Calcareous shale, siltstone, sandstone, chert, quartzite, and greenstone in the Vinini Formation in Lander, Eureka, Elko, and northern Nye Counties, and the Clipper Canyon Group in the northern Toquima Range are the core rocks of unit DOts. Difficulties in identifying distinct paleogeographic settings within Ordovician slope facies rocks are discussed in Finney and Perry (1991) and Finney and others (1993). On a regional scale, the distinction between this unit and rocks traditionally mapped as the Valmy Formation (DCs) is the preponderance of shale and siltstone of cratonal derivation that is present in the Vinini rocks but less common in the Valmy rocks. Both rock units contain bedded chert, massive quartzite, and greenstone (Finney and Perry, 1991) in many places. Many lower Paleozoic rocks grouped here likely formed in a basinal rather than slope setting, but the presence of more common siliciclastic horizons of shale, siltstone, and sandstone distinguish them as a regional grouping from the lower Paleozoic Basin assemblage rocks. Whether this is a function of distinct paleogeographic settings of coeval units as interpreted by early workers, or is actually an age distinction of older (Valmy) versus younger (Vinini) Ordovician rocks, as suggested more recently for at least the Roberts Mountains (Finney, Perry, and others, 1993), remains to be determined on a regional scale. Originally thought to be primarily Ordovician, studies and biostratigraphic data have demonstrated that this unit consists of tightly imbricated Devonian, Silurian, and Ordovician rocks (Coles and Snyder, 1985; Noble and Finney, 1999). The distinction between units DOts and DCs as currently mapped on a regional scale is ambiguous in many places. Identifying the numerous occurrences of Devonian and Silurian rocks that are embedded within this unit on a regional scale would significantly enhance our understanding of the complex structural history of these rocks. These rocks are everywhere in structural contact with other Paleozoic rocks including units IPMcl, Pacl, Dc, MDst, DSt, DSc, and Dcd. Stratigraphic correlation has been made between rocks of the Vinini Formation and the Carbonate shelf sequence in Nevada (Finney and Perry, 1991) on the basis of occurrence of quartzite that is coeval with the shelf unit Ocq. While this does suggest a connection between the Ordovician rocks of this composite unit and North America, the quartzite was deposited along a 1,000-mile length of the margin (Ketner, 1986) and thus does not constrain the rocks of unit DOts to deposition along a specific section of the margin. These rocks are unconformably overlain sporadically by units Pacl and PIPacl, and post-Paleozoic cover rocks.