321 



In some sections it would appear that, as an accompani- 

 ment of cracking, there has been a rotation of sectors of the 

 grain, for the grain is not optically continuous throughout. 

 Along these cracks the quartz may show higher polarization 

 colours. 



, Quartz is also present in the slides in allotrimorphic 

 grains, or as a constituent of a graphic intergrowth with 

 microcline. This intergrowth may develop around the large 

 quartz or microcline crystals, and is obviously of later 

 crystallization. 



Microcline is present in subidiomorphic crystals, which 

 may show inclusions of biotite, plagioclase, and quartz. The 

 microcline twinning after the albite and pericline laws is very 

 finely developed, and the lamellae are often seen to overlap. 

 In sections parallel to (010) pericline twinning may be absent 

 or submicroseopically developed ; where clearly developed, 

 however, it cuts the trace of the (001) cleavage at an angle of 

 74°-76°. 



Extinction on (010) sections has a maximum value of 6°-7° 

 from the basal cleavage. Some sections show a very fine 

 perthitic intergrowth with plagioclase. The layers traverse 

 the microcline, and are optically continuous ; optically they 

 have the properties of albite. This is the typical microcline 

 microperthite structure. 



Plagioclase occurs in more or less tabular crystals of well- 

 developed form, and shows the characteristic albite lamellae. 

 Zoning is characteristic. The refractive index is > C. 

 Balsam, and most sections show R.I. <C quartz. 



In zoned sections showing no multiple twinning the 

 extinction from the (001) cleavage read as maxima: — 

 Outer zone ... ... ... ... +9° 



Intermediate zone ... ... ... 10° 



Central zone ... ... ... ... — 9° 



This corresponds to a range from Oligoclase (Ab 4 AnJ to 

 Andesine (Ab 3 An 2 ). 



Such zoned sections (010) in convergent light show the 

 emergence of a bisectrix to be practically normal to the 

 section. This is the obtuse bisectrix, and the negative 

 birefringence is clear. 



The felspar is an Oligoclase-Andesine, the average com- 

 position being nearer the oligoclase end. 



Some sections of the granite show the presence of a 

 more acid plagioclase than the above. This has the pro- 

 perties of oligoclase albite. It represents a later stage of 

 crystallization, but is of minor development. 



Biotite. — This mineral is developed in clusters of flakes 

 of elongated section showing the strong basal cleavage. The 



L 



