Letter 129: Adam Sedgwick to Darwin, 18 September 1831
Why is it important? After completing a geological field trip to North Wales with the Cambridge Professor of Geology Adam Sedgwick, the 22-year old Darwin returned home to Shrewsbury to find an invitation to travel to South America on H.M.S. Beagle. Sedgwick’s letter contains advice on preparing for the voyage, including books to acquire and specimens to be studied in Britain before departure.
Letter 168: Darwin to W.D. Fox, [May] 1832 and Letter 171: Darwin to J.S. Henslow, 18 May and 16 June 1832
Why are they important? In these two letters, Darwin describes his geological work during the Beagle‘s first landfall at St. Jago in the Cape Verde islands. Together they document his new enthusiasm for the work of Charles Lyell and the relish with which he set about doing geological fieldwork. He had started reading Lyell’s Principles of Geology after receiving the first volume as a present from captain FitzRoy. As he wrote to his cousin, William Fox, ‘Geology carries the day; it is like the pleasure of gambling, speculating on first arriving what the rocks may be; I often mentally cry out 3 to one Tertiary against primitive; but the latter have hitherto won all the bets.’ To Henslow he explained, ‘The geology [of St. Jago] was preeminently interesting & I believe quite new: there are some facts on a large scale of upraised coast (which is an excellent epoch for all the Volcanic rocks to [be] dated from) that would interest Mr. Lyell.’
Letter 272: Darwin to J.S. Henslow, [10-13 March] 1835
Why is it important? Darwin experienced a powerful earthquake at Concepcion, Chile on 20 February 1835: ‘I wish,’ he told Henslow, ‘some of the Geologists who think the Earthquakes of these times are trifling could see the way the solid rock is shivered.’ At the harbour, FitzRoy found that the earthquake had raised the land several feet in relation to sea level, which supported Charles Lyell’s claims that even significant geological change could be explained by reference to causes of the type and intensity operating ‘in these times’ (as Darwin put it). As Darwin explained later in this letter, he had become convinced that South America was elevated by just such processes: ‘I can now prove that both sides of the Andes have risen in the recent period to a considerable height.’
Letter 335: Charles Lyell to Darwin, 26 December 1836
Why is it important? Lyell’s three-part work, Principles of Geology, was enormously influential on Darwin’s thinking during the Beagle voyage. This letter marks the beginning of the personal friendship between the two men after Darwin’s return. Lyell expresses his admiration for Darwin’s work on the geology of South America and advises him not to waste his time in official positions.
Letter 771: Darwin to Leonard Horner, 29 August 
Why is it important? Darwin grapples with the geological topic of ‘craters of elevation’, about which Charles Lyell, who was Horner’s son-in-law, had an ongoing dispute with the French geologist Léonce Élie de Beaumont. Darwin expresses his strong preference for Lyell’s work, and makes a famous statement about the extent to which his thoughts had been influenced by his familiarity with Lyell. ‘I cannot say how forcibly impressed I am with the infinite superiority of the Lyellian school of Geology over the Continental. I always feel as if my books came half out of Lyell’s brains & that I never acknowledge this sufficiently, nor do I know how I can, without saying so in so many words—for I have always thought that the great merit of the Principles, was that it altered the whole tone of one’s mind & therefore that when seeing a thing never seen by Lyell, one yet saw it partially through his eyes.’
Letter 3247: To T. F. Jamieson, 6 September 
Why is it important? In 1838, two years after returning from his geological adventures on the Beagle voyage, Darwin travelled to the highlands of Scotland to examine one of Britain’s most renowned geological phenomena, the ‘parallel roads’ of Glen Roy (see the project’s page on Glen Roy here). He dismissed the prevailing argument that these level terraces in the side of the valley had been formed when a lake stood in Glen Roy. Instead, he interpreted them as former sea beaches created when the British Isles were elevated from the water. Darwin’s explanation of the parallel roads was contravened by the Swiss geologist-zoologist Louis Agassiz, who claimed that the roads might be lake beaches formed when a glacier had dammed the foot of the valley. In this letter to the Scottish geologist Thomas Jamieson, who had elaborated the glacial view, Darwin definitively abandons his former theory about Glen Roy as a ‘gigantic blunder.’
Letter 6496: Darwin to G.H. Darwin, [9 December 1868]
Why is it important? Darwin discusses one of the most significant scientific challenges he faced after publishing the Origin in 1859. The mathematical physicist William Thomson (later known as Lord Kelvin) had claimed that the sun and the earth had not existed long enough for evolution to proceed as Darwin had envisioned. Like Thomson, George Darwin had completed a training in mathematical physics at Cambridge. Darwin asks his son to consult Thomson’s recent treatise in natural philosophy to ‘see exactly what he says (if he says anything) how many millions of years ago the crust of the Earth first became solidified so that it cd. have supported living beings.’ As Darwin explained: ‘The brevity of the world troubles me, on account of the pre-silurian creatures which must have lived in numbers during endless ages, else my views wd be wrong, which is impossible — Q.E.D.’
Letter 9868: Darwin to A.B. Buckley, [23 February 1875]
Why is it important? In this touching letter written immediately after Charles Lyell’s death to his secretary, Arabella Buckley, Darwin recalls his debts to the man who had been one of the young Darwin’s favourite authors and who became his closest scientific adviser and friend in the decades after the Beagle voyage: ‘I think that [Lyell’s] sympathy with the work of every other naturalist was one of the finest features of his character. How completely he revolutionised Geology; for I can remember something of pre-Lyellian days.’ Darwin concluded ‘I never forget that almost every thing which I have done in science I owe to the study of his great works. Well he has had a grand and happy career, and no one ever worked with truer zeal in a noble cause. It seems strange to me, that I shall never again sit with him and Lady Lyell at their breakfast.’ Buckley later became a popular author of books including The Fairy-Land of Science.
Letter 13145: To Alexander Agassiz, 5 May 1881
Why is it important? The Harvard zoologist Alexander Agassiz did not accept Darwin’s theory on the formation of coral reefs. Darwin wrote to Agassiz, who had made a fortune investing in copper mining, ‘It still seems to me a marvellous thing that there should not have been much, and long continued, subsidence in the beds of the great oceans. I wish that some doubly rich millionaire would take it into his head to have borings made in some of the Pacific and Indian atolls, and bring home cores for slicing from a depth of 500 or 600 feet…’ Agassiz spent the final decades of his life using his fortune to visit most of the world’s reef systems and investigating their origin (though he was never convinced by Darwin’s theory).