Colross (also historically known as Belle Air and Grasshopper Hall) is a Georgian mansion that was formerly the center of an estate in the Old Town neighborhood of Alexandria, Virginia. Colross is currently the administration building for Princeton Day School in Princeton, New Jersey. The Colross property originally occupied the entire 1100 block of Oronoco Street. The property was originally developed as a plantation by Alexandria merchant John Potts, who began constructing the mansion 1799–1800. In 1803, the property was purchased by Jonathan Swift, also an Alexandria merchant and a city councilman. Swift continued the mansion's construction during his ownership. Thomson Francis Mason (1785–1838), son of Thomson Mason (1759–1820) and grandson of Founding Father George Mason (1725–1792) of Gunston Hall, purchased Colross following Swift's death in 1824. Mason served as a judge of the Criminal Court of the District of Columbia and as mayor of Alexandria. During his ownership, Mason made Colross his chief homestead and made substantial modifications and additions to the mansion. After a series of successive ownerships, the area around Colross became heavily industrialized, and the mansion was purchased by John Munn in 1929. Between 1929 and 1932, the mansion was transported brick by brick to Princeton. In 1958, Colross was sold to Princeton Day School.