OBSERVATIONS ON THE FELDSPARS 203 
face of the microcline and the (oor) face of albite on each side is 
33° as measured with the simple reflection goniometer. 
A microscopic examination confirms the above. A section 
parallel to the base joo1{ shows the gridiron structure for the 
microcline and plates of albite with extinction angles of about 
33°. The albite plates parallel to the (110) face have extinction 
in the opposite direction from those parallel to the (110) face. 
Close examination reveals a tew narrow twin lamellae in each set of 
albite plates. Fig. 2 is a diagrammatic drawing of a thin section 
cut parallel to (oor). 
The question arises as to whether this intergrowth can be called 
perthite or not. In the original perthite the albite plates are 
polysynthetically twinned and are 
parallel or approximately parallel 
to the a-axis. There is before mea 
specimen from the original locality, 
Perth, Canada (Fig. 3), which shows 
a coalescence of albite plates practi- 
Cally forming a plate parallel to boas at 
eet PT 
the (110) face. I have also noted [5 
albite plates parallel to (110) in a 
microcline-perthite from Rincon, -@e=. a? : 
Cal. On account of the gradation 
between the typical perthite and 
the peculiar perthite, it seems ad- 
visable to extend the term perthite 
to cover such cases. It should 
also be mentioned that normal per- 
thite occurs among the Port Henry specimens. These specimens 
consist of microcline with plates of albite set parallel to the }100$ 
face. They show polysynthetic albite twinning. 
Fic. 3.—Perthite 
ALBITE WITH BRACHYPINACOIDAL PARTING 
Some cleavable albite of pale-green color from Port Henry, 
N.Y., consists of two kinds of specimens. Part of the specimens 
break in plates parallel to basal joro} cleavage while others break 
in plates parallel to the side pinacoid joro{ and have a lamellar 
