James V. O'Connor points his cane at a low wall of glossy granite, flecked with black, pink, white and gray. Look, he says, at how huge the splats of color in the stone are.

The flashy granite in front of the Capital Hilton Hotel is the ideal material for an entrance, where it's important to show a pretty face: "Bigger crystals give you, 'I'm richer, I'm cuter,' " he says. "They will catch your eye faster."

His tour group of nine out-of-town teachers and their spouses nod their heads, look again. O'Connor, 55, the former city geologist who may know more than anyone else about the raw materials that built the nation's capital, had already told them the granite was quarried in North Carolina. That was a nice factoid, but now he is making them really see.

O'Connor's cane is for pointing, not walking. So far, he does not need it for assistance. But under his floppy straw hat, his hair is falling out. His life is organized in three-week cycles, timed around chemotherapy treatments for esophageal cancer. He recently updated his will.

"I've been under some nasty chemical treatments for cancer," he tells the teachers, who are here for a National Science Foundation teacher awards program. "Today is my first day out in two months."

His skin is pale from his time indoors. He's dropped some weight but still lumbers down the street like a friendly bear, stopping every few feet to make a point or a joke. His manner is easygoing: His favorite compliment is "neat." Even when he criticizes something--modern "shoebox" buildings, for example--he does it with humor, not derision.

As he leads his group around, O'Connor urges them to notice the hidden world behind the world they see. See the ancient fossils on this limestone column. Look at this slope on 17th Street NW that once was a forested valley. Notice the green lines spray-painted on the street to warn that there is sewer work going on below.

The tour group takes two hours to walk one square block, because there is so much he wants to show them. The city is a giant, free classroom, O'Connor tells his group. Just look!

He points to a thumb-sized shape in a beige-gray column. The rock, 300 million years old, is filled with fossils. That teardrop shape was once a living thing.

"That's a critter?" asks one teacher, amazed.

He is legendary among people who have taken his tours for knowing something about what's on or under every street in the city. He can explain what Washington looked like when dinosaurs roamed, can tell the age of a brick by its wrinkles, is fond of asking questions such as, "If you were a street tree in D.C., would you rather be on the number streets or letter streets?" (The east-west letter streets are better, because they get more sun.)

For most people, a stone is an inert surface. For O'Connor, it's a vista into the past.

"A rock is a frozen environment," he tells the tour group. "The job of a geologist is to unravel that environment. Was it a dune? Was it a beach? Was it a river?"

Strength of Faith In January, O'Connor mentioned to a few people that he felt tired. Some wondered whether it might have to do with being a late-in-life parent: He and his wife, Rosanne, have a 4-year-old daughter adopted from China. Then, in March, he couldn't seem to swallow food. He began to hiccup and couldn't stop.

The cancer was discovered quickly after that, and he began treatment. First, a cold-laser treatment to clear the esophagus so he could eat. The doctors ordered him to shun the sun because the treatment made his skin ultra-sensitive to light. Having to stay indoors, he said, "was probably the most depressing thing" that's happened since the diagnosis. He still covers up from head to toe in the sun.

He is now midway through eight chemotherapy treatments. The first week after each he feels fine, goes out on the town. The next week, he plummets, tells his friends to stay away. The third week, he recovers.

Although he does not often express emotion, O'Connor is facing the possibility of death squarely, said those who know him. The scientist in him searches out everything he can find about cancer. The spiritual man--the one who trained for the Jesuit priesthood before he decided to become a geologist instead--finds solace in deep religious faith.

"He has a deep and rich faith in the connectedness of things and the natural order of things," his wife said. "The order is connected to a divine being. Jim lives that as something concrete. That's not to say he doesn't have fears."

Or tough times. He threw up every day for a week. He burned his palms when he tried to go out in the sun too soon after his laser treatment. He has to give himself injections. He's also gotten a small thrill out of seeing his own X-rays.

"Some of that has been interesting from a scientific standpoint," he said,"for a few minutes."

His voice sounds as if he arrived from Boston yesterday, but he has been here nearly 30 years. For most of his career, he taught at the University of the District of Columbia and was the official city geologist. Along the way, he developed the unusual talent of linking science to the local environment.

