W. Sxey.—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 continudus 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 difference 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 s 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. TIL, Art. XL. 
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