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The Onondaga group discussed in this paper includes all rock units between the top of the Oriskany (Ridgeley) sandstone and the base of the Marcellus formation of the Hamilton group. It corresponds in part to the "Corniferous" of older reports and drillers' records. Within the area described it contains two separable lithologic members, an upper shale and a lower chert.
In southeastern West Virginia the group is represented mainly by the Huntersville chert, which crops out in a belt about 120 miles long in Pocahontas, Greenbrier, Monroe, and Mercer counties. In Pocahontas and Greenbrier counties it is exposed on both flanks of the Browns Mountain anticline which lies west of the Allegheny Front. In Virginia and elsewhere in West Virginia it is confined to the Valley and Ridge physiographic province, where it has been traced as far south as Saltville, Virginia. The formation contains impure chert, highly silicified shale, silicified mudrock, and, commonly, one or more prominent glauconitic sandstones. In general the Huntersville has few fossils, but at some localities there occur numerous species which the author has identified and recorded. The Hunte sville chert occupies a stratigraphic position held in northeastern West Virginia and northwestern Virginia by a shale of Onondaga age which is believed to correspond with the Needmore shale of southern Pennsylvania. The chert appears to grade laterally into this shale, although where both units are represented, the shale invariably overlies the chert. Thus, the shale is partly younger than the chert. The upper Onondaga contact is apparently conformable, but an unconformity at the base reaches great magnitude at several localities in Virginia.
Wells pentrating deep horizons in West Virginia, western Pennsylvania, and eastern Ohio encounter a limestone and chert unit at the general Onondaga level. This unit is the eastern subsurface extension of the Columbus limestone of Ohio and, probably, the analogue of the Onondaga limestone of New York. It is also the precise equivalent of the Huntersville chert. Commercial gas was discovered in it (1936) along the Chestnut Ridge anticline of southwestern Pennsylvania, and a recent Oriskany test well on the same structure encountered rock pressure of 3,275 pounds in the Huntersville. Most likely the chert is a reservoir only because of brecciation. The extreme brittleness of the Huntersville is attested on the outcrop by strong fragmentation wherever it is folded. This fact suggests its potential value as a gas and, possibly, an oil reservoir wherever deformation has caused brecciation at depth.
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