Letter, from Sir Charles Lyell to Sir John Herschel, dated at 16 Hart St. Autograph letter signed by sender. When he received John Frederick William Herschel's request to give a lecture on American geology he did not treat it seriously. Now R. I. Murchison has made a similar request so thinks he could work up a popular lecture on the Mississippi Delta. Will keep off his old ground. 5 July 1846