I, a Geologist (Part 1): The geology of Charles Darwin
Stephen K Donovan and Cor F Winkler Prins (The Netherlands)
Darwin … devoted 1383 pages of notes to geological topics, compared with only 368 pages to biological topics.” – (Rhodes 1991, pp. 194-195)
Like the writer, Johann Goethe, who inscribed himself in the guest book of Karlsbad – present day Karlovy Vary, in the Czech Republic – as “J.W. Goethe, Geognost”, Charles Darwin considered himself a geologist (“I, a geologist” citation from his notebooks in Herbert 2005, p. 2), and rightfully so. He was a true savant and an amateur in the positive sense of the word, as commonly used in the seventeenth and eighteenth centuries (see Rudwick 2004, p. 23), This is despite the fact that he belonged, for some time after his return with HMS Beagle, to the geological elite (Rudwick 1982). This may be deduced, for example, from the fact that he was asked to write the part on geology of a naval manual (Darwin 1849).
Early geological influences: Edinburgh, Cambridge and Lyell’s Principles
Henslow then persuaded me to begin the study of geology [in 1831] … [T]he sagacious Henslow ¼ advised me to get and study the first volume of the Principles, which had just then been published, but on no account to accept the views therein advocated.” (Darwin 1983, pp. 39, 59)
Darwin’s geological education took a new direction as he read volume one of Lyell’s Principles of Geology (Fig. 1) on-board HMS Beagle. However, his instruction in the new science actually began when he attended formal courses in natural history and chemistry, and made friendships with geologists as a student at the universities of Edinburgh and Cambridge. Darwin was in Edinburgh from 1825 to 1827, where he had been sent by his father, Robert Waring Darwin (1766-1848), a physician, to study medicine, which was a kind of family tradition.
In this, Darwin was unsuccessful, but while there, he was introduced to geology by taking Robert Jameson’s course of lectures in his final academic year (1826-1827) (Secord 1991, p. 134). Jameson (1774-1854; Fig. 2) was Regius Professor of Natural History from 1804 until his death.
He had studied in Freiburg under Abraham Gottlob Werner (1749-1817) and held Wernerian views, at least at the time when he was teaching Darwin. The Wernerian theory of the history of the Earth considered all rocks to have been deposited or precipitated from a universal ocean. Darwin found Jameson’s lectures “¼ incredibly dull. The sole effect they produced on me was the determination never as long as I lived to read a book on geology ¼” (Darwin 1983, p. 28) – hardly the reaction expected from a man who would be awarded the Wollaston Medal, the highest accolade of the Geological Society of London, in 1859. However, Darwin was a hard working student in Jameson’s classes and developed a firm grasp of many aspects of geological knowledge at that time (Secord 1991, pp. 135, 138).
Darwin must also have received part of his geological education during the chemistry course (1825-1826) of Thomas Charles Hope (1766-1844) (Secord 1991, p. 139), a renowned lecturer, known for the discovery of the element strontium and the elucidation of why icebergs float. Mineralogy and geology would have formed part of Hope’s chemistry course, and his views were Huttonian, not Wernerian. Huttonian theory, recognizing the rock cycle and uniformitarianism, forms the basis of any modern geological synthesis.
As noted by one of his students, “¼ Dr Hope thinks that the Huttonian better accounts for the appearance of nature than the Wernerian theory” (quoted by Secord 1991, p. 139). Hope and Jameson were in dispute over the rival theories in the University and Darwin’s later Lyellian geological education was undoubtedly influenced by Hope’s ideas and his superior lecturing technique (Secord 1991, pp. 141-142).
Darwin’s distaste for a medical career (Darwin 1983, p. 31) led to a move to the University of Cambridge to study for the Church. He graduated, BA in 1831, but continued to pursue his interests in natural history, which led to him becoming naturalist on board HMS Beagle. His geological interests were fired by close association with, again, two notable savants, the Rev John Stevens Henslow, Regius Professor of Botany, and the Rev Adam Sedgwick, Woodwardian Professor of Geology.
Henslow (1796-1861; Fig. 3) was a polymath, with expertise in “botany, entomology, chemistry, mineralogy and geology” (Darwin 1983, p. 36). Darwin’s older brother, Erasmus, thought that the Professor was a man who knew every branch of science (Darwin in Jenyns 1862, p. 51). For example, Henslow had formerly been Professor of Mineralogy at Cambridge (Secord 1991, p. 142). He was Darwin’s mentor, introduced the young man to geology, persuaded Sedgwick to take him into the field and, most importantly, nominated his young protégé for the post of (unpaid) naturalist aboard HMS Beagle. Darwin was third choice after Henslow himself and Leonard Jenyns, Henslow’s brother-in-law. It is not too much to suggest that, without Henslow, there would have been no Origin of Species (1859). His own geological publications included accounts of the Isle of Man and Isle of Anglesey, and the discovery of phosphate deposits in East Anglia.
