GEOLOGY AND PETROLOGY OF ENOGGERA GRANITE. 141 



association with the ferro-magnesiau minerals, both as well- 

 shaped individual crj^stals and somewhat irregular clusters. 

 Magnetite and Apatite are also present in small amount. ~ ' 



Order of consolidation. — Normal. 



Name. — Biotite Granodiorite. 



It is difficult to reconcile this rock mineralogically with 

 either the Pink Phase or the Grey Phase. The abundance 

 of quartz and the great excess of biotite over hornblende 

 are points in comon witli the former, while the absence of 

 pink orthoclase. the nature of the plagioelases, and the 

 abundant pyrites are more like the latter. 



A comparison of the soda and lime content in analyses 

 E. 1, E. 2, and E. 4 would lead us to expect in this rock 

 plagioelases intermediate in character between those of the 

 Pink and Grey Phases, and the calculated norms also pre- 

 dict such a result. However, measurements of the extinc- 

 tion analyses of suitable sections of felspars (JMichel-Levy's 

 method) in many specimens, actually give the following 

 result : — 



Pink Phase — Phenocrysts, 5 degrees; ground mass, 



10 degrees. 

 Grey Phase — Phenocrysts, 9 degrees ; ground mass, 



15 degrees. 

 (?) Hybrid — ^Mostly 14 degrees; a few" 9 degrees; 

 some zoned. 



The plagioelases in this rock, and particularly the 

 more basic, frequently show corrosion, which suggests that 

 they are xenoerysts from the Grey Phase. 



This rock has, in addition, other features of special 

 interest. The most remarkal)le of these is the abundance 

 of primary pyrites, which occurs as individual crystals, 

 sometimes of comparatively large size, and clusters of 

 crystals scattered sporadically through the rock. Pyrites 

 occurs, too, in the "vughs" to be described below. The 

 pyrites is present in such amount and oxidises so rapidly 

 on exposure to the air, producing dirty brown stains on the 

 face of the rock, that the value of the rock as a building 

 stone is very greatly lowered. Indeed it has of late been 

 superseded by other granites brought from greater dis- 

 tances. In describing this rock Professor Richards states: 



