438 ON CERTAIN MODES OP OCCURRENCE OF GOLD IN AUSTRALIA. 



districts in Victoria. It is probable that in depth all snch veins will 

 be found connected with the neighbouring intrusive rocks, as at the 

 Upper Cape, Mount Davenport, and Specimen Gully at the Cape-Kiver 

 diggings, Queensland. 



8. In quartz veins traversing metamorphic rocks of undetermined 

 ages, in some cases associated with dykes of plutonic rock, in others 

 apparently free from such influence, e. g. the Cape and Peak Downs 

 diggings, Queensland. 



9. In quartz veins traversing Lower Silurian rocks without any 

 apparent connexion with plutonic dykes. 



These cases are only met with in Victoria, to which colony, I 

 believe, the Lower Silurian rocks are confined. These veins are 

 generally thicker than those associated with " dykes," but do not 

 afford one quarter the yield of gold per ton (the gold from this 

 class of vein also contains, as a rule, far less silver than those con- 

 nected with " dykes "), are not nearly so pyritous, and rarely have 

 an admixture of calcspar "gangue." 



EXPLANATION OF THE PLATES. 

 Plate XVII. 



Fig. 1. Section of an auriferous quartz-vein, enlarged 2^ diameters. 



The gold is represented in black ; and it will be observed that the 



coarsely crystalline quartz of the outer walls is entirely free from the 



precious metal. 

 Fig. 2. Section of a highly pyritous auriferous diorite. The black shows the 



pyrites in which fragments of the constituents of the diorite are 



enclosed. This section is enlarged about 60 diameters. 



Plate XVIII. 



Fig. 3. Section of a highly pyritous auriferous felsite from Victoria. It is en- 

 larged about 60 diameters. The quartz vein, which is a mere streak 

 in the original section from which it was enlarged, is evidently con- 

 temporaneous with the rock ; but the pyrites in it is much more de- 

 composed, and the commencement of a secondary vein is evidently being 

 set up. 



Fig. 4. Section, enlarged about 9 diameters, of an auriferous quartz-vein in a 

 porphyrite ; the materials of the vein having evidently been formed 

 from the decomposition of the ingredients of the bounding rock, 

 which is highly charged with decomposed copper and iron pyrites. 

 The black in the vein is chiefly iron- and copper-ore, though metallic 

 gold is visible in minute specks. 



