OCCURRENCE OF GOLD IN AUSTRALIA. 438 



Ulrich thinks it likely that gold is generally disseminated more or 

 less abundantly through these peculiar varieties of rock as far as 

 they extend. 



The case of the Peninsular Reef at Portobello does not offer any 

 better prospect to the miner than the " Greens " and " Tunnel " so- 

 called reefs ; so that all present evidence goes to show that so diffused 

 auriferous pyrites are not of direct practical value. 



Where, however, mechanical concentration has taken place in the 

 shape of alluvial deposits from such pyritous rocks, they have been 

 found to pay the miner — e. g. the alluvial deposits of Paddy and 

 Sharpers Gullies at the Lower Cape, Queensland, the Gooroomgam 

 diggings, and the pyritous granites of Bowenfels and Hartley, New 

 South Wales. 



The question when the auriferous pyrites were deposited in these 

 rocks is an interesting one, which requires extended and careful 

 microscopic investigation to solve. My own opinion is that most 

 of such pyrites are contemporaneous with the consolidation of the 

 rocks in which they occur, although some may owe their origin to 

 the passage of solutions through the rock at a subsequent period. 

 I have examined a large number of sections of pyritous diorites, 

 felsites, and granites of the auriferous series fromYictoria, New South 

 "Wales, and Queensland, and only in a few instances find portions of 

 the matrix rock imbedded in the pyritous crystals, a copy of a pho- 

 tograph of one of which is given in PL XVII. fig. 2. 



Here both felspar crystals and portions of felspar and altered 

 hornblende similar to the other portions of the rock are enclosed in 

 the pyrites, and felspar crystals also run into the pyrites, as if the 

 consolidation had been nearly contemporaneous, and not as if the 

 pyrites had filled decomposed portions of the rock, as calc-spar is 

 seen to do in some places ; but this never contains enclosures of 

 felspar or any other constituent of the rock substance. 



Professor Judd, in treating of the similar occurrence of gold in the 

 Schemnitz districts of Hungary*, says, " In every instance we find 

 proofs that the more deeply seated masses of andesitic lava have, in 

 consolidating, assumed a highly porphyritic or granitiform structure, 

 and that the action upon these of acid gases and vapours has 

 resulted in the decomposition of the mass, with the diffusion of 

 valuable metallic ores throughout the substance of the rock and 

 their accumulation in considerable quantities wherever a suitable 

 fissure occurred in it." 



To apply this form of reasoning to such cases in Australia it 

 would be necessary to show that the portions of the rock lying 

 below or without the line of decomposition ceased to contain auri- 

 ferous pyrites, which is by no means the case ; the zone of decom- 

 position is generally at or near the water-level of the district in 

 rocks difficult to decompose ; in softer rocks it may go much below 

 this zone ; but when the undecomposed rock is reached, the spora- 

 dically developed pyrites still continue a constituent, and indeed 



* Quart. Journ. Geol. Soc. August 1876, p. 323. 



2g2 



