The historical background of "Hutton's unconformity," the junction of inclined schist and sandstone beds discovered by James Hutton in 1787 on the Isle of Arran, off the west coast of Scotland, is described. It is shown that the prominent gash between beds at this locality, which for 150 years has been assumed to be the plane of the unconformity, actually occurs four feet below the unconformity, and that the fourfoot bed between the gash and the true plane of unconformity is composed of Dalradian (Precambrian) schist impregnated with calcareous material.