8 9 



later stages of its consolidation. The granite gneiss 

 contains quartz while the granite pegmatite is often quite 

 rich in this mineral. 



The gneissic granite series to the north of the hill, by 

 a gradual decrease in quartz, seems to merge into the 

 corundum-bearing series which overlies it and which form 

 the summit and southern slope of the hill. The corundum - 

 bearing series is a complex of diverse though closely related 

 rock types, differentiation products of one highly alkaline 

 and aluminous magma and representing one phase of 

 plutonic activity. These different rock types usually 

 occur in irregularly sinuous, rather ill-defined bands, the 

 gradual transition from one type to another being a dis- 

 tinctive characteristic of this occurrence. They are 

 usually foliated, the strike being N. 75 E. with a dip to 

 the south at an angle of io°-i2°. These rocks are inter- 

 sected by dykes and masses of syenite pegmatite, which are 

 very frequently parallel to the foliation, merging into the 

 normal or finer grained types. Superimposed upon this 

 corundiferous series and represented by small and infre- 

 quent outcrops protruding through the sand plain to the 

 south of the hill are the crystalline limestones of the 

 Grenville series. As elsewhere through the region, the 

 nepheline bearing rocks are intermediate in position 

 between the crystalline limestones and the granite-gneiss 

 batholith. The following types have been selected as the 

 more important representatives of this igneous complex, 

 although it must be understood that no sharp line exists in 

 nature between these several varieties. 



1. Craigmontite. — This is a very nepheline-rich syenite 

 containing corundum. The rock is prevailingly pinkish in 

 colour, owing to incipient alteration of the nepheline, and 

 is rather coarse in grain. Under the microscope it is seen 

 to be composed of nepheline, oligoclase, muscovite, biotite, 

 calcite, magnetite and corundum. The corundum (as 

 also in raglanite) occurs in well defined crystals, often with 

 characteristic, barrel-shaped outline and so disposed in 

 the rock that their longer axes are often at right angles to 

 the foliation. Smaller individuals viewed with the micro- 

 scope are often irregular shaped, owing to magmatic 

 corrosion, usually surrounded by a corona or mantle of 

 muscovite. An analysis of the rock is given on p. 96. 

 The "mode," or actual mineralogical composition, as 

 calculated from this analysis, is as follows: — - 



