194 
NOTES ON THE MOUNT LYELL MINE. 
By E. D. Petrrs, Jun., M.E., M.D. 
The western coast of Tasmania possesses but few indentations that can 
serve as reasonable harbours. Of these the most important one, com- 
mercially speaking, is Macquarie Harbour, on which is situated the port 
of Strahan. The country north of Strahan was subject to enormous 
fluctuations of level during early Silurian days, and deep seas alternated 
with wide, but shallow, lagoons, which in time rese into rugged hills, 
only again to disappear beneath the waters and lose their identity under 
the mud and pebbles that filled up the depressions and levelled the sea 
floor as with a smoothing iron. The mud and silt brought down by the 
arcient rivers, hardened into clay, slate, and schist, the pebbles were 
cemented into the ubiquitous conglomerate; the blanketed sea floor, 
unable to lose its heat by radiation, sank deeper and deeper, causing the 
crumpling and upheaving that led to the last cycle of mountain-building, 
and the general configuration of the country became perhaps something 
as we now see it, though no doubt much lowered and scored, as well as 
filled up, by erosion and glacial action. Let us for a moment go back 
toa period just before this last upheaval, and imagine a shoal pond or 
series of ponds nearly filled with the pebbles brought into it by the foam- 
ing rivers of that period, and undergoing, in common with the region 
surrounding it, a slow subsidence, say something like the coast of Norway 
at present, which I believe amounts to a considerable number of inches in 
a century. Owing to causes that we have no time to consider, the roar- 
ing mountain torrents were diverted or suppressed, and were replaced 
by more sluggish and feebler streams, that flowed through extensive 
bands of a schist rock before entering our chain of ponds. This belt of 
schist contained then, as it does still, specks of sulphides of iron and 
copper—pyrites—scattered through it, and although the amount of 
these sulphides in a single cubic yard of the rock was very small, yet 
their aggregate in even a single quarter of a mile of the belt was 
enormous, as can easily be determined at the present day. Pyrites, 
under these conditions, decomposes very rapidly, and forms soluble 
sulphates of iron and copper, whilst a considerable proportion of any 
silver that may be present is also dissolved by the waters of the stream. 
Even gold will go into solution to a minute extent, especially if the 
waters of the stream contain a little chlorine. This extremely dilute 
‘*mineral water” enters our pond, or ponds, through many different 
little trickling rills, and we can easily imagine the flow to be so slow, 
and the evaporation from the extensive surface of shoal water to be so 
great, that the solution is percep+ibly concentrated in its sluggish passage 
toward the outlet of the chain of lagcons. The evaporation might even 
equal the supply, in which case we should have a ‘‘ great salt lake,” with 
a decided admixture of the metals referred to above. But there are no 
evidences to warrant any such conclusion, whilst there are strong grounds 
for believing that the amount of salt in the water was not only very 
small, but that the lakes hadan outlet. Buta new element must now be 
introduced, without which [ fear that the wealth now locked up safely 
in the Mount Lyell mine would have quietly flowed out of the ponds 
exactly as it went into them, and eventually have gone to augment the 
metallic contents of the oceans. In most parts of the world, though 
apparently very rarely in Australia or Tasmania, nearly any local 
geologist could point out to you aswamp or peat bog into which streams 
discharge that have percolated through slate or schistose rocks carrying 
pyrites. If yeu watch carefully at the point where the sluggish, acrid 
waters of the stream begin to mingle with the black, peaty liquor of 
