W. E. Ford — Optical Study of the Amphiboles. 191 



general these analyses show, together with the decrease in the 

 angle of extinction, a decrease in the percentages of Si0 2 , MgO 

 and CaO and an increase in the percentages of A1 2 3 , Fe 2 3 , 

 FeO and the alkalies. This variation in composition is by 

 no means regular and the relations existing between the com- 

 position and the angle of extinction are not at all clear. The 

 only relationship that seems at all close in character is that 

 between the angle of extinction and the total amount of iron 

 present, there being an increase in the percentage of total iron 



Figs. 10, 11. 



that follows closely the decrease in the extinction angle. The 

 correlation between these two is shown graphically in figure 

 10. That the alkalies have little effect upon the angle of 

 extinction is seen by comparison of analyses I and V and III 

 and Till. It will be noted further that the decrease in the 

 value of the extinction angle is accompanied by a fairly uni- 

 form increase in the value of the mean index of refraction ; see 

 figure 11. 



The hornblende analyses are given on p. 190, also arranged 

 in the order of decreasing extinction angles. These analyses 

 have been studied in various ways but without finding any 

 reasonable correlation between their percentage composition 

 and the angle of extinction. The variations in the percentages 



