The caption of this photo reads "Puerto Rico Mine-furnace at mine, 3-ft thick walls", in Mexico. Notice the two cables coming down the side of the wall of the structure that's to the right and behind the furnace. I wonder if we're looking at the southern most terminus of the aerial tramway? It makes sense because some sources say the ore was crudely processed prior to shipment. This photo was provided to me by the national Park Service. Raymond Skiles, the photographer, is a wildlife biologist at Big Bend National Park.