American Girl: Scenes From a Small Town Childhood.

Mary Cantwell Random House, $20, 209 pp.

All over America the small towns are disappearing. Their main streets are spotted with boarded-up storefronts; their eating places have been moved to highway strips. In mid-America the declining numbers of family farms have made the towns negligible as trading centers. On the coasts, and along the river valleys, industry and shipping have deserted the small mill and factory towns; railroads, buses, and airlines no longer serve most of them. And everywhere the Wal-Marts and the malls have replaced the downtown where people used to meet at the general stores, the drugstores with ice cream counters, and the movie theater.

Fortunately, small towns, their charms and their limitations, will live on in nostalgic fiction and reminiscence. Mary Cantwell's American Girl is an example of the latter. The strong and lasting effect of an old-fashioned childhood is vividly illustrated in the last chapter: "Once, when I was walking west with my then-young daughters, I sniffed that unmistakable low-tide scent that comes off the Hudson and said, |Doesn't that smell like home!' |Mother,' my older daughter said, 'this is home.'"

But, of course, New York wasn't and isn't home to Mary Cantwell, as every. one brought up in a small town will understand. She loves New York where she has lived far longer than she had lived in her hometown of Bristol, Rhode Island, but, "My country is 200 miles away by Amtrak and all it would take is the sale of an apartment and the packing up of some books and furniture for me to live there again .... It would be as if nothing had ever happened, as if nothing had ever changed. Bristol, my Bristol, would blot out everything that came after it."

She evokes Bristol, the Bristol of her youth for us in one haunting description and anecdote after another. There is the sense of place--the ever-present salty smell of the harbor, the world of white Victorian houses and leafy trees, the common that divides comfortable Bristol from the poorer Bristol of the Portuguese and the immigrant Italians who live near the rubber factory where "the air was yellow and stank of rubber."

There is the bustling life of the big two-family house--Granny and Gampy and Aunt Esther downstairs, Mother and Father, Mary and her sister Diana upstairs. "Downstairs food and upstairs food are different. Downstairs tends towards baked beans, clam chowder, codfish cakes, johnny cake and apple pie. Upstairs is usually a roast, a green vegetable, a yellow vegetable, a starch and no dessert. Downstairs and upstairs look different, too."

There is swimming in the harbor just across the street and the seawall from the house. And skiing on the wet snow in winter down the hill on a nearby street--"so cold, so still and the only sound is a long s-l-i-i-sh." There is the lawn bordered with rose bushes and hydrangeas and flower plots planted with phlox, pansies, sweet william, peonies and tulips and, along a back fence, lilacs and hollyhocks.

Place and the sights, sounds, and smell of it, mark small-town childhoods and the impression is lifelong. And the people of a small town extend one's family in a mysterious way. They are never forgotten. So Ruthie, a lifelong best friend who "married one of the Catholic Connerys and never left town." is mingled in Mary Cantwell's memory with those who lie in the cemetery like the Portuguese grocery boy of the wild blue eyes who died of trichinosis, Miss Nerone, the remarkably encouraging high school teacher, and many others.

But there are subthemes to the nostalgic in this memoir and they bring out the wit and compassion that marked the author's "At Home" column in the New York Times. One is that of the title American Girl--what it was like to be a girl growing up in the 1940s and 50s before the sexual revolution had taken hold. These are the girls who learn about maleness from the Boy Scout Manual, who surreptitiously search their parents' bedrooms for information about the sex life of the married, whose terror is "The worst thing that could happen to anybody"--pregnancy like that which drove bold, foul-mouthed, fierce Margie and her mother from Bristol forever.

There is the first date with first love Norman--a Friday night movie at the Pastime Theater--the chaste evenings in the glider on the porch "with a boy whose shirt smelled of starch and fresh air and whose young, flushed face smelled of his older brother's after-shave." There are the dances at which, like all nice girls, she held back "to leave a little space between my stomach and theirs"--sometimes to no avail and "that was torture." There are the on again, off again diaries recording kisses and comparing them. There is the glamour of cheer-leading, the glory of the high school junior prom, and the climax of the graduation ceremony, prelude to life elsewhere.

Another subtheme is the mixture of ethnic and religious backgrounds that divide the town and the chasm between the year-round Bristol residents and the summer people "down the Ferry" on Narragansett Bay. Although Irish-Catholic Mary Cantwell, because of her family's income and relatively comfortable status, lived in what she called "the country of the blue-eyed," she knew, nevertheless, from an early age that Catholics "come out of the second drawer" in a New England town. And she also learned early that, although she was a junior member of the Bristol Yacht Club which "smelled of teak, leather, and salt which in coastal New England is the smell of money," and although "we looked the same, we dressed the same," a magnetic field enclosed the rich "as if it were a fence" impossible to pierce. Nevertheless, she does not envy The Ferry people for it is she, not they, "who was a Bristolian."

In only 209 pages Mary Cantwell has given us all this of the town she loves--a warm and lovely portrait of place and people in a time gone forever, a portrait sketched in graceful, evocative prose.