 18as 
Fig, 9. — External Albite and Pericline bands. 
cleavage or with the edge ooi :oio. In albite it is about as 
shown in the figure, viz., 13° to 22°; in oligoclase it is 4"; in 
andesine it is 0°; in labradorite from 2° to 9° in the opposite 
direction, and for anorthite it is 18° in the same direction as 
for labradorite (fig. 16.) 
When the polysynthetic twinning, albite or pericline, is 
again enveloped by a Carlsbad twinning, a thin section mani- 
fests it by the occurrence of two pairs of bands on one side 
which extinguish or polarize in sympathetic alternation, differ- 
ently from two pairs of bands on the other side. Generally 
the darkened line which separates the Carlsbads can be seen. 
It is heavier than the other dark lines, and is apt not to agree 
with them strictly in direction, or to be otherwise irregular. 
The twinning of anorthoclase and microcline is character- 
istic. They combine the albite and pericline types, producing 
a rectangular quadrillage on all sections of the zone 001:100, 
except on that which is parallel to the plane of association for 
the pericline law, and on all sections in the zone 001 :oio ex- 
cept on that parallel to the face of association for the albite 
law. In other words, the section parallel to 010 is identified 
by the disappearance of the albite made and that parallel to 
the plane of association of the pericline made by the disap- 
pearance of the pericline marks. This last plane in anortho- 
clase, shown by the trace of its made, makes an angle of — 78° 
to — 75° with the edge 001 :oio. It hence makes a large angle 
with the base 001, and it is practically perpendicular to the 
