​CASTALIA, or Fons Castalius, a celebrated fountain in Greece, now called the Fountain of St John, which rises in a chasm of Mount Parnassus, in the neighbourhood of Delphi. It was sacred to Apollo and the Muses, and its water was used in the religious purifications of the “Pythian Pilgrims.” From its connexion with the Muses it is sometimes referred to by late Greek writers (e.g. Lucian, Jup. Trag. 30) and Latin poets (e.g. Ovid, Am. i. 15. 36) as a source of inspiration, and this has passed into a commonplace of modern literature. According to some authorities the nymph Castalia was the daughter of Achelous; according to others the water of the spring was derived from the Boeotian Cephissus.