Volcanic Tiiff of Pejark Mavslt. 5 



The lake has an area of two square miles, ^^ and is' situated about 

 two and a-half miles north-west of the township of Terang. Its 

 circular form and low banks of volcanic tuff gently sloping into 

 .the surrounding plains on all sides, are very suggestive of volcanic 

 origin. The fact that the banks are raised at all, unless they are 

 •of aeolian origin, makes it difficult to understand how they could 

 .have been formed if thei lake is occupying a depression resulting 

 .from the withdrawal of material from below through the activity 

 of neighbouring volcanoes. 



It seems more reasonable to expect a gradual slope towards the 

 lake by a sagging of the strata, instead of the reverse. Again, the 

 .fiymmetrical shape, which has been assumed, in place of one 

 .more or less irregular, as in the case of lakes situated in areas 

 w^here the evidence supports an origin by subsidence, is more con- 

 sistent with a volcanic origin. There is no evidence that this 

 lake basin was formed in a depression in volcanic tuff. The tuff 

 appears to have been deposited on a comparatively level land sur- 

 face, through which the volcanic forces burst an opening. Mahomy 

 and Graysoni'^ point out with regard to Lake Bullenmerri, which 

 has only an area of a little over two square miles, is bounded by 

 steep sides, and has its floor lying 700 feet below the highest 

 part of the rim, that the formation of such a basin by the sinking 

 •of its floor has never been actually observed, but that there are 

 instances of the production of similar depressions by paroxysmal 

 .explosions. 



If it can be jaroved that the accumulations of tuff round the lake 

 in situ thinned out as they receded frofcn the lake itself, a strong 

 piece of evidence would bet established in favour of the basin being 

 .an explosion vent. 



With regard to this, and several other points, Mr. A. J. ]\Ierry 

 very kindly went to considerable trouble to ascertain what data 

 ■were known from well sinkings in the neighbourhood. The result 

 'Of his inquiries went to show that the well sinkings near Lake 

 Keilambete all indicated a gradual reduction in the thickness of 

 the tuff away from the lake, thus supplying the important evidence 

 'required. It is not certain, however, if this tuff extends to, and 

 is continuous with, the Pejark bed. On the geological quarter 

 sheet, buckshot gravel is seen to be the superficial deposit interven- 

 ing between the former and the Pejark Marsh, and the nature of 

 the underlying Ijeds is not disclosed. 



13. Intercolonial Exhibition Essays, Melbourne, 1866. 



14. Loc. supra cit., p. 13. 



