Forty years ago, a few pioneers from Brooklyn settled in apine-bush wilderness of dirt roads and dark nights. For six months,while they built homes of brick and stone nearby on what is now Bayview Drive, they lived in a mansion that predated the American Revolution.

Early Sunday morning, they saw the mansion and a piece of their lives destroyed by a fire that police said yesterday was started by two Mastic teenagers.

The two youths were arrested Tuesday night. One of them, Robert Marciano, 16, of 25 Harrison Ave., was arraigned yesterday in First District Court in Hauppauge before Judge Armand Araujo on a charge of fourth-degree arson. He was released in his parents' custody and is to return to court March 19.

Police said Marciano and a 15-year-old classmate at William Floyd High School, whose name was withheld because he is a juvenile, told officers they broke into the building at about 10 p.m. Saturday. They said they started a fire out of cardboard and paper because they were cold, Det. Sgt. John Ryan of the Suffolk County Arson Squad said. "When it started to get too big for them, they just ran," he said.

The Mastic Beach Fire Department was summoned at 11:55 p.m. and called in seven other companies. Four hours later the original mansion, which had become part of Bayview Community Hospital and later a county health clinic before it was closed in 1982, was a heap of smoldering rubble. A brick addition to the hospital was damaged by smoke. Ryan estimated damage at $200,000.

Patrick Messinetti, who lives next door, put the loss much higher: "It was a very sickening feeling," he said.

Messinetti, 69, was 30 when he and Michael DiPierro left the city in 1946 at the urging of Dr. Frank Calabro, who had purchased what was then known as the Lawson house as a summer residence. Calabro, Messinetti's brother-in-law, decided to move his office there, but was reluctant to move so far away by himself. "We sold our houses in the city, Mike and I, and our families moved into the hospital," Messinetti said. "There was 80 broken windows. Mike and I fixed the heat, fixed the windows and painted the place and made it livable."

The house was a child's paradise. "There were five or six families and we each had a big, tremendous room with fireplaces and we had a tremendous dining room, and all the mothers would cook," recalled Adrienne Dionisio of 228 Bayview Dr., Messinetti's niece, whose family shared the mansion. She explored the ice house and the barn and sneaked up to the attic, where she found shackles that had been used to secure hands and feet.

Her son, Joe, born in the hospital in 1963, snapped pictures of the fire scene yesterday. "Everyone knew it was coming," he said. "There were so many people hanging out here. You could see the graffiti on all the walls."

Dr. Erol Caypinar of Center Moriches said Bayview Community Hospital Inc., of which he is chairman, purchased the building from Calabro in the late 1960s and rented it to the county as a community hospital and, in 1972, as a health clinic. The building was closed in 1982 when the clinic moved to Montauk Highway. Several organizations were interested in using the building, he said, and Calabro's children had said they wanted to buy back the house.