One crisp dawn in May, just because, she swam across the Hudson. The river was close to three miles wide at Poughkeepsie, where Vassar was. She hid her bike in the bushes.

"Oh, I liked Vassar all right," says Mary Marvin Breckinridge Patterson in a voice full of good breeding and boarding schools. "But sometimes on that campus with all of those women, I just wanted to get out of there."

That was in 1925 or so, when her life as the New York-society-girl-gone-gleefully-astray was still in its formative years, before she rode horses through Appalachia and cowcatchers across Africa. Before broadcasting, which she did for CBS during World War II. when she interviewed the women who ran the firehouses of London, they gave her macaroni, pickled onions and tea. And called her Miss Breckinridge, though most everyone else called her Marvin.

Now, at 74, Mary Marvin Breckinridge Patterson smokes her cigarettes and takes her tea surrounded by the brass andirons and Persian miniatures of her Northwest Washington home. It is stuffed with these and other trappings from an American Brahmin life, a life whose restrictions she has seemed not so much to reject as ignore.

And now, at 74, she also finds herself surrounded -- for a day or two, anyway -- by photographers and reporters wanting to hear about the diamond mines of Kimberley, the Kikuyus of Nairobi, the ruins of Zimbabwe.

She took pictures of it all back in 1932, lugging her old Graflex from Capetown to Cairo on the steamers and trains of British Africa. Nearly half a century later, they're in a new book.

There was a noisy party for it at the Smithsonian last night.

It's called "Olivia's African Diary" and is written by Olivia Stokes Hatch, the longtime Washington friend whose mother thought Marvin might like to come along to Africa.

"We lived across the street from each other on Massachusetts Avenue," Marvin Patterson remembers. "And her mother said to my mother, 'Wouldn't it be nice to have someone to sit up with Olivia?'"

Yes it would everyone decided. So off went Marvin and Olivia and Olivia's parents, sailing from Southampton, England, on July 15, 1932. Olivia, whose father had been asked by the Carnegie Corp. to lecture at African universities, was 24. Marvin, a young photographer, was 26.

Olivia kept track of things in her brown leather diary, stamped on its cover with "My Trip Abroad." Inside were passages like this: "We saw three huge giraffes, many hartebeeste, wildebeeste, zebras, ostriches and some queer huge birds." And this: "A delightful morning of letter writing followed by an afternoon nap and then tea, some tennis, then sundowners. . . ." f

Well, the diary sat on a dusty shelf for decades. Then two years ago, a friend from the Smithsonian had lunch with Olivia and Marvin, heard about the faded old diary and said publish, publish! Marvin could supply the pictures.

In between, the life of Mary Marvin Breckinridge Patterson has been a curious mixture of Junior League and Istambul, debutante and Daniel Boone. cBorn in Manhattan, walked in Central Park by a governess, educated at Brearley, descended from a vice president and attorney general, she might have been expected to marry a Yalie and settle down to chair the Corcoran Ball happily ever after.

Which she also did. Still, she rode horses for the Frontier Nursing Service in Kentucky and broadcast World War II coverage from Amsterdam before the invasion. She got that job from Edward R. Murrow, an old friend.

"One night I was having dinner with Ed and Janet Murrow in London," she remembers. "And they said, 'What have you been doing?' And I said, 'Taking pictures.' And he said, 'Why don't you come on the air and talk about it?'"

"And so," says Marvin Patterson, "I said yes. I thought Mother and Dad would like to hear what their daughter was up to."

Today, she shrugs off the adventures, perhaps not the sort for self-analysis.A quiet rebel? An early feminist? She just did it, that's all. "My father wished I would have been more conventional," she offers. "But mother had a good deal of spirit."

Explains her friend Olivia, who's sitting nearby: "Marvin just has gumption."

She also has gray-blond hair now, a strong jaw, a sense of stoicism. She is dressed conservatively in green shirt-waist dress, short black pumps, some green stones at her throat.

"My father," she continues, "probably would have liked me to marry a nice gentleman and settle down."

It took a while. There was the war, and before that, Africa, and before that, Kentucky, the stop her parents disapproved of most. "I think they were afraid I'd fall in love with some handsome mountaineer and never come back," she says.

Instead, she fell in love with Jefferson Patterson, a career foreign-service officer and highly eligible Washington bachelor. They knew each other from parties here, but met again in Berlin. She was 34 and working for Cbs; he was 49, charming and chauffeur-driven.

After six weeks, as she was leaving for Amsterdam, he proposed to her on the way to the train station. Out of earshot of the chauffeur naturally.

They were married in Berlin, less than a fortnight later. And all Washington buzzed.

Last night, Washington buzzed again, this time at the Smithsonian book party. Thre was African tribal music and stuffed mushrooms. There was Donald Sole, the ambassador of South Africa, and Kathy Hotvedt of the Junior League. There were native turbans and there were minks.

"Makes life more interesting," said Marvin.