Because of its importance as a stratigraphic marker horizon, the Eureka Quartzite is depicted on this map wherever it is mapped separately from the Ordovician carbonate shelf rocks. It represents depositional sequence 3 of Cook and Corboy (2004). It is not differentiated from the rest of the Ordovician (OCc) in White Pine or Clark Counties, but is shown in Elko, Eureka, Nye, and Lincoln Counties. Rocks mapped as the Ely Springs Dolomite are included with the Eureka Quartzite in southern Nye County. The Eureka Quartzite depositionally overlies the Pogonip Group (OCc), and is overlain by either the Hanson Creek Formation or the Ely Springs Dolomite (SOc).