When HMS Beagle made its first landfall in January 1832, the twenty-two-year-old Charles Darwin set about taking detailed notes on geology. He was soon planning a volume on the geological structure of the places visited, and letters to his sisters confirm that he identified himself as a ‘geologist’. For a young gentleman of his class and income, this was a remarkable thing to do. Darwin's conversion to evolution by selection has been examined so intensively that it is easy to forget that the most extraordinary decision he ever made was to devote his life to the study of the natural world by becoming a geologist. It is only slightly less astonishing that he should have decided to align his work with Charles Lyell's controversial programme of geological reform, which had almost no followers in England.

On occasion Charles Darwin can seem our scientific contemporary, for the subjects he engaged remain engaging today, but in his role as author he belongs to the past. It is not customary today for scientists to write book after book, as Darwin did, or for these books to serve as the primary vehicle of scientific communication. For Darwin, however, the book was central. He wrote at least eighteen, depending on what one counts; in his Autobiography he entitled the section describing his most important work ‘An account how several books arose’; and in his personal Journal, begun in August 1838 after he had come to a mature sense of himself, he organized entries around his books. A characteristic entry is that for 1846: ‘Oct. 1st. Finished last proof of my Geolog. Observ. on S. America; This volume, including Paper in Geolog. Journal on the Falkland Islands took me 18 & 1/2 months:–’. Further, almost always he had a book under way: when one was complete, the next was begun. He called them the milestones to his life.

1990 marked the 150th anniversary of the publication of Charles Darwin's first major scientific theory. The paper, first presented by Darwin to the Geological Society of London on 7 March 1838, was entitled ‘On the Connexion of Certain Volcanic Phenomena and on the Formation of Mountain-Chains and the Effects of Continental Elevations.’ The paper was a remarkable attempt to develop a global tectonic synthesis. It was the culmination of a period of intensive geological activity by Darwin – then twenty-nine – who had returned from the Beagle voyage only eighteen months previously. The present article reviews the development of Darwin's views, their impact upon his contemporaries, their role in shaping his later views on the origin of species, and their significance in scientific theory-making. It draws, in part, on Darwin's unpublished geological notes and jottings. This paper, and the papers by Sandra Herbert and James Secord that accompany it, were delivered at a symposium which I organized at the Geological Society of London on 31 October 1988 to mark the 150th anniversary of the reading of Darwin's paper.