Volcanic islands

An introduction by Gordon Chancellor

Volcanic islands (1844) was the second part of Darwin's planned 'geology of the Beagle'. It included his descriptions of the geology of Australia and South Africa and sits between Coral reefs (1842) and South America (1846). He published his descriptions of the Falklands in a separate paper in 1846 (Darwin 1846). Although they are islands the Falklands are geologically continental and form a nice link between Darwin's work on South America and South Africa. Unsurprisingly all three of Darwin's geology books overlap. I believe a modern reader's enjoyment of both Coral reefs and Volcanic islands is bound to be enhanced if they are read alongside Patrick Armstrong's masterful account in his Darwin's other islands (2004).

Darwin brought out an unrevised edition of all three geology books in 1851 and a 'second edition' of Volcanic islands combined with South America in 1876 (Darwin 1876). Ward Lock published a posthumous edition of the three geology books together in 1890, each with a useful introduction by Darwin's friend John Judd (Darwin 1890).

Volcanic islands took some time to write. As is well known from his Autobiography, Darwin had the idea to publish a 'Geology of the Beagle' at his very first field location, the Cape Verde islands, in February 1832. Darwin started work on the book fifteen months after returning to England, drafting the chapters on Galapagos and Ascension in January 1838 (contrary to Judd's statement that work started in 1843; in fact that was revision of earlier drafts, see below). St Helena followed in February, Africa and Australia in April, New Zealand and the Cape Verdes in May. Darwin was then unwell and over the summer also switched his focus to the 'parallel roads' of Glen Roy (Darwin 1839). He also at this time opened new private notebooks, which he later labelled 'D' and 'M', and in September 'happened to read Malthus for amusement' and discovered natural selection.

On 14 September 1838 Darwin 'began Craters of Elevation' and 'began Corals' on 5 October. At this stage he certainly intended to publish Coral reefs and Volcanic islands as one book, but at some point realised that Coral reefs needed separate treatment. Over the winter of 1838-9 Darwin joined the Royal Society and married his cousin, then revised his 'earthquake paper' and returned to Coral reefs in February 1839. By May he was working on his monumental coral map and his first book, Journal of researches, was published in August. His first child, William, was born in December.

Darwin returned to Coral reefs and erratic boulders in January 1840 but did not pick up Coral reefs again until the July following the birth of his first daughter Anne in March 1841. He sent the manuscript of Coral reefs to the publishers in January 1842 and corrected the proofs in May, three years and seven months after starting the geology. Twenty months had been devoted to his massive compilation of facts for Coral reefs and to many other geological papers, not to mention his work on species and psychology. In that time Darwin had also moved to the village of Down and become a family man. Sadly his second daughter Mary was born and died in 1842.

After Mary's funeral Darwin returned to Volcanic islands. He had his servant Syms Covington shorten the manuscript (see Correspondence, vol. 2, p. 437 note 62), then started revising in July 1843. By October he recorded having spent sixteen months on the book, which he finished in January 1844, sending the manuscript off on the 5th (Correspondence, vol. 3, p. 395). According to a letter of 6 June 1846 only 143 copies had been sold and Darwin bought up the 76 remaindered copies in 1863. Unfortunately, the Smith Elder archives do not survive from before 1853, so it is impossible to know if this includes the 1851 edition.

The book itself consists of seven chapters and an appendix. The chapters are not arranged in the order that the Beagle visited the islands except that the first is for the Cape Verdes. The second is Fernando Noronha, Azores, Tahiti and Mauritius, with the non-volcanic St Paul's Rocks somewhat incongruously included. The third chapter is for Ascension, the fourth for St Helena and 'craters of elevation' and the fifth for the Galápagos. The sixth chapter discusses the different types of lava found on oceanic islands and also discusses the global distribution of these islands. The final chapter deals with those parts of Australia and South Africa visited by Darwin, but curiously he seems to have forgotten the section he wrote on New Zealand in May 1838 (see above) as all he published on that country was a footnote list on p. 142 of his rock specimens.

The chapters are all of a similar length, except the second which is short and the Ascension chapter which is noticeably the longest. The Appendix contains George Sowerby's descriptions of Darwin's Tertiary molluscs from the Cape Verdes, extinct land snails from St Helena and Palaeozoic brachiopods from Tasmania. William Lonsdale contributed descriptions of Darwin's Palaeozoic corals from Tasmania. None of these were illustrated. The book itself is quite well illustrated, with a chart of Ascension and a sprinkling of woodcuts of landscape views, drawings of specimens, sketch sections and maps.

