OBSERVATIONS ON THE FELDSPARS 207 
SUMMARY 
A peculiar intergrowth of microcline and albite is described. 
The albite plates are parallel to the unit prism faces. Plates 
parallel to (110) are in twinning position (albite twinning) with 
respect to the plates parallel to (110). The term perthite may be 
extended to include any regular intergrowth of albite with ortho- 
clase or microcline, either of the latter being in excess ane pew tte 
when albite is in excess). 
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7a—Plan 7b—Side elevation 
Fic. 7.—The space-lattice of orthoclase 
Lamellar albite with easy planes of separation parallel to the 
side pinacoid {oro} are explained by a parting in this direction. 
Here we have cleavage and parting in the same direction, and the 
variation in the side pinacoidal cleavage often noted in the literature 
may thus be explained. 
Several kinds of plagioclase varying from albite to labradorite, 
and including Ab,,An,, AbsAn,, Ab,An;, Ab,An,, and Ab,An,, show 
parting parallel to the rhombic section due to polysynthetic peri- 
cline twinning. The surfaces are dull and make slight angles with 
the cleavage surfaces. In these specimens we have examples of 
parting which are not parallel to either actual or possible 
crystallographic faces. As far as I know similar cases have 
not been recorded. 
The imperfect separation in the feldspars parallel to the (110) 
and (110) faces is cleavage rather than parting. This cleavage 
is almost universally present and is of great assistance in orienting 
feldspar cleavages. 
