GEOLOGY AND PETROLOGY OF EXOGGERA GRANITE. 147 



Microscopic. — Holocrystalline, porphyritic, the propor- 

 tion of phenocrysts to ground mass corresponds to the 

 "dopatic fabric" of Iddings. The ground mass is finely 

 microerystalline, and made up almost entirely of quartz 

 and felspar, often intergrown. Quartz occurs in two dis- 

 tinct generations. The phenocrysts of this mineral are all 

 rounded and embayed. They vary from 2-5 mm. in dia- 

 meter, are more or less fractured, bear signs of corrosion, 

 and show very definite "reaction rims" just outside the 

 crystals themselves. Comparatively large rounded inclu- 

 sions (or cross-sections of ver}^ deep embayments) made up 

 of quartz-felspar aggregates similar to that of the ground 

 mass is a feature of these quartz phenocrysts. The felspar 

 phenocrysts are considerably altered to muscovite and other 

 micaceous products, with the result that some of them are 

 indeterminate, but in some cases enough remains of the 

 original minerals to show that both Orthoclase and Plagio- 

 clase are present. The latter shows traces of Albite twin- 

 ning and appears, from measurements of extinction angles, 

 to be an intermediate andesine. Zoning is fairly definite 

 in some of these phenocrysts. Inclusions of apatite were 

 observed. Ferro-magnesian minerals are but poorly 

 developed, but fragments of Biotite largely altered into 

 colourless secondary minerals and magnetite were observed. 

 Magnetite occurs in irregular patches and idiomorphie 

 crystals, and seems to be pseudoraorphus after Pyrites^ 

 as some remnants of this mineral are present in the interior 

 of the magnetite crystals. Further alteration has resulted 

 in patches of Limonite. Other minerals present in small 

 amount are Apatite and Zircon. 



Name. — Quartz Porphyry (Granodiorite Porphyry), 



{D.4) 163. 



Dyke on road. Constitution Hill, Taylor Range. (See 

 Mierophotograph Plate III., No. 12.) 



Megascopic. — A holocrystalline porphyritic rock made 

 up of numerous phenocrysts of brown felspars, showing 

 zoning and fewer smaller phenocrysts of quartz and a dark 

 lerro-magnesian mineral set in a fine brown ground mass. 



Microscopic. — Rock very decomposed. Holocrystalline 

 porphyritic (sempatic fabric of Iddings). The ground 

 mass is a very fine-grained confused aggregate of felspathie 



