20 
Tlic American Geoh<rist. 
January, 1898 
The striations of the pcricline made, when visible in a section 
parallel to oio, make different angles with the basal cleavages 
which are visible in the same section, according to the feldspar 
examined. This angle varies from o° for andesine, to — 18° 
for anorthite, in one direction and in the other direction it 
varies to + 13° and +21° for albite. It may be represented 
for all the triclinic feldspars by the diagram below (fig. 16), 
which, shows the face 010 of a simple crystal. 
— it° Anorthite 
iOO 
\Ldbradorite 
"-2° J 
' 0" Andesuie 
+ ** ° Ollgoclase 
+ 13°^ 
+ 21"/ 
Albite 
■ TS'AnoHhoclase 
+100" jMicrocUne 
Fig. 16.— Angles of the Pericline Bands on 010. 
The plagioclases are also subject to twinning on the Carls- 
bad, Manebach and Baveno plans, and albite also on the Roc 
Tourne plan. This last consists of two albite macles, again 
twinned as couples by the union of the reentrant angle formed 
by the faces loi, loi, of one, upon the salient angle of the 
other formed by the same faces. The double crystal thus 
formed is approximately a parallelogram, flattened parallel to 
GIG, one of the twinned pairs being thinner than the other, 
as shown by figure 17. 
In thin section the twinning lines of albite are fine and far 
apart, often irregular and interrupted; those of oligoclase 
are very clear and of very regular widths, one of the 
systems being much more fine than the other — so 
fine, indeed, that sometimes it is impossible to perceive 
the width. In labradorite the lamellae are equally 
clear and definite, but the width varies much from one 
lamella to the other, and in the same lamella (rarely) 
from' one point to another. In anorthite the albite lamellae are 
