to Calcium and Magnesium Metasilieates. 29 



cation of the above paper, however, better facilities for refrac- 

 tive index determinations have been acquired and the refractive 

 indices of the two compounds have been redetermined. The 

 measurements were made on polished plates of the crystalline 

 aggregate, experience having taught that even under such 

 conditions it is possible to determine 7 and a in sodium light 

 with the reducing attachment of the Abbe-Pulfrich total 

 refractometer, while ft can also be ascertained if the individual 

 grains are sufficiently large. For wollastonite the new values 

 are, y^ = l'632±-002 ; /3 Na = l--628=b"003 ; a Na = 1-616 + 002 ; 

 ry — a— 016, 7 — j3 = -004:, j3— a — '012. The refractive indices 

 of pseudo-wollastonite are : 7 Na =l"650=b'002 ; a Na =l*609±003 ; 

 birefringence 7 — a = *041. The birefringence of both wollas- 

 tonite and pseudo-wollastonite was furthermore checked by 

 direct measurement in the thin section ; 7— a for wollastonite 

 being *014 and for pseudo-wollastonite *043.* 



Magnesium Metasilicate. — Like the calcium metasilicate, 

 this compound has also been described in a special paper in 

 which the optical characteristics are considered together with 

 the other properties. Four different forms or phases were 

 there mentioned bearing mono tropic relations to each other, 

 the monoclinic Mg-pyroxene being the one stable form. In 

 the course of the past winter, however, still another phase has 

 been discovered, orthorhombic in symmetry and in general 

 aspect and development not unlike that of olivine crystals. 











Table V. 









No. 



Letter 



Symbol 



Miller 



4 



1 



P 



1 



c 



(?) 







001 



.. 





2° 00' 



2 



a 





00 



100 



90° 



00 



90 00 



3 



b 





0oo 



010 







00 



a 



4 



m 





00 



110 



44 



04 



a 



5 



n 





00 2 



120 



23 



07 



a 



6 



I 



(?) 



»t 



250 



17 



56 



a 



7 



k 



(?) 



3oo 



310 



70 



44 



a 



8 



r 



(?) 



2oo 



210 



66 



54 



a 



9 



s 





11 



111 



43 



20 



38 58 



10 



e 





12 



121 



24 



58 



52 11 



11 



P 





10 



101 



90 



00 



29 09 



12 









1 



111 



46 



48 



40 21 



13 



i 





12 



121 



28 



30 



53 10 



14 





(?) 



i<> 



103 



90 



0*0 



11 55 



15 





(?) 



*o 



103 



90 



00 



12 55 



* Opportunity may liere be improved to correct several of the optical data 

 in the paper on the Lime-Silica Series of Minerals (this Jour. , xxii, 293-302, 

 1906). At top line of p. 297 read : a = 1-609 ± '003, 7 = 1 -620 ± '002, instead 

 of the values given ; on p. 298, 11 lines from the top, e = 1'554± '002 and 

 w = l-544±-002; on p. 299, 8 lines from bottom, 1-585 and 1*621; and 2 

 lines from bottom, a = 1-590 ; on last line, p. 299, read "019 instead of "025. 



