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Darwin's geological formation

Darwin's Deli - the natural selection Darwin became interested in natural history while studying medicine at the University of Edinburgh (1825-27) and subsequently read geology at the University of Cambridge (1828-1831) under the close supervision of Adam Sedgwick. Almost immediately afterwards, he joined HMS Beagle as an unpaid naturalist. Initially this was for two years, but Darwin eventually spent nearly five years at sea (1831-1836). The main purpose of the voyage was to survey the coastlines of southern South America. In addition, the Admiralty had requested that Captain FitzRoy should make geological maps of coral islands and record their shape and slope in order to provide information that might test the recently proposed theory that they were underlain by volcanoes. Darwin’s voyage culminated in a trilogy of publications on Zoology (1840), Volcanic Islands (1844) and Geology of South America (1846). Following the return of the Beagle Darwin became a Fellow of the Geological Society of London (December 1836) and very soon after, a Society secretary (February 1838) - a prominent role that he initially declined. In a letter (March 1837) to the new President, William Whewell, Darwin described how his time was fully occupied, writing up the natural history of the Beagle voyage:

“I think it is the duty of every geologist to follow your example, and that of the others, which you named, in sacrificing his time to the Society. But as I am just at present situated, I really cannot accept the office.— I have to write the third volume of Capt. FitzRoy's account of our expedition. He is already before hand with me, and I fear that even with giving up all details in Natural History, I shall be the cause of delay…”
In the same correspondence Darwin cited both his recent membership of the Society and his ignorance of English geology as further reasons for not accepting the post. Nevertheless, Whewell successfully persuaded him and Darwin served as secretary for three years. In 1840 Darwin began to suffer from ill health and was unable to participate actively in Geological Society meetings. His last fieldtrip was to North Wales in 18423. Darwin’s work on coral reefs4 and the geology of the Andes5 did, however, receive special praise from the Society when he was awarded its highest accolade, the Wollaston Medal, in 1859. His work on volcanic islands6 was largely unknown perhaps because English geology in the early to mid 19th Century was dominated by stratigraphers, palaeontologists and geomorphologists like Sedgwick, Murchison, Lyell and Owen.

At the core of the publication were Darwin’s findings from the Galapagos. He compared his observations with those from other ocean-islands, such as Ascension and the Azores, as well as findings by naturalists on other voyages. The penultimate chapter, entitled Trachyte and Basalt -Distribution of Volcanic Isles contains ideas fundamental to modern-day petrology, such as crystal settling. Darwin also commented that most ocean islands are volcanic and how many archipelagos are linear. In fact, he may have been one of the first scientists to appreciate the presence of “hot spot tracks”. However, following publication of Volcanic Islands, Darwin moved away from petrology and did not actively promote his ideas1.

Although Darwin had been interested in the origin of volcanic islands prior to the Beagle voyage, he had little field knowledge of igneous rocks. His experience consisted of visits to Salisbury Crags and a trip to North Wales with Sedgwick3. On board the Beagle his geological equipment included just a small hammer, pocket lens, contact goniometer and a blowtorch (for fusing rocks and minerals to determine their composition from the colour of their enamel). Darwin’s small on-board library included the Traité de Géognoise by d'Aubuisson de Voisins7, A description of active and extinct volcanos [sic] by Daubeny8, von Humboldt’s Personal Narrative (1799-1804)9, Principles of Geology by Lyell10 and Considerations on volcanos [sic] by Scrope12. Most had been recommended by Sedgwick, who wrote on 18 September 1831:

— I really dont know what to say about books— No. 1 Daubeny. No. 2. a book on Geology— D'aubuissons work is one of the best tho' full of Wernerian nonsense.— I dont think Bakewell a bad book for a beginner—For fossil shells what is to be done?— Go to the Geological Society and introduce yourself to Mr Lonsdale as my friend & fellow traveller & he will counsel you— Humboldts personal narrative you will of course get— He will at least show the right spirit with wh. a man should set to work— There is a small paper printed by the Geol. Socy containing directions for travellers &c—Lonsdale will give you a copy: but it is a mere horn book hardly worth your looking at— Study the Geological Socys. collection as well as you can—& pay them back in specimens— I am to propose you when the meetings begin.

Darwin clearly learned much from these publications and his field notes and diaries show that his ability to identify volcanic rocks increased rapidly during the voyage. In 1831 there were no systematic guidelines for naming volcanic rocks. Coarse-grained plutonic rocks were referred to as “granites” or “syenites” and fine-grained volcanic rocks as “basalt”, “greystone”, “phonolite” or “trachyte”.

Indeed, a review of Darwin’s four geological specimen notebooks has revealed that, despite visiting volcanic islands (Cape Verde and Fernando de Norohna) early on, it was not until reaching Rio de Janeiro in September 1833 that Darwin described any specimens of volcanic rock as “basalt”. But by the time the Beagle reached the Galapagos in 1835, Darwin frequently referred to his samples as trachytes and basalts, and it is clear that by this stage of the voyage he was aware of the significance of differentiating between them. His notes contain descriptions of the appearance of rocks including details of their “cellular” (vesicular) nature and also the abundance and grain size of feldspar. This was later highly important in his theory of magmatic evolution6.

Editor's note: References can be found at the foot of the main article.