ADAMS A BARLOW] ALKALI SYENITES OF EASTERN ONTARIO 41 



The hornblende is green in colour, the pleochroism and absorption 

 being as follows: — d = pale greenish yellow, b and c = very deep 

 green. The absorption is C = b > a. The maximum extinction ob- 

 served in the sections of the rock was 19°. It is an alkali hornblende, 

 containing less iron than hastingsite, but like it, as shown by the cal- 

 culation of the analysis of the rock, belonging to the division of the 

 syntagmatites. 



The plagioclase is present only in very small amount and is in some 

 cases untwinned, while in other cases it shows a faint, polysynthetic 

 twinning. In thin sections it bears a very close resemblance to the 

 nepheline, and when untwinned it is difficult in all cases to distinguish 

 the two minerals. When a section is treated with acid and etched, how- 

 ever, the plagioclase is seen to occur in individuals of a more or less 

 rounded form, or with curving outlines, lying between the nepheline 

 grains or enclosed in the latter. The feldspar isolated from another 

 variety of the rock in the same occurrence was found to be albite, and 

 this feldspar has, therefore, been taken as albite in calculating the mode 

 of the rock. 



The amount of cancrinite present varies very considerably in dif- 

 ferent specimens of the rock. In the specimen analyzed about 5 per 

 cent was found. In other specimens more is found, although in no case 

 is it very abundant. It is clear and colourless, but is at once distin- 

 guished from the nepheline when examined between crossed niçois by its 

 much higher polarization colours, which in thin sections frequently rise 

 to a blue of the second order. It is free from interpositions, and in con- 

 vergent light is seen to be uniaxial and negative. It also shows a slight 

 but distinct dispersion of the bisectrices, giving a brownish and a bluish 

 tint on either side of the position of maximum extinction. When separ- 

 ated by Thoulet's solution, the mineral was found to have a specific 

 gravity between 2.44 and 2.48, and to be readily decomposed when heated 

 with dilute hydrochloric acid, with the evolution of carbonic dioxide, and 

 with subsequent gelatinization. The cancrinite occurs in the nepheline 

 in the form of narrow strings or more rarely in little bunches of grains. 

 These usually follow the course of minute cracks or cleavage lines, but 

 also are frequently seen to follow the boundaries of individual grains of 

 nepheline on their contact with grains or other minerals. Thus be- 

 tween crossed niçois they appear as a brilliant edging about hornblende 

 individuals or about calcite inclusions in the nepheline, the small pris- 

 matic individuals of cancrinite l)eing arranged with their longer axes 

 at right angles to the contact or to the course of the crack, as the case 



