[ADAMS A BARLOW] ALKALI SYENITES OF EASTERN ONTARIO 19 



Microcline in rather unusual in the nepheline S3'enite and much of 

 it presents a somewhat indefinite and distorted mesh which is not dis- 

 tinctive. Much of the microperthite has the very fine and interrupted 

 twinning lamellie characteristic of anorthoclase with which it is prob- 

 ably identical. Most of what has been considered to be orthoclase also 

 shows quite a perceptible intergrowth of other feldspars, the potash feld- 

 spar being, however, predominant. A white and reddish mineral, which 

 was proved on analysis by Dr. Harrington to be orthoclase, fills certain 

 little cracks traversing the sodalite on lot 25, concession XIV of Dun- 

 gannon. It is mostly dull, but in places shows cleavage surfaces with a 

 pearly lustre. The reddish portions probably owe their colour to the 

 decomposition of pyrite, occasional grains of which still remain. The 

 mineral is regarded as secondary. The specific gravity at 18° C. was 

 fcnnd to be 2.555, and analysis gave the following percentage composi- 

 tion : — 



SiO^ 63.00 



AI263 18.93 



Fel^^O, .59 



Cab .08 



MgO .09 



K^O 12.08 



Na^O 3.67 



Loss on ignition 1.00 



99.44 



An analysis of the microperthite of the corundum-bearing syenite 

 pegmatite of Craigmont in the to^vnship of Eaglan is given on page 71. 



ScapoHte. — This mineral is a frequent and often abundant con- 

 stituent of the nepheline syenite, occurring in clear colourless grains 

 which meet the accompanying nepheline and feldspar grains with a per- 

 fectly 'sharp outline, there being no evidence that the mineral is the re- 

 sult of alteration or weathering. The double refraction is much stronger 

 than in the nepheline and feldspars, the interference colours seen in the 

 thin sections being red, blue, and yellow. In this it resembles cancrinite, 

 from which it can generally be distinguished by its habit, the cancrinite 

 usually filling in cracks and the small interspaces left after the cr3'stalli- 

 zation of the other constituents. 



Biotite. This is the chief iron-magnesia constituent of these rocks, 

 but it is usually present in subordinate amount. It occurs in the usual 

 small scales and plates, some of which exhibit good crystal boundaries. 

 The hand specimens show an almost black mica which has usually a dis- 



