193 



ridges north and south of the Leg of Mutton Lake, and the 

 other evidences detailed above, which can only be accounted 

 for by the third (central) crater shown in the reconstruction. 

 Woods gives much detail regarding the order of eruption, 

 but the writer could not discover any definite evidence on the 

 matter. There was a brief explosive phase prior to the 

 basalt flow, and that probably came from the western crater, 

 as did the lava. Possibly the order of activity was: — 1, an 

 explosive outburst from the Mount Gambier crater; 2, a brief 

 effusive phase at the same focus; 3, renewed explosive activity 

 at that crater ; 4, explosive eruption of Blue Lake crater ; 5, 

 explosive activity at Leg of Mutton crater. Still, there is 

 no positive evidence against the idea that phases 3, 4, and 5 

 were contemporaneous. 



9. The Subsidences. 



(a) Extent, Cause, and Time of Collapse.— As pointed 

 out in the introduction to these notes, only remnants of the 

 original cones now exist to mark the centres of volcanic 

 activity at Mount Gambier. The actual extent of the sub- 

 sidences is clearly shown in fig. 1 (general view), fig. 7 (plan), 

 and fig. 8 (section). 



It is difficult to imagine the reason for such extensive 

 collapses. Woods suggests the draining away to the south- 

 ward of a great mass of lava that had formed an underground 

 reservoir, at comparatively shallow depths. He quotes evi- 

 dence in favour of this (p. 250), which I have not had the 

 opportunity of investigating. 



To take the case of the Blue Lake subsidence, a huge 

 block of rock, 170 acres in extent, and comprising about 

 300-ft. thickness of level-bedded limestones, 20 to 50 ft. of 

 dense basalt, and possibly an average thickness of 350 ft. of 

 tuffs, has disappeared abruptly and precipitately downwards 

 into the earth. 



The surrounding limestone country contains many 

 caverns and sink holes, such as those known as Umpherston's 

 "Caves," but none of these are at all comparable in size to 

 those of the lakes. This suggests a genetic relationship 

 between the underground spaces into which the collapse took 

 place and the volcanic activity itself. Both, in turn, may 

 be related to the line of crustal weakness which is suggested 

 by the linear arrangement both of the craters and of the 

 subsidences. 



In other areas similar great crater depressions have been 

 formed by a violent final explosion, blowing away the whole 

 "roof" of the volcano. If that had been the case at Mount 



