Jed Cartwright and the Lost Gold Mine

"I still say there's gold left in the Bluebird Mine," the voice said. "Old Man Potter got that gold from somewhere!"
"Yeah, but that was thirty years ago," a second voice replied. "Potter died without leaving a clue to the mine's whereabouts."
"Potter made a map," the first voice asserted, "and I know how to find it. Think of all that gold sittin' there, jest waitin' for someone to come along and find it! It's as good as ours."
"And jest how are you goin' to go about findin' the map?"
The boys stared at each other, wide-eyed. Just then, Seth leaned against the counter, and a small glass bottle crashed to the floor. The voices stopped.
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When Jed and his new brother Nathan accompany Pa Cartwright on a trip to the Colorado Territory, they learn of a gold mine that has been lost for more than thirty years. Setting out to find it, the boys discover that someone else knows the mine's whereabouts. And that someone is willing to kill to protect the secret.