86 THE ROYAL SOCIETY OF CANADA 



the later injection are quite fresh. In thin section, there appears 

 to be a very narrow gradational phase along the contact, from which 

 it may be deduced that this later magma was injected before the rock 

 had entirely crystallized, in which case these patches might represent 

 portions of the later rock which had found their way into the rock 

 and had in some way displaced the unconsolidated magma and had 

 crystallized in its place. 



Arguments in favour of either viewpoint can be advanced. The 

 fact that the minerals contained in the centre of these patches resemble 

 very closely the minerals which make up the later intrusion and also 

 the fact that these patches do not occur in sections cut at some distance 

 from the injection, make it appear as if these areas were caused by the 

 later intrusion. 



On the other hand, it is difficult to see just how these small 

 amounts of molten material were able to force their way into the 

 earlier rock without affecting this rock at the contact more than they 

 have done. The chemical composition of these patches is very 

 nearly the same as that of the groundmass of the normal type and as 

 the later injection is apparently of the same composition also, the 

 mineralogical similarity of the patches and the pegmatite is not so 

 significant as it appears at first. 



The rock as a whole is thus composed essentially of augite, horn- 

 blende, barkevikite, and biotite in a groundmass of labradorite and 

 glass, and can, therefore, be described as a fourchite. 



Later Injection.— Although, the contact between the normal type 

 and the later intrusion is quite sharply defined in the hand specimen, 

 in thin section it is marked only by an abrupt cessation of the ferro- 

 magnesian phenocrysts with a correspondingly marked increase in 

 the more acid minerals which constitute the groundmass of the 

 normal type, Basic constituents are by no means lacking in this 

 phase, but they are subordinate to the groundmass. These con- 

 stituents are titanite, barkevikite, and biotite, while the ground- 

 mass consists of labradorite, heulandite, and an isotropic base with 

 some calcite which may or may not be secondary. Magnetite is 

 also present. 



Of the basic constituents, titanite and barkevikite occur in the 

 same way as in the normal type, that is, in small idiomorphic crystals. 

 These, however, are much smaller, usually being less than 0.05 mm. 

 in length, and are not nearly so abundant as in the earlier intrusive. 



Augite and hornblende are very rarely present, and their place 

 is taken by long laths of biotite. Some of these laths are 1.0 mm. in 

 length and only 0.03 mm. wide, but they are usually much smaller, 



