The Mariquita Chert of the Bermeja Complex in southwestern Puerto Rico contains radiolarians ranging in age from early Tithonian to late Aptian. Local stratigraphic and structural relations indicate that Puerto Rican ultramafic rocks are as old as or older than the chert; they are thus probably Jurassic or older. These rocks are the first pre-Cretaceous rock assemblages between Cuba and La Desirade (Lesser Antilles) to be identified, and they document the presence of pre-Cretaceous oceanic crust in the northeastern Caribbean. Subduction of the Caribbean plate beneath the North American plate, ending in the Early Cretaceous, destroyed most of this old oceanic crust and created a belt of metamorphosed volcanic and ophiolitic rocks now exposed in Cuba, Hispaniola, and Puerto Rico. The maximum radiometric ages from this belt are 127 m.y. Younger rocks were superimposed on the older belt when subduction polarity reversed and the North American plate was subducted beneath the Caribbean plate from 110 to 45 m.y. ago.