The cryptic perimeter of Precambrian sialic North America lies within the cordillera of the western United States. Between this perimeter and the Pacific basin is an assembly of displaced terranes. Ages and origins of segments of the sialic margin, orogenic features of the continental foreland, and times of attachment of displaced terranes provide a partial record of the plate tectonic evolution of western North America. The same area records Mississippian and Early Triassic tectonic events, probably passive margin-arc collisions. An active margin developed late in the Triassic, accompanied by a continental arc and foreland deformation in a zone that spanned most of what is now the Great Basin. The shortened foreland cover may have been underlain by an extensive d collement. The sialic margin in California and Idaho is Mesozoic and the probable product of tectonic removal of tracts of North America and earlier accreted terranes.