The bladelike shape of the large quartz diorite dike at Cascade Pass, Washington, and its trend, which is normal to regional metamorphic structures and folds, suggest emplacement by forceful intrusion along an ac fracture. Lit-par-lit structures and contact breccias along the margins indicate some sloping. K feldspar of possible deuteric origin is present in the main phase of the dike but is rare or absent in the chilled margins, suggesting that chilling prevented its formation. However, K feldspar is concentrated marginally in the top of the dike adjacent to an explosion breccia. K metasomatism of the hornfelsed schist fragments of the breccia suggests that the breccia formed during the deuteric stage in the cooling of the pluton. Hornfels occurs at the contact of the pluton, but farther away only small glomeroblastic patches of actinolitic hornblende and/or biotite superimposed on a schistose fabric show the thermal metamorphism. This outer portion of the contact aureole widens considerably in a deep valley, suggesting that the dike widens with depth.