IF". E. Ford — Optical Study of the Am/phiboles. 189 



11 12 13 14 15 16 17 18 



SiO a 



58-22 



45-79 



48-38 



42-74 



40-52 



39-95 



39-52 



40-85 



Ti0 2 





1-20 



0-05 



1-08 



1-71 



1-68 



10-31 



8-47 



Al a 3 



i : 37 



11-37 



10-83 



5-48 



10-99 



17-58 



11-22 



9-89 



Fe 2 3 



0-04 



0-42 



076 



11-92 



9 64 



7-25 



1-22 



8-85 



FeO 



0-61 



0-42 



1-56 



11-46 



9-83 



2-18 



8-81 



3-96 



MnO 



004 



0-39 



0-04 



0-06 



tr. 



tr. 



0-06 



0-12 



MgO 



2397 



21-11 



20-78 



11-60 



11-82 



14-15 



13-31 



12-47 



CaO 



12-95 



12-71 



12-24 



12-72 



12-33 



11-96 



1093 



12-16 



K 2 



04 



1-69 



1-38 



0-56 



0-68 



1-98 



1-07 



0-63 



Na 3 



024 



2-51 



2-69 



225 



2-38 



3-16 



2-95 



2-01 



H 2 



217 



0-67 



0-91 





050 



0-41 



0-59 



0-19 



F 



0-17 



2-76 



1-82 







03 





0-28 













NiO 0-13 





010 



99-82 10106 101-44 9969 100-40 10046 100-00 99'98 

 0=F 0-07 116 0-76 01 



99-75 99-90 10068 100-45 



11. Tremolite from Switzerland. Kreutz, Min. Mitt., xxvii, 251, 1908. 



12. Hornblende from Grenville, Penfield and Stanley, loc. cit. 



13. Pargasite from Pargas, Finland. Kreutz, Min. Mitt., xxvii, 249, 1908. 



14. Hornblende from Chester, Mass., Dnparc and Pearce, Bull. Soc. Min., 

 xxxi, 119, 190S. 



15. Soretite from Koswinsky in northern Urals. Dnparc and Pearce, Bull. 

 Soc. Min., xxvi, 131, 1903. 



16. Hornblende from Bilin, Penfield and Stanley, loc. cit. 



17. Kaersutite from Kaersut, Greenland. Washington, this Journal, xxvi, 

 198, 1908. 



18. Kaersutite from Linosa, Washington, ibid., p. 192. 



It was felt that with the data at hand it would be of value 

 to discover, if possible, any regularity in the chemical changes 

 that accompany the variation in the angle of extinction, c /\6, 

 measured on b (010). Some sixteen analyses of normal amphi- 

 boles were used for this purpose. These analyses were 

 arranged in a series with decreasing angles of extinction. It 

 was at once seen that they could be divided into two distinct 

 groups, each of which covered much the same range of varia- 

 tion in the extinction angles but were quite distinct from each 

 other chemically. The first series showed extinction angles 

 ranging from 20° to 8° with percentages of silica varying from 

 about 58 per cent to 51 per cent. The second series showed a 

 variation iu extinction angles from about 33° to 9° with silica 

 percentages from about 44 per cent to 36 per cent. That is, 

 we may have two amphiboles of distinctly different composi- 

 tion, one belonging to the tremolite-actinolite group and the 

 other to the hornblende group, which have nevertheless the 

 same angles of extinction ; compare, for instance, analyses IV 

 and XIY, p. 190. These two series of analyses were separated 

 from each other and each studied by itself. 



The analyses of the tremolite-actinolite series are given on 

 p. 190, arranged with decreasing angles of extinction. In 



Am. Jour. Sci.— Fourth Series, Vol. XXXVII, No. 218. — February, 1914. 

 14 



