ORDOVICIAN SYSTEM, in geology a term introduced by Lapworth in 1879 to include those rocks—well developed in the Welsh region formerly inhabited by the Ordovices,—which had been classed by Murchison as Lower Silurian and by Sedgwick as Upper Cambrian (see SILURIAN). In the early part of last century all the rocks which lie beneath the Carboniferous Lime stone were grouped together under the general name of Transi tion Series and it was not till 1831 that Sedgwick and Murchison made the first serious attempt to reduce them to order. Sedgwick started work in the Snowdon district and Murchison began upon the Welsh borders. By 1835 they had advanced so far that Mur chison gave the name of Silurian system to the group of rocks with which he was dealing, while Sedgwick called his group the Cambrian series. At that time it was supposed that the Cambrian lay entirely below the Silurian. Subsequently it was shown that the two formations overlap, the upper part of Sedgwick's Cam brian being the same as the lower part of Murchison's Silurian. A prolonged controversy followed which has left its effects in a confusion of nomenclature even to the present day. It has, how ever, long been recognised that the Cambrian and Silurian of Sedgwick and Murchison include three natural groups, and in 1879 Lapworth proposed that these groups should be called Cam brian, Ordovician, and Silurian. Murchison's terminology is still often used, especially in Germany, and the three groups are called the Cambrian, the Lower Silurian and the Upper Silurian.
Although under all the variations of terminology the three groups are now universally recognised there are still differences of usage with regard to their precise limits. Whereas in extra British areas there is a general concurrence of opinion as to the horizon at which the base should be drawn (zone of Dictyonema flabelliforme), most British geologists still adhere to the older classification, based largely upon structural considerations, in which the base is taken above instead of below the Tremadoc. There is also some divergence of opinion as to the horizon at which the upper limit should be drawn, the fossil evidence sug gesting that it should include beds up to the time of appearance of the true Pentamerids and Monograpti, though mainly on litho logical grounds it is generally taken below this level.
The various strata composing the System may be divided into several contemporaneous facies or types of deposit, controlled by different physical conditions at the time of formation, and since these same physical conditions also govern the nature of the organisms living in different parts of the sea, each facies of deposit tends to have a distinctive set of fossils.
The three principal facies can be recognized: I. Shallow water marine facies (a) grits, sandstones and shales indicative of heavy sedimentation; fossils mainly trilobites and brachiopods.
(b) Shelly limestones evidently laid down in clean water ; fossils mainly brachiopods, cystids, ostracods, and bryozoa.
(b) black shales; fossils, blind trilobites or trilobites with over developed eyes. Often occurring in association with (c) fine black shales; fossils mainly graptolites.
The natural relationships of these different types are indicated by their field relations, the shallow water facies of either type may have a considerable lateral extension running parallel with the old coast line, but in a direction approximately at right angles passes into one or other facies of the deeper-water type, the subdivisions of which are often intercalated into one another, though the type with the most widespread development is undoubtedly that of the Graptolite Shale. The graptolites do not really belong to the black shale since it is likely that they were planktonic or pseudo-planktonic, but since quietness of the waters and absence of coarse sediment are the determining factors governing the dis tribution of both, the two are almost invariably associated. Life.—The life of the Ordovician period presents a distinct advance in type upon that of the preceding Cambrian ; as a whole it may be said to be characterised by three main features: (a) Acme of development of the trilobites. (b) Rise and domin ance of the articulate brachiopods over inarticulate types. (c) Incoming in force and great development of the graptolites. The fauna consists on the one hand of a trilobite-brachiopod fauna characteristic of the shallow water facies, and a graptolite fauna characteristic of the deeper water facies, and it is by means of these great groups of organisms that the rocks belonging to the System have been divided up into a series of life zones. The grap tolites utilised as zone fossils differ in different parts of the world, but though the Index Fossil may be different, there is a very wide agreement in the General Assemblage found at the same horizon within the Graptolite Shales all over the world, and there is like wise the same change from the early many branched forms to those with fewer branches, and from the pendent to the scandent direction of growth. The cell form too changes in the same man ner from the straight to the slightly sinuous, and eventually to the decidedly sinuous type. Hence Clonograptus, Dichograptus, Tetra graptus and Didymograptus are characteristic of the Lower Ordo vician ; Nemagraptus, Dicellograptus and Climacograptus charac terise the Middle Ordovician, and Dicellograptus, Dicranograptus, Climacograptus and Diplograptus the Upper Ordovician. The articulate brachiopods include the various subgenera of Orthis, Strophomena Leptaena, and Plectambonites, and, more particu larly towards the top, forms with a definite internal brachial apparatus such as Atrypa and Zygospira make their appearance.