Ford and Bradley — Study of a Ldbradorite. 153 



c (001), making note of the direction of tlie line of cleavage 

 of J (010). 



The average values of a series of readings of the extinction 

 angles of the two sections made in each case with the direc- 

 tion of cleavage of the other face were as follows : 







Measured 



Calculated 



on J (010) 



= 



-24° 37 



— 25° 33 



on c (001) 



=: 



— 12° 13 



-11°58 



In the second column are given the theoretical values of the 

 extinction angles of the respective sections, calculated by 

 Mallard's formula, as given by Schuster.* The agreement of 

 the measured and calculated values is fairly satisfactory. Both 

 sections when viewed in convergent polarized light show the 

 emergence of an optic axis just beyond the field of the 

 microscope. 



Laboratory of Mineralogy, 



Sheffield Scientific School, Yale University, 



New Haven, June, 1910. 



*Min. Petr. Mitth., v, 192, 1883. 



