162 Notes on the Area of Intrusive Rocks at Dargo. 



do so now is shown by the observation made by Mr. 

 H. Y. L. Brown in the Alison mine at Costerfield, where a 

 mammillary, or stalactitical crust, had been deposited on the 

 roof of one of the drives, which contained ffold, top^ether with . 

 ores of iron and antimony. * 



Such sediments as those of Silurian times could not lose 

 their saline waters until they became elevated above the 

 sea level as dry land, and this seems only to have been the 

 case with them during the earlier part of the Devonian age, 

 as a consequence of the great terrestrial movements to which 

 I have before referred. So long as the saline waters 

 remained within the sediments they would afford materials 

 for mineral regeneration. The gold in solution, or possibly 

 also diffused in a metallic, finely-divided state within certain 

 beds, if precipitated by organic substances, would be a source 

 of supply under a series of reactions which are conceivable, 

 as also the final deposition of the gold, together with silica, 

 in the fissures which gave passage to the solutions. 



It would be idle to attempt to sketch out the course of 

 such reactions in the absence of knowledge as to the effect of 

 the very different conditions of temperature and pressure 

 at great depths within the earth. It suffices for my purpose 

 if I have been able to indicate the possible source of the 

 gold and the mode of its final resting-place in the quartz 

 reefs. The experiments of Daintree and Wilkinson suggest 

 the precipitation of the gold in solution by organic materials 

 in the sediments ; and as to the formation of the quartz 

 reefs, together with the gold, the experiments made by 

 Bischoff "f* show that the mutual reaction of solutions of 

 gold chloride and alkaline silicates may have played a part. 



The following are the conclusions to which I have arrived 

 on the foregoing subjects : — 



(1.) The Silurian sediments included a certain amount of 

 the waters of the seas in which they were laid down, and 

 thus contained some of the materials for their mineral 

 regeneration and the formation of metalliferous lodes. 



(2.) The folding, compression, metamorphism, and invasion 

 by plutonic rocks of these sediments occurred at the close 

 of the Silurian age, followed by — 



* J. Cosmo Newbery, B.Sc, Laboratory (page 175) — Keports of Progress, 

 Part IV., Geological Survey of Victoria. 



■f Lehrbuch der Chemischen und Physicalischen Geologie (Part III., 

 page 843). 



