JAMESTOWN, CALIF. -- Jim Halstead, on his knees in the shallow, muddy waters of Woods Creek, was determinedly digging out fist-sized rocks that dotted the creek bottom, thinking about the gold that he calculated might lay beneath them.

Minutes later, he saw the first sign that he had come to the right place. A piece of white quartz bisected by a thin line of gold washed into his sluice box.

It was a good omen, enough to catch the interest of the veteran prospector and excite the tourists from Illinois spending the day learning his techniques.

Halstead, 38, has sought gold for half of his life, first with friends who "used to go up in the hills and pretend like we were miners" and later, more seriously, on his own. Today, he works for Ralph Shock, owner of a Jamestown store that sells mining supplies, buys gold and takes customers out on expeditions to teach them how to find their own.

The discoveries are never a sure thing. Even though Shock tells customers that gold has turned up in every stream in his territory, located in the gold country that was the destination for many Eastern adventurers in the 1849 Gold Rush, he also warned those attending a morning orientation session, "There's a lot of guesswork."

"In 1849, they had no idea where the gold was," Shock said. "They had . . . guesswork. Most people were not lucky then, and they're not lucky today."

Shock himself was plagued by bad luck when he opened his business 14 years ago. How-to books sent him on fruitless searches of river banks, and old-time prospectors entertained him with their life stories but were unwilling to share their secrets.

"For two years, I could tell you every place gold was not," recalled Shock, 57, who left an auto-leasing company in San Francisco after the oil crisis of the mid-1970s to live in the westernmost hills of the Sierra Nevada.

But gradually, he learned the business, in part because he offered to buy large quantities of gold from prospectors who could help to point him in the right direction.

Today's prospectors strike it rich at least as often as their 1849 predecessors, said Shock, who wears gold around his fingers, wrists and neck. "You just don't hear of the big finds here because of security."

But of the more than 7.7 million ounces of gold that the government estimates were mined last year, most of the "big finds" belonged to commercial mining companies, not individuals working their own claims. By comparison, prospectors in 1849 mined less than 2 million ounces.

Those commercial operations, concentrated in Nevada, California and a few other Western states, use modern mining techniques that often enable them to extract and process gold from low-grade deposits that would have escaped the notice of prospectors using traditional methods.

"The grizzled prospector with a burro is no longer a significant participant in the search," the U.S. Geological Survey says in a publication that extols the thrills of finding gold and then warns of the "long and possibly discouraging campaign of preliminary work" that generally precedes any discovery.

"A successful venture does not necessarily mean large profits," the agency cautions.

Such warnings have done nothing to discourage the old-time prospectors or the newcomers at work in Woods Creek, just down the road from one of the big commercial mines.

Richard Rowe, working with Halstead, is an electrician who moved from Oklahoma to California for a job in construction. "Now I've got gold fever," he said. So he spends his days helping Halstead, learning from him but anticipating a return to his trade when prospecting slows in the fall.

Another of Shock's employees, who asked to be identified only as "Tex," said prospectors have "got to be careful" not to reveal too much about themselves. He has spent four years teaching novices how to search for gold.

"It looks easy, but it takes time," Tex said, surveying half a dozen new students who swirled water and creek-bed pebbles in their pans, trying to wash out all of the stones and hoping that small nuggets of gold would emerge.

"If it was easy, everyone would be doing it," he said. "You're not going to come out for a day or an hour and get rich. It's just for the experience."

But sometimes the experience is profitable. Shock told of a tight-lipped couple who wanted to sell him 11 pounds of gold nuggets. Eager as they were to sell their gold, they refused to tell him where they had found it or even what their names were.

Halstead said that, on his best day, discovery of 3 1/4 ounces of gold netted $1,900 for his students. In 1981, working on his own, he tapped into a hole in Woods Creek that eventually produced 400 ounces.

Halstead made his living as an independent prospector for more than a decade, he said, taking on a more predictable sideline as a woodcutter only after his wife gave birth to twin sons six years ago. Later, he went to work for Shock.

"We didn't get rich, but we made a good living," he said. "I had my heydays and I had my bad days . . . . It equals out to just like a job -- $8 to $10 an hour. But to put a real dollar sign on it would be hard."

Over the years, he has probed nearly every creek in seven counties where the riches of 1849 were concentrated, convinced that each holds gold. The number of specific caches there that he has tucked away in his mind's catalogue is "beyond imagination," he said.

Customers eventually will see many of those spots, but Halstead has plans for the ones they don't see.

"On my days off," he said, "I'm right out there doing it again."