[Named after Hamilton Island] Type locality: NW tip of Hamilton Is., 800 ft ESE of triangulation station Beth near NW end of Kupreanof Is. Is very thin-bedded dark-gray aphanitic limestone locally dolomitic. Crops out on Hamilton Is., N side of Hamilton Bay, and NE Keku Islets. Probably several 100 ft thick (unreliable due to complex folding). Conformably overlies Burnt Island Conglomerate; in some areas disconformably overlies Pybus Fm (thin unit of Burnt Island may be present). Conformably overlain by Hound Island Volcanics; on Hamilton Island upper beds of Hamilton Island Ls interbedded with lower beds of Hound Island Volcanics. Similar to argillite member of Hyd Fm. Fossils abundant; mostly HALOBIA; mostly late Karnian; age is Late Triassic. HALOSIA suggests deep-water marine environment. Coeval with shallow water Cornwallis Ls and part of Hound Island Volcanics (also of Hyd Group).

Source: GNU records (USGS DDS-6; Menlo GNULEX).