Proceedings of the American Academy of Arts and Sciences. 

 Vol. XXXIV. No. 12. — February, 1899. 



ON THE OPTICAL CHARACTERS OF THE VERTICAL 

 ZONE OF AMPHIBOLES AND PYROXENES; AND 

 ON A NEW METHOD OF DETERMINING THE 

 EXTINCTION ANGLES OF THESE MINERALS BY 

 MEANS OF CLEAVAGE PIECES. 



By R. a. Daly. 



With Three Plates. 



