By describing the history of a well-known boundary, St. George (whose other fine books include The Panama Canal, 1989) expertly demonstrates how a narrowly defined subject can illuminate broader historical issues. Beginning with the 1730's, she details the troubled history of the Pennsylvania-Maryland border, which led in 1763 to the employment (by descendants of Lord Baltimore and William Penn) of Charles Mason and Jeremiah Dixon to survey the boundary. Interspersing narrative with excerpts from Mason's diary, St. George depicts the often harrowing circumstances of the survey, which took five years to complete, then discusses the role of the boundary in the Revolution, the Underground Railroad, the Civil War, and even in one of North America's most severe earthquakes. A well-sourced narrative that ably interweaves personalities and events, most arrestingly in the case of George Washington, who began and ended his career along the Mason-Dixon line. The result is entertaining history from a fresh perspective. Not seen: maps, b&w photos, prints, bibliography, index.