Dressed in a blue blazer and bright red pants, Nicholas Lord Fairfax, the 14th baron of Cameron, was sitting yesterday on a patio that overlooks the Potomac River when a descendant of famed U.S. Chief Justice JOHN Marshall approached.

"Oh," said the descendant, retired Marine Brig. Gen. St. Julien Marshall, "there are the trousers."

Lord Fairfax, a 22-year-old English barrister who was in Fairfax County yesterday for a party celebrating the publication of a book about the county named after his ancestors, smiled at jmarshall and said, "So they are telling everyone to find me by my trousers."

The young lord said later in reference to his title and its utility in America, "People are bound to make a fuss about it, especially here in this county."

Nearly 400 members and guests of the Historical Society of Fairfax County showed up yesterday afternoon in wilting heat at the Madeira School off Georgetown Pike to make a fuss over Lord Fairfax, the marquis and Marquise de Chambrun (direct descendants of the American Revolutionary hero Lafayette) a book entitled "Fairfax County: a History."

The book, 800 pages long with nearly half a million words, has been in the making for nearly four years. More than 3,000 copies, the first printing, were sold out before they could even be flown into the county from St. Louis last week.

Lord Fairfax, who has been working at the Washington law firm of Covington & Burling for the last six months as a "foreign lawyer," is a descendant of Lord Bryon Fairfax, who in 1649 was given part ownership of what is now Northern Virginia and part of West Virginia by King Charles II.

After signing scores of copies of the just-published county history, Lord Fairfax went to the podium at the Madeira School's auditorium and said he is "sorely tempted" to imitate the claim of American Indians in Maine to land that once was theirs.

In return for his presence, the historical society gave Lord Fairfax a T-shirt emblazoned: "Fairfax is for Lovers."

Douglass S. Mackell III, the new president of the society, said the romantic T-shirt seemed the most appropriate gift for a young, unmarried lord.

Among the crowd that gathered to honor the book, which is being published by the Fairfax County Office of Comprehensive Planning, were four descendants of John Marshall, one of Thomas Jefferson, one of George Mason and one of George Washington's newphew Bushrod Washington.

The wife of Bushrod Washington's descendant, Lydia Washington, came to the party outfitted in a red, white and blue brocade and satin dress that she had made by hand for the Bicentennial. Mrs. Washington spent much of her time near a doorway of the auditorium's lobby because she said the dress, though lovely, was hot.

"Fairfax County: a History," which gives a detailed history of Fairfax County from 1649 to the present, was written by five authors and includes painting, political cartoonsand maps. A second printing will be available July 12 at a cost of $15.