In the vicinity of Trail Creek, central Idaho, a dominantly argillaceous sequence bearing Lower through Upper Ordovician (Arenigian, Llandeilan, Caradocian, and Ashgillian) and Silurian (Llandoverian and Wenlockian) graptolites is thrust over a thicker sequence of argillite and quartzite on the E. in which only early Caradocian graptolites have been found. The partly contemporaneous sequences, differing in lithic detail, belong to the Phi Kappa Formation as mapped by Umpleby, Westgate, and Ross. At the base of the upper thrust plate is massive quartzite 60 ft thick, with interbeds of shale containing a rich late Arenigian fauna, Above the quartzite lies mostly argillaceous rock about 600 ft thick that has a well developed succession of Llandeilan, Caradocian, and Ashgillian graptolite zones. The uppermost Ordovician underlies 47 ft of Silurian shale thinly interbedded with chert and marble and yielding middle Llandoverian graptolites at its base and Wenlockian graptolites at its top. About 250 ft of non-fossiliferous siltstone overlies the Silurian shale before being overthrust by Arenigian shale. The sequence in the lower thrust plate is shale and argillite interbedded with dark gray quartzite and has prominent quartzitic members in its upper part. An early Caradocian graptolite fauna was collected throughout a stratigraphic interval of more than 2000 ft in the lower thrust plate. The Saturday Mountain Formation 30 mi N. of Trail Creek contains both shelly and graptolite facies and is correlative with the higher Caradocian and possibly Ashgillian parts of the Phi Kappa. Argillaceous rocks that contain equivalent graptolite faunas are widespread in central and northern Nevada. They include 3 new formations in the northern Independence Range: the Basco Formation in the central Independence Range; the Petes Summit, Sams Spring, Perkins Canyon, and Gatecliff formations in the Toquima Range; and the undivided Valmy and Vinini formations. The succession of graptolite assemblages in Idaho and Nevada closely resembles those in Britain but it has a few Australian genera that have not been reported from Europe. At Trail Creek equivalents of the standard graptolite zones of Didymograptus extensus (4), Glyptograptus teretiusculus (8), Nemagraptus gracilis (9), Climacograptus peltifer (10), Dicranograptus clingani (12), Pleurograptus linearis (13), Dicellograptus anceps (15), Monograptus convolutus (20), and Cyrtograptus murchisoni (26) are recognized and their characteristic assemblages are illustrated.