Metamorphic and Plutonic Rocks at Omeo. 123 



An examination of a thin slice of this sample by the 

 microscope shows that it is composed of two kinds of mica, 

 two felspars and quartz, and that, therefore, in accordance 

 with the classification of Rosenbusch, which I follow, it is a 

 granite. 



The crystals of orthoclase are larger than those of the 

 accompanying plagioclase. They are also more converted 

 into mica. Instances occur of intergrowth with quartz. 

 Various stages of alteration can be followed out in this slice, 

 from a conversion of the edge of the crystal more or less 

 into muscovite, to the complete conversion into that mica. 

 Intermediate stages show portions of felspar still intact. In 

 one eroded crystal, the section of which was approximately 

 parallel with OP (001), I observed a number of angular 

 fragments of plagioclase. These had the appearance of being 

 parts of a former whole, and if so, would indicate more than 

 one generation of triclinic felspars. For the plagioclase 

 crj^stals in this rock, which are subordinate in number to 

 those of orthoclase, are better formed, are smaller, and are 

 less altered, and may, therefore, be considered as formed at 

 a later period than the orthoclase. The obscuration of these 

 triclinic felspars indicate oligoclase rather than albite. 



In one or two instances I observed the environment of a 

 simple crystal by a margin which was twinned. Muscovite 

 mica is in a few large crystals which appear to be of an. 

 older generation than the remaining small flakes, or aggre- 

 gates of flakes, which are certainly alteration products. Yet 

 even some of the larger flakes of muscovite extend into the 

 felspars. 



The magnesia mica is brown in colour, and distinctly 

 pleochroic. It was one of the earlier-formed minerals, but 

 is present only in small amount. The crystals are in places 

 crushed and broken, and the isolated flakes are tattered or 

 torn across, and in the latter case I observed, where the 

 fracture was filled in with minute flakes of muscovite. This 

 mica has in some cases dark pleochroic halos surrounding 

 microliths such as I have betore described. 



An inspection of the two analyses, Nos. 8 and 9, shows a 

 great similarity of composition, and this, together with the 

 mineral composition of the two rocks and their proximity to 

 each other, strongly suggests the conclusion to which I have 

 before referred, that the crystalline granular rocks shown 

 between the letters (x) and (x') in the section may, per- 

 haps, be unusual forms, in which the intrusive rocks have 



