THIS IS COAL, and this is a rock, and this is a rock, but this is more coal," the young man said to the boy as they poked through a dirt-filled sifter box. "We'll keep the coal, but not the rocks. Rocks like these don't tell us anything because they've always been here. But people brought the coal, and people are what we're trying to find out about."

The people into whose affairs they were prying once lived at Mount Clare mansion, country seat of the other Charles Carroll ("The Barrister") and now the centerpiece of Carroll Park in Baltimore's Pigtown section.

The lad had shown up that morning with his parents. They were welcomed, as are all visitors to the city's archeological digs, and the next thing the boy knew he was hard at work with twoscore other volunteers, professionals and paid crew.

"We don't put up keep-out signs because we have no right to do that even if we wanted to," said research director Carmen Weber. "This is the people's project. This ground belongs to them." She smiled. "And besides, we get an awful lot of work out of walk-ons."

The dig is the latest of a dozen conducted by the Baltimore Center for Urban Archaeology, which last Monday broke ground for what will be the first museum devoted solely to the archeology of a modern city.

Some projects have been salvage work, trying to snatch bits of history from beneath the bulldozer's blade; others, like Mount Clare, are designed to research and restore the city's heritage. Of which Crabtown is fiercely proud: For instance, while there has been a certain amount of gentrification going on in Pigtown recently, no one wants to change the name of the neighborhood (through which they used to drive swine from the railroad to the slaughterhouse).

Last summer's dig excavated Margaret Tilghman Carroll's orangery (greenhouse), which inspired -- and partly stocked -- the one at Mount Vernon. The shovels also turned up traces of Mount Clare's outbuildings, bowling green and orchard; this summer's follow- up includes an attempt to define the orchard and other plantings well enough that they can be at least partly reproduced.

Excavation is done in a checkerboard of meter-square test pits plus broad swaths from which the turf is delicately removed by a mammoth landscape grader. For the time being, of course, that raises hob with the terraced lawn, but that's graciously overlooked by the Society of the Colonial Dames of America, which has maintained the mansion since 1917 and is happy to see it getting some of the attention the place deserves as the oldest (1760) residence in Baltimore and the only surviving pre-Revolutionary manor house within a major American city.

Mount Clare is remarkable for having survived at all, but all the more so for having been bought fro the Carroll heirs by the city a century ago, when it had fallen into desuetude and was being used as the clubhouse of social and shooting society. ("Baltimore has always been forward-looking," said a spokesman for Mayor William Donald Schaefer, who created the archeological center.) The rooms, recently restored to their authentic, and rather unusual, pastel colors, are said to contain more original furnishings and family possessions -- including portraits by Charles Willson Peale -- than any comparable dwelling.

The diggers got a bonus recently when the closely-spaced rootstains of what may have been a vineyard and certainly was a large and formal planting were found on the slope below the orchard. "This is a find," said field supervisor Gary Norman, who extended the dig into July to allow for the mapping, pollen studies and ground transections that will be necessary to confirm the discovery.

The work week on all digs is Wednesday through Sunday "because that's most convenient for most volunteers," said Elizabeth Comer, city archaeologist. While walk-on volunteers are not discouraged, "we appreciate it if people will call a day or so in advance, because sometimes we get swamped. Too much help is a nice problem to have, but we can't really deal with more than about 25 volunteers at a time."

She also asks that children under 14 be accompanied by a parent "who's also going to dig," and that volunteers be prepared to work at least four hours.

While Baltimore's "y'all come" approach to archaeology is unique, there's nothing casual about the way the work is done. All of the techniques and constraints of the science are faithfully applied, including painstaking record- keeping and laboratory analysis.

The fun part comes from doing physical labor of lasting importance in company with nice folks. And from watching a nine-year-old boy's eyes widen when he's told the button he just found probably popped off the pants of a Civil War soldier before his father's father's father was born.

BALTIMORE: YOU CAN DIG IT -- Working hours are 8 to 4, Wednesday through Sunday. The Mount Clare dig probably will continue through August. Volunteers should call ahead: 301/369-1866.

MOUNT CLARE TOURS -- The manor house is open from 11 to 4 Tuesday through Saturday and noon to 4 Sunday. Admission is $2 adults, $1 students, 50 cents for children under 12. 301/837-3262.