During his famous Beagle voyage, Darwin collected rocks, fossils and other geological specimens. No previous geologist had amassed such a detailed set of data. He identified raised beaches and remains of marine organisms high above the sea, understanding their significance as evidence of the uprising of landmasses. He also witnessed an earthquake and volcanic eruptions, concluding that both are related to movements of molten rock deep in the Earth. In this 1909 lecture, Sir Archibald Geikie, then President of the Royal Society, outlines Darwin's geological findings and explains how these underpinned his developing ideas. We learn of Darwin's theory of coral reef formation, and his fascination with the activities of earthworms. Finally the lecture considers the importance of Darwin's geological studies in formulating his theory of evolution by natural selection, leading to his masterpiece On the Origin of Species.