SILURIAN, a term applied in 1835 by Sir Roderick Murchison, director-general of the Geological Survey of Great Britain, to those rocks of South Wales (the country of the old Silures), which were formerly included as °grauwacke° in the Transition System of rocks. The term is also applied to the period during which this system of rocks was laid down, now recognized as the third period of the Paleozoic Era. In the typical region the rocks are divided as follows: • Approx. average thickness (feet) 6. Ludlow Group 1,900 Upper Silurian 5. Wenlock Group 1.600 4. Llandovery Group 3.000 (3. Bala and Cardoc Group 6.000 Lower Silurian 2. I landeflo Group 3,000 11. Arenig Group 4,000 19,500 While Murchison was studying these strata in South Wales, the renowned Adam Sedgwick studied the rocks of North Wales, making out their true succession, and naming them °Cam brian,° from the ancient name of the district, Cambria. These were also divided into an upper and lower division, and as the study con tinued year after year, higher and higher strata were added to the top of the Cambrian series. Likewise, in the study of the Silurian strata, Murchison included lower and lower strata, and thus the two systems overlapped. It finally appeared that Murchison's Lower Silu rian and Sedgwick's Upper Cambrian were equivalents, and this gave rise to a long con troversy as to the use of the proper term. This controversy outlived the two principals, and has only recently been settled by the adoption of Lapworth's name °Ordovician° for the middle series. Thus the term Silurian is restricted to day by most geologists to the Upper Silurian of Murchison.
In North America the following subdivisions are recognized: Upper Silurian or Cayugas group. Manlius limestones, Bertie limestones, Salina formation.
Middle Silurian or Niagaran group.
Guelph formation, • Lockport dolomites. Rochester shales, Clinton formation.
Lower Silurian or Oswegan group. Medina sandstone, Oneida conglomerate or sandstone.
Ordovician precedes and Devonian follows the Silurian. The formations are typically ex posed in the State of New York, where the lower members are conglomerates and sand stones, the middle shales, limestones and dolo mites and the upper shales and shaly lime stones. The formations are best exposed along the Niagara River, where most of them are visible. The aggregate thickness here is 2,100 feet, though the upper member (Manlius) is partially absent. Eastward in New York some
strata thin out, while others are absent through erosion. The Silurian strata are well de veloped in Michigan, Ohio, Indiana, Illinois, Wisconsin, Kentucky and Tennessee, though generally much thinner than in New York. They are also more calcareous. In the Ap palachians the Silurian strata are frequently represented by shore deposits. Throughout most of their extent, the strata are richly fossilifer ous, containing sponges, graptolites, corals, brachiopods, mollusks and triliobites. Fish are also found, but are rare. Toward the end of Silurian time the remarkable crustaceans known as eurypterids (q.v.) became abundant and con stituted a marked feature of the life.
As a result of the Taconic Revolution, at the close of the Ordovician (q.v.), the seas were much restricted at the beginning of the Silurian, in the United States. During the Clinton coastal swamp conditions prevailed over much of Eastern United States, and great sedimentary beds of iron ore were laid down, from New York to Alabama. These are very extensively mined in New York and at Birmingham, Ala. During mid-Silurian time the interior con tinental sea was very greatly enlarged, and was united with the waters covering parts of Europe, probably by a channel across the Arctic region. This permitted migration from European seas into central North America, so that we find many Silurian fossils of this region also common in the European localities (Gotland, etc.). The widespread Niagara lime stone was formed in this sea. Migration along an Atlantic coast line between Western Europe and eastern North America also took place in Silurian time, as shown by the similar faunal characteristics of the Silurian beds of Maine arid the island of Anticosti to those of southern England.
In late Silurian time the waters of the in terior sea were entirely enclosed and life be came extinct, the sea water itself becoming highly concentrated, probably under desert con ditions. In this enclosed basin the salt-bearing strata of the Salina formation were laid down, and these now constitute a great commercial source of salt.
Widespread marine formations in Europe show that that continent was also widely covered by shallow seas during the period. A central land mass probably separated a northern and a southern sea. See GEOLOGY; SALINA FORMATION; SALT.