The Cape Verdes chapter is limited to a description of the island of St Jago [Sao Tiago] and should be read in conjunction with the recent study of Darwin's work there by Pearson and Nicholas 2007. These authors have clearly shown, contrary to the standard view of Darwin as a committed Lyellian 'uniformitarian', that he was quite comfortable adopting a 'catastrophist' explanation for the formation of the valley behind Porto Praya. It is also clear from the section on 'valleys near the coast' that Darwin preferred a marine explanation for these vallies. He over-extended this preference in his 1839 interpretation of the 'parallel roads' of Glen Roy (actually formed by an ice-dammed glacial lake) and in his highly controversial estimate of geological time in the Origin, in which he treated the erosion of the Weald as an almost entirely marine process, more or less ignoring the far more important erosion by rivers.

In his discussion of St Jago Darwin focused on the lavas overlying the limestone bed from which came the fossils described by Sowerby. Darwin was keenly interested to use the limestone as a dramatic piece of evidence for uplift of the island as this supported his view of the Earth's crust as subject to constant vertical mobility. On p. 95 Darwin, obviously referring to his second visit to St Jago in 1836, explicitly related his view of the island to his great 1838 paper on crustal mobility:

A conjecture… occurred to me, when, - with my mind fully convinced, from the phenomena of 1835 in South America, that the forces which eject matter from volcanic orifices and raise continents in mass are identical, - I viewed that part of the coast of St Jago [with the lava overlying the upraised limestone].

Today it is more clearly understand how the crust is highly mobile in a horizontal sense as the tectonic plates move relative to each other. Darwin's volcanic islands sit on oceanic crust and are relatively young i.e. less than 150 million year old (except St Paul's Rocks, which are anomalous), whereas Australia and South Africa are ancient continental crust, parts of which are billions of years old. In a remarkably prescient passage Darwin showed that he understood the global significance of Ascension and St Helena:

These facts seem to show, that an island or an archipelago is in process of formation in the middle of the Atlantic: a line joining St Helena and Ascension, prolonged, intersects this slowly nascent focus of volcanic action (p. 92 footnote).

Of course Darwin could not have known that the mid-Atlantic Ridge of which these islands are surface expressions was the spreading centre marking the split some 150 million years ago between the two sides of the Atlantic Ocean.

Darwin's subject is serious rather than entertaining but occasionally there are some stylistic flourishes, such as the analogy he drew between some lava with white streaks which 'immediately reminded [him] of the appearance in badly kneaded dough, of balls and drawn-out streaks of flour' (p. 12). One is also rather struck by his use of the first person, which although typical of the style of many of Darwin's books would never be tolerated by a scientific editor today. On one page he uses the word 'I' at least nine times, as exemplified by the following delightful passage from the end of the Cape Verde chapter:

On the shores of Quail Island, I found fragments of brick, bolts of iron, pebbles…united …into a firm conglomerate…..I may mention, that I endeavoured with a heavy hammer to knock out a thick bolt of iron, which was embedded a little above low-water mark, but was quite unable to succeed (p. 22).

The second chapter consists of relatively short essays on the islands which Darwin spent less time on, but which are vital parts of the Beagle voyage story (Chancellor and van Wyhe 2009). In decreasing order by length these are geological portraits of Mauritius, Tahiti, Terceira (Azores), St Paul's Rocks and Fernando Noronha. Darwin's pre-occupation with crustal mobility and his coral theory is betrayed by the statement that on Tahiti he 'looked in vain for any signs of recent elevation' (p. 28).

The third chapter deals with Ascension. It was on this island that Darwin first heard from his sisters of Adam Sedgwick's opinion that Charles was headed for a place among the leading men of science. Sedgwick told Darwin's father of the acclaim Charles had received at the Cambridge Philosophical Society and at the Geological Society of London, from the readings at those Societies in November 1835 of extracts from letters from Darwin to Henslow and also of the interest shown in Darwin's fossils at the Cambridge British Association meeting at Cambridge in 1833. Darwin, in his Autobiography, described how he was so excited at his sisters' news that he 'clambered over the mountains of Ascension with a bounding step and made the volcanic rocks resound under my geological hammer!' (p. 81)

Perhaps the statement in Volcanic islands on p. 49 that Darwin saw a 'mass divided into even layers half an inch in thickness, which were so compact that when struck with a hammer they rang like flint' was the rock referred to in the Autobiography?

The fourth chapter deals with 'prison-like' St Helena, and island which had already been studied by the geologist Robert Seale. Darwin described features of the island in great detail, such as the rim of horizontal lava clinging to the inside of a crater 'like the ice round a pool, from which the water has been drained' (p. 82). He also discussed the island's superficial deposits which contained extinct land snails, sea bird's eggs and bones and the remains of trees which clothed the island a few centuries before his visit. These issues were important to Darwin as he was keen to know whether St Helena, a very remote island, proved his belief that oceanic islands do not have separate 'creations' of species, but must receive them from the continents by migration.

