Palaeozoic Geology of Victoria, 95' 



surrounding rocks is not clear, tlie occurrence is included ten- 

 tatively with the basal beds of the Upper Palaeozoic. 



The Rhyolites and Associated Tuffs. — These roc^Ls 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 the. 

 main anticlinal fold. They vary in thickness 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 flows. 



These rocks, under the name of quartz-j^orphyries and felsites, 

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

 in this region, and they were invariably found 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, tliat of) Snowy Bluff' 

 in the Moroka Valley, to the north of Wellington, has been carefully 

 described by both Murray (9) and Howitt (4). 



The rhyolitic rocks there rest on conglomerates, and are estimated 

 at about 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 appearance. Numerous inclu- 

 sions of indurated slate and shale are also common. 



Tliin sections of some of these rocks have shown 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 outlinei so characteristic of 

 tuffs, but seldom so well preserved. Their 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. 



