B. P. D. Graham — Optical Properties of Hasting site. 543 



hastingsite, when examined under similar conditions, was 

 found to have an axial angle, 2 V =■ 17° 12' ; and allowing for 

 the difference in refractive index between the two minerals, 

 this would give for hastingsite, 2 V= 16°. 'From the nature 

 of the method employed in arriving at this value, it can at the 

 best be considered only as an extremely rough approximation ; 

 but it serves to indicate the probable order of the axial angle, 

 which is much smaller than at first suggested. 



The mean refractive index was determined by Schroeder 

 van der Kolk's method, using thin cleavage fragments placed 

 in the position of least absorption, i. e., for light vibrating 

 across the prism. The liquids employed were methylene 

 iodide and naphthalene monobromide, which mix to form a 

 clear solution. The refractive index of the resulting mixture, 

 after adjusting as nearly as possible to that of the mineral, 

 was determined in the usual manner by means of a hand refrac- 

 tometer. The mean of several determinations gave 1*69 as the 

 index of refraction of the mineral for light traversing it in this 

 direction. 



Geological Department, McGill University, Montreal. 



