Volcanic Tuff of Pejark Marsh. 5 



The lake lias an area of two square miles, ^^ and isi situated about 

 two and a-half miles north-w^est 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 the 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 

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

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

 where 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. Mahoiny 

 and Grayson^^ 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 

 o(f 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 proved that the accumulations of tufl round the lake 

 in situ thinned out as they receded froim 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. Merry 

 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 tufl away from the lake, thusi 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 beds is not disclosed. 



13. Intercolonial Exhibition Essays, Melbourne, 1866. 



14. Loc. supra cit., p. 18. 



