''Geological Observations on the Volcanic Islands, visited during the Voyage of H.M.S. Beagle'' is a book written by the English naturalist Charles Darwin. The book was published in 1844, and is based on his travels during the second voyage of HMS ''Beagle'', commanded by captain Robert FitzRoy. It is the second book in a series of geology books written by Darwin that also includes ''

The Structure and Distribution of Coral Reefs ''The Structure and Distribution of Coral Reefs, Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836'', was published in 1842 as Charles Darwin's first monogr ...

'' (published in 1842) and ''

Geological Observations on South America ''Geological Observations on South America'' is a book written by the English naturalist Charles Darwin. The book was published in 1846, and is based on his travels during the second voyage of HMS ''Beagle'', commanded by captain Robert FitzRoy ...

'' (published in 1846). The text contains seven chapters, and includes observations made during Darwin's travels to the volcanic island of St. Jago in

Cape Verde , national_anthem = () , official_languages = Portuguese , national_languages = Cape Verdean Creole , capital = Praia , coordinates = , largest_city = capital , demonym ...

, the Fernando de Noronha archipelago,

Ascension Island Ascension Island is an isolated volcanic island, 7°56′ south of the Equator in the South Atlantic Ocean. It is about from the coast of Africa and from the coast of South America. It is governed as part of the British Overseas Territory o ...

, the island of

Saint Helena Saint Helena () is a British overseas territory located in the South Atlantic Ocean. It is a remote volcanic tropical island west of the coast of south-western Africa, and east of Rio de Janeiro in South America. It is one of three constitu ...

, the Galápagos Islands, James Island, New Zealand,

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

, Van Diemen's Land, and the

Cape of Good Hope The Cape of Good Hope ( af, Kaap die Goeie Hoop ) ;''Kaap'' in isolation: pt, Cabo da Boa Esperança is a rocky headland on the Atlantic coast of the Cape Peninsula in South Africa. A common misconception is that the Cape of Good Hope is t ...

. The book includes one of the earliest accounts of the process of

magmatic differentiation In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of (u ...

. While observing a basaltic

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...

in the Galápagos Islands, Darwin observed that "crystals sink from their weight" Available on-line at
Internet Archive
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p. 117
of his ''Geological Observations on the Volcanic Islands'' … (1844), Darwin noted the precipitation of albite crystals within basaltic lava on Santiago Island (or "James Island") of the Galápagos Islands. He then cited a work by "von Buch" (German geologist and paleontologist Christian Leopold von Buch (1774–1853)): ''Physikalische Beschreibung der canarischen Inseln'' hysical description of the Canary Islands(Berlin, 1825), which was translated into French as: Léopold von Buch, with C. Boulanger, trans., ''Description physique des Îles Canaries'' (Paris, France: F. G. Levrault, 1836). Darwin cite
pp. 190–191
of that translation, where Buch mentions a similar geological formation on the island of Tenerife in the Canary Islands and where he mentions a "M. de Drée", who had found experimentally that feldspar crystals form and precipitate in molten lava: ''"Les expériences de M. de Drée, dans lesquelles il a fait fondre diverses laves dans un creuset, ont prouvé que dans une telle masse fluide, les cristaux de feldspath devaient tendre à se précipiter au fond."'' (The experiments of M. de Drée, in which he melted various lavas in a crucible, proved that in such a fluid mass, crystals of feldspar should tend to precipitate to the bottom.) "M. de Drée" was Étienne-Marie-Gilbert, Marquis de Drée (1760–1848) Fr ">:fr:_Étienne-Gilbert_de_Drée_.html" ;"title=":fr: Étienne-Gilbert de Drée "> Fr a French amateur geologist who in 1808 had presented to ''L'Institut national de France'' his paper ''"Mémoire sur un nouveau genre de liquéfaction ignée qui explique la formation des laves lithoïdes"'' (Memoir on a new type of igneous liquefaction that explains the formation of stony lavas). (Summarized (in French) in: Etienne-Marie-Gilbert Drée (1808 March 28
"Mémoire sur un nouveau genre de liquéfaction ignée qui explique la formation des laves lithoïdes,"
''Nouveau Bulletin des Sciences'', 1 : 137–142.)
On pp. 16–17 of the memoir
Drée stated: ''"En plaçant le morceau de porphyre dans le creuset no. 6, je suis certain que ce morceau touchoit au fond du creuset, et cependant on voit dans le produit tous les cristaux réunis dans la partie supérieure, preuve que la liquéfaction a été assez complète pour permettre l'élévation des cristaux de feld-spath (1)."'' (In placing the piece of porphyry in crucible number 6, I'm certain that that piece touched the bottom of the crucible, and yet one sees in the product all of the crystals clustered in the upper part — proof that the liquefaction was complete enough to permit the rising [i.e., floating] of the crystals of feldspar ⁽¹⁾.) Thus, although Drée had expected the feldspar crystals to float, they instead sank to the bottom of the molten lava — just as had happened in the lavas of James Island and Tenerife. and that this "throws light on the separation of the high silica versus low silica series of rocks." This was the first proposal of the

fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of the ...

hypothesis of magma differentiation that was further developed and demonstrated in the 20th century. Charles Darwin seated crop

The geologist Archibald Geikie praised the book, calling it "the best authority on the general geological structure of most of the regions it describes," and that Darwin was "one of the earliest writers to recognize the magnitude of denudation to which even recent geological accumulations have been subjected." A second edition of the book, published in 1876, combines ''Geological Observations on the Volcanic Islands'' with ''Geological Observations on South America''.

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