sentences:  Uplifted portions of the forearc, composed of Eocene igneous basement surmounted by reef terraces of Eocene and younger age, produce the island chain from Guam north to Ferdinand de Medinilla in the Marianas.Similarly, the Bonin or Ogasawara Islands are mostly composed of Eocene igneous rocks.There is no accretionary prism associated with the IBM forearc or trench.