﻿On a Neiv Alkali Hornblende. 79 



they really do, on account of the fact that the low double 

 refraction and deep color of these sections causes the 

 hyperbolas to be ill-defined, while the whole field is very 

 dark. The dispersion, however, makes itself evident in 

 the varying colors on the sides of the hyperbolas. When, 

 however, a gypsum plate giving a red of the first order is 

 inserted above the objective the hyperbolas become a little 

 better defined, although still not sufficiently definite to 

 allow the axial angle to be accurately measured. The 

 axial angle is found to be over 30^, possibly as much 

 as 45°, which, however, is still very small for hornblende, 

 being about one-half the usual value. Our thanks are 

 due to Professor Eosenbusch for his assistance in working 

 out these optical relations. 



On examining a large series of thin sections of nephe- 

 line syenites representing most of the important occur- 

 rences hitherto discovered, only two rocks were found 

 which contain a hornblende at all similar to that above 

 described. The first of these is the nepheline syenite 

 from the Corporation Quarry at Montreal, in which 

 hornblende with the same small axial angle, low double 

 refraction, intense color and pleochroism, large extinction 

 angle and high specific gravity, occurs intergrown with 

 the augite. The second is the hornblende described by 

 Hackman under the name of arfvedsonite and which 

 occurs intergrown with aegerine in the nepheline-syenite 

 from Umptek in the Kola peninsula^ This mineral, 

 however, differs from typical arfvedsonite in having an 

 extinction of about 40° as well as in several other 

 important respects. It possesses, moreover, a very small 

 axial angle, although this fact is not noted by Hackman, 

 while in true arfvedsonite the axial angle is very large. 

 This Kola hornblende is much lighter in color than 

 the hornblende from either of the above mentioned 

 Canadian localities. 



1 " Petrographische Beschreibung des Nephelinsyenites vom Umptek," von 

 Victor Hackman. Kuopio, 1894, p. 14. 



