160 Notes on the Area of Intrusive Rocks at Dargo. 



to deposit those combinations whicli had remained in 

 solution the longest, or which had been taken up from the 

 rock masses through which they circulated. I think reasons 

 can be shown for believing that silica would be amongst 

 those substances, and also gold. 



If the quartz reefs were formed^ as I believe, towards the 

 close of the period of plutonic and volcanic activity, it may 

 not be necessary to assume any great elevation of temperature 

 or of pressure to account for their formation. Indeed, as I 

 have before said, the deposition of the quartz from solutions, 

 and the production of the ores found in the contact lodes, 

 would be most likely to take place at a falling temperature, 

 when the solutions were no longer so well able to carry their 

 mineral burdens. 



But there is another view which must be considered when 

 looking round for the probable source of the silica which we 

 now see as the quartz of the reefs. 



We learn from the investigations of Mr. J. Cosmo 

 Newbery, C.M.G'., that the waters percolating from the sur- 

 face downwards are charged with ammoniacal compounds of 

 which the acid carbonate is the most energetic in its action 

 on the silicate rocks. He has shown tliat ammonia can be 

 obtained from almost any, if not all, of our springs and sub- 

 terranean waters, and that "such ammoniacal solutions, 

 especially that of the acid carbonate, can carry away silica 

 in solution, and penetrating to great depths become no 

 doubt one of the active agents in metamorphism." * 



It is probable that such agencies which are now active 

 may have been just as powerful in the time when these 

 quartz reefs in question were formed. That time was one of 

 volcanic activity, and probably of a land surface, and if such 

 were the case the waters percolating down into the earth 

 would be ammoniacal, and at times strongly so. 



It is, however, not probable that one set of reactions only 

 was concerned in the formation of the quartz reefs. The 

 silica may have been present, not only in solutions in the 

 manner suggested by Mr. Newbery's experiments and 

 researches, but also as a residuum in much older solutions 

 which had taken part in metamorphic processes. 



* Eeports of Progress Geological Survey of Victoria, Part IV. and Part V. 

 Laboratory Eeport, page 166, et infra. "Formation of Hyalite by the Action 

 of Ammonia"— Transactions of the Koyal Society of Victoria, Vol. XV., 

 page 49. 



