In a neighborhood that has seen better days, in a city that may never again see anything like those salad days now wistfully remembered, there is a building that can be called the birthplace of modern America. It bears a plaque that reads:

"Home of the Model T. Here at his Highland Park plant, Henry Ford in 1913 began the mass production of automobiles on a moving assembly line . . . . Mass production soon moved from here to all phases of American industry and set the pattern of abundance for 20th century living."

No vehicle has been produced there since 1973, and the complex probably will be razed by developers. A commercial civilization is aggressively unsentimental about things that are casualties of capitalism's dynamism, and hence often has only a watery memory of what made it what it is. But the passing of Highland Park is especially poignant. Ten thousand men flocked there looking for work on Jan. 6, 1914, the day after Henry Ford announced that he would pay $5 for an eight-hour day--double the prevailing basic wage for a nine-hour day.

When it became clear that there were not nearly enough jobs for all the applicants, the crowd rioted and police sprayed it with freezing water, a melancholy end to day one of labor's new golden age. Ford's new pay policy was not, of course, altruistic. His assembly line was going to get more from the men to whom he was going to give more. And he was going to make something even more important than automobiles, something indispensable to the automobile industry: a middle class that could afford to buy automobiles.

At Highland Park, the time needed to assemble a Model T was quickly cut from 14 hours to 93 minutes. A fly- wheel magneto had been assembled by one man who took 20 minutes to do it. Ford broke the task into 29 operations and cut assembly time to four minutes. Critics were quick to deplore this:

"As to machinists, old-time, all- round men, perish the thought: The Ford Company has no use for experience, in the working ranks anyway. It desires and prefers machine-tool operators. . . ."

But as a modern scholar has noted, more complaints probably came from sociologists than from the working ranks. Few people who actually did it thought that spending 20 minutes assembling a magneto was soul-enlarging craftsmanship they should regret abandoning.

Granted, the Ford Company was not run by soft spirits. When on Christmas Eve, 1915, a subordinate had the temerity to wish a senior executive a merry Christmas, the executive looked up from his work, thought a bit, and replied grudgingly: "Well, all right."

But the public was not inclined to render moral or aesthetic judgments against the men and processes that already had reduced the price of a "Tin Lizzie" from $950 in 1909 to $600 in 1913. The future was here: by 1912 a whippersnapper running for the New York State Senate conducted an "automobile campaign." What would young Franklin Roosevelt do next?

Today, in America's biggest market --California, the freeway state, where the future often is foreshadowed--half the new cars sold are Japanese. The thing that Ford's assembly line revolutionized--productivity--is lagging behind that of Detroit's competitors; and Detroit's wages, which productivity once justified, are higher than those of Detroit's competitors. So unemployment is at Depression levels, especially among the children and grandchildren of blacks who came north to find opportunity in Henry Ford's factories. Stagnation seems so permanent that some stores advertise special long-term credit arrangements for the unemployed.

It may be not only foolish to expect, but misguided to hope, that America's automobile industry regains its old vigor. Perhaps other nations now have a substantial and unassailable advantage in automobile manufacturing, and Americans should cheerfully drive Datsuns into a future of newer industries. Nationalism and the national interest may now diverge where automobiles are concerned.

But the road to that future is bound to be bumpy, steep and winding in a nation in which the automobile industry is the primary purchaser from 2,000 companies. And anyone with a spark of feeling for the romance of this nation's muscular history must be saddened and sobered by the aura of vanished supremacy that hovers like a ghost over the shell of Highland Park.