174 



4. General Description. 



Mount Gambier is the best known of a series of small 

 volcanic hills in the south-eastern corner of South Australia. 

 These hills may be classified in three groups : — 



(i.) Mount Gambier, The Bluff, Mount Muirhead, and 



Mount Burr. 

 (ii.) Mount Edwards, Mount Mclntyre, and Mount 



Graham, 

 (iii.) Mount Schank. 



Groups (i.) and (ii.) show a linear arrangement in a north- 

 north-west direction, while Mount Schank stands alone to 

 the southward. The linear direction referred to is parallel 

 with the present coastline, with a series of past coastlines, 

 with a rather remarkable and extensive series of consolidated 

 dune ridges/ 6 ) and possibly with the direction of a fault-line, 

 running parallel to and adjoining The Bluff, Mount Muir- 

 head, etc. This suggested fault-line has not been carefully 

 investigated, but a casual examination suggests it as well 

 worthy of study, from both the geological and physiographic 

 points of view. 



(a) The Surrounding Country. — The bed rock of the area 

 consists of a series of marine tertiary limestones (Jan- 

 jukian),( 7 > which mark the site of the ancient ''Murray Gulf," 

 and which extend over thousands of square miles to the north, 

 west, and east of Mount Gambier. This series is, in places, 

 from 1,800 to 2,000 ft. in depth, ( g ) is richly fossiliferous, and 

 is believed to be in places underlain by the carbonaceous 

 mudstone series of the Jurassic period; there is, however, no 

 record of the occurrence of fragments of this formation in the 

 ejectamenta at Mount Gambier. The limestones remain on 

 the whole remarkably level-bedded. Their wide level surface 

 rises gently from the sea, broken only by the series of parallel 

 ridges referred to, by occasional low inliers of early palaeozoic 

 and older rocks, and by one or two "breaks" which may be 

 due to comparatively late fault-scarps. 



The limestones include red and cream-coloured dolomites 

 and a polyzoal limestone (all three used as building stones), 

 and, in places, flints are extremely abundant. The surface 

 is of extreme topographic youth; there is an almost complete 



(6) See reference No. 2 ; also referred to in various maps and 

 reports of the Geological Survey of South Australia, e.g., Bulletin 

 No. 4, plate facing p. 25 (1915). 



(7) Memoirs of the National Museum, Melbourne, No. 5, 

 Frederick Chapman, 1914, p. 48. 



(8) The Portland (Vict.) Bore penetrated these limestones to 

 a depth of 2,265 ft. Ann. Rep. Sec. Mines, Vict., 1895, p. 60. 



