194 PROCEEDINGS OF SECTION C. 



2. The flow structure so well illustrated in Mr. Duixa's paper 



also shows an absence of fracture at the edge. If Mr. 

 Dunn's explanation were correct, one would expect that 

 as the rim was just the lower portion of the bubble the 

 flow line would have been continuous in the bubble 

 and rim, whereas the illustrations show that the flow lines 

 are concentric with the edge of the rim. 



3. Even granting for the sake of argument the possibility of 



Mr. Dunn's hypothesis accounting for the button form it 

 completely breaks down when applied to the dumb-bell 

 and oblong forms, a double bubble as suggested by 

 Mr. Dunn* being incapable of producing these results. 



4. Bubbles such as those hypotheticated by Mr. Dunn have 



never been observed in existing active volcanoes. 



5. During the period in which Australites were formed no lavas 



having compositions in the least comparable with those 

 • of the Australites are known to have been extruded in 

 Australia. 



In support of the second hypothesis, the following arguments may 

 bo advanced :— 



1. The forms of Australites are in all cases modifications of those 



which theoretically can be assumed by a liquid rotating 

 in vacuo. 



Professor Kerr Grant has statedf that the following 

 forms are possible : — 



(1) The sphere — possible only when there is no 



rotation. 



(2) The oblate spheroid — stable at low speeds of 



rotation, 



(3) The prolate spheroid — stable, if at all, only at 



high speeds of rotation. 



(4) The apioid or pear-shaped figure of revolution. 



(5) The dumb-bell or hour-glass figure of revolution. 

 The departure of the form of Australites from the above 



theoretical forms is due to the resistance of the atmo- 

 sphere. The button form would be derived from the 

 spheroidal form by the flowage of material from the front 

 of the rapidly travelling body, and this would form the 

 encircling ring. The flange noticeable on some of the 

 other forms would also be due to the same cause. 



2. A year or two back Mr. D. J. Mahony exhibited to the members 



of the Koyal Society of Victoria micro photographs 



* R'Jcords of th3 G-^ol. Surv. of Vicfcoria. Vol. 11, pt. 4, p. 202. 

 t Proc. Biy. Soc. Vic. Vol. X.K( (a)w s-Jriosi), Pt. [1. 19J8, p. 44t. 



