The end of the oil boom in western North Dakota has been just as hard for North Dakotans to comprehend as its beginning was, ten years ago. In 2006, when oil prices were high, prospectors discovered the Parshall Oil Field, triggering a period of furious drilling across the state. Two summers ago, prices began to drop, but it wasn’t until last winter, when the price per barrel fell below fifty dollars, that people really began to wonder whether the season of bounty was over. Government officials quoted rosy forecasts that oil production would remain steady, but another summer passed and production declined. One night in November, a local television station ran a segment in which a bulldozer fed whole trailers, which had been abandoned by oil workers, into a crusher. The trailers broke like eggs.

Walking through the city of Parshall, it is easy to pick out the structures that were built for the boom, with their small windows and thin walls, from those that seem built to outlast it. In this latter category is the Paul Broste Rock Museum, which was established in 1966 by a local farmer, Broste, who fancied himself an artist and a collector of rare stones, and who, in his words, desired “to have a good building so that the collection could be preserved for posterity.” To call the building “good” is an understatement. Broste’s inspiration was the Parthenon, although the end result is a cross between Spanish mission and English cottage. The exterior walls are mortared granite, five feet thick at the base. The doors are made of steel and the windows of heavy glass. To finance the construction, Broste sold his house and his land holdings, and when the museum was finished he moved in. He called the hill on which it was built “the Parshall Acropolis.”

When Broste died, in 1975, management of the museum transferred first to the city, which hired some halfhearted curators, and then to another local rock collector, Robert Jacobson. When Jacobson died, it went to his widow, Doris, who offers tours by request. I called her one morning earlier this year. The museum was closed for the winter, she said, but I could come by in an hour. It was a cold, gray day; inside the building, the lights flickered warmly. Jacobson, who is eighty-one, led me through a maze of locked glass cases containing thousands of colorful specimens: opals; agates; geodes; diamonds; tiger’s eye; fluoride crystals; a dinosaur egg; two halves of a petrified redwood stump; green glass from a bottle factory in Thermopolis, Wyoming, destroyed by fire; lead glass from the viewing window of a bunker at the site of the Trinity nuclear test; and, by far the most valuable rock in the museum (coveted by the Smithsonian, Jacobson claimed), a rutilated quartz crystal from Brazil, polished into a perfect sphere the size of a bowling ball.

Broste’s rock collection is the largest in the state, and one would be hard pressed to find his rarer treasures elsewhere in the country. In 1998, John Hoganson, of the North Dakota Geological Survey, inventoried the collection and deemed it “priceless.” It is indeed remarkable, but as I toured the museum I was struck less by its value than by the idea of it. Many of the rocks—six hundred and eighty of them—have been polished into orbs and balanced on steel structures, which Broste welded to look like trees. It is as though a giant’s game of marbles is waiting to be played. “Paul wanted to make his stones round like the earth, so they had no beginning and no ending and they were infinite,” Jacobson said. She led me into a side gallery called the “infinity room” and shut the door. In the center of the room was a constellation of spheres. The walls were mirrored, and when I looked past my reflection, I saw an endless corridor of swirling rock and steel.

Broste never married, nor did he have children. He once wrote, of the first years that he spent on his homestead claim near Parshall, “Sitting alone in a one-room shack, as one homesteader put it, ‘I could be dead and buried and nobody would know anything about it.’ ” Apart from its geological attractions, the museum also contains trinkets from Broste’s childhood—a rocking chair, a meticulous sketch of his family tree, a life-size photograph of his mother, and oil paintings by Broste of his father, his grandfather, and his uncle Ollie. One would think, looking at it all, that he had an ego. But the museum is less a shrine than a time capsule—intent on preserving memory, capturing it in objects, encasing it in stone. When I asked Jacobson why Broste was so fixated on infinity, she said, “I don’t know. He must have been a very smart man.”

If the building fails, it will be the roof that gives out first. It is flat and drains poorly, and the grout that holds the outer stone is beginning to loosen. To fix it would cost sixty thousand dollars. Jacobson usually receives royalties from oil companies, which lease some of her land, but recently the checks stopped coming. The city’s revenue has declined, too, and although it may fluctuate as production rates rise and fall, the boom, at its original scale, is unlikely to return. “I don’t think anybody was prepared for it to drop off as quickly as it did,” Jacobson said. She led me to the final case, where dendrite crystals sprawled like frost over a mudstone slab. It was hard not to reflect on the boom’s brevity in the midst of so many rocks. And it was easy to imagine the quiet of the museum returning to the prairie, when the derricks and pump jacks become a thing of the past.