Symphlagades, two fabulous floating rocks at the entrance of the Euxine, which, when driven by the winds, crushed every vessel that attempted to pass between them; the ship Argo (q.v.) managed to pass between them, but with the loss of part of her stern, after which they became fixed.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Symonds, John Addington * Symphony