DOA AUSTIN F. ROGERS 
structure. In albite the basal cleavage is perfect and the side 
pinacoidal cleavage, imperfect, hence the lamellar specimens are 
abnormal. A close examination of the two kinds of specimens 
shows polysynthetic twinning according to the albite law. In 
the basal cleavage of the first-mentioned variety the twinning lamel- 
lae are fairly uniform. Fig. 4 represents the basal cleavage of the 
variety with lamellar structure. Most of the twinning lamellae 
are very narrow, but every fifth to tenth one is much wider. On 
tracing the wider lamellae to the edge of the specimen there is 
usually noticed a parting plane. These parting planes are covered 
with a thin film of chlorite. In a specimen 
about 1 cm. wide there are about twenty-two 
broad lamellae and about twenty parting 
planes. The films of chlorite are probably the 
result, not the primary cause, of the parting, 
for the normal albite has finely divided chlorite 
all through it and not concentrated in films. 
If my observations are correct, there is 
both cleavage and parting in the same direc- 
tion, namely, parallel to {oro}. I know of 
no other similar case on record but there 
seems to be no reason why the same direction 
cannot be one of molecular disturbance as 
well as one of weak cohesion. 
PERICLINE PARTING IN PLAGIOCLASE 
(Bee os llone thule Cleavable white plagioclase from Amelia 
ning on albite (mag- 
mined) Co., Va., shows a dull surface with bright 
spots. The bright spots are cleavage surfaces 
parallel to the base {oor}. The dull surface is a plane of separa- 
tion parallel to the rhombic section, for pericline twin lamellae 
are visible on the imperfect joro} cleavage. A glass slip was 
cemented on the dull surface and the angle between it and the 
cleavage measured with the reflecting goniometer. The angle 
is +9, which places the plagioclase as albite-oligoclase, AbsAnxy. 
Fig. 5 is a side elevation showing the cleavage, parting, and peri- 
cline twin lamellae. 
