CASTALIA or FONS CASTALIUS, a celebrated fountain in Greece, now called the Fountain of St. John, which rises in a chasm of Mount Parnassus near Delphi. It was sacred to Apollo and the Muses, and its water was used in the religious purifica tions of pilgrims. From its connection with the Muses it is men tioned by late Greek writers (e.g., Lucian, Jup. Trag. 3o) 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.