Frederick Law Olmsted Sr., the genius landscape architect who did more than any other person to humanize America's rapidly growing cities during the 19th century, today has a friend indeed in Rep. John F. Seiberling, the conservationist congressman from Ohio.

Seiberling, in alliance with New York Sen. Daniel Patrick Moynihan on the opposite side of the Hill, has taken the lead in pushing for adoption of the Olmsted Heritage Landscapes Act, a straightforward bill with a simple purpose: to study the vast Olmsted legacy and, with the all-important aid of state and local governments and private groups and individuals, to begin to put it in order.

It was touching and, in a way, depressing to witness Seiberling's lonely performance last Tuesday as he conducted a hearing on the Olmsted bill. During MX week in the House (Democrat Seiberling predictably voted on the losing side twice), with such astronomical financial stakes ($1.5 billion for 21 missiles, or more than $71 million each), here was a congressman passionately prodding an oddly recalcitrant Interior Department to accept a mere $2.5 million, to be spent over the course of 10 years, just to give some long overdue attention to one of the nation's preeminent cultural treasures.

The performance was inspired. From time to time other subcommittee members would drop in, but the show, like the bill, clearly belonged to Chairman Seiberling.

He told of his privileged boyhood at Stan Hywet Hall, the Akron estate whose beautiful grounds were designed by an Olmsted associate. He recounted his long (and successful) "one-man crusade" to save the grounds from destruction by the private foundation to which he and his family had donated the estate. He told how driving by the old Hillandale grounds in Northwest Washington, today cut into condos, makes him feel "a little bit sick, and certainly sad," because he remembers well "how those rolling hills looked in the springtime." He quoted poets (Gerard Manley Hopkins, A.E. Housman, Robert Frost) during his frequent, spirited interchanges with the mostly favorable witnesses who formed his responsive audience. He said, "Every time we lose a thing of beauty made by nature and man together, we lose a little bit of our civilization." More than once his voice cracked and tears welled in his eyes, and more than once the hearing room was filled with spontaneous applause.

Seiberling's cause is just: Olmsted was a giant. His works stretch almost literally from sea to sea. Though best known as the designer (with Calvert Vaux) of Central Park in New York, he spread his talents far and wide, from Boston to Baltimore to Washington (the splendid U.S. Capitol grounds are his) to Chicago to Yosemite Valley in California, and to hundreds of places in between. And as extensive as are the parks, estates, campuses, suburbs and government grounds that he and his partners designed directly, it might be argued that an even greater legacy was the tremendous extent of the Olmsted influence. Not idly is he known as the "father of American landscape architecture."

Even if limited to works of his direct descendants -- his stepson John Charles, his son Frederick Law Olmsted Jr. and others who carried on the work of the firm well into the 20th century -- the numbers are impressive. According to Charles E. Beveridge of American University, editor of the voluminous Olmsted papers deposited at the Library of Congress, the firm accounted for some 3,000 projects after Olmsted Sr. retired in 1895, including 650 parks, parkways and playgrounds designed for public use, 150 educational campuses and 150 residential communities.

Many of the Olmsted sites have not fared well over the years. Some have been destroyed altogether. Others have been altered unfortunately by simple ignorance of the methods and intentions of their designers. (The views Olmsted so sympathetically laid out on the Capitol grounds, for instance, are being obliterated by ill-conceived plantings.) Other sites have been changed for the worse by being partially or wholly converted to more active recreational uses with cavalier disregard for their overall structure.

Today many more are endangered, or on their way out. In Atlanta, trees in a string of parks in the Olmsted-designed suburb of Druid Hills will be cut to make way for a highway serving the proposed Carter Presidential Library. A hilly, privately owned 228-acre Olmsted site in White Plains, N.Y., is to be converted into a multiuse commercial and residential development. Many and perhaps most others, according to witnesses at the Seiberling hearing, are being very poorly maintained.

So what is this bill that the Interior Department finds onerous? Basically it requires but an inventory of a great national heritage -- a simple listing of locations, dates and so on, an examination of the original designs and their execution, and a report on current conditions. In this procedure the federal government's role would be crucial but rather minimal: It would set standards and make recommendations for preserving such "historic designed landscapes" (an important provision because parks have received much less attention than buildings in preservation processes), provide technical assistance to those in state and local governments and private groups who would be doing most of the actual work, and serve as a sort of clearinghouse for the results thus obtained.

As a young man, in 1850, Olmsted visited a new park in Birkenhead, England, across the Mersey River from crowded Liverpool. "Five minutes of admiration and a few more spent in studying the manner in which art had been employed to obtain from nature so much beauty," he wrote, "and I was ready to admit that in democratic America there was nothing to be thought of as comparable with this People's Garden."

From the beginning of Central Park in 1857 until 1895, when he retired from the fray with a sadly addled brain, Olmsted did more than his share to redress the balance. But if from Olmsted's time onward parks have been popular in democratic America, their maintenance has been something else again. Olmsted himself learned this lesson well during his titanic struggles with the Tweed ring and other New York politicos during the decades it took to build Central Park. Today, development pressures, budget crunches and plain ignorance make this as true as, perhaps truer than, ever.

Would it not be a wonder if, by paying some close respect to the places that Olmsted and his followers designed and built, people in hundreds of cities, suburbs and towns could be inspired again by his great vision? This is at least a possibility opened up by a modest piece of legislation about which John Seiberling said, with Olmstedian spunk, "If they opponents in the administration don't ask for it, maybe we will give it to them, anyway."