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



break below the Upper Devonian formations. The quartz: 

 veins cross, or are contained in the former, but do not, 

 wherever the contact of the two formations can be observed, 

 pass up into the latter; indeed^ the lowest beds of the 

 Upper Devonian series are made up largely in places of 

 quartz pebbles derived from the denuded Silurian rocks.* 



A.S an illustration of the Silurian tracts in which aurifer- 

 ous quartz reefs are found, I take the district north of 

 Dargo for brief reference. A mental picture of it will be of 

 a great tract of highly-inclined, alternating quartzose and 

 argillaceous beds, rising to over three thousand feet above 

 its lowest valleys. 



The total thickness of these Silurian rocks is much greater 

 than the depth from the highest mountain summit to the 

 deepest valley, for within them there are no traces of the 

 nearness underfoot of the plu tonic rocks. 



This great mass of sediments, which covers more than two 

 hundred square miles between the Dargo and Wonnangatta 

 rivers is traversed, as may be seen in the workings of mines, 

 as well as in natural and artificial rock sections, by joints 

 and fissures, the results of innumerable compressions, dislo- 

 cations, elevations, and depressions by which the strata have 

 been affected. Very many of these lie in the direction of 

 the strike, but others cross it, as well as the dip or the 

 cleavage, at various angles. They all form, when taken in 

 the aggregate, as compared with the great mass of mountains, 

 a more or less connected network of separations in the rocks. 

 Many of them are only planes of discontinuity, but others 

 have been filled by vein quartz from several feet in thick- 

 ness down to the width of scarce more than a sheet of paper. 

 The fissures thus filled by "reefs" of quartz have in their 

 turn been faulted, so that in many places the following of 

 them in mining is a matter of great difficulty. 



In certain localities the quartz is more mineralised than in 

 others, and here it is more usual to find the reefs payably 

 auriferous. It may prove that in the area referred to, as in 

 others, where it has been shown to be the case by the valuable 

 researches of Mr. R. A. F. Murray, the auriferous quartz reefs 

 lie within a certain band, according with the strike of the 

 sedimentary rocks. In the district of which I am now 

 speaking, and of which Grant may be taken as the centre. 



* The occurrence of such conglomerates suggests that the lowest beds of the 

 Upper Devonian series may in places be auriferous. 



