W. Skey. — Formation of Gold Nuggets in Drift. 381 



is also deposited by pyrites, as I attempted to show in the paper just alluded 

 to, and if we assume that the strength of the gold solutions forming these 

 varieties of gold respectively was not greatly different, it is only reasonable 

 to suppose that the gold masses formed in this manner in drift would attain 

 the greatest dimensions, for in the first place this gold in depositing would 

 certainly aggregate more as the pyrites in the drifts or river beds would be 

 less continuous and more sparsely distributed than that in reefs. 



Secondly, the supply of gold to pyrites lying in these drifts or river beds 

 (and so exposed to rapidly changing waters) would be far more copious than 

 to pyrites cooped up in a rocky fissure, and so in contact only with water 

 stagnant or nearly so. 



And, thirdly, as regards the generally superior quality of these nuggets to 

 gold found in the reef, it will, I think, appear from the following considera- 

 tions that such a difierence in favour of drift gold is to be expected. 



I have previously shown'" that silver is deposited with greatest rapidity 

 and certainty upon pyrites from solutions which are alkaline from presence 

 of the fixed alkalies or alkaline earths, and that as such solutions are passed 

 from this condition to an acid one the silver present in them is retained in 

 solution ; any gold, however, that may be mixed with such silver is deposited 

 upon this reducing agent, no matter which of these conditions the solvent 

 is in. 



Now this alkaline condition is precisely that in which, as far as we can 

 ascertain, our lodes or rocks must have been at the time of the deposition of 

 the gold and silver now found in them, and this alkalinity would especially 

 manifest itself in those reefs which traverse rocks of a basic nature, such as 

 diorites or serpentines : hence, by the way, the large proportion of silver 

 alloying the gold found in these reefs, as compared with that alloying the gold 

 found in the lodes of our schists or older formations. 



But though the waters percolating our reefs must be to a more or less 

 extent of an alkaline nature the drainage waters issuing from them will lose a 

 portion of this alkalinity as they are exposed to the air, or to the products of 

 decomposing organic matters, from having absorbed a quantity of carbonic or 

 other acids (sulphuric, humic, etc.), thus in some measure, according to the 

 distance such waters have travelled from their springs, will their condition 

 be changed until their alkalinity may give way to neutrality, or even acidity, 

 either of which conditions are, as I have stated, unfavourable to the liberal 

 deposition of silver along with gold from such waters. Hence it is apparent 

 that from the instant the waters percolating rocks or lodes leave them to form 

 springs, etc., they are continually passing from a favourable condition to one 

 eminently unfavourable for the deposition upon pyrites of what silver they 

 * Trans. N.Z. Inst., Vol. III., Art. XL. 



