What was Charles Lyell doing when he died in 1875?

At the time of his death in 1875, Charles Lyell was engaged in revising his seminal work, the Principles of Geology, for its 12th edition. Following the death of his wife in 1873, Lyell had dedicated himself to his daily scientific work as a way to cope with his loss. He passed away on February 22, 1875, in London and was subsequently buried in Westminster Abbey.12