194 



Gambier, one would expect to find evidence of same in an 

 arrangement of abundant scattered blocks of limestone, basalt, 

 and bedded tuffs around the depressions. Huge blocks of 

 basalt and limestone certainly occur, but these are occasional 

 only, and are situated in all cases in the proximity of one or 

 other of the three ancient craters. The idea of accounting 

 for the depression by explosion has not been previously put 

 forward, and is here mentioned only in order to be dismissed 

 for lack of evidence. 



Woods believed that the subsidence was in part con- 

 temporaneous with the volcanic activity, but no evidence 

 could be found for this, and the writer, on general appear- 

 ances, prefers to agree with Professor Howchin that the sub- 

 sidence may have occurred "even after the volcano ceased to 

 eject material." It is, of course, possible that the collapses 

 were not in all cases abrupt and precipitate, but may have 

 occurred rather as a series of collapses, concentric to the 

 margins of the vents. 



(b) The Blue Lake. — This is the most remarkable and 

 most fascinating of the subsided areas. Notwithstanding 

 the fact that much of the native vegetation that clothed its 

 precipitous sides has now disappeared (possibly since the 

 advent of the rabbit), its beauty still deserves the enthusi- 

 astic admiration given by its discoverer and by early inves- 

 tigators. This is particularly so on a bright day, when the 

 beautiful blue colour of the water is most marked. 



The first person to venture on the lake was Governor Sir 

 R. G. McDonnell (Ref. No. 2, p. 247). When one ventures 

 in a boat on the lake for the first time, the unusual crystal 

 clearness and coldness of the water, the known great depth, 

 and a memory of its mode of origin, combined with the for- 

 bidding rampart of cliffs that everywhere surrounds the lake,, 

 quite justify the epithets of "weird," "uncanny," and "awe- 

 inspiring" to the imaginative mind. 



The average depth of the water is 250 to 280 ft. ; the 

 height of the cliffs averages 250 ft. above the water. The 

 shape is an irregular oval, and the area is about 170 acres. 

 The water supply of Mount Gambier town is derived from 

 a well sunk alongside the margin of this lake, and is limited 

 only by the capacity of' the pumping plant. The high ridge 

 north-east of the Leg of Mutton Lake is utilized as a storage 

 reservoir, to provide the necessary "head." 



If one followed the general terminology of physiographic 

 text-books, the Blue Lake would be termed a "caldera." 

 The typical caldera given by Professor W. M. Davis, in his 

 "Physical Geography" (pp. 212, 216), is quite similar to the 



