436 PROCEEDINGS OF THE AMERICAN ACADEMY. 



This scheme of color and absorption applies to all the zones, the colors 

 being simply modified in intensity. 



The specific gravity was determined in methyl iodide solution at 1 6° C. 

 The average for two unaltered crystals is 3.275. An outer light-colored 

 zone gave 3.195, and an inner darker zone 3.230. The difference be- 

 tween the last two was too small to permit of the separation of the light 

 and dark zones. I doubt that the lightest zone is more than one per cent 

 of the whole. There are no important inclusions in the mineral. 



M. Pisani of Paris made an analysis of the hornblende ; it resulted as 

 follows : — 



SiO. 45.20 



TiOa 0.84 



AloOg 7.34 



Fefis 7.55 



FeO 15.80 



MnO 1.52 



CaO 12.30 



MgO 8.40 



NaoO 0.80 



KoO 0.37 



Loss on ignition 0.70 



100.82 



The analysis does not lend itself to calculation in a satisfactory way. 

 There is considerable divergence in the proportions of the oxides from 

 an old analysis by Rammelsberg of a Philipstad hornblende with a spe- 

 cific gravity suggestively close to that of our hornblende.* 



It will be seen that the most noteworthy feature of the analysis is the 

 high percentage of ferrous iron, a fiict which correlates the mineral with 

 hastingsite, which also has an unusually great proportion of this oxide, 

 as well as an extraordinarily small optical angle.f 



That a high content of ferrous iron (plus MnO) always means a cor- 

 respondingly small optical angle cannot be asserted ; pargasite affords a 

 case sufficiently clear to invalidate any such claim. Yet it does seem that 

 there is some intimate relationship between the amount of the oxide and 

 the optical angle. The analogy of another group of allied silicates is in 

 striking corroboration of this conclusion. Thus Hiutze t gives a table 



* See Hintze, Handbuch der Mineralogie, 1894, p. 1223. 



t Cf. Adams, Canadian Record of Science, 1896, Vol. VII. p. 77. 



} Handbucli, p. 9G4. 



