186 



particularly the case when the materials have become, to some 

 extent, set and consolidated and covered with vegetation. In 

 the earlier period, while the material is still merely loose 

 fragments, the erosion of a winter should leave distinct traces. 

 At present, for instance, the loose faces of the interior wall 

 below the Tower are being rapidly worn away. 



In noting the many sections of the stratified ash, both 

 radial and normal to the foci, watch was kept for any signs 

 of contemporaneous erosion. In no case was there found any 

 suggestion of a runnel or valley formed in the ash-beds during 

 deposition. This is suggestive of the idea that no wet season 

 occurred during the period of explosive activity. 



The stratified beds are numerous, but show no consistent 

 evidence of rhythm in deposition, though Woods (Ref. 2, 

 p. 254) recognized three series of layers, varying in each series 

 from coarser material below to finer material above. The 

 occurrence of the numerous and varied layers is probably 

 due to the intermittence and variation in force of the ex- 

 plosions, and to the shifting of the winds. Summing up the 

 evidence as presented in the field, I would suggest as a 

 working hypothesis that the whole period of activity at Mount 

 Gambier was something more than, say, six weeks and less 

 than six months. 



8. The Number of Craters and the Order of Eruption. 



(a) Previous Opinions. — The question of the number of 

 craters that existed at Mount Gambier is one on which there 

 has been considerable divergence of opinion, and while it is 

 purely of theoretical and academic interest, very careful con- 

 sideration has been given to this point. The opinions of 

 previous observers are given below, as far as possible in their 

 own words. 



The earliest available account of the geology of South 

 Australia was published by Thomas Burr, in 1846, when the 

 Province was but ten years old, and seven years after Mount 

 Gambier had been first visited by white men (see Ref. No. 1). 

 The brief account there given of Mount Gambier was kindly 

 brought under my notice by Mr. L. Keith Ward, the Director 

 of Mines of South Australia. 



Thomas Burr says, concerning the craters: — "This moun- 

 tain has three craters, which lie in a direction nearly east 

 and west. The western crater < 17 ) is divided into two portions 

 — that to the east contains a lake of great depth containing 

 fresh water. The middle crater ( 18) is much smaller and 



(17) Browne and Valley Lakes. 



(18) Leg of Mutton Lake. 



