Silurian

Silurian, in geology, a series of strata which is here understood to include those Palaeozoic rocks which lie above the Ordovician and below the Devonian or Old Red Sandstone, viz. the Llandoverian (Valentian of C. Lapworth), Wenlockian and Ludlovian groups of Great Britain with their foreign equivalents. A word of caution is necessary, however, for in the early history of British stratigraphy the exact delimitation of " Silurian" was the subject of a great controversy, and the term has been used with such varying significance in geological literature, that considerable confusion may arise unless the numerous interpretations of the title are understood. The name " Silurian " was first introduced by Sir R. I. Murchison in 1835 for a series of rocks on the border counties of England and Wales - a region formerly inhabited by the Silures. Murchison's Silurian embraced not only the rock groups indicated above, but others below them that were much older, even such as are now classed as Cambrian. About the same time A. Sedgwick proposed the term Cambrian for a great succession of rocks which includes much of Murchison's Silurian system in its upper part; hence arose that controversy which left so lasting a mark on British geology. In 1850 A. d'Orbigny suggested the name " Murchisonian " for what is here retained as the Silurian system. As a solution of the difficulties of nomenclature, Professor C. Lapworth in 1879 proposed the term Ordovician systems (q.v.) for those rocks which had been the Lower Silurian of Murchison and the Upper Cambrian of Sedgwick. An approximate correlation of the usages of the title " Silurian" is here given in tabulated form: - The Silurian rocks are almost wholly of marine origin and include all the usual phases of sedimentation; shales and mudstones, marls and limestones, sandstones and grits are all represented in Great Britain and in most other countries where the Silurian is known. The majority of the rocks were deposited in the comparatively shallow waters of epicontinental seas, the graptolitic shales and sponge-bearing cherts being perhaps the representatives of the deeper waters. Locally, glauconitic limestones and ironstones (Clinton beds) indicate special conditions; while the isolation and desiccation of certain marine areas (New York) towards the close of the period gave rise to beds of red sandstone, red marls, gypsum and rock salt. The hydraulic limestone (Water Lime) of New York was probably a brackish-water formation. In Sweden and elsewhere some of the limestones and shales are distinctly bituminous.

Distribution

In the preceding Ordovician period several wellmarked marine provinces are indicated by the fossil contents of the rocks. At the beginning of Silurian time a general transgression of the sea - which had commenced at the close of the Ordovician - was in progress in the N. hemisphere (Europe and the Appalachian region). This culminated at the time when the Wenlock beds and their equivalents (Niagaran and Oesel beds) were forming at the bottom of a, great periarctic sea or shallow ocean. It is thus found that the same general characters prevail in the Silurian of Britain, N. America, Scandinavia and the Baltic region, Russian Poland (Podolia, Kielce, Galicia), the Arctic regions, New Siberia (Kotelny), Olenk district, Waigatsch, N. Zembla, Tunguska, Greenland, Grinnell Land and China. The Bohemian region, comprising central Bohemia, Thuringia, Fichtelgebirge, Salzburg, Pyrenees, Languedoc, Catalonia, South Spain, Elba and Sardinia, alone retained some of its marked individuality. Later in the period a gradual withdrawal of the sea set in over the N. hemisphere, affecting the British area (except Devon), the left of the Rhine, Norway and the Baltic region, N. Russia, Siberia and the Ural region, Spitzbergen, Greenland and the W. states of N. America. Thus the later Silurian conditions heralded those of the succeeding Devonian and Old Red Sandstone, and there is generally a gradual passage from one set of rocks to the other (Downtonian of Great Britain). The Silurian rocks may occur in close continuity with the upper Ordovician, as in S. Europe; or, as in the typical region, the Llandovery beds may rest unconformably upon older rocks; in N. America also there is a marked unconformity on this horizon. A large part of N. America was apparently land during part of Silurian time; the lower members are found in the E. alone, while the Cayaguan division is found to extend farther E. than the middle or Niagaran division, but not so far W. The falls of Niagara owe their existence to the presence of the hard Lockport and Guelph beds resting upon the softer Rochester shales. Most of the essential information as to the distribution of Silurian rocks will be found in a condensed form in the accompanying table and map; but attention may here be drawn to the upper Silurian (Ludlovian) limestone of Cornwallis Island, the mid-Silurian limestone of Grinnell Land and the lower Silurian limestone of New Siberia. Limestones of lower and middle Silurian age are found also in Timan, Tunguska and elsewhere in N. Russia. Rocks of this system in S. America have been only superficially studied; they occur in the lower regions of the Amazon, where they bear some resemblance to the Medina and Clinton stages of N. America, and in Bolivia and Peru. Little is known of the Silurian rocks. recorded from N. Africa.