ORDOVICIAN (from Lat. Ordorices, name of an ancient British tribe of North Wales), or LOWER SILURIAN SYSTEM. A division of geologic time following the Cambrian and preceding the Upper Silurian or Silurian proper. The Ordo vician comprises rocks originally classed by Murchison as the lower portion of his Silurian system. It was considered to be of suffieient im portance to be classed as a separate division by Lapworth in 1879, who proposed for it the term ordovician. The type section of the American ordovician is found in New York State. and eon sequently time names of many of the subdivisions are locality terms used in New York. It is as follows: The Ordovician rocks are chiefly limestones, with the exception of the upper and lower mem bers. which may be very shale, indeed the Ilud son River shale: form a very prominent scrip- of rocks in the Appalachian States. The Trenton rocks are widely distributed over the continent. The Ordovician strata are often found fringing the Arch:can areas, being sometimes separated from them by hut a thin strip of Cambrian. Thus there are belts of Ordovician rocks around the Yew York .),,Iirondaeks: from central New York Avestward to Wisconsin and 'Minnesota; along the line of the Appalachians on the eastern and some times on the western slope from Vermont to Ala bama; around the V-shaped or Lauren tian of Canada; and in the Central States, in Ohio, Kentucky, Indiana, and Tennessee. Ordo xician rocks are also known in the Uinta. Wa satch, and Rocky Aloun(ains. In Europe the Ordo vician ria.ks form a large area extending from Iceland into Ilussia. They are of considerable thickness in I :Feat Britain and \\ ales. Additional areas are found in Bohemia, Germany, France, Portugal, Spain, and Northern Afriea.
In neither North .A111(9'1(11 nor Europe do we find any marked break between the Cambrian and ord.% jejan cm:. but t he faunal changes are well shown. There was during the I )rdovician times a great interior sea over the United States. ;and the coastal States were also under water, but in the southwest much dry land was known. At
the close of the Ordovician there were great dis turbances. The mountains along the New York New England border were formed and gave rise to the so•called Taconic ranges and the Creel Much faulting and folding necom panic(' the uplift. There was also developed a line of uplift in Ohio. I:entucky, and Tennessee. which domed the rocks up into a low, broad an known as the Cincinnati arch.
The life of the ordovician. while being in ad vance of the Ca whrian t IMP, was not of a high order. Ilany seaweed: have eryptogams probably existed. though only scanty remains had been found. Being land plants, their preservation would be doubtful in marine sedi ments. Foraminifera and Radiolarin were abun were also sponges. In the Hudson slates graptolites were very common.
along the ponds many representatives of the Tetraeoralla„ and also large cup corals like Streptelasnm, while Columnaria Was a eommon compound one. Cystidean erinoids are numerous, and many starfishes and even sea-urchins are known. The trilobites were Very well deVelOped. hut the genera were largely different from those of Cambrian age. The brachiopods were repre sented by many well-known genera. especially Orthis, and Many groups of Mollusc, existed. Pleurotoniaria among the gastropods, Orthoeeras among the cephalopods were numerous. Fishes Were the only representatives of the vertebrates, and of these were found armored ostraeoderms in t he Ordovician sandstones of Colorado. Fish teeth are known in the European Ordovieian.
Among the useful minerals of the Ordovician are great quantities of building stone, including limestone and marble. in Ohio and Indiana great supplies of petroleum (q.v.) and natural gas occur in the rocks of the Cincinnati arch. Zinc aural frail ores are mined in southeastern :Missouri and the Upper Nllxsissippi Valley in 'Wisconsin, Iowa, and Illinois. Along the contact between the Cambrian and Silurian in rooks in many parts of the Appalachians there are found deposits of limonite ores, many of which are mined.