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 limestone＝Upper Orthoceratite limestone of Sweden. (B) Orthoceratite (Vaginaten) limestone＝Orthoceratite limestone of Sweden, Glauconitic limestone, Glauconitic sand (Greensand). The last-mentioned reposes on Cambrian Dictyonema shales. While the Ordovician rocks in Scania, the Baltic provinces and north-central Russia are undisturbed and level-bedded, those on the western side of the Scandinavian axis and in the Urals have suffered movement and are metamorphosed into schists, phyllites, quartzite, marble, &c.; and, especially in Scandinavia, have been extensively thrust. The Bohemian Ordovician, “stage D” of Barrande, consists mainly of greywackes and shales with some ironstone beds and eruptive rocks in the lower parts. In Germany the only large areas are found in the Thuringer Wald, Fichtelgebirge, Frankenwald and Vogtland, where they consist principally of unfossiliferous greywackes and shales with some oolites and glauconitic ironstone (chamosite) in the lower part. They are divisible into the Hauptschiefer or Lederschiefer and the Ober-Thuringit beds above, and the Griffelschiefer and Unter-Thuringit beds below, which rest upon the Leitmitzschiefer of the Euloma-Niobe (Cambrian) horizon. Across northern Russia Ordovician rocks cover a great area; they consist of clays, bituminous and calcareous shales, sands and marls, which in the Ural region have been metamorphosed; the Bukowka sandstone of Russian Poland is of this age. In north-west France this system is represented in Brittany and Normandy by the slates of Riadan, the grès de May, the schistes à calymènes (with an ironstone bed at the base) and the gres armoricain. In the Ardennes are the schistes de Gembloux, resting upon graptolitic shales of Arenig age. Sandstones and shales occur in Languedoc, and various rocks in the Pyrenees. In the Iberian peninsula Ordovician rocks are widely spread, represented by sandstones, slates and shales covering the whole of the period; they are well developed in Asturia and Galicia. In the eastern Alps about Graz are found calcareous shales with crinoids, the “Schockelkalk” and “Semriacher” shales; the Marthener beds of the Carnic Alps are of this age. In China (Kiang-su, Kian-chang), in Burma (Mandalay) and in the Himalayas (Niti and Spiti) Ordovician fossil-bearing rocks are known.

On the North American continent Ordovician rocks cover a very large area in the central, eastern and northern parts (north of lat. 30°). As regards the classification and correlation of the strata, which change in character from point to point, as is natural over so large an area, much remains to be done. In the table the divisions of the system that obtain in the New York district are enumerated; but in each state there is a local nomenclature for the beds. Thus in Iowa, Wisconsin and Minnesota we find (1) Lower Magnesian limestone, St Peter’s sandstone; (2) Trenton limestone, Galena limestone; (3) Hudson river shales; in Arkansas, the California or Magnesian limestone, Saccaroidal limestone, Izard limestone and Polk Bayou limestone; in Oklahoma, the Arbuckle limestone, Simpson series, Viola limestone and Sylvan shales; and in east Tennessee, the Chickamauga limestone, Athens shale, Tellia sandstone, Sievier shale and Bays sandstone. In Massachusetts there are enormous series of schists which have been assigned to this period. In west Virginia are the Martinsburg shales (1000 ft. or more). In Canada the Ordovician rocks (Quebec group) are thickly developed. In the upper division there are the lowest of the Anticosti limestones, the Hudson river beds, and Trenton limestone; to the middle division belong the Coenograptus shales; and the lower division consists of the Levis shales with Sillery beds at the base. In Nova Scotia and New Brunswick are the lower and upper divisions of the Cobequid group, a series of shales, quartzites and conglomerates with igneous rocks. In the polar regions Ordovician rocks are represented by the Trenton limestone in Boothia and King William’s Land; by limestones with Caryocystis granetum in east Greenland; and in the Barrow Straits by beds with Asaphus and Maclurea.

In North Africa Ordovician rocks are probably present, and in New Zealand the Arorere series (Wanaka group), and in Australia (Victoria) the graptolitic, gold-bearing shales and slates belong to this period. During this period there appears to have been a general tendency for the sea to transgress on the land, a tendency which increased towards its close, especially in the northern hemisphere (Europe and the Appalachian regions). One of the results of this movement was the interchange and commingling of many previously separated faunal groups. About the beginning of the period the sea withdrew from the land in Texas and south of the Rocky Mountains. The folding of the Appalachians was in progress early in Ordovician times and later in the period the first symptoms of the Scandinavian and British folding set in.

