To quote a contemporary observer:
"The work of driving the first part of the adit to Poldice mine was commenced in 1748 and extended to the West boundary of Poldice less than 20yrs. The cost of this was born by the adventurers of Poldice Mine. The adit together with its branches extendeds a distance of over 40miles. Its average depth from surface is about 35fathoms. It empties itself into the Carnon Valley a little above what was then the high water mark.
The water flowing out of the adit on average, gave 1.08 grams of copper to the gallon and the acid equivalent to 2.5 grains of H2SO4 to the gallon. "
Data courtesy of Ben Sum, Helston (8/6/20)
CRO = Cornwall Records Office (AKA Kresen Kernow, Redruth)
Buckley, J A (2000). The Great County Adit. Pool, Camborne, Cornwall: Penhellick Publications
[CRO] Geological Mining Map of Gwennap - R Symons, 1845
[CRO] County Adit District - Thomas, 1818 (Copied and Annotated, 1984)