of the Buchcm District. 37 



volcanic and terrestrial conditions terminating in the general 

 subsidence indicated by the transition from the Lower to the 

 Upper Buchan beds. 



The Diabase rocks which I have described in these 

 notes, I find at Murendel to have penetrated the felsitic 

 Lower Buchan beds; at the Back Creek, to underlie 

 the calcareous Upper Buchan beds, with apparently 

 passage-beds connecting the two; and at the Snowy River, 

 to be clearly and unmistakably in this position as regards 

 the same formation. It seems, therefore, unavoidable to 

 arrive at the conclusion that the Diabases of the Buchan 

 district represent volcanic rocks, which date from the period 

 of time marked by the change of material which distin- 

 guishes the Lower from the Upper Buchan beds. In other 

 words, they represent at places near Murendel the " necks," 

 while at the Snowy River they represent the flows them- 

 selves. 



In some previous notes on the interbedded volcanic rocks 

 of the Snowy Bluff,* I came to the conclusion that as 

 they appeared to have originally consisted of the minerals 

 plagioclase, augite, magnetite, and very rarely a little olivine, 

 with some traces of a basis, they were to be regarded as 

 highly altered basalts, or having regard to their being of 

 upper paleozoic age, melaphyrs. In studying the Diabase 

 rocks of Buchan and the Snowy River, I observed many 

 striking similarities of composition and structure to the 

 above, and I again examined the thin slices I had prepared 

 of the Snowy Bluff rocks, and compared them with those 

 discussed in this paper. The results I may thus state : — 



1. The geological age of the two groups is — Upper paleo- 

 zoic, those of the Buchan district being Middle Devonian, 

 whilst those of the Snowy Bluff, are probably Upper 

 Devonian. 



2. Both rocks, to judge from the least altered samples, 

 were composed of the minerals plagioclase, augite, and 

 magnetite. In those of the Snowy Bluff traces of olivine are 

 so rare, occurring, so far as I have observed, in only one of 

 the many interbedded sheets, that its presence may be said 

 to be the exception, proving the rule of its absence. The 

 olivine-bearing rocks of the Murendel may yet prove to 

 belong to the Diabase group of that district, and so become 

 a parallel to those of the Snowy Bluff. 



* Progress Report, Geological Survey of Victoria, Part III., p. 75. 



