GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 23 



The normal syenite always contains quartz, the average amount 

 being 12 to 20 per cent in the sUdes examined, and when it is greater 

 than 20 per cent the rock is no longer regarded as normal, but rather 

 granitic syenite to granite. Microperthite almost invariably makes 

 up from one-third to two-thirds of the rock. Orthoclase is much 

 less constantly present in variable amounts. Plagioclase (albite 

 to andesine) never fails in amounts up to 35 per cent. When the 

 rock is relatively rich in plagioclase it is a monzonite, though here, 

 for convenience, classed with the normal syenite. Common horn- 

 blende with yellowish green to dark green pleochroism usually 

 occurs in variable quantities up to 25 per cent, while green mono- 

 clinic pyroxene (usually diallage) frequently makes up 2 to 20 

 per cent of the rock. Enstatite was observed in only one slide. 

 Both hornblende and pyroxene sometimes occur in the same rock. 

 IMagnetite (ilmenite), apatite and zircon in small quantities rarely 

 fail. Biotite, garnet, zoisite and titanite are more sporadically 

 present in small amounts. 



The following table will serve to show the mineralogical varia- 

 tions of thin sections from carefully selected specimens of the 

 normal syenite. 



