In the Okanogan Highlands of Washington a complex plutonic mass called the Colville batholith intrudes folded and dynamometamorphosed sedimentary and volcanic rocks of late Paleozoic and Triassic age. Along the sharply discordant contact the wall rocks are much fractured and granulated, but contact metamorphism is slight or absent.
The batholith is remarkably heterogeneous both structurally and petrographically. A central mass of structureless granodiorite grades outward into a belt of foliated igneous rock which commonly shows intricate swirling of the foliation. These swirled rocks grade into a peripheral belt of variable but well-foliated migmatitic gneisses characterized by severe granulation of the constituent minerals. Over broad zones this rock is a mylonite; locally recrystallization has produced types resembling metamorphic granulites. That the crushing was protoclastic and not due to dynamic metamorphism following batholithic solidification, is proved by the relations with the wall rocks and by the widespread cementation of the broken materials by films and stringers of undeformed quartz and microcline.
Along the contact between the approximately contemporaneous Osoyoos and Colville batholiths occurs a narrow belt of heterogeneous syenite with highly complicated internal structure. This is believed to be a hybrid rock formed by the action of magmas and emanations from both batholiths upon a thin wall rock septum.
Deep erosion revealed the Colville rocks over wide areas. Flows of spilite and keratophyre were poured out, and after another erosion interval were covered by Tertiary sediments and volcanics. These were then folded and deeply eroded. Ice sheets covered the area during the Pleistocene.