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Contents of 3.3 How do we know how to determine rock origins?

• The current threefold genetic classification of rocks arose from late 18th- to early 19th-century geologic debates.
• Neptunists, led by A. Werner, asserted that nearly all rocks came from precipitates. Agreed with a view by many at the time, that Earth was internally cold.
• Vulcanists, following Hutton, used careful observation to refute Neptunists' claims by demonstrating the presence of igneous rocks and showing evidence of Earth's internal heat.