Obsidian Bombs in Australia. 33 



a button, a fact to which the term, " oVjsidian button," frequently 

 applied to them, is due. This obsidianite may be said to consist 

 of a flattened spherical centre surrounded by a marginal rim, 

 which on the one side forms the continuation of the spherical 

 surface, and when viewed from that side, as a rule, no indication 

 of it is seen. This side shows generally a number of successive 

 circular flat grooves, sometimes forming a distinct spiral. The 

 other side exhibits the rim surrounding the central portion, 

 which is always more or less pitted. These pittings, which are 

 usually scarce on the rim and under side, are small and frequently 

 of perfect hemispherical shape. The rim is sometimes quite 

 smooth and regular on the outer edge, forming a complete ring, 

 or else broken and irregular, but it always shows a depression or 

 trough all round. With two exceptions, for one of which see 

 PL III., Fig. 8, this is the only form observed with this rim. 

 Most of the others are round (Figs. 4 and 5, PI. III.), oval or elon- 

 gated (Figs. 1 and 2, PI. III.) in general form, the latter ones 

 being contracted in the centi-e, at times roughly resembling a 

 dumb-bell. The top and bottom are more or less convex, the 

 bottom, or smaller section, being always of greater convexity 

 than the top. They are both pitted to a greater or lesser extent, 

 sometimes like that in the specimen next described, or in a more 

 regular manner. The sides are rudely corrugated or furrt)\ved in 

 an approximately vertical direction, and slope at various angles, 

 but they can be traced in a series of specimens to gradually 

 become merged into part of the lower convex surface, in which 

 instance the obsidianite assumes the form of two hemispheres of 

 unequal diameter joined together, and we have a more or less 

 spherical body. The corrugations are then much less conspicuous 

 and at times almost obliterated. The sides show fewer pittings 

 than either top or bottom. Without doubt the most interest- 

 ing form met with is that of the hollow obsidianites. The one 

 from Kangaroo Island has been fully described by Stelzner, but 

 no description appears to have been given of the Horsham speci- 

 men (Figs. 1 and 1a., PI. IV.). This obsidianite. in its greatest 

 diameter across the peripheral ridge, measures 59 mm., and at 

 right angles to this, 52 "5 mm. In shape it is approximately 

 spherical, but it has the characteristic form just mentioned of 

 two hemispheres of unequal diameters joined together. The 



