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HOLLAND: MICA DEPOSITS OF INDIA. 



to that plane. These two statements are represented diagrammat'cally 

 by figs, i and 2. 



Fig. 1. Fig. 2. 



m, Traces of the prism faces. 

 b, Traces of the clinopinacoid or its equivalent. 

 EE, Leading line of the percussion- figure. 



By subjecting a sheet of mica to a gradually increasing pressure 

 with a blunt punch, another six-rayed figure, the so-called pressure- 

 figure, is produced with rays approximately bisecting the angles of the 

 percussion-figure (see plates VI and VII). Mica-crystals are often 

 found in which these pressure-figures have been produced by natural 

 earth pressures when embedded in the rocks (see plate VI), and the 

 crystals often split along these lines, forming pseudo-crystal faces, 

 which are inclined at about 67 to the basal cleavage-planes. The 

 occurrence of numerous fissures parallel to the pressure-figure lines is 

 the cause of the fibrous mica so frequently found in these so-called 

 gliding planes. These fibres are often found in muscovite aligned 

 most perfectly at right angles to the leading ray of the percussion- 

 figure. 



Allusion has already been made in a foot-note to the fact that the 

 rays of the percussion-figure do not (as was generally supposed on 

 account of the statements of Max Bauer) intersect one another at 

 angles of 6o°. Dr. T. L. Walker, who first called attention to this 

 fact, has measured the angles of the percussion-figures in a large 

 number of micas from various parts of the world, and has found that 

 ( 8 ) 



