>o6 



Mil. W. A. ilJLCHAEDSON : A MICllOMEXEIC [voL lxxix, 



characteristically perhaps to white mica. Often Avhole areas of 

 orthoclase are replaced by scaly aggregates of muscovite. 



Orthoclase always forms the largest crystals present, and it is 

 also the dominant mineral in the general average, amounting to 

 31o per cent. In the ma]) (fig. 1) it is seen to group itself in a 

 regular manner on application of the method already used in the 

 case of quartz. Exceptions over half of 1 per cent, are indeed 

 someAvhat more numerous, but in no case does the variation exceed 

 the probable limit of sampling. In general, the belts for quartz 

 and orthoclase coincide : the negative quartz-areas being also 

 positive orthoclase-areas. There is one exception, for the western 

 negatiA r e orthoclase zone is extended over the whole of the St. 

 Stephen's area, and this area is, therefore, a zone Avhere both 

 quartz and orthoclase fall below the general average. 



Fig A. Orthocl/isc. 



Wcrocline 



Symbols as in jig 2. 



Fig. 5. Pl/igiocuse. 



Symbols <as in fig 2. 



Plagioclase. — Complete examination of the slices reveals the 

 presence of two rather sharply contrasted types of plagioclase. 

 One of these is confined to the Luxullyan area. It occurs in large 

 crystals, strongly zoned, and centrally altered. The refractive 

 index of all the plagioclase is less than that of the e-ray of quartz ; 

 but in some zoned crystals, situated at the edge of a slice, the 

 centres had refractive indices higher than Canada balsam. Where 

 good sections parallel to (010) were present, the extinction at the 

 centre Avas either nearly or a small negative angle, but increased 

 to a large positive value at the extreme outside zone. The 

 maximum extinction-angles of the albite lamella? confirmed these 

 results, indicating that the centres have a composition nearly that 



