Ordovician

Ordovician, or Ordovian, from the Ordovices, an ancient tribe of central Wales, is a name recently applied to a great system of rocks known by Sedgwick as Upper Cambrian and by Murchison r.s Lower Silurian; but containing a distinct assemblage of fossils. They consist of greywackes, sandstones, grits, flagstones, shales or slates, with limestones in the upper part, and important, contemporaneous lavas and tuffs; and they cover large area in Wales, Shropshire, the Lake district, South Scotland, and the Isle of Man, passing conformably downwards into the Cambrian (q.v.). Their most characteristic group of fossils is that of the graptolites (q.v.); but trilobites (q.v.), such as Asaphus, Ogygia, and 'Jrinucleus, brachiopods, such as Ort'his, and the gastropods Murchisonia and Euomphalus were abundant. In the limestones corals for the first time "become numerous. The system is subdivided as follows: - Lower Llandovery Series. - 1,000 feet. Grits and sandstones. Lata and Coradoc Series. - 0,000 to 12,000 feet. Sandstones, slates, and giits, with Bala and Conistoa limestones. Llamleilo ffa