[facsimile of the author's drawings, from a letter to Charles Warren Stoddard of 9 January 1874; courtesy of the Henry W. And Albert A. Berg Collection, New York Public Library]
Clemens, a star on the lecture circuit, made several lucrative tours between 1866 and his final around-the-world lecture tour of 1896.
His "lectures" usually consisted of readings from his works interspersed with anecdotes and reminiscences. Often he scribbled an outline of his talk, using pictographs to remind himself of the sequence of his subjects.
He used the notes reproduced here for a lecture about his years in Nevada delivered in London in 1874. The key to the episodes in Roughing It to which the drawings relate may be found in volume 6 of the Mark Twain Project's edition of Mark Twain's Letters (2002), pp. 17-18.