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The U.S. Geological Survey (USGS) presents the downloadable version in PDF format of a paper by R.G. Tysdal in two parts entitled "Revision of Middle Proterozic Yellowjacket Formation, Central Idaho, and Revision of Cretaceous Slim Sam Formation, Elkhorn Mountains Area, Montana." The document originally appeared in no. 1601 of the "U.S. Geological Survey Professional Paper" series. The goal of the first part of the paper is to correlate Middle Proterozoic strata in the Lemhi Range of Montana with strata in the Salmon River Mountains and reconcile differences of interpretation and nomenclature. The second paper is an outgrowth of mineral-resource assessment studies conducted in the Helena National Forest of west-central Montana.