Wright — Two Microscopic- Petrograjphical Methods. 391 



arnpbibole or pyroxene (B= b) parallel to the clinopinacoid 010, 

 on which the extinction angle must be measured, is also suita- 

 ble to reveal the optical character of the mineral and the 

 double refraction (7— a). 



Determination of the plagioclase feldspars : 



Cleavage faces parallel to the basal pinacoid 001 of the soda- 

 lime feldspars (acid plagioclase), albite to basic andesine, show 

 the optic normal more or less distinctly, and can be determined by 

 the above method as such without further trouble. With this aid 

 in the diagnosis, the separation of the acid plagioclases by the 

 cleavage method on plates parallel to the base (striae after the 

 albite law) is rendered more certain, as the ambiguity of 

 the sign of extinction is largely eliminated. (The feldspars 

 Ab 1 An — AbgAn, are positive; Ab 8 An x — Ab 22 An 10 , negative; 

 Ab 22 An 10 — Ab x An 4 , positive; Ab 1 An 4 — Ab An„ negative.) 



In his " Etude sur la Determination des Feldspaths," Michel 

 Levy has plotted a curve illustrating the extinction angles on 

 plagioclase feldspars cut perpendicular to the optic normal 

 (B, n m ). Unfortunately, the values are so nearly equal in the 

 acid plagioclases that an exact separation of the same is not 

 possible by this method alone. The ambiguity, however, 

 arising from the plus or minus character of the angle between 

 albite and oligoclase and andesine can, in general, be eliminated 

 by the method described above. In the determination of the 

 plagioclase feldspar by the method of Michel Levy in conjunc- 

 tion with the above, a plate cut perpendicular to the optic nor- 

 mal (highest interference color) is first selected, its angle of 

 extinction (angle between the least ellipsoidal axis and the 

 twinning lamellae) measured, the direction of the acute bisec- 

 trix noted and its relative value, whether a or c, ascertained by 

 means of the quartz wedge. By combining the extinction 

 angle of any plagioclase with its optical character, the feldspar 

 can in general be determined accurately. 

 Michigan Geological Survey, Houghton, Michigan. 



