PUBLIC LIVES; A Polite Defender of SoHo's Cast-Iron Past

THERE is a copy of ''Final Exit,'' the manual of assisted dying, on a bureau in the bedroom of the preservationist Margot Gayle, not far from the aged copying machine she uses to assist her in her battle, and since Mrs. Gayle is 90 and troubled by arthritis and cannot take her daily one-mile walk without the assistance of a companion, it seems only right to ask:

Is the lady thinking of checking out?

A laugh and a denial from Mrs. Gayle, busy copying. Just reading, she says. But should things ever come to that, she considers it her own decision.

''It is my life, after all,'' she says.

A very active life. Mrs. Gayle was celebrated this month with a birthday dinner attended by the most influential preservationists in New York, she has published a new book on cast-iron architecture, and she has just had lunch with the architect Philip Johnson, who wrote the book's preface. They dined at the Four Seasons, which Mr. Johnson designed.

''A memorable lunch,'' says Joyce Mendelsohn, an enthusiastic writer a few decades younger than Mrs. Gayle, who accompanied her. ''Very exciting!''

Mrs. Gayle, a less excitable sort, adds: ''We compared notes on our 90's, our hearing aids, our arthritis. He has a hearing aid that cost $6,000. I have one that cost $600, and I think mine works better.''

Then a flash of the famous Mrs. Gayle manners: ''Oh, no, you mustn't put that in.''

A request, somewhat plaintive, to turn away from the woman to the work, which is also characteristic: ''Do you mind if we talk about cast-iron architecture?'' she asks. ''It's my all-consuming passion.''

To whom do we owe the preservation of the steel-faced buildings that are the face of SoHo?

Many -- though not the advocate herself -- would attribute it to the work of Mrs. Gayle. She founded the Friends of Cast-Iron Architecture in 1970; she led tours of cast-iron buildings, where she distributed little magnets so visitors could ''test for iron''; she campaigned to have SoHo declared a landmark district.

''Margot Gayle is the only reason we have a SoHo,'' says Brendan Sexton, president of the Municipal Art Society. ''The only person who comes close or who shares with Margot that honor is Jane Jacobs, who stopped Robert Moses from putting an expressway through what is today SoHo and TriBeCa. Margot turned her eye on the cast-iron district and it appeared like magic.''

Mrs. Gayle's new book is ''Cast-Iron Architecture in America: The Significance of James Bogardus'' (Norton), written with her daughter, Carol. It speaks of those days when lower Manhattan, which was most of Manhattan, suffered two great fires. (The fire of 1835 leveled 674 buildings.) An inventor, James Bogardus, concerned about finding a fireproof building material, originated the use of cast iron for the exteriors, and championed it in America's rapidly growing cities. Manhattan has three Bogardus buildings.

''We have 139 iron-front buildings in SoHo today,'' Mrs. Gayle says. ''More than any other place in the world.''

Why save them? What of the notion that change is natural to life and to cities?

''I suppose that's true, that change is natural,'' Mrs. Gayle says. ''But why not let people in the future enjoy some of the things we thought were extremely fine?''

Margaret McCoy Gayle was born in Kansas City. Her father was an executive in the automobile business, and the family moved so frequently that she attended a different school every year. When she was 13, the family spent a year in London.

''Like Edith Wharton spent so much of her youth!'' Ms. Mendelsohn says. ''Was it there you became interested in architecture?''

''At that age, I couldn't have cared less,'' Mrs. Gayle answers.

Mrs. Gayle earned a master's degree in bacteriology from Emory University, but could not get a job.

''Because you were a woman?'' Ms. Mendelsohn asks.

''The Depression,'' is the response.

Mrs. Gayle's first concern was politics. She was so active in her work to repeal the poll tax in Georgia that she earned the nickname Poll Tax Margot. She had not been particularly interested in marriage, preferring a career, but she married nonetheless. Her husband, William, was an accountant, and they raised two daughters in New York, where she worked as a radio writer and became active in local politics. She ran for a City Council seat in 1957 with the slogan, ''We need a woman in the City Council,'' a famously male bastion at the time. She lost.

Mrs. Gayle turned her attention to rescuing the endangered buildings of New York, organizing her neighbors from the kitchen of her Greenwich Village apartment. Her first triumph was the Jefferson Market Courthouse around the corner, which is now a library. Her most recent victory, and the one she is proudest of, is obtaining landmark status for the Bennett Building, a cast-iron structure on Nassau Street, which she says is the largest iron-front building in the city.

What does Mrs. Gayle believe would have happened to SoHo had it not received landmark status?

''It would have been urban renewed, as they say,'' Mrs. Gayle said. ''This is what happened north of Houston Street, where they knocked down all the older buildings and put up Washington Square Village and the I. M. Pei towers and then the Bobst Library, over Washington Square.''

The tall Bobst Library had been criticized as an ugly piece of work which literally cast a shadow on the park. What is Mrs. Gayle's opinion?

''I'd rather not say.''