Silurian, the name of one of the geologic ages, the age of mollusks and other invertebrates. The name is derived from that of the ancient Silures, who inhabited that portion of England and Wales where these rocks abound. The formation lies upon the Cambrian of Sedgwick, according to some classifications, and immediately below the Devonian. Murchison includes in it the upper Cambrian of Sedgwick. The subdivisions of the Silurian age differ in Europe and America, and also in different parts of the same continent. In North America the transition of the rocks and life from the lower to the upper Silurian is abrupt. In Great Britain the transition in life is gradual, although the rocks are unconformable in stratification. In Bohemia there is no break in the rocks, but there is marked change in the life. Dana has adopted the subdivision into periods and epochs derived 'from the succession of rocks in the state of New York, where the strata are well displayed, and have been carefully studied. In this arrangement the lower Silurian, beginning from below, includes the primordial or Cambrian, the Canadian, and the Trenton periods; the upper Silurian embraces, in the same ascending order, the Niagara, Salina, lower Helderberg, and Oriskany periods.

The Oris-kany formation was until recently placed as the lowest period of the Devonian age; but from the relations of its fossils it has been transferred to the Silurian. The Cambrian period has two epochs, the Acadian and the Potsdam. The Canadian period has the calciferous, the Quebec, and the Chazy epochs. The Trenton period embraces the Trenton, Utica, and Cincinnati epochs; the Niagara period, the Medina, Clinton, and Niagara epochs; while the Salina, lower Helderberg, and Oriskany periods have each one epoch, correspondingly named. The lower Silurian animal fossils are sponges, radiates, mollusks, and articulates; among the last are numerous trilobites, a species of which found near Braintree, Mass., in the Acadian formation, was 20 in. long. The calciferous and Quebec epochs of the Canadian period are remarkably rich in fossils and economic products, the latter including copper and silver ores. In Newfoundland the Quebec formation reaches a thickness of 6,600 ft., the upper half being sandstone and shales and the lower half mostly limestones. The Trenton period, abounding in fossils and economic products, among which is petroleum, has its formation along the Appalachians and over a large part of the Mississippi basin, including the galena limestone of Wisconsin and other states.

Trenton limestone has been found in the arctic regions, upon King William's island, North Somerset, and Boothia. The Niagara formation in North America covers a large part of the interior of the continent, and the arctic and other parts of British America, and also contains petroleum. At Niagara falls 85 ft. of limestone rest on 80 ft. of shale, and near the falls the shale is covered with 165 ft. of limestone. The Salina period includes the rocks which yield the salt brines of central New York. Through the Mississippi basin the Salina formation is for the most part absent. This formation contains numerous beds of gypsum, which are not stratified like the other rocks, and have been formed by the action of sulphuric acid upon limestone, the sulphuric acid being derived from sulphur springs. The Oriskany period contains no land plants in New York, but at Gaspe, province of Quebec, a small species of lycopodium or ground pine has been found. The most common animal fossils are bivalve mollusks. In Maryland there are five species of crinoids, but in New York they are rare. The rocks of both the lower and upper Silurian are widely distributed over the globe, although the lower are the most extensive.

The upper Silurian in Europe, besides invertebrate fossils, contains the vestiges of the earliest fishes, some of which are of the shark tribe; so that although the Devonian is the age of fishes, they really originated in the Silurian. It was formerly thought that the Silurian formation contained the earliest vestiges of organic life, but organic remains have recently been found in older formations. (See Geology, vol. vii., p. 694, and Palaeontology, vol. xii., pp. 811, 813, 816).