In 1908, Walcott renamed the Prospect Mountain Limestone of Hague (1883), the Eldorado Limestone. The type locality is Eldorado Tunnel near the end of Prospect Ridge near Eureka. The formation was amended by Wheeler and Lemmon (1939) who split off the uppermost beds as the Geddes Limestone. It is perhaps important to note that the beds at the type locality are altered, highly brecciated, and vertical to overturned to the west (Nolan and others, 1956).

The Eldorado Dolomite is considered early Middle Cambrian in age. The Eldorado is equivalent to the Pole Canyon Limestone.

The Eldorado Dolomite is commonly a massively bedded, blue gray limestone and thinner bedded (1-3 feet thick), light gray to dark blue or black, coarsely crystalline dolomite. A vuggy and porous, light-gray, coarsely crystalline dolomite is the most common lithology in the Eldorado (Roberts and others, 1967).

In the type section area near Eureka, the massive, blue-gray limestones of the Eldorado are often intensely deformed and hydrothermally altered. Shearing has caused recrystallization of the unit to a coarsely crystalline streaked limestone marble that is often bleached white (Nolan and others, 1956).

In the northern and central Schell Creek Range, the Eldorado is a thin to thick-bedded, dark to light gray, brown or white, fine-grained to aphanitic limestone. The limestone is commonly in beds of 20 feet or more in thickness, shows faint color lamination, and contains common oolites and pisolites and possible Girvanella (Young, 1960; Dechert, 1967). Dechert (1967) reports streaks and thick interbeds of dolomite in successions over 100 feet in the upper portion of the Eldorado which grade both laterally and vertically into limestone.

Thicknesses measured within the Eldorado are structural, and sections are often visably repeated along small and large-scale thrusts. A thickness of 2,500 feet is estimated in the Eureka area. A thickness of 500 feet of Eldorado Dolomite is estimated in the Mount Lewis area of the Shoshone Range (Gilluly and Gates, 1965). 1,550 feet were measured in the northern Schell Creek Range (Dechert, 1967), 1,846 feet has been measured in the Egan - northern Schell Creek - Cherry Creek Range area (Young, 1960), and about 3,171 feet were measured in a probable fault repeated section in the north-central Schell Creek Range near Duck Creek Dam (Young, 1960).

The Eldorado Dolomite is exposed within the Eureka area, and in central Shoshone, northern Egan and Schell Creek, and southern Cherry Creek Ranges.

The presence of oolitic and Girvanella-bearing limestones suggests that the Eldorado is a shallow water shelf facies. The details of depositional setting are poorly understood.