312 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



(010). It seems to have been taken for granted by some writers that 

 this is not true.* 



It is the object of this paper to make a systematic statement of the 

 relation between the extinction on (110) and on (010), and to indicate 

 under what conditions the former may be used to determine the latter. 

 First, there will be deduced a formula to express the extinction on any 

 plane of the vertical zone when the optical angle 2 Fand p, the angle of 

 extinction on (010), are known, and a graphic representation of possible 

 extinctions in that zone will be attempted. Then specific tables will be 

 introduced to show how 2 V, p, and 6' are related for any variety of 

 pyroxene or negative amphibole, where 6' represents the extinction on a 

 cleavage plate of that variety. Secondly, a method is proposed for the 

 determination of p by means of 6' and a new angle of extinction, 6", 

 found after turning the cleavage piece through a certain angle about the 

 vertical axis. 



Figure 1 



In Figure 1, let ECB be the plane of symmetry of any monocHnic 

 crystal characterized by parallelism between (010) and the optical plane. 

 Let E C G hQ any plane in the vertical zone cutting the plane of sym- 



* Rosenbusch, Physiographie der petrographisch wichtigen Mineralien, 3te 

 Auflage, 1892, p. 550. Zirkel, Lehrbuch der Petrographie, 2te Aufl., Bd. I. p. 302. 



