﻿78 Canadian Record of Science. 



blende in composition. The second variety, which occurs 

 in a series of exposures about two miles to the east of the 

 village of Bancroft, is quite different in character, having 

 a small axial angle with high extinction and a much 

 stronger pleochroism in the bluish tints suggestive of 

 arfvedsonite. 



A number of additional thin sections have been pre- 

 pared and in the present paper the results of a further 

 investigation of the optical properties and chemical 

 composition of this second variety of hornblende are 

 presented. 



Hornblende — The mineral occurs in hypidiomorphic 

 grains, which show the usual hornblende cleavages ; it is 

 optically negative, a being the acute bisectrix, but the 

 double refraction is weak. 



It possesses, as has been mentioned, a strong pleo- 

 chroism as follows : 



a = yellowish green, b and c = deep bluish green. 



The absorption is c = b > a. b and c, if not quite equal 

 in absorption, are nearly so, hence sections cut at right 

 angles to the acute bisetrix show but little pleochroism 

 and are nearly isotropic, c lies nearest the vertical axis, 

 but whether toward the acute angle ^3 or on the opposite 

 side cannot be determined as the mineral does not possess 

 a good crystalline form ; it makes with the vertical axis a 

 large angle the extinction amounting to 30°. The plane 

 of the optic axes is tlie clinopinacoid, and there is a 

 strong dispersion — red greater than violet. What drew 

 especial attention to this hornblende in the first instance 

 was the fact that it appeared to be nearly uniaxial. 

 When a section, cut at right angles to the acute bisectrix, 

 is examined between crossed nicols in convergent light, a 

 black cross is seen somewhat thickened toward the inter- 

 section of the arms. This cross, on revolving the stage, 

 divides into two hyperbolas, but these separate from one 

 .another but very little, and appear to separate less than 



