ORIGIN AND MODE OF OCCURRENCE OF GOLD-BEARING VEINS. 157 



Certain formations ..and classes of rock are associated with certain 

 metals, for instance, granite with tin, clay slate with copper, 

 quartz porphyry with silver, and limestone with lead, and although 

 such an arrangement has been shown to have many exceptions, 

 these, only tend to prove the rule. 



It is w^ell known Iioav the tin, copper, and lead lodes of 

 Cornwall generally alter the leading metals when the formation 

 changes, and we have ourselves seen how gold veins form no 

 exception to such a rule, but not only generally occur with certain 

 rocks, but also depend for their richness on the different belts of 

 country they pass through, — the same lodes being always poor in 

 one kind of rock and richer in another in the same district. 



The Charters Towers reefs in Queensland, and others mentioned 

 in this essay are instances of such influence being exerted by 

 different rocks on lodes. 



It may be considered as a fact that the rocks that are associated 

 with auriferous lodes are principally those that contain magnesian 

 minerals — such as hornblende, olivine, augite, and biotite, — all of 

 which abound in those rocks that contain or are in close proximity 

 to gold veins j and this is not only known to be the case in 

 Australasia, but seems to be. so elsewhere. 



As to those minerals that are found in conjunction with gold 

 in veins, iron pyrites is by far the most common, after which 

 come galena, zinc blende, arsenical pyrites, and copper pyrites. 

 ISone of these, however, hold such a prominent place as iron 

 pyrites, in fact, most of the gold found in our veins is either in 

 iron pyrites or was in it before the decomposition of the pyrites 

 set it free. 



Iron pyrites exist in many of our rocks to a great extent ; 

 granites and other rocks that are commonly associated with our 

 mineral veins are often largely impregnated with it, and where 

 gold is found disseminated through such rocks, it has doubtless 

 been chiefly derived from the pyrites. 



It will be clearly seen, therefore, how lateral secretion accounts 

 for the formation of auriferous lodes. 



That mineral waters have dissolved the metals contained in the 

 rocks adjoining the lodes or close to them, and re-deposited the 

 same in the veins, seems most feasible, and more in accordance 

 with observed facts than any other theory that has been advanced. 

 Of course, such deposits as dyke lodes or ore channels may be 

 formed either by lateral secretion or igneous injection, so far as 

 the main body of the lode is concerned, but the metalliferous parts 

 of the lodes are generally veins of quartz or some other matrix, 

 and these have been formed in the dyke or channels by the 

 process of lateral secretion in every instance, whether the main 

 body of lode was so or not. Tf metals are found as well in other 



