[Proc. Roy. Soc. Victoria, 2 4 (N.S.), Pt. I., 1911.] 



Art. IX. — On a Lmihurgife Rock occitrring as a Volcanic 

 Flag at Bahvijn, near Doiicaster. 



By F. chapman, A.L.S. 



AND 



A. O. THIELE. 



(With Plates XL. and XLI.) 

 [Kead 8th June, 1911.] 



General Description of the Locality and Occurrence. 



The bedrock of this district is formed of more or less folded 

 miidstones and sandy shales, belonging to the silurian (mel- 

 bournian) series. The highest points in the neighbourhood are 

 at Doncaster, Yarraibat Avenue, and Balwvn township, on the 

 410, 380 and 320 contour lines respectively. In common with 

 other intermediate localities, these eminences are capped with 

 red tertiary sands of kalimnan age (loAver pliocene) : remnants 

 of the vast sheet of sand-rock which at one time formed a cover 

 to the great Yarra or Nillumbik peneplain^ of this part of 

 Victoria, before it was dissected by the present river system. 



The Balwyn occurrence of the basic lava herein described, and 

 later shown to be a basic dole-rite related to the limburgites, is 

 situated in a paddock of the Whitethorn Estate (now chiefly 

 comprised in an allotment belonging to Mr. J. R. Lewis2), at 

 about 800 feet down the Whitethorn Road, north from the 

 Belmore Road, and about 200 feet east from the road. The 

 exposure presents the appearance of a low, but distinct mound 

 of oval or lenticular outline, rising very slightly above the con- 

 tour of the Silurian rock of the immediate vicinity, and sloping 



1 For description of this peneplain see J. T. Jutson, Proc. Roy. Soc. Victoria, vol. xxiii, 

 n.s., pt. 2, 1911, p. 447. 



2 For facilities given for access to the ground our best thanks are tendered to this 

 gentleman. 



