83 



The Eocene limestone, which occurs immediately below 

 the basalt, has been described by Tate and Dennant.^^) 



Geological Notes on the extinct volcanoes of Mount Gam- 

 bier and Mount Schank have been published by the Rev. W. 

 Howchin, F.G.S.t^) 



T. S. Hall, M.A., mentions the bedded tuffs of Mount 

 Gambier in his "Note on the Deposition of Bedded Tuffs." '■'^^ 



A type of basalt from Mount Gambier has been briefly 

 described by J. C. Moulden.(5) 



Chas. Chewings '^^ describes a basalt, from the same 

 locality, containing phenocrysts of felspar, and represents a 

 special type of lava. 



The present paper includes a complete analysis of the 

 Mount Gambier vesicular olivine basalt, with petrographical 

 descriptions of several varieties of allied rocks. Certain im- 

 portant differences are to be found in the lavas at that local- 

 ity, their structures varying from glassy to holocrystalline, 

 and from a very vesicular variety to a more or less compact 

 rock. A detailed petrographical description has been made 

 on the olivine nodules occurring in the ash-beds, with inter- 

 esting results described hereafter. 



II. — Vesicular Olivine Basalt. 



Rock 1. 



This rock was selected from the central portion of the 

 lava band, above the boathouse, on the northern extremity 

 of the Blue Lake. It represents the general type of speci- 

 men usually met with. 



Macroscopic Characters. 



In hand specimens the rock is dark-grey in colour. It 

 is fine-grained, containing many small visible phenocrysts of 

 olivine about two millimetres in length. Phenocrysts of 

 augite are also present, but are very difficult to see, even 

 \vith the aid of a lens. Vesicular structure is a prominent 

 feature of the rock, the vesicules varying from a few milli- 

 metres up to a centimetre or more in diameter in the com- 

 pact varieties, and they contain little or no crystalline mate- 

 rial. There is no apparent decomposition in the rock. 



The specific gravity of the rock is 301 at 16'4° C. 



(2) Trans. Roy. Soc, S.A., 1896. 



(3) Trans. Roy. Soc, S.A., vol. xxv., 1901, p. 54. 



(4)Proc. Roy. Soc, Vic, vol. xx., 1907, p. 21. 



(5) Trans. Roy. Soc, S.A., vol. xix., p. 70. 



(6) Geologic Slid- nnd Central-Australiens. Inaugnral-Di.sser- 

 tation zur Erlangung der Doktorwtirde — 1894. 



