SILURIAN SYSTEM (from Lat. Silures, a people of ancient Britain). A division of the Paleozoic group of rocks established by Murchi son (q.v.) to include the strata between the Archaean and Devonian systems. It was subse quently restricted to the two formations now known as the Ordovician or Lower Silurian and Upper Silurian. These two extend from the up per limits of the Cambrian to the base of the Devonian. Silurian rocks are extensively veloped in both the United States and Europe. The rocks of the Silurian system in America are divided as follows: The rocks are largely limestones, but there are also beds of shales and some sandstones inter stratified with them, While there was some dis turbance at the end of the Ordovician era. at the same time it was not sufficiently extensive in America to change materially or increase the ex tent of the land surface which existed in the Ordovician times. Silurian rocks are present in great thickness in the Eastern States, especially along the Appalachian region. The lowest forma tion, or that known as the Oneida, is a conglom erate which appears in central New York, thin ning toward the eastern shore line, but is very thick along the Appalachian ranges as far south as Tennessee. Owing to its great hardness, it forms many prominent ridges, notably the Sha wangunk Mountains of New York, also the crests of the Kittatinny Mountains, and the ridges at Delaware Water Gap.
Overlying the conglomerate is a great deposit known as the Medina sandstone, which was formed in shallow water and shows many rip ple marks. It extends from Central New York with decreasing thickness toward Ohio, but in eastern Pennsylvania the beds aggregate 1800 feet. Overlying this is the Clinton shale, which is well known from New York down into Georgia, and westward into Wisconsin. in which region it changes into limestone, indicating that the Silurian seas were deeper in that area than they were further 'east. A subsequent deepen ing of the water over a still greater area is indi cated by the formation of the Niagara limestone, which is well developed in the gorge of Niagara River, and whose resistance to erosion causes the abrupt descent of the Niagara River at the Falls. This formation ranges over a very large territory westward to Wisconsin, and then south ward through Illinois into Missouri and West Tennessee. Small areas are also found in Iowa, the Black Hills, and Nevada. Following this great limestone deposit there comes a series of shallow water deposits of salt. gypsum, and shale of the Salina stage, which are well de veloped in New York and Ohio, but thin out in Pennsylvania. ln sonic localities an argillaceous limestone was deposited during the same period, to which has been given the name of water-lime on account of its value in the manufacture of cement. On top of these beds lie great beds of limestones due to the deepening of the water in which the Silurian sediments were being de posited. To this great limestone series has been given the name of Lower Helderberg. It is proba ble that the depression made at this time sub merged some areas which had been dry land since Ordovician times, as in some cases we find the Lower Helderberg rocks resting directly on Ordovieian strata. The Lower Helderberg rocks are abundant in New York, where they form the bold escarpment of the Helderberg Mountains near Albany. but are also known to extend south ward through Pennsylvania to Virginia, while additional deposits are known in western Ten nessee and Maryland.
Silurian beds are well developed in Europe, China, Northern Africa, South America, and Australia, as well as in North America. At the termination of the Silurian there was a gradual transition into the Devonian, so that it is often difficult to determine the boundary line between the rocks of the two systems.
The plant life of the Silurian, so far as re vealed by the fossil remains, was scanty. Sea weeds were abundant, but land plants are rarely found. Among the animals there was a great development of invertebrates. Sponges were present in force, but the graptolites had di minished. Both the hydroid corals and the time corals were very important, the former be ing especially important as reef builders. Fa vosites and Halysites are two well-known fossil corals of the Silurian rocks. There was a marked increase of erinoids and also starfishes, while even the sea-urchins were fairly abundant. The trilobites also continued to flourish, although not as numerous as those of the Ordovician; among the common genera were Calymene, Dal manites, and Liehas. Some insects have also been found, such as scorpions, and prove that there roust have been land vegetation. The brachiopods continued in countless numbers, and the genera were quite different on the whole from those of the Ordovician. The most im portant were Atrypa, Spirifera, and Pentamerus. The bivalve mollusks were similar to those of the Ordovician, but other orders shoaled more or less change. Among the pteropods a very abundant form is the Tentaculites, whose remains occur in great numbers in certain strata of the lower Helderberg series. The only vertebrates that are known to have existed were fishes such as ostracoderms and sharks,• but their remains are rather fragmentary.
The economic minerals of the Silurian are fairly diversified. In the rocks of the Clinton age we find a very persistent bed of the hema tite iron ore known as the Clinton or fossiliferous iron ore. Wherever the Clinton rocks are found this ore is known to occur and forms the basis of the iron industry at Birmingham, Ala., where a deposit four miles long and from 12 to 20 feet thick is worked. In the rocks of the Salina group we find the deposits of gypsum and rock salt, the latter material being of great economic value in the State of New York. Many of the Silurian rocks are also excellently adapted for building purposes, and of these the Medina sandstone, named from its occurrence at Medina, New York, is specially well known.
BIBLIOGRAPHY. MllrchiSo11, Si/aria (London, Bibliography. MllrchiSo11, Si/aria (London, 1559) ; Geikie, Text-Book of Geology (ib., 1893) ; Dana, of Geology (4th ed., New York, 1896) ; Scott, Introduction to the Study of Geology (ib., 1902) ; and the following trea tises: Barton, "Notes on the Stratigraphy of a Portion of Central Appalachian, Virginia," American Geologist, vol. x. (Rochester, 1892) ; I'rosser, "The Thickness of the Devonian and Silurian Rocks of Central New York," Geological Society of America Bulletin, vol. iv. (ib., 1893) ; Weller, "The Silurian Fauna Interpreted on the Epieoutinental Basis," in Journal of Geology, vol. vi. (Chicago, 1898) ; Clarke, "Note on the Silura-Devouie Boundary," in Science, arum series, vol. xii. (New York, 1900) ; Schuehert, "Lower Devonie Aspect of the Lower Helderberg and Oriskany Formations," Geological Society of A m erica.), Bulletin., vol. xi. (Rochester, 1900). See CLINTON STAGE; SALINA STAGE, etc.