I had always known that as a naturalist, Darwin was also a geologist. Most geologists know that he was the first to propose the hypothesis that as tropical islands subside they turn into atolls (more on this later). And I always like to tell people that Darwin had a well-worn copy of Charles Lyell’s Principles of Geology along on the voyage. Principles, for all practical purposes, was the first geology “textbook”, and at the time of the Beagle’s departure it was hot off the presses (published in 1830). Lyell’s book illustrated how to “read the rocks” – interpreting what geology can tell us about the past. (This idea, often summarized as “the present is the key to the past”, was first proposed by James Hutton in 1780. Unfortunately, his book was virtually unreadable, so it took a while for the idea to sink in.) Later in the voyage, in a correspondence to W.D. Fox (noted in the diary footnotes), Darwin states “I am become a zealous disciple of Mr Lyells views, as known in his admirable book.” Furthermore, in his autobiography Darwin states:
“I had brought with me the first volume of Lyell’s Principles of Geology, which I studied attentively; and this book was of the highest service to me in many ways. The very first place which I examined, namely St. Jago in the Cape Verde islands, showed me clearly the wonderful superiority of Lyell’s manner of treating geology, compared with that of any other author, whose works I had with me or ever afterwards read.”
What I did not know until reading Darwin’s Beagle Diaries is that he was selected for the Beagle trip because Captain Fitzroy wanted a geologist to help study South America, and that by far, the majority of the observations Darwin recorded on the trip were on geology. If his career had ended shortly after the voyage, Darwin would have almost certainly gone down in history as a geologist. This emphasis on geology is clear in the section of his autobiography on Santiago:
“The investigation of the geology of all the places visited was far more important [than its natural history], as reasoning here comes into play. On first examining a new district nothing can appear more hopeless than the chaos of rocks; but by recording the stratification and nature of the rocks and fossils at many points, always reasoning and predicting what will be found elsewhere, light soon begins to dawn on the district, and the structure of the whole becomes more or less intelligible.”
“It then first dawned on me that I might perhaps write a book on the geology of the various countries visited, and this made me thrill with delight. This was a memorable hour to me, and how distinctly I can call to mind the low cliff of lava beneath which I rested, with the sun glaring hot, a few strange desert plants growing near, and with living corals in the tidal pools at my feet.”
I suppose biologists might say that Darwin turned to a more noble pursuit, moving from the inanimate to the animate, and indeed his studies (as reflected by his contributions to science) turned more to the living world in later years. But I would argue that geology is what gave him the necessary foundation to make the bold leaps forward in On the Origin of Species. Being a geologist, one is acutely aware of the immensity of time and the potential for slow & steady change. I have no doubt this pursuit played a crucial role in Darwin’s development as a scientist.
Interestingly, when he was asked to join the Beagle expedition in 1831 Darwin was studying with Adam Sedgwick. Sedgwick in know in geology circles for defining the Cambrian and Devonian periods of the geologic time scale – which, believe it or not, produced an enormous controversy in their own right in the mid 1800’s. Ironically, he was later to become an outspoken opponent to evolution and the concepts outlined in On the Origin of Species.
Since Cape Verde was the first place Darwin went ashore, it is the first real place that we see him describing the rocks. There are many quotes from his diary such as:
“the little time I was out of my cabin, I spent geologising on Quail Island” (Jan 18)
“Geology is at present my chief pursuit & this island gives full scope for its enjoyment. — There is something in the comparative nearness of time, which is very satisfactory whilst viewing Volcanic rocks.” (Jan 21)
He frequently refers to the volcanic rock of Santiago, already using the rock itself to surmise the volcanic nature of the islands (it is another series of hot spot islands). In Voyage of the Beagle he describes how “a perfectly horizontal white band in the face of the sea cliff, may be seen running for some miles along the coast, and at the height of about forty-five feet above the water.” Upon closer inspection he finds this layer to be wedged between two basaltic lava flows and “to consist of calcareous matter, with numerous shells embedded, most or all of which now exist on the neighbouring coast”. The implications about the uplift of the land surface recorded in these elevated beach sediments is not lost on Darwin.
Shell fossil (a) and uplifted shell layer between two lava flows (b) near Praia (from Charles Darwin Geologist at Santiago (Cape Verde Islands): A Field Reappraisal, by G. Pasquare et al., 2010)
And just because I want to fit it in here somewhere – one last quote from that correspondence with W.D. Fox mentioned earlier:
“Geology is a capital science to begin, as it requires nothing but a little reading, thinking & hammering.”
Say it again, Brother Darwin! (RJV)