The third oldest period of the Paleozoic Era, spanning an interval from about 412 to 438 million years before the present. The Silurian system includes all sedimentary rocks deposited and all igneous and metamorphic rocks formed in the Silurian Period. Both the base and top of the Silurian have been designated by international agreement at the first appearances of certain graptolite species in rock sequences at easily examined and well-studied outcrops.