88 
ECONOMIC GEOLOGY 
Mt. Lyell schists (porphyrite lavasand volcanic ash which have 
been crushed into sericite schist), with the massive conglomer- 
ates and quartzites of the West Coast Range of Tasmania. 
The conglomerates are 
probably Devonian and 
occur E. of the Mt. Lyell 
schists, which are thrust 
over the conglomerate by 
a long and complex fault, 
associated with cross- 
faults and thrustplanes. 
In places wedges of con- 
glomerate have been 
thrust into the schists. 
The Mt. Lyell Mine is in 
a bay of schist which has 
been nipped between 
three faults, and was so 
shattered that it was 
completely permeated by 
ore-bearing solutions. 
The ore is an irregular 
boat-shaped mass of iron 
pyrites which contained 
over seven million tonsa 
It was at the surfac: 800 
feet long by 200 feet 
broad; it widened below 
to 300 feet, but at lower 
levels decreased in size, 
and ended abruptly, at 
the depth of about 750 
feet, over the under- 
thrust conglomerate. The 
ore on the footwall side 
contained about 2-35 per 
cent. of copper with a 
little gold and silver; its richness was probably due to the 
solutions being nearly stagnant and giving time for the 
precipitation of the metals. The ore along the hanging 
wall was so low grade that it only paid to mine as fuel and 
Lf