20 ROYAL SOCIETY OF CANADA 



tinctly gi-eenish colour in transmitted light. The thin sections under 

 the microscope have a very strong pleochroism from pale greenish yellow 

 to very deep almost opaque greenish brown. Basal sections are dis- 

 tinctly bisaxial, but with a very small axial angle. In some of the 

 coarser phases of the rock, as well as occupying certain miarolitic cavities, 

 as in the north-west corner of Faraday, some of these biotite cr}'^stals 

 are veiy large and well formed. An analysis of the biotite (lepido- 

 melane) occurring in the nepheliue syenite on lot 16, range IX of the 

 township of Monmouth is given on page 35. 



Hornblende. — Although less common than the biotite this mineral 

 is occasionally the prevailing ferro-magnesian constituent. 



Dr. Adams, in his first description of the Dungannon occurrences, 

 made mention of the fact that two distinct varieties of hornl)lende, both . 

 green in colour, could often be distinguished in the same hand specimens, 

 and this has been found to be true of outcrops in the south-western 

 and north-eastern extensions of the band. The individuals are, as a rule, 

 much larger than those of the biotite and present a nearer approach to 

 perfection of crystallographic outline. The prismatic cleavages at 

 angles of about 124° are often well seen. 



The first variety has a large axial angle, with strong pleochroism 

 in tints varying from pale yellow to deep green. Before the blowpipe 

 it fuses with intumescence to a black glass, giving at thei same time a 

 strong soda flame. This variety probably contains a considerable quan- 

 tity of soda, but approaches common hornblende in composition. 



The second variety, the type specimen of which was obtained from 

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

 croft in Dungannon township, has a small axial angle with a high ex- 

 tinction angle and a much stronger pleochroism in the bluish tints sug- 

 gestive of arfvedsonite. 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 pos- 

 sesses very strong pleochroism as follows: a= yellowish green, b and c 

 = deep bhiish green. The absorption is c = b > a, b and 

 c, if not quite ecjual, 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 I) or 

 on the opposite side cannot be determined, as the inineral does not pos- 

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

 angle, the extinction amounting to 30*^. The plane of the optic axis 

 is the clinopinacoid, and there is a strong dispersion — red greater than 

 violet. When a section, cut at right angles to the acute bisectrix, is ex- 



