Complex sentence: In the 19th century, during a search for the tomb of James I, Arbella's lead coffin was found in the vault of Mary, Queen of Scots, placed directly on top of that of the Scots queen.
Split into simpler sentences: In the 19th century, during a search for the tomb of James I, Arbella's coffin was found in the vault of Mary, Queen of Scots. Her lead coffin is placed directly on top of that of the Scots queen.