204 Walker— Crystal Symmetry of the Mica Group. 



I have to thank Professors F. W. Clarke, Washington, D.C., 

 S. L. Penfield, New Haven, Ct, G. Tschermak, Yienna, F. J. 

 Wiik, Helsingfors, and A. Lacroix, Paris, for their kindness in 

 sending me specimens of phlogopites. 



Indian Museum, Calcutta, July, 1898. 



Fig.l 



Fig. 1 



FUf.S 



EXPLANATION OF FIGURES. 



Figure 1. — Cleavage plate from a crystal from Upper Burma, showing the posi- 

 tions of the percussion figure and optical plane. 



Figure 2. — Etching figures on phlogopite from Pargas, obtained by hydrofluoric 

 acid. [After Wiik.] * 



Figure 8.— Etching figures on biotite obtained by hydrofluoric acid. [After 

 Wiik.]* 



Figure 4. — Etching figures on zinnwaldite, showing twinning, obtained by hydro- 

 fluoric acid. [After Wiik.] * 



Figure 5. — Etching figures on rose-red lepidolite from Mursinsk, obtained by 

 action of hydrofluoric acid. [After Wiik.] * 



Figure 6. — Etching figures on muscovite from Kimito, obtained by action of 

 hydrofluoric acid. [After Wiik.] * 

 *loc. cit. 



