280 N. R. Junner : 



sericite and pyrites, much of which has been oxidised to limonite 

 and hematite. One large, simply twinned, phenocryst of orthoclase 

 remains. It is partly replaced by sericite and granular chloritfe 

 and a little carbonate. 



Other idiomorphic felspar crystals are seen to be entirely re- 

 placed by carbonates, chlorite (pennine), and sericite. The quartz 

 phenocrysts still remain clear, although greatly corroded and par- 

 tially sericitised in places. Small lenticular vesicles, infilled with 

 carbonates and quartz, are occasionally present. Veinlets of car- 

 bonates, chlorite and a little quartz traverse the rock. Carbonates 

 are abundant throughout the ground mass of the rock, especially 

 replacing felspar laths. The alteration of the tuffs and agglome 

 rates, that occur near by here, has been previously described. 



Near the Echo tunnel, in the Maroondah aqueduct, and also near 

 the Maroondah weir, carbonation, sericitisation and pyritisation 

 have taken place in the basal volcanic rocks underlying the normal 

 dacites. 



Secondary tourmaline and biotite have been mentioned as forming 

 in the rhyolites near Archer's Look-out, and in the tuffs near here 

 fluorite is occasionally present. 



This alteration has been ascribed by the writer to the action of 

 vapours released from the neighbouring granodiorite. 



5.— Field Relations and Origin of the Igneous Rock. 



A. — Eelationshijy of the gronodiorift to the sedimentary and 

 volcanic rocks. 



Wlierever seen the granodiorite is intrusive into the Silurian 

 sediments, and has altered the latter for some distance from th» 

 junction. Xenoliths of hornfels are also common in the grano- 

 diorite near the contact. 



The relationship of the plutonic rock to the igneous rocks, how- 

 ever, is not so evident. Selwyn stated that hornblende granite 

 passed into porphyry near Mount Monda, and Ferguson also 

 believed that there was a gradual passage between the plutonic and 

 volcanic rocks near Gembrook.i However, Professor Skeata has 

 Bhown clearly that near Gembrook and Macedon, the granodiorite 

 is intrusive into the normal dacite, and that the latter rock has 

 been rendered gneissic in places near the contact. Whenever the 

 two rocks come into relation in the Healesville area marked con- 



1 " Notes on certain Geological Features of the Parishes of Geniltrook North and Naugana." 

 Prog. Rept. Vict. Geol. Surv., No. 8, 1894. 



