392 MR. A. J. R. ATK1N ON THE GENESIS [Nov. I904, 



Towards the end of Tertiary times a greatly-increased rainfall 

 took place, which washed the last remains of the decomposed 

 quartz-reefs and surrounding rocks into the valleys, together with 

 the last of the Tertiary gravels, which are at the present day 

 found associated with the gold in the lowest-known placers. 



The present filled condition of these deep cuts is due to deposition 

 of material in later Glacial and Pleistocene times. In the open 

 workings of lower Williams' Creek there is an interesting section 

 of these formations. Above the old drift-workings is a streak of 

 flat schist-pebbles, separated from the Tertiary gravels by a seam 

 of Glacial clay. This streak, about 2 feet thick, indicates a reces- 

 sion of the ice, and was deposited while Williams' Creek brought 

 down the waters from the melting ice-caps on the surrounding 

 mountains, together with the rock-detritus from their sides. 



It would be interesting to know whether this streak carried 

 much gold farther up. This would be likely, as the upper part of 

 the creek must have had very little gravel in it at this time, and so 

 would offer facilities for the gold being washed down on to the first 

 stratum of Glacial sediment. The auriferous upper streaks some- 

 times found along this creek are to be attributed to slight reces- 

 sions of the ice-cap : their limited extent showing merely a short 

 duration of the period when the creek was bringing down material 

 from its higher reaches. 



Although, viewed in the above light, the occurrence of surface- 

 bonanzas is unlikely, it must not be forgotten that the reefs 

 which originated the placers still exist. Deeper exploration will 

 probably show an enriched zone deposited by the deep ascending 

 waters which gave the reefs birth, in no way connected with the 

 secondary enrichments which have made the placers famous, and 

 are in most gold-veins of doubtful continuity. 



Discussion. 



Mr. H. W. Monckton asked whether gold had ever been found 

 in a calcite-reef. 



Mr. Bedford McNeill remarked that the paper was a very 

 interesting example of the generally-accepted theory of ' secondary 

 enrichment ' as applied to a particular ore-occurrence. Naturally, 

 one would have preferred to have visited the locality before dis- 

 cussing the paper. Our present views were mainly the outcome of 

 the comparatively-recent work of Posepny and others; but, given 

 low-grade auriferous iron-pyrites and given descending oxidizing 

 waters, there was no doubt that the chemical changes alluded to did 

 take place. In this connection, the experiments mentioned by Mr. T. 

 A. Rickard, 1 as having been commenced by Daintree in 1871 in 

 Dr. Percy's laboratory at the Royal School of Mines, should not be 

 overlooked. A number of small bottles, each containing solution of 

 chloride of gold, were taken, and to each a crystal of the more common 

 metallic sulphides was added, such as pyrites, galena, blende, etc. 



1 Trans. Am. Inst. Min. Eng. vol. xxii (1893) p. 313. 



