Tg neons Rocks. 281 



tact alteration of the dacite is usually absent. However, near 

 Nyora the dacite contiguous with the granodiorite has been ren- 

 dered gneissic, and in the ranges to the west of Narbethong a 

 gneissic dacite was seen by the writer near the contact with grano- 

 diorite. Clearly, in the Healesville district as elsewhere in Vic- 

 toria, the intrusion of the granitic rock took place after the 

 extrusion of the dacite. 



Near Malory's falls rhyolite is apparently superposed on grano- 

 diorite. No alteration of the rhyolite near the contact is visible in 

 hand specimens. However, thin sections of the rhyolite show the 

 development of secondary biotite and blue tourmaline, and the 

 writer attributes this alteration to the action of vapours given off 

 from the cooling plutonic rock. Fluorite has also been formed in 

 the tuffs near Malory's falls. More certain evidence of the subse- 

 quent intrusion of the granodiorite, e.g., apophyses or dykes from 

 it passing into the rhyolites was not obtainable. In tlie sequel, it 

 will be show^n that the dacite is younger than the rhyolite, adding 

 further support to the sequence suggested above. 



Secondary biotite has also been formed in the obscure flinty 

 rocks, probably rhyolites. from near Mount St. Leonard. Grano- 

 diorite outcrops on the east Vjank of Meyer's creek, close by here, and 

 it is probable that the alteration in the rhyolite was brought about 

 ■owing to the intrusion of the plutonic rock. 



B. — Relatio/isli/ps of the volcanic rockt< to one another. 



That the pyroclastic rocks were associated with the rhyolitic out- 

 burst, and were previous to the dacitic activity is certain from 

 the following facts : — 



1. Everywhere in the area, the tuffs occur marginal to the 

 dacites, and clearly underlie them at certain points. 



2. Near Wade's Look-out and Badger weir, quartz porphyry 

 .and rhyolite pebbles and lapilli, from the agglomerates and tuffs, 

 .are often seen to have been caught up in the lowest layers of the 

 dacite. 



3. The tuffs throughout the area are composed of rhyolitic 

 debris, and andesitic or dacitic ejectmenta are generally absent 

 from them. 



A traverse up the Acheron river from St. Fillans shows that 

 the dacites overlie the rhyolites in this area. The latter rocks, 

 which, near their contact with the Silurian sediments contain 

 little biotite. become richer in this mineral close to the dacites. 



