[ADAMS A barlow] ALKALI SYENITES OF EASTERN ONTARIO 39 



the ordinary fundamental gneiss presenting the usual characters. This 

 layer of corundum-bearing rock consists of : — 



(1) Nepheline syenite, in places passing into an almost pure nephe- 

 line rock; 



(2) White or grey alkali syenite; 



(3) Pine grained pinkish alkali syenite like that found in the 

 township of MJethuen and elsewhere, frequently having a streaked 

 character. 



These three rocks often occur interbanded with one another and fre- 

 quently pass into one another. In some places the red syenite can be 

 seen to cut across the nepheline syenite. 



All these rocks locally hold corundum in abundance. In some 

 places bands of a dark micaceous rock conforming to the dip and strike 

 are seen associated with the syenite; these, however, in certain occur- 

 rences can be distinctly seen to consist of altered basic dykes. 



It is also to be noted that the sloping face of the corundum-bearing 

 rock dips to the south beneath the sandy plain at its foot, through which 

 runs a branch of the York river. Along this stream and elsewhere lime- 

 stone is exposed at several places, showing that in all probability these 

 corundum-bearing rocks here as elsewhere lie at the contact of the in- 

 trusive bathyliths of gneiss with the limestones of the Grenville series. 



The corundum workings at Craigmont take the form of five cuts or 

 trenches running up the face of the hill in parallel lines. The mill in 

 which the corundum is concentrated from these syenites was producing 

 305 tons of corundum per month in the summer of 1906. 



Nephelme Syenite. 



Typical specimens of two extreme phases of the nepheline syenite 

 from this locality were selected for analysis, one of these being very 

 rich in nepheline and the other being rich in plagioclase and contain- 

 ing proportionately less nepheline. Both varieties contain corundum 

 in well defined crystals, often having a barrel shape, the long axes of 

 which lie at right angles to the foliation of the nepheline syenite. 



Corundum-bearing nepheline syenite (rich in nepheline). — When 

 examined in thin sections this rock is seen to be composed of nepheline, 

 oligoclase, muscovite, biotite, calcite, magnetite and corundum. Some 

 of the nepheline is comparatively fresh, but most of it has undergone 

 more or less alteration, the resultant products consisting of a brilliantly 

 polarizing aggregate of minute scales of muscovite, developed along cer- 

 tain irregular lines and cracks. The plagioclase is likewise somewhat 



