Fort Cassin layers included in Brainerd and Seely's division D. 375 ft thick. Contains Ophileta at base "and the Fort Cassin fauna in a very few ft at the very top, the intervening layers being essentially unfossiliferous." In author's opinion, the line between division C and division D should have been made above the Ophileta bed; and "there is certainly a much greater affinity between Fort Cassin fossils and those of the rocks above them than with those of the Calciferous, and as latter name applies to beds having peculiar lithological characters, and containing a very limited fauna, it appears to me much more natural to associate the later fauna with that of the rocks above, and place the limestones there also, or else consider them as distinct from those below or above, and use a distinct name, as Fort Cassin, or Philipsburg formation, or any other appropriate one."

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

Fort Cassin Formation of Bascom subgroup [informal] of Beekmantown Group. In study area includes (ascending) Ward Siltstone, Sciota School [Sciota], Emerson Schoolhouse [Emerson School] Members. Wing Conglomerate of Seely (1906) occurs along the top of the Fort Cassin except where it has been removed by a later paleokarst. Overlies Lemon Fair Formation (new) and underlies Providence Island Formation.
["Subgroup" not recognized as a formal stratigraphic rank term (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005, 2021). Considered informal and should not be capitalized.]

Source: Modified from GNU records (USGS DDS-6; Reston GNULEX).