Obsidian Boinhs in Australia. 35 



extent be hypothetical, but what is now advanced is the result 

 of the examination of numerous specimens from all parts. Of 

 these, a number seemed to suggest a series in the development of 

 button forms commencing with the elongated or round types. In 

 the first case, if we assume on ascent or descent, as the case may 

 be, of the plastic material during rapid horizontal rotation, the 

 tendency will naturally be, in these elongated bodies, to cause 

 the material to flow towards the ends by centrifugal force, and 

 at the same time the resistance of the atmosphere might have 

 the effect of pushing back the outside portion of the obsidian 

 from the front or advancing side, and so producing the rim. 



Should this action be continued further, the material will go on 

 accumulating at the ends until the centre becomes so weakened 

 and reduced that a separation takes place, resulting in the forma- 

 tion of two incomplete buttons, then if plasticity and flight are 

 still retained, it is reasonable to infer that two complete forms 

 will result. Finally, the rim will be pushed so far as to cause a 

 complete separation between it and the ellipsoid centre (Fig. 9, 

 PI. III.), and, if conditions are still favourable, a new rim will 

 be formed, and the opei'ation repeated. These solid centres are 

 not uncommon, but only two separated rims have come under my 

 notice. 



With the other type, the process would be simpler, comprising 

 only a pushing back of the plastic material in order to form the 

 rim, and give the characteristic button-shape. This is practically 

 the same explanation as that offered by Professor Stelzner to 

 account for the rims, and he compares it to the pushing on of a 

 glove-linger or the form produced by tiring a leaden bullet into 

 sand. These forms might then be said to represent various 

 stages in the production of the button obsidianites, and depends 

 primarily upon the shape of the oi'iginal fragment, upon the 

 degree of plasticity, and, finally, upon the rate of ascent or descent 

 and rapidity of rotation. That they all reached the earth's 

 surface in a solid state seems apparent, because in no instance has 

 any alteration from their symmetrical form been observed, and 

 some modification would have resulted had they landed in a soft 

 or plastic state. Professor Stelzner does not think the assump- 

 tion of rotatory flight necessary for the production of form as 

 suggested by Darwin, and he compares it to the causps which 



