As North America was being pushed west for the last 180 million years volcanic island arcs that were built on the subducting plate, plus pieces of different parts of the oceanic lithospheric plate, were being sutured to the western, leading edge of North America. These tracts of rocks that are now part of the continent but were originally formed elsewhere are called exotic terranes, or (because they have been added to the edge of the continent), ___ terranes.