The composite Cornucopia stock, a satellite of the Wallowa batholith, intrudes a heterogeneous assortment of eugeosynclinal sediments and volcanic rocks of greenschist-facies regional metamorphism. Important field evidence for emplacement of magmas is furnished by contact relationships, wall-rock deformation, and structural features of the granitic rocks. The pluton contains at least five distinct granitic rocks. A related series of explosion breccias, intrusive breccias, and porphyry dikes forms part of the sequence.

Willow Lake-type layered rocks are notable in a biotite trondhjemite unit but are more striking in parts of different intrusions of cordierite trondhjemite. Taubeneck and Poldervaart (1960) defined Willow Lake-type layered rocks as being composed of alternating bands of felsic and mafic constituents in which the minerals are characteristically elongated and oriented roughly perpendicular to the banding. Detailed mapping of Willow Lake-type layered rocks in Cornopia stock was instrumental in belated field recognition of the composite character of the trondhjemites. Elsewhere, Willow Lake-type layered rocks should also be useful in unraveling intrusive histories of some igneous complexes.

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