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2.04.6 • ,RESPONSE continued the mining zone, it will not be considered here. The Lewis shale consists mainly of a dark gray, homogeneous marine shale. It is conformable with the underlying Williams Fork Formation. The contact with the Williams Fork is not sharply defined, and there- fore it was arbitrarily placed at the top of the beds, which consist mainly of sandstone, sandy shale, and coal as differen- tiated from overlying beds of mainly gray shale (Bass et al. 1955) . The Lewis shale is found immediately north and east of the site. The Williams Fork formation consists of a sequence of sand- stones, siltstones, mudstones, shales, and coals lying between the Lewis shale and the Trout Creek sandstone. Included are a lower unit and an upper unit separated by the ledge forming Twenty Mile sandstone. The upper unit of the Williams Fork is 7 roughly 600 ft. thick and includes the five coal beds within the mining zone of the proposed Hayden Gulch mine as well as sev- eral beds below the zone. The coals are separated by silt- stones, mudstones, and sandstones. At the base of the upper unit is the massive, wellsorted, medium grained, white to tan Twenty Mile sandstone. The Twenty Mile is about 100 ft. thick in the mine area and forms distinctive ledges below the southern end on the mine site. The lower unit of the Williams Fork is about ]000 ft. thick in the mine area. It consists of 400 ft. to 500 ft. of grey marine shale with a few sandy zones, underlain by a sequence of sandstones, siltstones, mudstones, shales, and coal seams. Below the Williams Fork is the Iles formation. it is capped by the 100 ft.thick Trout Creek sandstone, which is similar in composition to the Twenty Mile. The Trout Creek forms white ledges about I mi. to the south of the site. Below the Trout