NOTES ON SOME IGNEOUS ROCKS OF JAPAN 



643 



From the above figures, the following values for the azimuth of 

 the axis B against the edge P/M on P (!) and the true angle- 

 distance (&)) are given: 



-12.3 19 31 



For calculation of &>, %= 1.5785 and £ = 0.141 were adopted as 



the mean index of refraction and Mallard's constant, respectively. 



From the mean values of £ and «, 4> and X were given as follows: 





1 



-o. 3 c 

 -5-S c 



II 



•o° 17' 

 ■4° 23' 



The values under I are results approximately obtained by the 

 construction of the stereographic projection and those under II by 

 calculation. 



Plotting the latter values 

 on Becke's diagram, which 

 indicates the relation between 

 the orientation of the optic 

 axis B of plagioclase and its 

 corresponding chemical com- 

 position, the composition of 

 the present mineral would be 

 identified as Ab 4 An 96 — 

 Ab s An 9S , as shown in Fig. 12. 



The mineral is optically 

 negative with r as the acute 

 bisectrix. The optic angle 

 measured in cedar oil (n y = 

 1. 5 15) with yellow light, is 



2H a =QO° 11.5' 



and its true angle is 



2V„=8 5 ° 39' 



Chemical characters. — The mineral is easily attacked by hydro- 

 chloric acid, and the powdered sample is readily decomposed with 



100 



Fig. 12. — In the figure, A is the anorthite 

 from Vesuvius, B is the anorthite from 

 Tarumai, and C is the bytownite from 

 Naeroedal. 



