Two sequences of shelly-faunal limestones and two of graptolite-bearing limestones of latest Early Ordovician age were studied to determine their similarities and differences. To enable detailed comparison, one graptolite zone, that of Didymograptus protobifidus in the Marathon Limestone in the Marathon region, Texas, and in the Vinini Shale in central Nevada, and its correlative zone in the "shelly" limestones, the El Paso Limestone in the Franklin Mountains in west Texas and the Garden City Limestone in northeastern Utah were used.

The graptolite-bearing successions are similar in thickness, limestone type, faunal content, and percentage of limestone. Shale and sandstone comprise half of both successions. Dolomite is prominent in the Vinini Shale but little is present in the Marathon Limestone. The latter sequence bears several sedimentary features which suggest shallow-water deposition, whereas the Vinini Shale does not.

The shelly-faunal limestones are similar to each other in thickness, limestone type, faunal content, nature of occurrence of dolomite and chert, and sedimentary features. Chert is more common in the Garden City Limestone than in the El Paso Limestone, but dolomite is more abundant in the latter unit.

The shelly-faunal sequences differ from the graptolite-bearing sequences in thickness, percentage of limestone, limestone types, faunal content, and amount of chert and dolomite present.