In the social history of American industry, the words "garment district" evoke whole chapters. The Garment District, along New York's Seventh Avenue, is more than a place; it's a style of life, a boisterous, confusing world of brash executives, famous designers, noisy delicatessens and sweating men pushing racks of dresses along traffic-choked streets.

That's the old Garment District. The new one, centered in a single vast warehouse-like building that squats beside the Stemmons Freeway here, could hardly be more different. The cool, self-contained world of the Apparel Mart, where almost every well-known name in the clothing industry has a showroom, bears as much resemblance to New York's Garment District as a suburban shopping mall does to an old downtown.

The Dallas-Fort Worth area has become one of the nation's leading centers of apparel manufacturing and marketing, and the industry is a mainstay of the Texas economy. About 85,000 Texans are employed in apparel manufacturing, mostly in the Dallas area, and seasonal shows at the Apparel Mart are generating an estimated $3 billion a year in orders from retailers.

New York is still the undisputed industry leader, the place where trends are set, new fashion lines are first shown, and retailing styles are developed. With 192,000 employes in manufacturing, New York is also the leading production center, but statistics compiled by the American Apparel Manufacturers Association show that the number of garment workers declined by 36 percent in New York in the 1970s while growing by 42 percent in Texas.

"We'll never pass New York, not in our lifetime," said Marvin E. Segal, executive director of the Southwest Apparel Manufacturers Association. "But here in this building, we have everything a store needs."

"This building" means the Apparel Mart, where Segal's office is on a balcony overlooking an immense, four-story showroom and theater that seats up to 4,000 persons. The Apparel Mart is one of those monuments to Texas superlatives--biggest, most expensive, fastest-growing--that thrive in the fast-money atmosphere here.

"The six-story Apparel Mart, which covers four city blocks on a 20-acre site, is now the largest buiding of its kind in the world," according a press release reporting one of the building's unceasing expansions. Its floor space "is a whopping 1.8 million square feet, larger than 37 football fields." And it's still growing: a 400,000-square-foot addition exclusively for menswear is scheduled to open in October.

Inside the cavernous structure are miles of corridors where retail buyers can find representatives of some 10,000 lines of clothing, shoes, western wear and accessories for men, women, and children. According to Apparel Mart officials, more than 100,000 buyers from retail stores attend seasonal shows here each year.

What the buyers get is the opportunity to see the latest lines of apparel, talk to representatives of the manufacturers and designers, and write orders without having to face the hurly-burly and expense of New York. Instead of darting from showroom to showroom through congested streets of Manhattan's West Side, buyers stroll through one building that groups manufacturers' showrooms by type of clothing.

"New York has really changed. It's not fun any more," said Segal, a native New Yorker. Retail outlets that regard themselves as fahion leaders, such as Neiman-Marcus, are obliged to buy in New York, he said, but others often prefer the convenience, safety, and lower cost of the one-stop Dallas market. So many retailers in the Central States, the Southwest and the South have responded to that appeal that the designers and manufacturers are flocking to Dallas in increasing numbers.

Among the names on the showroom doors are such fashion leaders as Adolfo, Bill Blass, J. G. Hook, Izod, Halston, and Pierre Cardin, as well as unknowns such as Jim Heilman, a Dallas designer trying to establish his own line of women's wear.

"Dallas is now a major market, a very significant market," said Robert Ward, president of the Evan-Picone division of Palm Beach Inc., a manufacturer of sportswear for men and women. "They get major attendance at their shows from regional and Southeast people who prefer to go there rather than New York. People come there to write business, and I would endorse it as a major market."

The Apparel Mart is only one unit of a mammoth complex of wholesale centers and showrooms known as Dallas Market Center, all built and owned by a single entrepreneur, Trammel Crow. Crow, who was formerly the accountant to developer and industrialist John Stemmons--the same Stemmons whose name is on the freeway alongside the Market Center--went out on his own 30 years ago, and is now a Texas tycoon in his own right. In the Apparel Mart, he charges permanent exhibitors monthly rents of $12 to $14 per square foot, and keeps his costs down by running an entirely non-union operation, acccording to Segal.

The Dallas Market Center describes itself as "the world's largest wholesale merchandise mart," attracting more than 500,000 buyers a year in the apparel, furniture, home furnishings, floor coverings and computer trades. Crow owns the buildings, the 150 acres of prime Dallas real estate on which they stand, and the food service company that runs the restaurants inside. He also the owns the 572-room Mart Hotel, under construction next to the Apparel Mart building.

In the Apparel Mart, he even installed a modeling agency to work the exhibitions. "He called me up," said Kim Dawson, the head of the agency, "and he said I ought to be in his building. And here I am."