Sedgwick (1785-1873; Fig. 4) was one of the preeminent British geologists of the first half of the nineteenth century, with a research programme founded in fieldwork on what would now be called the Lower Palaeozoic and the Devonian successions of England and Wales. His joint researches with Roderick Impey Murchison (1792-1871) included the definition of the Cambrian and Devonian Systems. Darwin probably met Sedgwick at a party at Henslow’s house, a popular meeting place for naturalists in Cambridge, and may have attended some of Sedgwick’s lectures (Secord 1991, p. 143). However, Sedgwick’s greatest influence on Darwin was undoubtedly in the field.
After rubbing shoulders with these and other savants at two of the leading universities in the British Isles, it is perhaps curious that Darwin’s scientific development is traced so strongly to another young natural historian who he was destined not to meet until after returning to Britain after five years on HMS Beagle. Charles Lyell (1797-1875) was author of the most influential series of three volumes in the history of the Earth sciences, his Principles of Geology (Lyell 1830-1833). The first volume formed part of the library of HMS Beagle when it sailed for South America in December 1831. Henslow was apparently not the only geologist to be sceptical of Lyell’s gradualistic position when the Principles was published (Secord 1991, p. 151), but Darwin found it the most valuable book on geology available on-board (Herbert 2005, p. 64).
The book was radical, rejecting any Biblical literalism such as a universal deluge, going beyond Hutton’s uniformitarian ideas and incorporating a wealth of field evidence for the explanation of geological features in terms of phenomena that are observable at the present day. The copy of volume one of the Principles (Fig. 1) on HMS Beagle improved Darwin’s eye, already trained by Sedgwick, so that the young naturalist could test the Lyellian method in the field at the first chance, namely on St Jago in the Cape Verde islands (Secord 1991, p. 151 and below). Darwin accepted that small changes can accumulate to have major effects as the basis of his subsequent research programme, while rejecting Lyell’s more speculative ideas such as time’s cyclicity.
Darwin in the field
Tell Professor Sedgwick he does not know how much I am indebted to him for the Welsh Expedition; it has given me an interest in Geology which I would not give up for any consideration. I do not think I ever spent a more delightful three weeks than pounding the North-west Mountains.” (Quotation from a letter of Darwin to Henslow in Barrett 1974, p. 155)
Fieldwork is the core process whereby geological data are gathered and Darwin had two masters of fieldwork to instruct him – Sedgwick in person and Lyell through the pages of the Principles. However, his early field experiences were less than auspicious – Jameson’s field excursion to Salisbury Crags did not impress the student Darwin (1983, p. 29). In the summer of 1831, Darwin, now a Cambridge graduate, took himself off into the field in the area to the west and northwest of his parental home in Shrewsbury, but lack of experience engendered indifferent results (Roberts 1996, 2001). His real initiation into field geology only came when he made a tour into North Wales as assistant to Adam Sedgwick, an association arranged by Henslow.
Sedgwick was one of the most complete field geologists of his day, in a discipline that included Lyell, De la Beche and Murchison, to name but three worthy of similar accolade. There is some uncertainty as to exactly where Darwin travelled with Sedgwick (Barrett 1974; Roberts 2001). Certainly, they rode northwest and then west from Shrewsbury, into North Wales, and on to Bangor and the Menai Straits. The traditional interpretation is that they separated here, Sedgwick to investigate the complex geology of Anglesey while Darwin travelled south to visit friends in Barmouth. However, as noted by Roberts (2001, p. 35), on the voyage of HMS Beagle, Darwin had a firsthand knowledge of the geology of Anglesey. It is at least likely that this was gained in Sedgwick’s company.
Darwin’s fieldwork in North Wales seems to have been the making of him as a geologist. Sedgwick taught him both basic techniques and terminology, such as the correct use of the compass and clinometer for determining ‘strike’ and ‘dip’ (Herbert 2005, Fig. 1.4). He also demonstrated his thinking on cleavage as being a structural expression unrelated to bedding, which was the most advanced idea on this subject at the time and is now recognised as being correct. And Sedgwick actually got Darwin involved in his research programme on the geology of North Wales (Fig. 5), such as looking for ‘phantom’ outcrops of the Old Red Sandstone.
This unit was marked on Greenough’s (1820) geological map of England and Wales, but Sedgwick and Darwin showed that there was no evidence of its presence in the field. This they did both in tandem and, sometimes, separately to cover more ground. In less than a month, Darwin’s expertise in field geology had progressed demonstrably such that Sedgwick could entrust him with such independent enquiries.
Given his excellent grounding in field geology, the five years that Darwin spent abroad gave him the chance to greatly expand his first-hand knowledge of geology, but in terranes somewhat different from those of North Wales. The story is told of how volume one of Lyell’s Principles was a great influence on Darwin during this period, but the library of specialist volumes on geology available to Darwin was larger than just this one book. Of those listed by Secord (1991, p. 150), it is perhaps not stretching things too far to suggest that A Geological Manual, by such a consummate field geologist as De la Beche (1831), may have been of at least as much practical value, even though Darwin did subsequently not acknowledge it as such.
