The Chisholm Shale was named by Walcott (1908) for exposures near the Chisholm Mine on the west side of the Ely Range about 2 miles west of Pioche in Sec. 21, T. 1 N., R. 67 E.

The Chisholm Shale is Middle Cambrian in age (Tschanz and Pampeyan, 1970). In southern Lincoln County the Chisholm Shale conformably overlies the Lyndon Limestone. Both of these intervals are represented by the lower portion of the Pole Canyon Limestone in sections farther to the north (Tschanz and Pampeyan, 1970).

The Chisholm Shale is a yellowish-brown, maroon or olive-green fissile, calcareous to clay shale which contains several thin brownish weathering argillaceous limestone interbeds several feet to several tens of feet in thickness (Tschanz and Pampeyan, 1970). Locally, up to 30 percent of the Chisholm is composed of fine-grained, gray, pink weathering limestone (Merriam, 1964). Girvanella algae, trilobites and brachiopods have been reported form the Chisholm Shale (Merriam, 1964). The Chisholm Shale resembles the upper portion of the Pioche Shale but in general is much less micaceous and finer grained.

The Chisholm Shale is 100 to 170 feet thick in the Pioche Hills (Merriam, 1964).

Like the Lyndon Limestone, the Chisholm Shale is present to the southern portion of the evaluation area, and in adjacent parts of Arizona and Utah. In the evaluation area it is exposed only within the Highland Range, and just to the south in the Pioche Hills.

The details of depositional setting are poorly understood for the Chisholm Shale. The presence of Girvanella algae, brachiopods and trilobites suggests shallow and perhaps locally restricted marine shelf sediments.