The granite intrusion of St. Agnes Beacon, and consequent metamorphism of the pre-existing rocks, 'refined' the local mineralogy into particularly high quality veins of, for example, copper ore and tin. Unsurprisingly, St. Agnes became a centre of the Cornish mining industry, and the evidence is still prominent in the landscape.
This was the engine house of Wheal Friendly ('Rich Mine'), accommodating a 60" steam engine which pumped water out of the workings.