WITH MUCH PROVINICIAL HOOPLA and a dash of nonsense, the Prince William County supervisors until recently had been waging a battle of passive resistance against a bill to preserve historically valuable land around the Manassas National Battle-field Park. But now they have actually presented the nation with their idea of "compromise." Though nobody with a serious concern for the preservation of this area has bought the plan, it probably does give the supervisors a flimsy claim that, yes, they have, too, acknowledged the heretofore vain attempts of others to get their ear on this matter.

Rather than hold a public hearing and then negotiate with Interior Department authorities or with Rep. Herbert E. Harris (D-Va.), sponsor of the preservation legislation, however, the supervisors have laid down their own rules. They want to delete one farm from the plan, allow easements rather than purchase some other tracts and provide a guarantee that there will be no more expansions. After all, said the county officials sarcastically. "While all areas of major historic significance are already within the park, is is true that troops from both sides did have to march to the site to battle. Therefore, left to its own devices the park could proceed to expand indefinitely throughout Northern Virginia to include every campsite and marching route."

Of course - and when that happens, the supervisors will no doubt have another plan for them to operate hot-dog stands, gift shops and maybe an industrial park in the name of Virginia's heritage. But instead of abiding the antics of an insular board infatuated with development of any kind, Virginia Sen. Harry F. Byrd (I-Va.) could do a great public service for concerned residents of the county as well as historians throughout the nation by lifting his senatorial "hold" of further action on Capitol Hill. With his help, the Manassas preservation bill could - and should - be enacted this year, before the land is irretrievably ruined.