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Encyclopedia Britannica - Main :: SHA-SIV |
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SILURIAN , in geology, a series of strata which is here under-stood 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 Walesa region formerly inhabited by the Silures. Murchison's Silurian em-braced 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 185o 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 representative's 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
York
Distribution.In the preceding Ordovician period several well-marked marine provinces are indicated by the fossil contents of the rocks. At the beginning of Silurian time a general transgression of the seawhich had commenced at the close of the Ordovicianwas 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
Guelph
drawn
Silurian Life.Our know-ledge of the life of this period is limited to the inhabitants of the seas and of the brackish waters of certain districts. The remains of marine organisms are abundant and varied. Graptolites flourished as in the pre-ceding period, but the forms characteristic of the Ordovician gave place early in the Silurian to the single-axis type (Monograptidae) which prevailed until the close of the period (Rastrites, Monograptus, Retiolites and Cyrtograptus). As in the Ordovician rocks, the graptolites have been largely employed as zonal indicators. Trilobites were important; the genera Calymene, Phacops and Encrinurus attained their maxi-mum development; Proetus, Bronteus, Cyphaspis, Arethusina may be mentioned from among many other genera. The ostracods Leperditia and Bey- richia are very abundant locally. A feature of great interest
P. oblongus), but the spire-bearing forms soon began to increase (Spirifer, Whitfieldia, Meristina, Atrypa). Other genera include Rhynchonella, Chonetes, Terebratula, Strophomena, Stricklandinia. The bryozoa, especially the bulky rock-building forms, were less in evidence than in the Ordovician. The echinoderms were well represented by the crinoids (Cyathocrinus, Crotalocrinus, Taxocrinus), some of which are found in a state of beautiful preservation at Dudley in England, Lockport (New York); Waldron (Indiana) in N. America and also in Gothland in the Baltic. Cystids were abundant, but less so than in the Ordovician; blastoids made their first appearance. Corals, mostly tabulate forms, flourished in great abundance in the clearer waters and frequently formed reefs (Favosites gothlandica, Halysites catenularia, Alveolites, Heliolites) ; tetracorallian forms include Stauria, Cyathophyllum, Cystiphyllum, Acervularia, Omphyma and the remarkable Goniophyllum. Sponges were represented by Astylospongia, Aulocopium, &c. The peculiar genera Silurian Period :/ Suggested distribution of Land & Water Abet de Lapparent . toe shaded portions md%o,te tiviensin - wised alien. rocks Moe been /send R. I. Murchison. A. Sedgwick. C. Lapworth. American. A. de Lapparent. E. Renevier. Silurian. c = ro c a ' ai (Upper Silurian cr.) i.0. m re, ^ d a of some authors.) uv (=I 4 ti v Z. to b,o o 4 ~ao-c .: 5 occ C C7 M Z . C5 (/1 a LLa se- e. E c Ordovician. = c - o (Lower Silurian o E of some authors.) 0 CL ti E Um re: x '1~ a U M ~, .a U Cambrian. ro Upper. , Middle. - s:i Lower. U Graptolite England d c, a North America v`e ;~... 3i ~i .~ Zones and Scotland. Scandinavian. Baltic Region. Bohemia. .0 (New York). (Britain). Wales. U~ p d Monograptus Downton Downtonian Upper Cardiola Eurypterus Stage E2 of Manlius leints.ardin- and and beds and upper beds. J. Barrande. limestone. y ,: ens,s. Ludlow Raeberry Cephalopod or a Rondout 3 ,o M. bohemicus. groups. Castle Gothland lime- Limestones Water Lime. C S 2 y 'a' beds. stone. with Cobleskill cephalopods. limestone. t7 (n M. Vilssoni. Red sandstone. Upper Oesel Salina beds of d k; o y x Eis' E >' Lower cephalopod beds: o Onondaga ea u limestone. dolomite with U eu ca ' ~ ZF tl Crinoid and coral and c 4 rock salt d o c o limestone. limestones. ~ and ^ ;; g c e ^ O gypsum. `,c Lower Cardiola a a '"E shales and Mega- harms p. :Q ~ a 8 harms limestones. C y r t o g r apt u s Lower Oesel Crinoid oz S'j Guelph
a s o M. testis. Wenlock Riccarton, and Blair, shales and lower beds: limestones. $ ^ dolomite. o Cyrtograptus Woolhope and brachiopod and dolomite N Z g Linnarssoni. groups. Straiton coral limestone and L' -o Lockport d beds. with sandstone. marl. E limestone. a y a ~ .2= Cvrtograptus ^.ty c7 b a Murchisoni, a Rochester shales. Z et . 5 .7~ Clinton beds. '5 o MR cy boo i-7 N 3 4 o ^ y CO v rb o m '~/ . q 8 n Rastrites Tarannon, Queensberry Rastrites shales Penlarnerus Stage Er of ,._'-5 a -0 o o a .`r maximas. Llandovery, beds. and Strickland- beds. J. Barrande. o~ `d Medina .~ o and inia marls. Graptolite a sandstone. Z a " ~' a u M. spinigerus. May Hilt Birkhill shales shales M m d ^ as `u a.a... , 3 G ty o N groups. and with N Oneida
M. gregarius. Graptolitic beds diabase E conglomerate. 2 N E Ey of the at the base. us 0 a Diplograptus Girvan area. Shawangunk 0 3 Z c N'o toes iculosus. ma o. grit. a Li E q c7 3 E Diplograplus p4 00 ar uminalus. Receptaculites and Ischadites occur in the Silurian. Foraminifera and radiolaria also left their remains in the rocks. The most highly organized animals of the Silurian period were the fishes which had already made their appearance in the Ordovician rocks of Colorado and Russia. The Silurian fish include selachians (Onchus, Thyestis), and the occurrence of remains of the obscure backboned ostracoderms (placoderms) is particularly worthy of notice (Pteraspis, Cephalaspis, Tremataspis, Cyathaspis, Thelodus, Lanarkia, Eukeraspis). Scorpions (Palaesphonus) have been found in Lanark, Gothland and New York. Plant remains are very fully represented; land plants have been recorded from the Harz and Kellerwald (H. Potonie, 1901), and large silicified stemsup to 2 ft. in diameterperhaps representing a gigantic seaweed (Nematophycus), have been found in Wales and in Canada. Pachytheca is a small spherical body often associated with Nematophycus. Girvanella is another obscure algal plant. As a natural result of the open character of the great Silurian periarctic sea referred to above, there are many points of resemblance between the fauna of the several regions of the N. hemisphere; this has been specially noticed in the community not only of genera but of species between Britain, Sweden and the interior of N. America (Iowa, Wisconsin, Illinois). ' Goniophyllum pyramidale is common to Iowa and Gothland; Atrypa reticularis, Orthoceras annulatum and not a few others are common to Europe and N. America. An extremely interesting circumstance is the admixture of a periarctic and Bohemian fauna in the Australasian region. In a general sense the Silurian period was one of comparative quiescence as regards crustal disturbances, and a relative sinking of the land was followed by a relative elevation
For further information, see articles on the CAMBRIAN, ORDOVICIAN, LLANDOVERY, \VENLOCII, LUDLOW Systems and Groups. (J. A. H.) End of Article: SILURIAN If you wish, you can link directly to this article.
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