British geologist who became the foremost advocate of the fluvial (fluvial process) theories of erosion. His prolific book writing made him very influential in his time.

In 1855 Geikie was appointed to the Geological Survey of Great Britain, under Sir Roderick I. Murchison. Ten years later he was elected a fellow of the Royal Society, and, when a separate branch of the Geological Survey was established for Scotland in 1867, Geikie became its director. In 1871 he became the first Murchison professor of geology and mineralogy at the University of Edinburgh.

In 1882 Geikie became director general of the Geological Survey of the United Kingdom, and he immediately reorganized and increased the survey work, which had lagged under the previous director. He served as president of the Geological Society of London (1891–92 and 1906–08) and of the Royal Society (1908–13). He was knighted in 1891.

His best-known works are The Scenery of Scotland (1865, 3rd ed. 1901), Life of Sir R.I. Murchison (1875), Text-Book of Geology (1882, 4th ed. 1903), The Founders of Geology (1897, 2nd ed. 1905), The Ancient Volcanoes of Great Britain (1897), and Outlines of Field Geology (1876, 5th ed. 1900).