OUR town lost an ardent advocate when genial giant Jim O'Connor died in the midst of his crusade to get Washingtonians to look around them and see how the federal city was made.

As the District's geologist, O'Connor loved to lead tours around town in which he used the stone with which Washington was built and paved to illuminate the history of both the city and the planet. A former professor of geology, over the years O'Connor led hundreds of workshops and tours. He concentrated on teaching teachers, because they'd multiply his message by passing it on to their pupils.

The message: Science is fascinating and fun and it's all around us -- just open your eyes. As a road cut reveals rock strata, the capital city's buildings expose slices of most U.S. states and a number of foreign countries.

When he died of cancer in 1999, O'Connor was compiling a detailed description of the city's geology and the origin, kind and character of stone used in virtually every building in town. The notes he left behind were a marvelous jumble of science, social and political history and random factoids, delightful to read but incomplete and unpublishable.

Some of his friends and admirers at the Geological Society of Washington last fall used his notes and interviews to put together "The Jim O'Connor Memorial Field Trip" covering some 50 buildings and monuments. From that marathon we've extracted an O'Connor mini-tour of the Mall, starting at the Capitol. It's a good rainy day exercise, because the colors and inclusions in stone are much more vivid when wet.

Fittingly, the Capitol Building contains stone from at least nine states, including the crumbly sandstone from Aquia Creek called Virginia freestone, which has broken many a maintenance budget. Because it was easily obtained and shapes nicely, freestone was used in many of Washington's early buildings, including the White House and the center section of the Capitol, both of which have had to be extensively refaced with marble after much of the sandstone turned back into sand.

The House and Senate wings are of dolomitic Massachusetts marble, the steps of granite from Minnesota and North Carolina, and the refaced exterior, columns and balustrades of marble and limestone from Colorado, Georgia, Maryland, Tennessee and Vermont.

The most interesting stone in the building is the calico marble, a k a "pudding stone," used in the Corinthian columns of Statuary Hall, just off the Rotunda. Named for the many rocks and pebbles imbedded in its incompletely metamorphosed matrix, it comes from Leesburg and Frederick. Or rather, it used to. Nobody uses it anymore because the pebbles and stones have a habit of popping out, leaving pits and craters that have to be filled with wax or plaster. But it's beautiful.

Moving downslope to the Capitol Reflecting Pool, which requires a long detour because of a maze of construction and security barriers, you'll find yourself on the track of 300-million-year-old worms, whose trails can be traced as they wind through the myriad fossils in the Indiana limestone of the walks and walls around the pool.

The National Gallery of Art West Building, whose exterior is pink Tennessee marble, was one of O'Connor's favorites. Inside, there's marble and limestone from Alabama, Missouri, Vermont and Carrara, Italy. There are fossils just about everywhere, and where there aren't, there usually are fossils of fossils -- patches in the marble walls, floors and columns where fossils have been reduced to undifferentiated blobs by eons of heat and pressure. The walls of the restrooms nearest the Mall entrance are black marble from New York's Lake Champlain -- it's such a maze of fossils it's almost possible to forget why you went in there. The exterior columns contain "fossil waves," patterns formed by wave action.

What's under the National Museum of Natural History was as interesting to O'Connor as what's on its outside walls, which are pink and white granite from Massachusetts, North Carolina and Vermont. Tiber Creek, which used to run across the Mall, still runs under it, and over the years it has undermined the museum's east wing, causing it to pull away from the building. It's being stabilized during the current massive renovation project. The Mall side of the building features petrified wood from Arizona, and the terrace where it sits is a gem, almost. It's unakite, granite veined with green epidote, which rock hounds regard as a semiprecious stone and often use in jewelry.

The slab-sided pink Tennessee marble walls of the National Museum of American History are dull even in rain, but the Mall-side terrace's pearl pink Minnesota granite is rich with largish, reddish feldspar crystals.

Surprise and disappointment attend close inspection of the Washington Monument, which has just undergone a zillion-dollar restoration of its exterior and remains closed while the interior's being redone and the elevators replaced. Large sections of the lower corners of the structure have been chiseled out and replaced with new-cut white marble that contrasts cruelly with the darker original Maryland and Texas marble. The National Park Service says the new stone eventually will weather in to match, but it bids fair to be a long wait. Even more noticeable from up close is the deterioration of the four courses of Massachusetts marble that were added when construction on the monument, abandoned in 1854, resumed in 1879. The courses, noticeably darker than the earlier and later stone, have spalled -- flaked off -- alarmingly. Replacing the deteriorated stones would have been too expensive and probably dangerous, the NPS says, but it has been impregnated with stabilizers. The rest of the monument is of marble from Cockeysville, Md.

Slipping down the west slope of the monument grounds, past the Constitution Gardens lake and between the Reflecting Pool (North Carolina granite) and the Vietnam Veterans Memorial (black granite), we come lastly to the massive brooding presence of the Lincoln Memorial. It's exterior is fine-grained Yule marble from Gunnison County, Colo. The statue of Lincoln, quite properly, is of Cherokee marble from Georgia, one of the states he recaptured for the Union, and rests on pink marble from Tennessee, another Confederate state. Round about stand columns of Indiana limestone. To those who know how to read them, these stones are as eloquent as the monument's inscriptions.

"Stones tell stories," O'Connor wrote in 1998. "The buildings of Washington, D.C., and virtually every community, are monumental textbooks."

Buildings and monuments like the Lincoln Memorial are a gold mine of historical and geological information.