Charles Darwin's Beagle Diary

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.”

Charles Lyell:

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.”