268 iV^. Ji. Junner: 



.The analysis is very similar to many others of Victorian dacites, 

 and calls for no special remarks. 



Specimen No. H45, from near the source of the Acheron river, 

 resembles closely the normal dacites described from the Macedon 

 and Dandenong areas. It consists of phenocrysts of zoned plagio- 

 clase (labradorite Aljj Anj), greatly corroded quartz, biotite, and 

 occasional crystals of hypersthene, in a microcrystalline ground mass 

 composed of quartz, plagioclase, ilmenite and ( ?) potash felspar. 

 Zircorn and apatite are accessories. Occasional phenocrysts of 

 untwinned felspar may be orthoclase. The biotite phenocrysts have 

 been attacked by the solidifying magma, and as a result they are 

 often fretted and contain internal cavities filled with quartz and 

 felspar. The hypersthene is decidedly pleochroic and includes 

 ilmenite, felspar and biotite. The absorption scheme is as 

 follows : — 



X light brown with a trace of pink. 

 Y brownish yellow. 

 Z light green. 



Sections cut parallel to 010 give a biaxial figure, having a mode- 

 rately large axial angle. Ilmenite occurs as idiomorphic crystals 

 in the ground mass of the rock, and inclnde<l in biotite and 

 hypersthene. 



Of particular interest is the occurrence in this section of two or 

 three irregular shaped phenocrysts of cordierite sliowing the charac- 

 teristic trilling. 



One section in particular (vide plate I., figure 4) shows the trill- 

 ing very finely. The cordierite has a refractive index greater than 

 the balsam (/x= 1.532), and is distinctly biaxial. It is not greatly 

 altered, but contains numerous linearly arranged inclusions in 

 certain cases. 



Specimen No. H44, Blacks' Spur road, is typical of the hypers- 

 thene free dacites. It consists of phenocrysts of colourless quartz, 

 beautifully zoned plagioclase (chiefly labradorite Abj Auj), and 

 deep brown six-sided biotite in a cryptocrystalline ground mass. 

 Accessory and secondary minerals are garnet, tourmaline, zircon, 

 pyrrhotite, ilmenite or magnetite (?) opal, chlorite, sericite, 

 quartz and sillimanite. Plagioclase phenocrysts are numerous. 

 They are usually well zoned, and show both albite and pericline twin 

 lamellae. Maximum symmetrical extinction angles of about 27o 

 indicate labradorite near Abj An^. Biotite is greatly chloritised, 

 noticeablv so close to the garnet crystals. It is concentrated round 



