20 HOLLAND: MICA DEPOSITS OF INDIA. 



(6) The subordinate rays of the percussion-figure meet the 



subordinate rays of the pressure-figure at angles of 93 

 and 33 . 



(7) The etch-figures produced by the action of hydrofluoric 



acid or by fused potash are bisected symmetrically by 

 the principal ray EE' of the percussion-figure. 



The physical properties of a plate of muscovite are thus ranged 

 symmetrically with regard to a single line, namely, the leading 

 ray of the percussion-figure ; and, as the optic-axial plane is, according 

 to Tschermak, slightly inclined to the basal cleavage-plane, the mineral 

 muscovite must be regarded as monoclinic in its crystallization. 

 In many points muscovite is imitated by its chemically near relative, 

 lepidolite ; but the asymmetric character of the etch-figures in phlogo- 

 pite and biotite, and the oblique disposition of their optic-axial planes 

 with reference to the leading percussion rays, suggest that their 

 crystal symmetry belongs to a lower grade. 1 But the micas frequently 

 show twin-lamellae parallel to the basal plane, and, as the general 

 effect of twinning is an apparent increase of symmetry, it is just 

 possible that the higher grade of symmetry exhibited by muscovite 

 may be due to twinning on a scale too minute to be detected by 

 our ordinary physical tests. 



A very interesting occurrence of natural pressure and percussion 

 figures was found by Dr. T. L. Walker at Gudladona in the Nellore 

 district. The rays of both figures appeared to radiate from an 

 inclusion in the mica, apparently an altered allanite ; on examina* 

 tion it was found that the rays of the compound figure intersected one 

 another as nearly as possible at angles of 30 , and that one of the rays 

 occupied the correct position of the principal ray of the artificially 

 produced percussion-figure. Regarding this compound twelve-rayed 

 figure as the result of the symmetrical intersection of the percussion 

 and pressure figures we have to meet the difficulty that the natural 

 percussion-figure, unlike that produced by artificial means, is formed 



Cf. Walker, toe. cii. 



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