Darwin the Geologist

In an autobiographic note Charles Robert Darwin (12, February 1809 - 1882) remembers a childhood wish

"It was soon after I began collecting stones, i.e., when 9 or 10, that I distinctly recollect the desire I had of being able to know something about every pebble in front of the hall door--it was my earliest and only geological aspiration at that time."

Darwin today is mostly associated with evolution, but his first scientific achievements and publications dealt - even against his own preconceptions- with geology.

Fig.1. "I a geologist", from the Notebook M, 1838, page 39, the full phrase as follows: "I a geologist have illdefined notion of land covered with ocean, former animals, slow force cracking surface &c truly poetical."

As a not yet theology student at Edinburgh University during the years 1825-1827 Darwin tried also various courses on natural science, and so he encountered geology in the form of Neptunism by Jameson's introducing lectures and the explanation of igneous veins being filled with material deposited by water from above. Darwin considered this idea as absurd and the teaching of Jameson as boring, and despite his ambitions in collecting minerals in early years during his remaining time at university he never again actively joined a lecture about geology.
The time at Cambridge was more productive; he joined various private organized geological-botanical excursions in the areas surrounding the city. In July 1831 he visited privatly the cave of Llanymynech near his hometown of Shrewsbury, and in August of the same year, after graduating from Cambridge University and pushed by his mentor and friend, the botanist John Henslow, he accompanied Professor Adam Sedgwick (1785-1873, considered one of the founding fathers of geology in England) on a one week geologic tour in North Wales. Twenty pages of notes made by Darwin during the tour are today conserved the Cambridge University Library.

Fig.2. Darwin's sketch of 1831 of the geological map of Shrewsbury. In his private autobiography he later remembered: "This tour was of decided use in teaching me a little how to make out the geology of a country..." (ROBERTS 2001).

When Darwin returned home at Shrewsbury, a letter from Captain Robert FitzRoy's offered him a position of gentlemen companion on board of the Beagle. FitzRoy was himself a gifted amateur geologist and was searching a talented naturalist with additional geological knowledge to sustain him in a personal task - the Beagle voyage, despite improving the nautical maps of South America, could be used also to gather geological evidence for the biblical flood, a worldwide phenomena considered possible - if not already proven- by most geologists at the time.
As a welcoming gift FitzRoy presented him with a copy of Charles Lyell's recently published "Principles of Geology" (first edition 1830-1832).
Darwin became strongly influenced by the Uniformitarianism-geology of Lyell, observing at his first stop during the Voyage of the Beagle on the Cape Verde islands (January 16, 1832 to February 8) sediments enclosed by lava flows and raised above the sea level, but with fossils similar to the shells in the sea nearby (implying no substantial change of acting natural forces and habitats over time), he applied the principles proposed by Lyell and became convinced of the slow, minute and gradual changes of earth surface.
In a letter to his sisters Darwin confessed that he "literally could not sleep for thinking over my [geology]".

Fig.3. Profile of the island of St. Jago (today Santiago) as seen by Darwin in 1832. Darwin was the first to study the geology of the Cape Verde Islands (from DARWIN 1876). Darwin recognized three distinct layers of rocks, a lower series with volcanic rocks composed of hyaloclastite breccias and magma dikes, a limestone with fossils and finally a cover of basaltic lava. Darwin also, trained by Sedgwick, noted the contact metamorphism between the former hot molten lava and the earlier limestone.
It is curious to note that Darwin adopted the geological terms used by the German geologists, here still the strong influence of Alexander von Humboldt works, read by the young Charles, is recognizable.

In South America Darwin studied the rocks of the Andes and the sediments of the vast plains. On February 20, 1835, Darwin was in the town of Valdivia, Chile, when at 11.30 in the morning, a massive earthquake struck:

"I happened to be on shore, and was lying down in the wood to rest myself. It came on suddenly, and lasted two minutes, but the time appeared much longer. The rocking of the ground was very sensible".

Darwin noted after the earthquake raised shell beds on the coast of the Pacific, similar to the fossils seen on the island of Cape Verde - was it possible that even the highest mountains were formed by innumerable single earthquakes acting trough deep time.

