194 Proceedings of the Royal Society of Victoria. 

 As types of this series of rocks we may take the following : — 



I. WiIliinif:;ongong Type. 



Neighbourhood of Upper Maceclon. 



(a) Willimigongong Creek, near junction with the Ordo- 

 vicians ; opposite Cheniston (No. 25). 



This i-ock is seen, when examined microscopically, to be 

 porphyritic, consisting of well developed, abundant 

 phenocrysts in a hypidiomorphic, granulitic base. 

 The phenocrysts consist of plagioclase and pyroxene. 



The plagioclase phenocrysts are usually irregular and 

 corroded ; but in some the outline is regular and the 

 angles are sharp. The twinning is coarse and is on 

 the albite type, often combined with the carlsbad 

 type. Zonal growth is commonly shown, but no 

 undulose extinction. Basal cleavage fragments show 

 extinction of 22' on the edge 001/010 ; but in one 

 case the extinction angle was 28" ; hence the plagio- 

 clase phenocrysts include members of the bytownite 

 series. 



The pyroxene phenocrysts are mostly of hypersthene, 

 which shows the typical pleochroisra. The mineral 

 is abundant and often has regular crystal outlines. 

 The crystals are smaller than the larger bytownites. 

 A few corroded crystals of monoclinic pyroxene 

 occur. 



Ilmenite is abundant and occurs in well developed 

 crystals, often as large as the hypersthenes. 



The base of the rock is partly a granulitic mosaic of 

 felspar and quartz and partly a pilotaxitic mixture of 

 the same constituents. The separation of the two 

 minerals in the granulitic patches is difficult, but the 

 presence of quartz is shown by the uniaxial character 

 of some of the granules. The larger plagioclase 

 microliths show simple or multiple twinnings, and 

 have straight extinctions. The majority are oligo- 

 clase. Small biotite crystals occur in zones around 

 some of the larger felspars in the base, and some of 

 it is scattered irregularly through the base. 



