Charles Darwin provided one of the first detailed explanations for the diversity of igneous rocks. Building on many observations made during the Beagle voyage, Darwin hypothesized that density differences among crystals within a mass of partially molten rock would result in their physical separation by sinking and floating. Such a process, he proposed, could be responsible for the separation of compositionally distinct lavas from a single source. This idea, in modified form, lies at the heart of the modern science of igneous petrology. Darwin also speculated that partial melting of rocks in the deeper regions of the Earth's crust could produce basaltic melts. However, due to his lack of knowledge of the melting points of the silicate minerals, and his misinterpretation of a puzzling field locality at Bahia in Brazil, he wrongly believed granitic gneiss to be the progenitor of these basalts. Despite this error, Darwin's igneous speculations show a characteristic blend of detailed observation and broader theorizing. Most interesting of all, striking analogies can be found between Darwin's igneous work and his theory of natural selection, which he developed at about the same time.

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