6 



T. L. Walker — Percussion Figures of Mica, 



separated localities. The letter 

 angle opposite the clinopinacoidal 

 are averages of about half a dozen 







Optical pi 



ane. 



1. Muscovite — 



J_oo Poo (mica, 



1st kind) 



2. 



u 



i( 



<' 



" 



3. 



u 



u 



(1 



a 



4. 



u 



u 



a 



a 



5. 



'' 



u 



" 



" 



6. " 



u 



a 



" 



a 



7. 



a 



u 



a 



" 



8. Lepidolite— 



" 



u 



a 



" 



9. 



II 



" 



( " 



2d kind) 



10. 



(1 



i( 



a 



" 



11. " 



u 



u 



a 



a 



12. Zinnwaldite— 



a 



t( 



It 



a 



13. Biotite— 



L 



'1 



a 



«< 



14. 



ik 



*' 



(( 



" 



15. Magnesiaglimmer^ 



(C 



a 



a 



(Anomite ?) 











16. Phlogopite— 



u 



u 



" 



a 



17. 



(1 



u 



a 



a 



18. 



ii 



(( 



a 



a 



19. 



u 



" 



a 



a 



y^ is used to designate the 

 edge. The numbers given 

 measurements in each case. 



Murray Bay, P. Q Canada=52° 53' 

 Acworlh, N. H., U. S. A. =53° 20' 

 Locality unknown. =53° 42' 



Hereroland, S. W. Africa =54= 32' 

 Peterboro Co., Ontario =54° 56' 

 Marienberg, Saxony 55° 32' 



Uto, Sweden 55° 57' 



Wolkenburg, Saxony 

 Perm, Russia 

 Penig, Saxony 

 Mursinsk, Urals 



Zinnwald, Saxony 



Arendal, Norway 

 Siberia 



Lake Baikal 



Radigara, Ceylon 

 South Burgess, Ont. 

 Radigara, Ceylon 

 Radigara, Ceylon 



59° 12' 

 59° 07' 

 59° 45' 

 60° 16' 



59° 12' 



practically 60° 



60' 52' 

 61° 53' 

 62° 27' 

 63° 28' 



The above table shows that the angle / varies from 52° 58' in 

 muscovite from Murray Bay, Canada, to 63° 28' in phlogopite 

 from Kadigara, Ceylon. It is noteworthy that all the micas 

 examined, when arranged in a series according to the magni- 

 tude of the angle ;f, then all the muscovites are together in 

 one group which is quite separated from all the other micas ; 

 then follow the lithia micas in a second group, the biotites in 

 a third and finally the phlogopites. We notice, however, that 

 in this respect, the lepidolites and zinnwaldites form only 

 one group, though generally regarded as distinct species. Pos- 

 sibly such measurements as the above may be found useful in 

 the determination and classification of micas. E^o measure- 

 ments have been made of paragonite, lepidomelane or ano- 

 mite. In these species it may be different to obtain figures on 

 which satisfactory measurements could be made. 



The figures were produced with a large blunt darning 

 needle. The smooth back of a small book formed a satisfac- 

 tory foundation. Frequently after producing the figures on a 

 somewhat thick sheet of mica, one may obtain good material 

 for measurements by separating a very thin film from the 

 bottom of the plate. 



The so-called pressure figures, which are caused in mica 

 plates by application of gradual pressure, using a slightly 



* Too dark and brittle for observation. 



