Silurian System

Silurian System, a name given by Murchison in 1835 from the Silures (q.v.) in South Wales, where this system is well developed. The sedimentary strata consist principally of grits, slates, dark shales, flagstones, sandstones, and conglomerates, and interbedded with these occur occasional calcareous bands, and more or less lenticular beds of limestone. The following table gives the succession of the Silurian system of Britain:

The strata attain a thickness of more than 20,000 feet, and have a wide distribution. In Wales the basement beds rest conformably on the upper members of the Cambrian system, while a well-marked unconformity separates the Lower from the Upper Silurian. Silurian rocks are exposed at the surface in many of our upland areas: thus, they form a large part of the high grounds of Wales, Westmorland, and Cumberland, and the major portion of the Southern Uplands of Scotland, and they likewise extend into the Scottish Highlands. In the hilly parts of Ireland they are also well developed at the surface; nor can there be much doubt that the same strata, buried under younger systems, extend throughout the larger part of the British Islands. The deposition of the Lower Silurian was marked by the appearance of considerable volcanoes in Wales, Westmorland, Ayrshire, and the south-east of Ireland.

On the continent of Europe Silurian strata have an extensive development. They occupy large tracts in Scandinavia and the basin of the Baltic, and probably continue eastwards under the great plains of northern Russia, for they crop up along the flanks of the Ural Mountains. In middle and southern Europe they rise again and again to the surface in mountain-regions, from which it may be inferred that they underlie vast areas in the surrounding low grounds. In North America Silurian rocks extend westwards from the mouth of the St Lawrence by the great lakes into the far north-west, and south-west by the Alleghanies into Alabama. As more or less isolated areas rise to the surface in the interior of the continent, it is probable that Silurian strata, concealed under younger formations, extend throughout the interior continental basin; and they reappear in the Rocky Mountains. The system has been recognised as entering into the formation of the Cordilleras of South America. Silurian strata have also been detected in the Himalayas and other parts of Asia, in Australia, and New Zealand.

The Silurian rocks which occur in mountainous areas are usually much indurated and dip at high angles, being frequently much contorted and dislocated. In the Highlands of Scotland and in Norway they have even been subjected to such disturbance that they have over wide areas acquired a more or less schistose or foliated character. In other low-lying regions, however, as in Russia, the strata are not indurated, and occur in horizontal or gently inclined positions.

Life of the Period.—Plant-remains consist almost exclusively of seaweeds, only a few traces of land-plants having been met with. These indicate a cryptogamic flora—ferns and lycopodiaceous trees. The lower forms of animal life were represented chiefly by sponges (Amphispongia, Astylspongia, &c.). Amongst the Cœlenterata were numerous forms of graptolites and corals. The former are eminently characteristic of the Silurian, and abound in certain thin bands of black shale. Those with two rows of cells are mostly confined to the Lower Silurian, while the single-rowed graptolites are upon the whole most common in the Upper Silurian. Corals abounded—certain limestones appearing to be almost wholly made up of their remains. Some common or characteristic rugose forms were Omphyma, Zaphrentis, Cyathophyllum, &c. Prominent tabulate forms were Halyites, Favosites, &c. The living Alcyonarian corals (Heliopora) were represented by Heliolites. Crinoids or sea-lilies were very numerous—their jointed stems and arms entering largely into the composition of many of the limestones. Cystideans, which attained their maximum development in Silurian times, star-fishes, and brittle-stars appear to have been less abundant. Annelid-tracks are common on the surfaces of beds, and the filled-up burrows of sea-worms frequently occur. Now and again jaws of annelids are likewise met with, and occasionally their tubular cases (Serpulites, Spirorbis) are seen attached to shells, corals, &c. Among the most characteristic Silurian fossils were the Trilobites, which made their first appearance in Cambrian seas and reached their greatest development in the Silurian age. They lived on in much diminished numbers through the Devonian and Carboniferous periods, and became finally extinct in Permian times. Some of the more widely distributed Silurian forms are Ogygia, Trinucleus, Asaphus, Illænus, Phacops, Calymene, &c. Other remarkable Arthropods were the Enrypterids (an extinct order allied to the existing king-crabs). One of these (Pterygotus) was seven or eight feet in length. Ostracods (Beyrichia) and Phyllopods (Ceratocaris) likewise occur in Silurian rocks. Here also we meet with the first scorpion (Paleophoneus) and the earliest insect (Paleoblattina, a form of cockroach). Various lace-corals

(Fenestella) and other Polyzoa occur, but by far the most abundant forms amongst Molluscoids were the Brachiopods. Characteristic types are Discina, Orthis, Leptæna, Pentamerus, Rhynchonella, Strophomena. Lamellibranchs were much less numerous—Modiolopsis, Ctenodonta, and Orthonota are examples. Amongst Gastropods the more common genera are Euomphalus, Bellerophon, Holopella, Murchisonia, and Pleurotomaria. The Cephalopods are well represented by straight and coiled chambered cells: amongst the former is Orthoceras, many species of which are known; the latter show such types as Cyrtoceras, Lituites, Phragmoceras, Nautilus, &c. Vertebrates make their earliest known appearance in the Upper Silurian. The remains consist of bony bucklers or head-shields of ganoid fishes (Pteraspis, Cephalaspis), the defensive spines of some cestracodont, and fragments of shagreen-like skin and plates.

Physical Conditions.—The Silurian strata appear to have been deposited chiefly in shallow seas, which here and there, however, may have been moderately deep. No certain indications of true oceanic conditions have yet been met with. During the formation of the Lower Silurian strata the entire area of the British Islands, with the exception, perhaps, of some of the Archæan tracts of the north-west, appears to have been under water. At this time active volcanoes, forming groups of islets, were scattered over the area of what is now North and South Wales, the south-east of Ireland, and southern Ayrshire. Considerable earth-movements affected the British area at the close of the Lower Silurian period—the bed of the sea being here and there elevated, so that islands of considerable size came into existence at the beginning of the Upper Silurian period. Eventually, however, a movement of depression supervened, and the islands referred to were submerged and gradually buried under the sedimentary accumulations of the Upper Silurian sea. Judging from the geographical distribution of the Silurian strata, we are led to conclude that enormous areas of what are now our continents were during the accumulation of those rocks overflowed by shallow seas. The main land-masses of the period seem to have been grouped chiefly in boreal regions and were composed essentially of Archæan rocks. In Europe and North America alike the old land-surface lay towards the north, but the great continental ridges may have risen here and there to the surface in other places so as to form groups of islands, as in Bavaria and Bohemia in Europe, and in the region of the Colorado and Park ranges in America.

The temperature of the seas was such as to allow of the migration of closely allied and apparently identical species of molluscs, crustaceans, &c. over vast regions. In arctic lands fossils occur which are met with likewise in the Silurian rocks of Wales; more than this, many Silurian species were truly cosmopolitan, ranging from the extreme north across the equator to Australia. To have allowed of such world-wide distribution the temperature of the seas must have been singularly uniform. But while not a few Silurian species were cosmopolitan, many others appear to have had a more restricted range. Thus it may be inferred that, however uniform the climatic conditions may have been, the uniformity nevertheless was only comparative, and that even in Silurian times the oceanic areas had their distinct life-provinces. It is remarkable that in the Lower Silurian of Scotland large erratic blocks of gneiss and other crystalline rocks occur, and similar blocks are met with in the Silurian strata of the Lake Superior region. It is difficult to see how such blocks could have been transported without the agency of floating ice. See also the articles MURCHISON and GEOLOGY.