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 It draws, in part, on Darwin's unpublished geological notes and jottings.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.