The Coeur d'Alene district of this report is that area in northern Idaho between the Montana State line and longitude 116°17', and between latitudes 47°26'15" and 47°33'45".The country rock of the area consists of the Prichard, Burke, Revett, St. Regis, Wallace, and Striped Peak Formations of the Precambrian Belt Series. These formations are composed of fine-grained quartz-rich rocks that have been mildly regionally metamorphosed and which, in conse• quence now belong to the green schist facies. The ubiquitous sericite was oriented during the folding and metamorphism and the rocks now consist of quartzose slate, sericite phyllite, sericite quartzite, and in a few areas, schistose quartzite and sericite schist.Igneous rocks of the area include monzonite stocks and related monzonite dikes, which were probably intruded at the same time as the Idaho batholith, and biotite lamprophyre, diabase, and porphyritic hornblende-augite-olivine-dikes, the latter perhaps varieties of the diabase dikes. The two stocks are part of a line of northeastward-striking intrusives that apparently lie along a major fracture. This fracture is probably one of the controlling factors in the origin of the Coeur d'Alene district.Several sets of faults can be recognized; there is no certainty that any are older than the shear zones that contain ore deposits, but the great strike-slip faults-the Osburn and Placer Creek faults-have offset mineral belts.Idaho, within which the Coeur d'Alene district is the only important mining area, has ranked second in lead and in zinc production for many years. The first recorded production from the Coeur d'Alene district proper was