Bibliography: p. 22.

The Sheridan district, covering approximately 60 square miles, is on the western flank of the Tobacco Root Mountains, Madison County, Montana. The exposed metamorphic bedrock is divided into the following major mappable units: quartzo-feldspathic gneiss assemblage, amphibolite-hornblende gneiss assemblage, antho-phyllite gneiss assemblage, intermediate gneiss assemblage, and marble. Detailed study of 150 thin sections, 23 modal analyses, and 24 chemical analyses indicates that these metamorphic rock assemblages were derived from a 9,000-foot sequence of graywacke, clay-rich sandstone, sandstone, calcareous shale, dolomitic limeston interbedded with chert or pure sandstone, basic sills and flows, and ultrabasic intrusive bodies. Geologic map patterns delineate a series of slightly overturned, isoclinal antiforms and synforms, which plung N. 10À W., 24À NW. In the northern part of the map area the nose of the Wisconsin Creek antiform is overturned, detached, and refolded, thereby producing a nappe structure. In the southern half of the Horse Creek synform, Eclipse Gulch antiform and Bitney synform the hornblende-granulite subfacies rank of metamorphism was locally obtained. Temperature of metamorphism is estimated to have been 600ÀC.; depth of burial, based on structural evidence, approached 8 miles. Metamorphic rank decreases to the north where the sillimanite-almandine-orthoclase subfacies predominates. Metamorphic rocks to the northeast of the Noble fault belong to the sillimanite-cordierite-muscovite-almandine subfacies, cordierite-amphibolite facies. Joint systems within the Sheridan district are a result of both Precambrian and Laramide deformations. Dominant, nearly vertical, joint planes having average trends of N. 75À W., N. 45À W., N. 2À E., and N. 65À E. are believed to have originated in the Precambrian, on the basis of close agreement with theoretical trends. Dominant fracture systems probably controlled the intrusion of the Tobacco Root batholith and the orientations of the Noble, Booth, and Parson faults, which are all associated with the Laramide orogeny. Later Tertiary intrusive stocks of quartz monzonite and quartz-monzonite porphyry are associated with widespread mineralization. Mineralization was controlled by the contact of stocks with marble beds and by existing fracture systems. Major veins especially favored a N. 30À E. fracture trend.