Page:EB1911 - Volume 20.djvu/268

Rh ORDOVICIAN SYSTEM, in geology, the group of strata which occur normally between the Cambrian below and the Silurian above; it is here regarded as including in ascending order the, , and  or  series (qq.v.). The name was introduced by C. Lapworth in 1879 to embrace those rocks—well developed in the region formerly inhabited by the Ordovices—which had been classed by Sir R. Murchison as Lower Silurian and by A. Sedgwick in his Cambrian system. The term is convenient and well established, but Lower Silurian is still used by some authors. The line of demarcation between the Ordovician and the Cambrian is not sharply defined, and beds on the Tremadoc horizon of the Cambrian are placed by many writers at the base of the Ordovician, with good palaeontological reasons.

The rocks of this system include all types of sedimentation; when they lie flat and undisturbed, as in the Baltic region and Russia, the sands and clays are as soft and incoherent as the similar rocks of Tertiary age in the south of England; where they have been subjected to powerful movements, as in Great Britain, they are represented by slates, greywackes, quartzites, chlorite-, actinolite- and garnet-schists, amphibolites and other products of metamorphism. In Europe the type of rock varies rapidly from point to point, limestones, shales, sandstones, current-bedded grits and conglomerates or their metamorphosed equivalents are all found within limited areas; but in northern Europe particularly the paucity of limestones is a noteworthy feature in contrast with the rocks of like age in the south, and still more with the Ordovician of North America, in which limestones are prevalent. In the Highlands of Scotland, in north-west England, in Wales and Ireland, there are enormous developments of contemporaneous lavas and tuffs and their metamorphosed representatives; tuffs occur also in Brittany, and lavas on a large scale in Nova Scotia and New Brunswick.

Distribution.—The Ordovician system is widely distributed. The accompanying map indicates roughly the relative positions of the principal land-masses and seas, but it must be accepted with reserve.



A study of the fossils appears to point to the existence of definite faunal regions or marine basins. The Ordovician rocks of the British Isles seem to have been deposited in a North Atlantic sea which embraced also the north of France and Belgium. Confluent with this sea on the east was a rather peculiar basin which included Bohemia, southern France, Spain, Portugal, the eastern Alps, Thuringia, Fichtelgebirge and the Keller Wald. Another European basin, probably separated from the Bohemian or Mediterranean sea in early Ordovician times, lay over the Baltic region, Scandinavia, the Baltic provinces and north Germany, and communicated eastwards by way of Russian Poland and central Russia with far eastern

waters, embracing China, Siberia and the Himalayas; concerning the last-named marine area not much is known. In the opposite direction, the Baltic basin may have communicated, through Greenland, with the North American and Arctic seas. Over central and eastern North America another large body of water probably lay, with open communications with the north and west, and with a more constricted connexion with the Atlantic sea. The lagoonal character of some of the rocks of the Tunguska region of Siberia may perhaps be indicative of continental border conditions in that quarter.

Some of the principal subdivisions of the Ordovician rocks are enumerated in the table. Owing to the universal distribution of the graptolites, the correlation of widely separated areas has been rendered possible wherever the muds and shales, in which their remains are preserved, are found. Where they are absent the correlation of the minor local subdivisions of distant deposits is more difficult. In Great Britain, through C. Lapworth and his school, and J. E. Marr and the Cambridge school, and in Scandinavia and the Baltic region, through W. C. Brögger, S. A. Tulberg, F. Schmidt and others, the most elaborate subdivision of the Ordovician rocks has been attained.

In the Baltic provinces of Russia, F. Schmidt describes the following stages, in descending order: (Stage F) the Lyckholm and Borkholm zones, a highly fossiliferous series, equivalent to the Middle Bala of Britain; many of the limestones are largely formed of Rhabdoporella and other calcareous algae. (E) Wesenberg zone = Bala. (D) Jewe and Kegel zone. (C) Itfer beds, Kuckers Shale (bituminous limestones and marls = Brandschiefer), Echinosphaerite