A dusty, neglected envelope turned out to be a Bronx treasure chest.

Hundreds of photos dating to 1907 and illustrating a century of urbanization were found in a filing cabinet during a renovation of Borough President Ruben Diaz Jr.’s office.

Workers are seen setting down the first stone slabs of the landmark Bronx Borough Courthouse. And farmland hugs the stone wall and underpass of the Grand Concourse at 175th Street.

Borough historian Lloyd Ultan thinks the fotogs were engineers with first Borough President Louis Haffen’s Bureau of Highways.