From abstract: The report is limited to the geology of lode deposits in the northwestern part of the Boise Basin which are in or near mines that were in operation at the time of visit, in 1930. Owing to the recent inactivity of the formerly rich placer (leposits, there is nothing essential regarding them to add to Lindgren's report published in 1898. The area studied is underlain by granitic rock of the Idaho batholith, which is cut by dikes of Miocene(?) age. These dikes are dacite porphyry (intruded early) ; rhyolite porphyry, granophyre porphyry, and granite porphyry (closely related in character and age) ; and several basic varieties (of which some, at least, are of relatively late origin). Diorite porphyry dikes, of undetermined age but probably older than all of those named above, are also present.