Marshall Hall, the 256-year-old plantation house across the Potomac from Mount Vernon that was considered one of Maryland's finest large early colonial houses, was totally destroyed yesterday in a fire deemed arson by state fire marshals.

Demolished with the brick mansion was a turn-of-the-century carousel building, the only remnant of the Victorian amusement park which surrounded the house until the National Park Service closed and razed the amusement park two years ago.

The gate of the barbed-wire fence surrounding the mansion was broken open and the two fires apparently set after 1 a.m., according to Park Service officials and state fire marshal William Mitchell. A small 18th century brick outbuilding and the old Marshall family graveyard were not damaged.

Only an early mantel, a door frame, a cabinet and the brick walls remain of the house built in 1725, several years before George Washington was born and 29 years before the construction of Mount Vernon was begun, Park Service officials said yesterday. No estimate of the dollar value of the damage will be available until next week.

"It's very upsetting to those of us in historic preservation, because it was a fine building with great prospects," Mount Vernon resident director John Castellani said yesterday.

At the time of the fire, the Park Service was just finishing an architectural study and restoration plans for the house, listed on the National Register of Historic Places as the largest house in southern Maryland built prior to 1740.

"We were surprised to find so much of the original house still intact, including some early floors, mantels, and magnificent panelling hidden behind sheetrock walls," said to Park Service project coordinator David Sherman. The amusement park had used the buildings as offices from 1895 until 1979.

Although the Park Service had no definite plans for Marshall Hall, officials were considering a number of potential uses for the mansion, including as a possible new headquarters for National Colonial Farm, the Park Service-funded "living history" farm located about a mile away in another section of Piscataway Park.

If the brick walls are found to be structurally sound, they will be saved and the house could someday be rebuilt, Sherman said.

In 1650, William Marshall of London obtained a royal patent for the original 500 acres along the Potomac. His descendants later officially bought the land from the Piscataway Indian Nation, which had lived there for centuries. The family of merchants and traders owned the land for more than 200 years before being forced to sell it after the Civil War.

The Marshall and Washington families were close, establishing a public ferry across the Potomac between the two estates and even allowing intermarriage among their slaves, according to Mount Vernon librarian Ellen McCallister.