A reconnaissance geochemical stream-sediment sampling survey was conducted in a 1,150-km2 (440 mi2) area in the central part of the Okanogan Range, adjacent on the west to a 2,000-km2 (800 mi2) area previously studied. About 75 percent of the bedrock is composed of a mosaic of elongate northwest-trending Mesozoic plutons, the average composition of which is probably granodiorite. These intrude a highly recrystallized eugeosynclinal assemblage of Paleozoic and Mesozoic age that makes up about 15 percent of the area. The remaining 10 percent consists of andesite and dacite flows and flow breccias of Tertiary age that unconformably overlie the older rocks. The entire area is irregularly mantled by glacial drift.
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State/Local Government Series
Reconnaissance geochemical survey of gully and stream sediments, and geologic summary, in part of the Okanogan Range, Okanogan County, Washington