In the Middle Ordovician strata of southwestern Virginia, a distinctive succession of yellow-drab and gray mudstones, shales, and limestones occurs between the overlying Martinsburg formation and the underlying red Moccasin formation. For many years, part of this succession was classified with the Martinsburg formation and part with the Moccasin formation. This practice could not be condemned on faunal grounds because the fauna of the sequence between the Moccasin and Martinsburg is so sparse and equivocal in general character. Altered volcanic ash beds which are especially characteristic of the succession range as low as the middle part of the Moccasin formation and are also present in the lower Martinsburg formation. Mathews (1934, p.48) proposed the name, Eggleston, and defined it to include the relatively unfossiliferous transitional beds and intercalated bentonites younger than the upper maroon Moccasin formation and older than the Trenton beds of the lower Martinsburg formation in Giles County, Virginia. Until the present study was undertaken, few detailed studies of the Eggleston, as a separate unit, had been made. The almost barren beds had drama little or no interest from the paleontologist. The intercalated bentonites were studied, only as a part of the Moccasin-Martinsburg succession, and an attempt was made to correlate these related beds with similar beds in other localities. The name, Eggleston, has been accepted and is used in much of the literature published since 1935 but no attempt has been made to correlate the entire succession of beds lying between the Moccasin and Martinsburg formations with similar succession throughout southwestern Virginia. The writer has made a detailed study of this succession between New River and the Tennessee line to determine the relationship of faunas to the intercalated bentonites, and to correlate the beds of the succession with those at other localities in southwestern Virginia. / Master of Science