No type locality designated. Named for French Creek, Carbon Co, WY, Ts15 and 16N, Rs80 and 81W in the Northern Rocky Mountain region. Exposures are scattered. Consists largely of dark-brown to blackish-gray phyllites with thin beds of schistose quartzitic rocks and some laminae rich in magnetite and hematite. Pyrite cubes altered to limonite are abundant. Is 2,000 ft thick. Is younger than Towner greenstone (new). Succeeded by acidic intrusives of pre-Cambrian age. Assigned to the pre-Cambrian, early Algonkian and Proterozoic.

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

Revised in that French Slate is assigned as the upper formation of the upper part of the Libby Creek Group of the Snowy Pass Supergroup (first used) in the Medicine Bow Mountains of south-central WY in the Northern Rocky Mountain region. Conformably overlies Towner Greenstone, middle formation of the upper part of the Libby Creek. Is truncated by the Cheyenne Belt along its upper contact. Geologic map. Stratigraphic chart. Consists primarily of laminated black ferruginous slate and phyllite. Laminae contain layers of muscovite, chlorite, quartz, and opaque minerals alternating with quartz-rich layers with minor muscovite and chlorite. Two thick lenses of iron-formation. Has been complexly folded and crenulated along the shear zone. Is about 610 m thick. Thought to have been deposited in a deep marine or prodelta basin in Proterozoic time.

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

Assigned as the upper formation (of three formations) of the upper part of the Libby Creek Group of the Snowy Pass Supergroup in the Medicine Bow Mountains, WY in the Northern Rocky Mountain region. Snowy Pass Supergroup is newly defined in this report. Overlies Towner Greenstone of upper part of Libby Creek Group of the Snowy Pass. Is 610 m thick. Correlation chart; geologic map. Of Early Proterozoic age.

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