Notes on the Area of Intrusive Rocks at Bar go. 157 



portion of the crust gave way, and elevation was the result. 

 The accumulation of the immense thicknesses of the Silurian 

 formations, estimated by Dr. Selwyn as being at least 35,000 

 feet,* implies a long-continued period of depression. 



My own observations in Gippsland have shown me reason 

 to believe that the south-eastern part of Australia, as 

 evidenced in the Australian Alps, was subject to extensive 

 elevation at tlie close of the Silurian age, which culminated 

 in a volcanic period, evidenced by the Snowy River porphy- 

 ries, which are stratigraphically between the Upper Silurian 

 strata and the Middle Devonian beds of Buchan. 



The compression of the Silurian formations into acute 

 folds, together vrith elevation of the crust, would tend to 

 give room to the imprisoned plutonic magmas when forced 

 into the rising crust of the earth. Such movements as 

 these must have been necessary to produce the results which 

 denudation has laid bare at Dargo and the adjoining areas. 



The contact rocks produced by the action of the plutonic 

 masses upon the adjacent sediments are mainly varieties of 

 schistose and crystalline hornfels, normal in their character, 

 and not differing materially from similar rocks which have 

 been observed and described in other parts of the world. 



One of the most interesting features in connection with 

 the Dargo and neighbouring areas are the auriferous quartz 

 lodes at the contacts. Their interest and importance do not 

 arise out of their economic value, which is small, but from 

 the light which their study is calculated to throw in the 

 future, not only upon their own origin and formation, but 

 also on that of auriferous quartz reefs generally. 



In these notes I have described the relations of the 

 contact quartz lodes and veins to the adjoining formations, 

 and I shall now speak of the quartz reefs in the Silurian 

 tracts at a distance from the intrusive areas. 



The quartz reefs and smaller veins in the Silurian forma- 

 tions of North Gippsland were formed after the sediments 

 were invaded by the plutonic rocks, and before there had 

 been complete subsequent cooling and consolidation. The 

 limits of this space of time are fixed by the folding together 

 of the Silurian strata"!" and the complete stratigraphical 



* Intercolonial Exhibition Essays, 1886. Notes on the Physical Geography 

 and Geology of Victoria, p. 11. 



t In parts of North Gippsland the Upper Silurian beds have been folded in 

 this manner, as well as the Lower Silurian. 



