French geologist remembered for discovering the volcanic nature of the Auvergne region of central France (1751). Controversy ensued over the nature of columnar basalt in the region. Guettard initially concluded that it was not volcanic but later became convinced otherwise. He prepared the first mineralogical map of France (1746) and identified trilobites in the slates of Anjou. Guettard also discovered sources of kaolin and petuntse, which made possible the manufacture of Sèvres porcelain.