Charles Darwin, Geologist

Charles Darwin, Geologist
On the 150th anniversary of his publication of Charles Darwin’s On the Origin of Species in 1859 (24 November 2009), we can appreciate his impact more if we remember that he was as much a geologist as a zoologist. His first paper, read to the Geological Society in May, 1837, was about the geological formation of coral atolls. After leaving South America and the Galapagos Islands, the HMS Beagle had travelled to many Pacific islands built of coral. There he worked out how the islands form by the slow growth and death of corals over many millennia, and are given life by the transport of plant seeds and landing of birds and insects.
Darwin himself did not even write the zoology of the Beagle’s 1831–1936 British Admiralty expedition. Its volumes, published from 1838 to 1843, were authored by Richard Owen (fossil mammals), George Robert Waterhouse (living mammals), John Gould (birds), Leonard Jenyns (fish) and Thomas Bell (reptiles). But Darwin, as the editor, wrote a geological introduction to the Fossil Mammalia and a geographical introduction to the Mammalia.
Before the voyage, scientists already knew that the Earth’s surface had formed by slow geological processes over countless eons contrary to the Biblical (Noachian) flood. They had been studying fossils of extinct plants and animals that were known, by the geological layers in which they were found, to have predated modern forms. They knew that the fossils that were most dissimilar to modern ones were in deeper, older layers and that those most like modern species were in the newer layers. They generally (though not universally) recognized that some sort of “transmutation” of species occurred, and had proposed various theories about the forces driving it and the mechanisms by which it occurred. In 1830, Charles Lyell had brought these ideas together in his widely acclaimed volume 1 of Principles of Geology.
Darwin was not the official naturalist on the voyage: that post was filled by naval officers who made the official biological collections. Darwin was merely a paying passenger, although a highly placed one who became close friends with the captain, Robert Fitzroy, himself a trained scientist and biological specimen collector. A condition of Darwin’s passage was that he was given facilities for a private collection of fossils and animals. Before the voyage, Lyell had asked Fitzroy to make specific geological observations—the enthusiasm for which he shared with his passenger, the young Charles Darwin.
It was Fitzroy who had given Darwin his own copy of Lyell’s first volume before they sailed. Darwin eagerly obtained Lyell’s volumes 2 and 3 during the voyage, as soon as they were published. Darwin’s Narrative (1839) is filled with geological musings of how what he saw had come to be, largely by fitting his observations to Lyell’s framework, but also relying on the journals of previous expeditions such as Humboldt’s and Kotzebue’s. Thus, along the Argentine coast, Darwin used fossil mammal bones underlying fossil marine shells to theorize about geology: that they had been swept down a Tertiary river into the sea, covered with sediments, subsequently raised by geological uplift above the sea surface and later eroded to become the cliff face on a high point of land where he found them. Rounding Cape Horn, he surmised that the archipelago might be a row of mountains that had become submerged. Further up the coast of Chile, he confirmed his guess by noting the age and location of strata where he found marine fossils of various ages, from the seashore high up into the Andes. He worked out how crustal uplifting and earthquakes (one of which he experienced in Valdivia) and volcanoes had combined to raise the Andes from below sea level to the 7,000 meter high Mount Aconcagua, which he climbed (not to the top, but high enough). He realized that the forces that raised the Andes were the opposing forces to those that had submerged Tierra del Fuego. It was geology, also that led him to challenge previous assumptions, including Lyell’s, that species were “immutable” (i.e., fixed, not changing) because that would mean that either they had been created at different times in different places, or had crossed geological barriers such as salt water or solid rock, both of which geology had shown to be impossible. Geology, not finches, proved that species were not immutable: they evolved.