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The object of this paper is twofold-to present evidence found in southwestern Montana concerning the .great Jurassic base-leveling and its bearing on the solution of certain ·stratigraphic problems involving late Paleozoic, Triassic, and Jurassic formations, and to set forth the relations of those formations to· beds in western Wyoming. The conclusions briefly summarized are as follows: Prior to the encroachment of the sea from the northwest in late Jurassic time prolonged erosion and base-leveling occurred over much of the Rocky Mountain region. From the Idaho State line near Yellowstone National Park northward to the vicinity of Helena the erosion surface thus produced truncates beds of Triassic and Carboniferous age, through a stratigraphic range of about 1,000 feet. The Quadrant quartzite at the type locality in the northwestern part of Yellowstone Park is approximately equivalent to the Amsden and Tensleep formations in Wyoming. The Park City (Pennsylvanian and Permian) , Dinwoody (Lower Triassic), and Chugwater (largely Triassic) of the Wyoming section are in part represented by the Teton formation of Yellowstone Park. The quartzitic cherty basal beds of the Teton, containing the phosphate rock, are equivalent to the Phosphoria formation of Idaho, which corresponds to the upper part of the Park City formation. In western Montana north of latitude 45° 30' the overlying Triassic shaly limestone and red shale were removed by the Jurassic erosion and the Ellis formation (Upper Jurassic) rests on the pt>"rtly eroded Phosphoria formation.

Relations of late Paleozoic and early Mesozoic formations of southwestern Montana and adjacent parts of Wyoming