1911 Encyclopædia Britannica/Silurian

by John Allen Howe

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:—

	R. I. Murchison.	A. Sedgwick.	C. Lapworth.	American.	A. de Lapparent.			E. Renevier.
Silurian.  (Upper Silurian   of some authors.)	S i l u r i a n.	Silurian.	Silurian. (Salopian).	Niagaran (S. D. Dana). Ontaric or Siluric (Emmons, &c.), 1844.	S i l u r i e n.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Bohemian (2nd ed. Traité). Gothlandien (3rd-5th ed.).	S i l u r i q u e.
Ordovician.  (Lower Silurian  of some authors.)		C a m b r i a n.	Ordovician.	Silurian (Dana). Champlainic (Hall, Emmons, &c.).			Ordovician.
Cambrian.  Upper.  Middle.  Lower.	Cambrian.		Cambrian.				Cambrian.

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) 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 well-marked 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 arc 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.

Silurian Life.—Our knowledge 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 preceding 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 maximum development; Proehis, Bronteus, Cyphaspis, Arethusina may be mentioned from among many other genera. The ostracods Leperditia and Beyrichia are very abundant locally. A feature of great interest is the first appearance of the remarkable Eurypterid crustacean Eurypterus, which occasionally reached the length of over a yard, and of the limulids, Neolimulus and Hemiaspis. The cephalopods were the predominant molluscs, especially Orthoceras and various abbreviated or coiled orthoceras-like forms (Cyrtoceras, Phragmoceras, Trochoceras, Ascoceras); there was also a Nautilus, and an early form of goniatite has been recorded. Gasteropods include the genera Platyceras, Murchisonia and Bellerophon; the pteropod Tentaculites is very abundant in certain beds. The pelecypods were not very important (Cypricardinia, Cardiola interrupta, C. cornucopiae).

Next to the cephalopods in importance were the brachiopods : in the lower Silurian pentamerus-like forms still continued (P. Knightii, 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, Cyalhopkyllum, Cystiphyllum, Acervularia, Omphyma and the remarkable Goniophyllum. Sponges were represented

by Aslylospongia, Aulocopium, &c. The peculiar genera

Approximate Correlation of Silurian Rocks.

Graptolite Zones (Britain).

England and Wales.

Scotland.

Scandinavian.

Baltic Region.

Bohemia.

Western Europe.

E. Alps.

North America (New York).

Canada.

Nova Scotia.

New Brunswick.

Asia.

Australia.

Monograptus leintwardinensis.

Downton and Ludlow groups.

Downtonian and Raeberry Castle beds.

Upper Cardiola

Eurypterus beds.

Stage E2 of J. Barrande.

Manlius limestone.

beds and upper

Cephalopod or

Rondout

M. bohemicus.

Gothland limestone.

Limestones with

Water Lime. Cobleskill

cephalopods.

limestone.

M. Nilssoni.

Red sandstone.

Upper Oesel

Salina beds of

Lower cephalopod

beds:

Onondaga

limestone.

dolomite

with

Crinoid and coral

and

rock salt

limestone.

limestones.

and

gypsum.

Lower Cardiola

shales and Mega-

lomus limestones.

M. testis.

Wen lock

Riccarton,

Cyrtograptus

Lower Oesel

Crinoid

Guelph

and

Blair,

shales and lower

beds :

limestones.

dolomite.

Cyrtograptus

Wool hope

and

brachiopod and

dolomite

Linnarssoni.

groups.

Straiton

coral limestone

and

Lockport

Cyrtograptus

beds.

with sandstone.

marl.

limestone.

In a

Murchisoni,

Rochester

shales.

Clinton beds.

Rastriles

Tarannon,

Queensberry

Rastrites shales

Pentamerus

Stage Ei of

maximus.

Llandovery,

beds.

and Strickland-

beds.

J. Barrande.

Medina

and

inia marls.

Graptolite

sandstone.

M. spinigerus.

May Hill

Birkhill shales

shales

groups.

and

with

Oneida

851

M. grcgarius.

Graptolitic beds

diabase

conglomerate.

of the at the base. Diphgraptus

Girvan area.

Shawangunk

vesiculosus.

grit

Diplo^raptus atuminatus.

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 stems—up to 2 ft. in diameter—perhaps 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 affecting wide areas in the N. hemisphere. Local oscillations, such as those taking part in the formation of the Salina beds, &c., were naturally taking place, but the folding of the Scandinavian mountains and in the N. highlands of Scotland continued throughout the period accompanied by a great amount of thrusting. Volcanic activity was quite subordinate in Silurian times; flows of diabase occurred at the commencement of the period in Bohemia, and evidence of minor basaltic flows and tuffs is found at Tortworth in Gloucestershire and at a few localities in N. America.

For further information, see articles on the Cambrian, Ordovician, Llandovery, Wenlock, Ludlow Systems and Groups.  (J. A. H.)