A somewhat weathered block of blueschist, not quite in situ, but abandoned in the quarry from which it was excavated. In my experience, the rock here in the quarry is much finer-grained than that at the jetty. The portion of the pod that was closer to the river may have gone through more prolonged or more intense metamorphism than the remnants here, farther to the south, but I'm simply speculating. The darker rings evident in the photo are probably lichen or some similar biological thing, and the rust stains should be fairly easy to explain for those who've seen the gorgeous pyrite in earlier photos (e.g. here and here).