MINERALOGICAL AND CHEMICAL CHARACTERS. 



19 



the angle indicated by the letter k in figure 2, varies from about 

 51 in some muscovites to 64° in some phlogopites, whilst the lithia- 

 micas and biotite approximate more nearly to 60 . 1 



Z7 



27 



\ 30° 



30* / 



f 



33"\ 



fzz 



\\\ 



f *s* 



^ 



&$ 



l y^ / 



\ ^-v. 



33'/ 



\33' 



/ 



\ 



s/30" 



30" Y, 



27° 



27° 



Fig. 3. Diagram showing the average relations of the pressure-figure, percus* 

 sio?i-figure and optic-axial plane in Indian muscovites. 



The writer has examined the percussion and pressure figures 

 in some 30 Indian muscovites and has found the relations graphically 

 expressed in figure 3 and on plates VI and VII to represent to 

 the nearest degree the average of some 200 determinations. The 

 principal results may be summarised as follows : — 



(1) The principal ray of the percussion-figure EE' lies in the 



plane of symmetry and at right angles to the optic- 

 axial plane. 



(2) The principal ray is cut perpendicularly by the ray PP' of 



the pressure-figure, which lies in the optic-axial plane. 



(3) The angle k between the rays FF' and GG' averages 53-' 55' 



(nearly 54°). 



(4) The remaining angles of the percussion-figure are, to the 



nearest degree, each 63 . 



(5) The rays of the pressure-figure intersect one another as 



nearly as can be measured at angles of 6o°. 



1 T. L. Walker, " The crystal symmetry of the minerals of the mica group." 

 Amer. Journ. Set., Vol. VII, 1899, p. 199; " Percussion-figures on micas." Rec. 

 Geol. Surv. Ind., Vol. XXX (1897), p. 250. 



( 9 ) 



