158 PROCEEDINGS OP THE ROYAL SOCIETY OP QUEENSLAND. 



\vithin this area, although mining for arsenic, gold, silver, 

 copper, lead, and bismuth-^ has been carried out at the 

 neighbouring Samford massif some few miles away. 



On p. 161, Part I., Rands was quoted as having 

 described "'very minute specks of gold" in one of the dykes 

 which form the extensive system of intrusions between th^' 

 south-eastern edge of the Enoggera granite and the Brisbane 

 River at Indooroopilly. Up to that time (November, 1914) 

 no other minerals of economic value had been found asso- 

 ciated with these intrusions, but in the year 1918, a dis- 

 covery of silver-bearing galena was made at Finney's Hill, 

 near Indooroopilly, associated with the "Rhyolitic" intru 

 sions. Up to the present some 280 tons of lead and 45,75-^ 

 oz. of silver have been produced from the area, whit'.; 

 the presence of copper, zinc, and bismuth in smaller amount:5 

 has been proved. Ball has mapped the intrusives in this 

 area in great detail, as he considers them the "sole guides 

 in searching for new shoots of ore."-" He states further 

 that "a clear case is presented at Indooroopilly in favour 

 of a magmatic derivation, the metalliferous solutions being 

 an extreme differentiate of the plutonic igneous mass from 

 which the felsitic dyke rocks arose — the presence of :^ to ^ 

 per cent, bismuth is, in my opinion, decisive evidence." 



A specimen of the rhyolite ("felsite"), which actually 

 "ontained silver-bearing galena, was kindly supplied by 

 Mr. Ball, sliced and microphotographed. {See Plate III., 

 No. 8.) The rock is in all respects like the other intrusive 

 rhyolites of the neighbourhood, except that veins of second- 

 ary quartz are even more pronounced than usual. 



The value of the Hybrid rock (commercially known as 

 the ''Enoggera Granite") as a building stone, and for 

 purposes of "pitching," '"kerbing, " and road-making has 

 been very fully dealt with by Professor Richards in a paper 

 read before this Society in July, 1918.''° 



CONCLUSION. 

 In conclusion. I wish to thank Professor H. C. 

 Hichards, D.Sc, for the interest he has shown in this work 

 (which was originated at his suggestion) and for his helpful 

 advice on many points. 



'" L. C. Ball, Qld. Govt. Miii. .Journ., Vol. XXI. p. 266. 

 "•L. C. Ball, Qld. Govt. Min. Journ., Vol. XXI. p. 267. 

 '""The Building Stones of Queensland," Proc. Roy. Soc. Vol. 

 XXX., p. 101 et seq. 



