210 Adams and Harrington — Alkali Hornblende and 



Art. XXII. — On a new Alkali Hornblende and a titaniferous 

 Andradite from the NepJieline- Syenite of Dungannon, 

 Hastings County, Ontario / by Frank D. Adams and B. 

 J. Harrington, McGill College, Montreal. 



In a paper which appeared in the number of this Journal 

 for July, 1894, the discovery of a large area of nepheline- 

 syenite in the township of Dungannon, in the Province of 

 Ontario, was announced and the geological relations and min- 

 eralogical characters of the mass briefly described. 



One of the many peculiarities of this rock is the absence 

 from it of the mineral pyroxene, which is usually the chief 

 iron-magnesia constituent in rocks of this class, its place being 

 taken by hornblende and mica, but even these minerals are 

 present in comparatively small amount. Of the hornblende 

 two varieties, occurring in different parts of the mass, were 

 distinguished. The first, from near the York river, has a large 

 axial angle with strong pleochroism in tints varying from pale 

 yellow to deep green, and although containing a considerable 

 amount of soda, probably approaches common green hornblende 

 in composition. The second variety, which occurs in a series 

 of exposures about two miles to the east of the village of Ban- 

 croft, is quite different in character, having a small axial angle 

 with high extinction and a much stronger pleochroism in the 

 bluish tints suggestive of arf vedsonite. 



A number of additional thin sections have been prepared 

 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 pleochroism as 

 follows : 



a = yellowish green. 6 and c = deep bluish green. 



The absorption is c = 6 > a . 6 and c, if not quite equal in 

 absorption, are nearly so, hence sections cut at right angles to 

 the acute bisectrix show but little pleochroism and are nearly 

 isotropic, c lies nearest the vertical axis, but whether toward 

 the acute angle /9 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 the clinopina- 



