OCCTTEEENCE OF GOLD IN ATJSTEALIA. 435 



as the main cracks of cooling of the igneous rock, many of which 

 were probably opened again and again, forming chimneys for the 

 escape of gaseous emanations from below. 



That the gold in a reef may have been introduced at different 

 periods, we have constant evidence in practical mining all over 

 Australia, where the precious metal is usually found in some par- 

 ticular part of the larger veins, either near the hanging or foot wall, 

 in the centre, or in " shoots," dipping at an angle across the strike 

 of the lodes, the remainder of the reef being entirely or compara- 

 tively barren. In small veins this is also often a very marked 

 feature. 



Figure 1, which is enlarged two and a half diameters, from 

 a section of an auriferous quartz reef, shows clearly that the 

 first deposit from either wall was barren quartz of a crystalline 

 character — that then a break took place in the centre, which was 

 accompanied or followed by a very rich deposit of gold, and this 

 gold of the filiform and semicrystalline appearance so characteristic 

 of that associated with intrusive rocks, and which is called by the 

 diggers "spider-leg" gold. 



With regard to the age of the gold in Australia, I think all the 

 evidence goes to show that the auriferous veins were chiefly formed 

 during the earliest era of great volcanic agitation of which the 

 stratified rocks give evidence in their interstratified ashes and lavas, 

 as well as the volcanic cores or dykes ; and this period was assuredly 

 the Devonian *. 



In my last report as Geologist to the Queensland Government, I 

 pointed out that the miners of one of the diggings near Peak Downs 

 were finding waterworn gold in a Carboniferous conglomerate 

 containing Glossopteris, making a " bottom " of an underlying con- 

 formable shale abounding in Glossopteris. Mr. Wilkinson, the 

 Government Geologist of New South Wales, has lately recorded a 

 parallel fact as occurring at Gulgong, thus showing that some auri- 

 ferous quartz veins, at least, were formed prior to the Carboniferous 

 period in Australia. 



The only reason, in my opinion that waterworn gold has not 

 been more frequently found in similar conditions is, that marine 

 and lacustrine deposits, such as the Carboniferous and all the 

 Mesozoic and older Cainozoic strata of the continent, are chiefly built 

 up of sediments not derived from the rocks on which they rest. 

 Only beaches or locally filled fjords of Carboniferous or Mesozoic sea- 

 coast, where auriferous reefs cropped out, and had a chance of 

 extensive abrasion, would be likely to contain drifted gold. 



Between the Palaeozoic and Cainozoic periods in Australia there 

 appears to have been an almost entire cessation of the volcanic 

 forces. 



* In my previous paper on Queensland geology, the metamorphic series is 

 provisionally grouped below the Devonian ; but I much doubt whether it is 

 not in most cases, if not in all, simply Devonian strata altered by contact of 

 large masses of intrusive rocks, principally granite, as it is nowhere found at 

 any considerable distance from such masses. No Lower-Silurian fossils have 

 yet been found in any part of the colony. 



