Eocene rhyolite, rhyodacite, and dacite flows and associated plugs and dikes are exposed in several areas across the evaluation area. In the northern Simpson Park Mountains, a small patch of flow banded and autobrecciated rhyolite and minor hypersthene andesite flow breccia occur along Fyfe Canyon in T.25 N, R.48 E. These rocks have been informally termed the volcanics of Fyfe Canyon (Gilluly and Masursky, 1965). They appear to rest on the Ordovician Vinini Formation. The base is not exposed and the structure of the volcanics is difficult to discern because of highly contorted flow layering and brecciation. A thickness of 500 feet has been estimated (Gilluly and Masursky, 1965).

The base of the Fyfe Canyon Volcanics is composed of a few feet of light-gray, porphyritic, scoriaceous breccia containing volcanic boulders as much as several feet in diameter. Overlying the basal breccia, are rhyolite flows and glasses ranging from black obsidian through gray, greenish gray, and pink flow banded to brecciated rhyolite. No tuffs are present in the sequence (Gilluly and Masursky, 1965). K-Ar dating of biotite gave an age of 36.8 Ma (Gilluly and Masursky, 1965) which stradles the Oligocene/Eocene boundary. Similar rhyolites of Eocene age are present to the east and south in the Simpson Park Range in T. 25 N., R.49 E. and T.23 N., R.48 E.

To the north in southwestern Elko County, rhyolitic to dacitic flows and domes are exposed in the Mount Blitzen Quadrangle and quadrangles to the west. This sequence includes the Walker Mountain rhyodacite that is porphyritic with sanidine, plagioclase, quartz, biotite and small amounts of hornblende in a glassy to aphanitic groundmass (Coats, 1985). Groundmass makes up 10 to 60 percent of the rock that has been dated at 35.4 +/- 1.1 Ma (McKee and others, 1976) and is considered Eocene here. Also present in the area are unnamed and undated rhyolites with small percentages of quartz, sanidine, plagioclase and biotite phenocrysts (Coats, 1985). The thickness and extent of these rocks is poorly documented.

Farther to the north in Elko County, the Ottawanah Rhyolite of Coats (1971) is exposed in the Owyhee, Mountain City, Hat Creek and south of the Mount Velma Quadrangles (Coats, 1985). This sequence is up to 400 feet of rhyolitic domes and welded tuffs that have been dated at 39.6 +/- 2 Ma (Coats, 1971; Coats, 1985). This volcanic sequence unconformably overlies the andesite of Summit Creek and is unconformably overlain by the andesites of Jones Creek (Coats, 1971). Most of the sequence is welded tuff with phenocrysts of plagioclase, quartz, biotite, sanidine, and quartz in a vitric to cryptocrystalline groundmass (Coats, 1971).