176 



accumulations of the extensive swamp areas and the sand of 

 the ancient dune ridges. The most fertile areas are those 

 where basaltic "ash" fragments have been deposited. 



(b) Mount Gambier. — The immediate neighbourhood of 

 Mount Gambier might best be described by reference to the 

 sketch given in fig. 1, which has been drawn in part from 

 an aerial photograph taken by Mr. Arthur, of Mount Gambier. 



The present-day "Mount Gambier," which gives its name 

 to the town that nestles on its northern side, is but a remnant 

 of what was at one time a considerable pile of volcanic 

 material, mainly fragmentary. The series of cinder cones 

 extended about 180 chains in the direction of its greatest 

 length (roughly S.E.-N.W.), and was about 80 chains in 

 width. 



A series of extensive collapses took place along the line 

 of the cones, so that the higher portions have almost wholly 

 disappeared, and only the outer, lower slopes remain intact. 

 The collapsed area is in part occupied by lakes, the surfaces 

 of which are about 70 ft. below the level of the town and 

 the surrounding plain. The highest point, on which stands 

 the Centenary Tower, is about 650 ft. above sea level (about 

 500 ft. above the level of the town). 



The area of elevation and collapse, now largely clothed 

 by native and introduced vegetation, presents features of 

 remarkable variety from the scenic point of view. The 

 depressions, as shown in the sketch, are six in number: — 

 (i.) Blue Lake; (ii.) Leg of Mutton Lake; (iii.) Valley Lake 

 (broadly connected with iv.); (iv.) Browne or Crater Lake; 

 (v.) the Punch-bowl; (vi.) the Moorak depression. 



5. The Volcanic Materials. 



(a) The Lava and its Extent. — In the very early stages 

 of the volcanic outburst there was a small lava flow. This is 

 exposed in section in the western and eastern walls of the 

 Blue Lake, and in the eastern wall of Valley Lake, with a 

 thin deposit of volcanic ash separating it from the underlying 

 limestones. These sections are beautifully clear — almost 

 diagrammatic in outline (see fig. A, pi. x.). 



The basaltic lava is locally known as the "blue rock," 

 in distinction to the beds of stratified tuff, which latter, by 

 a curious etymological twist, is locally known as "the lava." 



The lava is thickest at exposures B and C (fig. 2), thinning 

 towards D, and doubtless flowed in the direction from A 

 towards D. As suggested by Professor Howchin (Ref. No. 4), 



