Geologist, specialising in volcanic geology and the microscopic examination of rocks. Born in Edinburgh and educated at the High School and University there, Geikie joined the United Kingdom Geological Survey (1855) and went on to lead the survey in Scotland (1867). He worked with life-long friend Sir Roderick Murchison (1792 - 1871) to produce a geological map of Scotland, published in 1862. Between 1870 and 1881 Geikie was Professor of Geology at the University of Edinburgh, and then Director-General of the Geological Survey until 1901. He was knighted in 1891 and served as President of the Geological Society (1891-2), Joint Secretary of the Royal Society (1903-8) and its President (1909).
His younger brother, James Geikie (1839 - 1914), succeeded him at the University.