316 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



umn headed " 62|°." This table is regarded as capable of more general 

 application than the one published by Harker,* inasmuch as his refers 

 only to that small class of amphiboles which have the acute optical angle 

 situated about the axis of elasticity lying next to the vertical axis (op- 

 tically 2)ositive, represented by pargasite). It will be observed that for 

 amphiboles with small optical angle (up to 60°), any one of the three 

 variables may be determined if the other two are known, but that, con- 

 sidering the instrumental errors of reading, the larger values of the 

 optical angle cannot with much safety be determined on account of the 

 slow variation in the corresponding angles of extinction on (010) and 

 (110). When 2 Fis equal to 70°, p and 0' are for any negative species 

 nearly equal. 



The following is the similar table (Table II.) for pyroxenes, where the 

 cleavage angle is taken at 92° 54'. It is simply a slight extension of 



that of Harker. t 



TABLE IT. 



We are not yet, however, in a position to make universal use of cleav- 

 age pieces for the purpose of finding the value of p. There are many 



* Extinction-angles on cleavage-flakes. Mineralogical Magazine, 1894, Vol. X. 

 p. 2.39. 



t Op. cit., p. 240. 



