250 HAROLD L. ALLING 
POTASH-SODA-LIME FELDSPARS 
Orn" EXTINCTION ANGLES 
P Or 
Fic. 12.—The diagrams, 1 to 4, are attempts to show the extinction angles 
of the potash-soda-lime feldspars by means of isogenic lines. Diagram 5 shows 
the approximate average composition of natural feldspars. The intersection of this 
compositional line and the proper isogenic lines determines the composition of the 
specimen under examination. Diagrams 6 and 7 show the specific gravities of the 
orthoclase-barbierite-anorthite and the microcline-albite-anorthite systems. All of 
these diagrams are tentative. 
