WASHINGTON (AP) _ Film archivist Susan Dalton said she felt like she'd opened Aladdin's cave when she stepped into a truck carrying more than 1,600 early American films donated by Australia's national film archive.

''They have basically given us a big chunk of our history and we love them for it,'' said Dalton, of the American Film Institute, which helped coordinate the transfer. ''There's magical treasures in those boxes.''

Some of the films, which arrived at the Library of Congress by refrigerated truck Wednesday, are believed to be the only remaining copies. They will be divided among the major American film archives for restoration.

The films - with such titles as ''The Jazz Monkey,'' ''The Cowboy Editor,'' and ''Chinese Cottage Industry'' - were donated by the National Film and Sound Archive of Australia. They date from before 1900 to the 1950s.

The films were found during a drive by Australia to preserve its own cinematographic history, Dalton said.

''They went around the country in a van to get all the nitrate film out of the hands of individuals,'' she said. ''They were very successful.''

Nitrate-based film, which was manufactured from the late 1880s until 1951, decomposes under the influence of heat, moisture and acids. Some of it was destroyed in attempts to recover silver from the emulsion. As a result, less than half the films shot before 1951 have survived.

Many of the original titles donated by Australia have not yet been identified, Dalton said.

Unexpected gems in the Australian collection are ''Once Every Ten Minutes'' and ''Peculiar Patients' Pranks'' (both 1915), apparently the only copies of these early works of silent film comedian Harold Lloyd.

''It's a miracle, really,'' Dalton said. ''These Harold Lloyd films - we had no idea that they existed at all. We just assumed they were gone forever.''

Among the earliest ''lost'' films in the collection are ''An Indian Sunbeam'' (1912), with Broncho Billy Anderson, the world's first cowboy film star; ''Among the Mourners,'' a 1914 Keystone comedy starring Charlie Chase; and ''Bringin' Home the Bacon'' (1924), one of the earliest features starring Jean Arthur.

The films were sent to Australia for exhibition when they were made and it was considered too expensive to ship them back to the United States afterward, Dalton said. More than 98 percent are on nitrate stock and most are ''orphan films'' no longer in copyright or produced by independent companies that have disappeared.

The Australian archive realized it did not have the money to preserve the films and decided last October to offer them to the United States.

''We are sorry to part with these films but we would not have been able to preserve them for at least 20 years,'' said Ann Baylis, acting deputy director of the Australian archive.