]38 The Monoclinic Pyroxenes of New York State. 



sides a still better series is obtained, but not a perfectly regular 



one. 



Indices of Refraction. 



These were determined by the Kohlrausch method,* on speci- 

 mens from all localities furnishing sufficiently fresh material. 

 The measurements were made on polished surfaces, but the reflec- 

 tions afforded were often rather weak. For the sake of compari- 

 son the indices of refraction for yellow light are given below : 



a ft y 



Eussel 1.6626 1.6718 1.6940 



Port Henry 1.6683 1.6730 1.6902 



DeKalbt 1.6674 1.6745 1.6961 



DeKalb.... 1.6749 1.6852 1.7013 



Sing Sing 1.6778 1.6848 1.7025 



Pitcairn 1.6806 1.6843 1.7036 



Diana 1.6888 1.6932 1.7108 



It will be seen that the index of the Russel diopside is the 

 lowest of the series, and lower in fact than any recorded in 

 Hintze's llandbuch der Mineralogie, where a number of determi- 

 nations made on pyroxenes from various localities are given. 



Axial Angles. — The axial angle is known to vary with the 

 chemical composition in the same manner as the extinction angle, 

 the increase of the angle with the corresponding increase of iron 

 and alumina does not seem to be as regular, however, as the en- 

 largement of the extinction angle, and also, the axial angle does 

 not increase with the extinction angle. 



The following table may serve to illustrate this fact : 



cAt. 2 Vna. Sum of FeO, Fe^Os, Al^Og. 



Eussel 37° 58° 56' 3.71% 



DeKalb 40° 59° 30' 1.52% 



Sing Sing 40° 10 59° 4.91% 



Port Henry 41° 56° 30' 3.12% 



Pitcairn 41° 30' 59° 40' 4.391% 



Diana 41° 30' 60° 40' 14.07% 



The same disagreement between the extinction angle and the 

 axial angle, is shown b}'' the following measurements J of pyrox- 

 enes from other localities. 



* Methylene iodide with index of 1.732 was used. 



t Determined by K. Zimanyi, see Ref. 60. 



X Hintze, Handbuch der Mineralogie, p. 1026-1029. 