Darwin included in the St Helena chapter a discussion of the formation of sea cliffs by the constant pounding of the waves and demonstrated how from offshore soundings one could calculate the amount of rock removed from an island:

The mind recoils from an attempt to grasp the number of centuries of exposure, necessary to have ground into mud and to have dispersed the enormous cubic mass of hard rock which has been pared off the circumference of this island (p. 92).

St Helena was the clearest example Darwin had seen of what he chose to label craters of elevation and for this reason he included his discussion of such craters in this chapter (Chancellor 1990). He could not accept the 'collapsed blister' explanation for such islands as promoted by Élie de Beaumont. Lyell and Darwin declared that he could not 'believe that the great central hollows have been formed by a simple dome-shaped elevation' (p. 94). Darwin's theory, worked out at almost exactly the time he discovered natural selection after reading Malthus, was that these islands were formed by differential uplift, the outer rims being elevated more than the centres, because the various fissures and orifices in the centre allowed the escape of pressure from beneath.

The fifth chapter was the first ever geological description of the Galápagos Islands and even today should not be ignored by any serious student of that archipelago. The modern geologist will be struck by how Darwin perceived the significance of the linear arrangement of the volcanoes on Albemarle [Isabela], Narborough [Fernandina] and other islands. Darwin saw how these roughly NW/SE lines were crossed at right-angles by a complementary series of lines which we today understand to be the result of shearing forces due to the Galápagos Islands moving away from two more or less perpendicular spreading centres (the West Pacific Rise and the Cocos Ridge).

Darwin was obviously excited about the Galápagos, even though the lava there was only 'slowly clothed with a poor vegetation, and the scenery has a desolate and frightful aspect' (p. 114). He reserved his most poetic descriptions for the archipelago, comparing the lavas of Chatham [San Cristobal] Island with those of Albemarle: "So rugged that they may be compared to a sea frozen during a storm; whilst the great stream of Albemarle Island is almost as smooth as a lake when ruffled by a breeze". (p. 105)

Chapter six is a technical discussion of the varying compositions of lavas encountered on oceanic islands. Pearson 1996 has shown clearly how advanced Darwin was in his thinking about the varieties of lavas, which on oceanic islands were generally of dark colour and 'basic' (that is low silica) composition and would be called basalts. The dark colour is due to the higher proportion of the heavier minerals containing iron and magnesium in basalts which, because they have less 'sticky' silica tend to flow more easily. Darwin encountered several occurrences, usually near the volcanic orifice, as for example on James Island [Santiago] in the Galápagos, of lighter lavas with more silica which would be termed trachyte. Pearson makes clear how Darwin understood that this 'fractionation' was the expected result of the heavier minerals sinking in the molten lava before eruption, but unfortunately Darwin's groundbreaking work on this was largely ignored. Perhaps it would have had more impact if he had published it as a separate paper.

Always trying to relate the particular to the general, Darwin finished chapter six with a short section on the distribution of volcanoes in which he made the link to his work on Coral reefs. He concluded 'that a vast majority of the volcanoes now in action stand either as islands in the sea, or near its shores' (p. 126) and he was not afraid to contradict the great authority of Von Buch's classification of volcanoes into 'central' and 'linear' types, which certainly did not correspond to what Darwin described in the Galápagos. (p. 100)

The final chapter deals with New South Wales and Tasmania, King George's Sound (Western Australia) and the Cape of Good Hope, in other words Australia and Africa. Darwin's descriptions are important contributions, even though he always tended to interpret landscape as elevated sea-bed, as for example in the Blue Mountains behind Sydney, and in that case was completely wrong. In recent years Patrick Armstrong has published detailed accounts of Darwin's geology in all these places, and Nicholas and Nicholas 1989 have presented a superb account of all Darwin's work in Australia.

Volcanic islands is one of Darwin's least-read books, as he himself must have realised. It is too detailed for the general reader and the later re-printings would probably never have appeared had it not been for the fame of its author. As Judd quoted in his introduction to the 1890 edition, Darwin wrote to Fox on 28 March 1843 that writing the book was 'uphill work' which 'cost money' and resulted in something 'not read even by geologists'.

Now that the author is regarded as one of the greatest scientists who ever lived, it is likely that more and more people will take the trouble to read Volcanic islands. On one level it is a technical treatise for geologists, but on another it is a series of pen portraits of exotic countries by a great travel writer. Finally, it provides yet further insights into the mental evolution of one of the most influential thinkers who has ever lived.

Gordon Chancellor

October 2008

Darwin, C. R. 1844. Geological observations on the volcanic islands visited during the voyage of H.M.S. Beagle, together with some brief notices of the geology of Australia and the Cape of Good Hope. Being the second part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. London: Smith Elder and Co. Text Image PDF F272