Middle Paleozoic rocks change facies from E. to W. across central Idaho reflecting sedimentation in different environments of the Cordilleran geosynclinal complex and its eastern margin. A thin cratonic assemblage of Upper Devonian Jefferson Dolomite, punctuated by local diastems, unconformably oversteps Ordovician Kinnikinic Quartzite and Pre-cambrian Belt strata in southwestern Montana and in the Beaverhead Range along the Idaho-Montana border. W. of the southern Lemhi Range the importance of the Devonian regional unconformity diminishes, and variable amounts of Upper Ordovician Fish Haven and Silurian Laketown dolomites are preserved below Jefferson Dolomite. The basal quartzite-carbonate succession of Kinnikinic Quartzite and Fish Haven, Laketown, and Jefferson dolomites thickens markedly in the Lost River Range toward the W. This miogeosynclinal succession is replaced farther W. in the Wood River region by Lower Ordovician to Silurian graptolitic shale, chert, and quartzite, the Phi Kappa-"Trail Creek" sequence of eugeosynclinal affinity. About 3500 ft. of interbedded shaly limestone, dolomite, shale, and quartzite of the Middle-Upper Ordovician Saturday Mountain Formation and the Silurian "Laketown Dolomite" are present in the Bayhorse region and appear to be transitional in facies between the time-equivalent, quartzite-dolomite sequence on the E. and the graptolitic shale-chert-quartzite sequence on the S. Since newly measured sections imply approximately N.-S. facies boundaries and thickness trends, the graptolitic shale-chert-quartzite sequence in the Wood River region lies due N. and on depositional trend with transitional facies in the Bayhorse region. This eastward invasion of the graptolitic shale-chert- quartzite sequence into the miogeosynclinal carbonate belt is attributed to thrusting as has been demonstrated for corresponding facies contrasts in Nevada. Contrary to classical concepts of geosynclinal sedimentation, most of the middle Paleozoic detrital rocks on the W. side of the miogeosynclinal carbonate belt and intertongued with the carbonates are of quartz sand and mature detrital fines apparently derived from eastern cratonic sources rather than from unknown orogenically active borderlands or volcanic island arcs on the W. In contrast, late Paleozoic orogenic movements in the eugeosynclinal belt caused uplifts from which a thick wedge of coarse detritus forming the Milligen and Wood River formations spread eastward to intertongue and grade into finer clastics and carbonates of the "Brazer Limestone" near the craton margin. Late Paleozoic orogenic activity in central Idaho represents a northern continuation of the Antler orogeny of Nevada. The Antler orogeny thus had extremely widespread and profound effects on tectonostratigraphic patterns of the Cordilleran geosyncline: for the first time in the Paleozoic history of Idaho and Nevada it replaced the craton as a principal source for detrital sediments; in Idaho, the orogeny brought to a close the eugeosynclinal episode of essentially depositional continuity extending from at least early ordovician to late Devonian.