Just outside the center of this once-booming mill town, a towering slab of white stands as a quiet memorial to a part of the area's industrial history. But the 80-foot (24-meter), crescent-shaped wall of a now-defunct marble quarry also shares space with one of North America's most unique geological structures. Just a few yards from where workers blasted a pit that yielded as much as 200 tons of stone per day, a short span of marble arches over a roaring brook to form the only natural marble bridge on the continent. These two worlds