While the first part (1842) focused on the structure and distribution of coral reefs and the second (1844) described the volcanic islands visited during the voyage, this third instalment is devoted exclusively to South America, where Darwin spent the longest period of the expedition. It discusses South America's geological and seismic history, the mineral content of lava and granite, and the deformation of metamorphic rock. Separate chapters cover different geographical areas, and topics include the formation of the East and West Coasts and the pampas, the plains and valleys of Chile, and the structure of the Cordillera. Geological Observations also includes detailed maps of South America and sketches of the terrain.

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