Darwin’s first independent chance to apply his received wisdom from Sedgwick, Lyell and others was on Quail Island (Ilheu de Santa Maria), in the harbour of Porto Praya, St Jago (São Tiago) in the Cape Verde Islands (Herbert 1991, pp. 164-174; 2005 pp. 143-156). Darwin’s investigations (1839b) were exemplary for a relatively inexperienced field geologist, including, as they did, observations on rock types, stratigraphy (bedding) and structure (dip and strike, faults and uplift). Eventually, it lead to a general overview of southern South America (Fig. 6).
However, Darwin’s geological research was not always successful as an application of Lyellian principles to his own observations. In 1838, Darwin examined a phenomenon known as the parallel roads of Glen Roy in the Scottish Highlands, south of the Great Glen. Darwin described these features, preserved high up on the sides of the glen, and determined that they were a series of ancient marine strandlines from multiple lines of evidence (Darwin 1839a). Darwin disagreed with earlier theories that suggested that these features marked former lake levels within Glen Roy and suggested that the parallel roads provided evidence of changes in the relative elevations of the Scottish mainland relative to sea level, which supported his ideas on vertical tectonic actions.
However, Agassiz read a paper to the Geological Society in November 1840 that showed the parallel roads represent successive shorelines of a freshwater, glacial lake that had been dammed by ice. Darwin had a strong reaction to Agassiz’s analysis and, after more than 20 years (Rudwick 2008, p. 537, footnote 3), repudiated his own publication: “This paper was a great failure, and I am ashamed of it” (Darwin 1983, p. 48). However, it brought the action of former glaciers sharply to Darwin’s attention and, in 1842, he examined glacial features in North Wales for himself (Darwin, 1983, p. 58).
Darwin and vertical movements of the crust
Even when writing on coral reefs Darwin approaches the subject as a geologist.” (Brower, 1976, p. 213)
The theory of plate tectonics dominates modern ideas concerning the evolution of the Earth’s crust. Stated simply, the internal heat of the Earth laterally propels large and rigid portions of the crust, called plates. These move apart, slide past each other and collide in a variety of settings and geometries that have determined the principal physiographic features of the planet. Many of these features are elevated (hills and mountains) or depressed (basins and oceanic trenches). Nonetheless, they are the products of lateral plate movement, which may be translated into vertical motion, most commonly close to inter-plate boundaries. However, the explanation of vertical motion, driven by indeterminate vertical processes, dominated tectonic theory before the early 1960s (Oreskes 1999). Darwin’s ideas on tectonics belonged to this prevailing school, recognising the importance of vertical, not horizontal forces. His attempt to produce an explanatory theory foundered for not having a measurable process to drive the observed pattern (Rhodes, 1991).
Darwin’s ideas on tectonic processes were mainly derived from his observations of the South American continent. This included, in particular, the Andes, but also the Chilean earthquake, which he had himself witnessed (see Rudwick, 2008, p. 487), and oceanic islands visited during the voyage of HMS Beagle (Darwin, 1838a, b). He presented his tectonic theory of continental crustal uplift to the Geological Society in London on 7 March 1838 (Darwin, 1838c, 1840; Rhodes 1991). His theory of mountain building was not the first such theory, any more than his theory of the origin of species by natural selection was the first theory of evolution. Essentially, Darwin considered the processes of formation of mountain chains and volcanoes to be closely related. Their formation was Lyellian – slow increments over a great length of time producing major elevations of the crust. Rhodes (1991, p. 206) summarised the theory as follows:
- The elevation of mountain chains was cumulative rather than catastrophic.
- The process of mountain building continues today.
- This force “has been in action with the same average intensity (volcanic eruptions being the index) since the remotest periods” (Darwin 1840, p. 625).
- Parallel mountain ranges may be of demonstrably different ages.
However, Darwin’s theory caused little interest when published in a climate of conflict between Catastrophists and Gradualists (Rhodes, 1991, p. 215 et seq.). It was an attempt at a Gradualist global synthesis of crustal evolution, something that would not be attained until the plate tectonic synthesis over a century later (Oreskes, 1999). Essentially, Darwin’s idea lacked a demonstrable driving mechanism. For similar reasons, Wegener’s (1915) ideas on continental drift, a theory even more appealing than that of Darwin, was nonetheless widely rejected until the physical driving mechanism was identified as part of the plate tectonic synthesis (Oreskes, 1999).
Darwin’s The Structure and Distribution of Coral Reefs (1842) was his most public pronouncement of his tectonic theory, although, to an uninformed observer, it might, at first, appear to be a contribution to the biological rather than geological sciences. It extended his theory from the continental to the oceanic realm (Rhodes 1991, p. 197). The book is concerned with vertical movements of the crust that encourage the growth of shallow water coral reefs, both elevation and subsidence, occurring slowly over long periods of time (Fig. 7). That is, the formation of reefs is inferred to have been a Lyellian process and was summarised as follows:
… it has already been shown … that the movements [of the crust] must either have been uniform and exceedingly slow, or have been effected by small steps, separated from each other by long intervals of time, during which the reef-constructing polypifers were able to bring up their solid frame-works to the surface … (p. 146) we may finally conclude, that the subterranean changes which have caused some large areas to rise, and others to subside, have acted in a very similar manner” (Darwin 1842, p. 145).
The second part of this article can be found at: I, a Geologist (Part 1): The geology of Charles Darwin.
About the authors
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