The lowest member of the Late Precambrian Signal Hill Formation (a massive, gray, lithic sandstone), exposed at the eastern extremity of the Avalon Peninsula near St. John's, Newfoundland, is cut by a system of thin veins containing quartz, calcite, and whitish prehnite. The veins, confined to a narrow stratigraphic horizon, have been found, so far, along a strike distance of 4 km. Prehnite also forms small diffuse patches in the rock, producing a distinctive mottling effect; it is associated with authigenic quartz, sericite, chlorite, and small amounts of possibly metasomatic calcite.The prehnite-bearing mineral assemblage is consistent with mineral characteristics of the prehnite–pumpellyite facies, usually considered to be the result of burial metamorphism. The patchy interstitial prehnite is believed to have been formed by a reaction between authigenic laumontite and calcite. The prehnite-bearing assemblage may have been formed under a lithostatic load not greatly exceeding the present maximum thickness of the overlying part of the Cabot Group – about 4.5 km. The formation of both the interstitial prehnite and the prehnite-bearing veins was contemporaneous with the tectonic stresses responsible for the open folding of the Cabot Group.