22 



Sodalite is almost entirely absent from the nepheline 

 syenites of this area. It does occur, however, in the 

 highest hill southwest of Coldwell, on Pic island and about 

 two miles (3-2 km.) north of mile post 78. 



Hornblende is much the most abundant ferromagnesian 

 constituent. There is a green and a brown variety. 

 The optical properties of the brown hornblende suggest 

 barkevikite, although no confirmatory chemical analysis 

 was undertaken. The colouring of the individuals is 

 by no means uniform, but pale interiors with deeply 

 coloured borders are the rule; often the crystals have a 

 spotted appearance. Pleochroism is very marked, vary- 

 ing from greenish yellow to chestnut brown in the brown 

 variety, and in the green hornblende from straw yellow 

 to deep green. Poikilitic structure mainly with feldspar 

 is common. Pyroxene, ranging in composition from 

 deep green aegirine-augite to pale coloured diopside, and 

 often surrounded by a border of hornblende is usually 

 present even in specimens that are rich in hornblende. 

 The pleochroism of the aegirine-augite is very strong and 

 from yellow to grass green. Aegirine-augite is especially 

 characteristic of varieties rich in nepheline. Frequently 

 it forms a rim around the paler coloured diopside. Biotite 

 is by no means a common constituent, although in one 

 locality (west part of Coldwell peninsula) it is the chief 

 ferromagnesian mineral. Magnetite as an inclusion is 

 always present, and comparatively large apatite crystals 

 are common. Occasionally muscovite, sphene, pyrite and 

 purple fluorite are noticeable. 



Essexite, Olivine Gabbro and Picrite. — The basic 

 rock of the Coldwell massif are undoubtedly the oldest of 

 the series. They are very variable in composition. The 

 more common type seen in the neighborhood of Coldwell 

 is a dark grey rock of medium texture with gleaming 

 crystals of biotite. The dark coloured constituents 

 represent more than three-fourths of the whole rock mass. 

 Thin sections show augite, olivine, biotite, hornblende, 

 labradorite, some orthoclase, occasionally nepheline and 

 much magnetite with apatite as the chief accessory con- 

 stituent. 



Most of the dykes of the region are small, ranging from 

 a couple of inches (5 cm.) to four feet (i-2m) in width. 

 They are usually of a slate grey colour and very fine grained. 

 Many of them are intermediate in composition between 



