1894. PROCEEDlNdS OF THE NATIONAL MUSEUM. 31 



tioii (lifificult, but occasional sections sliow parallel cleavage lines giv- 

 ing an extinction angle of 35^. Others showing cleavage lines nearly 

 at right angles give an angle of extinction =0°. A form with cleavage 

 cracks making an angle of 63°, evidently from the orthodiagonal zone, 

 shows the emergence of an optic axis with finely colored rings. Indi- 

 cations of an alteration process appear in some portions of the section 

 in a clouding and opacity extending inward from the cleavage cracks. 

 These portions under a higher power exhibit a finely fibrous structure 

 which is develoi)ed in the individual granules and which suggests that 

 alteration to amphibole is taking place. As compared with the Amer- 

 ican jadeites described by Clarke and Merrill* the Mogoung specimen 

 shows chemical and optical differences which correspond to those 

 already mentioned, as distinguishing it from the jadeites described 

 by Schoetensack and Damour {loc. cit.), viz: smaller percentages of 

 the elements replacing Na and Al and microscopically a finer texture. 



Proc. U. S. Nat. Mus., xi, 1888, 115. 



