Who labeled the era following the Civil War the "Gilded Age"?

http://www.google.com/#q=+%22Gilded+Age%22

Andrew Carnegie I think

The term "Gilded Age" was coined by the American author and social critic Mark Twain. He used this phrase in his book "The Gilded Age: A Tale of Today," which he co-wrote with Charles Dudley Warner. The term refers to the late 19th century in the United States, a time of great economic growth and prosperity, but also a period of social inequality and political corruption.