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 . 3o) and Latin poets (erg . 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 .