154 Notes on the Area of Intrusive Rocks at Dargo. 



The occurrence of such quartz veins at the contacts and 

 in the schists adjoining, as well as in the plutonic rocks, 

 ■sug-aests the source of the silica. 



It seems that in plutonic rocks, such as the quartz diorites 

 of Dargo and Omeo, the most basic of the constituent 

 minerals have heen the first to crystallise out of the magma 

 in definite forms, thus leaving it more silicious after each 

 successive crystallisation. The gradually increasing acidity 

 of such a magma may be inferred from the observations 

 which I have recorded on the quartz diorites of Noyang,* 

 where the successive intrusive rocks are increasingly 

 silicious, and the latest of them are dykes composed of 

 quartz and felspar only. The study of the Noyang rocks 

 shows also that which has been abundantly proved else- 

 where — namely, that the quartz consolidated from a plastic 

 condition, which was almost certainly that of a colloid con- 

 taining a certain but relatively small proportion of alkaline 

 water, some of which can still be found in the minute fluid 

 cavities, which in places fairly swarm in the now crystallised 

 quartz. In these cases the silica was the last constituent to 

 crystallise, and it moulded itself to the forms of the other 

 minerals, and filled in their interspaces just as the quartz 

 filled in the vein fissures, and moulded the schorl prisms in 

 the case of the quartz veins in question. 



It seems to me, therefore, more than probable that such 

 quartz veins as these represent some of the residual silica 

 of the plutonic magma, after the compound minerals 

 had crystallised out, and that this residuum was squeezed 

 out while in a colloid state into every adjoining fissure and 

 plane of separation. No high temperature v/ould, on this 

 view, be necessary to produce these dyke-like quartz veins, 

 for the exudation of the still colloidal silica was brought 

 about by the reduction of temperature, which caused the 

 plutonic magma to solidify. 



On the strength of these grounds, I conclude that the 

 quartz veins, which I desire to distinguish from those which 

 are auriferous, solidified from the residual colloidal silica of 

 the plutonic masses. 



It may be said with great truth that these quartz veins 

 are of plutonic origin, for they diff'er but little, except in 



* " The Eocks of Noyang." Transactions Eoyal Society of Victoria, Vol. 

 .XX., p. 18. 



