196 NOTES ON THE MOUNT LYELL MINE. 
possible to form some slight opinion as to its probable extent in depth. 
Take any ordinary lagoon, or shallow swamp-hole, and we may say in a 
_ general way that its length or breadth is many times its depth. An 
ordinary shoal pond of this kind might be perhaps 10ft. deep, and 
certainly 500ft. to 5,000[t. in length. And if it were 100ft. deep we 
should expect its length and breadth to be very considerable. But the 
Mount Lyell mine had an original depth—present width—of over 300ft., 
so that we may reasonably expect that it will continue in depth—former 
horizontal extent—to a distance far beyond our powers of penetration. 
The experience of such mines all over the world bears out these theories 
to perfection, and I caunot, at present, recollect a single large body of 
pyrites that has ‘“‘played out” in depth. The 200it. tunnel in the 
Mount Lyell mine shows that at this depth the deposit is thicker than 
at the surface—a plain indication that we are as yet working at or near 
to its upper edge, for it is likely tu go on increasing gradually in thick- 
ness until the centre of the former swamp-hole is reached, after which it 
will gradually grow thinner, as the pond shoaled towards its farther 
bank. The filling of an ordinary pond will usually give us a more or 
less lens-shaped body, and it is a notable fact that this is so commonly 
the shape of these deposits that they are often called ‘‘ pyritic lenses.” 
While this great body of sulphides was deeply entombed under hundreds 
of feet of more recently formed facts, it is probable that but few changes 
occurred within itself, as air and water, the two great agents of decom- 
position, were practically absent. But as soon as one edge of the deposit 
was raised to the surface, the unceasing action of these powerful agents 
began, never to entirely cease until every particle of sulphides in the 
entire deposit have been modified up to their highest possible pitch of 
oxidation and hydration. In the vase of iron pyrites this final stage is a 
hydrated oxide of iron, or frequently a hematite. The copper of the 
pyrites, being easily changed into a soluble sulphate, is soon dissolved 
and carried away by water. With these facts in mind, let us examine 
the present upturned edge of the deposit at some point where it has not 
been covered by gravel and debris rolling upon it from the hill above 
it, and see how actual results correspond with our theories as to what 
ought to take place. Here we find, just as we should expect, an immense 
mass of oxide of iron, partly hydrated and partly nen-hydrated. It 
contains no copper—copper as 1 have explained being so extremely 
soluble that it quickly disappears in a wet climate—the silver veins, 
is, on the whole, increased. The latter phenomenoa may seem peculiar 
at first sight, but it is strictly as we should expect from our knowledge 
of the properties of this metal. Gold is almostinsoluble, whilst copper 
is extremely, and iron slightly, soluble, as sulphates, and the sulphur 
itself all disappears by oxidation or solution. Hence from the mere 
change of the sulphides into an oxide, we have a decided diminution in 
weight, and a consequent concentration of the gold. But a still more 
potent agent assists the enrichment of this surface ore, or gossan, in gold, 
especially very close to the surface. This is the removal of oxide of 
iron mechanically in minute particles, either as a dry dust, by the 
wind, or more deeply by the trickling of drops of water through the 
easily permeable gossan, and the mechanical removal of particles of 
oxide of iron as fine scum, to be deposited, perhaps, a few feet further 
on, as a bed of iron-echre or iron-sinter, entirely free from gold. Plenty 
of such material is fouod near the surface of this and similar deposits. 
Thus certain portions of the gossan lose all their copper and sulphur, and 
much of their iron, and having lost 2 3rds to 5-8ths, or perhaps even 
19-20ths of these elements, become correspondingly richer in gold. The 
accuracy of this explanation is very curiously and beautifully attested by 
the presence of a substance that [ have not yet mentioned, but that 
exists in the Mount Lyell mine ore to an extent so minute, to be sure, 
that only chemical analysis assures us of its presence, but that yet does 
