1855.] CLARKE — OBSIDIAN BOMBS IN AUSTRALIA. 403 



3. On the Occurrence of Obsidian Bombs in the Auriferous 

 Alluvia of New South Wales. By the Rev. W. B. Clarke, 

 M.A., F.G.S. 



[Abridged.] 



In Mr. Darwin's * Geological Observations on the Volcanic Islands 

 visited during the voyage of H.M.S. Beagle* (p. 38), that author 

 describes and figures a volcanic bomb of green obsidian from the plain 

 between the Rivers Darling and Murray, and which he received from 

 Sir T. L. Mitchell. 



The explanation given by Mr. Darwin appears to be satisfactory 

 as to the origin of that individual specimen, and it is confirmed by 

 the parallel examples cited from M. Beudant. Sir T. L. Mitchell's 

 specimen would seem either to have been drifted from a very long 

 distance, or, which is more likely, from the known habits of the 

 aboriginals, to have been dropped by one of them, who probably 

 found it in the trap-hills of the Lachlan to the north-eastward. 

 This specimen was unique as to AustraUa, until recently. 



During the last two years, several similar specimens have been 

 found in the auriferous detritus of the western and northern Gold- 

 fields. 



The first which I met with was found in the cradle of a gold-washer 

 on the Turon River, who dug it from a depth of 30 feet below the 

 surface. This was a small, irregular, roundish substance, 0*6 inch 

 in diameter, having a specific gravity of 2*7 at a temperature of 66°. 

 It was undivided, and more like those examples described by M. 

 Beudant than the figure given by Mr. Darwin. A similar specimen, 

 but of a rude elliptical form, ^ inch diameter in the major axis, and 

 having a sp. gr. of 2*57 at a temperature of 63°, was found in the 

 washing-stuff of the Uralla, or Rocky River, County of Hardinge, 

 New England District. 



From the same locality were derived two other specimens, which 

 I have examined. These are perfectly round, having diameters 

 respectively, including the rim, of ^ inch and about f inch. These, 

 except in shape and in the extension of the cells over the rims as well 

 as in the nucleus, agree with Mr. Darwin's figure. Their colour is 

 also bottle-green, and they are translucent ; the surface appearing 

 black in the thicker portions, as in Mr. Darwin's specimen. But this 

 blackness does not arise from any difference in the composition ; it is 

 merely the effect of greater opacity. The external concentric rings 

 are evident in all three. The specific gravity of those from the Uralla 

 is, respectively, 2*42 and 2*51 at a temperature of 63°. The smaller 

 very much resembles a button without the shank ; and, from this 

 appearance, the diggers call them "button stones." They appear 

 as if they had been cast in a mould ; but there is no reason to doubt 

 the imputed origin. 



The alluvia of the Uralla, which rises in granite and flows through 

 rocks of that class for many miles, consist of mud and sand (sur- 

 rounding "boulders" of granite, in the decomposing exfoliative 



