descant

"The professor's lecture began as a quick overview, but soon turned into a lengthy descant on the history of the Roman Empire. "

"During the Christmas concert, a young girl sang a beautiful descant above the main melody of "Silent Night," adding a soaring harmony to the familiar tune. "

"My professor could descant for hours on the history of the Roman Empire, even when the bell rang. "

"During the Christmas carol, my sister decided to descant, adding a higher, flowing melody above the main tune. "