“More than two hundred and fifty million years ago the mountains of Great Britain fit together like a jigsaw puzzle,” the Appalachian novelist Sharyn McCrumb has written. These twin mineral trails are reminders of the ordinary miracle of continental drift, the kind of thing you learn about in grade school but can so easily forget as an adult that to recall it instills a fresh sense of awe. Legend has it that serpentinite was a valuable commodity to ancient Celts and some even believed it contained mystical powers.

Beneath the Appalachian Trail, an underground thread of earth-green mineral runs from Georgia up to Nova Scotia-geologists and locals refer to it as “the serpentine chain.” It is mirrored by an almost identical vein of verdant serpentinite all the way across the Atlantic Ocean, this one running through the mountains of Ireland, Wales, and Scotland, all the way up to the Arctic.