Small discontinuous bodies of Miocene basalt, basaltic andesite and andesite flows are present across the entire evaluation area. In Lincoln County, Miocene basalt flows less than 100 feet in thickness are present in the eastern Wilson Creek and central Fairview Ranges (Ekren and others, 1977). These flows are gray and dark gray to black weathering, massive to sparsely porphyritic, olivine basalt with phenocrysts of olivine and labradorite in an intergranular groundmass of clinopyroxene and plagioclase. These flows vary in age from about 8 to 17 Ma (Ekren and others, 1977).

Basalt and andesite flows, dikes, and plugs of Miocene age are also present in the Eureka district near Richmond Mountain, where they are as much as 700 feet thick. These dark flows are porphyritic with phenocrysts of plagioclase, augite, hypersthene and olivine (Nolan, 1962).

East of Cortez, the Cortez Mountains are capped with basaltic andesite flows of Tertiary age that attain a thickness of 350 feet in Fourmile Canyon (Roberts and others, 1967). Diabase dikes cut Paleozoic rocks such as the Vinini Formation south of Brock Canyon. Small basalt plugs with labradorite, olivine, augite and magnetite phenocrysts intrude the Humboldt Formation near the north end of Pine Valley, and are considered Late Miocene in age (Smith and Ketner, 1976). Miocene basalt cones and flows are also present to the south, along both the eastern (Garcia Flats) and western (Table Mountain) flanks of the Sulphur Spring Range.

Swarms of northwest-trending Miocene diabase dikes are present along the northwestern Cortez Mountains south of Little Cottonwood Creek and throughout the Roberts Mountains as feeders for thick basalt flows in the Shoshone, Cortez and Simpson Park Mountains (Gilluly and Masursky, 1965).

In Elko County, Miocene basalts are exposed in several areas. Two small exposures are present in the western portion of the county near Midas and along the Utah line in the Dairy Valley Quadrangle. Near Midas, an older Miocene basalt is olive-green to brown-gray and microcrystalline with sparse plagioclase phenocrysts in a groundmass of plagioclase microlites, augite, magnetite and small amounts of glass. This basalt is correlated with the 14.8 Ma flows in the Sheep Creek Range to the south of the Midas area (McKee and Silberman, 1970; Coats, 1985). Much younger olivine basalt is also present in the Midas area and has been dated at 6.3 +/- 2.3 Ma (Zoback and Thompson, 1978; Coats, 1985). In the Dairy Valley Quadrangle, the basalts are vesicular with sparse plagioclase, olivine, and magnetite microphenocrysts and plagioclase microlites. The age of these basalts is undetermined but they are correlated with 12.8 +/- 0.4 Ma rocks near Goose Creek along the Idaho-Utah border by Coats (1985).

One of the most widespread basaltic volcanics in Elko County was originally mapped as the Banbury Formation (Coats, 1964; 1971) and is now considered older than the Banbury and assigned to the Big Island Formation (Coats, 1985). The Big Island Formation is described separately below.