Asparagus Island with its shafts and adits from Samphire Island.

Porthcadjack. I used to like to eat my sandwiches from this location on top of the crest of Samphire Island.
I think these were no more than exploratory mines, sunk more in hope than expectation. Nonetheless you have to admire the skill and the bravery! Access only by boat or from above; neither easy at all!
Where you can see the orange lichen, such as under my foot at the top of Samphire Island, that would be granite. Much of the other rock is Devonian sandstone. Places such as this with a mish-mash of rock types might indicate mineral presence as well which is why the prospective miners had hopes. Much of the tin, copper etc is to found right on the edge of granite country.