CATARACTA. - A feminine Latin formation from the Greek masculine πετάρακτης. It properly means a river cascade, and in particular that of the Nile, hence the well-known verse of Ps 41:8: *abyssus abyssum invocat in voce cataractarum tuarum*. From this it came to be used for a dam, a barrier constructed to create a waterfall, as in Gen 7:11: *catractae caeli apertoe sunt* (for the flood). By analogy, it later came to signify any kind of shutter, such as those of a door, castle, etc., and then any kind of barrier or grille that blocked an exit or passage.
This must be the sense to be attributed to the *duplex cataracta* mentioned by the legates of Hormisdas as existing at the tomb of St Peter in a letter written to that Pope from Constantinople on 29 June 519: «si beatitudini vestrae videtur, sanctuaria apostolorum Petri et Pauli [i.e. relics] secundum morem ei [i.e. to Justinian] largiri praecipite, et si fieri potest ad secundam cataractam ipsa sanctuaria deponere vestrum est deliberare» (A. Thiel, *Epist. Roman. pontificum a s. Hilario ad Pelagium II*, Braunsberg 1867-68, p. 873). Pope Vigilius also mentions a *cataracta* at the altar of the church of St Peter in Constantinople (letter in PL 69, 55).
It has been thought that *duplex cataracta* meant a double window, but this is incorrect, since there are no secure examples of *cataracta* in the sense of a window. Others have proposed that it refers to two small doors or grilles placed vertically one above the other, both below the level of the Constantinian altar, but this interpretation has also been disproved by recent excavations. It can only be said that these *cataractae* were barriers preventing too close an approach to the relics of the Apostle placed inside and beneath the altar. The usage is better explained by what St Gregory of Tours notes (though not with complete accuracy): «Sepulcrum (s. Petri) sub altari collocatum valde rarum habetur. Sed qui orare desiderat, reseratis cancellis quibus locus ille ambitur, accedit super sepulcrum et sic finestella parvula patefacta, immisso introrsum capite, quae necessitas promit efflagitat» (*In gloria martyrum*, 27: MGH, *Scriptores meroving.*, I, p. 504).