During the entire voyage (1831-1836) Darwin encountered various outcrops with magmatic and volcanic products, and he became fascinated by these rocks. On the Galapagos Islands he carefully studied the viscosity of lava flows -

"The degree of fluidity in different lavas does not seem to correspond with any apparent corresponding amount of difference in their composition"

-this is an erroneous conclusion, the viscosity of lava in fact depends in certain degrees of the amount of silica. However he correctly postulates that a mineralogical differentiation of magma is possible by segregation of minerals by gravity - a fundamental point to explain the different lava types found on earth.

"Much of the difficulty which geologists have experienced, when they have compared the composition of volcanic with plutonic formations, will, I think, be removed, if we may believe, that most plutonic masses have been, to a certain extent, drained of those comparatively weighty and easily liquefied elements, which compose the trappean and basaltic series of rocks."

In the five years of the voyage, Darwin wrote 1.383 pages of notes about geology - compared to a mere 368 pages of notes on plants and animals.

After returning home, in 1838 Darwin hold his first scientific discourse of the geology of the Andes at the Royal Geological Society and published some preliminary results about volcanic phenomena observed in South America. His major contributions to volcanology are two later books: "The structure and distribution of coral reefs", published in 1842, and the "Geological Observations on the Volcanic Islands" published in 1844, followed 1846 by the "Observations on South America". The first book covers the distribution, structure and formation of coral-riffs by sinking volcanic islands, the second contains the descriptions of the visited volcanic islands, like Ascension, St. Helena, the Galapagos, and a short notification about the geology of the South Africa and Australia, finally the last books covers the continent Darwin explored and studied most.

Fig.4. Geologic map of Patagonia (Darwin, circa 1840, unpublished, from ZAPPETTINI & MENDIA 2009).

Darwin also published some minor papers (not to mention the volumes of the "The Zoology of the Voyage of H.M.S. Beagle" dedicated to the collected fossil remains), in 1846 a description about the geology of the Falkland Islands, in 1838 about some phenomenon's connected to the volcanism in South America, in 1841 about erratic blocks distribution and some unstratified sediments found in South America and in 1845 the observations about the dust that can be found, transported by wind, on ships crossing the Atlantic ocean.

After the Beagle experience however Darwin quickly retired from active geological research.
In July 1838 he visited Glen Roy in Scotland, and published his opinion on the origin of parallel terraces in some valleys in the paper "Observations on the Parallel Roads of Glen Roy, and of Others Parts of Lochaber in Scotland, with an Attempt to Prove that They are of Marine Origin." However just 2 years later the new theory of ice ages attributed the former lake beaches to a glacial origin, Darwin despite accepting the idea of ice ages remained aggrieved by the complete confutation of his former theory.
In 1842 he visited Cwm Idwal in North Wales, one of the last of geological excursion before his ill health forced him to an apparent quiet country life.

Geology played a major role in Darwin´s life and scientific (also on biology) work: The slow subsidence of coral reefs, the rising of the Andes by earthquakes, the fossil relatives to modern species in South America, these geological observations enabled Darwin to grasp two fundaments needed for his scientific theory: the deep time of Earth and the slow, but perpetual changes of earth itself.
If geology was able to such profound modifications over time, so had biology, to adapt and survive to the ever changing environment.

Bibliography:

DARWIN, C.R. (1876): Geological Observations on the volcanic islands and parts of South America visited during the Voyage of H.M.S. "Beagle". 2nd edition Smith, Elder & Co., London: 647
CHIESURA, G. (2010): A Santiago sulle orme di Darwin. Darwin - Bimestrale di Scienze No.40: 32-36
ROBERTS, M. (2001): Just before the Beagle: Charles Darwin's geological fieldwork in Wales, summer 1831. Endeavour Vol. 25(1): 33-37
SEWARD, A.C. (2006): Darwin And Modern Science. The Echo Library, Teddington: 489
TOSATTI, G. (2008): Charles Darwin geologo. Atti Soc. Nat. Mat. Modena 139: 205-219
ZAPPETTINI, O. & MENDIA, J. (2009): The first Geological Map of Patagonia. Revista de la Asociación Geológica Argentina 64 (1): 55 - 59

Online Resources:

Charles Darwin: A Genius in the Heart of London. (Accessed 12.02.2011)
The Complete Work of Charles Darwin Online (11.06.2010): Geology of The Voyage of The Beagle. (Accessed 12.02.2011)