biology of snowy river area 153 



Tuff Beds— Volcanic Pisolites 

 Volcanic tuff is exposed in cuttings along the Oelantipy-Wul- 

 gulmerang Road near Boundary Creek, where it forms compact 

 fine-grained rocks of pink or yellowish colour. One bed of reddish 

 coloured tuff exposed south of Boundary Creek contains small 

 spherical pellets of hardened ash set in a cement of the same 

 material. (Plate 12, lower). These pisolites vary in size from 

 one-sixth inch to one-half inch in diameter, and appear to 

 have been slightly flattened by the weight of the overlying strata. 

 Joints in the tuff bed pass through some of the pellets. In section, 

 these pellets have a concentric structure, and it is suggested that 

 they have been formed by trickles of water, or by wind rolling 

 some nucleus over a surface of soft volcanic mud. Their appear- 

 ance does not suggest that they were formed in the atmosphere 

 and fell as mud balls as was recorded at Vesuvius by Perret (5). 

 The name Accretionary Lapilli in preference to Volcanic Pisolites 

 has been suggested by Wentworth and Williams (6). 



Middle Devonian Limestones : 



The Buchan limestones which overlie the porphyries further 

 south were not examined during this survey. 



Sub-Basaltic Deposits 



(a) Carbonaceous Mudstone. — Boulders of black carbonaceous 

 mudstone containing fragmentary plant remains were found 

 in Butcher's Creek, about one mile above its junction with Back 

 Creek. Time did not permit a search for the outcrop from which 

 the boulders were derived, but in view of the soft crumbly nature 

 of the mudstone, it cannot be more than a few chains distant. 

 Quartz grains from l-2mm. are common in the mudstone, and two 

 almost perfect bi-pyramidal crystals were found in one small 

 specimen. The plant remains, mainly stems and woody tissue, 

 have not yet been determined. 



This bed is probably of small local extent, and may be of the 

 same age as a sub-basaltic plant bed described by Howitt (3) 

 from a deep lead at Mayford on the Upper Dargo River, and 

 identified by McCoy as Miocene. 



(b) Quartzite and Opal.— Quartzite occurs directly beneath 

 the basalt a quarter of a mile north of Currie Creek, and also 

 further south between Gelantipy and W Tree. Small patches of 

 common opal and wood opal occur at W Tree also beneath the 

 basalt, Silicification of sediments forming the quartzite and opal 

 was probably caused by solutions derived from the basalt at the 

 time of its eruption. 



