Igneous Rocks. 277 



Using H. C. Richards' analysis of the biotite from the dacite 

 near Mount Dandenong, with slight adjustment of the relative pro- 

 portions of FeO and MgO to fit the percentages of these constituents 

 in the rock, we can determine approximately the mineral composi- 

 tion of the rock. Thus : — 



Total 99.85 



Much of the felspar is changed to kaolin, so that this mineral will 

 account for a certain amount of the surplus AUOg and H^O. The 

 percentages of iron oxides, magnesia, and lime are all low, and 

 there is a corresponding small percentage of lime-bearing felspar 

 and femic minerals present in the rock. Microscopical examination 

 shows that the felspar phenocrysts are orthoclase, containing micro- 

 perthitic inclusions of soda felspar, accounting for the moderately 

 high percentage of soda in the rock. 



Microscojyical relations. — A section of the rock analysed showed 

 phenocrysts of quartz and kaolinised orthoclase in a microcrystal- 

 line ground mass consisting of quartz, orthoclase and biotite. 

 Zircon and apatite are accessories. Secondary minerals include 

 chlorite, kaolin, hematite, sericite and a little biotite, tourmaline 

 and brown opal. A micrometric analysis of the rock showed that 

 the ratio of ground mass to phenocrysts was approximately 1.1 :1. 

 The orthoclase phenocrysts are often idiomorphic and are sometimes 

 rounded by corrosion of the ground mass. They occasionally show 

 the characteristic cross fracture of sanidine. Carlsbad twinning is 

 not uncommon. The phenocrysts of quartz are occasionally hex- 

 agonal in section, but are more often rounded and embayed by 

 the ground mass. Numerous cracks and abundant glass inclusions 

 are present in the quartz. Biotite occurs sparingly as phenocrysts, 

 but is abundant in aggregates of minute flakes, in the ground mass 

 of the rock. 



Section No. H50, Archer's Look-out, shows phenocrysts of 

 quartz, sanidine, and altered biotite in a microcrystalline ground 

 mass consisting of the same minerals, together with tourmaline. 



