Like JesiLyn I was fascinated by this photo. Here's what I found on Wikipedia:
"Rookhope is a former lead and fluorspar mining village in County Durham, in England. A local landmark is the Rookhope Arch at Lintzgarth, a few hundred yards west up the valley; one of the few remaining parts of the two mile (3 km) Rookhope Chimney. This "horizontal" chimney (parallel to the ground, which actually rises steeply to the moors) was used to carry poisonous flue gases from the Rookhope lead smelting works up onto the high moor. Periodically, lead and silver carried over in the gases and deposited in the chimney were dug out and recovered, rather than going to waste."