In 1907, geologists Nelson Horatio Darton and C. C. O'Harra (of the South Dakota School of Mines) theorized that Devils Tower must be an eroded remnant of a laccolith. A laccolith is a large mass of igneous rock which is intruded through sedimentary rock beds without reaching the surface, but makes a rounded bulge in the sedimentary layers above. This theory was quite popular in the early 20th century, since numerous studies had earlier been done on laccoliths in the Southwest.