Palaeozoic Geology of Vicloria. 9b" 



surrounding rocks is not clear, the occoi'rence is included ten-- 

 tatively with the basal beds of the Upper Palaeozoic. 



The Rhyolites and Associated Tuffs. — These rocks occur as a well- 

 defined sheet of great extent. They are readily traced in the field, 

 consistently near the base of the series along either limb of tha- 

 main anticlinal fold. They vary in tliickness from less than 1000 

 to over 2000 feet. In the thinnest portions they would appear to 

 represent a single flow, but in Mt. Wellington where they form a 

 bold escarpment on the western face, they are composed of suc- 

 cessive beds of pyroclastic material and rhyolitic fiows. 



These rocks, under the name of quartz-porphyries and felsites, 

 were recognised by Howitt and Murray, in widely separated areas- 

 in this region, and they were invariably fo4.in-d occupying a position 

 towards the base of the series. Howitt recognised the volcanic 

 nature of the rock, and that there were both effusive and pyroclas- 

 tic representatives. One of the finest sections, that oi\ Snowy Bluff ' 

 in the ]\[oroka Valley, to the north of Wellington, has been carefully 

 described by both Murray (9) and Howitt (i). 



The rhyolitic rocks there rest on conglomerates, and are estimated 

 at alx)ut 100 feet in thickness. One of the striking features of 

 much of the igneous material in the vicinity of Wellington is that. 

 rocks of the outward appearance of quartz-porphyries, are often 

 crowded with water-worn pebbles of quartz and quartzite, often 

 producing quite a conglomeratic apj^earance. Numerous inclu-. 

 sions of indurated slate and shale are also common. 



Ihin sections of some of these rocks have show-n that the igneous 

 material of which they are in part composed, is pyroclastic. One 

 particularly fine example from the northern shore of Lake Karng,. 

 Mt. Wellington, showed the tuff character remarkably well (micro- 

 photo.). It contains angular and broken fragments of quartz and' 

 felspar set at all angles in a fine microcrystalline base, containing 

 beautifully preserved tubes of irregular outline so characteristic of 

 tuffs, but seldom so Avell preserved, llieir bent and twisted shapes 

 and broken cusp-like forms are particularly striking in the section. 



The fact that material of this nature is often admixed with 

 waterworn pebbles of the old rocks, points to the conclusion that 

 explosive volcanic action was practically contemporaneous with the- 

 deposition of part at any rate of the basal conglomerate beds. 



Another section from the southern shore of the lake has been 

 referred to in a previous paper (13). It has the character of a-- 

 rapidly-cooled lava and shows very fine perlitic structure. 