Volcanic Activity.—This period was one of great volcanic activity in several widely separated regions. “In Ayrshire and the south-western districts (of the southern uplands), where the volcanic constituents attain a great development, they consist of basic lavas (diabase, &c.), with intercalated tuffs and agglomerates. A characteristic feature of these lavas is the development of ellipsoidal or pillow-structure in them. This volcanic platform appears to underlie the Silurian region over an area of at least 2000 sq. m., inasmuch as it comes to the surface wherever the crests of the anticlines bring up sufficiently deep parts of the formations. It is thus one of the most extensive as well as one of the most ancient volcanic tracts of Europe” (Sir A. Geikie, Text-book of Geology, 4th ed. vol. ii. p. 951). In the west of England and in Wales there was also a very active volcanic centre. In the Snowdon district thousands of feet of contemporaneous felsitic lavas and tuffs occur in the Bala beds; while in Cader Idris, the Arenig Mountains and the Arans there are similar eruptions of felsitic and rhyolitic lavas, tuffs and agglomerates—probably many of them submarine—interstratified in the Arenig formation. In the Lake district a great series of lavas and ashes—the Borrowdale series—was erupted during the middle of the period; the earlier effusions were andesitic, the later ones felsitic and rhyolitic. In Ireland the Arenig lavas of Tyrone resemble some of those in Scotland. Volcanic rocks (porphyrites, syenites and lavas) occur in considerable force in the Ordovician rocks of Nova Scotia and New Brunswick and New Zealand. Tuffs of this age are found in Brittany, and diabase in Bohemia.

Ordovician Life.—Compared with the preceding Cambrian period, the Ordovician is remarkable for the great expansion in numbers and variety of organisms, apart from the fact that fossils are better preserved in the younger formations.

All the great classes of mollusks were represented, the most numerous being the brachiopods, which, in addition to the simple forms of the Cambrian, began at this time to develop spire-bearing genera (Chonetes, Orthis, Orthisina, Strophomena, Crania, Schizotreta, Porambonites, Rafinesquina, Leptaena, Zygospira). The gasteropods now developed all the leading types of shell (Pleurotomaria, Omphalotrochus); but both this class and the pelecypods (Lyrodesma, Ctenodonta, Modiolopsis) were subordinate in importance to the cephalopods. These mollusks were probably the most powerful living creatures in the Ordovician seas; straight-shelled, slightly curved, and nautiloid forms predominated (Orthoceras, Cyrtoceras, Gyroceras, Trocholites, Endoceras, Litoceras, Lituites, Actinoceras). Some of the straight shells were of enormous size, 12 to 15 ft. long and as much as 1 ft. in diameter, in the widest part. Trilobites were present in great abundance, and in this period they reached the climax of their development. In the lower stage we find Agnostus, Calymene, Asaphus, Illaenus, Placoparia; on the Llandeilo horizon, Calymene, Asaphus, Megalaspis, Dalmanitis; and, at the summit, Trinucleus and Homalonotus. In the transition zone between Ordovician and Cambrian, Ceratopyge, Euloma, Niobe, flourished. Other important genera are Ogygia, Cheirurus, Harpes, Acidaspis. Ostracods (Leperditia, Beyrichia), cyprids (Bairdia, Macrocypris), phyllocarids (Ceratiocaris, Peltocaris), cirripeds (Lepidocoleus), and, later, eurypterids represented other crustacean groups. The bryozoans, Stomatopora, Monticulipora, Phylloporina, Fenestella and others, were abundant and frequently formed beds of limestone. Among the echinoderms the cystoids were the most prominent (Pleurocystis, Aristocystis) and at this period reached their climax; crinoids (Archaeocrinus, Dendrocrinus) became more important; while ophiuroids, echinoids (Bothriocidaris) and asteroids (Taeniaster, Palaeaster) made their appearance. Corals (Streptelasma, Columnaria) were scarce, and sponges (Aulocopium, Caryospongia, Archaeocyathus) were not particularly important; Receptaculites, Ischadites, are well-known fossils doubtfully referred to this group. Radiolaria assisted in the formation of certain beds of chert, and foraminifera have been observed. The remarkable group, the graptolites, evidently inhabited the seas in countless numbers and have left their remains in the dark shales of this period all over the world. At this time the diprionidian forms alone were represented by such genera as Tetragraptus, Phyllograptus, Didymograptus, Dicellograptus, Diplograptus and others. Of great interest are the earliest known indications of vertebrate life in the form of dermal plates and teeth of fishlike organisms from the Ordovician of Colorado. The terrestrial life of the period is very meagrely represented by the remains of land plants, mostly poorly preserved in certain sandstones, and by scorpions and several orders of insects, Protocimex (Sweden), Palaeoblattina (Colorado).

One of the most striking facts brought out by the study of the distribution of Ordovician fossils is the wide range of the northern or “periarctic” faunal assemblage. This periarctic fauna prevails over the whole world—so far as our present knowledge shows—with the exception of the peculiar Bohemian or Mediterranean region, which