434 It. DA1NTREE ON CERTAIN MODES OF 



appear in larger proportion, as they are then more readily dis- 

 tinguishable than in the decomposed rock, where they are usually 

 represented by iron-oxide or empty cubical cavities *. 



Below the zone of decomposition, however, we generally lose a 

 class of auriferous veins which has proved very misleading to the 

 miner, though usually very rich in gold. These usually follow the 

 line of jointing in the rock, and are, in my opinion, simply due to 

 the decomposition of the auriferous pyrites and of the country rock 

 and the redeposit of such of the decomposed material as passed into 

 chemical solution in local fissures. 



An instance of this is well shown in enlarged microscopic section, 

 fig. 4. This is enlarged 9 diameters, and is from the Black Snake 

 district in Queensland, where such veins are numerous. It will be 

 seen that the vein (the first constituents of which seem to have 

 consisted almost entirely of quartz) has been opened, apparently by 

 the elastic force of crystallizing minerals, both from the sides and 

 the centre, the crystallizing mineral holding within it fragments of 

 detached quartz. 



Looking at this vein, however, teaches distrust of the reasoning 

 that pyrites even containing included detached portions or crystalline 

 constituents of the bounding rock would be formed contempo- 

 raneously with such rock. 



As the pyritic constituent in the intrusive rocks of the Black 

 Snake mining- district is chiefly copper pyrites, the auriferous veins 

 sometimes contain as much as 20 per cent, of copper ; and black 

 patches of the vein shown in fig. 4 consist principally of oxide of 

 copper and metallic copper with some oxide of iron ; the gold is 

 rarely visible to the naked eye. 



Figure 3, of a pyritous felsite from the Alexandra diggings in 

 Victoria, will give a good idea of the probable commencement of 

 such a vein as is shown in fig. 4 in a more advanced stage. It is 

 enlarged 60 diameters. 



It is probable that to this mode of formation are to be attributed 

 all such horizontal veins as those depicted in Mr. Selwyn's Exhibi- 

 tion Essay, 1866, of Shakespeare's Heef, Gaffneys Creeks. 



Besides these veins, however, there are associated with the in- 

 trusive auriferous plutonic rocks others of far more practical 

 importance, as many are likely to be persistent in depth and 

 generally of greater width, namely those formed by hydrothermal 

 action, which preceded and was contemporaneous with the intrusion 

 of the rock, and continued in some instances long after the intrusive 

 rock itself had cooled down. 



These fill cracks formed by the explosive power of the gases, into 

 which, in some cases, the fused material did not penetrate, as well 



* Near Baynton's Station, in Victoria, a dense basalt is impregnated with 

 magnetic pyrites ; and all the copper-ore of the Okatiep and other mines of 

 Namaqualand is obtained from highly felspathic dykes which show no signs of 

 decomposition at a short distance from the surface, the ores (pyrites and peacock 

 ore) being distributed through the rock partly in small grains, some of them 

 not larger than a pin's point, and partly in masses which may attain a weight of 

 several cwt. 



