Abstract

Latah County lies within the Coeur d’Alene Mountains and the Columbia Plateau. The plateau is mantled with loess which is dissected into the mature Palouse topography. The origin of the Palouse Hills is considered in relation to certain newly discovered closed depressions on the tops of some of the hills. Folding of the Clearwater monocline followed deposition of the loess and rejuvenated Potlatch River.

The mountains are underlain by metamorphosed formations of the Belt series, by rhyolite and rhyodacite flows correlated with the Seven Devils volcanics, and by the Thatuna batholith correlated with the Idaho batholith. Metamorphism of the Belt rocks was dynamothermal followed by retrograde changes. The principal intrusive rock is granodiorite grading into adamellite, tonalite, and granite. Thirty Rosiwal analyses are tabulated. The order of crystallization is normal, but quartz, microcline, orthoclase, albite, and sphene have been introduced by late replacement. Most of the granite and adamellite resulted from replacement of the other types. The Gold Hill stock is more like the igneous rocks in the Coeur d’Alene district. The principal type is syenite with considerable monzonite and syenodiorite. Dike rocks with the stock include adamellite porphyry, adamellite aplite, pyroxenite, dark syenitic pegmatite, magnetite dunite, and magnetitite.

The Columbia Plateau is underlain by Columbia River basalt. Younger flows fill valleys in the Columbia River basalt and also occur 300 feet above the plateau.

The principal economic resources are residual granitic and transported clays. The geologic conditions and mineralogy suggest weathering rather than hydrothermal origin. Certain basaltic clays resulted from hydrothermal alteration.

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