First published use in Ruby River basin, Madison Co, MT, Montana folded belt province. Name attributed to J. A. Dorr and W. H. Wheeler during their fossil flora collecting in 1948. Passamari, an Indian name meaning stinking water, refers to Ruby River, which was, until the turn of the century, called the Passamari River. No type locality designated. Described as generally thick lacustrine shale containing abundant fossil flora. Thickness not given. Overlies Montana group; underlies coarse conglomerates and sandstones, tuffaceous rocks and volcanic ash equivalent in age to Madison Valley formation. Mapped as Tertiary sediments. Fossil descriptions and plates. Is nearly contemporaneous with shales of Bridge Creek, OR but younger than shales of Florissant, CO area. Late Oligocene age.

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