Most D.C. public school science teachers have taken a class or a field trip with him, as have dozens in Maryland. For years, he has led public tours for the Smithsonian Resident Associates and the Audubon Naturalist Society, in which he uses the built environment--structures, manhole covers, even grave stones--to talk about geology, history, social trends. He's taught a similar class for the U.S. Department of Agriculture Graduate School.

When the Anacostia Museum wanted to develop an Afro-centered nature trail, it turned to O'Connor. When the museum wanted to run "environmental justice" bus tours of pollution sites in poor neighborhoods, he filled the bill. Last summer, he received a grant to hold outdoor geology workshops for children at several D.C. housing projects.

Two years ago, UDC fired him as part of a broad downsizing purge. He got by with a patchwork of freelance jobs until he was hired last fall by the D.C. Health Department as the city's hydrogeologist, which means he works to assess and protect the District's underground water. He works from home now, on a reduced schedule.

"He gets this job, and everything is turning up and--whammo!" said Raymond Rye, a friend and geologist in the paleobiology department at the Museum of Natural History. "That's what hits me."

Pillars of Support

The news buzzed via e-mail and telephone lines this spring through O'Connor's expansive network of friends and admirers--scientific types, environmentalists, historians, park rangers, former students.

Cards, e-mails, video get-well greetings flooded in. Replacements were found for most of his tours; a few had to be canceled. People come to help from Hospice Caring, a Montgomery County volunteer group. Friends telephone his house in Kensington for intense, two-hour conversations.

People swap stories about him--the time he got 30 teachers to lie down on the steps of the Lincoln Memorial to look at the curves of the Potomac River shoreline. The time he came to a chapel wedding in field clothes, with the permission of the bride, who was also a scientist. The way he can unearth learning sites anywhere--who else knew, for example, that you can see fossils in a neighborhood of warehouses in Bladensburg?

Many of his friends, like him, are trained to draw connections between cause and effect. They railed about the illogic of it all.

"No way," said Julia Washburn, chief ranger at Rock Creek Park. "He doesn't smoke, doesn't chew tobacco. We like to look for reasons why people are ill."

These days, many of his friends are in limbo. O'Connor's daily needs are taken care of, for now. He looks pretty good, all things considered. Maybe, if he worsens, they could organize a meal delivery network. But now, what role can they play?

"We all feel kind of helpless," Rye said. "We want to do something but are not sure what we can do."

The act of finding substitutes for O'Connor on tours brought home to some people the rarity of his combination of talents. In science these days, the money is in high-tech laboratory studies, not outdoor field work. Those scientists who work outside often do so far away, in more glamorous places. It's hard to find someone who is a scientist, who is steeped in the local environment, who can explain it in everyday language and vivid metaphor, who can incorporate poetry and sculpture along the way.

"A person with all that talent--with all that incredible information in his head," said Jane Huff, education director of the Audubon Naturalist Society. "What's going to happen to this incredible knowledge?"

Friends have been pushing O'Connor to put more of his knowledge on paper. Two former students--a married couple who met in one of his classes--hope to videotape him talking about the natural and human-crafted world.

His desk is piled with papers. There is the Smithsonian grant to develop a science-in-your-neighborhood educational program he calls "stone sleuthing." There is the new walking tour of Washington he is designing for hydrologists that will include the multiple sump pumps in the Ronald Reagan Building. There is the groundwater plan the city is supposed to submit to the federal government in July.

He is working on the curriculum for next year's classes for Montgomery County science teachers. "Whether I will be able to do them," he said, "depends on what happens."

"There's a lot to be done," O'Connor says, "before I retire in one way or another."

CAPTION: During the tour, Jim O'Connor answers a question from Tom and Iiona Sunday, right, from South Carolina.

CAPTION: Jim O'Connor points out brick features on a downtown District building. Chemotherapy has reduced the frequency of his popular walking tours.

CAPTION: Martha McIntyre, left, and Tom and Iiona Sunday look at the grain in the stone in a column on a building at 16th and I streets NW on a tour led by Jim O'Connor.