Notes on the Area of Intrusive Rocks at Dargo. 159' 



veins of quartz occur throughout the mass of the Silurian 

 formations from the summits of the mountains down to the 

 bottoms of the deepest valleys. The great thickness of these 

 rocks, which has been denuded durino- the lonof continuance 

 of terrestrial conditions in the Australian Alps^ was also 

 similarly traversed by quartz veins, as is proved by tiie 

 quartz gravels of the old rivers of Middle Miocene age which 

 are now situated almost on the summits of the mountains, 

 more or less covered up by flows of basaltic lava.* 



To complete the mental picture, one must also conceive 

 the Silurian strata with their quartz veins, extending down- 

 wards to the plutonic rock masses — at whatever depth below 

 the present surface these may be situated. This inference 

 is fully justified by that which one can observe at places 

 where, as at Dargo, denudation has laid bare the contact 

 of the two formations. 



In endeavouring to explain the formation of the quartz 

 reefs in this vast mass of Silurian sediments, which is only 

 the remains of a once much larger mass, it seems to me that 

 one is forced to assign as a cause the action of solutions 

 which have derived their silica and their gold also from 

 the strata through which they have percolated. Herein 

 they are distinguished from these quartz veins to whicli I 

 have before referred, which are found in the plutonic I'ocks, 

 or in the schists immediately adjoining them. 



If I am correct in saying that they were formed during the 

 interval of time from the close of the Upper Silurian period to 

 the close of the Middle Devonian period, then it is probable 

 that their formation was due to causes which lay between 

 two extremes. That is to say, to solutions intermediate in 

 character between those which existed at the time of the 

 invasion of the sediments by the plutonic rocks, or to those 

 which existed at the time when the plutonic action had 

 abated or had almost died out. The former would be 

 mineralised solutions acting under a high temperature and 

 great pressure ; the latter would be solutions remaining after 

 a long course of mineral regeneration, and under conditions 

 of much lowered temperature and pressure. The action of 

 solutions of the former kind seems to have been towards the 

 regeneration of the sediments as metamorphic schists. As 

 to the latter, we may suspect that they were most likely 



*The plant beds contain, among others, Cinnamomum polymorphoides 

 (M'Coy). 



