Jakob Johannes Sederholm

Jakob Johannes Sederholm was a Finnish petrologist born July 20, 1863, in Helsinki, Finland, who died June 26, 1934, and who developed the concept of migmatites, rocks of mixed composition in which layers of granitic rock are interleaved with metamorphic rock, which he viewed as the product of igneous magma intruding into metamorphic rocks at depth; he is most associated with his studies of migmatites and also researched the Jotnian geological period, the mineral myrmekite, and rapakivi granite formations. He studied geology in Helsinki, Stockholm and Heidelberg, served as director of the Geological Survey of Finland for forty years from 1893 to 1934, sat as a member of the Diet of Finland, led the League of Nations Commission of Enquiry in Albania from 1921 to 1923, and served twice as chairman of the Economic Society of Finland. He received the Murchison Medal from the Geological Society of London in 1928 and the Penrose Medal from the Geological Society of America the same year, 1928; the mineral sederholmite is named in his honor.