A FURTHEE NOTE ON OBSIDIAN BUTTONS. 43 



inquiries, sajing that he had consulted Dr. A. C. Law- 

 son, Associate Professor of (jreolooiy and T\!ineralooy, 

 and tliat their joint suggestion Avas, that the button-shaped 

 forms described and figured in the paper of Messrs. 

 Twelvetrees and Petterd miglit possibly be due to tlie 

 formation of spherulites in a hiva witli obscure flow- 

 structure, this structure being brought out by, weathering ; 

 but this suggestion appeared to have been offered with 

 some hesitation in the absence of any o}»portunity of 

 examination of specimens. The next thing to be done 

 was to attempt to supply specinxensfor personal examina- 

 tion, and an application to the Trustees of the Tasmanian 

 Museum resulted in niv being enabled to forward to 

 San Francisco three obsidian buttons from a collection 

 made many years ago near Glen.elg, in Yictoiia, and 

 presented to the Museum. 



By the last mail from America I received a second 

 letter from Professor LeConte, in which he says that, 

 after careful examination of the specimens, he gives up 

 the theory of their possible concretionary origin. His 

 letter continues as follows : — 



" I cannot think they have any relation to volcanic bombs : their 

 meteoric origin seems to me out of the question. Professor Law8on 

 throws out the following suggestion : — ' May they not be the result 

 of the bursting of bubbles on the surface of some liquid stiffly- 

 viscous lava, ready to solidify ? The bursting of such a bubble 

 would probably leave a mound-like centre surrounded by an 

 elevated ring-like margin, sharply elevated at first, but quickly 

 becoming more rounded by gravity and by cohesive shrinkage, 

 before setting completely. Thus might arise the appearance of the 

 fiat side. Subsequently the little ring and mound sejrdrate from the 

 lava-mass by ponchoidal fracture, forming the hemisiiherical side. 

 The fracture is supposed to be determined by inequality of surface 

 tension produced by the bursting of the Inibble.' 



" You see it is a mere suggestion, but I can think of nothing better 

 to offer. As to their mode of occurrence, it is easy to see that their 

 j^orin would favour wide distribution by mechanical means, and their 

 singiilarity , by human agency. 



" Many thanks for these valuable additions to our Museum." 



The suggestion thus offered by Professor LeConte 

 claims attention as being the nearest approach to a satis- 

 factory solution of a difficult problem that has yet been 

 put forward. It is necessarily conjectural, for the exact 

 conditions attending the bursting of bubbles of interstitial 

 steam or gas near the surface of a rapidly cooling glassy 

 volcanic magma have never been witnessed by any hitman 



