Rocks described as Silurian in the Utah-Nevada area have been studied and processed for conodonts. Only the Roberts Mountains Formation of central Nevada has yielded noteworthy faunas. Conodonts found throughout the formation indicate that the greater part of this stratigraphic unit in the Roberts Mountains, at Lone Mountain, and in the Cortez Mountains is at least latest Silurian (youngest Ludlovian).

The age of the conodonts from the Roberts Mountains Formation has been determined by the occurrence of the same fauna as that found in the Sutherland River Formation of Devon Island. Unpublished information brought to the writers' attention suggests that this latter fauna may be Early Devonian. If so, the middle and upper parts of the Roberts Mountains Formation may be Early Devonian. Definite Early Devonian conodonts have been obtained from overlying rocks in Antelope Valley and in the Cortez Mountains.

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