NOTES ON THE MINERALS OF JAPAN. 261 



100.68 



This mica is most likely zinnwaldite. 



b.) Small silver-white six-sided plates attached to the 

 crystals of potash-felspar. Their diameter is about 1.5 cm. or 

 less. The cleavage-surface is much folded parallel to the 

 liues of pressure- figure. Twinning is sometimes observed. The 

 optical plane is parallel to a line of the pressure figure. The 

 angle of optical axes is large. This variety sometimes over- 

 laps a similarly oriented six-sided biotite plate, so as to form 

 a broad outer zone to it. This mica is probably mus- 

 covite. 



c.) Minute six-sided plates like (b), but less than 0.8 cm. in 

 diameter and with curved cleavage-lamellae. They often show, 

 in crossed niçois, a subdivision into six triangular fields and 

 a zonal structure with zones of different orientations. 



d.) Brownish-grey six-sided or irregular plates with small 

 angles of optictil axes. This variety is sometimes 5 cm. in dia- 

 meter. It is altered, along cleavage as well as on the sur- 

 face, into a scaly silver-white mica, with a large angle of 

 optical axes and sometimes forming druses of small tabular 

 crystals. 



