8i 



list of minerals found in the thin sections of this rock. 

 The corundum is often very abundant. Some of the 

 individuals show a nearly perfect crystallographic develop- 

 ment, but the mineral as a general rule occurs as imperfect 

 crystals or as irregular grains. It frequently shows very 

 distinctly the parting planes parallel to the faces of the 

 rhombohedron and the base. The colour varies even 

 within the same individuals; the mineral has usually a 

 distinct and often pronounced sapphire blue colour, but 

 the corundum is sometimes white or brown in colour. 

 The individuals of corundum frequently have a distinct 

 corona of muscovite surrounding them, although this is 

 not invariably present. The corundum is by no means 

 uniformly distributed through the rock. A large portion 

 of the rock is completely free from this mineral, while other 

 areas, rather ill-defined in shape, contain a very high per- 

 centage of it. In outcrops exposed to the weather the 

 corundum becomes very conspicuous, weathering out as it 

 does in pronounced relief from the surface of the rock. 

 In some places characteristic barrel-shaped hexagonal 

 crystals, several inches in length, may be seen on these 

 weathered surfaces. In the freshly broken rock, on the 

 3ther hand, the corundum is detected only with difficulty, 

 unless it assumes the usual bluish colour which enables 

 it to be readily distinguished from the other constituent 

 minerals of the rock. 



In the exposures these rocks are seen to be cut through 

 in various directions by dykes of fresh red pegmatite 

 composed chiefly of red feldspar (orthoclase) , microperthite 

 and quartz, with a little hornblende. In some places 

 imperfect crystals of this last-named mineral can be found 

 which measure from 4 to 6 inches (10 to 15 cm.) in diameter. 

 These dykes are evidently differentiation forms of a red 

 syenite, which occurs in considerable volume immediately 

 to the south of these exposures, and which is probably a 

 somewhat quartzose type of the normal red variety of 

 alkali syenite. 



An analysis of this rock was made by Prof. Norton- 

 Evans, with the following results, under I. — 



35064—6 



