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I'm searching for a sci-fi short story. Back in about 1978, I read a story I'd swear was called "Footprints in the Sands of Time." I'm pretty sure it was written earlier, since it was in a class on classic science fiction. I think it was in a collection of short stories too. I've googled this and similar titles, but no luck.
The plot involves someone going back in time and leaving a capsule of radium behind to prove they made the trip. Time was described as a "coiled spring," where the successive turns were separated by 60 million years (I think).