56 ROYAL SOCIETY OF CANADA 



trate the feldspars. The corundum occurs as individuals about half an 

 inch long which are especiall}-- abundant in streaks following the strike 

 of the rock. In the thin sections it is invariably seen to lie embedded 

 in the muscovite, in the form of very irregular-shaped grains, rounded 

 or corroded in appearance. Each corundum grain has a single large in- 

 dividual or muscovite enclosing it, or occasionally there are two or more 

 smaller grains of corundum within the single muscovite individual which 

 look as if they had originally formed part of a larger indi\idual. The 

 muscovite outline often in a general way conforms to that of the corun- 

 dum core which it contains, as shown in figure. 



The corundum has the usual high index of refraction and a double 

 refraction, which usually gives yellows and reds of the first order, but in 

 pome cases, even in good sections, gives a blue of the second order. As 

 has been mentioned elsewhere, the colours which the mineral gives in 

 thin sections are usually higher than would be expected, on account of 

 the fact that, owing to its hardness, it always remains thicker in the 

 finished section, than the minerals with which it is associated and can 

 be seen distinctly standing up from the surface, if the slide is examined 

 before the cover glass is placed upon it. It often contains minute opaque 

 black inclusions, which, in the basal sections, appear as irregularly round- 

 ed plates, but in sections parallel to the vertical axis of the crystal as rods. 

 In the last mentioned sections the extinction of the corundum is parallel 

 toi the direction of the rods, so that the latter are inlaid parallel to the 

 base. It is impossible to determine from the thin sections whether 

 there is a constant orientation in the case of the corundum and mus- 

 covite. In one case, however, the basal plane of the corundum, marked 

 by the inclusions, lay in the direction of the vertical axis of the enclos- 

 ing muscovite individual. In addition to the inclusions just mentioned, 

 the corundum occasionally encloses minute grains of a deep green iso- 

 tropic mineral which have the characters of spinel. And in one or two 

 cases, larger grains of this mineral are seen associated with the corundum. 



Along the south-eastern, side of the Blue Mountain, the reddish 

 syenite crosses an arm o!: Kasha[)og Lake, and in most places holds a small 

 amount of a mineral which is now completely changed to a gieseckite-like 

 alteration product. This is yellow on the weathered surface, but in 

 fresh surfaces of the rock it has a pale green or a pink colour. It has 

 a hardness of four and is quite dull and lustreless. A rough quantita- 

 tive analysis of a specimen by Prof. 0. E. Leroy, M.Sc, showed it to pos- 

 sess the following chemical composition: — 



SiOa, 45 p.c; AlgO., (with a little FeaO,), 38 p.c; MgO, 3.6 p.c; 

 CaO, 8.2 p.c. ; H^O, 7.8 p.c ; K^^, not determined. 



