64 ROYAL SOCIETY OF CANADA 



or none of the muscovite " corona '' surrounding them, while others, 

 manifestly imperfect and much corroded, have quite a wide mantle of 

 this so-called alteration product surrounding them. Under the micros- 

 cope the thin sections show the rock to be made up largely of plagioclase. 

 The extinction angles are those characteristic of andesine. This optical 

 determination is subsequently confirmed by its separation with the Jieavy 

 solution, and chemical analysis of the material thus obtained. Corundum 

 is quite plentiful in tlie thin sections examined. 



Scapolite is also present usually in comparatively small amount, 

 the larger individuals occupying the interspaces between the feldspars, 

 with little to suggest its derivation from them. Some smaller grains 

 enclosed in the feldspar may be secondary. The mineral shows the 

 characteristic cleavages with parallel extinction and strong double refrac- 

 tion. Biotite is present in occasional small individuals, showing very 

 marked pleochroism from yellowish to very dark greenish brown and 

 very pronounced absorption. Muscovite occurs both intergrown with 

 the biotite and as zones or mantles of variable width surrounding most 

 of the corundum individuals. It is regarded as a primary constituent 

 formed as aready explained at a time immediately preceding the com- 

 plete solidification of the magma. Occasional grains and imperfect crys- 

 tals of magnetite and a still smaller amount of calcite complete the list 

 of minerals noticed in the thin sections. 



An analysis of this rock from this locality was made by Prof. 

 Norton-Evans, with the following results (under I) : — 



I. IT. 



SiO, 



AIA 



FeA 



FeO 



CaO 



MgO 



K2O 



Na,,0 



CO2 



H,0 



99.66 



Deducting the excess of alumina present as corundum, which was 

 determined by trial, neglecting the loss on ignition (H„0) and deduct- 

 ing the amount of lime (CaO) necessary to form the calcite with the 



