[ 



Notes on the Area of Intrusive Rocks at Dargo. 155 



bulk or in the final conditions of their crystallisation, from 

 the silica of the holocrystalline rocks,* 



I may sum up my remarks by saying that these pi atonic 

 quartz veins were formed at the cessation of the invasion of 

 the sediments by the great igneous masses, and when these 

 latter were crystallising and the temperature had begun to 

 lower. They have not produced mineral changes in the 

 rocks containing them, nor are they ore-bearing. 



The auriferous quartz veins of the kind found at Dargo 

 were formed at a later time, Avhen the temperature had fallen 

 still more, and when the cooling solutions deposited their 

 mineral or metallic burdens in the fissures they permeated, 

 or were precipitated by other solutions percolating from the 

 bounding rocks or from above. When these auriferous 

 veins occur in the plutonic rocks, the " country" bounding 

 the fissure is generally found to have been much altered, 

 and to be also more or less charged with the same ores that 

 enrich the lode. In these cases the Assuring and formation 

 of the lode is clearly subsequent to the consolidation of the 

 rocks. 



Although the interval between these formative processes, 

 that of the plutonic and auriferous quartz veins, was no 

 doubt vast, for the cooling of the plutonic rocks and of the 

 heated sediments must have extended over what we should 

 call ages of time, yet both were parts of the same great 

 sequence of events which commenced towards the close of 

 the Silurian age, and extended far into the Devonian period 

 before it terminated. 



Conclusion. 



I have found it always advantageous to summarise the 

 conclusions to which the study of any subject has led. By 

 so doing a clearer view of the field of inqnirj^ is gained, the 

 connection of the various observed facts becomes more 

 apparent, relations show themselves which were not before 

 seen, and it not infrequently happens that it is possible to 

 frame a tentative hypothesis explaining the observed facts. 



* I have been gratified to find that these views, to which the study of 

 metamorphism in the Gippsland Alps has led me, are substantially those 

 which Professor Lehmann has recorded in his magnificent work on the origin 

 of the crystalline schists. Among other passages which are worth the most 

 Berious consideration by geologists, I note his remarks on the subject of 

 those quartz veins which I venture to term "plutonic." See p. 56 — Die 

 Entstehung der Altkrystallinische Schiefergesteine— Bonn — 1884. 



